Advances in Psychology Volume 65 Stimulus-Response Compatibility: An Integrated Perspective

STIMULUS-RESPONSE COMPATIBILITY An Integrated Perspective ADVANCES IN PSYCHOLOGY 65 Editors: G.E. STELMACH P.A. VRO...

Author: Robert W. Proctor

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STIMULUS-RESPONSE COMPATIBILITY An Integrated Perspective

ADVANCES IN PSYCHOLOGY

65 Editors:

G.E. STELMACH P.A. VROON

NORTH-HOLLAND AMSTERDAM * NEW YORK OXFORD * TOKYO

STIMULUS-RESPONSE COMPATIBILITY An Integrated Perspective

Edited by

Robert W. PROCTOR Department of Psychological Sciences Purdue University West Lafayette, Indiana, U.S.A.

T. Gilmour REEVE Motor Behavior Center Auburn University Auburn, Alabama, U.S.A.

1990

-

-

NORTH-HOLLAND AMSTERDAM NEW YORK OXFORD TOKYO

NORTH HOLLAND ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 P.O. Box 21 1, 1000 AE Amsterdam, The Netherlands

Distributors for the United States and Canada: ELSEVIER SCIENCE PUBLISHING COMPANY, INC. 655 Avenue of the Americas New York, N.Y. 10010, U.S.A.

ISBN: 0 444 88092 5

0ELSEVIER SCIENCE PUBLISHERS B.V., 1990 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science Publishers B.V./ Physical Sciences and Engineering Division, P.O. Box 1991, 1000 BZ Amsterdam, The Netherlands. Special regulations for readers in the U.S.A. - This publication has been registered with the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the U.S.A. All other copyright questions, including photocopying outside of the U.S.A., should be referred to the copyright owner, Elsevier Science Publishers B.V., unless otherwise specified.

No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. pp. 3-30, 261-276: copyright not transferred. Printed in The Netherlands

V

FOREWORD T h e p r i n c i p l e o f compatibility in psychology a n d human f a c t o r s was developed as an o u t g r o w t h o f psycho-acoustic research b e g u n during World War I I . In 1946, I was called on t o leave m y position a t t h e U n i v e r s i t y o f Iowa (Head o f t h e V i o l i n Department in t h e School o f Music a n d D i r e c t o r o f t h e Psycho-Acoustic L a b o r a t o r y in t h e Psychology Department) t o d i r e c t psycho-acoustic research f o r t h e U n i v e r s i t y o f California's Division o f War Research in San Diego, California. At the Division, t h e war-time mission i n c l u d e d all aspects o f s o u n d Cavigation a n d r a n g i n g (sonar). Research a n d development were in p r o g r e s s o n u n d e r w a t e r acoustics, equipment development, a n d tactics. T h e missing research segment o f concern in t h e sonar system was t h e operator. Sonar i s an operator-equipment-environment i n t e r a c t i v e system used o n ships t o d e t e c t objects in t h e sea. In i t s a c t i v e mode o f operation, an u l t r a s o n i c p u l s e is t r a n s m i t t e d u n d e r w a t e r in a g i v e n d i r e c t i o n . Reflect i o n s f r o m solid objects a r e detected by t h e equipment, h e t e r o d y n e d t o 800 hertz, a n d presented as audio signals t o t h e o p e r a t o r . Progress of t h e signal t h r o u g h t h e w a t e r g i v e s r i s e t o r e v e r b e r a t i o n that, along w i t h a n y i n h e r e n t sea noise, causes masking o f t h e reflected signal. T h e sonar operator's t a s k s a r e t o d e t e c t t h e reflected signal in t h e noise, t h e n t o i d e n t i f y i t s p r o b a b l e source, a n d t o r e p o r t t h e source t o higher ship authority. Each o f these t a s k s involves v a r y i n g degrees o f u n c e r t a i n t y , a n d time is o f t h e essence in success t o w a r d e f f e c t i v e operations. I n t e n s i v e t r a i n i n g is r e q u i r e d t o accomplish t h e tasks. E f f e c t i v e performance by t h e sonar o p e r a t o r r e q u i r e s e a r l y detect i o n o f masked signals. Thus, t h e psycho-acoustic research a t t h e D i v i sion o f War Research focused o n signal detection. The results of this research generated a theory o f backward, o r residual, masking a n d were p u t t o use by t h e f l e e t worldwide. With t h e e n d o f World War II, t h e N a v y ' s Bureau o f Ships absorbed t h e Division o f War Research as t h e N a v y Electronics L a b o r a t o r y . The Laboratory's responsibilities were broadened t o i n c l u d e all electronic systems-related research a n d development, w i t h consequent b r o a d e n i n g of human f a c t o r s involvement. I was requested t o remain a n d develop a Human Factors Division. As indicated earlier, t h e t a s k o f d e t e c t i n g masked audio signals was d i f f i c u l t . However, no o t h e r modality o f signal d i s p l a y h a d been available on ships. Now as a peace-time research e f f o r t , it seemed desirable t o s t u d y a n d evaluate t h e addition o f a v i s u a l display, used simultaneously w i t h t h e a u d i t o r y display, in an e f f o r t t o enhance signal detection.'

ISee Small, A . M., & Gales, R . S . (1950). Recognition o f u n d e r w a t e r sounds. In U n d e r w a t e r acoustics research: A s u r v e y r e p o r t o n basic problems (pp. 123-1371, Washington, DC: Committee o n Undersea Warfare, National Research Council.

vi

Foreword

T h e o n l y available real-time visual d i s p l a y o f sonar signals was t h e cathode r a y t u b e w i t h A-scan. ( T h e 5 - s c a n t u b e was in i t s i n f a n c y . ) With A-scan, t h e light beam traces a horizontal l i n e f r o m l e f t t o right w i t h v e r t i c a l p e r t u r b a t i o n s , t h a t is, spatial excursions of t h e beam indic a t i n g t h e presence a n d level o f b a c k g r o u n d noise a n d signals, if present. Detection research was conducted w i t h s k i l l e d subjects, as i n e a r l i e r tests, but now u s i n g simultaneous bimodal d i s p l a y o f t h e t e s t materials. T h e r e s u l t s o f t h i s research on bimodal d i s p l a y were inconsistent, w i t h signal detection t h r e s h o l d s v a r y i n g f r o m p l u s o r minus t h r e e decibels, compared w i t h audio alone. In examining t h e t e s t p l a n a n d r e s u l t s f o r possible reasons f o r t h i s inconsistency, it was realized t h a t o u r d i s p l a y conditions i n v o l v e d "incompatible" a t t r i b u t e s . T h a t is, t h e c h a r a c t e r i s t i c o f t h e a u d i t o r y stimulus was variation in amplitude o f vibration, leading t o perception o f loudness changes, whereas t h e c h a r a c t e r i s t i c o f t h e c o n c u r r e n t v i s u a l stimulus was deviations f r o m t h e cathode r a y beam trace, leading t o perception of spatial position changes. Compatibility w o u l d require, therefore, t h a t comparable stimulus charact e r i s t i c s b e i n v o l v e d in t h e d u a l sensory display, r e q u i r i n g t h a t t h e v a r i able c h a r a c t e r i s t i c o f t h e v i s u a l stimulus should b e i n t e n s i t y , leading t o perception o f b r i g h t n e s s changes. Thus, t h e p r i n c i p l e o f compatibility was formulated. In 1951, I presented a p a p e r a t t h e Ergonomics Research Society in England on Compatibility as a Principle i n Multi-Sensory Displays. I recall t h a t Paul M. F i t t s , who was in attendance, p r o n o u n c e d formulation o f t h e compatibility p r i n c i p l e as a landmark o f g r e a t significance w i t h broad applicability. In t h e n e x t few years, h e a n d h i s students concent r a t e d on t h i s research area, a n d we met several times d i s c u s s i n g issues. T h e compatibility p r i n c i p l e soon f o u n d application in psychological a n d human-factors research a n d application i n v o l v i n g n o t o n l y stimulus-stimulus compatibility, but also stimulus-response a n d response-response compati bi Iit y .

T h e l i t e r a t u r e on compatibility effects is expanding, w i t h research a n d applications r e p o r t e d f r o m t h e U n i t e d States, Europe, a n d Asia. Among t h e areas of i n t e r e s t a r e information processing, t h e motor system, learning, language, memory, human performance, a n d person-machineenvironment design. The development of t h i s book is indeed timely, therefore, a n d I believe a v e r y s i g n i f i c a n t c o n t r i b u t i o n . I trust the reader w i l l understand, therefore, m y exhilaration w i t h these developments a n d m y expression of deep appreciation t o t h e e d i t o r s a n d a u t h o r s o f t h e p r e s e n t volume.

A r n o l d M. Small, S r . Professor Emeritus U n i v e r s i t y o f Southern California Los Angeles, California

vii

PREFACE T h e t e r m "stimulus-response compatibility" was popularized by t h e research o f Paul M. F i t t s a n d h i s colleagues, i n w h i c h assignments o f stimuli t o responses were manipulated. F i t t s adopted t h i s terminology f r o m a presentation by A r n o l d M. Small t o t h e English Ergonomics Research Society in 1951. Stimulus-response (S-R) compatibility r e f e r s t o t h e finding t h a t c e r t a i n mappings o f stimuli t o responses p r o d u c e f a s t e r a n d more accurate responding t h a n d o o t h e r s . Compatibility effects also a r e f o u n d f o r stimulus-stimulus ( S - S ) relations a n d response-response (R-R) relations. Since t h e i n i t i a l study, p u b l i s h e d by F i t t s a n d Seeger in t h e 1953 volume o f t h e Journal of Experimental Psychology, a wealth o f research on compatibility effects has been conducted. T h i s research has r a n g e d f r o m basic investigations in laboratory s e t t i n g s t o applied research in f i e l d settings. Because of t h e d i v e r s i t y of research on compatibility effects, p r o g r e s s t o w a r d a comprehensive, theoretical u n d e r s t a n d i n g o f t h e effects has been slow. Compatibility effects l o n g have been r e g a r d e d as t h e p r o v i n c e o f t h e areas o f human performance a n d human f a c t o r s . One reason f o r t h i s is t h a t many studies focused p r i m a r i l y on w h i c h o f several arrangements a n d assignments o f stimuli a n d responses was most compatible. However, w i t h t h e r i s e in p o p u l a r i t y o f c o g n i t i v e psychology o v e r t h e p a s t t w o decades, an increasing p r o p o r t i o n o f t h e research o n compatibility effects has been devoted t o d e f i n i n g t h e i r fundamental n a t u r e . T h e knowledge t h a t has been gained f r o m these studies indicates t h a t compatibility effects r e f l e c t basic c o g n i t i v e processes t h a t b e a r on a b r o a d r a n g e o f issues in c o g n i t i v e science. T h e recent research on compatibility effects f a l l s i n t o several b r o a d categories, i n c l u d i n g (a) mental representation, (b) psychophysiological indices a n d neurophysiological mechanisms, (c) motor performance, a n d (d) applications t o human f a c t o r s . T h e research o n representation a n d coding has established t h a t S-R compatibility effects r e f l e c t t h e n a t u r e o f t h e codes used t o t r a n s l a t e between stimuli a n d responses. F o r many situations, t h e coding is predominantly spatial. However, coding can o c c u r on t h e basis o f o t h e r salient features o f t h e stimulus a n d response sets. T h e research on neurophysiological bases a n d psychophysiological indices suggests t h a t c e r t a i n compatibility phenomena can b e related t o t h e s t r u c t u r e o f t h e b r a i n a n d t h a t psychophysiological measures can b e used t o discriminate effects t h a t appear t o b e similar when o n l y reaction times a r e examined. T h e research on motor performance has h i g h l i g h t e d t h e pervasiveness o f S-R compatibility effects a n d shown them t o b e independent f r o m t h e processes i n v o l v e d i n t h e i n i t i a t i o n a n d c o n t r o l o f movements. Finally, models developed f o r human-factors purposes show t h a t S-R compatibility effects r e f l e c t general aspects o f t h e humaninformation processing system. These models also show t h a t t h e r e l a t i v e compatibilities o f a l t e r n a t i v e assignments i n a v a r i e t y o f situations can b e p r e d i c t e d f r o m a common set o f processes.

viii

Preface

I n short, t h e p r o g r e s s made along several f r o n t s o f S-R compatibility research suggests t h a t we a r e o n t h e v e r g e o f a comprehensive, theoretical u n d e r s t a n d i n g o f compatibility phenomena. T o date, l i t t l e attempt has been made t o p r o v i d e a comprehensive o v e r v i e w t h a t integrates t h e views t h a t have been emerging f r o m t h e d i f f e r e n t lines o f research. T h e p u r p o s e o f t h e p r e s e n t volume is t o summarize t h e major f i n d i n g s w i t h i n each o f t h e categories o f research a n d t a k e an i n i t i a l step t o w a r d p r o v i d i n g an i n t e g r a t e d p e r s p e c t i v e . T h e theoretical issues addressed should b e o f i n t e r e s t t o researchers in perception, cognition, a n d action, as well as t o c o g n i t i v e scientists in general. Moreover, t h e p e r s p e c t i v e developed has applied implications t h a t should b e o f value t o humanfactors engineers. T h e p r e s e n t volume reflects t h e e f f o r t s o f numerous i n d i v i d u a l s . T h e most obvious a r e t h e c o n t r i b u t o r s , whom we would l i k e t o t h a n k f o r t h e i r excellent c h a p t e r s . Also, George E. Stelmach, t h e series co-editor, a n d K. Michielsen, o f N o r t h - H o l l a n d Publishers, p r o v i d e d encouragement f o r u s t o p r e p a r e t h e volume. O u r g r a d u a t e students, Daniel J. Weeks, Lanie Dornier, L a r r y P. Wiley, a n d M a r k A. Guadagnoli, p r o v i d e d h e l p f u l discussions a n d assisted t h e p r o j e c t i n various ways. B e t t y e Campbell a n d h e r staff, M a r k i e G a r d n e r a n d Bonnie Phillips, a t A u b u r n U n i v e r s i t y typed and printed the final manuscript. Janet S u g g assisted w i t h t h e placement o f f i g u r e s in t h e m a n u s c r i p t . Finally, t h i s p r o j e c t was s u p p o r t e d i n p a r t by a g r a n t f r o m t h e L i f e Sciences D i r e c t o r a t e o f the A i r Force O f f i c e o f Scientific Research (#AFOSR-88-0002). We would l i k e t o t h a n k A l f r e d R. Fregly, t h e manager o f t h e AFOSR p r o g r a m i n cognition, a n d John Jonides, consultant t o t h e program, f o r t h e s u p p o r t .

R o b e r t W. Proctor Purdue University T. Gilmour Reeve Auburn University

ix

CONTENTS Foreword

..............................................................

v

A r n o l d M. Small, S r . Preface

..............................................................

Contents

.............................................................

List of Contributors

..................................................

vii ix

...

xi11

P A R T I: ISSUES I N STIMULUS-RESPONSE C O M P A T I B I L I T Y 1. T h i n g s T h a t Go T o g e t h e r : A Review o f Stimulus-Response Compatibility a n d Related Effects ...............................

.3

Earl A . A l l u i s i a n d Joel S . Warm

2. T h e Effects o f an I r r e l e v a n t Directional Cue on Human Information Processing

.................................

.31

......................

.89

J. R i c h a r d Simon P A R T II: MENTAL REPRESENTATION

3 . Spatial Stimulus-Response C o m p a t i b i l i t y . . Carlo UmiltS a n d Roberto Nicoletti 4.

Spatial Coding a n d Spatio-Anatomical Mapping: Evidence f o r a Hierarchical Model o f Spatial Stimulus-Response Compatibility .............................. Gabriele Heister, Peter Schroeder-Heister, Walter H. Ehrenstein

5. Some Aspects o f Spatial Stimulus-Response Compatibility in A d u l t s a n d Normal C h i l d r e n

.117

and

. . . . . . . . . . . . . . . . . . .145

Elisabetta LBdavas

6.

T h e Salient-Features Coding P r i n c i p l e f o r Spatial- a n d Symbolic-Compatibility Effects

. . . . . . . . . . . . . . . . ... .163

T. Gilmour Reeve a n d Robert W. P r o c t o r PART 1 1 1 : 7.

PSYCHOPHYSIOLOGICAL INDICES A N D NEUROPHYSIOLOGICAL MECHANISMS

Stimulus-Response Compatibility Viewed f r o m a C o g n i t i v e Psychophysiological Perspective.. . . . . . . . . . . . . . . . . .. . ,183 Theodore R. Bashore

Contents

X

8. Cerebral Evoked Potentials: E a r l y Indexes o f Compatibility E f f e c t s .........................................

.225

R i c h a r d Ragot 9.

Psychological a n d Neurophysiological Factors in Stimulus-Response Compatibility ..............................

.241

John B r e b n e r

10. Attentional Processes in Spatial Stimulus-Response Compatibility

..................................................

261

Mieke Verfaellie, Dawn Bowers, a n d Kenneth M. Heilman PART

IV:

MOTOR PERFORMANCE

11. Stimulus-Response Compatibility a n d t h e Programming o f Motor A c t i v i t y : Pitfalls a n d Possible New Directions ......................................

.279

Howard N . Zelaznik a n d Elizabeth Franz 12. Response Selection a n d Motor Programming: E f f e c t s of Compatibility a n d Average V e l o c i t y ...........................

.297

Will A. C . S p i j k e r s 13.

Rapid Responses w i t h t h e L e f t or R i g h t Hand: ResponseResponse Compatibility Effects Due t o lntermanual Interactions ............................ . . . . . . . . . . . . . . . . . . . . . . .

311

H e r b e r t Heuer 14.

Perceptual-Motor Processing in Speech..

......................

.343

Peter C. Gordon PART

V: APPLICATIONS

TO HUMAN FACTORS

15. Stimulus-Response Compatibility a n d Human Factors ............. .... B a r r y H. Kantowitz, Thomas J. T r i g g s , a n d Valerie E. Barnes 16. T h e Mental Model i n Stimulus-Response Compatibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ray E. E b e r t s a n d Jack W. Posey 17. T o w a r d an Engineering Model o f Stimulus-Response Compatibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bonnie E. John a n d Allen Newell

389

Contents

xi

P A R T V I : A N I N T E G R A T E D PERSPECTIVE

18. Research on Stimulus- Response Compatibility : Toward a Comprehensive A c c o u n t . . ...........................

.483

Robert W. Proctor and T . Gilmour Reeve AUTHOR I N D E X SUBJECT I N D E X

......................................................

495

......................................................

This Page Intentionally Left Blank

xiii

L I S T OF CONTRIBUTORS E a r l A. A l l u i s i

O f f i c e o f t h e S e c r e t a r y o f Defense (Research a n d Advanced Technology) T h e Pentagon Washington, D C

Valerie E. Barnes

Battelle Human A f f a i r s Research Centers Seattle, Washington

Theodore R. Bashore

Department o f P s y c h i a t r y Medical College o f Pennsylvania a t -EPPI Philadelphia, Pennsylvania

Dawn Bowers

Department of Neurology U n i v e r s i t y o f Florida Gainesville, Florida

John B r e b n e r

Department o f Psychology T h e U n i v e r s i t y of Adelaide Adelaide, South A u s t r a l i a

Ray E. E b e r t s

School of I n d u s t r i a l Engineering Purdue University West Lafayette, Indiana

Walter

H.

Ehrenstein

Elizabeth Franz

l n s t i t u t f u e r Arbeitsphysiologie an d e r U n i v e r s i t a e t D o r t m u n d Dortmu nd, West Germany Motor Behavior L a b o r a t o r y Department o f Physical Education, a n d Recreation Studies Purdue University West Lafayette, Indiana

Health

Peter C. Gordon

Department of Psychology Harvard University Cambridge, Massachusetts

Kenneth M. Heilman

Department of Neurology U n i v e r s i t y o f Florida and Gainesville Veterans Administration Medical Center Gainesville, Florida

Gabriele Heister

A b t e i l u n g Neuropsychologie Universitaetsspital Zuerich Zuerich, Switzerland

H e r b e r t Heuer

Fachbereich Psychologie der Philipps-Universitaet M a r b u r g , West Germany

List of Cpntributors

xiv

Bonnie E. John

Department o f Psychology Carnegie Mellon U n i v e r s i t y P i t t s b u r g h , Pennsylvania

B a r r y H. Kantowitz


Elisabetta L i d a v a s

Dipartimento di Psicologia Universita di Bologna Bologna, I t a l y

Allen Newell

Department of Computer Science Carnegie Mellon U n i v e r s i t y P i t t s b u r g h , Pennsylvania

Roberto Nicoletti

Dipartimento di Psicologia Generale Universita di Padova Padova, I t a l y

Jack W. Posey

School o f I n d u s t r i a l Engineering Pu r d u e U n i v e r s i t y West Lafayette, Indiana

Robert W. Proctor

Department of Psychological Sciences Purdue University West Lafayette, I n d i a n a

Richard Ragot

C e n t r e National d e l a Recherche Scientifique UA654-LENA-Hopital d e la Salpetriere Paris, France

T . Gilmour Reeve

Motor Behavior Center Department o f Health a n d Human Performance Auburn University A u b u r n , Alabama

Peter Sch roeder- Heister

Zentrum Phi losoph ie u n d Wissenschaftstheorie Universitaet Konstanz Konstanz, West Germany

J. Richard Simon

Departments of Psychology a n d I n d u s t r i a l and Management Engineering U n i v e r s i t y o f Iowa Iowa C i t y , Iowa

A r n o l d M. Small;

Sr.

Will A. C . Spijkers

Human Factors Department I n s t i t u t e o f Safety and Systems Management U n i v e r s i t y of Southern California Los Angeles, California I n s t i t u t e f o r Psychology U n i v e r s i t y of Technology Aachen, West Germany

List of Contributors Thomas J . T r i g g s


Carlo Umilt6

l s t i t u t o di Fisiologia Umana U n i v e r s i t s di Parma Parma, I t a l y

Mieke Verfaellie

Memory Disorders Research C e n t e r Boston Veterans Administration Medical Center a n d Boston U n i v e r s i t y School o f Medicine Boston, Massachusetts

Joel S. Warm

Department o f Psychology University of Cincinnati Cincinnati, Ohio

Howard N. Zelaznik

Motor Behavior L a b o r a t o r y Department o f Physical Education, Health, a n d Recreation Studies and Department o f Psychological Sciences Purdue Universty West Lafayette, Indiana

xv


PART I ISSUES IN STIMULUS-RESPONSE COMPATI B I L l T Y T h e t w o chapters o f P a r t I p r o v i d e e x t e n s i v e reviews of research on stimulus-response compatibility a n d related compatibility effects. C h a p t e r 1, by A l l u i s i a n d Warm, takes a h i s t o r i c a l approach a n d l i n k s compatibility research t o t h e e a r l i e r studies o f population stereotypes. T h e c h a p t e r p r o v i d e s examples of v a r i o u s t y p e s o f compatibility effects a n d discusses possible mechanisms t h a t u n d e r l i e t h e effects. C h a p t e r 2, by Simon, describes his d i s c o v e r y o f what has come t o b e r e f e r r e d t o as t h e "Simon effect," w h i c h is t h a t reaction times t o symbolic stimuli a r e affected by t h e relation of an i r r e l e v a n t stimulus location t o t h e r e l e v a n t response location. T h e c h a p t e r also reviews t h e progression o f Simon's research, s t r e s s i n g t h e logic b e h i n d his experiments a n d t h e c r u c i a l findings. T h e chapters i n P a r t I p r o v i d e a good i n t r o d u c t i o n t o compatibility research, w i t h many o f t h e topics a n d issues t h a t a r e i n t r o d u c e d in these c h a p t e r s b e i n g p u r s u e d l a t e r i n t h e book.


STIMULUS-RESPONSE COMPATIBILIN R. W. Proctor and T.G. Reeve (Editors Elsevier Science Publishers B. V. (dorth-Holland). 1990

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THINGS T H A T GO TOGETHER: A REVIEW OF STIMULUSRESPONSE C O M P A T I B I L I T Y A N D RELATED EFFECTS EARL A . A L L U l S l O f f i c e o f t h e S e c r e t a r y o f Defense (Research a n d Advanced Technology) JOEL S. WARM Department o f Psychology University of Cincinnati A p r i m a r y goal o f c o g n i t i v e psychology is t o broaden a n d deepen o u r u n d e r s t a n d i n g o f t h e phenomena a n d mechanisms o f human behavior. T h e operational t e s t o f t h e v a l i d i t y o f t h a t u n d e r s t a n d i n g lies in t h e successful application o f t h e p r i n c i p l e s d e r i v e d therefrom, f o r example, i n human-factors engineering. T h u s , c o g n i t i v e psychology a n d humanf a c t o r s e n g i n e e r i n g have a common b o n d - - t h e one produces t h e basic u n d e r s t a n d i n g a n d p r i n c i p l e s t h a t t h e o t h e r validates t h r o u g h successful applications. In these applications, a p r i m a r y goal o f human f a c t o r s e n g i n e e r i n g [,s t o enhance t h e design a n d i n t e g r a t i o n o f hardware, software, a n d

Iivewa r e ” components a n d interfaces i n t o e f f e c t i v e human -machine systems. T h e demands made on t h e liveware o r human-performance components i n such systems a r e generally viewed in stimulus-response (SR) terms. In t h i s context, t h e c o r r e c t paradigm involves n o t merely single stimulus a n d response p a i r i n g s , but r a t h e r sets o r alphabets o f stimuli a n d responses t h a t a r e matched o r coded i n t o S-R ensembles. It follows t h a t t h e t o p i c o f things t h a t go together--alphabets, S-R ensembles, optimum coding, S-R compatibility, a n d related effects--is, o r should be, c e n t r a l n o t o n l y t o t h e i n t e r e s t s of t h e c o g n i t i v e psychologist, but also t o t h e r e p e r t o i r e o f e x p e r t i s e o u t o f w h i c h t h e human-factors specialist c o n t r i b u t e s t o t h e design a n d use o f human-machine systems. T h a t t o p i c i s t h e focus o f t h i s c h a p t e r . Ensembles a n d C o m p a t i b i l i t y E f f e c t s In o r d e r t o have a usable code, it is necessary t o have a set o f discriminable stimuli. B u t t h a t is n o t s u f f i c i e n t . These stimuli must also b e a r r a n g e d i n t o an alphabet. T h e stimulus alphabet m u s t t h e n b e matched t o a response alphabet (an alphabet o f responses d r a w n f r o m some set o f discriminable responses) t o f o r m a code o r S-R ensemble.

T h e S-R ensemble must b e learnable a n d usable. T h a t is, t h e humans who c o n s t i t u t e t h e system’s liveware must b e able t o learn t o associate the symbols in t h e response alphabet w i t h those in t h e stimulus alphabet a n d t o d o so well enough t o p e r f o r m c o n s i s t e n t l y a n d c o r r e c t l y .

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E.A. Alluisi and J.S. Warm

I t has l o n g been established t h a t t h e ease a n d e x t e n t t o which these t w o objectives a r e attained is a f u n c t i o n o f t h e stimuli a n d responses used. T h i s dependence of performance o n t h e specific p a i r i n g s o f t h e stimulus alphabet w i t h a response alphabet (i.e., on t h e specific ensemble) i s what. F i t t s a n d Seeger (1953) called an S - R compatibility effect, a f t e r a 1951 suggestion o f A r n o l d Small's. It has likewise been long established t h a t S-R compatibility effects a r e r e l a t i v e l y l a r g e in magnitude. T h e y a r e especially l a r g e in comparison w i t h t h e effects e i t h e r o f variations in t h e amounts o f improvement p e r S-R ensemble w i t h practice, o r o f practice i t s e l f (Deininger & Fitts, 1955; F i t t s & Deininger, 1954). T h e effects are consistent a n d independent o f t h e performance measure employed. The e r r o r s associated w i t h a g i v e n S-R ensemble a r e positively correlated w i t h t h e reaction times ( R T s ) associated w i t h t h e same ensemble. Thus, an ensemble t h a t produces slow performance also tends t o produce inaccurate performance. Deininger a n d F i t t s (1955) observed t h a t although individuals slow down t o some degree while p e r f o r m i n g a p o o r l y encoded task, t h e y d o n o t slow down enough t o overcome t h e effects o f t h e poor encoding. Instead o f gaining accuracy a t t h e expense o f speed, t h e human sacrifices b o t h speed and accuracy in t h e process o f re-encoding t h e information. S-R compatibility effects a r e also r e l a t i v e l y stable, b u t t h e y a r e accentuated u n d e r conditions o f workload stress. For example, extraneous a c t i v i t y , such as a secondary task, affects performance least w i t h t h e most compatible ensembles a n d degrades performance most w i t h t h e least compatible p a i r i n g s (Garvey & Knowles, 1954).

In brief, t h e use o f highly compatible S - R ensembles tends t o optimize b o t h human performances a n d t h e system performances t h a t are dependent upon those human performances. Thus, t h e discovery a n d use of t h e mechanisms u n d e r l y i n g S-R compatibility effects is basic t o t h e development o f effective human-factors applications. Such applications would t e n d t o produce enhanced human-machine system performances t o t h e e x t e n t t h a t t h e system's Iiveware ( o r human-performance) components a n d interfaces were designed t o e x p l o i t t h e benefits o f u s i n g highly compatible S-R ensembles. Maximizing Compatibility In some o f t h e earliest studies, especially those u s i n g motorresponse alphabets, r e l a t i v e S-R compatibility appears t o approach a maximum (a) when t h e p a i r i n g s of stimulus and response alphabets correspond t o one another in a d i r e c t physical sense a n d (b) when t h e p a i r i n g s o f elements in t h e ensemble agree w i t h s t r o n g i n d i v i d u a l o r population stereotypes, where such stereotypes e x i s t (see Deininger & Fitts, 1955, pp. 319f). Operationally, such stereotypes a r e specified by tables o f t h e empirically determined frequencies w i t h which a set of permissible responses a r e made t o each stimulus in free-response situations where none o f t h e responses has been predesignated as "correct" o r "appropriate. Thus, correspondence a n d stereotypy a r e among t h e earliest o f i d e n t i f i e d mechanisms or techniques f o r maximizing compatibility. I'

Things That Go Together

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Correspondence Between Alphabets T h e emphasis o n physical correspondence between stimulus a n d response alphabets is p r o b a b l y a r t i f a c t u a l . T h e e a r l y generalizations w i t h t h i s emphasis a r e based n e a r l y e x c l u s i v e l y o n t h e r e s u l t s o f experiments t h a t focused o n perceptual-motor performances. Most o f t h e studies, a n d especially t h e e a r l i e r ones, used ensembles formed o f l i g h t - p a t t e r n e d stimulus alphabets p a i r e d w i t h m o t o r - p a t t e r n e d response alphabets. F o r example, Paul F i t t s a n d h i s associates g e n e r a l l y r e q u i r e d t h e i r subjects t o move a s t y l u s q u i c k l y in t h e d i r e c t i o n indicated by a coded visual stimulus (Deininger & Fitts, 1955; F i t t s & Deininger, 1954; F i t t s & Seeger, 1953). O n t h e o t h e r hand, G r a n t a n d h i s coworkers- r e q u i r e d k e y - p r e s s i n g responses t o light stimuli (Anderson, Grant, & Nystrom, 1954; M o r i n & Grant, 1955; N y s t r o m & Grant, 1954). Garvey and his colleagues also used p u s h - b u t t o n responses t o light stimuli ( G a r v e y & Knowles, 1954: G a r v e y & Mitnick, 1955; Knowles, Garvey, & Newlin, 1953) and, in some cases, also used a u d i t o r y stimulus symbols (Knowles e t al., 1953). A l l o f these studies indicate t h a t r e l a t i v e S - R compatibility t e n d s t o increase in p r o p o r t i o n t o t h e degree o f d i r e c t physical correspondence between t h e p a i r e d stimulus a n d response alphabets. T h i s observation can b e safely generalized t o all o t h e r ensembles employing similar t y p e s o f perceptual-motor performances. Even so, it is p r o b a b l y o v e r l y l i m i t i n g in i t s reference o n l y t o t h e p h y s i c a l aspects o f correspondence, especially when t h e emphasis i s n o t specifically on t h e motor aspects o f performance. T h e observation could, a n d p r o b a b l y should, b e broadened f u r t h e r t o i n c l u d e a t least t h e "dimensionality" aspects o f correspondence, a n d perhaps even t h e idea o f conceptual correspondence (see Alluisi, 1961). Conceptual correspondence: Dimensionality. A recommendation has been made elsewhere t o p r e s e r v e t h e multidimensional aspects o f a combination-stimulus alphabet by p a i r i n g it w i t h a conceptually matching multidimensional-response alphabet (Muller, Sidorsky, Slivinske, Alluisi, & Fitts, 1955, p. 90). T h e recommendation was based i n i t i a l l y o n an experiment by Slivinske, Bennet, a n d lrby (see M u l l e r e t al., 1955, pp. 74-83), but it is also s u p p o r t e d by t h e r e s u l t s o f o t h e r contemporary studies. F o r example, M o r i n a n d G r a n t (1955) measured R T w i t h t h r e e d i f f e r e n t S-R ensembles. A l l t h r e e ensembles employed t h e same stimulus alphabet o f 10 l i g h t s a r r a n g e d in a horizontal row, a n d t h e same response alphabet o f 10 k e y - p r e s s i n g b u t t o n s also a r r a n g e d in a horizontal r o w below t h e l i g h t s . However, t h e t h r e e S - R ensembles consisted o f d i f f e r e n t p a i r i n g s of t h e stimulus a n d response alphabets: (a) I n t h e direct-response ensemble, t h e k e y f o r a g i v e n light was d i r e c t l y below it; (b) in t h e reverse-response ensemble, t h e dimensionality was retained, but reversed, so t h e left-most k e y was p a i r e d w i t h t h e r i g h t - m o s t light, t h e r i g h t - m o s t k e y w i t h t h e left-most light, a n d so f o r t h ; a n d (c) in t h e randomly p a i r e d ensemble, t h e k e y s were randomly assigned t o t h e l i g h t s f o r each subject. Performance was b e s t w i t h t h e d i r e c t - r e s p o n s e ensemble, but n o t much worse w i t h t h e reverse-response ensemble. It was considerably degraded w i t h t h e random-response ensemble.

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Conceptual correspondence: L a t e r a l i t y . T h e r e s u l t s o f more recent studies a r e consistent w i t h t h e e a r l i e r f i n d i n g s a n d s u p p o r t as well t h e idea t h a t if conceptual correspondence i s maintained in t h e c o n s t r u c t i o n o f S-R ensembles, S-R compatibility effects w i l l t e n d t o b e maximized. F o r example, in another motor-response study, subjects pressed one o f t w o k e y s o n t h e i r l e f t o r right, w i t h t h e i r hands crossed o r uncrossed, when one o f t w o stimuli appeared on t h e l e f t o r right, o r above o r below a f i x a t i o n p o i n t (Wallace, 1971). Results showed t h a t S-R compatibility t e n d s t o b e maximized when t h e S-R ensemble i s c o n s t r u c t e d t o maintain correspondence o f ,:patiat laterality, t h a t is, o f t h e concepts o f "leftness" a n d " r i g h t n e s s . Thus, l e f t - k e y responses t o t h e l e f t stimulus, a n d r i g h t - k e y responses t o t h e right stimulus, p r o d u c e d t h e s h o r t e s t RTs, regardless o f t h e h a n d used o r w h e t h e r t h e hands were crossed o r not. O t h e r studies have also demonstrated t h i s t e n d e n c y f o r S - R compatibility t o b e maximized when t h e S - R ensemble is c o n s t r u c t e d w i t h alphabets t h a t a r e matched t o correspond in spatial l a t e r a l i t y . In one, similar t o t h e preceding, subjects c o u l d n o t see t h e k e y s o r t h e i r hands, a n d so depended solely on kinesthetic cues f o r response positioning (Wallace, 1972). In another, t h e S - R ensemble t h a t p r o d u c e d t h e s h o r t e s t R T s matched t h e l a t e r a l i t y o f an a u d i t o r y (ear-side) stimulus alphabet w i t h t h a t of a k e y - p r e s s i n g (key-side) response alphabet (Callan, Klisz, & Parsons, 1974). T h e effects were g r e a t e s t in t h e Donders' b - t y p e reactions, n e x t in t h e c-type, a n d least in t h e a - t y p e reactions. T h e effects o c c u r r e d regardless o f t h e a u d i t o r y tone used (high o r low) o r h a n d crossing. T h e importance o f conceptual correspondence in t h e c o n s t r u c t i o n o f S-R ensembles, as contrasted w i t h t h e more-limited c r i t e r i o n of p h y s i c a l correspondence, can also b e i n f e r r e d f r o m numerous o t h e r studies. In one example, eye-movement R T was measured t o monaurally presented tones where one f r e q u e n c y signaled a l o o k - l e f t command a n d a second f r e q u e n c y signaled a l o o k - r i g h t command. Oculomotor RTs were f a s t e r when t h e tone was p r e s e n t e d in t h e ear t h a t matched t h e look-direction command ( f r e q u e n c y ) o f t h e tone (Bertera, Callan, Parsons, E Pishkin, 1975). T h e r e a r e many o t h e r examples i n t h e l i t e r a t u r e , i n c l u d i n g t h e r e s u l t s o f e a r l i e r studies on t h e coding o f numerical information such as those n e x t discussed. Conceptual correspondence: Numerical codes. Studies o f coding numerical information f o r use w i t h vocal a n d motor responses show an advantage f o r t h e S - R ensemble o f vocal number-naming responses t o Arabic-numeral stimuli o v e r e i t h e r (a) motor k e y - p r e s s i n g responses t o these stimuli, o r (b) e i t h e r vocal o r motor responses t o o t h e r symbolic codes r e p r e s e n t i n g numerical information (see A l l u i s i & Muller, 1956, 1958). T h i s is shown in t h e data o f F i g u r e 1.

In t h e t w o experiments r e p o r t e d i n F i g u r e 1, t h e informationh a n d l i n g performance o f 10 subjects was measured w i t h conventional A r a b i c numerals a n d s i x o t h e r symbolic v i s u a l codes. Motor ( k e y p r e s s i n g ) responses were used in one experiment, a n d vocal (numbernaming) responses were used in t h e o t h e r . T h e symbolic codes included a set o f s t r a i g h t - l i n e symbolic A r a b i c numerals, t h r e e sets o f o r d e r e d

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STIMULUS ALPHABETS .

A - ARABIC NUMERALS

0 -SYMBOLIC ARABIC NUMER

0 -SIMPLE INCLINATION -

\I - CLOCK INCLINATION

x - BINARY INCLINATION O - ELIPSE-AXISRATIO

EB r8

EY r 8

Y

Y

3z B 2

3z 0

v)

z Y

z f

a

E

E 3

2

1

0 1

2

3

4

5

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PRESENTATION RATE (BITS/SEC) F i g u r e 1. Rate o f information transmission as a f u n c t i o n o f t h e r a t e o f information presentation w i t h vocal (number-naming) a n d motor ( k e y p r e s s i n g ) responses t o seven, ten-element, symbolic visual codes. T h e solid diagonal l i n e represents t h e maximum possible transmission rate. Each data p o i n t is based on 1,000 stimulus presentations. F o r simplicity, t h e lines d r a w n r e p r e s e n t t h e data averaged o v e r t h e alphabets w i t h i n t h e t h r e e empirically e v i d e n t g r o u p i n g s o f t h e t w o numerical codes, t h e three inclination codes, a n d t h e s i n g l e codes of color a n d ellipse-axis ratio. (Revised f r o m A l l u i s i & Muller, 1956, 1958). symbols based on differences i n t h e visual inclination o f a l i n e (simple, clock, a n d b i n a r y ) , a set o f colors, a n d a set o f ellipses o f d i f f e r i n g axis ratios (five h o r i z o n t a l l y a n d f i v e v e r t i c a l l y o r i e n t e d ) . A l l symbolic alphabets were c o n s t r u c t e d o f symbols t h a t h a d been p r e v i o u s l y scaled a n d selected t o maximize d i s c r i m i n a b i l i t y . Each experiment employed b o t h


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self-paced a n d forced-paced rates o f information presentation, l a t t e r b e i n g v a r i e d f r o m 2 t o 6 bits/sec in unit steps.

with the

T h e data o f t h e forced-paced t r i a l s a r e shown i n F i g u r e 1. It is q u i t e a p p a r e n t t h a t t h e seven codes f a l l i n t o t h r e e g r o u p i n g s w i t h b o t h vocal a n d motor responses. T h e g r o u p i n g reflects how well t h e codes p o r t r a y ( a n d c o r r e s p o n d in t h e i r p o r t r a y a l o f ) t h e numerosity concept. T h e two numerical codes a r e s u p e r i o r t o t h e t h r e e inclination codes, a n d these a r e a l l s u p e r i o r t o t h e t w o remaining codes o f color a n d ellipse-axis ratio. Furthermore, t h e i n t e r a c t i o n o f stimulus codes w i t h response modes, i n d i c a t i v e o f v e r y s t r o n g S - R compatibility effects, is also q u i t e apparent a n d consistent w i t h p r i o r observations. Essentially similar S - R compatibility effects were f o u n d in t h e self-paced performances t h a t a r e n o t detailed here. Even w i t h t h e highly compatible number-naming responses t o t h e A r a b i c numerals, t h e importance o f t h e conceptual-correspondence aspect o f t h e S-R ensemble i t s e l f i s demonstrated in another s t u d y . Fitts and Switzer (1962) f o u n d t h a t t h e vocal "two" response t o t h e v i s u a l l y presented "2" in an ensemble composed o f t h e two-element v i s u a l alphabet, "1" a n d "2," p a i r e d w i t h t h e two-element vocal-response alphabet, "one" a n d "two," is made f a s t e r t h a n w i t h an ensemble consisting o f "2" a n d "7" p a i r e d w i t h "two" a n d "seven. " Conceptual correspondence: C e n t r a l processing. Wickens (1 987, in t h e r e c e n t l y p u b l i s h e d Handbook of Human Factors (Salvendy, 19871, discussed t h e concept o f "stimulus-central processing (SC) compatibility"--a concept t h a t h e based o n t h e notion t h a t t h e human has t w o d i f f e r e n t w o r k i n g memory systems f o r rehearsing o r t r a n s f o r m i n g v e r b a l a n d spatial t a s k s (see Baddeley & Hitch, 1974). He p o i n t e d o u t t h a t t h e p a r t o f c e n t r a l processing t h a t makes use of v e r b a l o r phonetic memory is more compatibly matched w i t h t h e a u d i t o r y stimuli a n d vocal responses, whereas t h e p a r t t h a t makes use o f spatial memory is b e t t e r matched w i t h spatial stimuli a n d responses (e.g., key-pressing responses t o l i g h t s ) . pp.

76-77),

T h e issue o f compatible modalities is more b r o a d l y addressed elsewhere (Wickens, 1984). T h e conclusion suggested i s t h a t p a i r i n g an a u d i t o r y - s t i m u l u s alphabet w i t h a speech-response alphabet is l i k e l y t o p r o d u c e a highly compatible ensemble f o r a v e r b a l t a s k . However, f o r a spatial task, a highly compatible ensemble i s more l i k e l y t o b e obtained k i t h t h e p a i r i n g o f a visual/spatial-stimulus alphabet w i t h a motor/ ( c o r r e s p o n d i n g l y s p a t i a l l - r e s p o n s e alphabet. T h e mediation is p r o v i d e d by t h e s h o r t - t e r m ( w o r k i n g ) memory system. T h i s suggests t h e use o f an S-C-R compatibility model t h a t p r e d i c t s t h a t human performance w i l l b e optimized when t h e stimulus alphabet (S), t h e c e n t r a l 'processing coding o f s h o r t - t e r m memory ( C ) , a n d t h e response alphabet ( R ) a r e compatibility matched. T h i s model is consistent w i t h t h e view t h a t it is r e a l l y t h e conceptual correspondence between stimulus a n d response alphabets, r a t h e r t h a n t h e physical correspondence, t h a t t e n d s t o optimize or maximize S-R compatibility. Of course, t h i s S-C-R approach t o optimizing S-R compatibility effects is essentially a special case o f t h e general stimulus-organism-response ( o r S - 0 - R ) orientation in experimental psychology ( c f . Woodworth, 1938; Woodworth & Schlosberg, 1954).


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Summary: Correspondence between alphabets. Thus, it appears t o b e reasonably well established t h a t S-R compatibility approaches a maximum when t h e p a i r i n g s o f stimulus a n d response alphabets c o r r e s p o n d t o one another i n a d i r e c t conceptual sense. I n t h e applicable cases, such as those t h a t employ motor responses t o stimuli w i t h d i f f e r e n t spatial locations, it is l i k e l y t h a t t h i s w i l l o c c u r when t h e alphabets c o r r e s p o n d t o one another in a d i r e c t p h y s i c a l sense as well. In selecting modalities f o r t h e stimulus a n d response alphabets, it i s p r u d e n t t o match them t o t h e c e n t r a l -processi ng , s h o r t - t e r m ( w o r k i n g ) memory, requirements o f t h e task. Population Stereotypes A n empirical t a s k is t y p i c a l l y used b o t h t o establish t h e existence o f a population stereotype a n d t o p r o v i d e i t s d e s c r i p t i o n . The task involves t h e d i s c o v e r y o r demonstration o f preferences, among subjects who a r e t h e members o r representatives o f some t a r g e t population, in t h e specific assignments o f response symbols t o stimulus symbols. As stated earlier, such stereotypes a r e specified operationally by tables o f t h e empirically determined frequencies w i t h w h i c h permissible responses a r e made o r assigned t o each stimulus symbol i n a free-response paradigm, :nd where "9,"" o f t h e responses has been predesignated as "correct" o r appropriate. Such preferences o r stereotypes a r e f o u n d f r e q u e n t l y , if n o t universally. T h e y may d i f f e r as a f u n c t i o n f; c u l t u r e o r p r a c t i c e . At times, t h e y a r e f o u n d t o b e c o n t r a d i c t o r y t o standard" design practices (see L u t z & Chapanis, 1955). Some contradictions may b e unavoidable, especially when c r o s s i n g c u l t u r a l lines. F o r example, t h e customary o r t y p i c a l room-light toggle-switch c o n t r o l is moved up t o a c t i v a t e a room light in t h e U n i t e d States, b u t down in t h e U n i t e d Kingdom. Stereotype8 in v e r b a l l e a r n i n g . Population stereotypes a r e used not o n l y in t h e s t u d y o f perceptual-motor skills, but also in t h e s t u d y o f language skills, where t h e y a r e b e t t e r known as association norms. There, inferences r e g a r d i n g t h e s t r e n g t h s o f i n d i v i d u a l stereotypes a r e f r e q u e n t l y made on t h e basis o f t h e s t r e n g t h of population stereotypes. Thus, if all o f t h e subjects in a sample f r o m some population d i s p l a y a single preference, it i s o f t e n assumed n o t o n l y t h a t t h e p r e f e r e n c e is u n i v e r s a l in t h e population, but also t h a t it is a v e r y s t r o n g p r e f e r e n c e in each i n d i v i d u a l member o f t h e population. The validity of that assumption is an empirical i s s u e - - i t may o r may n o t p r o v e t o b e t r u e - but, as F i t t s a n d Posner (1967) p o i n t e d out, "the rate a t which associations between p a i r s o f words a r e learned agrees closely w i t h predictions f r o m association norms" (p. 21). Such stereotypes r e p r e s e n t preferences embedded in l o n g - t e r m memory. T h e y a r e mediated by a central-processing mechanism. T h i s is n o t t h e same as t h e p r e v i o u s l y discussed mechanism o f s h o r t - t e r m ( w o r k i n g ) memory t h a t Wickens (1984, 1987) p r e s e n t e d as mediating t h e modality influences in maximizing S - R compatibility. T h e short-term memory mechanism deals w i t h t h e alphabet-correspondence issue; t h e l o n g - t e r m memory mechanism deals w i t h t h e issues o f preferences a n d stereotypes. In a normal adult, l o n g - t e r m memory can b e represented as including, in a f r e q u e n c y o r p r o b a b i l i t y sense, all t h e information t h a t

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has p r e v i o u s l y entered t h e person's c o g n i t i v e channel. Thus, f r e q u e n c y o f exposure, familiarity, appear t o b e important determiners o f stereotypes.

information-handling o r experience would

T h a t t h i s i s t h e case can b e i n f e r r e d f r o m t h e results o f t w o studies. In one t h a t dealt with aesthetic judgments, a multiple-R o f +.843 was f o u n d in p r e d i c t i n g r a n k - o r d e r preference o f appearance w i t h t h r e e frequency-based r a n k o r d e r i n g s o f English l e t t e r s ( A l l u i s i & Adams, 1962). In t h e second, a multiple-R o f +.672 was f o u n d in p r e d i c t i n g r a n k - o r d e r vocal d i s j u n c t i v e R T t o d i f f e r e n t English l e t t e r s ( c f . F i t t s 8 Switzer, 1962) w i t h t h e b e s t t w o o f several f r e q u e n c y and preferencebased p r e d i c t o r variables; namely, r a n k o r d e r f o r (a) f r e q u e n c y o f use in English a n d (b) preference o f appearance (Alluisi, 1963).

I t seems safe t o conclude t h a t Summary: Population stereotypes. where s t r o n g i n d i v i d u a l o r population stereotypes are found, a n d where t h e p a i r i n g s o f t h e elements in t h e stimulus and response alphabets a r e consonant w i t h t h e stereotypes, (a) t h e r e s u l t i n g S-R ensemble w i l l t e n d t o b e optimum, (b) S-R compatibility maximum, a n d (c) t h e r e s u l t i n g performances e i t h e r optimized o r maximized. Examples o f recognized population stereotypes relevant t o t h e design a n d use o f human-machine systems a r e l i s t e d by Woodson a n d Conover (1970, p. 1.301, a n d by Wickens (1987, pp. 89-90). T h e o t h e r side o f t h e coin is t h a t if s t r o n g i n d i v i d u a l o r population stereotypes e x i s t a n d if t h e y a r e violated i n t h e construction o f t h e S-R ensemble, human performances w i l l b e degraded r e l a t i v e t o what t h e y could have been--and p r o b a b l y t o levels below those t h a t were expected, predicted, o r planned in t h e design o f t h e workplace a n d system in which t h e ensemble is used. T h e human-factors specialist is well-advised t o follow t h e guidance o f V a n C o t t and Kinkade (19721, n o t o n l y in u s i n g t h e population stereotype ("what t h e operator expect:") i n designing control a n d display panels, b u t also in v e r i f y i n g w i t h operational personnel t o assure t h a t violation o f a population stereotype w i l l n o t b e incorporated" (p. 404) elsewhere in t h e design o f t h e system and i t s components.

S-R Compatibility Effects in Information H a n d l i n g T h e general topic o f S - R compatibility effects in information handling i s broad, has a relatively long history, and has been well covered elsewhere in many o f i t s aspects. In fact, a b r i e f section o n "Population stereotypes in responding t o directional cues" is included i n t h e chapter by F i t t s (1951, pp. 1306-1308) on "Engineering Psychology and Equipment Design," in t h e now classic Handbook of Ex,erimental Psychology (Stevens, 1951). More recently, sections on StimulusResponse Compatibility" appear in chapters on "Motor Control" (Keele, 1986, pp,. 30.8-30.100) in Boff, Kaufman, a n d Thomas (1986), on "Information Processing, Decision-Making, a n d Cognition" (Wickens, 1987, pp. 89-90) in Salvendy (19871, and on "Design f o r Action" in B a r b e r (1988, pp. 67-98). F u r t h e r , a section on "Compatibility," with the articulation o f a "Compatibility Principle" ("Minimize t h e amount o f information recoding t h a t w i l l b e necessary"), appears in "Software I n t e r f a c e Design" (Williges, Williges, & Elkerton, 1987, pp. 1419-1420).


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Also, e n t i r e t e x t s have been w r i t t e n on information t h e o r y a n d i t s applications in psychological research a n d t h e o r y , w i t h A t t n e a v e (1959), G a r n e r (1962), a n d Quastler (1955) b e i n g among t h e earliest. Other t e x t s have addressed psychological applications o f information theory; among them, B e r l y n e (1960), Broadbent (1958), F i t t s a n d Posner (1967), Luce (1960), a n d Senders (1958) span a r a n g e o f i n t e r e s t s p e r t a i n i n g t o t h i s topic. S t i l l o t h e r t e x t s have e i t h e r a m i n o r section (Dember & Warm, 1979, pp. 106-123), a c h a p t e r (Alluisi, 19701, o r a major section (Posner, 1986) devoted t o topics o f i n f o r m a t i o n - t h e o r y applications in psychological research a n d t h e o r y . These w o r k s a t t e s t t o t h e b r e a d t h o f impact o f i n f o r m a t i o n - t h e o r y concepts a n d measures in psychology. A l t h o u g h f e w o f them deal d i r e c t l y w i t h S-R compatibility effects, t h e y establish t h e foundation f o r t h e question, "How d o S-R compatibility effects influence human-information processing?" T a k e n together, these w o r k s suggest t h a t t h e influence is substantial. Information-Handling Rate Rates o f numerical information transmissions in forced-paced serial t a s k s have been measured in numerous experiments. In one (Alluisi, Muller, & Fitts, 1955, 19571, v i s u a l presentations o f t h e Arabic-numeral stimulus alphabet were matched t o t w o response alphabets, one vocal a n d t h e o t h e r motor. T h e highly compatible vocal response alphabet consisted of,,numbernaming w i t h t h e u;ual English names o f t h e numerals (i.e., one" f o r "1 "two" f o r "2, e t c . ) . T h e dimensionally compatible motor response alphabet consisted o f k e y - p r e s s i n g w i t h use o f a b a n k o f 10 keys, a r r a n g e d h o r i z o n t a l l y in t w o semicircles t o fit t h e n a t u r a l placement o f t h e 10 f i n g e r tips, numbered t o increase f r o m l e f t t o right (i.e., "1" f o r t h e l i t t l e f i n g e r on t h e l e f t hand, "2" f o r t h e r i n g f i n g e r , e t c . ) . ,I'

T h e experiment was designed p r i n c i p a l l y t o determine w h e t h e r t h e r a t e o f information h a n d l i n g in a forced-paced serial t a s k is a f u n c t i o n o f (a) t h e r a t e o f stimulus presentation (1, 2 , 3 stimuli/sec), (b) t h e u n c e r t a i n t y p e r stimulus (1, 2 , o r 3 bits/stimulus), o r (c) t h e j o i n t e f f e c t o f t h e t w o f a c t o r s expressed as t h e r a t e o f information presentation Each o f 10 h i g h l y p r a c t i c e d male subjects ( r a n g i n g f r o m 1 t o 9 bits/sec). responded t o 100 presentations of t h e stimuli in each o f t h e n i n e conditions w i t h b o t h o f t h e response alphabets. T h e r e s u l t i n g data a r e g i v e n in F i g u r e 2, in terms o f t h e rates o f information transmission. F i r s t , t h e interaction o f response mode w i t h t h e informationin vocal a n d motor presentation variables (i.e., t h e differences responses) is an S-R compatibility e f f e c t t h a t reflects t h e h i g h e r compatibility o f t h e vocal number-naming responses, o v e r t h e motor k e y p r e s s i n g responses, when p a i r e d w i t h t h e Arabic-numeral stimuli. It is n o t i n d i c a t i v e o f a n y general s u p e r i o r i t y o f vocal o v e r motor responses. Indeed, t h i s same k e y - p r e s s i n g motor-response mode is b e t t e r t h a n t h e number-naming vocal responses when p a i r e d w i t h spatial stimuli (rows o f l i g h t s ) t h a t c o r r e s p o n d p h y s i c a l l y t o t h e placement o f t h e k e y s (Muller, 1955). Thus, h i g h e r rates o f information transmission a r e obtained w i t h ensembles o f h i g h e r S-R compatibility.


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P r6 c

PRESENTATION RATE (BITS/SEC)

Figure 2. Rate of information transmission as a f u n c t i o n of t h e r a t e of information presentation w i t h vocal (number-naming) a n d motor ( k e y p r e s s i n g ) responses t o v i s u a l l y presented A r a b i c numerals a t t h r e e levels of stimulus complexity (1, 2 , 3 bits/stimulus) a n d t h r e e rates o f stimulus T h e solid diagonal l i n e represents t h e presentation (1, 2, 3 stimuli/sec). maximum possible transmission r a t e . Each data p o i n t is based o n 1,000 stimulus presentations. (Revised f r o m Alluisi, Muller, & Fitts, 1955, 1957) . Second, t h e data o f F i g u r e 2 also indicate t h a t o v e r these ranges o f stimulus complexities (1 t o 3 bits/stimulus) a n d presentation rates ( 1 t o 3 stimuli/sec), (a) increasing complexity a t a g i v e n stimulus-presentation r a t e produces an increased r a t e of information transmission generally, a n d especially so w i t h t h e highly compatible vocal response t o t h e A r a b i c numerals, b u t (b) increasing t h e r a t e o f stimulus presentation a t a g i v e n complexity generally produces a decrement, which is especially notable i n t h e less-compatible motor-response p a i r i n g s t o t h e A r a b i c numerals.


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S t a b i l i t y o f S-R C o m p a t i b i l i t y E f f e c t s S-R compatibility effects a r e q u i t e stable o v e r b o t h s h o r t a n d longt e r m practice. Indeed, t h e s t a b i l i t y o f t h e e f f e c t is among t h e earliestnoted c h a r a c t e r i s t i c o f t h e phenomenon ( D e i n i n g e r & Fitts, 1955; F i t t s & Seeger, 1953). T h i s s t a b i l i t y is also f o u n d in t h e data o f o t h e r studies. F o r example, A l l u i s i a n d M a r t i n (1957, 1958) f o u n d it in t h e persistence o f b e t t e r performances, as indicated by h i g h e r informationtransmission rates, obtained w i t h vocal (number-naming) responses o v e r motor ( k e y - p r e s s i n g ) t-fsponses t o t w o sets o f v i s u a l l y p r e s e n t e d A r a b i c numerals. One was a conventional" set o f AND-10400 numerals, a n d t h e o t h e r was a "symbolic" set based on an eight-element s t r a i g h t - l i n e m a t r i x . S h o r t - t e r m l e a r n i n g was measured t h r o u g h t h e responses o f 48 college students f r o m a pool o f volunteers. T h e g r o u p was randomly d i v i d e d i n t o t w o g r o u p s o f 24, one f o r t h e vocal responses a n d t h e o t h e r f o r t h e motor responses. Each subject responded during one session o f f i v e t r i a l s o f 200 stimulus presentations (100 each f o r conventional a n d symbolic A r a b i c numerals) on each o f 2 successive d a y s . Thus, when t h e 10 t r i a l s a r e p l o t t e d f o r each o f t h e f o u r combinations ( t w o stimulus t y p e s by t w o response modes), each data p o i n t represents 2,500 stimulus presentations. Results o f s h o r t - a n d l o n g - t e r m l e a r n i n g a r e essentially similar, as discussed below. L o n g - t e r m l e a r n i n g was measured similarly, but t h r o u g h t h e responses o f 10 o f t h e 48 subjects, a n d f o r a t o t a l o f 12, f i v e - t r i a l sessions. When t h e 12 sessions (blocks o f f i v e t r i a l s ) a r e p l o t t e d f o r t h e f o u r conditions, each data p o i n t also represents 2,500 stimulus presentations. A l t h o u g h t h e improvements in information-transmission rates ( i n bits/sec) r a n g e d f r o m about 30% t o 40%, on t h e average, o v e r t h e 12 sessions, t h e conventional A r a b i c numerals a r e c o n s i s t e n t l y s u p e r i o r t o t h e symbolic when p a i r e d w i t h vocal responses. N e i t h e r alphabet i s s u p e r i o r when p a i r e d w i t h motor responses. B o t h numeral t y p e s y i e l d b e t t e r performances w i t h vocal responses t h a n w i t h motor responses. Thus, t h e data show f o u r n o t q u i t e parallel lines, t h e bottom t w o o f which a r e i n t e r t w i n e d . I n t e r e s t i n g l y , t h e effects of p r a c t i c e a f t e r i n i t i a l learning, t h a t is, f r o m t h e 3 r d to t h e 12th session, a r e g r e a t e r w i t h t h e h i g h e r t h a n w i t h t h e lower-compatibility S-R ensembles. T h a t is, t h e t w o lines f o r t h e b e t t e r ensembles a r e diverging f r o m t h e t w o lines f o r t h e o t h e r (worse) ensembles ( A l l u i s i & Martin, 1958, p. 82). In contrast, F i t t s a n d Posner (1967, p. 24) show converging lines o f performance ( w i t h long-term practice--25 sessions spaced o v e r 3 months) in one high a n d one low compatibility condition f r o m t h e F i t t s a n d Seeger (1953) s t u d y . T h i s seeming contradiction o f f i n d i n g s c o u l d b e a reflection o f t h e subjects' attaining, o r a t least approaching, asymptotic levels o f performance in t h e h i g h - c o m p a t i b i l i t y condition w i t h t h e 3 months o f additional p r a c t i c e i n t h e l a t t e r s t u d y . Studies o f t h e i n t e r a c t i o n o f S - R compatibility a n d t h e r a t e o f gain of information suggest t h a t t h i s i s l i k e l y t o b e t h e case.

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S-R C o m p a t i b i l i t y a n d t h e Rate of Gain o f Information It was H i c k (1952) who i n i t i a l l y observed t h a t t h e r a t e o f gain o f information is a constant. T h a t is, a s t r a i g h t l i n e r e s u l t s when d i s j u n c t i v e R T is p l o t t e d as a function o f t h e amount o f information t r a n s m i t t e d ( H t ) p e r S-R event, a t least o v e r t h e r a n g e f r o m 1 t o 3 o r 4 bits/S-R e v e n t (but see Longstreth, 1988; L o n g s t r e t h & Alcorn, 1987; Longstreth, El-Zahhar, & Alcorn, 1985; Smith, 1968, pp. 83-85; Welford, 1987). T h e rate of gain (of information) is t h e slope of t h a t line. R T is g e n e r a l l y an increasing linear f u n c t i o n o f H t (e.g., Crossman, 1953; Gregg, 1954; Warm & Alluisi, 1971). T h a t is, t h e slope constant o f t h e l i n e fitting R T as a f u n c t i o n o f H t is a p o s i t i v e value Some researchers have f o u n d conditions generally, but n o t u n i v e r s a l l y . u n d e r w h i c h R T has failed t o increase w i t h H t (Leonard, 1959; Mowbray, 1960; Mowbray E Rhoades, 1959). O t h e r s have f o u n d conditions u n d e r w h i c h v a r i o u s f a c t o r s a p p a r e n t l y influence t h e degree o f increase (Alluisi, 1965; B r a i n a r d , Irby, Fitts, & Alluisi, 1962; F i t t s & Switzer, 1962; Smith, Warm, & Alluisi, 1966). I n t e r a c t i o n with t h e r a t e of g a i n o f information. H i g h degrees o f experience, familiarity, practice, a n d s k i l l w i t h t h e S-R ensemble have been among t h e conditions f o u n d t o b e associated w i t h b o t h high compatibility a n d low rates o f gain o f information. T h e t y p i c a l p a i r i n g o f number-naming vocal responses with v i s u a l l y presented A r a b i c numerals produces f o r a d u l t humans a highly compatible S-R ensemble, use o f which r e s u l t s i n a measured r a t e o f gain o f information t h a t approaches zero ( B r a i n a r d e t at., 1962). Thus, compatibility a n d information-gain r a t e i n t e r a c t (see Alluisi, Strain, & Thurmond, 1964). T h i s i s shown in t h e data o f F i g u r e 3 . T h e data o f F i g u r e 3 a r e based on t h e vocal responses o f 54 subjects a t t h r e e levels o f stimulus u n c e r t a i n t y (Hs) t o v i s u a l l y presented A r a b i c numerals. T h e stimuli f o r t h e Hs levels o f 1, 2, a n d 3 bits/stimulus were projections o f t h e AND-10400 A r a b i c numerals 1 a n d 2, 1 t h r o u g h 4, a n d 1 t h r o u g h 8, respectively. Ensembles o f low, intermediate, a n d high S - R compatibility were created by p a i r i n g responses w i t h stimuli in t h r e e ways: ( a ) In t h e h i g h - c o m p a t i t i l i t y conditions, t h e numerals w e r e assigned t h e i r usual names, t h a t is, one f o r 1, "two" f o r 2, a n d so f o r t h ; (b) in t h e intermediate condition, subjects were i n s t r u c t e d t o respond vocally w i t h the,,number name o f t h e displayed-numeral-plus-two, t h a t is, "three" f o r 1, f o u r " f o r 2, a n d so f o r t h ; a n d (c) t h e low-compatibility condition was created by randomly p a i r i n g numeral names w i t h numerals d i f f e r e n t l y f o r each subject. T h e t h r e e levels of Hs were combined f a c t o r i a l l y w i t h t h e t h r e e degrees o f S-R compatibility, a n d s i x subjects were assigned a t random t o each o f t h e n i n e conditions. Thus, no subject served u n d e r more t h a n one level o f Hs a n d one level of S - R compatibility, a n d subjects were t e s t e d in random o r d e r d u r i n g 1 - h o u r experimental sessions. The order o f t h e 80 stimulus presentations in each session was random a n d d i f f e r e n t f o r each subject, w i t h equal numbers o f presentations o f t h e assigned stimuli--40 each in t h e two-numeral condition, 20 in t h e f o u r , a n d 10 in t h e eight-numeral condition. Each subject's median R T f o r t h e 80 responses was t h e measure o f performance employed. Performance was essentially f r e e of e r r o r (less t h a n l%), so Hs i s i n t e r p r e t e d as equivalent t o H t .


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1.o

.8

.6

.4

COMPATIBILITY .2

O O

1

2

3

F i g u r e 3. Reaction time ( R T ) as a f u n c t i o n o f stimulus u n c e r t a i n t y (Hs) a t t h r e e levels o f S-R compatibility. Because performance was essentially e r r o r l e s s in t h i s case, Hs i s e q u i v a l e n t t o Ht, t h e amount o f information t r a n s m i t t e d p e r S-R event. (Revised f r o m Alluisi, Strain, & Thurmond, 1964). T h a t t h e t h r e e S-R ensembles r e p r e s e n t d i f f e r e n t levels o f S-R compatibility is e v i d e n t in t h e data o f F i g u r e 3. Means f o r t h e low, intermediate, a n d high c o m p a t i b i l i t y conditions a r e 743.1, 649.8, a n d 477.7 ms, r e s p e c t i v e l y . T h e differences among t h e means a r e s t a t i s t i c a l l y reliable, F(2, 45) = 54.40, p .001. T h a t R T increases w i t h Hs (= Ht, in t h i s case) is also e v i d e n t in t h e data o f F i g u r e 3. Means f o r t h e Hs levels o f 1, 2, a n d 3 bits/stimulus a r e 521 . I , 657.3, a n d 692.2 ms, respectively. The differences among t h e means a r e s t a t i s t i c a l l y reliable, F(2, 45) = 26.78, p < .001. T h a t t h e r a t e o f gain of information is i n v e r s e l y related t o t h e degree o f S-R compatibility is a p p a r e n t in t h e i n t e r a c t i o n o f R T w i t h Hs

16

€.A. Alluisi and J.S. Warm

o v e r t h e t h r e e S-R compatibility conditions shown in F i g u r e 3; t h e interaction is s t a t i s t i c a l l y reliable, F(4, 45) = 4.91 p < .005. T h e r a t e o f gain o f information (slope constant) i s 148, 83, a n d 23 ms. w i t h t h e low, intermediate, a n d h i g h - c o m p a t i b i l i t y S-R ensembles, respectively. Summary: S-R c o m p a t i b i l i t y e f f e c t s a n d t h e r a t e o f g a i n of information. R T is an increasing linear f u n c t i o n o f Ht, except in t h e case o f highly compatible S-R ensembles. In general, t h e degree t o w h i c h R T is influenced by H t is an i n v e r s e f u n c t i o n o f t h e degree o f S-R compatibility. T h a t is, t h e r a t e o f gain o f information (i.e., t h e slope o f t h e l i n e r e l a t i n g R T t o H t ) v a r i e s i n v e r s e l y w i t h S-R compatibility. As suggested elsewhere ( A l l u i s i e t al., 19641, it may even b e used as a measure o f S-R compatibility in much t h e same way t h a t "resistance t o e x t i n c t i o n " is used as a measure o f l e a r n i n g . Generality, Limitations, a n d Possible Mechanisms of S - R Compatibility E f f e c t s T h e e a r l i e r studies o f S - R compatibility effects have been emphasized in t h e p r e v i o u s sections of t h i s c h a p t e r . The f i r s t o f two purposes f o r t h i s emphasis is t o c a p t u r e r e l e v a n t studies t h a t a r e in d a n g e r o f b e i n g missed o r i g n o r e d . T h a t is t o say, t h e "corpus" o f t h e PsyclNFO System, t h e electronic databases consisting o f summaries o f t h e "world's l i t e r a t u r e in psychology a n d related fields," begins w i t h summaries p u b l i s h e d in Psychological Abstracts during 1967. Generally, papers p u b l i s h e d e a r l i e r t h a n 1965 a r e n o t included. Some t e x t s a r e i n c l u d e d but o n l y w i t h b r i e f annotations. Chapters o f edited books w i l l n o t appear until 1989 o r 1990. Thus, unless i n c l u d e d i n reviews such as this, papers p u b l i s h e d p r i o r t o t h e rnid-l960s, as well as p a s t t e x t s a n d t h e i r chapters, w i l l soon b e lost t o those g r o w i n g numbers of researchers w h o depend solely o r mostly on computerized searches of those excellent a n d otherwise comprehensive b i b l i o g r a p h i c databases o f t h e PsyclNFO System. T h e second p u r p o s e is t o lay a foundation f o r t h e remainder of t h i s t e x t by documenting, a n d i l l u s t r a t i n g t h r o u g h t h e citations a n d data o f these e a r l i e r studies, t h e major experimental paradigms, performance characteristics, a n d f i n d i n g s t h a t define t h e phenomenon known as "S-R compatibility effects." T h e l a t e r w o r k s o f t h e 1970s a n d 1980s r e p r e s e n t exploitations o f t h e phenomenon. T h e y e x p a n d i t s horizons, seek t o establish j t s limits a n d r e s t r i c t i o n s , a n d p r o v i d e a deeper u n d e r s t a n d i n g o f t h e phenomenon. I n so doing, t h e y c l a r i f y t h e c o n t e x t a n d e x t e n d t h e g e n e r a l i t y o f t h e phenomenon, a n d t h e y b e g i n t o establish i t s mechanisms. T h e f i n a l sections o f t h i s c h a p t e r consist o f b r i e f reviews of these l a t e r studies. Rate of Gain o f Information In h i s review o f choice RT, Smith (1968, pp. 83-85) c i t e d numerous studies t h a t showed t h e r a t e o f gain o f information t o b e i n v e r s e l y related t o S-R compatibility, but h e f o u n d l i t t l e o r no evidence o f t h e slopes e v e r reaching zero. One s t u d y did show b o t h zero a n d negative slopes, but t h e a u t h o r dismissed them as p r o b a b l y a r t i f a c t s o f p r a c t i c e a n d o f p r o c e d u r e - t h e t e s t i n g of all subjects i n an ascending o r d e r o f Hs levels (Alluisi, 1965).


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More recently, L o n g s t r e t h (1988) a r g u e d t h a t t h e relation between R T a n d Hs is n o t linear a t all, b u t r a t h e r curvilinear, c e r t a i n l y beyond u n c e r t a i n t y levels g r e a t e r t h a n 3 b i t s p e r stimulus. He a n d h i s colleagues ( L o n g s t r e t h e t al., 1985) suggested t h e data b e t t e r fit a power law, b u t t h i s view is n o t u n i v e r s a l l y accepted (see L o n g s t r e t h & Alcorn, 1987; Welford, 1987).

It should b e noted t h a t "Hick's Law" relates d i s j u n c t i v e R T to Ht, It says t h a t t h e t h e amount o f information transmitted (see Hick, 1952). relation i s l i n e a r - - t h a t R T is a s t r a i g h t - l i n e f u n c t i o n o f H t . O n l y u n d e r v e r y special conditions w i l l stimulus u n c e r t a i n t y (Hs, o r t h e amount o f information p e r stimulus event) equal H t . For example, w i t h equiprobable stimuli a n d e r r o r l e s s performance, Hs is equal t o H t . I n o t h e r cases, Hs is n o t equal t o Ht, a n d use o f Hs in t h e c o n t e x t o f Hick's Law would b e incorrect. Instead, in such cases, H t would have t o b e computed d i r e c t l y f r o m t h e m a t r i x t h a t records t h e responses made t o t h e stimuli presented. Even so, it would b e unreasonable t o expect t h e linear "Hickks Law" relation t o hold w i t h o u t limit. T h e position t h a t is l i k e l y t o p r o v e valid in t h e l o n g r u n i s essentially t h e same as t h a t stated in a p r i o r section based o n t h e f i n d i n g s of t h e earlier studies; namely, t h a t a t least w i t h i n t h e r a n g e o f H t f r o m 1 t o 3 o r 4 b i t s p e r S-R event,

. .

.

t h e relation between d i s j u n c t i v e R T a n d H t i s linear, t h e slope o f t h e l i n e interacts w i t h S-R compatibility, t h e r e is an i n v e r s e relation between t h e slope a n d t h e degree o f S-R compatibility, and t h e slope approaches zero as S-R compatibility approaches a maximum.

Scope o f S-R Compatibility Effects As noted p r e v i o u s l y (see Maximizing Compatibility), t h e earlier studies o f S-R compatibility focused mostly, b u t n o t exclusively, on perceptual-motor performances, f o r example, on l i g h t - p a t t e r n e d stimulus alphabets p a i r e d w i t h motor-patterned response alphabets. However, vocal-response a n d symbolic-visual alphabets were also widely used, a n d S-R compatibility effects were even demonstrated in t h e paired-associate learning o f nonsense syllables (Baddeley, 1961). Later studies have expanded t h e scope o f t h e phenomenon by exploring, measuring, a n d examining it w i t h d i f f e r e n t S-R ensembles a n d t a s k situations. For example, S-R compatibility effects have been found w i t h S-R ensembles t h a t d o o r d o n o t demonstrate color correspondence (responding w i t h a r e d k e y t o a r e d light, and a green k e y t o a green light; or, w i t h a r e d k e y t o a green light, etc.; Hedge & Marsh, 1975; also see Simon, Sly, & Vilapakkam, 1981). A u d i t o r y stimulus alphabets have also been used in numerous studies, many o f them by Simon a n d h i s colleagues (see B e r t e r a e t al., 1975; Callan e t a l . , 1974; Knowles e t al., 1953; a n d most o f t h e c i t e d chapter references by Simon a n d h i s colleagues). Stimulus o r response alphabets i n o t h e r modalities have also been used, i n c l u d i n g t h e tactual (Leonard, 1959), t h e kinesthetic (Wallace, 1972), a n d t h e oculomotor ( B e r t e r a e t al., 1975). Essentially

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identical S-R compatibility effects as a r e t y p i c a l l y f o u n d in d i s j u n c t i v e R T t a s k s have also been demonstrated in a watchkeeping t a s k ( K u l p & Alluisi, 1967) a n d in a memory scanning t a s k (Ogden & Alluisi, 1980). Shulman a n d McConkie (1973), c i t i n g F i t t s a n d B r i g g s (1959), contended t h a t t h e notions o f S - S compatibility a n d R-R compatibility a r e c o r o l l a r y t o t h e concept o f S-R compatibility. T h e y suggested t h a t a basis f o r S - S compatibility lies in t h e p a s t research o n d i v i d e d a n d selective a t t e n t i o n a n d in Garner's (1970) w o r k o n i n t e g r a l v s . separable etimulus dimensions. It seems reasonable t o agree t h a t S-R compatibility may b e considered along such dimensions as d i s c r i m i n a b i l i t y a n d separableness" (p. 375), since information processing a n d choice R T a r e known t o b e affected by stimulus d i s c r i m i n a b i l i t y a n d d i s s i m i l a r i t y (see A l l u i s i E Sidorsky, 1958; Morgan & Alluisi, 1967; T h u r m o n d & Alluisi, 1963). What m i g h t now b e called "S-S compatibility effects" a r e shown in a s t u d y t h a t predates t h e f i r s t publication o f t h e term, "S-R compatibility." Specifically, F i t t s a n d Simon (1952) used various v i s u a l stimulus p a t t e r n s formed by d i f f e r e n t i n s t r u m e n t a n d p o i n t e r aivangements p a i r e d w i t h a continuous d u a l - p u r s u i t t a s k . T h e y f o u n d t h a t performance i s b e t t e r when i n s t r u m e n t s a r e close t o g e t h e r a n d aligned h o r i z o n t a l l y . Also, performance i s g e n e r a l l y b e t t e r when p o i n t e r s a r e aligned in t h e 9 a n d 12 o'clock d i a l positions, but t h i s f a c t o r i n t e r a c t s w i t h i n s t r u m e n t alignment. T h e 9 o'clock position i s b e t t e r w i t h h o r i z o n t a l l y aligned instruments, but t h e 12 o'clock position is b e t t e r w i t h v e r t i c a l l y aligned instruments. Also, R-R compatibility effects a r e demonstrated by Shulman a n d McConkie (1973), who v a r i e d S-R combinations by c h a n g i n g t h e spatial mapping r u l e i.n a two-choice t a s k a n d v a r i e d R-R combinations by c h a n g i n g t h e specific set o f k e y - p r e s s i n g responses used. T h e i r r e s u l t s showed t h a t t h e effects of response d i s c r i m i n a b i l i t y a n d S-R compatibility are additive. T h e y i n t e r p r e t e d t h a t a d d i t i v i t y along t h e lines o f t h e S t e r n b e r g (1969) paradigm a n d concluded t h a t t h e t w o effects r e p r e s e n t independent c e n t r a l processes. In a more-recent demonstration o f possible response ( R - R ) compatibility effects, M i l l e r (1988) seeks t o resolve an issue raised by t h e r e s u l t s o f c u i n g experiments t h a t show a hand-preparation advantage i.e., R T w i t h f i n g e r s on t h e same h a n d t o b e f a s t e r t h a n w i t h f i n g e r s on d i f f e r e n t hands (Miller, 1982, 1985, 1987). He i n t e r p r e t s this t o b e an R - R compatibility effect, but Reeve a n d Proctor (1984, 1985) p o i n t o u t t h a t t h e h a n d advantage in such c u i n g experiments could b e i n t e r p r e t e d w i t h equal v a l i d i t y as an S-R compatibility effect. Miller's (1988) r e s u l t s show t h e usual advantage o f same v e r s u s d i f f e r e n t hands, but t h e evidence does n o t r u l e o u t t h e p o s s i b i l i t y t h a t it is an S-R compatibility effect. H e discusses t h e issue, but f a i l s t o resolve it. T h i s is but one o f many unresolved issues r e g a r d i n g possible mechanisms o f S-R compatibility effects. Possible Mechanisms o f S -R Compatibility E f f e c t s

As discussed p r e v i o u s l y (see Maximizing Compatibility), t h e e a r l i e r studies established t h a t S-R compatibility effects would t e n d t o b e maximized (a) when t h e elements in t h e stimulus a n d response alphabets a r e p a i r e d t o c o r r e s p o n d in a d i r e c t physical sense a n d (b) where t h e p a i r i n g s a r e consonant w i t h e x i s t i n g population o r i n d i v i d u a l stereotypes.


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(Recall, however, t h a t t h e notion o f correspondence in a d i r e c t physical sense was shown, in t h e c i t e d section, t o b e too c o n f i n i n g a concept, a n d t h e w o r d physical was replaced w i t h Conceptual.) Thus, in an i n i t i a l study, F i t t s a n d Seeger (1953) showed t h a t R T i s f a s t e r (S-R compatibility h i g h e r ) when t h e response k e y s in a motor-response alphabet a r e placed t o correspond p h y s i c a l l y w i t h t h e placement o f t h e spatially located l i g h t s in t h e stimulus alphabet. The underlying mechanism proposed by Simon a n d his colleagues (e.g., C r a f t & Simon, 1970; Simon, 1968; Simon & Rudell, 1967) t o account f o r t h i s finding a n d others, assumed t h a t human subjects have "a p o t e n t n a t u r a l t e n d e n c y t o react t o w a r d t h e major source o f stimulation" (Simon, C r a f t , & Small, 1970, p. 63). However, Wallace (1971) a r g u e d t h a t t h e mechanism i s one o f conceptual correspondence in l a t e r a l i t y . He based t h i s o n f i n d i n g s t h a t t h e effects ( w h i c h h e views as S-S compatibility, r a t h e r t h a n S-R) o c c u r when t h e spatial position o f t h e stimulus ( l e f t o r right) i s matched by t h a t o f t h e response key, regardless o f t h e subject's hands b e i n g crossed o r not. Cotton, Tzeng, a n d H a r d y c k (1977, 1980) disagreed. They a r g u e d f o r a mechanism based on f u n c t i o n a l differences in t h e t w o cerebral hemispheres--an explanation in terms o f c e r e b r a l l a t e r a l i t y f a c t o r s (but see Katz, 1981; Nicoletti e t al., 1982; Whitaker, 1980, 1982). T h e y c i t e d as s u p p o r t i n g evidence f i n d i n g s t h a t show oculomotor R T t o b e f a s t e r when tones signaling l e f t o r r i g h t - l o o k i n g a r e presented in t h e ear c o r r e s p o n d i n g t o t h e d i r e c t i o n o f t h e commanded eye movement, r a t h e r t h a n t h e opposite ear (see B e r t e r a e t al., 1975). Lupker and Katz (1982) reviewed these studies a n d concluded t h a t "unless more compelling data can b e produced, it seems much more reasonable t o r e t a i n t h e s t a n d a r d S-R compatibility explanation t h a n t o i n v o k e one based o n questionable hemisphere assumptions and supported by logically inconsistent data" (p. 98). Thus, it would appear t h a t conceptual correspondence, whatever i t s mechanisms, may y e t p r o v e t o b e t h e more v a l i d a n d parsimonious explanation o f S-R compatibility!

With more t h a n a dozen studies p u b l i s h e d o v e r t h e p a s t t w o a n d a half decades, Simon is easily one o f t h e most p r o l i f i c researchers o n t h e possible mechanisms o f S-R compatibility effects (see C h a p t e r 2, by Simon). He is senior a u t h o r o f 14 studies c i t e d i n t h i s c h a p t e r s reference list, a n d j u n i o r a u t h o r o f y e t another t w o ( C r a f t & Simon, 1970; LeMay & Simon, 1969). Also, these papers have stimulated many o t h e r studies, some s u p p o r t i n g , some opposing t h e i n t e r p r e t a t i o n s a n d mechanisms hypothesized f o r what has come t o b e called ( a f t e r Hedge & Marsh, 1975) t h e "Simon effect" i n S-R compatibility. For example, Simon a n d his associates d i v i d e d S-R compatibility effects i n t o spatial, symbolic, a n d "Simon-effect" compatibilities (Simon e t al., 1981). Spatial compatibility, they said, results from a correspondence between t h e spatial arrangement o f p a i r e d stimulus a n d response alphabets. T h e f i n d i n g s o f e a r l y studies t h a t employed motorp a t t e r n e d responses t o l i g h t - p a t t e r n e d stimuli a r e examples o f t h i s kind o f correspondence (e.g., Deininger & Fitts, 1955; F i t t s & Deininger, 1954; F i t t s & Seeger, 1953). Similarly, Simon e t al. (1981) said t h a t symbolic compatibility r e s u l t s f r o m a nonspatial (or conceptual?) correspondence between stimulus a n d response alphabets. Examples o f t h i s kind o f correspondence can b e f o u n d i n t h e e a r l y studies o f vocal responses t o numerical stimuli (see A l l u i s i F, Martin, 1957, 1958; A l l u i s i & Muller, 1956, 1958; A l l u i s i e t a l . , 1955, 1957).

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The "Simon-effect'' t y p e of compatibility results from conjoint correspondence among t h e responses and t h e relevant and irrelevant characteristics of :he stimuli. Generally, i n t h e experiments t h a t show t h e "Simon effect, both the responses and t h e irrelevant stimulus cues are spatial, whereas t h e relevant stimulus cues are symbolic o r nonspatial. For e.xample, a s t u d y might require a l e f t - o r right-handed response depending on t h e relevant color of a visual stimulus t h a t may appear (irrelevantly) on the l e f t o r r i g h t of t h e display. The findings of such studies typically show t h a t RT is faster when t h e spatial (irrelevant) and color (relevant) cues o r stimulus characteristics both signal t h e same response--a response t h a t corresponds spatially t o t h e irrelevant cue (see C r a f t & Simon, 1970; Simon, 1969, Simon & Craft, Such 1970; Simon, Hinrichs, & Craft, 1970; Simon & Rudell, 1967). demonstrations of t h e "Simon effect" have been interpreted "as reflecting a stereotypic tendency t o respond toward the source of simulation (Simon, 1969)" (Simon e t al., 1981, p. 64). T h e y can also be interpreted i n terms of expectancy (Whitaker, 1980, 1982) o r interference among conjoint o r conflicting conceptual correspondences in t h e p a i r i n g of stimulus and response alphabets t o form the S-R ensemble, o r of the cognitive coding o r recoding of the information t o produce t h e correct response (Brebner, 1979; Brebner, There i s other Shephard, & Cairney, 1972; Hedge & Marsh, 1975). evidence f o r t h i s view. For example, Nicoletti and his associates presented data f o r conceptual correspondence as an explanation of spatial Compatibility. Specifically, t h e y a t t r i b u t e d spatial compatibility "to t h e correspondence, o r lack of it, of t h e locational codes associated w i t h stimuli and responses" (Nicoletti & Umilti, 1984, p. 341; Nicoletti e t al., 1982). T h e y related t h e i r s t u d y t o t h e Teichner and Krebs (1974) view t h a t S-R compatibility effects represent t h e proportion of choice RT "attributable t o stimulus-response translation time" (p. 91), and concluded, "it is not s u r p r i s i n g t h a t even i n t h e adult S-R pairings on t h e same side of t h e body midline are predominant over S-R pairings t h a t cross t h e b o d y midline" (Nicoletti & UmiltB, p. 342). Spatial compatibility effects, especially those related to lateral correspondence, are apparently q u i t e robust. They occur with children as well as adults (Alluisi, 1965; Lidavas, 1983). and w i t h aging adults They occur even when t h e subject's (Simon & Pouraghabagher, 1978). head is rotated and t h e stimulus and response alphabet are arranged perpendicularly (Lidavas & Moscovitch, 1984). They occur w i t h f i n g e r responses, whether t h e hands are held palms up o r down (Heister, Ehrenstein, & Schroeder-Heister, 1986, 1987). They are similar in t h i s regard t o t h e inclusive general class of S-R compatibility effects, as Indeed, S - R demonstrated i n t h e earlier sections of t h i s chapter. compatibility effects may even be found with non-human animals, if Michaels (1988) 'is correct i n t h e i r being describable by Gibson's theory of affordances . Summary Things that go together--S-R ensembles based on stereotyped, corresponding, and S-R compatible alphabets--contribute t o optimum codes and human performances. They are, o r should be, central not only t o t h e repertoire of expertise from which t h e human-factors specialist cont r i b u t e s t o t h e design and use of human-machine systems, b u t also t o the


21

i n t e r e s t s o f c o g n i t i v e psychologists who aim t o e x p a n d o u r u n d e r s t a n d i n g o f human c o g n i t i v e behavior. When a set o f discriminable stimuli has been a r r a n g e d i n t o an alphabet, a n d t h e elements o f t h i s stimulus alphabet p a i r e d w i t h those o f a response alphabet, a code o r S - R ensemble w i l l have been formed. T h e ease a n d e x t e n t t o w h i c h t h e ensemble can b e learned a n d used consistently, c o r r e c t l y , a n d r a p i d l y is a f u n c t i o n o f t h e specific p a i r i n g s o f t h e stimuli a n d responses employed. T h e o b s e r v e d variations in human performances t h a t a r e based on t h e specific p a i r i n g s o f elements o f stimulus a n d response alphabets a r e known as S-R compatibility effects ( a f t e r F i t t s & Seeger, 1953, - b a s e d o n a 1951 suggestion o f A r n o l d Small's). It has been l o n g established t h a t S-R compatibility effects a r e r e l a t i v e l y l a r g e in magnitude. They are consistent, a n d independent o f t h e performance measure employed. They a r e r e l a t i v e l y stable but may b e accentuated u n d e r conditions o f workload stress. T h e y approach a maximum when t h e p a i r i n g s o f stimulus a n d response alphabets c o r r e s p o n d t o one another in a d i r e c t conceptual sense. In t h e applicable cases, such as those t h a t employ motor responses t o stimuli w i t h d i f f e r e n t spatial locations, it i s l i k e l y t h a t t h i s w i l l o c c u r when t h e alphabets correspond t o one another in a d i r e c t physical sense. And, in selecting modalities, it i s p r u d e n t t o match t h e stimulus a n d response alphabets t o t h e central-processing, s h o r t - t e r m ( w o r k i n g ) memory requirements o f t h e task, as well.

S - R compatibility effects also t e n d t o b e maximized when p a i r i n g s o f t h e elements in t h e stimulus a n d response alphabets consistent w i t h a n y s t r o n g i n d i v i d u a l o r population stereotypes t h a t found. Examples o f recognized population stereotypes r e l e v a n t t o design a n d use o f human-machine systems a r e l i s t e d by Woodson Conover (1970, p. 1.30), a n d Wickens (1987, pp. 89-90).

the are are the and

If s t r o n g i n d i v i d u a l o r population stereotypes exist, a n d if t h e y a r e violated in t h e c o n s t r u c t i o n o f t h e S-R ensemble, p o o r e r performances w i l l result. Thus, t h e human-factors specialist should follow t h e advice of V a n C o t t a n d Kinkade (1972) t o use population stereotypes in d e s i g n i n g c o n t r o l a n d d i s p l a y panels a n d t o v e r i f y w i t h operational personnel t h a t such stereotypes have n o t been violated, but r a t h e r have been incorporated in t h e system's design. T h e r e is an i n t e r a c t i v e relation between S-R compatibility effects a n d t h e r a t e o f gain o f information. Specifically, w i t h i n t h e r a n g e o f an amount o f information t r a n s m i t t e d ( H t ) between 1 a n d 3 o r 4 b i t s p e r S-R event, (a) t h e relation between d i s j u n c t i v e R T a n d H t is linear, (b) t h e r e is an i n v e r s e relation between t h e slope of t h e l i n e a n d t h e degree o f S - R compatibility, a n d (c) t h e slope approaches zero as S-R compatibility approaches a maximum. Of the possible mechanisms of S-R compatibility effects, correspondence a n d s t e r e o t y p y were t h e earliest o b s e r v e d a n d identified. T h e y a r e well-established as mechanisms. In recent years, many o t h e r possibilities have been investigated. A l t h o u g h none o f t h e o t h e r newer mechanisms postulated has been universally accepted, each has i t s adherents a n d s u p p o r t i n g data.

22


For example, a major extension o f t h e phenomenon known as S - R compatibility i s called t h e "Simon effect." T h i s is a compatibility e f f e c t t h a t r e s u l t s f r o m t h e conjoint correspondence o f t h e elements o f t h e response alphabet with those of both relevant and irrelevant characteristics o f t h e S/R alphabet elements. T h e Simon e f f e c t has o c c u r r e d most f r e q u e n t l y ( b u t n o t exclusively) w i t h a spatial response (e.g., l e f t o r right k e y press), a p a i r e d non-spatial relevant stimulus (e.g., r e d o r green light signaling l e f t o r right response, respectively), a n d an associated, but i r r e l e v a n t spatial stimulus characteristic (e.g., t h e r e d light sometimes on t h e left, sometimes o n t h e right). In such cases, R T for t h e l e f t keypress t o t h e r e d - l i g h t - o n - t h e - l e f t i s f a s t e r t h a n f o r t h e same keypress t o t h e r e d - l i g h t - o n - t h e - r i g h t , a n d so f o r t h . U n t i l these o r o t h e r mechanisms are so well established as t o b e u n i v e r s a l l y accepted ( o r n e a r l y so), t h e human-factors specialist should a t t e n d t o t h e mechanisms t h a t a r e known t o e x i s t . A t present, these are t h e empirically d e r i v a b l e stereotypes a n d correspondences, optimization o f which also tends t o optimize t h e beneficial effects o f S - R compatibility in human performances. Thus, in p a i r i n g stimulus and response alphabets t o f o r m S - R ensembles, t h e human-factors specialist should a t t e n d t o t h e stereotypes a n d correspondences, even t o t h e seemingly " i r r e l e v a n t " characteristics o f t h e stimulus a n d response elements a n d pairings, t h a t c o g n i t i v e psycholo g y has demonstrated t o b e important determiners o f human cognitive behavior. O n l y by doing so in t h e design o f t h e codes, displays and controls, human-machine interfaces, a n d w o r k stations t h a t a r e p a r t s o f any human-machine system, w i l l t h e human-performance aspects o f t h e system's performance t e n d t o b e optimized. And, o n l y by such use and t h e r e s u l t i n g optimization w i l l c o g n i t i v e psychology's u n d e r s t a n d i n g and explanation o f human performance b e validated. References T o w a r d optimizing man's tactile communication. Alluisi, E. A . (1961). Perceptual and Motor Skills, 12, 235-245. (1963). Frequencies, preferences, and choice reactions t o Alluisi, E. A . l e t t e r s . Perceptual and Motor Skills, 16, 109-110. Alluisi, E. A . (1965). g a i n of information.

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T H E EFFECTS OF A N IRRELEVANT DIRECTIONAL CUE ON HUMAN INFORMATION PROCESSING

J . RICHARD SIMON Department o f Psychology Department o f I n d u s t r i a l a n d Management Engineering U n i v e r s i t y of Iowa

A fundamental concern in c o g n i t i v e psychology a n d in human factors engineering is t o u n d e r s t a n d t h e f a c t o r s t h a t a f f e c t t h e speed o f t r a n s l a t i n g information f r o m a d i s p l a y i n t o an a p p r o p r i a t e c o n t r o l action. T h e t e r m S-R compatibility ( F i t t s & Seeger, 1953) i s commonly used t o describe t h e e x t e n t t o which t h e ensemble o f stimulus a n d response combinations comprising a t a s k results in a high r a t e o f information t r a n s f e r . Many t y p e s o f compatibility have been identified, e.g., symbolic, temporal, 'and spatial compatibility, t o name a few. Symbolic compatibility results f r o m a correspondence between stimulus a n d response codes t h a t simplifies t h e translation process; f o r example, translation i s simplified when a r e d l i g h t signals a r e d - k e y response a n d a green light signals a g r e e n - k e y response r a t h e r than v i c e versa (e.g., LeMay & Simon, 1969; Simon & Sudalaimuthu, 1979). Temporal compatibility effects may occur when processing is affected by t h e o r d e r in which b i t s o f information a r e presented t o t h e operator (LeMay & Simon, 1969). Spatial compatibility is, perhaps, t h e most common a n d results f r o m a correspondence between t h e spatial arrangements o f displays a n d controls; f o r example, compatibility is high when l e f t a n d right l i g h t s a r e associated, respectively, w i t h l e f t a n d right keys ( F i t t s & Seeger, 1953; G a r v e y & Mitnick, 1955; Simon E Craft, 1970b; Simon & Wolf, 1963). T h i s c h a p t e r deals w i t h another t y p e o f compatibility, which m i g h t b e classified as a v a r i e t y o f spatial compatibility. We w i l l describe a series o f related experiments t h a t demonstrate t h a t t h e location o f a stimulus p r o v i d e s a n i r r e l e v a n t directional cue t h a t affects t h e time r e q u i r e d t o process t h e meaning of t h e stimulus. In o t h e r words, t h e r e seems t o b e a s t r o n g stereotypic tendency t o respond i n i t i a l l y t o t h e directional component o f a stimulus r a t h e r than t o i t s symbolic content. Hedge a n d Marsh (1975) used t h e label "Simon effect" t o r e f e r t o t h i s phenomenon. We, too, w i l l use t h i s label as a convenient s h o r t h a n d designation for t h e stereotype. A n Accidental D i s c o v e r y O u r research on t h e Simon effect began q u i t e by accident (Simon & Rudell, 1967). We were o r i g i n a l l y interested in t h e phenomenon o f hemispheric dominance f o r speech a n d designed an experiment t o i n v e s t i gate t h e possible interaction between ear stimulated a n d handedness in an a u d i t o r y reaction time ( R T ) t a s k . Without g o i n g i n t o a detailed rationale, suffice it t o say t h a t we wanted t o t r y t o demonstrate t h a t r i g h t - h a n d e d subjects would respond f a s t e r t o a v e r b a l command in t h e i r right ear t h a n in t h e i r l e f t ear and t h a t left-handed subjects would respond f a s t e r t o a command in t h e i r l e f t ear than i n t h e i r right e a r .

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We selected t w o g r o u p s o f subjects. One g r o u p consisted o f 48 s t r o n g l y r i g h t - h a n d e d students, a n d t h e o t h e r o f 32 s t r o n g l y left-handed students. T h e subjects wore earphones a n d sat a t a table w i t h t h e i r right a n d l e f t i n d e x f i n g e r s each r e s t i n g on a telegraph k e y . T h e y were i n s t r u c t e d t o press t h e right-hand k e y as q u i c k l y as possible a f t e r hearing t h e w o r d right" a n d t o press t h e l e f t - h a n d k e y as q u i c k l y as possib l e a f t e r h e a r i n g t h e w o r d "left." A timer s t a r t e d when a command was presented a n d stopped when t h e subject pressed t h e k e y . Each subject responded t o t h e same series o f 132 tape-recorded,, commands, each comT h e commands mand consisting of t h e w o r d "right" or t h e w o r d l e f t . were presented t o e i t h e r t h e right ear o r t h e l e f t ear in a predetermined random sequence, so t h a t t h e subject h a d n o way o f knowing p r i o r t o t h e presentation o f each command which ear would b e stimulated o r what t h e command would be. A ready signal was presented 2 s p r i o r t o each command, a n d t h e r e was a 6-s i n t e r v a l between commands. T h e median R T was calculated f o r each subject f o r each o f t h e f o u r experimental conditions (i.e., "right" a n d " l e f t " commands t o t h e right a n d l e f t e a r ) . These data were subjected t o a n analysis o f variance. We were disappointed t o find that t h e p r e d i c t e d interaction between ear stimulated a n d handedness was n o t significant. T h e r e was, however, another e f f e c t - - t o t a l l y unexpected--but, as it t u r n e d out, much more i n t e r e s t i n g t h a n t h e e f f e c t t h a t was o u r o r i g i n a l concern. T h e l e f t half o f F i g u r e 1 p i c t u r e s t h i s effect, a s i g n i f i c a n t Command Ear Stimulated interaction. Note t h a t R T was markedly f a s t e r when t h e right" command was h e a r d in t h e right ear t h a n when it was h e a r d in t h e l e f t ear, and, similarly, R T t o t h e " l e f t " command was f a s t e r when it was h e a r d in t h e l e f t ear than when it was h e a r d in t h e right ear. Obviously, a n i r r e l e v a n t cue, t h e ear in which t h e subject h e a r d t h e command, was a f f e c t i n g t h e time r e q u i r e d t o process t h e symbolic content o f t h e command. A t t h i s point, we were s t i l l interested in trying t o demonstrate t h e hypothesized interaction between ear stimulated a n d handedness. We conjectured that, if t h e subject's u n c e r t a i n t y as t o t h e source o f t h e command were removed, t h e n t h e variance c o n t r i b u t e d by t h i s i r r e l e v a n t cue w o u l d b e eliminated, a n d we m i g h t b e able t o detect a n Ear Stimulated x Handedness interaction. We, therefore, conducted another experiment t h a t was essentially t h e same as t h e f i r s t except that, t h i s time, t r i a l s were presented i n blocks r a t h e r t h a n in t h e random-ear o r d e r employed p r e v i o u s l y . R i g h t - a n d left-handed subjects performed on t w o blocks o f t r i a l s . In one block, t h e commands were presented t o t h e right ear, and, in t h e o t h e r block, t h e commands were presented t o t h e l e f t ear. i n s t r u c t i o n s p r i o r t o each t r i a l block stressed t h e f a c t that t h e commands w o u l d , b e h e a r d in t h e right ear o n l y o r in t h e l e f t ear only, as t h e case may be. Again, t h e analysis p r o v i d e d no evidence f o r t h e p r e d i c t e d Ear Stimulated x Handedness interaction. B u t again, as in t h e f i r s t e x p e r i ment, t h e major source o f variance was t h e Command x Ear Stimulated Results o f these t w o interaction p i c t u r e d in t h e right h a l f o f F i g u r e 1. studies, then, clearly indicated t h a t t h e speed o f processing v e r b a l commands was affected by a cue i r r e l e v a n t t o t h e t a s k itself, t h a t is, t h e ear i n which t h e command was heard. R T was s i g n i f i c a n t l y f a s t e r when t h e

Effects of an Irrelevant Directional Cue

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content of t h e command corresponded t o t h e ear stimulated t h a n when it did not.

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An Isomorphic Association Between Ear Stimulated a n d lpsilateral Hand? A t t h i s point, t h e focus o f o u r research shifted. We decided that, r a t h e r t h a n p u r s u i n g o u r o r i g i n a l hypothesis, we would t r y t o determine t h e reason f o r t h e unexpected Command x Ear Stimulated interaction. O u r f i r s t t h o u g h t was t h a t t h e r e m i g h t b e a s t r o n g tendency t o associate r i g h t - e a r stimulation w i t h a r i g h t - h a n d response a n d left-ear stimulation w i t h a l e f t - h a n d response. If t h i s were t r u e , we reasoned, t h e n t h e interaction should not occur on a unimanual t a s k . If t h e interaction did occur on a unimanual task, t h i s would indicate t h a t it did n o t d e r i v e f r o m a simple isomorphic association between ear stimulated a n d ipsilateral hand.

In o u r n e x t experiment (Simon, 1968a), r i g h t - h a n d e d subjects moved a c o n t r o l handle t o t h e right o r l e f t f r o m a center position in response t o t h e same recorded series o f v e r b a l commands used in t h e f i r s t t w o experiments. Half o f t h e subjects performed w i t h t h e i r right hand, while t h e o t h e r half performed w i t h t h e i r l e f t hand. A timer, which measured RT, s t a r t e d when a command was presented a n d stopped when t h e subject moved t h e handle away f r o m t h e c e n t e r position. A n o t h e r timer, which measured movement time, s t a r t e d when t h e handle

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was moved f r o m t h e c e n t e r position a n d stopped when t h e 10 in. lateral movement was completed. Subjects were i n s t r u c t e d t o react a n d move as f a s t as t h e y c o u l d on each t r i a l . T h e R T data revealed t h e same response i n t e r f e r e n c e o b s e r v e d in p r e v i o u s experiments, demonstrating t h a t t h e e f f e c t was n o t d u e t o a n y simple isomorphic association between ear stimulated a n d ipsilateral hand. I n t e r e s t i n g l y , t h e same Command x Ear Stimulated interaction also o c c u r r e d f o r movement time. T h a t is, movements t o t h e right were f a s t e r when t h e "right" command was h e a r d i n t h e right ear t h a n when it was h e a r d in t h e l e f t ear, and, similarly, movements t o t h e l e f t were f a s t e r when t h e " l e f t " command was h e a r d i n t h e l e f t ear t h a n when it was h e a r d in t h e right e a r . These effects on movement time indicate t h a t d i s p l a y c h a r a c t e r i s t i c s can a f f e c t not o n l y R T but also t h e r a t e o f a s u b sequent b a l l i s t i c movement. T e n d e n c y to React T o w a r d t h e Source of Stimulation

It appeared a t t h i s p o i n t t h a t t h e i r r e l e v a n t c u e a f f e c t i n g R T m i g h t b e a n a t u r a l t e n d e n c y t o react t o w a r d t h e source o f stimulation. T o test t h i s notion, we designed a v e r y simple experiment (Simon, 1969). Subjects were i n s t r u c t e d t o move a c o n t r o l handle t o e i t h e r t h e right o r t h e l e f t f r o m i t s c e n t e r position, depending upon t h e ear in w h i c h t h e y h e a r d a tone. Subjects were told, " T h i s is a t e s t t o see how q u i c k l y y o u can react a n d move in response t o a tone t h a t y o u w i l l hear in e i t h e r y o u r right ear o r in y o u r l e f t e a r . " In one block o f t r i a l s , t h e y were i n s t r u c t e d to "...move t h e c o n t r o l handle m a y f r o m t h e side of t h e ear stimulated. In o t h e r words, when you hear t h e tone i n y o u r l e f t ear, move t h e c o n t r o l handle t o t h e right as q u i c k l y as possible, a n d when y o u hear t h e t o n e in y:ur right ear, move t h e c o n t r o l handle t o t h e l e f t In another b l o c k o f t r i a l s , subjects h e a r d t h e as quickly as possible. same sequence o f stimulus tones, but, t h i s time, t h e y were i n s t r u c t e d t o move t h e c o n t r o l handle toward t h e side o f t h e ear stimulated. Subjects reacted s i g n i f i c a n t l y f a s t e r when i n s t r u c t e d t o move t h e c o n t r o l handle toward instead o f away f r o m t h e side o f t h e ear stimulated (292 v s . 351 ms). These r e s u l t s can b e explained by p o s t u l a t i n g a n a t u r a l t e n d e n c y t o react t o w a r d t h e source o f stimulation. T h e necessity f o r o v e r r i d i n g t h i s s t e r e o t y p e b e f o r e r e s p o n d i n g t o t h e tones presented i n t h e "away" b l o c k o f t r i a l s would account f o r t h e slower information processing during t h a t block. Facilitation and/or Interference? L e t me now b a c k t r a c k t o o u r studies i n v o l v i n g R T t o v e r b a l d i r e c tional commands. O u r r e s u l t s t h e r e suggested t h a t t h e a u d i t o r y d i s p l a y p r o v i d e d t w o cues, one r e l e v a n t ( i . e . , t h e c o n t e n t o f t h e command) a n d t h e o t h e r i r r e l e v a n t (i.e., t h e ear stimulated), a n d t h a t t h e time r e q u i r e d t o process t h e f o r m e r c u e was somehow affected by t h e presence o f t h e l a t t e r cue. A t t h i s point, we f e l t t h a t t h e Command x Ear Stimulated i n t e r a c t i o n was d u e t o t h e i r r e l e v a n t c u e interfering w i t h information p r o cessing on t r i a l s in w h i c h it did n o t correspond w i t h t h e symbolic c o n t e n t o f t h e command. It was possible, however, t h a t t h e interaction was d u e t o t h e i r r e l e v a n t cue facilitating information processing on t r i a l s in w h i c h it corresponded w i t h t h e symbolic c o n t e n t of t h e command. It was also possible t h a t t h e i r r e l e v a n t c u e operated both t o f a c i l i t a t e r e s p o n d i n g o n t h e c o r r e s p o n d i n g t r i a l s a n d t o i n t e r f e r e w i t h r e s p o n d i n g on t h e noncorresponding trials.


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O u r n e x t experiment (Simon & Small, 1969) was designed t o d e t e r mine how t h e Command x Ear Stimulated interaction was produced. It was also designed t o determine t h e g e n e r a l i t y o f t h e interaction; t h a t is, was t h e phenomenon limited t o situations i n v o l v i n g v e r b a l commands, o r did it also occur when simple stimuli such as p u r e tones were used t o p r o v i d e t h e relevant directional information? O u r subjects’ task, t h i s time, was t o press t h e c o r r e c t one o f t w o f i n g e r keys as q u i c k l y as possible a f t e r h e a r i n g e i t h e r a h i g h - p i t c h e d tone (loo0 Hz) o r a low-pitched tone (400 Hz) presented t o one ear o r the other. T h e subjects did n o t know p r i o r t o h e a r i n g t h e tone which ear would b e stimulated o r what t h e tone would be. Half o f t h e subjects were i n s t r u c t e d t o press t h e right k e y when t h e y h e a r d t h e h i g h - p i t c h e d tone a n d t o press t h e l e f t k e y when t h e y h e a r d t h e low-pitched tone. T h e o t h e r h a l f o f t h e subjects were g i v e n t h e opposite t o n e - k e y r u l e . In addition t o t h i s monaural block, each subject also performed o n a block o f t r i a l s i n v o l v i n g b i n a u r a l stimulation. T h e o u t p u t SPL of t h e b i n a u r a l t r i a l s was reduced by 6 dB, so as t o y i e l d approximately t h e same loudness as o n t h e monaural t r i a l s . F i g u r e 2 shows R T t o t h e right a n d l e f t tonal commands presented t o t h e right ear, t h e l e f t ear, o r t o b o t h ears simultaneously. It can b e seen t h a t R T was markedly f a s t e r when t h e right command was h e a r d in

TONAL DIRECTIONAL COMMAND

F i g u r e 2. Reaction time t o tonal directional commands as a f u n c t i o n o f ear(s) stimulated.

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t h e right ear t h a n when it was h e a r d in t h e l e f t ear, and, similarly, R T t o t h e l e f t command was f a s t e r when it was h e a r d in t h e l e f t ear t h a n Clearly then, subjects responded s i g when it was h e a r d in t h e right. n i f i c a n t l y f a s t e r o n t r i a l s in which t h e symbolic content o f t h e command corresponded w i t h t h e ear stimulated t h a n o n t r i a l s in which it did not. T h i s r e s u l t demonstrated t h a t t h e Command x Ear Stimulated interaction, heretofore o n l y observed w i t h v e r b a l directional commands, also o c c u r r e d It when p u r e tones were used to signal t h e appropriate response. appears, then, t h a t t h e interaction reflects a basic and general phenomenon t h a t exists independently o f whether t h e command i s comIt also appears t h a t t h e interaction is municated v e r b a l l y o r nonverbally. unrelated t o p r i o r symbolic associations since, in contrast t o v e r b a l directional commands, t h e tones h a d no implicit directional significance. T h e major p u r p o s e o f t h i s experiment, however, was t o determine whether t h e Command x Ear Stimulated interaction was d u e t o a facilitation of information processing o n t h e corresponding t r i a l s o r t o an i n t e r ference w i t h information processing on t h e noncorresponding t r i a l s . On t h e monaural trials, t h e ear in which t h e tone was h e a r d p r o v i d e d an i r r e l e v a n t cue which, o f course, was absent on t h e b i n a u r a l t r i a l s . Therefore, b i n a u r a l R T was used as a baseline f o r evaluating t h e effect o f t h i s i r r e l e v a n t cue. Statistical tests revealed t h a t b i n a u r a l R T was s i g n i f i c a n t l y faster t h a n R T o n noncorresponding as well as corresponding monaural t r i a l s . On t h e noncorresponding monaural t r i a l s , t h e i r r e l e v a n t location c u e apparently conflicted w i t h processing t h e relevant symbolic On t h e corresponding monaural trials, t h e cue, r e s u l t i n g in slower R T . i r r e l e v a n t location cue coincided w i t h t h e relevant symbolic cue, b u t t h i s Results of t h i s correspondence did n o t facilitate information processing. experiment, then, suggested t h a t t h e Command x Ear Stimulated interact i o n was a r e s u l t o f i n t e r f e r e n c e w i t h information processing o n t h e noncorresponding t r i a l s r a t h e r t h a n a facilitation on t h e corresponding t r i a l s . (In l a t e r experiments, u s i n g a d i f f e r e n t experimental design, o u r results indicated t h a t t h e i r r e l e v a n t location c u e may p r o d u c e facilitation as well as i n t e r f e r e n c e [e.g., Acosta & Simon, 1976; Simon 8 Acosta, 1982; Simon & Craft, 1970a; Simon & Pouraghabagher, 19781). T h e Simon E f f e c t w i t h Visual Displays O u r n e x t experiment was concerned w i t h determining whether t h e same phenomena t h a t h a d been observed w i t h a u d i t o r y displays would also A stereoscope was occur w i t h visual displays ( C r a f t & Simon, 1970). modified by removing t h e lenses and i n s e r t i n g an opaque p a r t i t i o n t o d i v i d e t h e visual f i e l d i n t o t w o halves (see F i g u r e 3). Red a n d green stimulus l i g h t s were mounted in t h e center of each h a l f of t h e f i e l d . Subjects operated f i n g e r keys w i t h t h e i r right o r l e f t index fingers. Half of, t h e subjects were told, "If y o u see a r e d light, p u s h t h e r i g h t - h a n d key, a n d if y o u see a green light, p u s h t h e l e f t - h a n d key." T h e o t h e r half o f t h e subjects were g i v e n t h e opposite l i g h t - k e y associations. Subjects performed on t w o blocks o f t r i a l s , a monocular In t h e monocular block, a r e d light o r a block a n d a binocular block. green light was presented t o e i t h e r t h e right eye o r t h e l e f t eye in a


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F i g u r e 3. Modified viewer i n d i c a t i n g location o f stimulus l i g h t s when v i s u a l f i e l d is p a r t i t i o n e d a n d lenses a r e removed. random sequence. I n t h e b i n o c u l a r block, t h e r e d l i g h t s o r t h e g r e e n l i g h t s w e r e p r e s e n t e d t o b o t h eyes simultaneously. F i g u r e 4 shows t h e r e s u l t s o f t h i s experiment. Reactions t o t h e right command were f a s t e r when it was p r e s e n t e d t o t h e right e y e t h a n when it was presented t o t h e l e f t eye, a n d reactions t o t h e l e f t command were f a s t e r when it was p r e s e n t e d t o t h e l e f t eye t h a n when it was p r e sented t o t h e right eye. T h i s Command x Eye Stimulated i n t e r a c t i o n i s a d i r e c t parallel t o t h e Command x Ear Stimulated interaction o b s e r v e d in p r e v i o u s studies. Note, too, t h a t reactions on t h e b i n o c u l a r t r i a l s were f a s t e r t h a n on e i t h e r t h e c o r r e s p o n d i n g o r noncorresponding monocular t r i a l s , s u g g e s t i n g t h a t t h e i n t e r a c t i o n o b s e r v e d on t h e monocular t r i a l s was d u e t o i n t e r f e r e n c e o n noncorresponding t r i a l s r a t h e r t h a n t o f a c i l i tation on corresponding trials. Perceptual R a t h e r t h a n Sensory Explanation for t h e Simon E f f e c t B u t w h a t is t h e source o f t h i s interference? It c o u l d b e t h e eye stimulated p e r se o r it c o u l d b e an i r r e l e v a n t cue, such as t h e spatial locations of t h e stimulus lights, associated with t h e eye stimulated. Our

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F i g u r e 4.

Reaction time t o commands as a f u n c t i o n o f eye(s) stimulated.

n e x t experiment was designed t o isolate t h e c r i t i c a l f a c t o r . T h e proced u r e i n v o l v e d m o d i f y i n g t h e d i s p l a y so as t o eliminate t h e directional cue p r o v i d e d by t h e locus o f t h e stimulus. If t h e directional c u e was responsible f o r t h e interference, t h e n t h e Command x Eye Stimulated interaction should b e eliminated. If, on t h e o t h e r hand, t h e eye stimulated per se was t h e source o f t h e interference, t h e n t h e i n t e r a c t i o n should s t i l l occur. T h e d i s p l a y was t h e same as t h a t used in t h e p r e v i o u s e x p e r i ment except t h a t t h e opaque p a r t i t i o n dividing t h e v i s u a l f i e l d s was removed, a n d t h e lenses were reinstalled. Now, instead o f seeing a r e d o r g r e e n light in e i t h e r t h e l e f t o r right v i s u a l field, t h e subject saw t h e light in t h e c e n t e r o f t h e v i s u a l f i e l d . J u s t as in t h e p r e v i o u s e x p e r i ment, a n y one o f t h e f o u r l i g h t s could b e presented, t h e r e b y stimulating t h e right o r l e f t eye separately. However, since n o spatial o r directional c u e was now associated w i t h t h i s stimulation, subjects c o u l d n o t t e l l w h i c h eye was b e i n g stimulated. Subjects responded t o t h e same random sequence o f monocular t r i a l s used p r e v i o u s l y ; t h a t is, e i t h e r a r e d l i g h t o r a g r e e n light was p r e sented t o e i t h e r t h e right eye o r t h e l e f t eye. Results indicated a comp l e t e absence o f t h e Command x Eye Stimulated interaction, demonstrating conclusively t h a t t h e i n t e r a c t i o n t h a t was so prominent in t h e p r e v i o u s


39

experiment was due, n o t t o t h e eye stimulated per se, but t o t h e d i r e c tional c u e associated w i t h t h e eye stimulated. O u r n e x t experiment (Simon, Small, Ziglar, & Craft, 1970) was designed t o determine w h e t h e r t h e Command x Ear Stimulated i n t e r a c t i o n observed in o u r p r e v i o u s studies was d u e t o t h e ear stimulated per se o r t o a directional c u e associated w i t h ear stimulated. Obviously, these t w o factors h a d been confounded in o u r p r e v i o u s experiments. T h a t is, a signal t o t h e right ear appeared t o come f r o m t h e right side, a n d a signal t o t h e l e f t ear appeared t o come f r o m t h e l e f t side. T h e question was how t o unconfound sensory (ear stimulated) a n d perceptual (directional cue) f a c t o r s . Because we c o u l d think o f no w a y o f p r e s e n t i n g monaural stimulation w i t h o u t an associated directional cue, we decided t o eliminate ear stimulated as a f a c t o r . B y employing a p h a s e - s h i f t i n g device, it was possible t o stimulate b o t h ears simultaneously a n d y e t have it appear t o If t h e subjects as if t h e signal were coming f r o m one side o r t h e o t h e r . i n t r o d u c t i o n o f an i r r e l e v a n t directional cue on these b i n a u r a l t r i a l s p r o duced i n t e r f e r e n c e similar t o t h a t on t h e monaural t r i a l s , one m i g h t safely presume t h a t t h e directional cue was responsible f o r t h e i n t e r f e r e n c e on t h e monaural t r i a l s . Subjects performed on t w o blocks o f t r i a l s , one b l o c k i n v o l v i n g monaural stimulation a n d t h e o t h e r i n v o l v i n g b i n a u r a l On t h e monaural trials, subjects pressed a right- 0; l e f t stimulation. right" h a n d k e y in response t o e i t h e r a high- o r low-pitched tone (i.e., o r " l e f t " command) presented t o e i t h e r t h e i r right o r l e f t ear. On a b i n a u r a l b l o c k o f trials, subjects h e a r d t h e same random sequence o f tonal commands, but t h i s time t h e tone was p r e s e n t e d t o b o t h ears w i t h a p h a s e - s h i f t s e t t i n g o f e i t h e r 90" o r 270O. T h e l e f t half o f F i g u r e 5 p i c t u r e s t h e r e s u l t s f r o m t h e monaural block. T h e Command x Ear Stimulated interaction is, by now, a familiar one. T h e right h a l f o f F i g u r e 5 shows t h e r e s u l t s f r o m t h e b i n a u r a l block. With a 270" p h a s e - s h i f t setting, t h e tonal command appeared t o come f r o m t h e right, and, w i t h a 90' p h a s e - s h i f t setting, t h e tone appeared t o come f r o m t h e l e f t . Note t h a t t h e subjects responded f a s t e r t o t h e "right" command when it appeared t o come f r o m t h e right t h a n when it appeared t o come f r o m t h e l e f t a n d responded f a s t e r t o t h e " l e f t " command when it appeared t o come f r o m t h e l e f t t h a n when it appeared t o come f r o m t h e right. These r e s u l t s s t r o n g l y suggested t h a t t h e i n t e r a c t i o n on t h e monaural t r i a l s was n o t d u e t o ear stimulated per se, but t o t h e directional cue associated w i t h t h e ear stimulated. It should b e noted t h a t t h e interaction on t h e b i n a u r a l block was n o t as m a r k e d as o n t h e monaural block. Presumably, t h i s is because t h e i r r e l e v a n t directional cue p r o d u c e d by manipulating i n t e r a u r a l phase was n o t as p o t e n t as t h a t p r o d u c e d by monaural stimulation. Manipulating Potency o f t h e Directional C u e O u r n e x t experiments (Simon, C r a f t , & Small, 1971) used i n t e r a u r a l phase s h i f t as a means o f manipulating n o t o n l y t h e spatial locus o f a stimulus but also t h e potency o f t h e directional cue. If a directional c u e was, in fact, responsible f o r t h e i n t e r f e r e n c e w i t h information processing observed in p r e v i o u s studies, t h e n it should b e possible, by v a r y i n g t h e s t r e n g t h o f t h i s i r r e l e v a n t cue, t o a l t e r t h e amount o f interference. F i g u r e 6 shows that, w i t h a p h a s e - s h i f t s e t t i n g o f 90°, a b i n a u r a l tone appears t o come f r o m t h e l e f t . With a s e t t i n g o f 45", a b i n a u r a l tone

J.R. Simon

40

-

450r

Monaural Stimulation

Binaural Stimulation 0--

0

0

Right command Left command

0

375

Right

Left

EAR S T I M U L A T E D

I

I

2 70. ( Right 1

90° ( Lcf t 1

I N T E R A U R A L PHASE DIFFERENCE

F i g u r e 5. Reaction time t o monaural a n d binaural tonal directional commands as a f u n c t i o n of t h e i r actual a n d apparent source. (On monaural trials, t h e tone was presented t o e i t h e r t h e right o r l e f t ear, while on b i n a u r a l t r i a l s t h e tone was presented t o b o t h ears w i t h t h e i n t e r a u r a l phase d i f f e r e n c e adjusted so t h a t i t s apparent source was a t e i t h e r t h e right o r l e f t ear.) s t i l l appears t o come f r o m t h e left, b u t t h e directional cue is, presumSimilarly, w i t h a phase-shift s e t t i n g of ably, n o t as s t r o n g as a t 90'. 270°, a b i n a u r a l tone appears t o come f r o m t h e right. A s e t t i n g o f 315' also produces a tone t h a t appears t o come f r o m t h e right but, presumably, t h e directional cue i s n o t as s t r o n g as a t 270'. T h e subjects' t a s k was t o press a right- o r a l e f t - h a n d k e y as q u i c k l y as possible a f t e r h e a r i n g a high- o r a low-pitched binaural tone. T h e tones were presented in a random sequence a t phase-shift settings o f e i t h e r 90°, 270°, 45O, o r 315', all w i t h i n a single block o f t r i a l s . It should b e emphasized that, a t each o f these phase-shift settings, t h e tones a t each ear were identical w i t h respect t o frequency, sound p r e s sure, a n d moment of onset. T h e o n l y difference was t h a t o f r e l a t i v e phase, a n d it was t h i s cue t h a t led t o t h e apparent position o f t h e sound o n t h e l e f t o r right side. We p r e d i c t e d t h a t a Command x A p p a r e n t Source interaction would o c c u r f o r t h e 270'-90' maximum deviation condit i o n a n d also f o r t h e 315'-45' reduced deviation condition b u t t h a t t h e size o f t h e interaction would n o t b e as marked in t h e l a t t e r case as in t h e former. I n o t h e r words, we p r e d i c t e d a t r i p l e - o r d e r interaction o f Command x A p p a r e n t Source x S t r e n g t h o f Directional Cue.


41

INTERAURAL PHASE DIFFERENCE

F i g u r e 6. tings.

A p p a r e n t source o f tonal commands a t various p h a s e - s h i f t set-

T h e r e s u l t s f o r t h e maximum deviation condition ( t h e 270°-90' p h a s e - s h i f t settings) duplicated those f r o m t h e p r e v i o u s s t u d y . However, t h e reduced deviation condition ( t h e 315O-45O p h a s e - s h i f t settings) p r o d u c e d v i r t u a l l y identical r e s u l t s . In o t h e r words, t h e phase-shift settings t h a t we h a d selected f o r t h e reduced deviation condition did n o t p r o d u c e t h e p r e d i c t e d t r i p l e - o r d e r interaction. We recognized t h a t t h e relation between s t r e n g t h o f t h e directional c u e a n d i n t e r a u r a l phase d i f f e r e n c e may n o t b e linear, a n d so we decided t o t r y again, t h i s time f u r t h e r r e d u c i n g t h e i n t e r a u r a l phase difference, a n d therefore, presumably, also f u r t h e r r e d u c i n g t h e s t r e n g t h o f t h e i r r e l e v a n t directional cue. F o r t h i s experiment, p h a s e - s h i f t s e t t i n g s of 345O a n d 15" were selected f o r t h e reduced deviation condition (see F i g u r e 61, a n d these were again contrasted w i t h t h e 270°-90° maximum deviation condition. T h e l e f t h a l f o f F i g u r e 7 shows t h e data f r o m t h e maximum deviat i o n condition. Note t h e exact replication o f p r e v i o u s f i n d i n g s . The right half of F i g u r e 7 shows a similar s i g n i f i c a n t i n t e r a c t i o n f o r t h e reduced deviation condition, but t h i s time, j u s t as predicted, t h e i n t e r action was s i g n i f i c a n t l y smaller t h a n u n d e r t h e maximum deviation condit i o n . Thus, t h e r e s u l t s demonstrated t h a t we c o u l d a l t e r t h e size o f t h e Command x A p p a r e n t Source i n t e r a c t i o n by manipulating t h e s t r e n g t h of t h e i r r e l e v a n t directional cue. T h i s finding p r o v i d e s additional evidence

J.R. Simon

42

t h a t t h e i r r e l e v a n t cue o p e r a t i n g in t h i s a n d in p r e v i o u s studies was, in fact, a directional cue. T h e s t r o n g e r t h a t directional cue, t h e s t r o n g e r was t h e tendency t o react t o w a r d t h e source of stimulation. D E V I A T I O N OF A P P A R E N T

380

SOURCE

Maximum Deviation

FROkl M E D I A N P L A N E Reduced Deviation 00-

Right command

360

z

350

0

0 0

0

270° (Right 1

0

9O0(Left)

0

345O(Right)

I 5 O (Left)

I N T E R A U R A L PHASE DIFFERENCE ( A p p a r e n t Source of Command 1

F i g u r e 7. T h e p r e d i c t e d t r i p l e - o r d e r interaction between command, apparent source, a n d deviation o f apparent source f r o m t h e median plane. Reaction T o w a r d t h e Major Source of Stimulation T h e n e x t question we asked was, is t h i s stereotype a specific tendency t o react t o w a r d t h e relevant stimulus ( t h e command) o r i s it a more general tendency t o react t o w a r d t h e major source o f stimulation? I f it is a tendency t o react t o w a r d t h e major source o f stimulation, it should b e possible t o eliminate t h e Command x Ear Stimulated interaction by p r e s e n t i n g an i r r e l e v a n t stimulus t o t h e opposite ear simultaneously w i t h t h e relevant tonal command. It should also b e possible t o a l t e r t h e size o f t h e interaction by manipulating t h e d B level o f t h e i r r e l e v a n t stimulus t o t h e opposite ear. It m i g h t even b e possible t o reverse t h e d i r e c t i o n o f t h e interaction by increasing t h e d B level o f t h e ' i r r e l e v a n t stimulus t o exceed t h a t o f t h e relevant tonal command. To investigate some o f these notions, we conducted t h r e e e x p e r i ments similar t o those r e p o r t e d p r e v i o u s l y (Simon, Craft, & Small, 1970). In each experiment, subjects were t o l d t o press a right- o r a l e f t - h a n d k e y in response t o high- o r low-pitched tonal commands. In t h e f i r s t experiment, h a l f o f t h e t r i a l s in a block were exactly t h e same as those in p r e v i o u s experiments; that is, a tonal command was presented t o one ear o r t h e other, a n d t h e r e was no stimulus p r o v i d e d t o t h e opposite ear. On t h e o t h e r h a l f o f t h e t r i a l s , an 89 d B broad-band noise was presented


43

t o t h e opposite ear simultaneously w i t h t h e i n t r o d u c t i o n o f t h e mand. U n d e r t h e no-noise condition, we obtained t h e same observed so many times before. However, when t h e noise was t o t h e opposite ear, t h e Command x Ear Stimulated interaction nated.

tonal cominteraction introduced was elimi-

T h e n e x t experiment was designed t o determine whether we could a l t e r t h e size o f t h e Command x Ear Stimulated interaction by manipulating t h e i n t e n s i t y o f t h e stimulation t o t h e opposite ear. We reasoned t h a t if an 89 d B noise eliminated t h e interaction, t h e n a noise o f lesser i n t e n s i t y m i g h t reduce b u t n o t eliminate t h e interaction, a n d a noise o f g r e a t e r i n t e n s i t y m i g h t actually reverse t h e d i r e c t i o n o f t h e interaction. F i g u r e 8 shows t h e results o f t h i s experiment. U n d e r t h e no-noise With condition ( l e f t panel), we again observed t h e familiar interaction. 74 d B noise t o t h e opposite ear (center panel), t h e interaction was s t i l l significant b u t was s i g n i f i c a n t l y smaller t h a n u n d e r t h e no-noise condit i o n . W i t h t h e 106 d B noise t o t h e opposite ear (right panel), t h e i n t e r In o t h e r action was statistically significant in t h e opposite direction. words, now R T was faster t o t h e l e f t command when it was h e a r d in t h e right ear t h a n when it was h e a r d in t h e left, a n d R T t o t h e right command was faster when it was h e a r d in t h e l e f t ear t h a n when it was heard in t h e right ear.

475

-

450

-

425

-

400

-\ 0

/

375

sa

1 0 6 d B Noise

74 d B Noise

N o Noise

-

/

0 '

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0-0

Right command L e f t command

350 Right

L e f t Right EAR STIMULATED

L e f t Right

Left

BY TONAL COMMAND

F i g u r e 8. Tonal Command x Ear Stimulated interactions. T h e interaction on t h e no-noise t r i a l s [ l e f t ] was e i t h e r s i g n i f i c a n t l y reduced [center] o r reversed [right], depending on t h e i n t e n s i t y o f noise presented t o t h e ear which did n o t receive t h e tonal command.

J.R. Simon

44

Because we wanted t o establish t h a t the crucial factor i n these last two experiments was not t h e presence of the noise per se, but, rather, t h e locus of t h e noise, we performed an additional experiment. On a third of t h e trials, t h e commands were presented alone (i.e., no noise t o opposite ear); on a t h i r d of t h e trials, an 89 d B broad-band noise was presented t o t h e opposite ear simultaneously w i t h t h e command; and on t h e remaining third of t h e trials, t h e 89 d B noise was presented t o t h e sum eur as t h e tonal command. Figure 9 pictures t h e results. The data f o r t h e no-noise condition ( l e f t panel) duplicated previous findings. This time, however, t h e 89 d B noise t o t h e opposite ear (center panel) did more than eliminate t h e i n t e r (This represents t h e action; it actually produced a significant reversal. o n l y instance in t h e e n t i r e series of studies f o r which we failed t o replicate previous results.) Presentation of t h e noise t o t h e same ear as t h e command (right panel) accentuated t h e interaction, demonstrating t h a t t h e locus of t h e irrelevant noise was the crucial factor. In summary, t h e main results of these last t h r e e experiments can all be explained by postulating a strong natural tendency t o react toward t h e major source of stimulation. Changing t h e stimulus-intensity difference at t h e two ears altered t h e strength of an irrelevant directional cue and produced t h e observed shifts in t h e direction and magnitude of the Command x Ear Stimulated interaction.

89 dB Noise in Opposite Ear

No Noise -4

RqM

89 dB Noise in Same Ear

Right Command

Left

--

Right

Left Right

Left

EAR STIMULATED BY TONAL COMMAND Figure 9. Tonal Command x Ear Stimulated interactions. The marked interaction on t h e no-noise trials [left] was either reversed [center] o r accentuated [right], depending on whether the noise was presented t o t h e opposite ear o r t o t h e same ear which received t h e tonal command.


45

Mechanisms U n d e r l y i n g A u d i t o r y S-R Compatibility We conducted t w o additional experiments t o i d e n t i f y t h e f a c t o r s c o n t r i b u t i n g t o S-R compatibility in a u d i t o r y information-processing tasks (Simon, Hinrichs, & Craft, 1970). O u r e a r l i e r studies h a d shown t h a t reactions were s i g n i f i c a n t l y f a s t e r when t h e content o f t h e command c o r responded t o t h e ear stimulated t h a n when it did not. In almost all of these studies, however, subjects used t h e i r right h a n d to operate a right k e y a n d t h e i r l e f t hand t o operate a l e f t k e y . It i s n o t clear, therefore, whether t h e f a s t e r responding o n t h e corresponding t h a n on t h e noncorresponding t r i a l s ( t h e compatibility effect) was d u e t o ear/hand correspondence o r ear/response-location correspondence. I n o t h e r words, t h e r e m i g h t b e a tendency to associate r i g h t - e a r stimulation w i t h a righthand response a n d l e f t - e a r stimulation w i t h a l e f t - h a n d response; o r alternatively, r i g h t - e a r stimulation may t e n d t o e l i c i t a right- k e y response a n d left-ear stimulation may t e n d t o e l i c i t a l e f t - k e y response. The reader w i l l recall that, e a r l y i n o u r research program, we were concerned with ear/hand correspondence as a possible explanation f o r o u r f i n d i n g s a n d conducted a s t u d y t h a t demonstrated t h a t t h e compatibility e f f e c t s t i l l o c c u r r e d on a one-hand t a s k (Simon, 1968a). T h i s finding, however, did n o t eliminate t h e p o s s i b i l i t y t h a t ear/hand correspondence m i g h t c o n t r i b u t e t o t h e effect in a two-hand t a s k . We, therefore, designed an experiment t o unconfound these t w o possible sources o f a u d i t o r y S-R compati biIity

.

T h e experiment was similar in many respects t o those r e p o r t e d earlier. Subjects performed on t w o blocks o f trials, responding on one b l o c k w i t h t h e i r arms uncrossed a n d on t h e o t h e r block w i t h t h e i r arms crossed. Thus, o n t h e uncrossed block, subjects operated t h e right k e y with their right-index finger and the left key with their left-index finger, while, on t h e crossed block, t h e y operated t h e right k e y w i t h t h e i r l e f t - i n d e x f i n g e r a n d t h e l e f t k e y w i t h t h e i r r i g h t - i n d e x f i n g e r . On each trial, a high- o r low-pitched tone was presented t o one ear. Half o f t h e subjects were told, “If y o u hear t h e h i g h - p i t c h e d tone, press down t h e key w i t h y o u r l e f t - i n d e x f i n g e r as f a s t as y o u can. If y o u hear t h e low-pitched tone; press down t h e k e y w i t h y o u r r i g h t - i n d e x f i n g e r as fast as y o u can. T h e o t h e r h a l f o f t h e subjects were g i v e n t h e opposite tone-hand rule. A n analysis o f variance o f t h e uncrossed-arms t r i a l s revealed a s i g n i f i c a n t Command x Ear Stimulated interaction; t h a t is, R T was f a s t e r when t h e right-hand command was h e a r d in t h e right ear t h a n when it was h e a r d i n t h e l e f t ear (385 v s . 445 ms), a n d R T t o t h e l e f t - h a n d command was f a s t e r when it was h e a r d in t h e l e f t ear t h a n when it was T h e crossed-arms t r i a l s also heard in t h e right ear (386 vs. 457 ms). revealed a s i g n i f i c a n t interaction, b u t , t h i s time, t h e direction was reversed f r o m t h a t observed on t h e uncrossed trials; t h a t is, R T was f a s t e r when t h e r i g h t - h a n d command was h e a r d in t h e l e f t ear t h a n when it was h e a r d i n t h e right (405 v s . 465 ms), and R T t o t h e l e f t - h a n d command was f a s t e r when it was h e a r d i n t h e r i g h t ear t h a n when it was h e a r d in t h e l e f t (432 v s . 482 ms). T h e f a c t t h a t crossing t h e arms produced a reversal in t h e d i r e c t i o n o f t h e Command x Ear Stimulated interaction s t r o n g l y suggests t h a t ear/response-location correspondence r a t h e r t h a n ear/hand correspondence was t h e crucial f a c t o r accounting f o r t h e interaction. To clarify t h i s point, we combined a n d relabeled t h e means f o r t h e various treatment

46

J.R. Simon

combinations t o r e f l e c t t h e presence o r absence o f ear/response-location correspondence a n d ear/hand correspondence. T h i s analysis demons t r a t e d a marked e f f e c t o f ear/response-location correspondence (402 v s . 462 ms), a n d a minimal e f f e c t o f ear/hand correspondence (430 v s . 435 ms). It appears, then, t h a t t h e monaural tonal command p r o v i d e s an i r r e l e v a n t directional cue that tends t o e l i c i t a response on t h e same side as t h e ear stimulated (Simon, 1969). T h i s potent response tendency occurs regardless o f w h e t h e r t h e response is executed by t h e hand i p s i lateral o r contralateral t o t h e ear stimulated.

In t h e above experiment, t h e a u d i t o r y display p r o v i d e d t w o cues: one relevant ( t h e meaning o f t h e tonal command) a n d t h e o t h e r i r r e l e v a n t (the ear stimulated). Results indicated t h a t t h e time r e q u i r e d t o process t h e relevant c u e was affected by t h e presence o f t h e i r r e l e v a n t cue. In o t h e r words, t h e S-R compatibility phenomenon hinged on a stimulus dimension t h a t was i r r e l e v a n t to t h e t a s k itself. B u t what a r e t h e mechanisms u n d e r l y i n g a u d i t o r y S-R compatibility in a simpler t a s k i n v o l v i n g j u s t one relevant cue, t h e ear in which a tone is presented? In an earlier experiment (Simon, 19671, we had subjects respond t o a tone presented t o e i t h e r t h e l e f t o r right ear by p r e s s i n g a l e f t k e y

w i t h t h e i r l e f t h a n d o r a right k e y w i t h t h e i r right hand. On one b l o c k of t r i a l s (corresponding trials), t h e y were i n s t r u c t e d t o press t h e l e f t hand k e y when t h e y h e a r d a tone i n t h e i r l e f t ear and t h e right-hand k e y when t h e y h e a r d a tone in t h e i r right ear. On another block (noncorresponding t r i a l s ) , i n s t r u c t i o n s were t o press t h e l e f t - h a n d k e y when t h e y h e a r d a tone in t h e i r right ear a n d t o press t h e r i g h t - h a n d k e y when t h e y h e a r d a tone in t h e i r l e f t ear. Results indicated s i g n i f i c a n t l y slower responding on t h e noncorresponding trials, b u t since these t r i a l s i n v o l v e d a reversal o f b o t h ear-hand correspondence and earresponse-location correspondence, it was impossible t o determine t h e cont r i b u t i o n of each f a c t o r separately. We, therefore, designed a f a i r l y complex experiment t o achieve t h i s separation (Simon, Hinrichs, & C r a f t , 1970, Experiment II).

In t h a t experiment, subjects pressed one of t w o f i n g e r keys as q u i c k l y as possible depending on t h e ear in which t h e y heard a tone. Each subject performed on t w o blocks o f trials, responding on one block w i t h arms uncrossed and on t h e o t h e r block w i t h arms crossed. On one o f these t w o blocks, t h e subject followed i n s t r u c t i o n s designed t o produce ear/hand correspondence; that is, "If you hear t h e tone in y o u r right ear, p r e s s down t h e k e y w i t h y o u r r i g h t - i n d e x f i n g e r and if y o u hear t h e tone in y o u r l e f t ear, press down t h e k e y w i t h y o u r l e f t - i n d e x finger." On t h e o t h e r block, t h e subject followed i n s t r u c t i o n s designed t o p r o d u c e ear/hand noncorrespondence; t h a t is, "If you hear t h e tone i; y o u r right ear, press down t h e k e y w i t h y o u r l e f t index f i n g e r ...etc. Depending on which i n s t r u c t i o n s (corresponding o r noncorresponding) were p a i r e d w i t h which block (uncrossed o r crossed arms), t h e r e s u l t i n g treatment was either ear/response-location correspondence or ear/response-location noncorrespondence. Subjects were assigned a t random t o one o f t w o groups, a n ear/response-location correspondence g r o u p o r an ear/response-location noncorrespondence g r o u p . T h e first-mentioned g r o u p performed t h e i r uncrossed block w i t h corresponding ear/hand i n s t r u c t i o n s a n d t h e i r crossed b l o c k w i t h noncorresponding ear/hand instructions, whereas t h e l a t t e r g r o u p performed t h e i r uncrossed block w i t h noncorresponding

Effects of ear/hand instructions ear/hand i n s t r u c t i o n s .

and

an Irrelevant Directional Cue their

crossed

block

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corresponding

Reactions were s i g n i f i c a n t l y f a s t e r when b o t h ear stimulated a n d response k e y were o n t h e same side o f t h e b o d y midline (333 vs. 390 ms). In addition, reactions w e r e s i g n i f i c a n t l y f a s t e r when t h e h a n d c o r responded t o t h e ear stimulated (330 v s . 394 ms). Overall, then, r e s u l t s o f t h e Simon, Hinrichs, a n d C r a f t experiments (1970) suggested that, when t h e stimulus was simple a n d p r o v i d e d no information o t h e r t h a n i t s locus, t h e r e were a t least t w o components t o a u d i t o r y S-R compatibility: ear/hand correspondence a n d ear/response-location correspondence. However, when t h e stimulus p r o v i d e d b o t h a r e l e v a n t symbolic cue a n d an i r r e l e v a n t location cue, t h e n ear/response-location correspondence alone accounted f o r S-R compatibility. E f f e c t Across Sensory Modalities I n all o f t h e studies d e s c r i b e d t h u s f a r , b o t h t h e r e l e v a n t a n d t h e i r r e l e v a n t cues i n v o l v e d t h e same sensory modality; f o r example, a u d i t o r y information processing was influenced by an i r r e l e v a n t a u d i t o r y location cue, whereas v i s u a l information processing was a f f e c t e d by an i r r e l e v a n t v i s u a l location cue. It seemed reasonable t o ask w h e t h e r similar effects would o c c u r when t h e r e l e v a n t a n d i r r e l e v a n t cues i n v o l v e d d i f f e r e n t sensory modalities. Conceivably, subjects m i g h t find it easier t o separate relevant f r o m i r r e l e v a n t cues if t h e i r respective i n p u t s i n v o l v e d d i f f e r e n t sensory channels.

T h e experiment (Simon & C r a f t , 1970a) r e q u i r e d subjects t o operate a combination d i s p l a y - c o n t r o l panel consisting o f t w o small w h i t e t r a n s l u c e n t buttons, one on t h e l e f t a n d one on t h e right, t h a t c o u l d b e b a c k l i g h t e d by m i n i a t u r e b u l b s . Below a n d e q u i d i s t a n t f r o m these stimulusresponse bu)tons was a small b u t t o n t h a t p r o v i d e d t h e s t a r t i n g p o i n t f o r t h e subjects response. O n each t r i a l , subjects moved t h e i r r i g h t - i n d e x f i n g e r f r o m t h e s t a r t b u t t o n a n d pressed t h e b u t t o n t h a t l i g h t e d up. O n some t r i a l s , t h e light was presented alone while, on o t h e r trials, t h e light onset was accompanied by a 500-Hz tone p r e s e n t e d e i t h e r monaurally o r b i n a u r a l l y t h r o u g h earphones. T h e r e were, then, e i g h t treatment combinations, t h a t is, l e f t o r right light presented alone o r accompanied by a tone t o t h e l e f t ear, t h e right ear, o r t o b o t h ears. Each t r e a t m e n t comb i n a t i o n was p r e s e n t e d 21 times in a predetermined random sequence w i t h i n a single b l o c k o f t r i a l s . A n analysis o f variance o f t h e monaural t r i a l s revealed a s i g n i f i c a n t Response x Ear Stimulated interaction. Responses t o t h e l e f t light were f a s t e r when t h e tone was presented t o t h e l e f t ear t h a n t o t h e right ear (530 v s . 577 ms), a n d responses t o t h e right l i g h t were f a s t e r when t h e tone was presented t o t h e right ear t h a n t o t h e l e f t (485 v s . 511 ms). Clearly, t h e speed o f r e s p o n d i n g t o t h i s visual d i s p l a y was affected by t h e presentation o f c o n c u r r e n t a u d i t o r y stimulation t h a t h a d n o relevance t o t h e t a s k i t s e l f . Results, then, indicated t h a t t h e i r r e l e v a n t directional c u e p r o d u c e d an e f f e c t across sensory modalities. A second p u r p o s e o f t h i s s t u d y was t o determine w h e t h e r t h e monaural t o n e h a d a f a c i l i t a t i v e o r i n h i b i t o r y e f f e c t on information p r o cessing. B i n a u r a l t r i a l s p r o v i d e d a baseline f o r comparison since t h e y i n v o l v e d t h e presence o f an a u d i t o r y stimulus w i t h o u t t h e associated

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directional cue. A series of planned comparisons led t o the conclusion t h a t t h e monaural tone had both a facilitative and i n h i b i t o r y effect on visual RT. The left response was facilitated by t h e tone t o t h e l e f t ear but inhibited by. t h e tone t o t h e right ear and, similarly, t h e r i g h t response was facilitated by t h e tone t o the. r i g h t ear and inhibited by t h e tone t o t h e l e f t ear. These results d i f f e r e d from o u r previous findings ( C r a f t E Simon, 1970; Simon & Small, 1969), which had suggested t h a t the irrelevant directional cue interfered w i t h processing on t h e noncorresponding t r i a l s b u t did not facilitate responding on t h e corresponding trials. We speculated t h a t t h e d i f f e r e n t pattern of results may be due t o t h e f a c t that, i n t h e present study, the baseline trials were presented in t h e same block as the corresponding and noncorresponding trials, whereas, i n previous studies, t h e baseline trials were presented i n a separate block. Indeed, i n a later study (Simon & Acosta, 1982), we demonstrated t h a t indications of facilitation and/or interference are dependent on t h e context i n which t h e baseline trials are administered. When baseline trials were presented in t h e same block as the correspondi n g and noncorresponding trials, t h e directional cue produced both facilitation and interference. However, when baseline t r i a l s were presented in one block and corresponding and noncorresponding trials were mixed together i n another block, data suggested an interference effect only. The d i f f e r e n t pattern of results from t h e two experimental designs was attributed t o t h e degree of stimulus uncertainty within a t r i a l block (Garner, 1962). When t h e baseline trials were i n a separate block, uncertainty was less (i.e., there was uncertainty about t h e content of t h e command b u t not i t s source), and t h i s resulted i n faster RT, which would bias t h e p i c t u r e against concluding t h a t facilitation occurred on t h e corresponding trials. Finally, mention should be made about t h e no-tone trials, which were included t o provide a baseline f o r evaluating the effect of concurr e n t auditory stimulation on visual RT. Reactions on t h e no-tone trials were significantly slower than on t h e binaural trials. The reason f o r t h i s difference is not clear. It may be t h a t t h e auditory stimulus had an energizing effect on visual RT. Alternatively, it may be t h a t RT on t h e no-tone t r i a l s was slowed because of the absence of t h e tone; t h a t is, because a tone accompanied t h e l i g h t on three f o u r t h s of the trials, subjects may have momentarily delayed t h e i r response t o the l i g h t on t r i a l s i n which the tone was absent. Additional studies would be necessary t o decide between these alternative explanations. Reactions t o Onset and Offset of Lights and Tones Results reported thus f a r support t h e notion t h a t there exists i n humans a stereotypic tendency t o react toward t h e source of stimulation. However, w i t h one exception (Simon, 1969), the evidence concerning t h i s stereotype has been somewhat indirect. We have drawn inferences from situations where t h e processing of a relevant symbolic cue was affected by the presence of an irrelevant directional cue. The objective of o u r n e x t s t u d y (Simon, Craft, & Webster, 1971) was t o construct an information-processing task in which t h e stereotype could be observed more d i r e c t l y and from which differential predictions concerning i t s operation could be generated and tested. The task involved responding to the onset o r offset of one l i g h t on t h e simple two-light display-control panel described previously (Simon &


49

Craft, 1970a). If "stimulus" is defined as t h e element o f t h e d i s p l a y t h a t changes, a n d if t h e r e is, in fact, a stereotypic tendency t o react t o w a r d t h e stimulus, t h e n one would p r e d i c t t h a t when t h e stimulus i s a light onset, reactions t o w a r d t h e light t h a t goes on w i l l b e f a s t e r t h a n reactions t o w a r d t h e light t h a t remains o f f . Conversely, when t h e stimulus i s t h e o f f s e t o f one o f t h e t w o lights, one would p r e d i c t t h a t reactions t o w a r d t h e light t h a t goes o f f w i l l b e f a s t e r t h a n reactions t o w a r d t h e light t h a t remains on. Subjects were assigned a t random t o e i t h e r a stimulus-onset g r o u p o r a stimulus-offset g r o u p . Each o f these g r o u p s performed on t w o blocks o f t e s t t r i a l s . O n one block, t h e y pressed t h e stimulus-response b u t t o n t h a t changed and, o n t h e o t h e r block, t h e y pressed t h e stimulusresponse b u t t o n that did n o t change. Specifically, t h e onset g r o u p was told, " A t t h e b e g i n n i n g o f each trial, b o t h o f t h e b u t t o n s w i l l b e o f f . You w i l l hear a w a r n i n g tone and then, one second later, one o f t h e t w o b u t t o n s w i l l light up." O n one b l o c k (response t o changed element), t h e y were told, "Respond as f a s t as y o u can by p r e s s i n g t h e b u t t o n t h a t lights up." On t h e o t h e r block (response t o unchanged element), t h e y were told, "Respond as f a s t as y o u can by p r e s s i n g t h e b u t t o n t h a t doesn't light up." T h e o f f s e t g r o u p was g i v e n parallel i n s t r u c t i o n s . T h e y were told, " A t t h e b e g i n n i n g o f each t r i a l , b o t h o f t h e b u t t o n s w i l l b e l i g h t e d up. You will hear a w a r n i n g ton; a n d then, one second later, one o f t h e t w o l i g h t e d b u t t o n s w i l l g o o f f . On one b l o c k (response t o changed element) i n s t r u c t i o n s we';: t o "respond as f a s t as y o u can by p r e s s i n g t h e b u t t o n t h a t goes off, while on t h e o t h e r b l o c k (response t o unchanged element), i n s t r u c t i o n s were t o "respond as f a s t as y o u can by p r e s s i n g t h e b u t t o n t h a t remains lit." Results were j u s t as p r e d i c t e d . T h e onset g r o u p responded s i g nificantly faster t o t h e b u t t o n t h a t went on (changed element) than t o t h e b u t t o n t h a t remained o f f (unchanged element). T h e o f f s e t group responded s i g n i f i c a n t l y f a s t e r t o t h e b u t t o n t h a t went o f f (changed element) t h a n t o t h e b u t t o n t h a t remained on (unchanged element). Results, then, s u p p o r t e d t h e conclusion t h a t reactions a r e f a s t e r t o w a r d t h e source o f stimulation, t h a t is, t o w a r d t h e d i s p l a y element t h a t changes. A companion experiment (Simon, Craft, 5 Webster, 1971, E x p e r i ment II) investigated a u d i t o r y information processing f r o m an analogous t y p e o f display. Subjects wore earphones and pressed a right- o r l e f t hand k e y i n response t o t h e onset o r t h e o f f s e t o f a tone i n one ear. T h e experimental design was identical t o t h e p r e v i o u s study; t h a t is, subjects were assigned a t random t o e i t h e r a stimulus onset g r o u p o r a stimulus o f f s e t group, a n d each g r o u p performed on t w o blocks o f t r i a l s . On one block, subjects pressed t h e k e y on t h e side o f t h e ear in which t h e stimulus changed, and, on t h e o t h e r block, t h e y pressed t h e k e y o n t h e side o f t h e ear in which t h e stimulus did n o t change. T h e onset g r o u p was told, " A tone w i l l come on in one o f y o u r ears." On one block (response t o changed element), i n s t r u c t i o n s were t o "press t h e k e y o n t h e side where y o u hear t h e tone come o n . " On t h e o t h e r block (response t o unchanged element), i n s t r u c t i o n s were t o "press t h e k e y on t h e side where y o u d o n ' t hear t h e tone." T h e o f f s e t g r o u p was told, " T h e tone in one of y o u r ears w i l l g o o f f . " On one block (response t o changed element), t h e y were t o l d t o "press t h e k e y o n t h e side where you hear t h e tone g o o f f . " On t h e o t h e r block (response t o unchanged

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element), they,, were t o l d t o "press t h e k e y o n t h e side where y o u s t i l l hear t h e tone.

A c u r s o r y examination o f t h i s a u d i t o r y t a s k m i g h t suggest t h a t it is e x a c t l y comparable t o t h e v i s u a l t a s k in t h e companion experiment a n d that, therefore, t h e r e s u l t s o f t h e t w o experiments should b e comparable. In fact, we made d i f f e r e n t p r e d i c t i o n s f o r t h i s experiment. Specifically, we p r e d i c t e d that, f o r t h e tone-onset group, reactions would b e f a s t e r o n t h e side where t h e t o n e came o n (changed element) t h a n o n t h e side w h e r e t h e t o n e was absent (unchanged element). F o r t h e tone-offset group, however, we p r e d i c t e d t h a t reactions would b e f a s t e r on t h e side where t h e t o n e remained o n (unchanged element) t h a n on t h e side where t h e t o n e w e n t off (changed element). Results confirmed these p r e d i c t i o n s . F o r t h e onset group, react i o n s w e r e f a s t e r t o t h e changed element (320 v s . 386 ms), whereas, f o r t h e o f f s e t group, reactions were f a s t e r t o t h e unchanged element (488 v s . 578 ms). Why t h i s a p p a r e n t discrepancy between r e s u l t s o f t h i s p a i r o f experiments? Specifically, w h y , f o r t h e tone-offset group, were reactions f a s t e r o n t h e side o f t h e unchanged element? T h e f i n d i n g s a r e r e a d i l y i n t e r p r e t a b l e if one considers b o t h v i s u a l a n d a u d i t o r y displays in terms o f t h e subjects' perception r a t h e r t h a n in terms o f t h e p h y s i c a l stimulus per se. I n t h e case o f t h e v i s u a l display, t h e stimulus events a n d t h e subjects' p e r c e p t i o n o f them corresponded; t h a t is, a light e i t h e r went o n o r o f f , a n d t h i s i s w h a t subjects perceived. Similarly, on t h e tone-onset t r i a l s , subjects p e r c e i v e d t h e onset o f a monaural t o n e as t h e stimulus t o respond. O n t h e tone-offset t r i a l s , however, t h e r e was a discrepancy between t h e stimulus events a n d subjects' perception o f them. T h e s u b jects r e p o r t e d that, when a b i n a u r a l t o n e changed a b r u p t l y t o a monaural tone, t h e y did n o t p e r c e i v e t h e e v e n t as a tone o f f s e t in one ear. Rather, t h e y h e a r d t h e "onset" o f a tone in t h e o t h e r ear. When viewed in t h i s light, r e s u l t s o f t h e tone o f f s e t t r i a l s d o n o t r e p r e s e n t a d i s c r e p a n t finding. To t h e c o n t r a r y , t h e y b u t t r e s s o t h e r f i n d i n g s ( C r a f t G Simon, 1970; Simon, Small, Ziglar, & C r a f t , 1970) a n d s u p p o r t t h e notion o f a s t e r e o t y p i c t e n d e n c y t o r e a c t t o w a r d t h e apparent source o f stimulation. I t should b e p o i n t e d o u t e x p l i c i t l y t h a t t h e r e s u l t s o f these e x p e r i ments concerned w i t h onset a n d o f f s e t o f l i g h t s a n d tones c o u l d n o t have been p r e d i c t e d f r o m p r e v i o u s data concerned w i t h spatial S-R compatibility (e.g., Biederman 8 Kaplan, 1970; F i t t s C Seeger, 1953). It is well established and, in fact, it seems by now i n t u i t i v e l y obvious t h a t i n f o r mation processing w i l l b e slowed when subjects a r e t o l d t o r e v e r s e " n a t u r a l " S-R relations; f o r example, p r e s s t h e k e y on t h e opposite side o f t h e light" r a t h e r t h a n "press t h e k e y on t h e same side as t h e light. O u r i n s t r u c t i o n s , however, did not, on t h e surface a t least, r e q u i r e subjects t o r e v e r s e spatial S-R correspondence. Therefore, one m i g h t n o t p r e d i c t t h a t p r e s s i n g t h e b u t t o n t h a t l i g h t s - u p would b e f a s t e r t h a n p r e s s i n g t h e b u t t o n t h a t doesn't light-up o r t h a t p r e s s i n g t h e b u t t o n t h a t goes o f f would b e f a s t e r t h a n p r e s s i n g t h e b u t t o n t h a t remains lit-unless, o f course, one conceptualized t h e problem in terms o f a tendency t o react t o w a r d t h e stimulus source.


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Performance E f f e c t s in an O d d i t y T a s k

In another experiment (Simon & Craft, 19721, subjects were r e q u i r e d t o i d e n t i f y a n d respond t o t h e one element o f a three-element display t h a t was d i f f e r e n t f r o m t h e o t h e r t w o elements. Specifically, t h e d i s p l a y consisted of t h r e e stimulus-response b u t t o n s t h a t c o u l d b e l i g h t e d individually. T h e subjects performed o n t w o blocks o f t r i a l s , one b l o c k i n v o l v i n g l i g h t ( s ) onset a n d t h e o t h e r b l o c k i n v o l v i n g l i g h t ( s ) o f f s e t . F o r onset t r i a l s , t h e o r i g i n a l state o f t h e b u t t o n s was off, a n d t h e stimulus was t h e onset o f one o r t w o o f t h e b u t t o n s . Thus, t h e c o r r e c t response, t h a t is, p r e s s i n g t h e o d d b u t t o n , sometimes i n v o l v e d p r e s s i n g t h e l i g h t e d b u t t o n ( t h e element t h a t h a d j u s t changed) a n d sometimes t h e u n l i g h t e d b u t t o n ( t h e element t h a t was unchanged). Similarly, f o r o f f s e t t r i a l s , t h e o r i g i n a l s t a t e o f t h e b u t t o n s was on, a n d t h e stimulus was t h e o f f s e t o f one o r t w o o f t h e t h r e e b u t t o n s . Thus, t h e c o r r e c t response sometimes i n v o l v e d p r e s s i n g t h e u n l i g h t e d b u t t o n ( t h e changed element) a n d sometimes t h e l i g h t e d b u t t o n ( t h e unchanged element). If one conceives o f t h e stimulus as t h e changed element(s) of t h e display, a n d if one hypothesizes a s t e r e o t y p i c t e n d e n c y i n humans t o react t o w a r d t h e stimulus source (Simon, 1969; Simon, C r a f t , & Webster, 19711, t h e n t w o p r e d i c t i o n s follow. I n t h e onset b l o c k o f t h i s o d d i t y task, reactions t o a l i g h t e d b u t t o n should b e f a s t e r t h a n t o an u n l i g h t e d b u t t o n . In t h e o f f set block, t h e r e v e r s e finding should occur; t h a t is, reactions t o t h e u n l i g h t e d b u t t o n should b e f a s t e r t h a n t o t h e l i g h t e d b u t t o n . In o t h e r words, t h e c r u c i a l f a c t o r is n o t w h e t h e r t h e element t o w h i c h a subject responds i s o n o r off, but, r a t h e r , w h e t h e r t h e state o f t h a t element i s changed o r unchanged. Reactions should b e f a s t e r t o w a r d a changed t h a n t o w a r d an unchanged element because t h e r e is no necessity f o r o v e r r i d i n g an i n i t i a l i n c o r r e c t response t e n d e n c y b e f o r e e x e c u t i n g t h e c o r r e c t response. Results, again, were as predicted; t h a t is, subjects reacted s i g n i f i c a n t l y f a s t e r t o a d i s p l a y element when t h e state o f t h a t element h a d j u s t changed t h a n when it was unchanged (594 v s . 636 ms). T h i s effect, a f a s t e r reaction t o w a r d t h e source o f stimulation, o c c u r r e d o n t h e onset block (615 v s . 644 ms) as well as t h e o f f s e t b l o c k (573 vs. 628 ms). These f i n d i n g s i l l u s t r a t e t h a t i r r e l e v a n t cues i n a c h a n g i n g d i s p l a y can e l i c i t i n t e r f e r i n g response tendencies, w h i c h r e s u l t in slower information processing on a problem-solving t a s k .

A P e r s i s t e n t Phenomenon A l l o f t h e studies d e s c r i b e d t h u s f a r have i n v o l v e d observations g a t h e r e d during a single experimental session. While t h e e f f e c t o f t h e stereotype in t h a t single session has been extremely potent, one m i g h t ask w h e t h e r t h e phenomenon is s h o r t - l i v e d o r e n d u r i n g . Clearly, if t h e e f f e c t is a t r a n s i e n t one t h a t disappears q u i c k l y w i t h practice, it is n o t as important a behavioral phenomenon as if it p e r s i s t s i n d e f i n i t e l y . We decided, therefore, t o investigate t h e e f f e c t o f e x t e n d e d p r a c t i c e on t h e tendency t o react t o w a r d t h e stimulus source (Simon, Craft, & Webster, 1973). We also wanted t o conduct a formal analysis o f e r r o r s in a t a s k i n v o l v i n g c o n f l i c t i n g directional a n d symbolic cues. Such an analysis h a d been p r e c l u d e d i n p r e v i o u s studies by a combination o f a low e r r o r r a t e a n d a limited number o f observations f o r each subject. We f e l t t h a t an analysis o f t h e r e l a t i v e latency a n d f r e q u e n c y o f t h e d i f f e r e n t k i n d s o f e r r o r s would p r o v i d e valuable i n s i g h t s i n t o t h e mechanism u n d e r l y i n g t h e

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effect previously observed on correct response t r i a l s (i.e., slower i n f o r mation processing when the source and content of t h e stimulus did not correspond.) We had suggested (Simon, Small, Ziglar, & Craft, 1970) t h a t t h i s slower information processing on noncorresponding trials was due t o an initial tendency t o react t o t h e source of t h e stimulus rather than t o i t s meaning, and t h e necessity f o r o v e r r i d i n g t h i s initial response tendency produced t h e observed delay i n information processing. It would follow, then, t h a t e r r o r s on noncorresponding trials represent failures t o i n h i b i t t h e initial incorrect response tendency. It was, therefore, hypothesized that t h e latency of e r r o r responses on noncorresponding t r i a l s should be less than t h e latency of correct responses on these t r i a l s . Subjects performed 24 practice trials and 192 t e s t trials on each of 5 consecutive days. On each trial, subjects pressed a l e f t o r r i g h t key in response t o a high- o r low-pitched monaural tone. Figure 10 shows that, on each of t h e 5 days, RT was significantly faster when t h e r i g h t command was heard i n t h e r i g h t ear than when it was heard i n t h e l e f t ear, and R T t o t h e l e f t command was faster when it was heard i n t h e left ear than when it was heard i n t h e r i g h t ear. Thus, although overall performance improved w i t h practice, t h e Simon effect showed no sign of disappearing.

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Analysis of t h e e r r o r data b u t t r e s s e d o u r i n t e r p r e t a t i o n o f t h e Simon e f f e c t as r e p r e s e n t i n g an i n i t i a l t e n d e n c y t o react t o t h e source o f a stimulus r a t h e r t h a n t o i t s content. E r r o r s on noncorresponding t r i a l s were s i g n i f i c a n t l y more f r e q u e n t t h a n e r r o r s on c o r r e s p o n d i n g t r i a l s , which i s w h a t one m i g h t expect if e r r o r s r e p r e s e n t f a i l u r e s t o inhibit an i n i t i a l i n c o r r e c t response tendency. C o r r e c t responses on noncorresponding t r i a l s were also s i g n i f i c a n t l y slower t h a n e r r o r responses (368 v s . 321 ms), w h i c h reflected t h e additional time r e q u i r e d t o o v e r r i d e an i n i t i a l i n c o r r e c t response tendency. I r r e l e v a n t C u e Presented P r i o r to Relevant Stimulus In all o f t h e research d e s c r i b e d t h u s far, t h e directional c u e o c c u r r e d simultaneously w i t h t h e r e l e v a n t stimulus. T h a t is, t h e d i r e c tional cue was e i t h e r an i r r e l e v a n t dimension o f t h e stimulus itself, o r it was p r o v i d e d by a simultaneous but i r r e l e v a n t stimulus t o another sensory modality. T h e p u r p o s e o f o u r n e x t s t u d y (Simon, Acosta, & Mewaldt, 1975) was t o i n v e s t i g a t e t h e e f f e c t o f an i r r e l e v a n t directional c u e presented p r i o r t o t h e onset o f t h e r e l e v a n t stimulus.

Subjects pressed a l e f t - o r r i g h t - h a n d k e y depending on t h e ear in Half were i n s t r u c t e d t o p r e s s t h e k e y w h i c h t h e y h e a r d a stimulus tone. on t h e same side as t h e ear stimulated ( c o r r e s p o n d i n g S-R mapping), whereas t h e o t h e r half p r e s s e d t h e k e y o n t h e opposite side (noncorresponding S-R mapping). P r i o r t o t h e stimulus, subjects h e a r d a w a r n i n g tone in e i t h e r t h e l e f t ear, t h e r i g h t ear, o r b o t h ears. We p r e d i c t e d t h a t reactions would b e f a s t e r when w a r n i n g t o n e a n d response were on t h e same side o f t h e b o d y t h a n when t h e y were o n opposite sides. Assuming t h a t t h e locus o f t h e w a r n i n g tone h a d t h e p r e d i c t e d effect, we were also i n t e r e s t e d in d e t e r m i n i n g t h e persistence o f t h e effect. Presumably, t h e potency o f an i r r e l e v a n t c u e dissipates o v e r time. Hence, t h e i n t e r v a l between w a r n i n g a n d stimulus tones was manipulated w i t h t h e expectation that, as w a r n i n g i n t e r v a l increased, t h e r e m i g h t b e a r e d u c t i o n in t h e e f f e c t o f w a r n i n g t o n e locus o n R T t o t h e e n s u i n g stimulus. F i g u r e 11 p i c t u r e s t h e r e s u l t s . Note t h a t t h e p r e d i c t e d i n t e r a c t i o n o c c u r r e d in t h e noncorresponding S-R mapping t a s k but n o t in t h e c o r r e sponding t a s k . Note, too, t h a t t h e interaction was as p o t e n t a t 400 ms following t h e w a r n i n g tone as it was a t 200 ms.

Why did t h e expected i n t e r a c t i o n o c c u r in t h e noncorresponding t a s k but n o t in t h e c o r r e s p o n d i n g task? We believe t h a t t h e explanation lies in t h e r e l a t i v e s t r e n g t h o f competing response tendencies. In t h e c o r r e s p o n d i n g S-R mapping task, t h e i n s t r u c t i o n s corresponded with a s t r o n g n a t u r a l t e n d e n c y t o respond t o a stimulus in one ear by making an ipsilateral response. T h e r e was l i t t l e o r no t e n d e n c y t o make t h e comp e t i n g contralateral response. In t h i s situation, then, a n y additional response t e n d e n c y elicited by t h e w a r n i n g t o n e was n o t p o t e n t enough t o a f f e c t t h e balance between already unequal response tendencies associated w i t h t h e stimulus. In t h e noncorresponding task, however, t h e i n s t r u c t i o n s t o p r e s s t h e k e y on t h e opposite side o f t h e stimulated e a r conf l i c t e d w i t h t h e s t r o n g n a t u r a l tendency t o make an ipsilateral response. T h e slower R T o n t h e noncorresponding t a s k reflects t h e competition between t h e t e n d e n c y t o make t h e response as i n s t r u c t e d a n d t h e tendency t o make t h e response t h a t was n a t u r a l . T h e p o i n t is, however,

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Locus of Warning Tone F i g u r e 11. E f f e c t o f locus o f a w a r n i n g tone a n d w a r n i n g i n t e r v a l on R T f o r right a n d l e f t responses in noncorresponding a n d c o r r e s p o n d i n g S-R mapping tasks. Note t h a t t h e speed o f information processing on t h e noncorresponding t a s k is affected by t h e r e l a t i v e location o f t h e w a r n i n g t o n e in relation t o t h a t o f t h e response. t h a t in t h i s situation i n v o l v i n g r e l a t i v e l y equal but competing responses t o t h e stimulus, t h e added response t e n d e n c y elicited by t h e w a r n i n g tone was p o t e n t enough t o have a s i g n i f i c a n t e f f e c t . I n summary, then, these f i n d i n g s e x t e n d e d o u r u n d e r s t a n d i n g o f t h e Simon e f f e c t by specifying t h e r a n g e o f situations in w h i c h it operated. Whether o r n o t i n t e r f e r e n c e o c c u r r e d depended o n t h e n a t u r e o f t h e response t h a t was r e q u i r e d t o t h e r e l e v a n t stimulus. Furthermore, since t h e d i f f e r e n c e between t h e c o r r e s p o n d i n g a n d noncorresponding t a s k s was p u r e l y a difference in response assignment, t h e r e s u l t s suggested t h a t response selection was t h e stage affected by t h e i r r e l e v a n t directional cue. Stages of Processing T h e Simon, Acosta, a n d Mewaldt (1975) data did n o t establish conc l u s i v e l y t h a t t h e i r r e l e v a n t directional c u e affects o n l y t h e responseIn fact, some o f o u r e a r l i e r f i n d i n g s m i g h t b e i n t e r selection stage. p r e t e d t o suggest t h a t t h e i r r e l e v a n t cue affects both t h e stimulusencoding stage a n d t h e response-selection stage. Specifically, Simon a n d Small (1969) a n d C r a f t a n d Simon (1970) f o u n d t h a t b o t h c o r r e s p o n d i n g a n d noncorresponding t r i a l s were s i g n i f i c a n t l y slower t h a n baseline t r i a l s


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presented i n a b l o c k in w h i c h t h e directional c u e was absent. This f i n d i n g m i g h t suggest t h a t t h e i r r e l e v a n t directional c u e n o t o n l y t e n d e d t o e l i c i t a response t o w a r d t h e source o f stimulation but t h a t it also increased t h e time r e q u i r e d t o encode t h e r e l e v a n t symbolic information o n t h e c o r r e s p o n d i n g a n d noncorresponding t r i a l s . O u r n e x t experiment, then, employed t h e S t e r n b e r g (1969) a d d i t i v e f a c t o r s method t o t e s t t h e hypothesis t h a t t h e i r r e l e v a n t directional c u e affected t h e stimulus encoding stage. T h e method involves c o n d u c t i n g f a c t o r i a l experiments t o determine w h i c h variables p r o d u c e s i g n i f i c a n t interactions a n d w h i c h d o not. T h e variables t h a t i n t e r a c t a r e assumed t o a f f e c t a stage in common, whereas t h e variables t h a t d o n o t i n t e r a c t a r e assumed t o a f f e c t d i f f e r e n t stages. Furthermore, f r o m t h e n a t u r e o f t h e variables t h a t interact, one can d e f i n e t h e processing t h a t i s accomplished by a c e r t a i n stage. T h o u g h t h e a d d i t i v e f a c t o r s method involves some t h o r n y methodological problems (Pachella, 1974; Taylor, 19761, it has p r o v i d e d an extremely u s e f u l tool in information-processing research. O u r s t u d y (Acosta & Simon, 1976) i n v o l v e d h a v i n g some subjects p r e s s a l e f t - o r r i g h t - h a n d k e y i n response t o t h e c h a r a c t e r X o r 0. O t h e r subjects performed a unimanual t a s k t h a t i n v o l v e d m o v i n g a t o g g l e switch t o w a r d o r away f r o m t h e i r b o d y in response t o t h e same stimuli used in t h e bimanual t a s k . O n half o f t h e t r i a l s , a t r a n s l u c e n t Plexiglas p l a t e was placed in f r o n t o f t h e v i s u a l d i s p l a y t o r e d u c e t h e d i s c r i m i n a b i l i t y o r q u a l i t y o f t h e alphanumeric characters. Previous research indicates t h a t stimulus d i s c r i m i n a b i l i t y affects t h e stimulusencoding stage a n d does n o t a f f e c t a n y o t h e r processing stage (Biederman & Kaplan, 1970; Sternberg, 1967, 1969). Simultaneous w i t h t h e onset o f t h e alphanumeric character, subjects h e a r d a t o n e in e i t h e r t h e l e f t ear, t h e right ear, o r b o t h ears. If t h i s i r r e l e v a n t a u d i t o r y stimulus (i.e., t h e directional cue) affected t h e stimulus-encoding stage, i t s effects should i n t e r a c t w i t h discriminability, whereas, if t h e i r r e l e v a n t directional c u e affected another separate stage, t h e r e should b e no i n t e r a c t i o n w i t h discriminability. Results indicated t h a t when response- k e y locations were isomorphic t o t h e source o f t h e i r r e l e v a n t tone ( i . e . , bimanual task), t h e directional c u e h a d a p o t e n t e f f e c t on performance. However, when response-select i o n operations were altered by employing responses t h a t were n e u t r a l w i t h respect t o t h e source o f t h e i r r e l e v a n t tone (i.e., unimanual task), t h e directional c u e h a d no e f f e c t . In addition, d e g r a d i n g t h e q u a l i t y o f t h e v i s u a l stimulus r e s u l t e d in slower R T but did n o t a l t e r t h e e f f e c t o f t h e i r r e l e v a n t directional cue. A c c o r d i n g t o a d d i t i v e f a c t o r s logic, t h i s p a t t e r n o f f i n d i n g s suggested t h a t t h e i r r e l e v a n t directional c u e did n o t a f f e c t t h e stimulus-encoding stage but, instead, affected t h e responseselection stage. I n t e r a c t i o n Between Stereotypes T h u s f a r , most o f o u r investigations of t h e stereotype have i n v o l v e d situations in which t h e a l t e r n a t i v e commands, stimulus locations, a n d response locations were e i t h e r l e f t o r right. In retrospect, t h i s manipulation along t h e horizontal dimension may have been unfortunate, f o r it has necessitated elaborate experimental p r o c e d u r e s t o establish t h a t t h e Simon e f f e c t was n o t related t o factors such as hemispheric dominance, r e s p o n d i n g b o d y member, o r sense o r g a n stimulated. T h e major

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purpose o f o u r n e x t s t u d y (Simon, Mewaldt, Acosta, & Hu, 1976) was t o determine whether t h e stereotypic reaction t o w a r d t h e stimulus source o c c u r r e d in t h e v e r t i c a l dimension, ,y well. Evidence that t h e stereotype o c c u r r e d in a situation i n v o l v i n g u p " a n d "down" commands, stimulus locations, a n d responses would n o t o n l y establish t h e g e n e r a l i t y o f t h e phenomenon, but would p r o v i d e convincing evidence t h a t l a t e r a l i t y was n o t a f a c t o r in i t s production. A second p u r p o s e was t o investigate t h e e f f e c t of t h e frequency-space stereotype on a u d i t o r y R T . Mudd (1963) investigated spatial stereotypes associated w i t h various dimensions o f sound by h a v i n g subjects move a p e g in a panel f r o m one position t o another, in a manner suggested by t h e relation between t w o tones. He f o u n d t h a t subjects h a d a v e r y s t r o n g tendency t o associate h i g h - p i t c h e d tones w i t h an u p response a n d low-pitched tones w,ith a down response. We hypothesized t h a t if p i t c h , in fact, provides a n a t u r a l " indication o f t h e c o r r e c t response, t h e n subjects should react f a s t e r when t h e toneresponse r u l e (i.e., stimulus-response, o r S - R mapping) corresponded w i t h t h e frequency-space stereotype (i.e., high tone = u p response and low tone = down response) t h a n when it did n o t (high tone = down response a n d low tone = u p response). In o t h e r words, t h e c o r responding S - R mapping should b e "compatible" ( F i t t s & Seeger, 19531, in t h a t t h e ensemble o f stimulus a n d response combinations comprising t h e t a s k should r e s u l t in a h i g h e r r a t e o f information t r a n s f e r t h a n w i t h t h e noncorresponding mapping. A third p u r p o s e o f t h e s t u d y was t o i n v e s t i gate t h e possible interaction between t h e t w o stereotypes; t h a t is, does t h e s t r e n g t h o f t h e tendency t o react t o w a r d t h e stimulus source depend o n w h e t h e r t h e tone-response r u l e corresponds w i t h t h e "natural" f r e quency-space stereotype? Subjects were seated in a soundproof booth a t a table o n which a display-control panel was placed. T h e panel contained t w o small speakers mounted one above t h e other, 45 cm a p a r t . Midway between t h e speakers was a three-position toggle switch, which flormally rested in t h e center position, b u t could b e toggled up o r down. Subjects r e s t e d t h e i r elbow on t h e t a b l e a n d grasped t h e switch between t h e t h u m b a n d f o r e f i n g e r o f t h e i r p r e f e r r e d hand. T h e y were i n s t r u c t e d that, on each trial, t h e y would hear e i t h e r a h i g h - p i t c h e d tone (500 Hz) o r a low-pitched tone (200 Hz) emanating f r o m e i t h e r t h e t o p o r bottom speaker a n d t h a t t h e i r t a s k was t o make t h e c o r r e c t response as q u i c k l y a n d accurately as possible. Subjects performed on t w o blocks o f t r i a l s . On one block (i.e., c o r r e sponding block), t h e y were i n s t r u c t e d t o move t h e switch u p in response t o t h e h i g h - p i t c h e d tone a n d down in response t o t h e low-pitched tone. On t h e o t h e r b l o c k (i.e., noncorresponding block), t h e y moved t h e switch u p in response t o t h e low-pitched tone a n d down in response t o t h e h i g h - p i t c h e d tone. F i g u r e 12 p i c t u r e s t h e r e s u l t s . Note, f i r s t , that R T was faster in t h e corresponding S-R mapping t a s k ( l e f t panel) t h a n in t h e noncorresponding S-R mapping t a s k (right panel). Note, too, t h e significant Command x Command Location interaction in t h e corresponding t a s k ( l e f t panel). T h a t is, u p responses were f a s t e r when t h e h i g h - p i t c h e d tone emanated from t h e t o p speaker t h a n when it emanated f r o m t h e bottom speaker, a n d down responses were f a s t e r when t h e low tone came f r o m t h e bottom speaker t h a n when it came f r o m t h e t o p speaker. However, in t h e noncorresponding S - R mapping t a s k ( r i g h t panel), t h i s Command x Command Location interaction was n o t significant. T h e o n l y significant


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F i g u r e 12. Effect o f location o f tonal command on reaction time f o r up and down responses in corresponding and noncorresponding S-R mapping tasks. effect i n t h e noncorresponding t a s k was a main e f f e c t o f command, indicating t h a t u p responses were f a s t e r t h a n down responses (412 ms v s . 454 ms). T h e finding t h a t R T was f a s t e r when t h e S-R mapping c o r r e sponded w i t h t h e frequency-space stereotype is n o t p a r t i c u l a r l y s u r p r i s ing in view of Mudd's (1963) finding t h a t tone f r e q u e n c y has a s t r o n g n a t u r a l association w i t h t h e v e r t i c a l dimension o f space; t h a t is, high tones suggest a n u p response a n d low tones suggest a down response. In previous studies, we had employed h i g h - and low-pitched tones t o signal l e f t o r right responses, w i t h half o f t h e subjects i n s t r u c t e d t h a t a h i g h tone means right a n d a low tone means left, while t h e o t h e r h a l f were g i v e n t h e reverse S-R mapping (e.g., Simon, C r a f t , & Small, 1970; Simon, Craft, & Webster, 1973, Simon & Small, 1969). We h a d n e v e r detected a n y d i f f e r e n c e in R T between t h e t w o S-R mapping conditions. In t h e p r e s e n t study, however, where responses of u p o r down r a t h e r than right o r l e f t were involved, one S-R mapping condition was c l e a r l y "compatible" in t h e sense t h a t it resulted i n a h i g h e r r a t e of information

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t r a n s f e r ( F i t t s & Seeger, 1953). T h i s r e s u l t may have p r a c t i c a l applicat i o n s in t h e design o f a u d i t o r y displays f o r man-machine systems where, f o r example, a p p r o p r i a t e l y chosen tonal commands m i g h t facilitate t h e u p o r down movements o f a c o n t r o l o r a vehicle. T h e second finding, t h a t s t e r e o t y p i c reactions t o w a r d t h e stimulus source o c c u r r e d in t h e v e r t i c a l plane, i s of i n t e r e s t because it establishes t h e g e n e r a l i t y of a phenomenon t h a t has been demonstrated p r e v i o u s l y along t h e horizontal dimension. More importantly, it establishes c o n v i n c i n g l y t h a t t h e Simon e f f e c t is n o t related t o f a c t o r s such as l a t e r a l i t y o r hemispheric dominance. T h e interaction between t h e stereotypes is, theoretically a t least, t h e most i n t e r e s t i n g and, a t t h e same time, t h e most p u z z l i n g o f o u r results. Subjects reacted f a s t e r t o t h e up command when it emanated f r o m t h e t o p speaker t h a n when it emanated f r o m t h e bottom speaker, a n d t h e y reacted f a s t e r t o t h e down command when it came f r o m t h e bottom speaker t h a n when it came f r o m t h e t o p s p e a k e r - - b u t o n l y o n t h e b l o c k o f t r i a l s where t h e S-R mapping corresponded w i t h t h e frequency-space stereotype. T h i s reaction t o w a r d t h e source o f stimulation did n o t o c c u r when subjects h a d t o process commands t h a t c o n f l i c t e d w i t h t h e frequency-space stereotype (i.e., low-pitched t o n e = u p a n d h i g h - p i t c h e d t o n e = down). It appears t h a t processing t h e symbolic c o n t e n t o f a tonal command t h a t c o n f l i c t s w i t h one p o w e r f u l stereotype may have so t h o r o u g h l y engaged t h e information-processing mechanisms as t o eliminate t h e o t h e r stereotype. T h e reader w i l l recall t h a t t h i s s o r t o f complex i n t e r action between n a t u r a l response tendencies had been o b s e r v e d i n an e a r l i e r s t u d y (Simon, Acosta, E Mewaldt, 1975). I n t h a t study, subjects pressed a l e f t - o r r i g h t - h a n d k e y depending on t h e ear in w h i c h t h e y h e a r d a tone. Some subjects pressed t h e k e y on t h e same side as t h e e a r stimulated ( c o r r e s p o n d i n g condition), while o t h e r s pressed t h e k e y on t h e opposite side (noncorresponding c o n d i t i o n ) . T h e locus o f a w a r n i n g tone, w h i c h preceded t h e stimulus tone, affected R T o n noncorresponding t r i a l s , but n o t o n c o r r e s p o n d i n g t r i a l s . Those e a r l i e r f i n d i n g s , then, a r e similar to, b u t in a sense, opposite f r o m those obtained in t h e p r e s e n t s t u d y w h e r e t h e i r r e l e v a n t directional cue affected o n l y t h e c o r r e s p o n d i n g trials. How does one rationalize t h i s a p p a r e n t discrepancy? F i r s t , i n t h e e a r l i e r s t u d y , t h e directional c u e was relevant, whereas i n t h e p r e s e n t s t u d y , t h e symbolic c u e was r e l e v a n t . We know t h a t t h e r e a r e i m p o r t a n t differences in information processing, depending o n w h e t h e r t h e cue is directional o r symbolic (Simon & C r a f t , 1971). Second, in t h e e a r l i e r study, t h e i r r e l e v a n t directional cue was i n t h e w a r n i n g tone, whereas i n t h e p r e s e n t s t u d y , t h e directional c u e was an i r r e l e v a n t dimension o f t h e r e l e v a n t symbolic stimulus. We know t h a t t h e basic mechanisms u n d e r l y ing a u d i t o r y S-R compatibility a r e d i f f e r e n t when stimulus location is t h e r e l e v a n t c u e t h a n when location i s an i r r e l e v a n t dimension o f t h e r e l e v a n t symbolic cue (Simon, H i n r i c h s , & C r a f t , 1970). In retrospect, then, it seems reasonable t o a r r a n g e t h e treatment conditions o f t h e p r e s e n t experiment a n d o u r e a r l i e r s t u d y (Simon, Acosta, & Mewaldt, 1975) along a single compatibility dimension. At the high e n d o f t h i s continuum would b e t h e c o r r e s p o n d i n g condition o f t h e e a r l i e r s t u d y , w h i c h r e q u i r e d r e s p o n d i n g t o a directional cue by p r e s s i n g a k e y o n t h e same side as t h e e a r stimulated. Subjects could p e r f o r m t h i s t a s k in a r e f l e x - l i k e way, almost w i t h o u t t h i n k i n g , a n d it is n o t s u r p r i s i n g t h a t a directional c u e f r o m a w a r n i n g tone presented p r i o r t o t h e


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stimulus did n o t have a d i s c e r n i b l e e f f e c t on R T . A t t h e low e n d o f t h i s same compatibility continuum w o u l d b e t h e noncorresponding condition of t h e p r e s e n t s t u d y , in w h i c h subjects h a d t o process a symbolic c u e by employing a tone-response r u l e t h a t r a n c o u n t e r t o t h e p o t e n t f r e q u e n c y space stereotype. T h e r e q u i r e d symbolic t r a n s l a t i o n may have been so complex as t o overload t h e information-processing apparatus, t h u s p r e v e n t i n g t h e directional c u e f r o m h a v i n g a n y impact. Between these extremes would b e t h e noncorresponding condition o f t h e e a r l i e r s t u d y a n d t h e c o r r e s p o n d i n g condition o f t h e p r e s e n t study; in both, t h e stereotypic reaction t o w a r d t h e stimulus source o c c u r r e d . O u r results, then, p r o v i d e general s u p p o r t f o r t h e Sheridan a n d F e r r e l l (1974) s u g gestion t h a t t h e conception o f t h e o p e r a t o r as a limited channel o f i n f o r mation transmission is least accurate u n d e r conditions where each stimulus has, in effect, i t s own channel (high compatibility) a n d w h e r e t h e S - R mapping is a r b i t r a r y o r r u n s c o u n t e r t o a common stereotype. It is clear, however, t h a t much remains t o b e learned about information-processing mechanisms in situations t h a t i n v o l v e i n t e r a c t i n g s t e r e o t y p i c response tendencies. Time C o u r s e o f t h e Phenomenon O u r n e x t s t u d y (Simon, Acosta, Mewaldt, & Speidel, 1976) was conc e r n e d w i t h c h a r t i n g t h e time course o f t h e s t e r e o t y p i c t e n d e n c y t o respond t o w a r d t h e stimulus source and, also, t o localize i t s e f f e c t in relation t o t h e stages o f information processing. T w o d i f f e r e n t paradigms were used. I n one paradigm, subjects responded t o a r e d o r g r e e n light presented on t h e right o r l e f t side by p r e s s i n g a right o r l e f t k e y . T h e location o f t h e light p r o v i d e d t h e i r r e l e v a n t cue. Subjects were i n s t r u c t e d t o delay t h e i r r e s p o n d i n g t o t h e l i g h t until t h e y h e a r d a g o signal presented 0, 150, 250, o r 350 ms a f t e r t h e light. F i g u r e 13 shows t h a t t h e Command x Command Location interaction o c c u r r e d up t o a delay i n t e r v a l of about 250 ms. When t h e delay i n t e r v a l was increased t o 350 ms, t h e source o f t h e stimulus no longer h a d a n y detectable e f f e c t on R T . These r e s u l t s suggested t h a t t h e e f f e c t of t h e i r r e l e v a n t directional cue was localized in t h e l a t e r stages o f processing, perhaps in t h e response-selection stage. Since t h e t y p i c a l magnitude o f t h e Command x Command Location interaction is o n t h e o r d e r o f 60 ms, it is d i f f i c u l t t o conceive how t h e e f f e c t could p e r s i s t undiminished f o r 250 ms and, yet, have i t s onset e a r l y in t h e decision process. In another paradigm, subjects pressed e i t h e r a l e f t - o r r i g h t - h a n d k e y labeled " r e d " o r "green" in response t o t h e pitch o f a stimulus tone; t h a t is, low tone signaled r e d k e y a n d h i g h tone signaled g r e e n k e y . T h e location o f t h e tone ( l e f t o r right ear) p r o v i d e d t h e i r r e l e v a n t d i r e c tional cue. A l i g h t - e m i t t i n g diode (LED) capable o f p r o d u c i n g e i t h e r a r e d o r a g r e e n light was located b e h i n d each k e y . B y employing t h e LEDs, it was possible t o v a r y t h e labeling o f t h e keys f r o m t r i a l t o t r i a l , t o delay labeling t h e keys until a f t e r t h e stimulus tone was presented, a n d t o manipulate t h e i n t e r v a l between stimulus presentation a n d labeling o f t h e k e y s . F o r some groups, t h e r e d a n d g r e e n k e y s were designated (i.e., labeled) p r i o r t o t h e onset of t h e tone; t h a t is, t h e right k e y m i g h t b e r e d a n d t h e l e f t k e y green. Thus, upon onset o f t h e tone, subjects h a d all t h e information t h e y needed t o i n i t i a t e t h e i r response. For o t h e r groups, t h e keys were n o t labeled until some time a f t e r t h e tone had been presented, so subjects h a d t o delay t h e i r response until t h e y c o u l d determine w h i c h k e y was r e d a n d w h i c h k e y was g r e e n on t h a t

J.R. Simon

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Location of Command (Eye Stimulated) Figure 13. E f f e c t o f an i r r e l e v a n t directional c u e i n a choice reaction time t a s k i n v o l v i n g a g o signal. T h e basic t a s k i n v o l v e d p r e s s i n g e i t h e r a l e f t - o r a r i g h t - h a n d key, depending on t h e color o f a r e d o r a g r e e n command light. T h e light appeared in e i t h e r t h e l e f t o r t h e right half of t h e v i s u a l field, t h u s p r o v i d i n g an i r r e l e v a n t directional cue. Subjects w e r e i n s t r u c t e d t o delay e x e c u t i n g t h e i r response until t h e y h e a r d an a u d i t o r y g o signal, w h i c h was presented e i t h e r simultaneously w i t h t h e command light (Panel 1) o r following t h e light by 150, 250, o r 350 ms (Panels 2 t h r o u g h 4). Reaction time was measured f r o m t h e onset of t h e g o signal. particular trial. In t h i s way, subjects were p r e v e n t e d f r o m b e g i n n i n g t h e response-selection stage until t h e i n s t a n t specified by t h e e x p e r i menter. O u r idea was t h a t a n y i n i t i a l tendency t o react t o t h e source o f t h e tone w o u l d dissipate d u r i n g t h e enforced delay b e f o r e responding. We f e l t that, by i d e n t i f y i n g t h e c r i t i c a l i n t e r v a l a t w h i c h t h e stereotype no l o n g e r occurred, we c o u l d estimate t h e d u r a t i o n o f t h e e f f e c t a n d also localize it in t h e information-processing sequence. F i g u r e 14 shows t h a t t h e source o f t h e tonal command affected information processing only when t h e response keys were labeled p r i o r to, o r simultaneously with, t h e onset o f t h e tone. When t h e response k e y s were labeled a f t e r onset o f t h e tone, t h e i r r e l e v a n t directional cue


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h a d n o s i g n i f i c a n t effect. T h i s finding has i m p o r t a n t implications because it i s f u r t h e r evidence t h a t t h e i r r e l e v a n t c u e affects t h e response-select i o n stage; t h a t is, t h e directional c u e h a d an e f f e c t o n l y when subjects could proceed d i r e c t l y t o t h e response-selection stage. When t h e labeling o f t h e response k e y s was delayed, subjects w e r e p r e v e n t e d f r o m p r o ceeding t o t h e response-selection stage, a n d t h e directional c u e in t h e stimulus t o n e h a d n o e f f e c t .

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Location of Command (Ear Stimulated) Figure 14. E f f e c t of an i r r e l e v a n t d i r e c t i o n a l cue on t h e time r e q u i r e d to t r a n s l a t e p i t c h information i n t o a k e y p r e s s response. T h e basic t a s k i n v o l v e d p r e s s i n g e i t h e r a l e f t - o r a r i g h t - h a n d key, labeled "red" o r "green," in response t o t h e pitch o f a monaural tonal command. Each o f s i x g r o u p s (see Panels 1-6) p e r f o r m e d a d i f f e r e n t v a r i a t i o n o f t h i s basic task. F o r G r o u p 1, t h e r e d k e y a n d t h e g r e e n k e y were labeled b e f o r e t h e s t a r t o f t h e experiment, a n d these designations remained f i x e d throughout. F o r G r o u p 2, t h e keys were labeled 1 s p r i o r t o t h e onset o f each tonal command, a n d these labels v a r i e d f r o m t r i a l t o t r i a l ; i.e., on one t r i a l , t h e right k e y m i g h t b e r e d a n d t h e l e f t k e y green, whereas o n t h e n e x t t r i a l t h e r e v e r s e labeling m i g h t occur. Reaction times f o r Groups 1 a n d 2 were measured f r o m t h e onset o f t h e tonal command. For Groups 3, 4, 5, a n d 6, t h e k e y labels also v a r i e d f r o m t r i a l t o t r i a l . T h e labeling, however, o c c u r r e d e i t h e r simultaneously w i t h presentation o f t h e tonal command o r 150, 250, o r 350 ms a f t e r t h e onset o f t h e command. Reaction times f o r Groups 3 - 6 were measured f r o m t h e i n s t a n t t h e keys were labeled.

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J.R. Simon A B u f f e r Model of Response Selection

We have developed a model t h a t employs a scanning notion as a means o f d e s c r i b i n g t h e response-selection process. T h e model proposes t h a t a response " b u f f e r " is established f o r each acceptable response in a choice R T t a s k . - T h e r e f o r e , in a t a s k t h a t r e q u i r e s p r e s s i n g e i t h e r a l e f t - o r r i g h t - h a n d key, t w o b u f f e r s a r e created, one f o r t h e l e f t - a n d one f o r t h e r i g h t - h a n d response. Each b u f f e r contains a representational l i s t i n g o f t h e stimuli associated w i t h t h a t p a r t i c u l a r response. F o r example, if t h e stimulus "X" indicates a l e f t - k e y response a n d t h e stimulus 0" indicates a r i g h t - k e y response, a representation o f "X" is contained w i t h i n t h e l e f t b u f f e r a n d a representation of "0" i s contained w i t h i n t h e right b u f f e r . We assume that, once a presented stimulus has b-een identified, each o f t h e response b u f f e r s is searched i n a serial, self-termin a t i n g scan until t h e matching representation is located a n d t h e associated response i s selected. We f u r t h e r assume t h a t an i r r e l e v a n t d i r e c tional c u e associated w i t h t h e stimulus affects R T by biasing t h e subject t o search f i r s t t h e b u f f e r t h a t corresponds t o t h e i r r e l e v a n t cue. Thus, when t h e r e l e v a n t a n d i r r e l e v a n t cue correspond, R T w i l l b e r e l a t i v e l y f a s t because t h e c o r r e c t b u f f e r w i l l b e searched f i r s t . However, when t h e r e l e v a n t a n d i r r e l e v a n t c u e d o n o t correspond, R T w i l l b e r e l a t i v e l y slow because t h e i n c o r r e c t b u f f e r w i l l b e searched f i r s t . T h e above model accounts f o r t h e r e s u l t s o f t h e p r e v i o u s t w o experiments. F o r example, i n t h e f i r s t experiment, u n d e r t h e 150-ms delay, t h e g o signal o c c u r r e d w h i l e t h e subjects were s t i l l i d e n t i f y i n g t h e stimulus, a n d so response selection proceeded in t h e same fashion as in t h e no-delay condition; t h a t is, RTs reflected a p o t e n t Simon e f f e c t . However, when t h e g o signal was delayed f o r more t h a n 250 ms, t h e subject h a d time t o scan both response b u f f e r s a n d select t h e a p p r o p r i a t e response before t h e signal to execute was presented. Thus, under t h e long-delay conditions, R T no longer reflected t h e beneficial o r detrimental effects p r o d u c e d by scanning a p a r t i c u l a r response b u f f e r f i r s t . In t h e second experiment, t h e t y p i c a l Simon e f f e c t o c c u r r e d when subjects were able t o p r o g r a m t h e stimulus-response associations p r i o r t o stimulus presentation, t h a t is, when t h e response k e y s were labeled p r i o r to the trial. T h e s t e r e o t y p e also o c c u r r e d when t h e response keys were labeled simultaneously w i t h t h e tone. U n d e r t h i s l a t t e r condition, subjects a p p a r e n t l y f i r s t i d e n t i f i e d t h e k e y labels so t h e y could load t h e response b u f f e r s . T h i s additional step slowed R T but, once completed, subjects c o u l d proceed w i t h o u t delay t h r o u g h t h e stimulus-identification a n d response-selection stages. When t h e labeling of response k e y s was delayed, t h e sequence of operations was altered. Now, presumably, subjects f i r s t processed t h e tone, d u e t o i t s temporal p r i o r i t y , a n d t h e n proceeded t o i d e n t i f y t h e k e y lab:ls a n d load t h e b u f f e r s . Apparently t h e engagement of t h e subjects' c e n t r a l processing u n i t " in i d e n t i f y i n g t h e k e y labels a n d loading t h e b u f f e r s eliminated t h e i n i t i a l scanning bias p r o d u c e d by t h e locus o f t h e tone a n d t h e r e b y eliminated t h e Simon effect.

To elaborate a n d t e s t t h i s b u f f e r model o f response selection, we designed a t a s k t h a t i n v o l v e d t h e assignment o f unequal numbers o f stimuli t o k e y - p r e s s responses by t h e l e f t o r right h a n d (Mewaldt, Connelly, & Simon, 1980). Specifically, t w o visual stimuli were assigned t o a response by one h a n d (two-item set) a n d f o u r v i s u a l stimuli were assigned t o a response by t h e o t h e r h a n d ( f o u r - i t e m set). Each time a


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stimulus was presented, it was accompanied by a monaural t o n e t h a t p r o v i d e d t h e i r r e l e v a n t directional cue. Sometimes t h e t o n e was presented t o t h e ear c o r r e s p o n d i n g t o t h e response t o b e made on t h a t t r i a l (henceforth, called c o r r e s p o n d i n g trials), a n d sometimes t h e tone was presented t o t h e ear t h a t did n o t correspond t o t h e response (noncorresponding t r i a l s ) . T h e model makes p r e d i c t i o n s c o n c e r n i n g t h e relations among t h e R T s o b s e r v e d f o r t h e f o u r d i f f e r e n t combinations o f r e l e v a n t a n d i r r e l e v a n t information. These p r e d i c t i o n s a r e based o n t h e assumptions t h a t t h e locus o f t h e tone determines t h e b u f f e r searched f i r s t a n d t h a t R T is a d i r e c t f u n c t i o n o f t h e number o f items scanned d u r i n g response selection. Table 1 shows t h e presumed average number o f items t h a t m u s t b e scanned in o r d e r t o i d e n t i f y a stimulus f r o m a two- o r f o u r - i t e m set u n d e r conditions in which t h e i r r e l e v a n t directional c u e corresponds with t h e response a n d conditions i n w h i c h it does n o t . F o r example, consider t h e case in w h i c h a stimulus f r o m a two-item set is accompanied by a noncorresponding tone. T h e t o n e biases t h e subject t o search f i r s t t h r o u g h t h e f o u r - i t e m b u f f e r , w h i c h in t h i s instance is t h e i n c o r r e c t b u f f e r . We assume t h a t all f o u r items in t h i s b u f f e r a r e scanned b e f o r e t h e two-item b u f f e r is searched. T h e stimulus is f i n a l l y located as e i t h e r t h e f i r s t o r second item in t h e two-item b u f f e r . In t h i s example, then, t h e subject has t o scan an average o f 5.5 items (4 1.5) b e f o r e t h e c o r r e c t response can b e selected a n d executed. If a stimulus f r o m a t w o item set is accompanied by a c o r r e s p o n d i n g tone, t h e c o r r e c t b u f f e r (i.e., t h e two-item b u f f e r ) is searched f i r s t , a n d t h e stimulus is located a f t e r scanning an average o f 1.5 items. T h e average number o f items t h a t must b e scanned t o locate a stimulus f r o m t h e f o u r - i t e m set is calculated in t h e same manner, a n d these f i g u r e s a r e presented in t h e right column o f Table 1. +

Table 1 Predicted Average Number o f Items Scanned to Locate a Stimulus From a Two- o r F o u r - I t e m Set

Correspondence o f Tone a n d Response Noncor res pondi ng ( N ) Corresponding ( C ) N - C Difference

T w o - Itern Set 5.5 1.5 4.0

Four-Item Set 4.5 2.5 2.0

Note. Model assumes t h a t t h e location o f t h e i r r e l e v a n t tone determines t h e b u f f e r searched f i r s t a n d t h a t t h e search is self-terminating. T h e major p r e d i c t i o n d e r i v e d f r o m t h i s model i s t h a t t h e d i f f e r e n c e i n R T between c o r r e s p o n d i n g a n d noncorresponding t r i a l s should b e g r e a t e r f o r stimuli f r o m t h e two-item set t h a n f o r stimuli f r o m t h e f o u r item set. We base t h i s p r e d i c t i o n on t w o assumptions: (a) t h e location o f t h e i r r e l e v a n t t o n e determines t h e b u f f e r searched f i r s t , a n d (b) R T

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is a f u n c t i o n o f t h e number o f items scanned. As Table 1 indicates, t h e d i f f e r e n c e in items scanned between c o r r e s p o n d i n g a n d noncorresponding t r i a l s should b e g r e a t e r f o r stimuli f r o m t h e two-item set t h a n f o r stimuli f r o m t h e f o u r - i t e m set. O u r r e s u l t s confirmed t h i s p r e d i c t i o n . Furthermore, a f t e r we c o r r e c t e d f o r a 31 ms l o n g e r identification time f o r items f r o m t h e f o u r - i t e m set, o u r data (see Table 2) showed a n e a r l y p e r f e c t relation between t h e RTs f o r t h e v a r i o u s treatment conditions a n d t h e number o f items p r e sumably scanned in each condition. O u r f i n d i n g s , then, p r o v i d e f i r m s u p p o r t f o r o u r b u f f e r model o f response selection. Table 2 Reaction Time (in Milliseconds) A d j u s t e d t o Remove t h e Set-Size E f f e c t

Correspondence o f Tone a n d Response Noncorresponding (N) C o r r e s p o n d i n g (C) Mean N - C Difference

Note.

Two-Item Set

Four-Item Set

540 504 522 36

Mean

530

535

513 522 17

508

Adjustment i n v o l v e d s u b t r a c t i n g 31 msec f r o m RTs t o t h e f o u r - i t e m

set. T h e Logical Recoding Notion Hedge a n d Marsh (1975) used a paradigm similar t o o u r s t o i n v e s t i gate t h e e f f e c t of manipulating symbolic compatibility on reactions t o w a r d t h e stimulus source. I n one v a r i a n t o f t h e t a s k , t h e y p r e s e n t e d a g r e e n o r a r e d stimulus o n e i t h e r t h e l e f t o r t h e right side o f a v e r t i c a l panel. Subjects responded by moving t h e i r r i g h t h a n d f r o m a c e n t r a l s t a r t b u t t o n t o one o f a p a i r o f response keys, g r e e n on t h e l e f t a n d r e d o n t h e right. Subjects p e r f o r m e d on t w o blocks o f t r i a l s . In one b l o c k ( t h e compatible o r same-color condition), subjects were i n s t r u c t e d t o p r e s s t h e g r e e n k e y when t h e g r e e n stimulus appeared a n d t o p r e s s t h e r e d k e y when t h e r e d stimulus appeared. In another block ( t h e incompatible o r alternate-color condition), subjects were i n s t r u c t e d t o p r e s s t h e r e d k e y when t h e g r e e n stimulus appeared a n d t o p r e s s t h e g r e e n k e y when t h e r e d stimulus appeared. F i g u r e 15 shows in diagrammatic f o r m t h e S-R relations t h a t c h a r acterized t h e various k i n d s o f t r i a l s contained in t h e compatible a n d incompatible blocks. Diagrams (a), (b), (c), a n d (d) r e p r e s e n t t h e f o u r k i n d s o f compatible t r i a l s , whereas diagrams ( e l , (f), (g), a n d (h) r e p r e s e n t t h e f o u r k i n d s o f incompatible t r i a l s . In each diagram, t h e small squares r e p r e s e n t t h e l e f t a n d right windows that, when illuminated, displayed e i t h e r t h e g r e e n ( G ) o r t h e r e d (R) stimulus. The


COMPATIBLE S-R

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MAPPING (SAME-COLOR CONDITION 1

(C)

INCOMPATIBLE S-R

MAPPING ( ALTERNATE-COLOR CONDITION 1

--

F i g u r e 15. Diagrammatic representation o f t h e various S-R relations t h a t characterized t h e compatible a n d incompatible S-R mapping t r i a l s o f Hedge a n d Marsh (1975) and Simon e t a l . (1981). In each diagram, t h e small squares represent t h e l e f t and right windows that, when illuminated, displayed e i t h e r a green ( G ) o r r e d ( R ) stimulus. T h e circles represent t h e green ( G ) a n d r e d (R) response k e y s . T h e a r r o w in each diagram indicates t h e c o r r e c t response k e y f o r t h e p a r t i c u l a r t r i a l . small circles represent t h e t w o keys, green (G) on t h e l e f t a n d r e d (R) on t h e right. T h e a r r o w in each diagram indicates t h e c o r r e c t response key f o r the particular trial. Hedge a n d Marsh f o u n d t h a t , in t h e compatible block, t h e l e f t response was f a s t e r when t h e stimulus appeared o n t h e l e f t side t h a n when it appeared on t h e right ( a < c), a n d t h e right response was f a s t e r when t h e stimulus appeared on t h e right side t h a n when it appeared o n t h e l e f t (b < d ) . In o t h e r words, reactions were f a s t e r on t r i a l s in which t h e location o f t h e stimulus a n d response corresponded t h a n o n t r i a l s in which t h e y did n o t correspond. Hedge a n d Marsh used t h e label "Simon effect" t o r e f e r t o t h i s t y p i c a l finding. T h e i n t e r e s t i n g aspect o f t h e Hedge a n d Marsh findings, however, was t h e reversal o f t h e Simon e f f e c t i n t h e incompatible block, f o r which reactions were now f a s t e r on t r i a l s in which t h e location o f stimulus and response did n o t correspond than on t r i a l s in which t h e y corresponded. T h a t is, t h e l e f t response was f a s t e r when t h e stimulus appeared on t h e right side t h a n when it appeared on t h e l e f t (e < g), a n d t h e right response was f a s t e r when t h e

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stimulus appeared o n t h e l e f t side t h a n when it appeared on t h e right (f < h). Hedge a n d Marsh explained t h i s a p p a r e n t r e v e r s a l o f t h e Simon e f f e c t in terms o f a notion t h a t t h e y called logical recoding. "For a g i v e n logical r e c o d i n g ( i d e n t i t y o r reversal) o f t h e r e l e v a n t a t t r i b u t e (colour), r e s p o n d i n g was f a s t e r f o r t r i a l s i n w h i c h t h e recoding o f t h e i r r e l e v a n t a t t r i b u t e (position) was o f t h e same logical t y p e as t h a t o f t h e r e l e v a n t a t t r i b u t e , t h a n f o r t r i a l s in w h i c h t h e logical recoding o f t h e i r r e l e v a n t a t t r i b u t e was opposite in t y p e " (p. 435). I n o t h e r words, Hedge a n d Marsh suggested t h a t when subjects h a d t o t r a n s l a t e a r e d stimulus i n t o a g r e e n k e y press, t h e y also recoded o r r e v e r s e d t h e i r r e l e v a n t position c u e so t h a t t h e y now responded f a s t e r on t h e side opposite t h e location o f t h e stimulus. D i s p l a y - C o n t r o l A r r a n g e m e n t Correspondence We f e l t t h a t t h e concept o f logical recoding was n o t necessary t o explain t h e a p p a r e n t r e v e r s a l o f t h e Simon e f f e c t u n d e r incompatible S-R mapping i n s t r u c t i o n s . Rather, we hypothesized t h a t t h e Hedge a n d Marsh r e s u l t s c o u l d b e explained in terms o f a c o n f o u n d i n g w i t h a v a r i e t y o f spatial cfmpatibility, w h i c h we call "display-control arrangement c o r r e spondence. B y this, we mean a correspondence between t h e spatial arrangement ( l e f t a n d right) o f t h e g r e e n a n d r e d l i g h t s o n t h e d i s p l a y panel a n d t h e g r e e n a n d r e d response k e y s . We suggest t h a t , i n t h e Hedge a n d Marsh experiment, half o f t h e t r i a l s in each b l o c k were characterized by d i s p l a y - c o n t r o l arrangement correspondence, a n d h a l f were characterized by d i s p l a y - c o n t r o l a r r a n g e ment noncorrespondence. T h e diagrams in F i g u r e 15 show t h e d i s p l a y c o n t r o l c o n f i g u r a t i o n t h a t characterized t h e various t y p e s o f t r i a l s . Of t h e compatible t r i a l s , types, (a) a n d (b) i n v o l v e d a c o r r e s p o n d i n g arrangement; t h a t is, t h e stimulus light a n d response k e y o f t h e same color were o n t h e same side o f t h e console. T y p e s (c) a n d (d) i n v o l v e d a noncorresponding arrangement. Similarly, o f t h e incompatible trials, t y p e s (e) a n d (f) i n v o l v e d a c o r r e s p o n d i n g arrangement, whereas t y p e s (9) a n d (h) i n v o l v e d a noncorresponding arrangement. (Note that, in F i g u r e 15(a), f o r example, t h e g r e e n light appeared in t h e l e f t window w h i l e t h e right window was "empty." We characterize t h i s d i s p l a y - c o n t r o l c o n f i g u r a t i o n as c o r r e s p o n d i n g because t h e g r e e n light a n d g r e e n k e y a r e o n t h e same side, even t h o u g h t h e r e is n o t a v i s i b l e r e d stimulus in t h e right window t o c o r r e s p o n d t o t h e r e d k e y on t h e right.) Because one m i g h t expect f a s t e r reactions when t h e d i s p l a y - c o n t r o l arrangement i s c o r r e s p o n d i n g t h a n when it is noncorresponding, one would p r e d i c t t h a t t r i a l t y p e s (a) a n d (b) would b e f a s t e r t h a n (c) a n d (d) a n d t h a t t r i a l t y p e s (e) a n d (f) would b e f a s t e r t h a n (9) a n d (h). T h i s is e x a c t l y w h a t Hedge a n d Marsh f o u n d . Thus, we a r e s u g g e s t i n g t h a t a v a r i a t i o n i n d i s p l a y - c o n t r o l arrangement correspondence p r o d u c e d t h e Hedge a n d Marsh r e s u l t s a n d t h a t t h e r e was no need f o r them t o r e s o r t t o t h e more i n v o l v e d explanation in terms o f t h e Simon e f f e c t a n d i t s r e v e r s a l u n d e r incompatible S-R mapping i n s t r u c t i o n s (i.e., t h e notion o f logical recoding). O u r experiment, then, represented an attempt t o replicate t h e Hedge a n d Marsh r e s u l t s by deliberately manipulating d i s p l a y - c o n t r o l arrangement correspondence (Simon, Sly, & Vilapakkam, 1981). Subjects were assigned a t incompatible S - R mapping g r o u p .

random t o e i t h e r a compatible o r an The compatible g r o u p was told, "If a


67

green l i g h t appears, press t h e green k e y as f a s t as y o u y:n, while if a r e d light appears, press t h e r e d k e y as f a s t as y o u can. T h e incompatible g r o u p was told, "If a green light appears, press t h e r e d k e y as f a s t as y o u can, while if a r e d light appears, press t h e green k e y as f a s t as y o u can." Both t h e compatible and incompatible g r o u p s performed on t w o blocks o f trials, one b l o c k characterized by display-control arrangement correspondence a n d t h e o t h e r block characterized by display-control arrangement noncorrespondence. Specifically, r e f e r r i n g again t o F i g u r e 15, t h e compatible g r o u p received t r i a l t y p e s (a) a n d (b) in t h e i r c o r r e sponding block a n d t r i a l t y p e s (c) a n d ( d ) i n t h e i r noncorresponding block. T h e incompatible g r o u p received t r i a l t y p e s (e) a n d ( f ) in t h e i r corresponding block a n d t r i a l t y p e s ( 9 ) a n d ( h ) in t h e i r noncorresponding block. T h e reader w i l l note that, in t h e blocks characterized by d i s play-control arrangement correspondence, when t h e green stimulus was presented, it always appeared in t h e l e f t window, a n d when t h e r e d stimulus was presented, it always appeared in t h e right window. In o t h e r words, t h e spatial arrangement o f t h e green ( l e f t ) a n d r e d (right) l i g h t s o f t h e display corresponded w i t h t h e spatial arrangement o f t h e green ( l e f t ) and r e d ( r i g h t ) keys. I n t h e t r i a l blocks characterized by display-control arrangement noncorrespondence, when t h e green stimulus was presented, it always appeared in t h e right window, a n d when t h e r e d stimulus was presented, it always appeared i n t h e l e f t window. In o t h e r words, t h e spatial arrangement o f t h e green (right) a n d r e d ( l e f t ) d i s p l a y l i g h t s did n o t correspond w i t h t h e spatial arrangement o f t h e green ( l e f t ) and r e d (right) keys. The verbal instructions f o r the corresponding and noncorresponding blocks o f t r i a l s were identical. The experimenter ,,demonstrated by p o i n t i n g and, thus, p u r p o s e l y avoided r e f e r r i n g t o l e f t " o r "right" windows, t o ensure t h a t t h e subjects would respond in terms o f t h e relevant stimulus (color) r a t h e r t h a n in terms o f t h e redundant i r r e l e v a n t stimulus (stimulus location). In t h e Hedge a n d Marsh procedure, w i t h e i t h e r color appearing i n e i t h e r window, stimulus location was i r r e l e v a n t b u t n o t redundant. A n overall analysis o f variance confirmed o u r hypothesis; t h a t is, t h e corresponding block was s i g n i f i c a n t l y f a s t e r t h a n t h e noncorrespondi n g block (719 v s . 807 ms). T h i s correspondence e f f e c t o c c u r r e d in t h e compatible g r o u p (633 v s . 746 ms), as well as t h e incompatible g r o u p (804 v s . 868 ms), a n d t h e r e was no interaction between correspondence a n d compatibility. We t h e n relabeled o u r d a t a a n d analyzed them as Hedge a n d Marsh had done. F i g u r e 16 shows that, f o r t h e compatible S-R mapp i n g group, t h e r e was t h e t y p i c a l Simon effect. In o t h e r words, reactions were f a s t e r when t h e location o f t h e stimulus a n d t h e response c o r responded than when it did n o t . For t h e incompatible group, t h a t effect was reversed. T o summarize, we have shown t h a t a manipulation o f display-control arrangement correspondence produced t h e same p a t t e r n o f results as t h a t observed by Hedge a n d Marsh. It seems t o us more parsimonious t o explain t h e Hedge a n d Marsh f i n d i n g s i n terms o f t h i s main e f f e c t t h a n it is t o assert t h a t subjects recode (reverse) t h e i r r e l e v a n t position cue u n d e r conditions o f incompatible S - R mappings ( i . e . , t h e notion o f logical recoding). O u r experiment, however, did not eliminate logical recoding as a viable alternative explanation f o r o u r f i n d i n g s . That is, one m i g h t s t i l l

J.R. Simon

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COMPAT I BLE S-R MAPPING

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STIMULUS LOCATION

Figure 16.

Effect o f stimulus location on reaction time f o r l e f t a n d right responses u n d e r compatible a n d incompatible S-R mapping i n s t r u c t i o n s .

a r g u e t h a t t h e d i f f e r e n c e between corresponding a n d noncorresponding blocks in t h e compatible g r o u p was d u e t o t h e Simon effect, whereas t h e difference between corresponding a n d noncorresponding blocks in t h e incompatible g r o u p was d u e t o logical recoding. T h e problem is t h a t t h e r e a r e t h r e e k i n d s o f compatibility which a r e i n e x t r i c a b l y confounded in t h e Hedge a n d Marsh t a s k . T h e y a r e display-control arrangement correspondence, t h e Simon effect, and symbolic compatibility. Because o f t h i s confounding, t h e r e is n o way t h a t t h e Hedge a n d Marsh paradigm can b e used t o p r o v i d e a c r u c i a l t e s t o f whether t h e Simon e f f e c t reverses u n d e r incompatible mapping i n s t r u c t i o n s . We, therefore, employed a d i f f e r e n t paradigm which eliminated t h e factor o f display-control arrangement correspondence by p r e s e n t i n g t h e relevant visual stimulus f r o m a single window in t h e center o f t h e d i s p l a y panel. T h e i r r e l e v a n t directional cue was p r o v i d e d by a monaural tone presented simultaneously w i t h t h e relevant visual cue. O u r previous research had demonstrated t h a t responses t o a visual stimulus a r e f a s t e r


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when t h e location o f t h e i r r e l e v a n t tone a n d t h e response c o r r e s p o n d t h a n when t h e y d o n o t (e.g., Acosta & Simon, 1976; Mewaldt e t al., 1980; Simon & Craft, 1 9 7 0 ~ ; Simon & Pouraghabagher, 1978). With t h i s paradigm, then, it was possible t o independently manipulate t h e i r r e l e v a n t directional cue a n d compatibility o f S-R mapping w i t h o u t encountering t h e troublesome confounding o f these f a c t o r s w i t h display-control arrangement correspondence t h a t characterized o u r f i r s t experiment. T h e subjects' response in t h i s second experiment was t h e same as in t h e f i r s t ; t h a t is, t h e y pressed e i t h e r a r e d o r green k e y in response t o t h e onset o f a r e d o r green stimulus t h a t appeared in one c e n t r a l l y located d i s p l a y window. On each trial, t h e stimulus was accompanied by t h e simultaneous onset o f a tone presented t h r o u g h earphones t o t h e A compatible g r o u p was i n s t r u c t e d t o press subjects' right o r l e f t ear. t h e green k e y if t h e green light appeared and t o press t h e r e d k e y if t h e r e d light appeared, whereas an incompatible g r o u p was g i v e n t h e opposite S-R mapping i n s t r u c t i o n s . If a reversal o f t h e Simon e f f e c t were t o occur on t h i s t a s k u n d e r incompatible S-R mapping instructions, we would have clear evidence t o s u p p o r t t h e logical recoding notion. O u r results did n o t s u p p o r t t h e notion o f logical recoding. With compatible SR mapping, we obtained t h e t y p i c a l Simon effect; t h a t is, reactions were f a s t e r when t h e location o f t h e i r r e l e v a n t cue a n d t h e response However, w i t h incompatible S-R corresponded than when t h e y did not. mapping, t h e r e was no evidence o f a reversal o f t h e Simon effect; rather, t h e location of t h e i r r e l e v a n t c u e simply h a d n o effect. It appears t h a t t h e incompatible S-R mapping i n s t r u c t i o n s may have so loaded t h e information-processing apparatus as t o negate t h e e f f e c t o f t h e i r r e l e v a n t directional cue. Thus, evidence f r o m t h i s s t u d y s u p p o r t s e a r l i e r indications (Simon e t al., 1976) t h a t a complex interaction may o c c u r between d i f f e r e n t sources o f compatibility.

A Research Tool for S t u d y i n g A g i n g We have used t h e Simon e f f e c t as a tool, in combination w i t h S t e r n b e r g ' s a d d i t i v e f a c t o r s method, t o determine t h e e f f e c t o f a g i n g on t h e encoding a n d response-selection stages o f a choice R T t a s k . Young (average age 20 years) a n d o l d (average age 73 years) subjects pressed The a l e f t - o r r i g h t - h a n d k e y in response t o t h e onset o f an X o r 0. q u a l i t y o r d i s c r i m i n a b i l i t y o f these characters was manipulated so as t o affect t h e stimulus-encoding stage (Beiderman & Kaplan, 1970; Sternberg, 1967, 1969). A monaural tone presented simultaneously w i t h t h e characters p r o v i d e d an i r r e l e v a n t directional cue, which o u r p r e v i o u s research had shown t o a f f e c t t h e response-selection stage b u t n o t t h e stimulus-encoding stage (e.g., Acosta & Simon, 1976).

If aging affects t h e stimulus-encoding stage, t h e r e should b e a s i g n i f i c a n t Stimulus Quality x Age interaction. If a g i n g affects t h e response-selection stage, t h e r e should b e a s i g n i f i c a n t Directional Cue x Age interaction. If older subjects t e n d t o encode i r r e l e v a n t stimuli t h a t younger subjects ignore, t h i s should manifest i t s e l f as a t r i p l e - o r d e r interaction o f Age x Stimulus Q u a l i t y x Directional Cue. F i g u r e 17 shows t h a t each o f t h e t h r e e independent variables h a d a significant e f f e c t on RT; reactions t o t h e degraded stimulus were slower t h a n t o t h e i n t a c t stimulus; t h e i r r e l e v a n t directional c u e p r o d u c e d t h e t y p i c a l cross e f f e c t (Simon effect) i n each f i g u r e panel; a n d o l d subjects responded slower t h a n y o u n g subjects.

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Model representing information processing in the single-task

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RESPONSE

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Model A

Model B

ENCODING AUDITORY

Verbal Repat

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Model c AUDITORY

Verbal Repwt

VISUAL

Model D

Figure 19. Four possible models representing information processing in the dual-tasks condition.


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O u r f i n d i n g s indicated t h a t b o t h correspondence (i.e., location o f t h e tone in relation t o t h e c o r r e c t key-press response) a n d stimulus q u a l i t y affected visual RT, b u t these t w o effects did n o t i n t e r a c t . In addition, t h e correspondence e f f e c t was more pronounced in t h e dualtasks condition t h a n in t h e single-task condition. Finally, visual R T was slower in t h e dual- t h a n in t h e single-task condition. These r e s u l t s a r e all consistent w i t h Model D and inconsistent, in one way o r t h e other, w i t h t h e a l t e r n a t i v e models. We w i l l n o t spell o u t t h e complete reasoning here. Suffice it t o say t h a t t h e p a t t e r n o f main effects a n d interactions observed in t h i s s t u d y suggested t h a t t h e same model o f information flow described b o t h single- a n d dual-tasks conditions; t h a t is, visual a n d a u d i t o r y stimuli were encoded separately b u t shared capacity a t t h e response-selection stage. Resemblance t o O t h e r Phenomena T h e Simon effect bears a c e r t a i n resemblance t o o t h e r phenomena. For example, we speculated e a r l y o n that it m i g h t b e a manifestation of a basic o r i e n t i n g r e f l e x (OR) which, in i t s f u l l y developed form, can i n v o l v e a specific molar reaction o f t u r n i n g t o w a r d t h e source o f stimulat i o n (Razran, 1961). O r d i n a r i l y , however, t h e OR e v o k i n g capacity of a stimulus decreases w i t h repeated presentations, whereas t h e Simon e f f e c t is c e r t a i n l y n o t a t r a n s i e n t phenomenon (e.g., Simon e t al., 1973). Similarly, t h e Simon e f f e c t bears some resemblance t o t h e well-known Stroop phenomenon (Jensen & Rohwer, 1966). I n a t y p i c a l Stroop task, t h e stimuli consist o f names o f colors p r i n t e d i n o t h e r colors. Interference occurs when subjects are asked t o name t h e colors a n d i g n o r e t h e words. A t t h e superficial level, t h i s interference, produced by t h e i n c o n g r u i t y o f ink color a n d t h e v e r b a l content o f t h e color word, is similar t o t h e interference we have observed when t h e source o f a command does n o t correspond t o i t s symbolic content. In o u r research, however, t h e interference i s n o t related t o a c o n f l i c t between t w o symbolic cues b u t seems instead t o r e f l e c t a p r i m i t i v e innate tendency t o react t o w a r d t h e source o f stimulation. We believe t h a t t h i s p o t e n t behavioral p a t t e r n may b e analogous t o t h e t a x i s o r d i r e c t e d orientation reactions o f lower animals (Fletcher, 1966; Fraenkel & Gunn, 1961; Lorenz & T i n b e r gen, 1957). T h e necessity o f o v e r r i d i n g t h i s i n i t i a l response tendency before responding t o a relevant symbolic cue would account f o r t h e slower information processing we have observed. Some o f t h e tasks that we have employed bear n o resemblance t o t h e S t r o o p - t y p e t a s k and, yet, we have been able t o generate, test, a n d confirm predictions based o n t h e existence o f t h i s u n i q u e a n d p o t e n t response tendency (e.g., Simon & C r a f t , 1972; Simon, C r a f t , & Webster, 1971). Possible Practical Implications What about t h e practical implications o f o u r research t o engineers who a r e concerned w i t h t h e design a n d arrangement o f displays a n d controls? Clearly, designers should b e m i n d f u l t h a t t h e i r decisions conc e r n i n g t h e location o f a display on a console may, inadvertently, i n t r o duce i r r e l e v a n t directional cues t h a t i n t e r f e r e w i t h a n operator's a b i l i t y t o process relevant symbolic information f r o m t h e display. B u t can we d o more t h a n simply caution designers t o avoid interference f r o m i r r e l e v a n t directional cues? For example, can t h e directional cue actually b e utilized t o facilitate information processing? T w o studies were conducted t o explore t h i s p o s s i b i l i t y . In one s t u d y (Simon, 1968b), we attempted t o facilitate processing o f verbal directional commands by u s i n g t h e ear

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stimulated t o p r o v i d e a r e l e v a n t ( r e d u n d a n t ) directional cue. We asked w h e t h e r subjects would respond f a s t e r t o a d i s p l a y t h a t combined d i r e c t i o n a l a n d symbolic cues (i.e., monaural v e r b a l commands of "right" a n d " l e f t " in t h e ear c o r r e s p o n d i n g t o t h e c o n t e n t o f t h e command) t h a n t h e y would to displays tha! i n v o l v e d ei)Fer a symbolic c u e alone (i.e., b i n a u r a l v e r b a l commands o f right" a n d l e f t " ) o r a directional c u e alone (i.e., monaural p u r e tones). Results showed that, w h i l e responses t o t h e comb i n e d cues were f a s t e r t h a n t o t h e symbolic c u e alone (322 v s . 420 ms), t h e y were n o t f a s t e r t h a n t o t h e directional c u e alone (307 ms). Findings f r o m t h i s study, however, cannot b e used t o s u p p o r t generalizations conc e r n i n g t h e r e l a t i v e effectiveness of symbolic v e r s u s directional cues o r t h e general e f f e c t of combining these cues because q u a l i t a t i v e l y d i f f e r e n t stimuli (tones a n d v e r b a l commands) were used f o r t h e d i f f e r e n t conditions. A n o t h e r s t u d y (Simon & Craft, 1971) o b v i a t e d t h e above-mentioned difficulty by employing p u r e tones t o p r e s e n t b o t h directional a n d symIn separate blocks o f trials, subjects pressed a right- o r bolic cues. (a) high- o r l o w - p i t c h e d b i n a u r a l tonal l e f t - h a n d k e y in response to: commands (symbolic cue), (b) t h e e a r stimulated by a single p u r e tone (directional cue), a n d (c) high- o r low-pitched monaural tonal commands in t h e e a r c o r r e s p o n d i n g t o t h e symbolic c o n t e n t o f t h e command (combined directional a n d symbolic cues). T h e combined ( r e d u n d a n t ) cue condition p r o d u c e d s i g n i f i c a n t l y f a s t e r R T (295 ms) t h a n t h e directional c u e alone (304 ms) o r t h e symbolic c u e alone (371 ms). T h i s finding f i t s i n t o a small b o d y o f l i t e r a t u r e w h i c h suggests that, u n d e r c e r t a i n conditions, p r o v i d i n g r e d u n d a n t information in a d i s p l a y may f a c i l i t a t e i n f o r However, mation processing (e.g., Woodworth & Schlosberg, 1954). because t h e o b s e r v e d d i f f e r e n c e was so small, t h i s finding i s of p e r h a p s g r e a t e r theoretical t h a n p r a c t i c a l significance. Methodological Implications O u r research o n t h e stereotype has important methodological implications f o r studies o f hemispheric dominance a n d interhemispheric t r a n s f e r time, because it i l l u s t r a t e s how i r r e l e v a n t directional cues in t h e d i s p l a y i n t e r a c t w i t h t h e response location t o a f f e c t information processing (Bashore, 1981; Simon & Agens, 1980; Swanson, Ledlow, & Kinsbourne, 1978; Young, 1982). Researchers must b e especially a l e r t t o these d i r e c tional cues because t h e cues may b e confounded w i t h t h e v a r i a b l e t h a t is manipulated (Babkoff, Genser, & Hegge, 1985; Bradshaw & Umilt5, 1984; Cohen, 1973; Elias & Elias, 1976; G a r d n e r e t al., 1977; G a r d n e r & Ward, 1979; Genesee e t al., 1978; l z d e b s k i & Shipp, 1978; Klatzky, 1972). Consider, f o r example, a t a s k t h a t involves presentation o f a stimulus t o t h e right o r l e f t v i s u a l f i e l d in o r d e r t o stimulate t h e l e f t o r right hemisphere. T h e subject is asked t o respond by p r e s s i n g keys labeled Sup"same" o r "different," located o n t h e right a n d left, respectively. pose t h a t same judgments a r e s i g n i f i c a n t l y f a s t e r when t h e stimulus is presented t o t h e right v i s u a l f i e l d t h a n when it is presented t o t h e l e f t . A n i n v e s t i g a t o r m i g h t a t t r i b u t e t h i s finding t o t h e s u p e r i o r i t y o f t h e l e f t hemisphere o v e r t h e r i g h t when, actually, t h e d i f f e r e n c e c o u l d b e d u e t o f a s t e r reactions t o w a r d t h e source o f stimulation (e.g., Simon, 1970). In a similar fashion, r e s u l t s o f c e r t a i n a u d i t o r y asymmetry studies may b e affected by i r r e l e v a n t directional cues associated w i t h t h e ear stimulated Suppose, f o r example, t h a t speech sounds (e.g., Elias & Elias, 1976). a r e p r e s e n t e d monaurally a n d subjects p r e s s a l e f t - o r r i g h t - h a n d k e y t o r e g i s t e r a "yes-no" o r a "same-different" judgment. We would c l e a r l y


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expect t o find f a s t e r reactions w i t h t h e k e y on t h e same side as t h e ear stimulated (Simon & Rudell, 1967). Related Research T h e p u r p o s e o f t h i s c h a p t e r has been t o review o u r own research on a t y p e o f spatial S-R compatibility t h a t Hedge a n d Marsh (1975) have labeled t h e "Simon e f f e c t . " We have indicated how a serendipitous findi n g launched us on t h i s p r o g r a m o f research. We have t h e n d e s c r i b e d a series o f related experiments t h a t demonstrate t h a t t h e location of a stimulus p r o v i d e s an i r r e l e v a n t directional c u e t h a t affects t h e time r e q u i r e d t o process t h e meaning o f t h e stimulus. O u r r e s u l t s suggest t h a t t h e r e may b e a s t r o n g n a t u r a l t e n d e n c y t o respond i n i t i a l l y t o t h e directional component o f a stimulus r a t h e r t h a n t o i t s symbolic content. T h i s i r r e l e v a n t location c u e biases t h e subject t o search a p a r t i c u l a r response b u f f e r f i r s t , which, on c e r t a i n trials, produces a delay in R T . While o u r main aim h e r e has been t o summarize o u r own research, it seems a p p r o p r i a t e t o a t least mention related studies. T h e r e have been a number o f papers concerned w i t h r e p l i c a t i n g a n d / o r e x t e n d i n g o u r findi n g s (Anzola, Bertoloni, Buchtel, & Rizzolatti, 1977; Bauer & Miller, 1982; B e r n s t e i n & Edelstein, 1971; Bertera, Callan, Parsons, & Pishkin, 1975; Brebner, 1973, 1979; Brebner, Shephard, & Cairney, 1972; Callan, Klisz, & Parsons, 1974; Cotton, Tzeng, & H a r d y c k , 1977, 1980; Faber, v a n d e r Molen, Keuss, & Stoffels, 1986; Guiard, 1983; Hammond & Barber, 1978; Hasbroucq, 1987; Hedge & Marsh, 1975; Katz, 1981; Klisz & Parsons, 1975; Ladavas, 1987; Nicoletti & Umilta, 1984; Nicoletti, Umilta, & Ladavas, 1984; Nicoletti, Anzola, Luppino, Rizzolatti, & Umilta, 1982; Pieters, 1981; Puleo & Sheldon, 1977; Ragot, 1984; Ragot & Lesevre, 1986; Ragot & Renault, 1985; Ragot, Renault, & Remond, 1980; 1982; Riggio, Gawryszewski, & Umilti, 1986; Rastatter & Gallaher, Schaefer, Gunn, & Dubowski, 1977; Shulman & McConkie, 1973; Stoffels, v a n d e r Molen, & Keuss, 1985, 1986; Umilta & Nicoletti, 1985; v a n d e r Molen & Keuss, 1981; Wallace, 1971, 1972; Whitaker, 1982). O t h e r papers have suggested a l t e r n a t i v e explanations f o r t h e Simon e f f e c t (Heister & Schroeder-Heister, 1985; Kahneman, 1973; Kinsbourne, 1970; L i d a v a s & Moscovitch, 1984; L u p k e r & Katz, 1982; Nickerson, 1973; Proctor & Reeve, 1985; Smith & Brebner, 1983; Stoffels, 1988; Swanson e t al., 1978). O u r own explanation o f t h e phenomenon has been i n t e r p r e t e d too b r o a d l y on some occasions (e.g., Nicoletti e t al., 1982) a n d too n a r r o w l y on o t h e r s (e.g., U m i l t i & Nicoletti, 1985). We have n e v e r meant t o suggest t h a t all spatial S-R compatibility effects can b e explained in terms o f an innate tendency t o react t o w a r d t h e source o f stimulation. In fact, we have been c a r e f u l t o d i s t i n g u i s h experiments in which t h e stimulus location p r o v i d e s an i r r e l e v a n t c u e f r o m those in w h i c h t h e stimulus location i s the r e l e v a n t cue ( e . g . , Simon, Hinrichs, & C r a f t , 1970; Simon e t al., 1981). N o r d o we d e n y t h e role o f coding a n d Indeed, translation processes in e x p l a i n i n g some compatibility effects. o u r own research p r o v i d e s examples of t h e Simon e f f e c t i n t e r a c t i n g w i t h symbolic compatibility, t h a t is, w i t h t r a n s l a t i o n processes (e.g., Simon e t al., 1976). Umilta a n d Nicoletti (1985) conducted seyeral experiments designed t o t e s t t h e i r "coding hypothesis" against o u r attentional" explanation o f t h e Simon e f f e c t . T h e y conclude t h a t t h e i r r e s u l t s s u p p o r t t h e coding

J.R. Simon hypothesis. In fact, we would have p r e d i c t e d t h e same results. Specifically, we would expect t h e Simon e f f e c t t o occur when b o t h stimuli ( l e f t a n d right) as well as b o t h responses ( l e f t a n d right) a r e located o n t h e In o u r view, t h e Simon e f f e c t does n o t same side o f t h e b o d y midline. necessitate t h a t an actual movement b e made t o w a r d t h e source o f stimulation (see L u p k e r & Katz, 1982). Umilta and Nicoletti's finding o f a significant, t h o u g h reduced, Simon e f f e c t when t h e i r r e l e v a n t location cue did n o t v a r y w i t h i n a block o f t r i a l s was also n o t s u r p r i s i n g because we had obtained t h e same r e s u l t e a r l y in o u r research (Simon & Rudell, 1967). Finally, we have no problem w i t h t h e i r finding t h a t t h e Simon e f f e c t depends, in c e r t a i n cases, o n r e l a t i v e r a t h e r t h a n a b s o l u p location cues. We suggest that, r a t h e r t h a n thinking o f "coding" a n d attention" as t w o mutual!y exclusive explanations o f t h e phenomenon a n d t h e n trying t o design a crucial" experiment t o r e f u t e one explanation and s u p p o r t t h e other, t h e explanations should b e viewed as r e f l e c t i n g a difference in emphasis. Nicoletti a n d Umilta (19851, Wallace (19711, a n d others seem t o view t h e e f f e c t in terms o f a high-level c o g n i t i v e process t h a t involves coding t h e i r r e l e v a n t location cue a n d comparing it t o t h e code f o r t h e relevant cue. In contrast, we t e n d t o minimize t h e c o g n i t i v e component and view t h e e f f e c t in terms o f a more p r i m i t i v e reaction t o t h e location o f t h e stimulus t h a t biases t h e subject t o search a p a r t i c u l a r response buffer first. O u r view has enabled u s t o p r e d i c t results on tasks where t h e r e is no l e f t - r i g h t coding o f stimulus and/or response positions (e.g., Simon & Craft, 1972; Simon, Craft, & Webster, 1971). The continuing i n t e r e s t in t h e Simon e f f e c t by numerous investigators and t h e i r d i f f e r i n g explanations f o r it indicate t h a t t h e r e is s t i l l much t o b e learned about t h i s p o t e n t phenomenon. Summary T h i s c h a p t e r describes almost 40 related experiments concerned w i t h what we believe t o b e a basic and important human response mechanism, a tendency t o react t o w a r d t h e source o f stimulation. T h e following i s a b r i e f summary o f o u r s t e p - b y - s t e p attempt t o u n d e r s t a n d t h e basic n a t u r e o f t h i s phenomenon a n d t o explore some o f i t s practical, methodological, a n d theoretical implications. 1. We discovered q u i t e by accident t h a t R T t o v e r b a l "right" a n d "left" commands was s i g n i f i c a n t l y f a s t e r when t h e content o f t h e command corresponded t o t h e ear stimulated t h a n when it did not. In o t h e r words, t h e location o f t h e stimulus p r o v i d e d a n i r r e l e v a n t cue t h a t affected t h e time r e q u i r e d t o process t h e meaning o f t h e stimulus. This e f f e c t has been labeled t h e Simon e f f e c t .

2. Removing subjects' u n c e r t a i n t y as t o which ear would b e stimulated did n o t eliminate t h i s effect. 3. T h e Simon e f f e c t was n o t d u e t o a n y simple isomorphic associat i o n between ear stimulated a n d ipsilateral hand, because t h e same response i n t e r f e r e n c e also o c c u r r e d in a one-hand t a s k .

4. Subjects reacted f a s t e r when t h e y were t o l d t o move toward r a t h e r t h a n away f r o m t h e source o f a n a u d i t o r y signal, suggesting t h a t t h e r e was a n a t u r a l tendency t o react t o w a r d t h e source o f stimulation. T h e necessity f o r o v e r r i d i n g t h i s n a t u r a l response tendency would account f o r t h e slower information processing on t h e "away" block o f trials.


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5. Whether t h e i r r e l e v a n t directional c u e h a d a f a c i l i t a t i v e and/or i n h i b i t o r y e f f e c t on information processing seemed t o depend o n t h e e x perimental design; specifically, t h e e f f e c t depended o n t h e c o n t e x t in w h i c h baseline t r i a l s were presented.

6. T h e Simon e f f e c t o c c u r r e d when p u r e tones were used t o signal l e f t a n d right responses, i n d i c a t i n g t h a t t h e phenomenon did n o t depend on p r i o r symbolic associations attached t o v e r b a l directional commands.

7. T h e Simon e f f e c t was also o b s e r v e d in a v i s u a l R T t a s k t h a t paralleled t h e o r i g i n a l a u d i t o r y t a s k . T h i s e f f e c t was n o t d u e t o eye stimulated per se but, r a t h e r , t o t h e directional c u e associated w i t h eye stimulated. 8. Additional studies showed t h a t t h e i r r e l e v a n t c u e p r o d u c i n g t h e e f f e c t in t h e a u d i t o r y experiments was n o t t h e ear stimulated per se but r a t h e r t h e directional cue associated w i t h ear stimulated. In o t h e r words, perceptual f a c t o r s r a t h e r t h a n sensory f a c t o r s were responsible f o r t h e Simon e f f e c t . 9. B y manipulating i n t e r a u r a l phase s h i f t , we v a r i e d n o t o n l y t h e a p p a r e n t spatial locus o f a stimulus but also t h e p o t e n c y o f t h e d i r e c tional cue. T h e s t r o n g e r t h e directional cue, t h e s t r o n g e r was t h e t e n d e n c y t o react t o w a r d t h e source o f stimulation.

10. B y manipulating t h e i n t e n s i t y a n d locus o f b r o a d - b a n d noise presented c o n c u r r e n t l y w i t h t h e command, we eliminated, reduced, accentuated, a n d even r e v e r s e d t h e Simon effect. In o t h e r words, t h e stereotype involves a t e n d e n c y t o react t o w a r d t h e major source o f stimulation a n d n o t simply a t e n d e n c y t o react t o w a r d t h e r e l e v a n t stimul u s , t h a t is, t h e command. 11. T h e mechanisms u n d e r l y i n g a u d i t o r y S-R compatibility d i f f e r e d f o r d i f f e r e n t tasks. When stimulus location was t h e r e l e v a n t cue, t h e r e ear/hand corresponwere t w o components t o a u d i t o r y S-R compatibility: dence a n d ear/response-location correspondence. However, when stimulus c o n t e n t was t h e r e l e v a n t cue a n d location t h e i r r e l e v a n t cue, t h e n ear/response-location correspondence alone accounted f o r S-R compatibility effects. 12. T h e i r r e l e v a n t directional cue p r o d u c e d an e f f e c t across sens o r y modalities; t h a t is, speed o f r e s p o n d i n g t o a v i s u a l d i s p l a y was affected by t h e location o f a c o n c u r r e n t but i r r e l e v a n t a u d i t o r y stimulus.

13. Experiments i n v o l v i n g responses t o t h e changed element o f a two-element d i s p l a y (i.e., onset a n d o f f s e t o f l i g h t s a n d tones) p r o d u c e d r e s u l t s consistent w i t h t h e notion o f a s t e r e o t y p i c t e n d e n c y t o react t o w a r d t h e a p p a r e n t source o f stimulation. 14. I n an experiment i n v o l v i n g an o d d i t y task, subjects h a d t o i d e n t i f y a n d respond t o t h e one element of a three-element d i s p l a y t h a t was d i f f e r e n t f r o m t h e o t h e r t w o elements. I r r e l e v a n t cues f r o m t h i s c h a n g i n g d i s p l a y a p p a r e n t l y elicited a s t e r e o t y p i c response t h a t i n t e r f e r e d w i t h information processing. 15. T h e stereotype p e r s i s t e d r e l a t i v e l y undiminished f o r more t h a n 1,000 t r i a l s o v e r a 5 - d a y p e r i o d . Furthermore, analysis o f t h e latency

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a n d f r e q u e n c y o f d i f f e r e n t k i n d s o f e r r o r s s u p p o r t e d t h e conclusion t h a t t h e e f f e c t represented an i n i t i a l tendency t o react t o w a r d t h e source o f a stimulus r a t h e r t h a n t o i t s meaning.

16. A n i r r e l e v a n t d i r e c t i o n a l c u e presented p r i o r t o t h e onset o f a r e l e v a n t stimulus i n t e r f e r e d w i t h t h e processing o f t h a t stimulus. 17. Studies employing a d d i t i v e f a c t o r s logic suggested t h a t t h e directional c u e affected t h e response-selection stage a n d did n o t a f f e c t t h e stimulus-encoding stage. 18. A s t e r e o t y p i c reaction t o w a r d t h e stimulus source o c c u r r e d in a t a s k i n v o l v i n g high- a n d low-pitched "up" a n d "down" commands emanating f r o m a t o p o r bottom speaker. T h i s f i n d i n g p r o v i d e d c o n v i n c i n g e v i dence t h a t l a t e r a l i t y was n o t a f a c t o r in p r o d u c i n g t h e Simon effect. Results also revealed an interaction between t w o p o t e n t population stereotypes, a frequency-space stereotype, a n d a t e n d e n c y t o respond t o w a r d t h e source of stimulation.

19. T w o d i f f e r e n t paradigms were used t o c h a r t t h e time course o f t h e phenomenon and, also, t o localize i t s e f f e c t in relation t o t h e stages o f information processing. Results p r o v i d e d additional c o n v e r g i n g e v i dence t h a t t h e directional c u e affected response selection. 20. We developed a n d t e s t e d a b u f f e r model o f t h e response-select i o n process. Results s u p p o r t e d t h e notion t h a t response selection i n v o l v e d a serial s e l f - t e r m i n a t i n g search o f response b u f f e r s a n d t h a t t h e i r r e l e v a n t directional c u e determined t h e b u f f e r searched f i r s t . 21. We e x p l o r e d a n d rejected t h e suggestion that, u n d e r incomp a t i b l e S-R mapping i n s t r u c t i o n s , subjects m i g h t recode (i.e., reverse) t h e i r r e l e v a n t location cue a n d respond f a s t e r o n t h e side opposite t h e location o f t h e stimulus. O u r r e s u l t s indicated t h a t a confounded factor, d i s p l a y - c o n t r o l arrangement correspondence, p r o v i d e d a more parsimonious explanation for t h e a p p a r e n t r e v e r s a l o f t h e Simon e f f e c t . 2 2 . We have used t h e stereotype as a tool i n combination w i t h S t e r n b e r g ' s a d d i t i v e f a c t o r s logic to s t u d y performance changes w i t h a g i n g a n d t o examine information processing u n d e r dual-tasks conditions.

23. We suggested t h a t d i s p l a y designers attempt t o avoid i n t r o d u c i n g i r r e l e v a n t directional cues t h a t i n t e r f e r e w i t h information processing. B u t can designers f a c i l i t a t e information processing by making t h e directional cue relevant? We f o u n d t h a t p r o v i d i n g r e d u n d a n t information in a d i s p l a y may, u n d e r c e r t a i n conditions, f a c i l i t a t e information processing, but t h e e f f e c t is so small as t o b e o f d o u b t f u l p r a c t i c a l significance. 24. O u r f i n d i n g s have important methodological implications f o r researchers ex p l o r i ng hemispheric domi na nce effects, because ir r e l e v a nt directional cues may b e confounded w i t h t h e variable b e i n g manipulated. 25. We suggest t h a t t h e t e n d e n c y f o r humans t o react t o t h e source o f stimulation may b e analogous t o t h e t a x i s o r d i r e c t e d orientation react i o n s o f lower animals.


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P A R T II MENTAL REPRESENTATION T h e f o u r chapters o f P a r t II p r o v i d e evidence f o r c o g n i t i v e coding as a major f a c t o r in stimulus-response compatibility. A l l o f t h e chapters address aspects o f spatial-compatibility, in w h i c h spatial-location stimuli a r e assigned t o d i s c r e t e k e y p r e s s responses a t d i s t i n c t locations. C h a p t e r 3, by U m i l t i a n d Nicoletti, is organized a r o u n d a c u r r e n t t a x onomy o f compatibility effects. T h e c h a p t e r evaluates t h e roles o f egocentric a n d r e l a t i v e spatial dimensions f o r stimuli a n d responses, b o t h when stimulus locations a r e r e l e v a n t t o t h e t a s k a n d when t h e y a r e irrelevant. A l t e r n a t i v e hypotheses r e g a r d i n g t h e n a t u r e o f t h e effects a r e contrasted. C h a p t e r 4, by Heister, Schroeder-Heister, and Ehrenstein, proposes a hierarchical model in w h i c h spatial coding a n d spatio-anatomical mapping p l a y roles. C h a p t e r 5, by Lidavas, presents a similar hierarchical model a n d p r o v i d e s evidence t h a t p r e - v e r b a l c h i l d r e n show spatial-compatibility effects. C h a p t e r 6, by Reeve a n d Proctor, extends t h e investigation o f spatial c o d i n g t o more complex, four-choice It also demonstrates how compatibility effects can arise p r e c u i n g tasks. f r o m symbolic stimuli assigned t o keypress responses.


STIMULUS-RESPONSE COMPATIBILITY R. W. Proctor and T.G. Reeve (Editors

0 Elsevier Science Publishers B. V. (Idoiih-Holland), 1990

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S P A T I A L STIMULUS-RESPONSE C O M P A T I B I L I T Y CARLO U M I L T ~ l s t i t u t o di Fisologia Umana U n i v e r s i t a di Parma, I t a l y ROBERTO N l C O L E T T l Dipartimento di Psicologia Generale U n i v e r s i t a di Padova, I t a l y

F i t t s (1951) observed t h a t people, if allowed t o choose one response t o a stimulus among a set o f possible responses, were consist e n t in t h e i r choice. T o describe t h e phenomenon h e used t h e t e r m "population stereotype," meaning t h a t f o r most people t h e r e is an o p t i mal p a i r i n g o f t h e elements o f t h e stimulus set w i t h t h e elements o f t h e response set. T h e tasks t h a t conform t o such population stereotypes a r e easier t o perform, i n terms o f b o t h speed a n d accuracy, t h a n those t h a t d o n o t conform t o them. In subsequent studies ( F i t t s & Deininger, 1954; F i t t s & Seeger, 1953), performance in these t a s k s was shown n o t t o depend o n t h e p a r t i c u l a r sets o f stimuli a n d responses t h a t were used, but on t h e way in which i n d i v i d u a l stimuli a n d responses were p a i r e d w i t h each o t h e r . In o t h e r words, t h e t y p e o f stimulus-response (S-R) p a i r i n g adopted in t h e t a s k determined t h e speed a n d accuracy w i t h which t h e t a s k was executed. T h e t e r m "population stereotype" r e f e r r e d t o t h e f a c t that, g i v e n t h e occurrence o f a c e r t a i n element o f t h e stimulus set, subjects showed a consistent tendency t o select a p a r t i c u l a r element o f t h e response set. I n contrast, t h e t e r m "stimulus-response compatibility," w h i c h l a t e r became popular, r e f e r s t o t h e f a c t t h a t some S-R p a i r i n g s a r e easier t o use t h a n o t h e r s . F o r example, i n choice reaction time ( R T ) tasks, S - R p a i r i n g s t h a t y i e l d s h o r t e r latencies a n d lower e r r o r rates a r e said t o b e more compatible t h a n S-R p a i r i n g s t h a t y i e l d longer latencies a n d h i g h e r e r r o r rates. In s p i t e o f t h e p o p u l a r i t y enjoyed by t h e t e r m S - R compatibility, i t s use does n o t indicate a b e t t e r u n d e r s t a n d i n g o f t h e phenomenon t h a n t h e use o f t h e o l d e r t e r m "population stereotype." In fact, b o t h terms a r e merely d e s c r i p t i v e . Population stereotype describes t h e p r o b a b i l i t y w i t h w h i c h a response is chosen, whereas S-R compatibility describes t h e speed a n d accuracy w i t h w h i c h a response is emitted. Yet, g i v e n a c e r t a i n stimulus, t h e most probable response is also t h e fastest a n d most accurate one (Nicoletti E Umilt6, 1984). A Taxonomy of S-R Compatibility V e r y likely, t h e vagueness of t h e t e r m S-R compatibility, coupled w i t h t h e false impression t h a t t h e t e r m possesses an e x p l a n a t o r y value,

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r e n d e r e d it so p o p u l a r as t o b e employed t o describe a number o f a p p a r e n t l y dissimilar phenomena. Simon, S l y a n d Vilapakkam (1981; see also Hedge & Marsh, 1975) d i s t i n g u i s h e d t h r e e t y p e s o f S-R compatibility. T h e f i r s t was "symbolic compatibility," w h i c h r e s u l t s f r o m t h e v e r b a l labels associated w i t h t h e stimulus a n d t h e response. T h e seco n d was "spatial compatibility," w h i c h occurs when t h e position o f t h e stimulus signals t h e position o f t h e r e q u i r e d response. T h e third was t h e "Simon effect," w h i c h occurs when t h e position o f t h e stimulus, t h o u g h i r r e l e v a n t , biases t h e selection o f t h e response. In our previous w o r k (see, e.g., Nicoletti & U m i l t i , 1984; Umilta & Nicoletti, 1985) we made use o f t h i s taxonomy. However, v e r y recently, Kornblum, Hasbroucq, a n d Osman (in press) succeeded in p r o v i d i n g a much more comprehensive taxonomy, w h i c h w i l l t h u s b e adopted i n t h i s c h a p t e r . Pivotal t o K o r n b l u m e t a l . ' s (in press) taxonomy i s t h e notion of dimensional o v e r l a p o f t h e S - R ensemble, according t o w h i c h t h e stimul u s set may o r may n o t share categorical a t t r i b u t e s w i t h t h e response set. T o t h e e x t e n t t h a t it does, it generates v a r y i n g degrees o f dimensional o v e r l a p in t h e S-R ensemble. Note t h a t t h e notion o f dimensional o v e r l a p i s n o t r e s t r i c t e d t o t h e p h y s i c a l a t t r i b u t e s o f stimuli a n d responses but extends also t o t h e i r mental representations. A n o t h e r important aspect o f t h e taxonomy is t h e d i s t i n c t i o n between r e l e v a n t a n d i r r e l e v a n t dimensions o f t h e stimulus set. A stimulus dimension is r e l e v a n t when t h e r e q u i r e d response depends on t h e value o f t h e stimulus in t h a t dimension, whereas a stimulus dimension is i r r e l e v a n t if values on it a r e uncorrelated w i t h t h e r e q u i r e d response. B y combining dimensional o v e r l a p w i t h dimensional relevancy, K o r n b l u m e t a l . (in press) o b t a i n f o u r t y p e s o f S-R ensembles. Most of t h e following examples a r e taken f r o m t h e i r p a p e r . Examples of spatial S - R ensembles, more p e r t i n e n t t o t h e purposes o f t h e p r e s e n t chapter, w i l l b e p r o v i d e d in subsequent sections.

Type I Ensembles T h i s t y p e o f ensemble occurs when t h e r e is no dimensional o v e r l a p in e i t h e r t h e r e l e v a n t o r t h e i r r e l e v a n t dimension. T h a t is t o say, i n it t h e stimulus set does n o t share a n y categorical a t t r i b u t e s w i t h t h e response set. Because o f this, a n y S - R p a i r i n g is as easy as a n y o t h e r p a i r i n g , a n d t h e v e r y basis of S-R compatibility effects is l a c k i n g in t h e t a s k . A good example o f T y p e I ensemble is t h a t o f a condition i n F i t t s a n d Deininger's (1954) s t u d y i n w h i c h movements (response set) were p a i r e d w i t h p r o p e r names (stimulus set). Not s u r p r i s i n g l y , all S - R p a i r i n g s p r o v e d e q u a l l y good.

It is perhaps w o r t h n o t i n g t h a t T y p e I ensembles can b e more d i f f i c u l t t o find t h a n one would t h i n k . For instance, L i d a v a s (1987) f o u n d an e f f e c t o f S - R p a i r i n g in a t a s k i n which t h e subjects were r e q u i r e d to p r e s s t h e l e f t o r right b u t t o n in response t o a light shown above or below t h e f i x a t i o n p o i n t . H e r r i g h t - h a n d e d subjects did b e t t e r when t h e l e f t b u t t o n was p a i r e d w i t h t h e stimulus located below t h e f i x a t i o n point, a n d t h e right b u t t o n was p a i r e d w i t h t h e stimulus located above it, t h a n when t h e p a i r i n g s were reversed. T h a t means t h a t t h i s S - R ensemble showed an unexpected dimensional overlap.

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T y p e I I Ensembles T h i s t y p e o f ensemble is characterized by t h e presence o f dimensional o v e r l a p in t h e r e l e v a n t dimension. Therefore, it satisfies t h e condition f o r o b t a i n i n g S-R compatibility effects. Most o f t h e studies w i t h T y p e I I ensembles use spatial stimulus a n d response dimensions: A light is p r e s e n t e d in t h e l e f t o r i n t h e right visual f i e l d a n d t h e subject is r e q u i r e d t o p r e s s e i t h e r t h e l e f t o r t h e right k e y in response. R T is f a s t e r when t h e imperative stimulus a n d t h e response a r e on t h e same side t h a n when t h e y a r e on opposite sides (see, e.g., Nicoletti, Anzola, Luppino, Rizzolatti, & UmiltS, 1982; Simon, 1969). Similar spatial S-R compatibility effects a r e observed when t h e light stimuli a r e p r e s e n t e d in t h e u p p e r o r i n t h e lower v i s u a l f i e l d a n d t h e response keys a r e located one above t h e o t h e r (see, e.g., Nicoletti & Umilta, 1984, 1985). A c c o r d i n g t o o u r p r e v i o u s terminology, these effects i n v o l v e d "spatial compatibility. "

Also, nonspatial T y p e 1 1 ensembles p r o d u c e S-R compatibility effects. F o r instance, S t e r n b e r g (1969) a n d Blackman (1975) i n s t r u c t e d t h e i r subjects t o respond t o d i g i t s w i t h t h e name o f t h e digit o r w i t h Similarly, Sanders (1970) used vowels as t h e name of t h e n e x t digit. stimuli a n d t h e name o f t h e vowel o r t h a t o f t h e n e x t vowel in t h e response. I n all cases, t h e r e were c l e a r - c u t S-R compatibility effects, in t h e sense t h a t t h e name o f t h e stimulus gave f a s t e r R T s t h a n t h e In t h e o l d taxonomy, these effects were called "symbolic n e x t name. compatibility . " T y p e I l l Ensembles

In t h i s t y p e o f ensemble, t h e relevant stimulus dimension has no o v e r l a p w i t h a n y o f t h e response dimensions, whereas t h e i r r e l e v a n t dimension does. T y p i c a l l y , subjects a r e presented w i t h t w o geometric f i g u r e s o r t w o color l i g h t s a n d a r e i n s t r u c t e d t o p r e s s t h e l e f t k e y i n response t o one f i g u r e o r color a n d t h e r i g h t k e y i n response t o t h e o t h e r f i g u r e o r color. T h e stimuli a r e randomly shown in t h e l e f t o r right side o f t h e d i s p l a y . Despite t h e f a c t t h a t side is i r r e l e v a n t , R T is f a s t e r when t h e side o f t h e response k e y corresponds t o t h e side where t h e stimulus appears t h a n when stimulus a n d response o c c u r on opposite sides (see, e . g . , U m i l t i & Nicoletti, 1985; Wallace, 1971). Simon a n d Small (1969) obtained similar r e s u l t s i n t h e a u d i t o r y modality when t h e subjects h a d t o p r e s s t h e l e f t o r right k e y i n response t o h i g h o r low p i t c h e d tones, randomly d e l i v e r e d t o t h e l e f t o r right ear. Side o f stimulated ear was i r r e l e v a n t , b u t R T was f a s t e r when t h e side of stimulation corresponded t o t h e side of t h e response t h a n when it did not. B y following t h e o l d taxonomy, these effects would have been termed "Simon effects. " It is i n t e r e s t i n g t o note t h a t in all studies t h a t showed S-R comp a t i b i l i t y effects w i t h T y p e I I I ensembles, t h e i r r e l e v a n t dimension was spatial. T h a t a p p a r e n t l y means t h a t correspondence i n space is a p a r t i c u l a r l y p o w e r f u l way o f o b t a i n i n g an o v e r l a p between stimulus set a n d response set.

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92 T y p e I V Ensembles

In t h i s t y p e o f ensemble, t h e stimulus a n d response sets overlap' o n b o t h a r e l e v a n t a n d an i r r e l e v a n t dimension. When t h e relevant a n d t h e i r r e l e v a n t dimensions a r e d i f f e r e n t , t h e ensemble is characterized as T y p e I V - A , a n d when t h e t w o dimensions a r e equal, t h e ensemble is characterized as T y p e IV-€3. A n example o f a T y p e I V - A ensemble can b e f o u n d in a s t u d y by Hedge a n d Marsh (1975). I n t h e i r s t u d y t h e subjects performed a choice R T t a s k w i t h r e d o r g r e e n l i g h t s appearing t o t h e l e f t o r right o f a f i x a t i o n p o i n t . T h e responses consisted o f p r e s s i n g r e d a n d g r e e n k e y s positioned t o t h e l e f t o r right o f t h e b o d y midline. Color was t h e r e l e v a n t dimension f o r mapping stimuli o n t o responses, whereas side was t h e i r r e l e v a n t dimension. S - R compatibility effects a t t r i b u t a b l e t o b o t h dimensions were f o u n d . Examples o f T y p e I V - B ensembles a r e those t y p i c a l l y employed i n S t r o o p - t y p e t a s k s (see, e.g., reviews in D y e r , 1973, a n d in Jensen & Rohwer, 1966) f o r w h i c h t h e r e l e v a n t dimension i s t h e color o f t h e stimulus a n d t h e i r r e l e v a n t dimension is t h e meaning o f t h e stimulus (i.e., t h e w o r d "green" w r i t t e n in green o r r e d ink). A somewhat d i f f e r e n t example o f a T y p e IV-8 ensemble can b e f o u n d i n a s t u d y by Simon, Mewaldt, Acosta, a n d H u (1976b), i n w h i c h subjects were i n s t r u c t e d t o move a t o g g l e s w i t c h up i n response t o a h i g h - p i t c h e d tone a n d down i n response t o a low-pitched tone. T h e tones were emitted randomly by t w o loudspeakers positioned one above t h e o t h e r . Frequency o f t h e sound p r o v i d e d t h e r e l e v a n t dimension a n d position o f t h e loudspeaker p r o v i d e d t h e i r r e l e v a n t dimension. Simon e t al. showed t h a t b o t h dimensions p r o d u c e d S-R compatibility effects. Summary T h e taxonomy i l l u s t r a t e d above is meant t o encompass all t y p e s of S - R compatibility effects a n d w i l l b e applied h e r e t o t h e special case o f spatial S-R compatibility. T h e general p l a n o f t h e c h a p t e r is t h e following. F i r s t , t w o sections a r e devoted t o discussing spatial stimulus a n d response dimensions t h a t o v e r l a p t o p r o d u c e S - R compatibility effects. Then, in t h e subsequent section, we i l l u s t r a t e a phenomenon, namely, t h e dominance o f t h e l e f t - r i g h t o v e r t h e above-below dimension, which can shed light on how t h e stimulus dimensions a r e processed. Finally, in t h e last section, e x p l a n a t o r y hypotheses o f spatial S-R comp a t i b i l i t y a r e t a k e n i n t o consideration. Egocentric a n d Relative Stimulus Dimensions T h e v i s u a l choice R T studies t h a t w i l l b e considered in t h i s sect i o n employed ensembles o f T y p e I I , T y p e I l l , a n d T y p e IV. In all cases t h e o v e r l a p p i n g dimension f o r t h e stimulus a n d response sets was spatial, a n d i t s values were specified in terms of l e f t - r i g h t positions. In o t h e r words, t h e response set a n d t h e stimulus set always were comp r i s e d o f t w o elements, namely, a l e f t a n d a r i g h t position.


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When t h e stimulus set comprises a l e f t and a r i g h t element, l e f t and r i g h t values can be specified along two dimensions, one egocentric and t h e other relative. I n t h e case of t h e egocentric dimension, t h e stimulus can be on either t h e l e f t o r r i g h t side i n relation t o an egocentric reference axis, such as t h e body midline, t h e head midline, o r t h e vertical retinal meridian. (Note t h a t if t h e observer fixates a point without t u r n i n g t h e head o r t h e body, t h e three axes are aligned and t h e stimulus is on t h e l e f t o r r i g h t side in relation t o each axis). In t h e case of t h e relative dimension, t h e stimulus can be on either t h e l e f t o r right position i n relation t o an external reference point, such as t h e other stimulus. From now on we will use t h e term side" t o mean t h e position in relation t o one o r more of t h e egocentric axes (which are assumed t o be aligned) and t h e term "relative position" t o mean t h e position i n relation t o an external reference point (usually, t h e other stimulus). The two dimensions cannot be distinguished if t h e two stimuli are shown t o t h e l e f t o r r i g h t of t h e egocentric axes: The stimulus on t h e l e f t side occupies t h e l e f t relative position, and t h e stimulus on t h e Consider, however, a r i g h t side occupies t h e right relative position. display such as t h a t shown in Figure 1 . The stimulus can appear in two positions only (i.e., those marked by the two boxes), and both positions are on t h e r i g h t side. Side and relative position are

LVF

RVF

r Figure 1. Schematic diagram of stimulus display and response keys. I n t h e delay condition t h e two boxes preceded the stimulus by 500 ms. (RVF, r i g h t visual field o r r i g h t side; LVF, l e f t visual f i e l d o r l e f t side).

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unconfounded a t t h e box near t h e f i x a t i o n point, which is on t h e right side but occupies t h e l e f t r e l a t i v e position. Side a n d r e l a t i v e position a r e unconfounded also a t t h e box closer t o f i x a t i o n o n t h e l e f t side ( n o t shown i n F i g u r e 1). O f course, t h e d i s t i n c t i o n between t h e egocentric a n d t h e r e l a t i v e dimensions holds f o r t h e response set as well. T h a t is, f o r t h e positions o f t h e responses, t h e r e is also a l e f t o r right side a n d a l e f t o r right r e l a t i v e position. Side a n d position can b e p a r t i a l l y unconfounded by p o s i t i o n i n g t h e t w o responses on t h e same side in relation t o t h e egocentric axes. T h e issue o f response dimensions w i l l Here we w i l l confine b e dealt w i t h separately in t h e n e x t section. ourselves t o discussing stimulus dimensions, a n d t h e experiments t a k e n i n t o consideration were aimed a t d e t e r m i n i n g t h e role o f t h e t w o t y p e s o f l e f t - r i g h t dimension in p r o d u c i n g spatial S-R compatibility effects.

Left-Right Stimulus Dimensions for T y p e II Ensembles In t h e case o f T y p e I I ensembles, t h e dimensional o v e r l a p occurs in t h e r e l e v a n t dimension, which must b e processed intentionally because it is t h e value o f t h e stimulus i n t h a t dimension t h a t allows t h e selection o f t h e c o r r e c t response, ( i . e . , t h e c o r r e c t value in t h e response set). F o r example, if t h e stimulus is on t h e right side, t h e n t h e right response (compatible S-R p a i r i n g ) o r t h e l e f t response (incompatible S-R p a i r i n g ) must b e selected. I n terms o f t h e controlled-automatic dichotomy o f information-processing modes (see, e . g . , Schneider, Dumais, & S h i f f r i n , 1984; S h i f f r i n & Schneider, 19771, it can b e said t h a t in T y p e II ensembles t h e l e f t - r i g h t dimension u n d e r goes c o n t r o l l e d processing, w h i c h is comparatively slow, serial, e f f o r t ful, capacity-limited, subject t o interference, a n d can b e stopped. Note t h a t by s a y i n g t h a t in T y p e II ensembles t h e relevant dimension undergoes controlled processing, we d o n o t mean t o imply t h a t t h e position o f t h e stimulus a n d t h e position o f t h e response a r e encoded t h r o u g h a controlled process. We simply want t o stress t h e f a c t t h a t S-R p a i r i n g s a r e formed i n t e n t i o n a l l y . In most studies t h a t employed spatial T y p e I 1 ensembles, side a n d r e l a t i v e position were confounded, because t h e t w o stimuli were shown t o t h e l e f t o r r i g h t o f t h e f i x a t i o n p o i n t (see, e.g., Anzola, Bertoloni, Buchtel, & Rizzolatti, 1977; Brebner, Shepard, & Cairney, 1972; Simon, 1969). Therefore, these studies a r e uninformative about which dimension caused t h e S-R compatibility effects. Nicoletti e t al. (1982) a n d Umilt6 a n d L i o t t i (1987, Experiment 1) attempted t o show S - R compatibility effects p u r e l y a t t r i b u t a b l e t o t h e processing o f r e l a t i v e position. In these studies, t h e subjects made unimanual, discriminative, k e y p r e s s i n g responses t o t w o v i s u a l stimuli, b o t h shown t o e i t h e r t h e l e f t o r right o f t h e f i x a t i o n p o i n t (see F i g u r e 1 f o r an example o f such d i s plays). More .precisely, in t h e compatible condition t h e subject was i n s t r u c t e d t o p r e s s t h e l e f t k e y if t h e stimulus was shown i n t h e l e f t r e l a t i v e position (i.e., t h e position closer t o f i x a t i o n on t h e right side a n d t h e more p e r i p h e r a l position in t h e l e f t side) a n d t o p r e s s t h e right k e y if t h e stimulus was shown in t h e right r e l a t i v e position (i.e., t h e position closer t o f i x a t i o n on t h e l e f t side a n d t h e more p e r i p h e r a l posit i o n on t h e right side). In t h e incompatible condition, t h e assignment


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was reversed, so t h a t t h e l e f t k e y was used i n response t o t h e stimulus in t h e right r e l a t i v e position a n d t h e right k e y was used in response t o t h e stimulus in t h e l e f t r e l a t i v e position. In b r i e f , r e l a t i v e position o f t h e stimulus signaled t h e position o f t h e c o r r e c t response, whereas side was i r r e l e v a n t . T h e r e were S - R compatibility effects d u e t o t h e relat i v e dimension, whereas no effects a t t r i b u t a b l e t o t h e egocentric dimension c o u l d b e f o u n d . T h e r e s u l t s showed, therefore, t h a t S-R compatibility effects o c c u r o n l y f o r t h e dimension t h a t is submitted t o cont r o l l e d processing ( i . e . , t h e r e l a t i v e one in t h i s case). T o t h e b e s t o f o u r knowledge, t h e o n l y s t u d y t h a t attempted t o demonstrate S - R compatibility effects p u r e l y a t t r i b u t a b l e t o t h e processing o f t h e egocentric dimension, by u n c o n f o u n d i n g it f r o m t h e r e l a t i v e In dimension, was done by Umilt5 a n d L i o t t i (1987, Experiment 2 ) . t h e i r stimulus d i s p l a y (see F i g u r e 21, f o u r positions were m a r k e d by empty boxes, t w o on each side o f f i x a t i o n . T h e t w o boxes w i t h b r o k e n contours h a d t h e o n l y p u r p o s e o f m a r k i n g t h e r e l a t i v e position o f t h e stimulus, while t h e stimulus i t s e l f was always shown w i t h i n one o f t w o boxes w i t h continuous contours. In t h e compatible condition t h e subject pressed t h e l e f t k e y if t h e stimulus appeared on t h e l e f t side (i.e., w i t h i n t h e box w i t h continuous contours t o t h e l e f t o f f i x a t i o n ) a n d t h e right k e y if t h e stimulus appeared on t h e r i g h t side (i.e., within the

LVF I I I

RVF

I I

r

B

F i g u r e 2. Schematic diagram o f stimulus d i s p l a y a n d response k e y s . I n t h e delay condition t h e f o u r boxes preceded t h e stimulus by 500 ms. T h e stimulus was always shown in one o f t h e solid boxes. T h e dashed boxes marked t h e r e l a t i v e positions.

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box w i t h continuous contours t o t h e l e f t o f f i x a t i o n ) . T h e assignment was r e v e r s e d in t h e incompatible condition, so t h a t t h e response was t o b e emitted w i t h t h e l e f t k e y t o stimuli shown on t h e r i g h t side a n d w i t h t h e right k e y t o stimuli shown o n t h e l e f t side. Because t h e i n s t r u c t i o n s were g i v e n in terms o f l e f t a n d right sides, t h e egocentric dimension was relevant, whereas t h e r e l a t i v e dimension was i r r e l e v a n t . T h e r e were S-R compatibility effects d u e t o t h e egocentric dimension, whereas effects a t t r i b u t a b l e t o t h e r e l a t i v e dimension were absent. I n o t h e r words, it was confirmed t h a t S - R compatibility effects o c c u r r e d now t h e egocentric one, whose elements u n d e r o n l y in t h e dimension, went c o n t r o l l e d processing. It c o u l d b e a r g u e d t h a t t h e experimental conditions j u s t described come close t o making a T y p e I V - A o r a T y p e IV-B ensemble. T h i s is because, by unconfounding side a n d r e l a t i v e position, t w o stimulus dimensions a r e made available t h a t both can o v e r l a p w i t h t h e response dimension because t h e y b o t h comprise l e f t - r i g h t elements. Which dimension is r e l e v a n t a n d w h i c h is i r r e l e v a n t depends on w h e t h e r t h e subject i s i n s t r u c t e d t o process side o r r e l a t i v e position. It remains t o b e seen w h e t h e r these t w o dimensions a r e equal ( T y p e IV-B) o r different (Type IV-A). However, it is apparent t h a t e i t h e r dimension ceases t o h a v e any effects when it becomes i r r e l e v a n t a n d does n o t und e r g o controlled processing. Thus, these studies approximate but d o n o t q u i t e make T y p e IV ensembles in which, by definition, b o t h dimensions must p r o d u c e S-R compatibility effects.

L e f t - R i g h t Stimulus Dimensions f o r T y p e II 1 Ensembles B y u s i n g t h i s t y p e o f ensemble it can b e demonstrated t h a t spat i a l S-R compatibility effects a r e n o t r e s t r i c t e d t o t h e r e l e v a n t stimulus dimension but a r e also obtained w i t h an i r r e l e v a n t stimulus dimension, p r o v i d e d t h a t t h e i r r e l e v a n t dimension overlaps w i t h t h e response dimension. T h e r e l e v a n t stimulus dimension can b e a nonspatial f e a t u r e o f t h e stimulus, such as color, w h i c h signals t h e l e f t - r i g h t value o f t h e c o r r e c t element in t h e response set. For example, if t h e light is red, t h e n t h e l e f t response must b e selected; if t h e light is green, t h e n t h e right response must b e selected. T h e lights, however, appear randomly on e i t h e r t h e l e f t o r right side, a n d t h u s an i r r e l e v a n t stimulus dimension overlaps w i t h t h e response dimension. T h a t t h i s dimensional o v e r l a p is e f f e c t i v e is shown by t h e f a c t t h a t t h e r e d light is responded t o f a s t e r when it is presented on t h e l e f t t h a n t h e r i g h t side (compatible a n d incompatible condition, respectively) a n d v i c e versa f o r t h e g r e e n light. T o explain t h e e f f e c t o f t h e i r r e l e v a n t dimension, it seems necess a r y t o assume t h a t t h i s dimension is processed automatically, t h a t is, t h a t i t s processing takes place u n i n t e n t i o n a l l y a n d cannot b e bypassed. B y definition, automatic processing i s fast, parallel, r e l a t i v e l y immune t o interference, a n d f a i r l y effortless; it i s n o t limited by s h o r t - t e r m memory capacity and, perhaps, cannot b e stopped (see, e.g., Schneider e t al., 1984; S h i f f r i n & Schneider, 1977).

Also, with Type

as is t h e case f o r T y p e I I ensembles, in most of t h e studies I l l ensembles t h e t w o stimuli were shown t o t h e l e f t a n d


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right o f fixation, a n d t h u s side a n d r e l a t i v e position w e r e confounded (see, e.g., Simon, 1969; Wallace, 1971, 1972). T h e r e are, however, a few studies t h a t allow independent determination o f t h e r o l e o f t h e egoc e n t r i c a n d t h e r e l a t i v e dimensions w i t h T y p e I l l ensembles. I n a s t u d y by Umilts a n d Nicoletti (1985, Experiments 2 a n d 4), t h e r e l e v a n t stimulus dimension was color, a n d processing a stimulus position was n o t necessary f o r choosing t h e c o r r e c t response. In fact, subjects were i n s t r u c t e d t o p r e s s one b u t t o n f o r t h e g r e e n light a n d t h e o t h e r b u t t o n f o r t h e r e d light, regardless o f stimulus position. T h e t w o stimuli could appear i n t w o d i f f e r e n t positions but always w i t h i n t h e same visual f i e l d (as in F i g u r e I), t h u s u n c o n f o u n d i n g side a n d r e l a t i v e position. Umilt.5 a n d L i o t t i (1987, Experiment 3) did a similar experiment in w h i c h t h e relevant stimulus dimension was shape a n d t h e subject was r e q u i r e d t o make l e f t - r i g h t discriminative responses t o a square a n d a rectangle, i r r e s p e c t i v e of where t h e stimulus was shown. In t h e condition u n d e r consideration here, t w o boxes were randomly presented t o t h e l e f t o r right o f t h e f i x a t i o n m a r k (see F i g u r e 11, a n d then, a f t e r an i n t e r v a l o f 500 ms, t h e imperative stimulus ( t h e square o r t h e rectangle) appeared w i t h i n one b o x . Therefore, in t h e case of t h e d i s p l a y shown in F i g u r e 1, f o r instance, t h e subject knew 500 ms in advance t h a t t h e imperative stimulus was g o i n g t o appear o n t h e right side. What was u n c e r t a i n u n t i l stimulus onset was, besides i t s shape, w h e t h e r t h e stimulus was t o b e shown in t h e l e f t o r right r e l a t i v e position. Therefore, side a n d r e l a t i v e position were unconfounded. Note t h a t in b o t h studies t h e r e l e v a n t stimulus dimension (color o r shape) did n o t o v e r l a p w i t h t h e response dimension, whereas t h e i r r e l e v a n t stimulus dimension ( r e l a t i v e position) did. In b o t h studies t h e r e were S - R compatibility effects d u e t o t h e r e l a t i v e dimension a n d no effects a t t r i b u t a b l e t o t h e egocentric dimension. T h a t can mean t h a t o n l y r e l a t i v e position was automatically processed a n d p r o duced S - R compatibility effects.

Umilt6 a n d L i o t t i (1987, Experiment 4) also t r i e d t o show S-R compatibility effects a t t r i b u t a b l e t o t h e automatic processing o f t h e egoc e n t r i c dimension, n o t confounded w i t h t h e r e l a t i v e dimension. In one condition, t h e d i s p l a y o f F i g u r e 2 was shown and, a f t e r an i n t e r v a l of 500 ms, t h e imperative stimulus (again, a square o r a rectangle) appeared w i t h i n one o f t h e t w o boxes w i t h continuous contours. Therefore, i n t h e case o f t h e d i s p l a y shown i n F i g u r e 2, f o r example, t h e subject knew 500 ms i n advance t h a t t h e stimulus was g o i n g t o appear in t h e right r e l a t i v e position. What was u n c e r t a i n was w h e t h e r it was t o b e shown t o t h e l e f t o r right side o f f i x a t i o n . In t h i s experiment, too, t h e r e l e v a n t stimulus dimension (shape) did n o t o v e r lap w i t h t h e response dimension, whereas t h e i r r e l e v a n t dimension (side) did. T h e r e s u l t s showed S - R compatibility effects d u e t o t h e egocentric dimension a n d no effects d u e t o t h e r e l a t i v e dimension. It seems, thus, t h a t o n l y t h e egocentric dimension was automatically processed a n d p r o d u c e d S - R compatibility effects. It is w o r t h n o t i n g t h a t also these experiments could b e c o n s t r u e d as instances o f T y p e IV-A o r T y p e I V - B ensembles, depending on whether one considers t h e t w o dimensions t o b e d i f f e r e n t o r equal.

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Actually, t h e y would make r a t h e r special instances o f T y p e IV ensembles, because it i s t w o i r r e l e v a n t stimulus dimensions (egocentric a n d relative) that can o v e r l a p w i t h t h e response dimension, instead o f a r e l evant a n d an i r r e l e v a n t stimulus dimension, as is t y p i c a l o f T y p e IV ensembles. However, since one o f t h e t w o stimulus dimensions (i.e., t h e one t h a t becomes available p r i o r t o stimulus presentation), does n o t p r o d u c e S-R compatibility effects, we p r e f e r t o consider these as instances o f T y p e I l l ensembles. L e f t - R i g h t Stimulus Dimensions for T y p e I V Ensembles T h i s t y p e o f ensemble is characterized by t h e f a c t t h a t t h e r e is overlap between t h e stimulus a n d t h e response sets in b o t h t h e relevant and t h e i r r e l e v a n t dimensions. As already said, t h e experimental conditions described above, u n d e r t h e heading T y p e I I ensembles, could b e construed as belonging t o T y p e IV, in which one stimulus dimension was egocentric a n d t h e o t h e r relative. Depending on t h e instructions, t h e egocentric dimension was relevant a n d t h e r e l a t i v e dimension was irrelevant, o r v i c e versa. However, because t h e i r r e l e v a n t dimension did n o t produce S-R compatibility effects, we considered those t o b e Similarly, in t h e experimental conditions described T y p e II ensembles. u n d e r t h e heading T y p e I l l ensemble, t h e r e were t w o i r r e l e v a n t stimulus dimensions (again, one egocentric a n d t h e o t h e r relative) t h a t could overlap w i t h t h e response dimension. However, because o n l y one dimension, depending on t h e experiment, produced S-R compatibility effects, we considered those t o b e T y p e I l l ensembles. In t h e s t u d y by U m i l t i and L i o t t i (1987, Experiments 3 a n d 4) t h e r e were conditions t h a t were closer approximations o f T y p e IV ensembles. T h a t happened when t h e r e was n o delay between presentat i o n o f t h e boxes a n d presentation o f t h e imperative stimulus (see F i g ures 1 a n d 2 ) . Note that t h e o n l y d i f f e r e n c e w i t h respect t o t h e experimental conditions already discussed concerned t h e t i m i n g o f t h e display elements. More specifically, in these new experimental conditions, t h e information needed t o determine stimulus position i n t h e egoc e n t r i c dimension a n d in t h e r e l a t i v e dimension became available a t t h e same time. T h e results showed t h a t n e i t h e r t h e egocentric dimension n o r t h e r e l a t i v e dimension p r o d u c e d S - R compatibility effects. Rememb e r t h a t w i t h t h e delay o f 500 ms t h e r e were, instead, effects d u e t o e i t h e r t h e r e l a t i v e dimension (Experiment 3) o r t h e egocentric dimension (Experiment 4). U m i l t i a n d L i o t t i argued that, in t h e absence o f t h e delay, b o t h stimulus dimensions overlapped w i t h t h e response dimension, b u t t h e S-R compatibility effects t h e y produced cancelled each other out.

I t must b e r e i t e r a t e d t h a t t h e conditions w i t h o u t delay o f E x p e r i ments 3 a n d 4 o f U m i l t i and L i o t t i d o n o t fit exactly a T y p e IV ensemble. T h i s i s because t h e t w o stimulus dimensions t h a t overlapped w i t h t h e response dimension were b o t h i r r e l e v a n t (in fact, t h e relevant stimulus dimension was shape), r a t h e r than one relevant a n d t h e o t h e r irrelevant, as r e q u i r e d by a t r u e T y p e IV ensemble. It is also n o t clear w h e t h e r those conditions approximated a T y p e IV-B o r a T y p e IVA ensemble because it is d i f f i c u l t t o establish if t h e t w o stimulus dimensions were equal o r d i f f e r e n t . T h e important p o i n t here, however, is


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t h a t those experiments confirmed t h a t t h e subject can process automatically b o t h stimulus dimensions. T h i s p o i n t depends, o f course, on accepting t h e explanation o f f e r e d f o r t h e disappearance o f t h e S-R comp a t i b i l i t y effects in t h e absence o f t h e delay, namely, t h a t b o t h dimensions p r o d u c e d S-R compatibility effects t h a t cancelled each o t h e r o u t . T h i s p o i n t w i l l b e discussed i n t h e last section of t h e chapter, w h e r e i n t e r p r e t i v e models a r e t a k e n i n t o consideration. Egocentric a n d Relative Response Dimensions Anatomical Factors Before discussing t h e role o f egocentric a n d r e l a t i v e response dimensions, a p r e l i m i n a r y issue concerning possible anatomical f a c t o r s in spatial S-R compatibility must b e c l a r i f i e d . T h e studies considered so f a r have a l l employed choice R T tasks. However, t h e time t a k e n t o react manually t o a lateralized v i s u a l stimulus depends on t h e relation between t h e stimulus a n d t h e responding h a n d in simple R T tasks, as well. T h e r e can b e l i t t l e d o u b t t h a t i n simple R T t a s k s t h e directness o f t h e anatomical connections between t h e stimulated hemiretinae a n d t h e responding h a n d plays an important role (Anzola e t al., 1977; Berlucchi, Crea, D i Stefano, & Tassinari. 1977; Berlucchi, Heron, Hyman, Rizzolatti, & UmiltB, 1971; see C h a p t e r 7, by Bashore). Responses made w i t h t h e h a n d ipsilateral t o a stimulus p r e s e n t e d t o t h e l e f t o r right v i s u a l f i e l d a r e f a s t e r t h a n those made w i t h t h e contralatera1 hand, because n e u r a l connections a r e s h o r t e r in t h e f o r m e r t h a n in t h e l a t t e r case: lpsilateral responses a r e mediated by n e u r a l c i r c u i t s located w i t h i n a single hemisphere, whereas contralateral responses r e q u i r e an e x t r a time f o r t r a n s f e r o f information f r o m one hemisphere t o t h e other. T h i s e x t r a time is on t h e o r d e r o f a few milliseconds, w i t h ipsilateral responses b e i n g about 2-3 ms f a s t e r t h a n contralateral responses (also see review in Bashore, 1981). B y contrast, in choice R T tasks t h e influence o f t h e anatomical c o n n e c t i v i t y becomes almost immaterial a n d spatial S-R compatibility emerges as t h e most important determinant o f speed o f response. Note also t h a t t h e d i f f e r e n c e in speed o f response between compatible a n d incompatible S-R p a i r i n g s i s on t h e o r d e r o f several t e n s o f milliseconds instead o f j u s t 2 o r 3 ms (Anzola e t al., 1977; Bradshaw & UmiltB, 1984). T h e d i f f e r e n t i a l r o l e played by anatomical connections a n d by S-R compatibility i n simple a n d choice R T t a s k s was c l a r i f i e d by those experiments in which t h e subjects e i t h e r k e p t t h e i r hands in anatomical position o r crossed them so t h a t t h e l e f t h a n d was o n t h e right side o f t h e b o d y a n d t h e right h a n d was on t h e l e f t side. I n simple R T tasks, regardless o f h a n d position, t h e r i g h t h a n d was a few milliseconds f a s t e r i n r e s p o n d i n g t o stimuli shown in t h e right visual f i e l d t h a n in t h e l e f t v i s u a l f i e l d a n d v i c e versa f o r t h e l e f t h a n d (Anzola e t al., 1977; B e r l u c c h i e t al., 1977; also see reviews i n Bashore, 1981, a n d in Bradshaw & Umilt5, 1984).

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A q u i t e d i f f e r e n t situation arose in choice R T tasks. When t h e subjects crossed t h e i r hands, t h e right h a n d responded f a s t e r t o t h e stimuli p r e s e n t e d t o t h e l e f t v i s u a l f i e l d a n d t h e l e f t h a n d responded f a s t e r t o t h e stimuli p r e s e n t e d t o t h e right v i s u a l f i e l d . T h e d i f f e r e n c e was in t h e o r d e r o f t e n s o f milliseconds (Anzola e t al., 1977; B r e b n e r e t al., 1972; Nicoletti, UmiltB, & LBdavas, 1984; Wallace, 1971, 1972; also see reviews in Bashore, 1981, a n d i n Bradshaw & Umiltti, 1984). T h i s finding cannot b e explained in terms o f f i x e d anatomical connect i o n s a n d shows t h a t in choice R T t a s k s what matters is t h e relation between position o f t h e stimulus a n d position o f t h e e f f e c t o r . Crossed-Hand Effects Once established that, in choice R T tasks, o n l y t h e position o f t h e e f f e c t o r p l a y s a r o l e in S-R compatibility, it becomes i n t e r e s t i n g t o ask w h e t h e r t h e response dimension t h a t produces those compatibility effects i s egocentric o r relative. O f course, t h i s is t h e same question we asked b e f o r e f o r t h e stimulus dimension. Also in t h e case o f t h e effectors, egocentric dimension is d e f i n e d w i t h reference t o an egocent r i c axis, such as t h e b o d y a n d / o r t h e head midlines, whereas r e l a t i v e dimension is d e f i n e d in relation t o an e x t e r n a l reference point, such as t h e other effector. T h e question o f t h e r e l a t i v e importance o f these t w o response dimensions c o u l d n o t b e asked in most o f t h e studies about spatial S-R compatibility because t h e t w o e f f e c t o r s were positioned on opposite sides in relation t o t h e b o d y midline, a n d t h u s side a n d r e l a t i v e position were confounded. As a l r e a d y p o i n t e d out, t h e t w o dimensions can b e unconfounded, a t least p a r t i a l l y , by p o s i t i o n i n g t h e t w o effectors on t h e same side o f t h e b o d y midline. T h i s was done by Nicoletti e t al. (1982, Experiment 2; 1984, Experiments 2 a n d 31, w h o showed S - R compatibility effects a t t r i b u t a b l e t o t h e r e l a t i v e position o f t h e effectors. When subjects cross t h e i r hands so t h a t t h e right h a n d is located t o t h e l e f t o f t h e l e f t h a n d a n d t h e l e f t h a n d is located t o t h e right o f t h e right hand, spatial S-R compatibility a p p a r e n t l y reverses i t s d i r e c tion. R T becomes f a s t e r when t h e l e f t stimulus commands a response w i t h t h e e f f e c t o r located t o t h e left, even t h o u g h t h i s is t h e r i g h t hand, a n d v i c e versa f o r t h e right stimulus. As a l r e a d y p o i n t e d out, such a p p a r e n t reversal is i m p o r t a n t because it disproves a n y i n t e r p r e t a t i o n of spatial S - R compatibility effects i n terms o f anatomical connect i o n s a n d u n d e r l i n e s t h e importance o f t h e spatial relation between stimulus and effector. T h e f a c t t h a t such apparent reversal in t h e d i r e c t i o n o f S - R compatibility occurs also if t h e t w o hands a r e positioned on t h e same side o f t h e b o d y midline, namely, b o t h on t h e l e f t o r right side egocentrically (Nicoletti e t al., 1984, Experiments 2 a n d 3), can b e t a k e n as f u r t h e r evidence o f t h e r o l e o f t h e r e l a t i v e response dimension in p r o d u c i n g S - R compatibility effects. A n o t h e r i n t e r e s t i n g consequence caused by t h e c r o s s i n g hands i s t h a t R T becomes o v e r a l l longer t h a n when t h e hands anatomical (i.e., uncrossed) position (Anzola e t al., 1977; Greim, Mendicino, & Koening, 1979; Nicoletti e t al., 1982, Nicoletti e t al. (1984) showed t h a t t h e l e n g t h e n i n g o f R T w i t h

of the a r e in Klapp, 1984). hands


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crossed cannot b e a t t r i b u t e d t o bio-mechanical f a c t o r s (i.e., an a w k w a r d posture; see B e r l u c c h i e t al., 1977; Klapp e t al., 1979) but i s d u e t o a mismatch between t h e position o f t h e e f f e c t o r a n d i t s anatomical status. In o t h e r words, when, f o r example, t h e right h a n d becomes t h e l e f t effector, a c o n f l i c t arises between t w o representations, one positional a n d t h e o t h e r anatomical, o f t h e same e f f e c t o r . Because o f t h i s conflict, responses t o t h e l e f t stimulus a r e slower w i t h t h e crossed right h a n d t h a n w i t h t h e uncrossed l e f t h a n d (i.e., t h e crossed-hand effect). Remember, however, t h a t responses t o t h e l e f t stimulus a r e f a s t e r w i t h t h e crossed right h a n d t h a n w i t h t h e crossed l e f t h a n d (i.e., t h e a p p a r e n t r e v e r s a l o f S-R c o m p a t i b i l i t y ) . T h e explanation o f t h e crossed-hand e f f e c t in terms o f a c o n f l i c t between positional a n d anatomical representations o f t h e e f f e c t o r w i l l b e discussed in t h e last section. What is o f i n t e r e s t h e r e is that, also f o r t h e crossed-hand effect, one can ask w h e t h e r it is d u e t o t h e egocent r i c o r t h e r e l a t i v e dimension. T h a t is t o say, one can ask w h e t h e r t h e position o f t h e e f f e c t o r is determined w i t h reference t o t h e b o d y m i d l i n e o r t o t h e other effector. If w h a t matters is t h e egocentric dimension, t h e n t h e crossed-hand e f f e c t should o c c u r o n l y when t h e t w o hands a r e located on t h e contralateral sides across t h e b o d y midline. If w h a t matters is t h e r e l a t i v e dimension, t h e n t h e crossed-hand e f f e c t should o c c u r also when t h e t w o hands a r e crossed on t h e same side, t h a t is, when o n l y one o f them lies across t h e b o d y midline. In most o f t h e experiments t h a t employed a crossed-hand condition, t h e t w o dimensions were confounded because t h e subjects placed t h e i r hands by c r o s s i n g t h e b o d y midline, a n d t h u s each h a n d occupied t h e "wrong" position, b o t h egocentrically a n d r e l a t i v e l y . In t h e s t u d y by Nicoletti e t at. (1984), t h e t w o dimensions were unconfounded because t h e subjects k e p t t h e i r hands e i t h e r uncrossed o r crossed o n t h e same side o f t h e b o d y midline. It was f o u n d t h a t t h e crossed-hand e f f e c t depended on t h e r e l a t i v e position o f t h e effector. In conclusion, t h e r e s u l t s o f t h e f o r e g o i n g studies c o n v e r g e in showing that, also in t h e case o f t h e response set, t h e values o f t h e r e l e v a n t spatial dimension a r e specified w i t h reference t o t h e r e l a t i v e position o f t h e effectors. B y contrast, t h e role o f t h e egocentric response dimension in d e t e r m i n i n g t h e o v e r l a p w i t h t h e stimulus dimension has y e t t o b e demonstrated, i n t h e absence o f a c o n f o u n d i n g w i t h t h e r e l a t i v e dimension. Remember t h a t i n t h e case o f t h e stimulus set b o t h dimensions have, instead, p r o v e d e f f e c t i v e in p r o d u c i n g S-R comp a t i b i l i t y effects. Position o f t h e Response Goal So far, we have considered t h e response set by making reference t o t h e positions o f t h e effectors in general. However, t w o positional features of t h e response can b e distinguished, as suggested, f o r example, by G u i a r d (1984) a n d by Klapp e t al. (1979). One is t h e position of t h e e f f e c t o r (i.e., t h e h a n d i n t h e case o f a k e y - p r e s s i n g response) a n d t h e o t h e r is t h e position o f t h e response goal (i.e., t h e k e y ) . In spatial S-R compatibility studies, b o t h positional features have been t a k e n i n t o consideration. F o r instance, Wallace (1971, 1972) stressed

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t h e importance o f e f f e c t o r emphasized k e y position.

position,

whereas

Brebner et

al.

(1972)

A s t u d y by Riggio, Gawryszewski, a n d Umilta (1986) was aimed a t d i s e n t a n g l i n g t h e role o f t h e position of t h e e f f e c t o r a n d t h a t o f t h e response goal. In t h e i r Experiment 2, t h e hands were uncrossed but operated t w o sticks, which c o u l d b e e i t h e r uncrossed o r crossed a t t h e b o d y midline. In t h e uncrossed condition, t h e effectors ( i . e . , t h e t w o hands) a n d t h e response goals (i.e., t h e t w o keys) occupied t h e same position in b o t h egocentric (i.e., in relation t o t h e b o d y midline) a n d r e l a t i v e (i.e., in relation t o t h e o t h e r e f f e c t o r o r response goal) terms. I n t h e crossed condition, w h i c h was t h e c r i t i c a l one, t h e effectors a n d t h e response goals occupied opposite positions, again e i t h e r egocentrically o r r e l a t i v e l y . It was reasoned t h a t if t h e determining f a c t o r f o r S-R compatibility was t h e position o f t h e effector, responses t o stimuli presented on t h e side w h e r e t h e h a n d was located, but opposite t o t h e key, should b e f a s t e r t h a n those to stimuli presented on t h e o t h e r side Of w i t h respect t o t h e hand, but on t h e t h e same side o f t h e k e y . course, if t h e determining f a c t o r was t h e position o f t h e response goal, t h e n t h e p r e d i c t i o n would b e reversed. T h e r e s u l t s were unequivocal a n d demonstrated t h e importance o f t h e position o f t h e response goal. T h e position o f t h e e f f e c t o r h a d no influence. I n t h e crossed condition, t h e right hand, which was on t h e right side but operated t h e l e f t key, was f a s t e r f o r t h e l e f t t h a n t h e right stimulus, whereas t h e l e f t hand, which was on t h e l e f t side b u t operated t h e right key, was f a s t e r f o r t h e right t h a n t h e l e f t stimulus. T h i s r e s u l t is s u r p r i s i n g if one considers t h a t t h e subject c o u l d r e n d e r compatible (i.e., easier) t h e incompatible condition by coding t h e posit i o n o f t h e e f f e c t o r instead of t h a t o f t h e response goal. F o r example, in t h e incompatible condition, when t h e l e f t stimulus was shown, t h e subject h a d t o p r e s s t h e right k e y w i t h a movement performed by t h e l e f t hand, w h i c h was located o n t h e same side as t h e stimulus. Had t h e subject coded t h e position o f t h e hand, t h e S-R p a i r i n g would have become compatible because t h e l e f t stimulus would have been p a i r e d t o t h e left effector. However, t h e r e s u l t s clearly show t h a t S-R compatibility depended on t h e position of t h e response goal, which dominated o v e r t h e position o f t h e e f f e c t o r . I n t e r e s t i n g l y enough, also in t h i s experiment t h e responses w i t h t h e s t i c k s crossed were slower t h a n those w i t h t h e s t i c k s uncrossed, despite t h e f a c t t h a t t h e hands were always i n t h e anatomical uncrossed position a n d o n l y t h e s t i c k s t h e y h e l d crossed t h e midline t o reach t h e response k e y s . O f course, t h i s f i n d i n g rules o u t any possible explanat i o n i n terms o f bio-mechanical f a c t o r s .

A l l t h e studies reviewed i n t h i s section used T y p e I I ensembles. Thus, it can b e concluded that, f o r t h e response set, t h e relevant dimension o n .which t h e dimensional o v e r l a p o c c u r r e d was r e l a t i v e position of t h e response goal.


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T h e Dominance o f t h e L e f t - R i g h t Dimension O v e r t h e Above-Below Dimension T h e Phenomenon of L e f t - R i g h t Dominance

A series o f experiments conducted in o u r l a b o r a t o r y (Nicoletti & Umilta, 1984, 1985; Nicoletti, Umilta, Tressoldi, & Marzi, 1988) has e v i T h i s phenomenon i s t h e dominance denced an intriguing phenomenon. o f t h e l e f t - r i g h t dimension o v e r t h e above-below dimension in spatial S R compatibility. These experiments employed r a t h e r unusual instances o f T y p e I I ensembles, i n w h i c h t h e stimulus a n d response sets o v e r lapped in t w o r e l e v a n t spatial dimensions ( i . e . , above-below a n d l e f t right), instead o f one only, as happens w i t h r e g u l a r T y p e I I ensembles. T h a t is t o say, each element o f t h e stimulus set as well as each element o f t h e response set c o u l d unambiguously b e specified in e i t h e r t h e above-below o r t h e l e f t - r i g h t dimension. E i t h e r dimension was p e r f e c t l y p r e d i c t i v e o f t h e c o r r e c t S-R p a i r i n g . F o r example, t h e r e c o u l d b e t w o possible stimulus positions, f o r instance, t o p / r i g h t a n d bottom/left, and two response positions, again, top/right and bottom/left. T h e subject could, therefore, b e i n s t r u c t e d t o make use o f e i t h e r dimension f o r f o r m i n g t w o e x h a u s t i v e sets o f compatible a n d incompatible S-R p a i r i n g s w i t h t w o independent T y p e I I ensembles. When t h e i n s t r u c t i o n s were couched in above-below terms, t h e p a i r i n g s were t o p stimulus/top response, t o p stimulus/bottom response, bottom stimulus/bottom response, bottom stimulus/top response. When t h e i n s t r u c t i o n s were couched in l e f t - r i g h t terms, t h e p a i r i n g s were right s t i m u l u s / r i g h t response, r i g h t stimulus/left response, l e f t stimulus/left response, l e f t s t i m u l u s / r i g h t response. Note that, a l t h o u g h o n l y one o f t h e t w o dimensions was mentioned in t h e i n s t r u c t i o n s , these cannot b e considered t o b e T y p e IV-A ensembles ( n o doubt, t h e t w o dimensions were d i f f e r e n t ) , because b o t h dimensions were relevant, instead o f one relevant a n d t h e o t h e r i r r e l e v a n t , as r e q u i r e d by T y p e IV ensembles. Regardless o f t h e t y p e o f S-R ensemble, t h e spatial dimension most f r e q u e n t l y employed i n S-R compatibility paradigms i s t h e l e f t right one. However, S-R compatibility effects can b e easily shown also when t h e stimulus a n d response sets o v e r l a p on t h e above-below dimension ( L i d a v a s & Moscovitch, 1984; Nicoletti G Umilt5, 1984, 1985; Nicoletti e t al., 1988). For example, in t h e case o f a r e g u l a r T y p e I I ensemble, R T is s h o r t e r when t h e t o p stimulus commands a response w i t h t h e t o p t h a n w i t h t h e bottom key, a n d v i c e versa f o r t h e bottom stimulus. Furthermore, above-below S - R compatibility effects a r e o f more o r less t h e same magnitude as l e f t - r i g h t effects. Therefore, t h e r e seems t o b e no i n t r i n s i c d i f f i c u l t y i n u s i n g t h e above-below spatial dimension f o r mapping stimuli o n t o responses. T h e situation changes radically when stimulus a n d response sets overlap, n o t o n l y on t h e above-below dimension, b u t also on t h e l e f t right dimension, which too happens t o b e r e l e v a n t . If b o t h l e f t - r i g h t a n d above-below values a r e available i n t h e stimulus set a n d in t h e response set, t h e n t h e use o f t h e above-below dimension f o r mapping stimuli o n t o responses becomes more d i f f i c u l t , o r altogether impossible,

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as demonstrated by t h e weakening (Nicoletti & Umilti, appearance (Nicoletti & Umilti, 1985; Nicoletti e t al., compatibility effects in t h a t dimension.

1984) o r t h e d i s 1988) o f t h e S-R

In a paradigmatic experiment, subjects a r e e x p l i c i t l y t o l d t o use t h e above-below dimension o n l y f o r mapping stimuli onto responses. T h e t w o dimensions a r e combined orthogonally, so t h a t f o u r experiment a l conditions originate. In t h e f i r s t , t h e S-R p a i r i n g s a r e compatible f o r both t h e above-below a n d t h e l e f t - r i g h t dimensions. For example, when t h e t o p right light is shown, t h e subject responds by p r e s s i n g t h e t o p right k e y . In t h e second condition, t h e S-R p a i r i n g s are comp a t i b l e f o r t h e above/below dimension o n l y . F o r example, t h e t o p right light commands a response with t h e t o p l e f t k e y . T h e third condition comprises S-R p a i r i n g s t h a t a r e compatible f o r t h e l e f t - r i g h t dimension only. In t h i s case, t h e t o p right light commands a response w i t h t h e bottom right k e y . T h e f o u r t h condition is, instead, compatible f o r n e i t h e r dimension. For example, t h e t o p right light commands a response w i t h t h e bottom l e f t k e y . These were t h e experimental conditions t h a t demonstrated t h e dominance o f t h e l e f t - r i g h t o v e r t h e abovebelow dimension. T h a t is, despite t h e f a c t t h a t t h e i n s t r u c t i o n s were g i v e n exclusively in terms o f above-below S - R p a i r i n g s (i.e., in t h e f i r s t t w o examples above, t h e subject was t o l d t o use t h e t o p key, whereas in t h e second t w o examples h e was t o l d t o use t h e bottom key), S-R compatibility effects were p r e s e n t f o r t h e l e f t - r i g h t dimension o n l y o r were much s t r o n g e r f o r t h a t dimension. I n t e r p r e t a t i o n s of t h e Phenomenon Even t h o u g h t h e phenomenon o f t h e l e f t - r i g h t dominance is s t r o n g a n d reliable, i t s i n t e r p r e t a t i o n has p r o v e d t o b e r a t h e r elusive. The f i r s t possible explanation i s that, when t h e subject uses t h e hands f o r responding, t h e values o f t h e response set a r e more n a t u r a l l y specified in terms o f l e f t - r i g h t t h a n above-below positions. However, Nicoletti and U m i l t i (1985) could n o t s u p p o r t t h i s explanation because t h e l e f t right dominance was p r e s e n t also in a n experiment i n which t h e responses were emitted w i t h one hand a n d one foot instead o f t h e t w o It would seem t h a t t h e positions o f hand and foot a r e more hands. n a t u r a l l y specified along t h e above-below dimension t h a n t h e l e f t - r i g h t dimension.

A second explanation is based on t h e consideration t h a t i n t h e experiments t h a t showed t h e l e f t - r i g h t dominance, t h i s dimension extended across t h e b o d y midline ( i . e . , t h e egocentric a n d relative dimensions were confounded). I n contrast, above-below values were n o t specified in relation t o any egocentric reference axes. It could b e argued t h a t it was t h e availability o f unambiguous reference axes t h a t rendered l e f t - r i g h t values more salient and easier t o process t h a n above-below values. T h i s explanation is scarcely tenable, however, since it has been demonstrated t h a t S - R compatibility effects a r e o f t h e same magnitude i r r e s p e c t i v e o f whether t h e l e f t - r i g h t values a r e specif i e d across t h e b o d y midline (Nicoletti et a l . , 1982; U m i l t i & Nicoletti, 1985) .


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A third explanation o f t h e l e f t - r i g h t dominance was r e c e n t l y t e s t e d a n d rejected by Nicoletti e t al. (1988). We reasoned that, because d i s c r i m i n a t i n g between l e f t a n d right is d i f f i c u l t f o r a horizont a l l y symmetrical organism, it c o u l d b e t h a t a t t e n t i o n is selectively allocated t o t h e l e f t - r i g h t dimension, t h u s r e n d e r i n g values in t h a t dimension comparatively more salient. If t h i s hypothesis w e r e correct, s t r o n g e r S-R compatibility effects should b e f o u n d f o r t h e above-below dimension t h a n t h e l e f t - r i g h t dimension in t h e a u d i t o r y modality. This p r e d i c t i o n arises because sound localization is known t o b e more difficult, and, presumably more a t t e n t i o n demanding in t h e v e r t i c a l t h a n in t h e horizontal plane. I n s p i t e o f that, i n t h e experiment by Nicoletti e t al., t h e l e f t - r i g h t dominance was f o u n d also when acoustical stimuli were used in place o f v i s u a l stimuli. O f course, these r e s u l t s r u l e o u t t h e p o s s i b i l i t y t h a t t h e reason w h y a t t e n t i o n is selectively allocated t o t h e l e f t - r i g h t dimension is t h e d i f f i c u l t y human o b s e r v e r s experience in d i s c r i m i n a t i n g l e f t f r o m right. It may well be, however, t h a t a t t e n t i o n is selectively allocated t o t h e l e f t - r i g h t dimension f o r another, as y e t unknown, reason a n d t h u s l e f t - r i g h t cues a r e always more salient t h a n above-below ones, i r r e s p e c t i v e o f t h e modality tested. Relevant a n d I r r e l e v a n t Dimensions

It is w o r t h n o t i n g a c e r t a i n s i m i l a r i t y between t h e r e s u l t s o f t h e experiments t h a t demonstrate t h e l e f t - r i g h t dominance a n d t h e r e s u l t s of some o f t h e experiments discussed u n d e r t h e heading l e f t - r i g h t stimulus dimensions f o r T y p e I I ensembles, (i.e., U m i l t i E Liotti, 1987, E x p e r i ments 1 a n d 2 ) . In t h e s t u d y by U m i l t i a n d Liotti, t h e values o f t h e stimuius set were specified o n e i t h e r an egocentric o r a r e l a t i v e l e f t right dimension. Depending on t h e instructions, one dimension was r e l e v a n t a n d t h e o t h e r was i r r e l e v a n t . Despite t h e f a c t t h a t stimulus a n d response sets overlapped on b o t h dimensions, o n l y t h e r e l e v a n t dimension, regardless o f w h e t h e r it happened t o b e t h e egocentric o r t h e r e l a t i v e one, p r o d u c e d S-R compatibility effects. Actually, t h i s was t h e reason w h y those were assumed t o b e T y p e I I ensembles. Taken together, these r e s u l t s suggest t h a t a subject, if engaged in processing one spatial dimension in a c o n t r o l l e d way, i s unable t o process simultaneously another spatial dimension, n o t even automatically. I n t h e case o f t h e l e f t - r i g h t dominance, it can b e a r g u e d t h a t , f o r u n k n o w n reasons, t h e l e f t - r i g h t dimension p r e f e r e n t i a l l y a t t r a c t s attention and, because o f this, undergoes c o n t r o l l e d processing a t t h e expense o f t h e above-below dimension. Note t h a t spatial dimensions, regardless o f t h e i r n a t u r e ( i . e . , l e f t - r i g h t egocentric, l e f t - r i g h t relat i v e o r above-below), a r e processed automatically when t h e competing dimension t h a t undergoes c o n t r o l l e d processing is nonspatial. T h i s is t h e cause o f t h e spatial S-R compatibility effects obtained w i t h T y p e I l l a n d T y p e IV ensembles. A l t e r n a t i v e l y , a n d perhaps more likely, it could b e t h a t t h e automatic processing o f t h e i r r e l e v a n t ( o r nondominant) spatial dimension does occur, but i t s effects a r e cancelled o u t by t h e effects o f t h e c o n t r o l l e d processing o f t h e other, r e l e v a n t ( o r dominant) spatial dimension. A d i f f e r e n t situation arises when t h e t w o spatial stimulus dimensions t h a t can o v e r l a p w i t h t h e spatial response dimension(s) a r e b o t h

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irrelevant. In t h i s case, t h e outcome depends on t h e r e l a t i v e t i m i n g in t h e a v a i l a b i l i t y o f t h e t w o i r r e l e v a n t spatial dimensions (see Umilt5 G Liotti, 1987, Experiments 3, 4, a n d 5). If t h e t w o spatial dimensions become available one a f t e r t h e other, t h e n S-R compatibility effects can b e o b s e r v e d o n l y f o r t h e dimension t h a t is processed second. If t h e t w o spatial dimensions become available a t t h e same time, t h e n n e i t h e r Because i r r e l e v a n t spatial dimension produces S-R compatibility effects. dimensions can no d o u b t be, processed automatically, if n o competing spatial dimension i s simultaneously p r e s e n t (note, again, t h a t t h i s is t h e cause o f spatial S-R compatibility effects obtained w i t h T y p e I l l a n d T y p e IV ensembles), it must b e concluded t h a t t h e r e s u l t s o f t h e automatic processing o f t w o spatial dimensions cancel each o t h e r o u t . This p o s s i b i l i t y was already mentioned in t h e section about stimulus dimensions w i t h T y p e IV ensembles a n d w i l l b e discussed f u r t h e r in t h e n e x t section. E x p l a n a t o r y Hypotheses of Spatial S-R Compatibility Spatial C o d i n g Hypothesis f o r T y p e II Ensembles Wallace (1971, 1972; also, see Nicoletti e t al., 1982) proposed an explanation o f spatial S-R compatibility effects in terms of t h e correspondence, o r lack o f correspondence, between t h e spatial code t h a t specifies t h e position o f t h e stimulus a n d t h e spatial code t h a t specifies t h e position o f t h e e f f e c t o r . Response latencies a r e f a s t e r when t h e t w o positions s h a r e t h e same code t h a n when t h e y a r e coded differently. In o t h e r words, Wallace suggested t h a t t h e positions of stimulus a n d e f f e c t o r a r e related t o spatial codes a n d t h a t it is t h e o u t come o f a comparison between t h e i r representations in these codes t h a t y i e l d s t h e d i f f e r e n c e in R T between compatible a n d incompatible S-R pairings. T e i c h n e r a n d K r e b s (1974) extended t h i s hypothesis t o nonspatial t y p e s o f S-R compatibility a n d r e n d e r e d it more e x p l i c i t by i n d i c a t i n g a stage in processing a t w h i c h S-R compatibility effects emerge. These a u t h o r s d i s t i n g u i s h e d f o u r temporal components in choice R T tasks: (a) A constant r e p r e s e n t i n g t h e sum of v a r i o u s n e u r a l transmission lags, (b) t h e time r e q u i r e d f o r stimulus categorization, (c) t h e time r e q u i r e d t o p e r f o r m some t y p e of t r a n s l a t i o n between stimulus a n d response, a n d (d) t h e time r e q u i r e d f o r response selection. In t h e i r view, S - R compatibility effects o c c u r a t t h e level o f t h e third compo:ent a n d t h e degree o f S-R compatibility i n h e r e n t in a t a s k depends on . . . t h e p r o p o r t i o n o f choice reaction time t h a t is a t t r i b u t a b l e t o stimulusresponse t r a n s l a t i o n time" (p. 91). T h a t is t o say, S - R compatibility would depend on t h e number o f S-R c o d i n g transformations: T h e f e w e r t h e t r a n s l a t i o n s required, t h e h i g h e r t h e degree o f compatibility o f t h e t a s k . T h e h i g h e s t level o f compatibility is obtained when t h e coda t h a t specifies t h e value o f t h e imperative stimulus w i t h i n t h e stimulus set is t h e same as t h e code t h a t specifies t h e value o f t h e c o r r e s p o n d i n g response w i t h i n t h e response set. Welford (1976), too, has a similar view a n d speaks e x p l i c i t l y o f t r a n s l a t i o n mechanisms t h a t u n d e r l i e S - R compatibility effects. Note t h a t , according t o t h e spatial coding hypothesis, S-R compatibility effects a r e d u e t o a l e n g t h e n i n g o f R T f o r


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incompatible S-R pairings, r a t h e r t h a n t o a facilitation o f R T f o r comp a t i b l e ones. T h i s i m p o r t a n t point, however, is f a r f r o m b e i n g s e t t l e d (see Simon & Acosta, 1982; Simon, Acosta, Mewaldt, & Speidel, 1976a). T h e c o d i n g hypothesis is n o d o u b t more c o n v i n c i n g in t h e case o f T y p e I I ensembles (Nicoletti e t al., 1982; Nicoletti 6 U m i l t i , 1984). d e s p i t e t h e f a c t t h a t it was proposed by Wallace (1971, 1972) t o e x p l a i n spatial S-R compatibility effects obtained w i t h ensembles t h a t can b e classified as T y p e I l l . In t h e case o f T y p e I I ensembles, codes must b e assigned t o t h e values o f stimuli a n d responses in t h e o v e r l a p p i n g dimension. T h e n some s o r t of t r a n s l a t i o n f r o m t h e stimulus code t o t h e response code m u s t t a k e place because it is t h e v a l u e o f t h e imperative stimulus in t h e o v e r l a p p i n g dimension t h a t indicates t h e v a l u e o f t h e r e q u i r e d response. Therefore, it i s reasonable t o assume t h a t response latency i s f a s t e r when t h e t r a n s l a t i o n is simple t h a n when it is complex. A n o t h e r important consideration concerns t h e f a c t t h a t t h e t r a n s l a t i o n process performed on t h e r e l e v a n t dimension is v e r y l i k e l y t o o c c u r in a controlled mode. T h i s is because t h e subject is required, In p a r by t h e t a s k instructions, t o allocate a t t e n t i o n t o t h a t process. t i c u l a r , t h e subject i s e x p l i c i t l y t o l d t o use t h e stimulus code f o r selecting t h e c o r r e c t response. T h e coding hypothesis can e x p l a i n t h e lack o f S-R compatibility effects f o r a spatial dimension o f t h e stimulus, t h a t also o v e r l a p s w i t h t h e response dimension, but is d i f f e r e n t f r o m t h e one t o w h i c h t h e subject's a t t e n t i o n i s d i r e c t e d . (Remember t h a t t h i s second stimulus dimension p r o v e s i n e f f e c t i v e regardless o f w h e t h e r it i s r e l e v a n t o r irrelevant.) One has simply t o make t h e additional assumption t h a t t r a n s l a t i o n processes t h a t o c c u r in a c o n t r o l l e d mode (i.e., that are attended t o ) a n n u l t h e effects o f similar, a n d p o t e n t i a l l y competing, processes t h a t a t t h e same time o c c u r automatically, (i.e., t h a t a r e n o t attended t o ) . T y p e I l l a n d T y p e I V Ensembles A hypothesis t h a t a t t r i b u t e s t h e l e n g t h e n i n g o f incompatible response latencies t o a stage o f t r a n s l a t i o n f r o m t h e stimulus t o t h e response code is, however, much less c o n v i n c i n g f o r T y p e Ill a n d T y p e IV ensembles. In fact, t h e stimulus a n d response sets o v e r l a p o n an i r r e l e v a n t dimension, w h i c h need n o t b e processed. Therefore, no such t r a n s l a t i o n is necessary. I n t h e case o f S-R compatibility effects d u e t o an i r r e l e v a n t stimu l u s dimension that, however, overlaps w i t h t h e response dimension, t h r e e e x p l a n a t o r y hypotheses have been proposed. Actually, t h e f i r s t t w o t o b e considered below were proposed t o e x p l a i n spatial S-R comp a t i b i l i t y in general, n o t j u s t t h e effects d u e t o an i r r e l e v a n t stimulus dimension. However, it seems t o us t h a t spatial S-R compatibility effects obtained w i t h T y p e I I ensembles a r e s a t i s f a c t o r i l y accounted f o r by t h e coding hypothesis a n d a r e n o t i n need o f f u r t h e r explanations. A n i m p o r t a n t p o i n t t h a t should n o t b e overlooked is t h a t spatial S - R compatibility effects p r o d u c e d on a r e l e v a n t dimension (i.e., w i t h

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T y p e I I ensembles) a r e c o n s i s t e n t l y g r e a t e r t h a n those p r o d u c e d on an i r r e l e v a n t dimension (i.e., w i t h T y p e I l l a n d T y p e IV ensembles). The f o r m e r a r e o f about 50-70 ms, whereas t h e l a t t e r a r e n o t g r e a t e r t h a n approximately 25 ms ( f o r estimates o f t h e t w o t y p e s o f effects o b s e r v e d in comparable conditions, see Nicoletti e t al., 1982; Nicoletti & UmiltO, 1984, 1985; UmiltO & Liotti, 1987; UmiltO E Nicoletti, 1985). It seems, therefore, t h a t t h e d i s t i n c t i o n between spatial S-R compatibility effects in a r e l e v a n t o r in an i r r e l e v a n t dimension is n o t merely semantic.

All t h r e e hypotheses c l e a r l y p o i n t t o response selection as t h e stage a t w h i c h S-R compatibility effects emerge. T h i s i s no d o u b t a l i k e l y possibility, because it has been shown (see UmiltO 8 Liotti, 1987; Umilta & Nicoletti, 1985) t h a t these effects o c c u r o n l y f o r t h a t dimension t h a t becomes available w h i l e t h e response is b e i n g selected. The stimulus dimension t h a t can b e processed 500 ms b e f o r e response select i o n does n o t p r o d u c e a n y effects. F o r example, if information about side is p r e s e n t e d in advance, whereas information about r e l a t i v e posit i o n i s p r e s e n t e d simultaneously w i t h t h e imperative stimulus, t h e n S-R compatibility effects o c c u r f o r r e l a t i v e position o n l y . Conversely, if information a b o u t r e l a t i v e position i s p r e s e n t e d in advance, whereas information about side is p r e s e n t e d a t t h e same time as t h e imperative stimulus, t h e n S-R compatibility effects o c c u r f o r side. Similarly, Simon e t al. (1976a) f o u n d t h a t spatial S-R compatibility effects disapp e a r if t h e response is delayed f o r approximately 300 ms a f t e r stimulus onset. T h e t h r e e hypotheses also share, e x p l i c i t l y o r implicitly, t h e assumption t h a t t h e processes t h a t cause spatial S-R compatibility effects w i t h T y p e I l l a n d T y p e IV ensembles a r e automatic, t h a t is, d o n o t r e q u i r e attention. T h i s t o o seems t o b e a l i k e l y p o s s i b i l i t y because, as repeatedly said, t h e stimulus dimension t h a t overlaps w i t h t h e response dimension, a n d t h u s produces S-R compatibility effects, is n o t t a s k r e l e v a n t and, presumably, does n o t u n d e r g o processing in a controlled mode.

The Attentional Hypothesis. T o explain spatial S-R compatibility effects, Simon (1968; Simon & Rudell, 1967) i n v o k e d a basic n a t u r a l t e n d e n c y t o respond t o w a r d t h e source o f t h e imperative stimulus. When t h e side o f t h e stimulus does n o t match t h e side o f t h e response, comparatively l o n g e r R T s a r e obtained because t h e i n a p p r o p r i a t e response t e n d e n c y must b e i n h i b i t e d . When t h e side o f t h e stimulus a n d t h e side o f t h e response correspond, RTs a r e s h o r t e r because t h e basic response t e n d e n c y is consistent w i t h t h e r e q u i r e d response. T h i s is an attentional explanation, as attested by t h e f a c t t h a t Simon (1969) l i k e n e d t h e n a t u r a l t e n d e n c y t o respond t o w a r d t h e stimulus source t o an o r i e n t i n g response. T h e imperative stimulus would p r o v i d e a d i r e c tional cue, w h i c h t e n d s t o e l i c i t automatically a response on i t s same side (Simon, Hinrichs, E C r a f t , 1970; f o r a modified v e r s i o n o f t h e attentional hypothesis, see Verfaellie, Bowers, & Heilman, 1988). T h e attentional hypothesis becomes scarcely tenable if one considers t h a t spatial S-R compatibility effects can b e obtained w i t h T y p e I l l ensembles also when t h e i r r e l e v a n t dimension is relative, t h a t is, i n a condition in w h i c h b o t h t h e t w o stimuli a n d t h e t w o responses o c c u r


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o n t h e same side (see UmiIta & Liotti, 1987; Umilth & Nicoletti, 1985). If t h e d e t e r m i n i n g f a c t o r were t h e t e n d e n c y t o react in t h e d i r e c t i o n o f t h e imperative stimulus, n o effects should instead b e f o u n d because e i t h e r stimulus should cause a response t e n d e n c y t h a t i s identical f o r e i t h e r response. In o t h e r words, if t h e t w o stimuli appear in t h e right v i s u a l field, t h e n b o t h should e l i c i t a t e n d e n c y t o r e s p o n d t o w a r d t h e right side. Also t h e f a c t t h a t S-R compatibility effects vanish when t w o i r r e l e v a n t spatial stimulus dimensions a r e simultaneously available (see U m i l t i & Liotti, 1987, Experiments 3 a n d 4) creates problems f o r t h e attentional hypothesis. I n fact, t h e hypothesis cannot e x p l a i n why t h e stimulus t h a t appears o n t h e right side (see F i g u r e s 1 a n d 2 ) should n o t f a v o r t h e response located o n t h e same side, simply because t h a t stimulus occupies also t h e l e f t r e l a t i v e position.

A f u r t h e r difficulty encountered by t h e hypothesis concerns t h e fact t h a t S-R compatibility effects a r e absent in simple R T tasks. (Remember t h a t in simple R T tasks w h a t matters is anatomical connectivity, r a t h e r t h a n spatial S-R Compatibility.) It i s unclear w h y a stimulus shown in t h e right v i s u a l f i e l d should n o t p r o d u c e a response t e n d e n c y t h a t f a v o r s t h e response o n t h e right side o v e r t h e response on t h e l e f t side also w i t h simple R T s . T h e Dimensional O v e r l a p Hypothesis. According t o Kornblum e t al. (in press), S-R compatibility effects a r e p r o d u c e d w i t h an i r r e l e v a n t dimension of t h e stimulus because, d u e t o t h e o v e r l a p w i t h t h e response dimension, presentation o f a p a r t i c u l a r element o f t h e stimulus set causes t h e automatic activation o f a p a r t i c u l a r element o f t h e response set. If t h e response t h a t is activated by t h e stimulus i s t h e one assigned t o it on t h e basis o f t h e r e l e v a n t dimension, t h e n response latency w i l l show a b e n e f i t . If t h e activated response d i f f e r s f r o m t h e r e q u i r e d one, t h e n response latency w i l l show a cost. O f course, in t h e absence of o v e r l a p between t h e i r r e ! e v a n t stimulus dimension a n d t h e response dimension, n e i t h e r costs n o r b e n e f i t s w i l l b e f o u n d . S-R compatibility effects a r e due, in t h i s view, t o t h e automatic p r i m i n g , by an i r r e l e v a n t stimulus element, o f a response element. T h e response element t h a t is p r i m e d may o r may n o t coincide w i t h t h e response element t h a t must b e selected on t h e basis o f t h e r e l e v a n t stimulus element. K o r n b l u m e t al.'s ( i n p r e s s ) hypothesis a p p a r e n t l y f a r e s b e t t e r t h a n t h e p r e v i o u s one in t h e face of t h e empirical f i n d i n g s . It has no d i f f i c u l t i e s i n accounting f o r t h e S-R compatibility effects d u e t o t h e r e l a t i v e position o f t h e stimuli a n d / o r t h e responses. In fact, t h e r e i s no reason w h y t h e stimulus a n d response sets should n o t o v e r l a p o n t h e r e l a t i v e dimension. Thus, in t h a t dimension too, a p a r t i c u l a r stimulus element can activate a p a r t i c u l a r response element. T h e absence o f spatial S-R compatibility effects in simple R T t a s k s can b e easily explained, based on t h e f a c t t h a t i n those t a s k s t h e stimulus a n d response sets comprise one element o n l y . Therefore, t h e r e cannot b e a d i f f e r e n t i a l a c t i v a t i o n of response elements by a stimulus element.

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T h e absence o f S-R compatibility effects if t w o spatial dimensions a r e p r e s e n t a t t h e time when t h e response i s b e i n g selected seems t o b e more problematic f o r t h e hypothesis. One can argue, o f course, t h a t t w o d i f f e r e n t responses a r e p r i m e d a n d t h e n e t e f f e c t is null. However, t h a t implies t h a t one single stimulus can y i e l d t w o independent stimulus elements. T h i s is an a d hoc assumption, which is in need o f empirical s u p p o r t . T h e Coding Hypothesis. A d i f f e r e n t t a c k was t a k e n by Umilta a n d L i o t t i (1987; also see UmiltB & Nicoletti, 19851, who attempted t o account f o r spatial S-R compatibility effects p r o d u c e d by an i r r e l e v a n t stimulus dimension by e x t e n d i n g t o them t h e c o d i n g hypothesis. (Remember t h a t t h i s was also t h e o r i g i n a l proposal o f Wallace, 1971, 1972.) T h e basic idea is t h a t S-R compatibility effects a r i s e because of t h e presence, a t t h e moment when t h e response i s selected, o f t w o spat i a l codes. One i s t h e response code, w h i c h must b e formed in o r d e r t o p e r f o r m t h e t a s k . T h e o t h e r is a stimulus code, w h i c h is o f no r e l evance f o r p e r f o r m i n g t h e task, but is formed automatically. T h e stimulus code can influence t h e speed w i t h w h i c h t h e response code is formed. If t h e t w o codes a r e equal, t h e selection o f t h e response is facilitated. If t h e t w o codes a r e d i f f e r e n t , t h e selection o f t h e response is delayed. Note t h a t t h i s v e r s i o n o f t h e c o d i n g hypothesis ascribes t h e S-R compatibility effects p r o d u c e d by an i r r e l e v a n t stimulus dimension t o t h e stage in w h i c h response selection occurs, r a t h e r t h a n t o t h e stage in w h i c h a t r a n s l a t i o n occurs f r o m t h e stimulus code t o t h e response code. B y contrast, S - R compatibility effects p r o d u c e d by a r e l e v a n t dimension a r e t h o u g h t t o o r i g i n a t e a t t h e stage in which such t r a n s l a t i o n takes place. T h e d i s t i n c t i o n between t h e stage in w h i c h t h e response i s selected a n d t h e stage i n w h i c h t h e stimulus code is t r a n s l a t e d i n t o t h e response code is based on t h e consideration t h a t a t t h e t r a n s l a t i o n stage t h e spatial codes o f t h e stimulus set a r e mapped i n t o t h e spatial codes o f t h e response set, whereas a t t h e selection stage t h e a p p r o p r i a t e element o f t h e response set is chosen. T h e coding hypothesis can easily account f o r t h e S - R compatibility effects d u e t o r e l a t i v e stimulus position. Actually, t h i s is t h e reason w h y it was i n i t i a l l y put f o r w a r d (see Umilta & Nicoletti, 1985). In t h i s view, t h e r e l a t i v e position o f t h e stimulus is automatically p r o cessed, a n d t h i s processing g i v e s r i s e t o a spatial code, w h i c h can speed u p o r delay t h e p r o d u c t i o n o f t h e spatial response code. The same happens, o f course, when t h e stimulus dimension is side, instead of r e l a t i v e position, w h i c h is processed while t h e response code i s b e i n g formed (see Umilta & Liotti, 1987, Experiments 3 a n d 4).

If r e l a t i v e position a n d side a r e b o t h coded a t t h e moment when t h e response code is formed, a n d t h e outcome o f t h e t w o independent c o d i n g processes is d i f f e r e n t , t h e n t h e facilitation a n d i n h i b i t i o n effects balance each o t h e r o u t . T h e n e t r e s u l t is t h e absence o f spatial S - R compatibility effects (see, again, Umilt.5 & Liotti, 1987, Experiments 3 a n d 4). If r e l a t i v e position a n d side a r e b o t h processed b e f o r e p r e sentation o f t h e imperative stimulus, t h e n no spatial code is a c t i v e a t t h e moment of response selection. Also, in t h i s case, S - R compatibility effects vanish (see Umilta & L i o t t i , 1987, Experiment 5).

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As for t h e absence o f S-R compatibility e f f e c t s i n simple R T tasks, t h e explanation o f f e r e d by t h e c o d i n g hypothesis i s t h a t no spat i a l response code needs t o b e formed because t h e r e i s one response only. Therefore, even if a spatial stimulus code i s automatically formed, t h e v e r y basis f o r a n i n t e r f e r e n c e between t h i s code a n d t h e response code is lacking.

The Crossed-Hand Effect In a p r e v i o u s section, we d e s c r i b e d t h e consequences t h a t r e s u l t when t h e subject crosses h i d h e r hands. T h e t y p e o f ensemble ( T y p e I I o r T y p e I l l ) is immaterial because t h e spatial response dimension i s always r e l e v a n t . One consequence is t h a t spatial S-R compatibility a p p a r e n t l y reverses i t s d i r e c t i o n . T h e o t h e r is t h a t R T becomes l o n g e r o v e r a l l (i.e., t h e crossed-hand e f f e c t ) . T h e a p p a r e n t r e v e r s a l o f S-R compatibility poses n o problems f o r a n y o f t h e t h r e e hypotheses. For t h e f i r s t hypothesis, t h e t e n d e n c y t o r e s p o n d t o w a r d t h e stimulus influences t h e response t h a t is emitted in a p a r t i c u l a r position, n o t t h e h a n d t h a t emits t h e response. F o r t h e second hypothesis, a stimulus element activates automatically a response element t h a t is located in t h e c o n g r u e n t position, n o t t h e h a n d t h a t emits t h e response. F o r t h e third hypothesis, w h a t i s automatically coded i s t h e position o f t h e response, n o t t h e h a n d t h a t emits t h e response. B y contrast, t h e crossed-hand e f f e c t o f slowing o v e r a l l RTs can b e explained o n l y w i t h i n t h e f r a m e w o r k o f t h e c o d i n g hypothesis (see Nicoletti e t al., 1984; Riggio e t al., 1986). T h i s hypothesis maintains t h a t t h e anatomical s t a t u s of t h e e f f e c t o r i s coded along with t h e position o f the effector. If t h e t w o codes a r e d i f f e r e n t , as happens in t h e crossed-hand condition, a c o n f l i c t arises a t t h e stage o f response select i o n which, therefore, occurs w i t h a c e r t a i n delay. Summary In t h i s chapter, K o r n b l u m e t al.'s (in press) taxonomy was T h e most imporadopted t o c l a s s i f y spatial S-R compatibility effects. t a n t f e a t u r e o f t h e taxonomy is t h e d i s t i n c t i o n between r e l e v a n t a n d i r r e l e v a n t dimensions o f t h e stimulus set. A stimulus dimension is said t o b e r e l e v a n t when t h e r e q u i r e d response is signaled by t h e v a l u e of t h e stimulus in t h a t dimension. A stimulus dimension i s said t o b e i r r e l e v a n t if values on it d o n o t signal t h e r e q u i r e d response. In t h e f i r s t section, we discussed choice R T studies in w h i c h t h e l e f t - r i g h t dimension of t h e stimulus c o u l d b e e i t h e r r e l e v a n t (i.e., T y p e I I ensembles, according t o K o r n b l u m e t a l . ' s taxonomy) o r i r r e l e v a n t (i.e., T y p e I l l or T y p e IV ensembles). When t h e stimulus set comp r i s e s a l e f t a n d a right element, l e f t a n d right values can b e specified along t w o dimensions: T h e stimulus can b e on e i t h e r t h e l e f t o r right side i n relation t o an egocentric reference axis (i.e., t h e egocentric dimension); a l t e r n a t i v e l y , it can b e e i t h e r t h e l e f t o r right position in relation t o an e x t e r n a l reference p o i n t ( i . e . , t h e r e l a t i v e dimension). We showed t h a t these t w o dimensions can b e unconfounded a n d

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independently p r o d u c e S-R compatibility effects w i t h T y p e II, T y p e I I I , a n d T y p e IV ensembles.

I n t h e second section o f t h e c h a p t e r it was a r g u e d t h a t in simple R T tasks, spatial S-R compatibility i s immaterial a n d w h a t matters i s t h e directness o f t h e anatomical connections between t h e stimulated hemiretinae a n d t h e r e s p o n d i n g hand. B y contrast, in choice R T tasks, speed o f response depends p r i m a r i l y on t h e spatial relations between t h e position o f t h e response and t h e position of t h e imperative stimulus, t h a t is, o n spatial S-R compatibility. Also in t h e case of t h e response position, egocentric a n d r e l a t i v e l e f t - r i g h t dimensions can b e unconfounded. T h e studies discussed in t h i s section show S-R compatibility effects a t t r i b u t a b l e to t h e r e l a t i v e position o f t h e response. More p r e cisely, if one distinguishes between t h e r e l a t i v e position o f t h e e f f e c t o r a n d t h e r e l a t i v e position o f t h e response goal, it appears t h a t S-R comp a t i b i l i t y effects depend on t h e l a t t e r . T h e r o l e o f t h e egocentric dimension o f t h e e f f e c t o r o r response goal in p r o d u c i n g compatibility effects has y e t t o b e demonstrated. In t h i s same section, t w o phenomena t h a t a r e caused by t h e position o f t h e effectors were discussed: When subjects cross t h e i r hands, S-R compatibility a p p a r e n t l y reverses i t s direction, a n d R T becomes o v e r a l l longer. In t h e third section, t h e dominance o f t h e l e f t - r i g h t dimension o v e r t h e above-below dimension in spatial S-R compatibility was d i s cussed. T h i s phenomenon manifests i t s e l f w i t h special instances o f T y p e I 1 ensembles, in w h i c h t h e stimulus a n d response sets o v e r l a p in t w o spatial dimensions (i.e., above-below a n d l e f t - r i g h t ) , instead o f one, as happens w i t h r e g u l a r T y p e I I ensembles. A l t h o u g h S-R comp a t i b i l i t y effects a r e easily f o u n d when t h e stimulus a n d t h e response sets o v e r l a p o n t h e above-below dimension only, t h i s dimension i s n o t used f o r mapping stimuli o n t o responses if also l e f t - r i g h t values a r e available in b o t h t h e stimulus a n d t h e response set. Finally, in t h e f o u r t h section, several e x p l a n a t o r y hypotheses o f spatial S-R compatibility effects were t a k e n i n t o consideration. It was concluded t h a t S-R compatibility effects p r o d u c e d by e i t h e r a r e l e v a n t o r an i r r e l e v a n t stimulus dimension can b e s t b e explained w i t h i n t h e framework o f t h e c o d i n g hypothesis. In t h e case o f T y p e I I ensembles, spatial codes a r e assigned t o t h e values o f stimuli a n d responses in t h e o v e r l a p p i n g dimension. T h e n a t r a n s l a t i o n f r o m t h e stimulus code t o t h e response code takes place. T h i s is because it i s t h e value o f t h e imperative stimulus in t h e o v e r l a p p i n g dimension t h a t signals t h e value of t h e r e q u i r e d response. Therefore, response latency is s h o r t e r when t h e t r a n s l a t i o n i s simple ( i . e . , compatible S-R p a i r i n g s ) t h a n when it is complex (i.e., incompatible S-R p a i r i n g s ) .

In t h e case o f T y p e I l l a n d T y p e IV ensembles, spatial S-R comp a t i b i l i t y effects o c c u r because o f t h e presence, a t t h e moment when t h e response is selected, of t w o spatial codes. One is t h e response code, w h i c h must b e formed t o p e r f o r m t h e t a s k . T h e o t h e r is a stimu l u s code, w h i c h i s o f no relevance f o r p e r f o r m i n g t h e task, but is formed automatically. If t h e t w o codes a r e equal ( i . e . , compatible S - R p a i r i n g s ) , t h e selection o f t h e response is facilitated; if t h e t w o codes

Spatial S-R Compatibility are different (i.e., response is delayed.

incompatible S-R

pairings),

113 t h e selection o f t h e

In conclusion, t h e coding hypothesis a t t r i b u t e s t h e spatial S-R compatibility effects p r o d u c e d by an i r r e l e v a n t stimulus dimension t o t h e stage in w h i c h t h e response selection occurs, whereas it a t t r i b u t e s t h e effects p r o d u c e d by a r e l e v a n t dimension t o t h e stage in w h i c h t h e t r a n s l a t i o n f r o m t h e stimulus code t o t h e response code o c c u r s .

Acknowledgements Preparation o f t h e p a p e r was s u p p o r t e d by g r a n t s f r o m t h e Consiglio Nazionale delle Ricerche a n d t h e M i n i s t e r 0 della Pubblica l s t r u z i o n e t o C.U. T h e a u t h o r s a r e g r a t e f u l t o S. Kornblum, T. Hasbroucq, a n d A . O m a n f o r allowing t h e use o f t h e i r taxonomy.

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SPATIAL CODING AND SPATIO-ANATOMICAL MAPPING: EVIDENCE FOR A HIERARCHICAL MODEL OF SPATIAL STl MULUS- RESPONSE COMPAT I B I L IT Y GABRIELE HEISTER A b t e i l u n g Neuropsychologie Universitaetsspital Zuerich, Switzerland PETER SCHROEDER-HEISTER Zentrum Philosophie und Wissenschaftstheorie U n i v e r s i t a e t Konstanz, FRG WALTER H. EHRENSTEIN l n s t i t u t f u e r Arbeitsphysiologie an d e r U n i v e r s i t a e t Dortmund, FRG

Spatial stimulus-response (S-R) compatibility u s u a l l y denotes t h e reaction-time ( R T ) advantage f o r spatially compatible S-R p a i r i n g s o v e r incompatible p a i r i n g s in choice-reaction tasks. T h i s notion goes b a c k t o F i t t s a n d Seeger (1953), who c r e d i t e d t h e t e r m "compatibility" t o a suggestion by A. M. Small. It i s difficult t o g i v e a d e f i n i t i o n t h a t captures e v e r y t h i n g t h a t has been associated w i t h t h i s term. Quite generally, one may call S-R p a i r i n g s spatially compatible if, w i t h respect t o a c e r t a i n spatial characteristic, t h e stimuli a n d responses c o r r e s p o n d t o each o t h e r and incompatible if they have opposite spatial characteristics. A t y p i c a l example o f spatial compatibility is t h e situation in w h i c h subjects react t o a light stimulus in t h e right o r l e f t visual f i e l d by p r e s s i n g a r i g h t o r l e f t response b u t t o n w i t h t h e i r right or l e f t hand, respectively. One normally f i n d s a spatial S-R compatibility effect, in t h e sense t h a t r i g h t - h a n d reactions t o right l i g h t s a n d l e f t - h a n d reactions t o l e f t l i g h t s (compatible condition) a r e f a s t e r t h a n r i g h t - h a n d reactions t o l e f t l i g h t s a n d l e f t - h a n d reactions t o right l i g h t s (incompatible condition; Brebner, Shephard, & Cairney, 1972; C r a f t & Simon, 1970; Simon & Wolf, 1963; Wallace,1971; upright head conditions o f Lddavas & Moscovitch, 1984, Experim n t s 1 a n d 3, a n d o f Schroeder-Heister, Heister, & Ehrenstein, 1988).? Usually it is n o t specified w h e t h e r t h i s e f f e c t l T h e r e is a c e r t a i n ambiguity i n t h e notion o f S-R compatibility. B y S-R compatibility, one may designate (1) a f e a t u r e o f t h e experimental stimulus-response arrangement whose e f f e c t on which performance is measured (i.e., an independent variable, which is experimentally manipulated); (2) t h e e f f e c t t h a t compatible responses a r e f a s t e r t h a n incompati b l e responses (i.e., an observed r e s u l t ) ; (3) a c e r t a i n i n t e r p r e t a t i o n o f t h i s compatibility e f f e c t emphasizing i t s c o g n i t i v e c h a r a c t e r as opposed, e.g., t o neuroanatomic explanations (see Heister E Schroeder-Heister, 1985). I n t h e following, it w i l l always b e clear f r o m t h e c o n t e x t w h a t is meant.

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means (a) t h a t t h e r i g h t hand reacts faster t o t h e r i g h t l i g h t than t o the l e f t light, and t h e l e f t hand faster t o t h e l e f t light than t o t h e r i g h t light, (b) t h a t t h e right hand reacts faster than the l e f t hand t o t h e r i g h t light, and t h e l e f t hand faster than t h e r i g h t hand t o t h e l e f t light, o r (c) both of these possibilities. Similar examples could be given f o r stimuli i n the upper and lower visual fields (LBdavas & Moscovitch, 1984, u p r i g h t head conditions of Umilt6, Experiments 1 and 4; Nicoletti & Umilt6, 1984; Nicoletti, Tressoldi, & Marzi, 1988) and f o r auditory stimulation (Simon 1969; Simon & Rudell, 1967; see also Chapter 2, by Simon). However, because the pattern of results is similar f o r these d i f f e r e n t experimental conditions, i n the following section, i n which some basic results of compatibility research are t o be discussed, we will mainly deal with the paradigm of bimanual choice reactions t o r i g h t and l e f t lights. O u r considerations can be extended t o most other S-R arrangements i n a straightforward way. For t h e sake of simplicity, we will also r e f r a i n from distinguishing between spatial S-R compatibility effects f o r relevant stimulus location, f o r which t h e subject's task concerns the spatial distinction t h a t is i n question f o r t h e compatibility effect, and irrelevant stimulus location, f o r which t h e task concerns responding t o some other feature of the stimulus (such as i t s color o r i t s semantic content). What is Compatible or Incompatible in Spatial S-R Compatibility? With bimanual choice reactions t o r i g h t and l e f t stimuli, there are at least t h r e e d i f f e r e n t r i g h t h e f t distinctions on t h e response side, leading t o three d i f f e r e n t relations between stimuli and responses. First, we have a r i g h t h e f t distinction between t h e positions of the response keys. Then, we can distinguish between t h e r i g h t o r l e f t position of the responding hand. And, finally, there is t h e anatomical distinction between the right and the left hand. Correspondingly, the compatibility effect observed may be due t o the compatibility o r incompatibility of either (a) stimulus position and response key position, (b) stimulus position and response effector position, o r (c) stimulus position and responding hand. Obviously, all three possibilities are confounded i n t h e normal paradigm, in which the right key i s operated w i t h t h e right hand held i n right position. T h e confounding between responding hand and i t s position was resolved by Simon, Hinrichs and C r a f t (197Ob--auditory stimulation) and I n these studies, independently by Wallace (1971 --visual stimulation). subjects r e a c t e d ' w i t h t h e i r arms crossed, so t h a t the r i g h t hand was placed on t h e l e f t side and t h e l e f t hand was placed on t h e r i g h t side. T h e compatibility between stimulus position and t h e position of responding hand (or response key), and not t h e compatibility between stimulus position and right o r l e f t hand, was shown t o be crucial f o r t h e observed effect. (For applications of t h i s technique, see Anzola, Bertoloni, 1972; Callan, Klisz, & Buchtel, & Rizzolatti, 1977; Brebner et al., Parsons, 1974; Nicoletti, Anzola, Luppino, Rizzolatti, & Umilta, 1982.) Riggio, Gawryszewski, and Umilta (1986) resolved t h e confounding between position of response keys and position of responding hand. They studied responses w i t h hands held i n r i g h t o r l e f t positions, w i t h

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o n l y t h e r e s p o n d i n g index f i n g e r s crossed, so t h a t subjects pressed t h e l e f t k e y w i t h t h e i r r i g h t - i n d e x f i n g e r a n d t h e right k e y w i t h t h e i r l e f t index f i n g e r . T h e compatibility between stimulus position a n d position o f response keys, a n d n o t between stimulus position a n d position o f r e s p o n d i n g hand, caused t h e o b s e r v e d compatibility e f f e c t . This result was confirmed by an experiment i n w h i c h subjects used crossed s t i c k s t o p r e s s t h e response buttons, so t h a t even t h e index f i n g e r s o f t h e r e s p o n d i n g hands were on t h e i r normal right o r l e f t sides. Because Riggio e t al.'s (1986) p a p e r was t h e f i r s t s t u d y t h a t e x p l i c i t l y d e a l t w i t h t h e hand-position/key-position d i s t i n c t i o n (Klapp, Greim, Mendicino, & Koenig, 1979, Experiment 1, dealt w i t h it implicitly, see later), t h e empirical basis f o r t h i s d i s t i n c t i o n is s t i l l r e l a t i v e l y small. In particular, n o investigation has been c a r r i e d o u t f o r i r r e l e v a n t stimulus location. O n t h e stimulus side, too, t h e r e is a t least one confounding, namely between a p p a r e n t location o f t h e stimulus a n d t h e sensory o r g a n stimulated. C r a f t a n d Simon (1970, Experiment 2) used stereoscopic p r e sentation o f stimuli t o show t h a t t h e compatibility e f f e c t disappears if t h e a p p a r e n t position o f t h e stimulus is in t h e middle o f t h e v i s u a l field, alt h o u g h actually e i t h e r t h e right or t h e l e f t eye is stimulated. Therefore, o n t h e stimulus side, t h e apparent position o f t h e stimulus is essential f o r S-R compatibility. Analogous r e s u l t s were also obtained f o r a u d i t o r y stimulation, u s i n g phase transformation t o p r o d u c e an a p p a r e n t r i g h t h e f t location w i t h b i n a u r a l stimulus presentation (Simon, C r a f t , & Small, 1971; Simon, Small, Ziglar, & C r a f t , 1970a). T h e r e s u l t s f r o m t h e above-mentioned studies show t h a t spatial S-R compatibility is related t o t h e compatibility between p e r c e i v e d stimulus position a n d t h e position o f t h e response k e y . However, in most designs, t h e apparent a n d actual stimulus positions, as well as t h e positions o f t h e r e s p o n d i n g hands a n d response keys, a r e t h e same. Thus, we w i l l t r e a t t h e relation between stimulus position a n d response position as essential f o r spatial S-R compatibility. In o t h e r words, t h e c e n t r a l d i s t i n c t i o n on t h e response side t h a t we a r e addressing in t h i s p a p e r i s t h a t between response position (which may b e k e y position o r h a n d posit i o n ) a n d response effector (e.g., anatomically d e f i n e d right o r l e f t hand). T h e d i s t i n c t i o n between h a n d position a n d k e y position w i l l nevertheless b e i n c l u d e d in t h e hierarchical model proposed subsequently. Several explanations have been proposed t o account f o r t h e importance o f spatial positions. A c c o r d i n g t o Simon, t h e stimulus perceived is a command t o react in a c e r t a i n way, namely, t o p r e s s t h e right o r left button. Because, as a "population stereotype" (Simon & Rudell, 1967, p. 3001, a " ' n a t u r a l ' tendency t o respond t o w a r d thf source o f stimulation" (Simon, 1969, p. 174) i s assumed, t h e r e is t h e necessity o f o v e r r i d i n g t h i s i n i t i a l response tendency" (Simon e t al., 1970a, p. 314) when t h e content o f t h e command (i.e., t h e locus o f t h e c o r r e c t response) does n o t agree w i t h t h e locus o f t h e stimulus source, y i e l d i n g l o n g e r RTs. In e a r l i e r papers, Simon h a d considered t h i s stereotype t o express an o r i e n t i n g r e f l e x (Simon, 1968, 19691, but h e l a t e r abandoned t h i s i n t e r p r e t a t i o n because such an e f f e c t should v e r y q u i c k l y disappear w i t h practice, which is n o t t r u e f o r t h e spatial S - R compatibility e f f e c t (see Simon, 1970; Faber, v a n d e r Molen, Keuss, & Stoffels, 1986). Furthermore, Simon did n o t claim t h a t t h i s n a t u r a l tendency is "natural" in t h e s t r i c t sense: "It m i g h t v e r y well b e learned" (Simon, 1970, p. 51).

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A c c o r d i n g t o Wallace (1971), spatial S-R compatibility is d u e t o a comparison o f spatial codes f o r stimulus a n d response positions, leading t o l o n g e r RTs when t h e representations o f stimulus a n d response posit i o n s d o n o t coincide. T h i s c o d i n g i n t e r p r e t a t i o n resembles F i t t s a n d Seeger's (1953) t h e o r y t h a t S-R compatibility is related t o t h e r a t e o f L a t e r researchers have f u r t h e r information t r a n s f e r in reaction tasks. advanced Wallace's c o d i n g i n t e r p r e t a t i o n o f spatial S-R compatibility (in p a r t i c u l a r , Nicoletti e t al., 1982; Nicoletti & Umilt6, 1984, 1985; Umilt6 & Nicoletti, 1985; Umilt6 & L i o t t i 1987; see C h a p t e r 3 by UmiltA & Nicoletti) . However, whereas these l a t t e r researchers saw a fundamental d i s t i n c t i o n between Simon's approach, w h i c h t h e y called "attentional" (Nicoletti e t al., 1982; we followed t h i s perhaps u n f o r t u n a t e terminology in Heister, Ehrenstein, & Schroeder-Heister, 1986, 1987), a n d t h e coding hypothesis, Wallace did not. Rather, h e r e g a r d e d h i s coding t h e o r y as a more a b s t r a c t basis f o r Simon's "tendency t o respond t o w a r d t h e source o f stimulation" (see Wallace, 1971, p. 360). With respect t o t h e stage o f c o g n i t i v e processing in which S-R comp a t i b i l i t y effects a r e generated, t h e major position is t h a t t h e responseselection stage, r a t h e r t h a n t h e stimulus-encoding stage, is r e l e v a n t (cf. Sanders, 1980). T h i s hypothesis is s u p p o r t e d by r e s u l t s u s i n g Sternberg's (1969) a d d i t i v e f a c t o r s paradigm (see Acosta & Simon, 1976; Callan e t al., 1974; Simon, 1982; Simon, Acosta, & Mewaldt, 1975; Simon, Acosta, Mewaldt, & Speidel, 1976). However, t h e r e is also some evidence c o n c e r n i n g t h e importance o f t h e stimulus side, in p a r t i c u l a r t h e dependence o f t h e size o f t h e o b s e r v e d e f f e c t on t h e distance o f t h e stimuli (Ehrenstein, Heister, & Schroeder-Heister, in preparation; Gunia, 1987; Rabbitt, 1967; Simon e t al., 1971).

In t h e following, we w i l l r e f e r t o t h e coding hypothesis as t h e major theoretical explanation o f spatial S-R compatibility, while b e i n g aware t h a t "coding" has a r a t h e r vague meaning. We u n d e r s t a n d "coding hypothesis" t o b e used mainly as a label f o r theories assuming t h a t spatial S-R compatibility is a genuinely c o g n i t i v e phenomenon t h a t has t o d o w i t h t h e i n t e r n a l representation o f spatial relationships. Unimanual T w o - F i n g e r Choice Reactions S t a n d a r d designs f o r s t u d y i n g spatial S-R compatibility t y p i c a l l y i n v e s t i g a t e bimanual reactions t o lateralized v i s u a l o r a u d i t o r y stimuli. Sometimes unimanual movement reactions have also been employed, such as moving a handle t o t h e right o r l e f t (Simon 1968, 1969) o r moving t h e i n d e x f i n g e r f r o m a s t a r t i n g location t o one o f t w o response b u t t o n s (Bauer & Miller, 1982; Brebner, 1979; Cotton, Tzeng, & Hardyck, 1980; Results f r o m Katz (1981) indicate t h a t spatial S Hedge & Marsh, 1975). R compatibility effects also a r e obtained between stimulus locations a n d t h e t w o f i n g e r s o f one hand. In an experiment w i t h i r r e l e v a n t stimulus location a n d unimanual reactions, w i t h t h e h a n d h e l d i n a middle position, Katz showed t h a t compatible v i s u a l f i e l d / f i n g e r p a i r i n g s (e.g., stimulus in t h e right field, response w i t h t h e right f i n g e r ) were f a s t e r t h a n incomp a t i b l e ones, independent o f w h i c h h a n d was used f o r responding. In t h e following, we w i l l call t h i s k i n d o f spatial S-R compatibility finger Compatibility, as d i s t i n g u i s h e d f r o m hand Compatibility. I n t h e context of unimanual t w o - f i n g e r choice reactions, hand-compatibility effects denote R T advantages f o r compatible field/hand relations t h a t may b e obtained in addition t o f i n g e r - c o m p a t i b i l i t y effects. These l a t t e r effects a r e p r o b a b l y

Spatial Coding and Spatio-Anatomical Mapping p r e s e n t in most data f o r choice reactions w i t h t w o o r more fingers, a r e u s u a l l y n o t evaluated.

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T h e Comparison of Prone a n d Supine H a n d Orientations

In several experiments we have i n v e s t i g a t e d compatibility effects w i t h t w o - f i n g e r choice reactions, placing p a r t i c u l a r emphasis o n t h e d i s t i n c t i o n between spatial a n d anatomical f a c t o r s . T o d i s t i n g u i s h between spatial a n d anatomical influences, an analog t o t h e crossed-arms t e s t o f t h e bimanual case h a d t o b e developed. I n Heister e t al. (1986, 1987), we used t h e comparison between p r o n e h a n d position (response b u t t o n s pressed as usual f r o m above) a n d supine h a n d position (response b u t t o n s pressed f r o m below). When t h e h a n d is t u r n e d , t h e spatial relations between t h e r e s p o n d i n g f i n g e r s a r e reversed: T h e f i n g e r t h a t is spatially r i g h t w i t h t h e h a n d h e l d p r o n e i s spatially l e f t in t h e s u p i n e hand. Comparing t h e f i n g e r - c o m p a t i b i l i t y effects f o r b o t h positions can t h e r e f o r e indicate w h e t h e r t h e o b s e r v e d e f f e c t is d u e t o the position o f t h e responding f i n g e r o r t o some anatomical relation. These possibilities were confounded in Katz's (1981) experiment. Results f o r r e l e v a n t stimulus location. T h e main question o f o u r f i r s t s t u d y (Heister e t al., 1986) was w h e t h e r a spatial S - R compatibility e f f e c t occurs w i t h t w o - f i n g e r choice reactions when t h e palms a r e f a c i n g down a n d w h e t h e r it p e r s i s t s when t h e hands a r e t u r n e d o v e r . Eight female subjects h a d t o react t o w a r d a r i g h t o r a l e f t light w i t h t h e index o r middle f i n g e r of t h e i r right o r l e f t hand, w h e r e b y t h e r e s p o n d i n g h a n d was h e l d in a normal r i g h t o r l e f t position. T h e r e s u l t s showed a c l e a r - c u t f i n g e r - c o m p a t i b i l i t y e f f e c t when t h e palms were f a c i n g down a n d an even more pronounced e f f e c t when t h e palms were f a c i n g up. These r e s u l t s s u p p o r t t h e coding hypothesis of spatial S-R compatibility, which says t h a t t h e r e l a t i v e spatial positions o f stimuli a n d responses a r e encoded a n d compared i r r e s p e c t i v e o f t h e anatomical response organs. A n y explanation o f t h e r e s u l t s r e l y i n g on callosal c r o s s i n g time (pathway hypothesis) is immediately r e f u t e d because t h e movements o f t w o f i n g e r s o f one h a n d a r e i n i t i a t e d w i t h i n t h e same hemisphere. Because b o t h responses a r e g i v e n on t h e same side o f t h e b o d y midline, t h e r e s u l t s cannot b e explained by t h e tendency t o react towards t h e source of stimulation ( a t least n o t w i t h o u t making an a d hoc extension o f t h i s hypothesis). Finally, no r i g h t / l e f t classification o f t h e anatomical f i n g e r s can account f o r these results, because t h e spatial compatibility e f f e c t p e r s i s t e d f o r hands in supine position. T h a t is, such a classificat i o n is r e v e r s e d by turning t h e hands and, therefore, would p r e d i c t t h e converse e f f e c t w i t h t h e palm-up position. No hand-compatibility e f f e c t was observed in addition t o t h e s t r o n g f i n g e r - c o m p a t i b i l i t y effect, a l t h o u g h t h e r e s p o n d i n g h a n d was h e l d in lateral r a t h e r t h a n middle position, opposed t o Katz's (1981) design. T h i s negative r e s u l t may b e explained by t h e f a c t t h a t t h e experimental t a s k did n o t demand a choice between responding hands (which were o n l y changed between blocks o f t r i a l s ) , but between t h e responding f i n g e r s o f one hand.

Results f o r i r r e l e v a n t stimulus location a n d f i x e d v e r s u s a l t e r n a t i n g hands. In a subsequent s t u d y (Heister e t a l . , 1987), spatial S - R comp a t i b i l i t y effects f o r t w o - f i n g e r choice reactions were i n v e s t i g a t e d f o r

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i r r e l e v a n t stimulus location. Tests also were i n c l u d e d f o r determining w h e t h e r additional hand-compatibility effects w o u l d b e obtained w i t h i r r e l e v a n t stimulus location a n d w h e t h e r such effects may depend on h o l d i n g t h e r e s p o n d i n g h a n d f i x e d t h r o u g h o u t an experimental session. Eight new female subjects responded w i t h t h e i r index o r middle f i n g e r t o a bicolor diode t h a t emitted e i t h e r r e d o r g r e e n light i n t h e right o r l e f t visual field. T h e subjects were tested w i t h t h e palms f a c i n g down a n d t h e palms f a c i n g up, w i t h t h e r e s p o n d i n g h a n d b e i n g altered in b o t h cond i t i o n s between blocks o f t r i a l s . T h e y also were tested w i t h t h e palms f a c i n g down, but w i t h t h e same h a n d b e i n g used t h r o u g h o u t an e x p e r i mental session. T h e r e s u l t s showed a s t r o n g f i n g e r - c o m p a t i b i l i t y e f f e c t f o r all t h r e e conditions. T h e tendency f o r t h e f i n g e r - c o m p a t i b i l i t y e f f e c t t o increase f r o m t h e palm-down condition ( w i t h a l t e r n a t i n g hands) t o t h e palm-up condition, w h i c h was already p r e s e n t in t h e p r e v i o u s s t u d y (Heister e t al., 19861, now became s t a t i s t i c a l l y s i g n i f i c a n t . As in t h e p r e v i o u s s t u d y , no h a n d -cornpati bi I ity e f f e c t occu r r e d in t h e conditions w i t h a It e r n a t i ng hands. In t h e condition w i t h nonalternating hands, however, an a d d i tional, small hand-compatibility e f f e c t was obtained. These r e s u l t s again suggest t h e prevalence o f t h e spatial coding f a c t o r in S-R compatibility, because t h e compatibility e f f e c t f o r f i n g e r s depends on t h e r e l a t i v e spatial positions of t h e r e s p o n d i n g f i n g e r s w i t h respect t o each o t h e r a n d n o t on t h e i r anatomical relation. T h e small hand-compatibility e f f e c t f o r t h e condition w i t h t h e responding h a n d b e i n g f i x e d t h r o u g h o u t an experimental session can b e explained by a modified attentional hypothesis. A c c o r d i n g t o t h i s hypothesis, constant use o f one h a n d d i r e c t s more a t t e n t i o n t o w a r d t h e c o r r e s p o n d i n g side o f stimulation. However, t h e empirical basis f o r t h i s hand-compatibility e f f e c t is s t i l l too small t o allow f o r more t h a n a t e n t a t i v e hypothesis. Experimental w o r k t o elucidate t h i s e f f e c t is b e i n g performed p r e s e n t l y (Schroeder-Heister, Ehrenstein, & Heister, in p r e p a r a t i o n ) . Spatial Versus Anatomical F i n g e r Distance A n o t h e r experiment w i t h t w o - f i n g e r choice reactions tested t h e hypothesis t h a t t h e size o f t h e spatial S - R compatibility e f f e c t is d i s tance-dependent w i t h respect t o t h e response. T h a t is, we i n v e s t i g a t e d w h e t h e r a n d in w h a t manner t h e compatibility e f f e c t changes if t h e r e l a t i v e distance o f responses i s altered. A s an a l t e r n a t i v e t o t h e supine hands test, s u c h an i n v e s t i g a t i o n should discriminate between coding of response positions a n d anatomical r i g h t / l e f t classification o f f i n g e r s , if one distinguishes between t h e spatial distance of t h e response b u t t o n s A n example o f a n d t h e anatomical distance o f t h e responding f i n g e r s . anatomical distance is t h a t t h e second a n d f o u r t h f i n g e r s a r e nearer t o each o t h e r t h a n a r e t h e f i r s t a n d fifth, even if t h e y operate t h e same pair of buttons. If t h e size o f t h e observed compatibility e f f e c t o n l y depends on t h e spatial distance a n d n o t on t h e anatomical distance, one may i n f e r t h a t spatial coding is t h e e f f e c t i v e f a c t o r . If t h e size o f t h e o b s e r v e d e f f e c t o n l y depends on t h e anatomical distance a n d n o t on t h e

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spatial distance, t h e n an anatomical r i g h t - l e f t discrimination can b e conc l u d e d t o b e essential. F u r t h e r possibilities a r e t h a t t h e size o f t h e comp a t i b i l i t y e f f e c t depends on b o t h spatial a n d anatomical distance, o r t h a t t h e r e is no distance-dependence a t all. Therefore, t h e question o f t h e following experiment was w h e t h e r t h e f i n g e r compatibility e f f e c t remains stable when t h e response i s g i v e n w i t h d i f f e r e n t spatial a n d d i f f e r e n t anatomical distances between responding fingers. I n o u r p r e v i o u s studies, we used index a n d middle f i n g e r s f o r response, but now we chose t h e second ( i n d e x ) a n d f o u r t h f i n g e r ( r i n g ) o r t h e f i r s t (thumb) a n d fifth ( l i t t l e ) f i n g e r , respectively, so t h a t t h e third (middle) f i n g e r c o u l d b e r e g a r d e d as t h e middle o f t h e r e s p o n d i n g hand. To v a r y spatial a n d anatomical distance between f i n g e r s independently, in one o f t h e experimental conditions subjects placed t h e i r f i r s t a n d fifth f i n g e r on response b u t t o n s a t a n a r r o w separ a t i o n t h a t would normally b e used f o r t h e i r second a n d f o u r t h f i n g e r s . Six r i g h t - h a n d e d female college s t u d e n t s (aged 17 t o 19 years) s e r v e d as subjects. T h e stimuli w e r e t w o r e d l i g h t - e m i t t i n g diodes, positioned a t 5 O o f v i s u a l angle t o t h e l e f t a n d r i g h t o f t h e f i x a t i o n p o i n t a n d Response k e y s were t w o c i r c u l a r microswitches, presented f o r 100 ms. whose centers were separated by 45 mm ( n a r r o w b o x ) o r by 110 mm (wide box); these distances were average r e s u l t s f o r h a n d measurements t a k e n o f t h e subjects i n advance: T h e n a r r o w box was used f o r t h e i n d e x finger/ring-finger condition a n d f o r t h e t h u m b / l i t t l e - f i n g e r , narrow spacing condition; t h e wide box was used f o r t h e t h u m b / l i t t l e - f i n g e r wide ( o r n a t u r a l ) spacing condition. T h e subjects attended t w o sessions on d i f f e r e n t days. In one session t h e y responded w i t h t h e i r r i g h t hands, in t h e o t h e r session w i t h t h e i r l e f t hands. In each session, t h r e e conditions were investigated: (a) index-finger/ring-finger operating the narrow buttons, (b) t h u m b / l i t t l e - f i n g e r o p e r a t i n g t h e wide buttons, a n d (c) t h u m b / l i t t l e - f i n g e r operating the narrow buttons. T h e t h r e e conditions were tested in a sequence o f six blocks o f t r i a l s ( i . e . , t w o blocks f o r each condition), each consisting o f s i x p r a c t i c e t r i a l s followed by 60 t e s t t r i a l s (30 in t h e right field, 30 i n t h e l e f t f i e l d ) . I n t h r e e blocks o f each session, t h e subjects made spatially compatible responses; i n t h e o t h e r t h r e e blocks, t h e y made spatially incompatible responses. So as n o t t o d r a w t h e i r attention t o spatial relations, t h e i n s t r u c t i o n s were in terms o f index-, r i n g - , l i t t l e f i n g e r s , o r thumbs, r a t h e r t h a n l e f t o r right position o f t h e fingers. E r r o r s were few, a n d e r r o r t r i a l s were repeated a t t h e e n d o f each block. Mean RTs were subjected t o an analysis o f variance, w i t h response condition (a, b, c), responding h a n d ( r i g h t A e f t ) , f i e l d o f stimulus p r e sentation ( r i g h t / l e f t ) , a n d r e s p o n d i n g f i n g e r ( r i g h t / l e f t ) as w i t h i n - s u b j e c t factors (see Table 1 f o r cell means a n d c o r r e s p o n d i n g s t a n d a r d d e v i ations). T h e r e were t w o s i g n i f i c a n t main effects, one f o r experimental condition, F(2, 10) = 4.61, p < .05, a n d one f o r f i e l d o f stimulus presentation, F(1, 5 ) = 49.52, p < .001. Responses were f a s t e s t w i t h second a n d f o u r t h f i n g e r (Condition a: 281 ms) a n d slowest w i t h f i r s t a n d fifth f i n g e r p r e s s i n g t h e n a r r o w b u t t o n s (Condition c : 293 ms). Reactions were overall f a s t e r w i t h r i g h t - f i e l d t h a n w i t h l e f t - f i e l d stimulation (283 v s . 293 m s ) , which m i g h t b e i n t e r p r e t e d a s e x p r e s s i n g a left-hemisphere s u p e r i o r i t y f o r spatial decision tasks a n d is i n agreement w i t h o u r

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p r e v i o u s r e s u l t s f o r S-R designs w i t h r e l e v a n t stimulus location (Heister T h e s i g n i f i c a n t interaction between f i e l d a n d r e s p o n d i n g e t al., 1986). f i n g e r , F(1, 5) = 87.47, p < .001, expresses t h e S-R compatibility e f f e c t f o r f i n g e r s . T h e r e was a s i g n i f i c a n t change in t h e size o f t h i s compatibility e f f e c t between t h e d i f f e r e n t response conditions, as indicated in t h e t h r e e - w a y interaction between response condition, f i e l d o f stimulation, a n d f i n g e r , F(2, 10) = 9.06, p < .01 (see F i g u r e 1). Table 1 Mean Reaction Times [in milliseconds) a n d S t a n d a r d Deviations f o r Experimental Condition a (second a n d f o u r t h f i n g e r s o p e r a t i n g n a r r o w buttons), Condition b ( f i r s t a n d f i f t h f i n g e r s o p e r a t i n g wide distance buttons), a n d Condition c ( f i r s t a n d f i f t h f i n g e r s o p e r a t i n g n a r r o w buttons)

L e f t light

R i g h t light Responding f i n g e r

Left

Right

Left

Condition a Right hand

264 (37)

310 (39)

Left hand

259 (25)

314 (22) Condition b

Right hand

32 1 (37)

272 (28)

295 (34)

Left hand

Condition c Right hand

266 (30)

315 (35)

Left hand

282 (41 1

324 (27)

Note.

T h e numbers in parentheses a r e s t a n d a r d deviations.

Right

Spatial Coding and Spatio-Anatomical Mappins

CONDITION a

CONDITION b

125

CONDITION c

RTIms) 340

280

260 240

LEFT LIGHT

RIGHT LlGHT

LEFT LIGHT

RIGHT LIGHT

LEFT LIGET

R IGHT LIGHT

F i g u r e 1. Mean R T s f o r responses t o l i g h t s in t h e right o r l e f t v i s u a l f i e l d w i t h t h e right ( f i l l e d circles) a n d l e f t f i n g e r s ( u n f i l l e d circles), averaged o v e r b o t h hands. Condition a: Second a n d f o u r t h f i n g e r s o p e r a t i n g n a r r o w buttons; Condition b: F i r s t a n d fifth f i n g e r s o p e r a t i n g wide distance buttons; Condition c: F i r s t a n d f i f t h f i n g e r s o p e r a t i n g n a r r o w buttons.

U n d e r Condition b, in w h i c h t h e wide-distance b u t t o n s were pressed w i t h t h e f i r s t a n d f i f t h fingers, t h e compatibility e f f e c t was significantly smaller t h a n in t h e o t h e r conditions ( d i f f e r e n c e between incomp a t i b l e a n d compatible reactions in Condition a: 51 ms, Condition b: 26 ms, Condition c: 46 ms; see F i g u r e 1 ) . Because t h i s was especially t r u e f o r l e f t h a n d responses, t h e i n t e r a c t i o n between response condition, hand, field, a n d f i n g e r was significant, F(2, 10) = 7.57, p = .01. A s i g n i f i c a n t d i f f e r e n c e in t h e compatibility e f f e c t between right a n d l e f t hands took place o n l y in Condition b, in w h i c h t h e f i r s t a n d fifth f i n g e r s operated t h e wide-distance b u t t o n s (interaction between hand, field, a n d f i n g e r f o r t h i s condition: F(1, 5) = 11.21, p < .05). F u r t h e r research t o c l a r i f y w h y t h i s e f f e c t took place o n l y f o r l e f t - h a n d responses i s b e i n g performed. T h e most i n t e r e s t i n g r e s u l t is t h a t t h e r e was no d i f f e r e n c e in t h e size o f t h e S - R compatibility e f f e c t f o r Condition a (index f i n g e r / r i n g f i n g e r - - n a r r o w b u t t o n s ) a n d Condition c ( t h u m b / l i t t l e f i n g e r - - n a r r o w buttons), which d i f f e r w i t h respect t o anatomical f i n g e r distance, but n o t w i t h respect t o spatial f i n g e r distance. T h i s means t h a t t h e anatomical distance o f t h e r e s p o n d i n g f i n g e r s alone is n o t r e l e v a n t for the size o f t h e compatibility effect. B u t t h e r e was a s i g n i f i c a n t d i f f e r e n c e between finger--wide buttons) a n d Condition c Condition b ( t h u m b / l i t t l e ( t h u m b / l i t t l e f i n g e r - - n a r r o w b u t t o n s ) , w h i c h d i f f e r w i t h respect t o spatial but n o t w i t h respect t o anatomical f i n g e r distance f i n g e r distance, (interaction between condition, field, a n d f i n g e r in t h e subanalysis f o r data o f Conditions b a n d c : F(1, 5 ) = 15.49, p = .01). T h e S-R comp a t i b i l i t y e f f e c t was smaller when t h u m b a n d l i t t l e f i n g e r pressed t h e wide

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b u t t o n s than when t h e y pressed t h e n a r r o w buttons, which means t h a t o n l y t h e spatial (environmental) distance o f t h e responding f i n g e r s seems t o b e relevant to t h e size o f t h e effect. Therefore, we can conclude t h a t t h e spatial S - R compatibility e f f e c t f o r t w o - f i n g e r choice reactions i t s e l f essentially depends on t h e p u r e l y spatial r a t h e r t h a n anatomical relations o f t h e responses. Spatial C o d i n g a n d Spatio-Anatomical Mapping T h e Effectiveness of Subordinate Factors When t h e spatial r i g h t h e f t d i s t i n c t i o n on the stimulus side is eliminated by a r r a n g i n g t h e stimulus along t h e spatial top/down dimension, but response b u t t o n s a r e s t i l l o n t h e right o r l e f t side, t h e n t h e compatibility e f f e c t disappears (see Simon & Wolf, 1963; Soetens, Deboeck, Hueting, & Merckx, 1984; Wallace 1971, 1972--nevertheless, t h e r e may b e an association o f dominant h a n d w i t h t o p position, see LBdavas, 1987). However, a compatibility e f f e c t may b e obtained when on the response side t h e r i g h t h e f t d i s t i n c t i o n f o r response keys is eliminated, b u t stimulus l i g h t s a r e s t i l l o n t h e right o r l e f t side, because f o r t h e responses o t h e r r i g h t h e f t distinctions (i.e., position o f response effector or anatomical classification) s t i l l remain in force. Klapp e t a l . (1979, Experiment 1--crossed and uncrossed conditions) had subjects operate a single switch, which was mounted in a middle position, w i t h t h e i r thumbs f r o m t h e right o r l e f t side, a n d observed a spatial-compatibility e f f e c t ( R T advantage f o r compatible handlight relations). B y p e r f o r m i n g t h i s experiment w i t h crossed a n d uncrossed arms, Klapp e t a l . made s u r e t h a t t h e e f f e c t was n o t d u e t o t h e anatomical status o f t h e hands as right o r l e f t b u t t o t h e position of t h e responding hand. Furthermore, when t h e r i g h t / l e f t distinction f o r response keys, as well as f o r positions o f responding hands, is eliminated, a compatibility effect is observed f o r which t h e anatomical status o f t h e responding hand as right o r l e f t is relevant. Again, experiments by Klapp e t al. (1979) can b e used as evidence: T h e y had subjects respond t o l e f t a n d right stimuli, w i t h hands h e l d in t h e up/down dimension, by o p e r a t i n g a single A h i g h l y significant interaction b u t t o n w i t h t h e i r respective thumbs. between ( r i g h t o r l e f t ) f i e l d o f stimulation and (anatomically right o r l e f t ) responding hand was f o u n d b o t h f o r uncrossed and crossed arms (Klapp e t al., 1979, Experiment 1, up/down condition w i t h stimulus location relevant; Experiment 2, experimental condition w i t h stimulus location i r r e l e vant). These f i n d i n g s also were obtained when subjects l a y on t h e i r sides and t h e spatial r i g h t / l e f t positions o f t h e responding arms were These results show t h a t eliminated (Klapp e t al., 1979, Experiment 3 ) . those spatial relations t h a t are subordinate t o t h e basic stimulus position/response- k e y position relation (i.e., t h e stimulus-position/responseeffector position relation and t h e stimulus-position/anatomical-hand relation) can b e effective.

A corresponding r e s u l t f o r f i n g e r compatibility was obtained in o u r s t u d y w i t h two-finger choice reactions and orthogonal S-R relations (Ehrenstein, Schroeder-Heister, & Heister, 1989). T h e main question was whether, u n d e r conditions in which no d i r e c t r i g h t - l e f t association of


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stimulus positions a n d positions o f response effectors is possible, an anatomical r i g h t / l e f t classification o f r e s p o n d i n g f i n g e r s becomes essential. F o r t h i s purpose, an experimental design was chosen in w h i c h stimuli a n d responses w e r e p e r p e n d i c u l a r t o each o t h e r . In f o u r d i f f e r e n t sessions, subjects reacted t o r e d o r g r e e n l i g h t s in t h e right o r l e f t v i s u a l f i e l d ( i r r e l e v a n t stimulus location) by p r e s s i n g response k e y s t h a t w e r e a r r a n g e d in t h e following ways: (a) on t h e right o r l e f t side o f t h e b o d y midline ( c o n t r o l condition), (b) p e r p e n d i c u l a r t o t h e stimuli horizontal in t h e midsaggital plane, (c) same as in Condition b but pressed f r o m below (supine h a n d position), a n d (d) p e r p e n d i c u l a r t o t h e stimuli v e r t i c a l in t h e midsaggital plane, pressed w i t h t h e palms f a c i n g t h e b o d y . T h e r e s u l t s f o r Condition a ( t h e c o n t r o l condition) showed t h e usual spatial S-R compatibility e f f e c t f o r f i e l d o f stimulus presentation and responding finger. F o r Conditions b a n d d, a s t r o n g S-R compatibility effect was f o u n d f o r t h e same stimulus/finger p a i r i n g s . For Condition c, t h a t is, reactions w i t h supine h a n d position, R T advantages f o r t h e opposite s t i m u l u d f i n g e r p a i r i n g s were obtained. T h i s means t h a t those s t i m u l u d f i n g e r relations were f a s t e r t h a n t h e y would b e if one t u r n e d t h e h a n d b a c k i n t o t h e normal right o r l e f t position (while keeping i t s p r o n e o r supine orientation). These f i n d i n g s s t r o n g l y s u p p o r t t h e hypothesis t h a t an anatomical r i g h t / l e f t d i s t i n c t i o n becomes e f f e c t i v e if t h e r i g h t / l e f t d i s t i n c t i o n between response positions i s eliminated. The s h i f t o f t h i s relation u n d e r Condition c can b e i n t e r p r e t e d as showing that, u n l i k e t h e case o f t h e right a n d l e f t hands, t h e r e i s no canonical r i g h t / l e f t classification o f anatomical f i n g e r s . T h a t is, t h i s classification may depend o n h a n d o r i e n t a t i o n ( p r o n e v s . supine). One m i g h t suppose t h a t t h e effectiveness o f t h e anatomical r i g h t / l e f t d i s t i n c t i o n s f o u n d by Klapp e t al. (1979) c o u l d b e explained by neuroanatomical c o n n e c t i v i t y . However, t h e effects o b s e r v e d a r e much h i g h e r t h a n s t a n d a r d estimates o f t h e callosal c r o s s i n g time a n d h a v e t h e size o f a s t a n d a r d compatibility e f f e c t f o r w h i c h t h e positions o f response keys a r e important. Furthermore, t h e y o n l y a r e obtained, a t least in t h e size observed, if t h e r e is a choice reaction between t h e r e s p o n d i n g hands. If t h e r e is n o choice between t h e r e s p o n d i n g hands, a compatibility e f f e c t f o r hands is, if e x i s t e n t a t all, v e r y small (see Klapp e t al., 1979, Experiment 2). And, finally, o u r r e s u l t s w i t h orthogonal S - R relations r e f u t e t h i s hypothesis, because t h e responses o f t w o f i n g e r s o f one h a n d a r e i n i t i a t e d f r o m t h e same hemisphere. T h i s suggests t h a t t h e o b s e r v e d anatomical r i g h t / l e f t effects a r e c o g n i t i v e l y based, as a r e t h e o t h e r S-R compatibility effects mentioned. One may hypothesize t h a t t h e r e is a coding o f t h e right a n d l e f t h a n d as spatially right a n d left, respectively. T h i s code is based on t h e f a c t t h a t t h e anatomically right a n d l e f t hands a r e s p a t i a l l y right a n d left, when t h e hands a r e h e l d i n normal positions on t h e respective sides o f the body. T h i s code is t h e n kept, even when t h e hands a r e h e l d in non-normal positions, t h a t is, in middle positions o r on opposite sides o f t h e body. One may also speak of a " n a t u r a l association" o f t h e anatomically right a n d l e f t hands as spatially right a n d l e f t . Correspondingly, f o r t w o - f i n g e r choice reactions, we may assume t h a t t h e r e a r e codes f o r anatomical f i n g e r s as right o r l e f t (even if these codes a r e n o t u n i q u e a n d canonical as f o r right a n d l e f t hands), a n d t h a t these codes a r e k e p t even when t h e response keys a r e n o t mounted parallel t o t h e stimuli.

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T o a v o i d confusion, we propose t h e t e r m spatio-anatomical mapping (in s h o r t : mapping) f o r t h i s association o f spatial positions w i t h anatomical distinctions, a n d r e s e r v e t h e t e r m spatial coding f o r t h e coding o f response positions. We w i s h t o emphasize h e r e t h e c o g n i t i v e c h a r a c t e r o f spatio-anatomical mapping, w h i c h is n o t e x p l i c i t l y stated in Heister e t al. (19861, w h e r e t h i s t e r m was i n t r o d u c e d . 2 Evidence f o r spatial S-R compatibility effects in situations f o r w h i c h t h e response k e y s a r e n o t o r i e n t e d along t h e r i g h t / l e f t dimension can also b e f o u n d in neuropsychological investigations o f cerebral functioning, w h i c h we i l l u s t r a t e by some data o f o u r own. C o m p a t i b i l i t y E f f e c t s With O r t h o g o n a l S-R Relations in Divided-VisualF i e l d Studies of C e r e b r a l Lateralization Because S-R compatibility effects t a k e place n o t o n l y w i t h relevant stimulus location but also w i t h i r r e l e v a n t location, compatibility m i g h t a f f e c t e v e r y normal d i v i d e d v i s u a l - f i e l d s t u d y o f c e r e b r a l lateralization in w h i c h choice reactions a r e r e q u i r e d t o stimuli in t h e right o r l e f t visual f i e l d . Therefore, t h e R T asymmetry ( f i e l d effect), w h i c h u s u a l l y is used as an i n d i c a t o r o f t h e n a t u r e o f hemispheric processing, is d i f f i c u l t t o i n t e r p r e t because o f potential S-R compatibility effects. We f i r s t obtained evidence f o r such j o i n t effects o f cerebral lateralization a n d S-R compatibility by r e p l i c a t i n g a lexical decision t a s k u n d e r t h r e e d i f f e r e n t response conditions: (a) normal unimanual t w o - f i n g e r choice reactions, w i t h t h e hands positioned on t h e right a n d l e f t sides; (b) unimanual t w o - f i n g e r choice reactions, w i t h t h e response keys positioned one b e h i n d t h e o t h e r along t h e midline o f t h e experimental d e s k (medial position); (c) vocal responses (see Heister & Schroeder-Heister, 1987). T h e experiment w i t h lateral response position (a) showed a handcompatibi ‘ty e f f e c t t h a t s i g n i f i c a n t l y decreased w i t h medial response posit i o n (b).

5

Therefore, in a subsequent s t u d y on hormonal influences on hemis p h e r i c processing during t h e menstrual cycle (Heister, Landis, Regard, 6 Schroeder-Heister, 19891, we tried t o exclude such possible additional S-R compatibility effects by p l a c i n g t h e response b u t t o n s ( f o r bimanual reactions) one b e h i n d t h e o t h e r in medial position (i.e., orthogonal t o t h e stimuli along t h e midline o f t h e experimental d e s k ) . In t h i s way, t h e r i g h t / l e f t d i s t i n c t i o n f o r positions o f response k e y s was eliminated. However, in v i e w o f t h e r e s u l t s r e p o r t e d in t h e p r e v i o u s section, t h i s arrangement m i g h t n o t b e able t o fully eliminate S-R compatibility, because a d i s t i n c t i o n between right a n d l e f t h a n d is s t i l l possible. Therefore, a reanalysis of t h e data f r o m t h i s experiment is presented, w h i c h focuses o n compatibility effects. T h e basic hypothesis is t h a t spatial S-R compatibility effects s t i l l a r e obtained in addition t o c e r e b r a l effects a n d hormonal influences. 2The possible c o g n i t i v e representation o f anatomical d i s t i n c t i o n s is also t a k e n i n t o consideration by G u i a r d (1984, p . 20). 3Because o f t h e i n i t i a l main i n t e r e s t in interhemispheric relations, f i n g e r - c o m p a t i b i l i t y effects were n o t evaluated in Heister a n d SchroederHeister (1987).


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Twelve normally m e n s t r u a t i n g females aged 22 to 39 y e a r s took p a r t i n t w o experiments, one lexical-decision t a s k ( f u n c t i o n w o r d s v s . nonsense syllables) a n d one analogously c o n s t r u c t e d face-decision t a s k (photographically p r o d u c e d composites o f normal v s . scrambled faces). Half o f t h e subjects gave yes-decisions w i t h t h e f a r t h e r b u t t o n a n d nodecisions w i t h t h e n e a r e r one, a n d t h e o t h e r h a l f h a d t h e r e v e r s e d relation. T h e stimuli were presented f o r 130 ms in t h e right o r l e f t v i s u a l field. E v e r y subject took p a r t f o u r times i n e v e r y experiment in f o u r d i f f e r e n t phases o f t h e menstrual cycle. T h r e e subjects s t a r t e d in each o f t h e f o u r c y c l e phases ( f o r a more detailed d e s c r i p t i o n see Heister e t al., 1989). In Heister e t al. (1’989), t h e data were evaluated w i t h respect t o phase o f menstrual c y c l e w i t h o u t c o n s i d e r i n g t h e r e s p o n d i n g h a n d as a separate f a c t o r . T h e following reanalysis p r e s e n t s t h e data g r o u p e d w i t h respect t o o r d e r o f t e s t sessions (session 1 t o session 4) a n d effects o f r e s p o n d i n g hands (right v s . l e f t ) . T h i s reanalysis may also answer t h e question o f w h e t h e r p r a c t i c e a n d experience w i t h t h e t a s k s influence a possible S-R compatibility e f f e c t . F o r each o f t h e t w o experiments ( v e r b a l a n d non-verbal), an analysis o f variance was conducted f o r means o f t h e c o r r e c t RTs w i t h t e s t session (1 t o 41, f i e l d o f stimulus presentation (right/left), a n d r e s p o n d i n g h a n d ( r i g h t h e f t ) as w i t h i n - s u b j e c t f a c t o r s . O n l y r e l e v a n t data a r e presented here.

In t h e lexical-decision experiment, in addition t o an o v e r a l l p r a c t i c e e f f e c t (linear decrease o f RTs f r o m session 1 t o session 4) a n d a right f i e l d ( l e f t hemisphere) s u p e r i o r i t y , a s i g n i f i c a n t interaction was obtained between t e s t session, f i e l d a n d hand, F ( 3 , 33) = 3.43, p < .05. T h e r e was an S-R compatibility e f f e c t in t h e f i r s t t e s t session (compatible reactions [right f i e l d / r i g h t hand, l e f t f i e l d / l e f t hand] b e i n g 57 ms f a s t e r t h a n incompatible ones), w h i c h disappeared during t h e replications a n d even s h i f t e d t o t h e opposite i n t h e last session, in w h i c h incompatible reactions were 14 ms f a s t e r t h a n compatible ones (see F i g u r e 2). T h e analysis f o r t h e face-decision experiment also showed a p r a c t i c e e f f e c t and, as expected, a l e f t f i e l d (right hemisphere) s u p e r i o r i t y . In addition, t h e r e was a s i g n i f i c a n t interaction between f i e l d o f stimulus p r e sentation a n d r e s p o n d i n g hand, F ( 1 , 11) = 6.52, p < .05. T h i s interact i o n expresses an overall S - R compatibility effect, w i t h compatible responses b e i n g 15 ms f a s t e r t h a n incompatible ones (see F i g u r e 3).

W i t h small S-R compatibility effects, one can always object t h a t t h e y were actually pathway effects. However, i n t h e p r e s e n t case t h i s could n o t explain w h y t h e e f f e c t disappeared i n t h e lexical-decision t a s k a f t e r t h e f i r s t session a n d even s h i f t e d in t h e last session, because pathway effects should r e f l e c t h a r d w a r e and, thus, should b e stable. In general, these d i v i d e d v i s u a l - f i e l d studies o f cerebral lateralization showed evidence f o r S-R compatibility effects f o r t h e responding hand, although t h e response b u t t o n s were in t h e midsaggital plane a n d t h u s orthogonal t o t h e stimuli, a n d t h e position o f t h e stimuli was i r r e l e v a n t f o r t h e decision. I n addition, t h e r e were differences between t h e lexical-decision a n d t h e face-decision t a s k t h a t m i g h t have t o d o w i t h

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differences in t a s k d i f f i c u l t y : For t h e more d i f f i c u l t t a s k (face decisions), t h e S - R compatibility e f f e c t did n o t change d u r i n g f o u r r e t e s t sessions, whereas it did f o r t h e easier t a s k (lexical decisions). L E X I C A L DECISIONS RT(as) 900

..

850

.-

800

..

750

..

700

.-

650

.-

ct J /

I

I

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1

1

2

3

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EXPERIMENTAL SESSION

F i g u r e 2. Mean R T s f o r responses t o v e r b a l stimuli f o r compatible ( f i l l e d circles) a n d incompatible ( u n f i l l e d circles) f i e l d - h a n d relationships f o r experimental sessions 1 t o 4.

O u r r e s u l t s s u p p o r t t h e hypothesis that spatio-anatomical mapping becomes e f f e c t i v e if spatial coding o f response positions i s excluded by a r r a n g i n g response k e y s p e r p e n d i c u l a r t o t h e stimuli. One m i g h t object t h a t n o t t h e anatomical r i g h t / l e f t d i s t i n c t i o n , but r a t h e r t h e spatial posit i o n o f r e s p o n d i n g hands ( w h i c h were s t i l l on t h e right o r l e f t side, even t h o u g h t h e response b u t t o n s , b e i n g pressed w i t h t h e index fingers, were mounted in middle position) was essential f o r t h e e f f e c t observed. However, even if t h i s objection i s correct, w h i c h cannot b e decided on t h e basis o f t h e data available, t h e r e s u l t s show t h a t h a n d position, as d i s t i n g u i s h e d f r o m k e y position, can b e e f f e c t i v e . Finally, t h e r e s u l t s c o n f i r m t h e claim t h a t spatial S - R compatibility n o t o n l y i s obtained w i t h low-level t a s k s (such as r i g h t / l e f t o r red/green discrimination) but also w i t h h i g h e r - l e v e l tasks (such as lexical o r face decisions)--a t o p i c t h a t has n o t y e t been g i v e n a p p r o p r i a t e attention in compatibility research. T h i s means t h a t in neuropsychological studies o f t h i s kind, spatial S-R compatibility effects cannot b e t o t a l l y r u l e d o u t .


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:I 1 650

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I

1

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I

4

F i g u r e 3. Mean R T s f o r responses t o n o n - v e r b a l stimuli f o r compatible ( f i l l e d circles) a n d incompatible ( u n f i l l e d circles) f i e l d - h a n d relationships f o r experimental sessions 1 t o 4.

A Hierarchical Model of Spatial S-R C o m p a t i b i l i t y T h e T h r e e Factors a n d T h e i r Rank O r d e r We have d i s t i n g u i s h e d t h r e e kinds o f compatibility t h a t d i f f e r w i t h respect t o t h e spatial coding o f t h e response: (a) compatibility d u e t o t h e coding o f positions o f response keys (coding of key position), (b) compatibility d u e t o t h e c o d i n g of positions o f r e s p o n d i n g hands o r f i n g e r s (coding of effector position), a n d (c) compatibility d u e t o t h e coding o f t h e response e f f e c t o r s as right o r l e f t (spatlo-onotomical m p p i n g ) . A c c o r d i n g t o t h e r e s u l t s reported, w h i c h o f these compatibilities is e f f e c t i v e in a g i v e n situation is n o t an u n p r e d i c t i b l e f u n c t i o n o f t h e response conditions t h a t a r e used in an experimental design. Rather, t h e i r appearance follows a clear p a t t e r n : C o d i n g o f k e y position is t h e dominant f a c t o r (see Riggio e t al., 1986). However, if such a coding is impossible (as in K l a p p e t al., 1979, Experiment 1, crossed a n d uncrossed conditions, f o r w h i c h t h e r e was no spatial d i f f e r e n c e between t h e response goals), t h e n spatial coding o f e f f e c t o r position becomes t h e dominant f a c t o r . Furthermore, if an a p p r o p r i a t e coding o f positions of response effectors i s likewise impossible (as in Klapp e t al., 1979, Experiment 1, up/down condition, f o r w h i c h stimuli were in r i g h t / l e f t

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position but r e s p o n d i n g h a n d in up/down position, o r in t h e orthogonal S - R arrangements f o r t w o - f i n g e r choice reactions o f Ehrenstein e t al., 19881, t h e n spatio-anatomical mapping becomes t h e dominant f a c t o r . Thus, we propose a model t h a t assumes a hierarchical o r d e r o f t h e following f a c t o r s : (a) spatial coding o f response keys, (b) spatial coding o f e f f e c t o r positions, (c) spatio-anatomical mapping. These f a c t o r s j o i n t l y determine spatial S-R compatibility. Factor (a) i s dominant o v e r f a c t o r s (b) a n d (c), a n d f a c t o r (b) i s dominant o v e r (c). However, if one f a c t o r is n o t applicable, t h e n t h e f a c t o r o f n e x t lower r a n k becomes t h e dominant one. T h i s approach, w h i c h develops f u r t h e r an idea by Klapp e t al. (1979) on anatomical a n d environmental dimensions o f S-R compatibility, enables us t o t r e a t spatial S-R compatibility as a u n i f o r m phenomenon t h a t i s determined by several f a c t o r s w h i c h become e f f e c t i v e according t o t h e i r r a n k o r d e r . T h e h i e r a r c h i c a l model proposed is c e r t a i n l y n o t fully exhaustive. It p u t s p a r t i c u l a r emphasis on d i s t i n c t i o n s between ways o f coding spatial relations on t h e response side. D i s t i n g u i s h i n g ways o f coding spatial relations on t h e stimulus side may lead t o an extension o f t h e model. What we propose is o n l y a f i r s t attempt to, overcome t h e situation t h a t a f t e r more t h a n t w e n t y [now: thirty] years research t h e r e is n o m e t r i c f o r compatibility t h a t transcends p a r t i c u l a r experimental conditions" (Welford, 1980, p. 99). I n t e r a c t i o n s Between t h e Factors of t h e Hierarchical Model T h e hierarchical model a n d i t s r a n k o r d e r were motivated by t h e r e s u l t s t h a t show t h a t a c e r t a i n t y p e of compatibility becomes r e l e v a n t if c e r t a i n o t h e r s a r e n o t applicable. However, t h e r e a r e also r e s u l t s t h a t suggest t h a t t h e d i f f e r e n t f a c t o r s do i n t e r a c t . T h i s means t h a t t h e f a c t o r s o f t h e model n o t o n l y replace each o t h e r following a c e r t a i n r a n k o r d e r , but can j o i n t l y determine an o b s e r v e d e f f e c t (by p r e s e r v i n g t h i s order). With respect t o spatial coding v e r s u s spatio-anatomical mapping, t h e r e s u l t s a n d theoretical proposals by Nicoletti, U m i l t i , a n d LAdavas (1984) o n t h e slowing down o f reactions w i t h crossed arms can b e i n t e r p r e t e d i n t h a t way. T h e y showed t h a t w i t h simple R T s t h e r e is no slowing down o f reactions if t h e r e s p o n d i n g h a n d is h e l d on t h e opposite side o f t h e b o d y midline. T h e y concluded t h a t t h e longer o v e r a l l R T s u s u a l l y observed u n d e r crossed-arms conditions is d u e t o a mismatch between t h e spatial codes o f h a n d a n d h a n d position and, t h u s , is a c o g n i t i v e phenomenon. T h e same t h e o r i z i n g applies t o t h e slowing down o f reactions o b s e r v e d f o r supine as compared t o p r o n e h a n d o r i e n t a t i o n in o u r e x p e r i ments w i t h t w o - f i n g e r choice reactions. Because turning t h e hands causes a r e v e r s a l o f t h e spatial o r d e r o f t h e anatomical fingers, t h i s slowing down o f reactions can b e a t t r i b u t e d t o a mismatch between spatial codes o f anatomical f i n g e r s a n d f i n g e r positions. However, it was statist i c a l l y s i g n i f i c a n t o n l y w i t h r e l e v a n t stimulus location (Heister e t al., 1986), showing a n o n - s i g n i f i c a n t t r e n d w i t h i r r e l e v a n t stimulus location (Heister e t al., 1987). T h i s may b e d u e t o t h e f a c t t h a t anatomical r i g h t / l e f t codes a r e n o t so obvious a n d u n i q u e l y determined f o r responding f i n g e r s as f o r r e s p o n d i n g hands. A n e a r l y s i g n i f i c a n t increase o f RTs u n d e r t h e supine h a n d condition was also o b s e r v e d in Ehrenstein e t


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al. (1988, Condition c), w h e r e response k e y s were a r r a n g e d perpend i c u l a r t o t h e stimuli, so t h a t no a p p r o p r i a t e spatial code f o r t h e f i n g e r positions c o u l d b e formed. H e r e t h i s increase may b e d u e t o a mismatch between d i f f e r e n t anatomical f i n g e r codes f o r p r o n e a n d supine orientation. However, r e s u l t s a r e n o t y e t available w i t h simple R T s f o r p r o n e v s . supine h a n d orientation, by means of w h i c h one c o u l d c l e a r l y d i s t i n g u i s h between a c o g n i t i v e e f f e c t d u e t o a mismatch between d i f f e r e n t codes a n d a p u r e l y bio-mechanical d i f f e r e n c e between t h e t w o h a n d o r i e n tations. Even t h e generally slower reaction f o r t h e condition in w h i c h t h e f i r s t a n d fifth f i n g e r s pressed t h e n a r r o w b u t t o n s ( t h e f i n g e r - d i s t a n c e experiment r e p o r t e d in t h i s c h a p t e r ) m i g h t b e i n t e r p r e t e d in - a similar way. In t h i s condition, a mismatch may b e seen as o c c u r r i n g between t h e codes o f a n a r r o w spatial distance a n d a wide anatomical distance, leading t o t h e o v e r a l l increase o f R T s . However, t h i s idea m u s t b e cons i d e r e d v e r y t e n t a t i v e l y , because an explanation t h a t simply r e f e r s t o t h e mechanical d i f f i c u l t y o f t h e t a s k has n o t been experimentally excluded. Analogous hypotheses can b e applied t o Riggio e t al.'s (1986) r e s u l t s on t h e slowing down o f reactions w i t h crossed f i n g e r s o r crossed sticks. These f i n d i n g s may b e considered as b e i n g d u e t o a mismatch between t h e spatial codes f o r response goals a n d position o f response effectors. ( R i g g i o e t al. themselves explained t h e f i n d i n g s by a mismatch between t h e spatial codes f o r response goals a n d t h e anatomical r i g h t h e f t distinction. 1 O u r r e f e r r a l t o "interaction" o f f a c t o r s o f t h e hierarchical model is j u s t i f i e d in t h e s t r i c t statistical sense. When we consider t h e paradigm o f bimanual reactions, t h e slowing down o f reactions u n d e r t h e crossed-arms condition can b e i n t e r p r e t e d as a main e f f e c t o f experimental condition, as an i n t e r a c t i o n between r e s p o n d i n g h a n d a n d i t s position, a n d as an i n t e r action between compatibility w i t h respect t o hands a n d compatibility w i t h respect t o t h e i r positions, depending on how f a c t o r s a r e g r o u p e d in t h e analysis o f variance. Suppose t h a t i n a hypothetical experiment, we obtained t h e data displayed in Table 2 as r e s u l t s f o r r i g h t - h a n d a n d l e f t h a n d reactions t o right a n d l e f t l i g h t s w i t h uncrossed a n d crossed arms. Then, we can f i r s t i n t e r p r e t these data as e x p r e s s i n g a main e f f e c t f o r experimental condition (uncrossed v s . crossed arms). Second, we can a r r a n g e them as i n t h e u p p e r panel o f Table 3, i n t e r p r e t i n g them as an i n t e r a c t i o n between h a n d a n d h a n d position (as in t h e studies o f Nicoletti e t al., 1984, a n d B r e b n e r e t al., 1972). Finally, however, we may a r r a n g e them as i n t h e lower panel o f Table 3. With respect t o t h i s arrangement, we can i n t e r p r e t t h e data as e x p r e s s i n g an interaction b e t ween stimulus-hand compatibility a n d stimulus-hand position compatibility (i.e., between f a c t o r s (b) a n d (c) o f o u r hierarchical model.) This means t h a t t h e f i e l d - h a n d position compatibility effect, w h i c h is d u e t o spatial coding, takes place essentially o n l y u n d e r a compatible f i e l d - h a n d relation. T h i s arrangement o f data was proposed by Simon e t al. (1970b) a n d also used by Callan e t al. (1974). In t h e following, we put p a r t i c u l a r emphasis on t h e relations between f a c t o r s (a) a n d (b) on t h e one side a n d f a c t o r (c) o n t h e other, t h a t is, t h e relation between c o d i n g o f response k e y o r e f f e c t o r position a n d spatio-anatomical mapping.

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Table 2 Reaction Times in a Hypothetical Experiment for R i g h t Hand (RH) a n d L e f t Hand (LH) Responses t o R i g h t a n d L e f t L i g h t s f o r t h e Experimental Conditions o f Arms Held Uncrossed a n d Arms Held Crossed

Condition

L e f t Light LH RH

R i g h t Light LH RH

Mean

Uncrossed

300

350

350

300

325

Crossed

400

350

350

400

375

Table 3 Arrangement o f t h e Data o f Table 2 A c c o r d i n g t o t h e Factors Hand a n d Hand Position ( u p p e r panel), a n d A c c o r d i n g t o t h e Factors Stimulus-Hand Compatibility a n d Stimulus-Hand Position Compatibility (bottom panel)

Hand Hand Position

Left

Left Right

325 375

Stimulus-Hand Position Compatibility

Right

375 325

St imuI us - Hand Cornp a t ibi I ity Compatible Incompatible

Compatible Incompatible

300 400

350 350

D i r e c t Effects of 'Spatio-Anatomical Mapping

So f a r we have dealt w i t h t h e case in w h i c h spatial coding is replaced by spatio-anatomical mapping if c o d i n g is undefined, a n d w i t h t h e case where coding is t h e major factor, b u t mapping indirectly i n f l u ences coding by i n t e r a c t i n g w i t h it. T h e l a t t e r produces a slowing down o f reactions u n d e r crossed-arms o r supine-hand conditions. However, t h e r e a r e also situations in w h i c h coding i s possible in p r i n c i p l e ( i . e . , i s n o t undefined), but mapping replaces coding o r is a t least directly effect i v e by n o t o n l y i n f l u e n c i n g o v e r a l l RTs but also q u a l i t a t i v e l y o r q u a n t i t a t i v e l y i m p a i r i n g t h e coding e f f e c t i t s e l f . Ladavas a n d Moscovitch's (1984) r e s u l t s can be t a k e n as an example o f coding b e i n g replaced by mapping, a l t h o u g h a c o d i n g hypothesis makes sense. Schroeder-Heister


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e t al. (1989) showed t h a t mapping can inhibit c o d i n g w i t h o u t fully elimin a t i n g it. I n several experiments, Lddavas a n d Moscovitch (1984) s t u d i e d t h e relation between head tilt a n d frames o f reference by i n v e s t i g a t i n g t h r e e conditions of head. position: head h e l d upright, t i l t e d 9 0' t o t h e right, a n d t i l t e d 90' t o t h e l e f t . In t h e t i l t e d - h e a d conditions, stimuli a n d responses were always p e r p e n d i c u l a r t o each other, w i t h e i t h e r t h e stimuli a r r a n g e d along t h e horizontal dimension a n d t h e (bimanual) responses along t h e v e r t i c a l dimension o r v i c e versa. In h a l f o f t h e experiments, t h e responding hands were exchanged, similar to t h e crossed-arms t e s t . Ladavas a n d Moscovitch f o u n d an association between stimuli a n d r e s p o n d i n g hands ( n o t hand positions) o f t h e - following p a t t e r n : T h e r e was an R T advantage f o r those stimulus-hand p a i r i n g s t h a t correspond t o each o t h e r if t h e stimulus is encoded as right o r l e f t in t h e a p p r o p r i a t e frame o f reference (environmental v s . egocentric) t h a t makes a r i g h t h e f t d i s t i n c t i o n possible. T h i s r e s u l t is explained by t h e idea t h a t u n d e r t h e conditions o f t h e g i v e n experimental design, t h e c o d i n g o f response positions was replaced by t h e c o d i n g o f t h e anatomical r i g h t h e f t c l ssification, t h a t is, by spatio-anatomical mapping in o u r terminology.$ T h i s replacement o f spatial coding by spatio-anatomical mapping took place although a s o r t o f spatial coding is imaginable in principle: One may, f o r example, assume t h a t t h e positions o f stimuli a n d responses a r e coded in d i f f e r e n t frames o f reference, a r r i v i n g e i t h e r a t a top/down o r r i g h t h e f t classification f o r b o t h stimuli a n d respo ses (see Ladavas f, Moscovitch, 1984, Discussion o f Experiments 1 a n d 2 ) .

s

T o resolve t h e confound between head tilt a n d o r t h o g o n a l i t y o f stimuli a n d responses in LBdavas a n d Moscovitch's s t u d y , we i n v e s t i g a t e d t h e condition o f head t i l t e d alone (Schroeder-Heister e t al., 1989). We h a d subjects react u n d e r t h e conditions o f head h e l d upright, head t i l t e d t o t h e right, a n d head t i l t e d t o t h e l e f t (as i n Lddavas E Moscovitch, 1984), but w i t h stimuli always a r r a n g e d h o r i z o n t a l l y a n d responses b e i n g g i v e n always by t w o b u t t o n s mounted o n t h e r i g h t o r l e f t side o f t h e experimental desk. Subjects were tested b o t h w i t h uncrossed a n d w i t h crossed arms. In t h e cpright head condition, t h e usual spatial S - R compatibility effect between stimulus position a n d response position was obtained f o r b o t h uncrossed a n d crossed arms. In t h e t i l t e d - h e a d conditions, t h i s e f f e c t was s t i l l present, but s i g n i f i c a n t l y decreased f r o m t h e uncrossed-arms t o t h e crossed-arms conditions, w i t h o u t r e v e r s i n g t o 4Ladavas a n d Moscovitch (1984) vacillate between t h e i n t e r p r e t a t i o n t h a t it is essentially t h e condition of head tilt u n d e r w h i c h spatial coding is replaced by spatio-anatomical mapping ( p . 213, bottom o f l e f t column a n d bottom o f right column) a n d t h e i n t e r p r e t a t i o n t h a t head tilt in combination w i t h o r t h o g o n a l i t y o f stimuli a n d responses is responsible f o r t h e observed e f f e c t (p. 214, t o p o f right column). 5LBdavas a n d Moscovitch's (1984) r e s u l t s also seem t o express a tendency towards a slowing down o f reactions w i t h exchanged hands, which may indicate an i n d i r e c t influence of spatial coding in addition t o t h e dominant e f f e c t of spatio-anatomical mapping. However, t h i s cannot b e e x a c t l y established on t h e basis o f t h e p u b l i s h e d data.

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t h e opposite. In o t h e r words, w i t h crossed arms, R T s were n o t o n l y o v e r a l l slower, but t h e size o f t h e spatial-compatibility e f f e c t was diminished. B y n o t u s i n g experimental condition (arms uncrossed v s . crossed) but instead h a n d compatibility a n d position compatibility as f a c t o r s in t h e analysis of variance (see t h e hypothetical experiment discussed above a n d Tables 2 a n d 3), t h e r e s u l t s f o r t h e t i l t e d - h e a d conditions can also b e i n t e r p r e t e d as follows: B o t h a s i g n i f i c a n t position-compatibility e f f e c t a n d an (anatomical) hand-compatibility e f f e c t were obtained, as was a s i g n i f i c a n t i n t e r a c t i o n between these effects. T h i s shows t h a t besides c o d i n g o f response positions, w h i c h remains t h e dominant factor, spatio-anatomical mapping can b e d i r e c t l y present, now r e d u c i n g t h e coding e f f e c t u n d e r t h e crossed-arms condition. T h e i n t e r a c t i o n o f coding a n d spatioanatomical mapping expresses again t h e i n d i r e c t influence o f mapping by increasing t h e o v e r a l l R T s in t h e crossed-arms condition. We explained t h i s r e s u l t as i n d i c a t i n g t h a t spatial coding o f response positions is p a r t i c u l a r l y difficult when b o t h t h e head i s t i l t e d a n d t h e arms a r e crossed, leading t o a g r e a t e r u n c e r t a i n t y in coding spatial positions as right o r left. Because o u r r e s u l t s show t h a t head tilt alone does n o t eliminate spatial coding o f response positions as t h e dominant factor, t h e additional difficulty in stimulus encoding (choice o f an a p p r o p r i a t e frame o f r e f e r ence) in L i d a v a s a n d Moscovitch's (1984) experiments may lead subjects t o r e l y o n t h e clear anatomical r i g h t h e f t d i s t i n c t i o n f o r t h e i r responses. T h i s would mean t h a t response encoding may also depend on t h e condit i o n s o f stlmulus encoding, because these a r e t h e o n l y essential d i f f e r ences between o u r design a n d LBdavas a n d Moscovitch's design f o r t h e horizontal response arrangement. T h e f i n d i n g s o f L i d a v a s a n d Moscovitch (1984) a n d SchroederHeister e t al. (1989) fully s u p p o r t o u r hierarchical model. In a situation w h e r e spatial c o d i n g i s made difficult, spatio-anatomical mapping has t h e chance t o i n t e r f e r e o r , u n d e r extremely difficult conditions, even t o dominate o v e r spatial coding. Some Methodological Remarks In t h e p r e s e n t paper, we have placed o u r main emphasis on t h e response side o f spatial S-R compatibility, d i s t i n g u i s h i n g between various ways of c o d i n g t h e response: c o d i n g o f positions o f response keys, coding o f positions o f r e s p o n d i n g hands o r f i n g e r s , a n d coding o f (anatomically right o r l e f t ) hands o r f i n g e r s , t h a t is, spatio-anatomical mapping. Correspondingly, t h e techniques considered t o separate influences o f t h e various f a c t o r s were manipulations o f t h e response a r r a n g e ment. One o f t h e basic experimental methods t o d i s t i n g u i s h between p u r e l y spatial a n d spatio-anatomical aspects o f S - R compatibility is a comp a r i s o n between data f o r arms h e l d in normal position a n d f o r arms crossed. T h e r e is a general difficulty w i t h i n t e r p r e t i n g such data t h a t has implications f a r b e y o n d t h e crossed-arms paradigm.

A c c o r d i n g t o one possible i n t e r p r e t a t i o n , uncrossed-arms v s . crossed-arms a r e experimental conditions whose influence o n t h e manner o f c o g n i t i v e processing is experimentally investigated. If a d i f f e r e n c e in performance i s o b s e r v e d when t h e arms a r e h e l d crossed, t h i s d i f f e r e n c e


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i s i n t e r p r e t e d as e x p r e s s i n g a change in c o g n i t i v e processing. For example, t h e o v e r a l l slowing down o f reactions i n crossed-arms conditions can b e i n t e r p r e t e d as r e f l e c t i n g a c o g n i t i v e c o n f l i c t between anatomical a n d spatial codes t h a t is n o t p r e s e n t when t h e arms a r e h e l d normally, but t h a t arises when t h e arms a r e crossed. A c c o r d i n g t o another i n t e r p r e t a t i o n , t h e crossed-arms condition is j u s t a measurement device o f t h e experimenter t o d i s t i n g u i s h between effects d u e t o h a n d a n d t o h a n d position, w i t h o u t a f f e c t i n g c o g n i t i v e p r o cedures. A change in performance w i t h arms crossed i s n o t i n t e r p r e t e d as e x p r e s s i n g a change in information processing, but as r e v e a l i n g feat u r e s o f i t s permanent s t r u c t u r e t h a t cannot show up, a l t h o u g h present, w i t h hands in normal position. T a k i n g t h e same example as above, t h e slowing down o f reactions w i t h arms crossed is i n t e r p r e t e d as i n d i c a t i n g some general interaction o f anatomical a n d spatial codes t h a t is always present, but manifests i t s e l f in a specific w a y when t h e arms a r e crossed. One may t r y t o d i s t i n g u i s h experimentally,,between these t w o i n t e r between” uncrossed a n d pretations by p r o v i d i n g a baseline condition If t h e crossed-arms t e s t is a condition t h a t changes p r o crossed arms. cessing by causing a c o g n i t i v e c o n f l i c t between spatial a n d anatomical codes, an i n h i b i t i o n o f processing should b e expected w i t h arms crossed. However, if t h e crossed-arms t e s t i s n o t h i n g but a methodological i n s t r u ment demonstrating a permanent c o g n i t i v e i n t e r f e r e n c e of anatomical a n d spatial codes, processing should n o t j u s t b e i n h i b i t e d if b o t h codes a r e i n c o n g r u e n t (i.e., u n d e r t h e crossed-arms condition), but also f a c i l i t a t e d if b o t h codes a r e c o n g r u e n t (i.e., u n d e r t h e uncrossed-arms condition). Without detailed discussion, K l a p p e t al. (1979, Experiment 1) i m p l i c i t l y investigated t h i s question by s t u d y i n g a baseline i n t h e i r up/down condit i o n (hands o p e r a t i n g a b u t t o n f r o m above a n d below), w h i c h t h e y used in addition t o t h e i r crossed/uncrossed conditions (hands o p e r a t i n g a b u t t o n f r o m t h e right o r l e f t side w i t h arms crossed o r arms uncrossed). T h e o v e r a l l RTs i n t h e crossed-arms condition were s i g n i f i c a n t l y l a r g e r (by n e a r l y 100 ms) t h a n in b o t h t h e uncrossed-arms a n d t h e up/down conditions, whereas t h e r e was p r a c t i c a l l y no R T d i f f e r e n c e between t h e uncrossed-arms a n d t h e up/down condition. T h i s indicates t h a t t h e r e is i n h i b i t i o n o f processing u n d e r t h e crossed-arms condition, but no facilitation u n d e r t h e uncrossed-arms condition. T h u s , one may conclude t h a t something a c t u a l l y “happens” i n t e r n a l l y when one crosses arms. T h a t is, t h e crossed-arms t e s t manipulates t h e manner o f processing. However, s t u d y i n g baseline conditions l i k e t h e one j u s t mentioned, which is a r a t h e r neglected t o p i c i n compatibility research, cannot fully eliminate t h e basic difficulty. I n principle, one cannot exclude t h a t t h i s baseline condition i t s e l f represents an experimental manipulation t h a t may change processing a n d t h u s r e q u i r e s another ( “ h i g h e r o r d e r ” ) baseline condition t o establish w h e t h e r it is in f a c t a n e u t r a l condition between t h e uncrossed- a n d crossed-arms conditions, a n d so f o r t h ad infiniturn. Obviously, t h i s problem occurs n o t j u s t w i t h t h e crossed-arms t e s t in compatibility experiments, but w i t h all s o r t s o f changes o f t h e response arrangements t h a t a r e u n d e r t a k e n e i t h e r t o isolate o r t o exclude anatomy-related f a c t o r s as relevant f o r an o b s e r v e d effect. Besides t h e crossed-arms test, we have p a r t i c u l a r l y discussed t h e comparison between p r o n e a n d supine h a n d positions a n d t h e d i s t i n c t i o n between anatomical a n d spatial f i n g e r distance. Related questions can b e posed if one wants

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t o use t h e crossed-arms methodology t o establish effects d u e t o neuroanatomical c o n n e c t i v i t y (see Anzola e t al., 1977; Berlucchi, Crea, D i Stefano, E Tassinari, 1977; Bradshaw & Umilti, 1984; Levy, 1984). It seems t h a t t h e o n l y way t o cope w i t h these difficulties, a t least in p a r t , is t o consider baseline conditions on t h e stimulus side (e.g., by p r e s e n t i n g stimuli in middle position) independently o f manipulations on t h e response side. What we a r e facing h e r e w i t h respect t o S-R compatibility is t h e general problem o f experimental methodology: Certain methods o f observation change what i s observed in a way essential f o r t h e purpose o f t h e investigation. Summary

A consideration o f d i f f e r e n t r i g h t / l e f t relations on t h e response side distinguishes a t least t h r e e t y p e s o f spatial S-R compatibility: (a) compatibility between stimulus positions and positions o f response keys, (b) compatibility between stimulus positions a n d positions o f response effectors (hands o r fingers), a n d (c) compatibility between stimulus posiCompatibilities of tions a n d anatomically right o r l e f t response effectors. t y p e s (a) a n d (b?, may b e a t t r i b u t e d to an i n t e r n a l coding of response positions (called spatial coding") a n d compatibility o f t y p e (c) t o an i n t e r n a l coding o f anatomical r i g h t / l e f t classifications (called "spatioanatomical mapping"). Standard results in compatibility research show t h a t spatial coding i s dominant in determining compatibility effects a n d t h a t anatomy-related factors p l a y a subordinate role. For bimanual choice reactions t h i s has been established by t h e crossed-arms test, whereas it has been established f o r unimanual t w o - f i n g e r choice reactions by comparing p r o n e and supine hand orientations. As a f u r t h e r possibility t o separate spatial f r o m anatomical factors in unimanual t w o - f i n g e r choice reactions, we p r o pose t h e d i s t i n c t i o n between anatomical a n d spatial f i n g e r distance. Preliminary experimental data presented h e r e show t h a t t h e size o f t h e compatibility e f f e c t varies w i t h spatial and n o t w i t h anatomical f i n g e r distance. Notwithstanding t h e p r i o r i t y o f spatial coding, several results show t h a t spatio-anatomical mapping may b e effective if spatial r i g h t / l e f t d i s In particular, mapping is tinctions on t h e response side a r e eliminated. effective if response b u t t o n s a r e arranged in middle position perpend i c u l a r t o t h e stimuli. T h i s conclusion i s also supported by t h e reanalysis o f data o f a ,divided visual-field s t u d y o f cerebral lateralization. T o i n t e g r a t e t h e seemingly d i v e r g e n t findings, we have proposed a hierarchical model, according t o which spatial S-R compatibility is a j o i n t r e s u l t of t h e t h r e e factors mentioned above, which dominate each o t h e r according t o a c e r t a i n rank o r d e r and which can i n t e r a c t in various ways.


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Simon J. R. (1982). Effect of an auditory stimulus on t h e processing of a visual stimulus under single- and dual-tasks conditions. Acta Psychologica, 51, 61-73. Simon, J. R., Acosta, E., E Mewaldt, S . P. (1975). Effect o f locus of warning tone on a u d i t o r y choice reaction time. Memory 6 Cognition, 3 , 167- 170. (1976). Simon, J. R., Acosta, E., Mewaldt, S . P . , & Speidel, C. R. T h e effect of an irrelevant directional cue on choice reaction time: Duration of t h e phenomenon and i t s relation t o stages of processing. Perception d Psychophysics, 19, 16-22. Simon, J. R., Craft, J. L., & Small, A . M. (1971). Reactions toward t h e apparent source of an auditory stimulus. Journal of Experimental Psychology, 8 , 203-206. Simon, J. R., Hinrichs, J. V., & Craft, J. L. (1970b). Auditory S-R compatibility: Reaction time as a function of ear-hand correspondence and ear-response-location correspondence. Journal of Experimental Psychology, 86, 97-102. Auditory S-R compatibility: The Simon, J. R., 8 Rudell, A. P. (1967). Journal of effect of an irrelevant cue on information processing. Applied Psychology, 5 1 , 300-304. Simon, J. R., Small, A. M., Ziglar, R. A., & Craft, J. L. (1970a). ResDonse interference i n an information Drocessina t a s k : Sensorv verkus perceptual factors. Journal of Ex&imentalPsychology, 85; 311-314. Simon, J. R., 6 Wolf. J. D . (1963). Choice reaction time as a function of angular stimulus-response correspondence a n d age. Ergonomics, 6, 99-105. Soetens, E., Deboeck, M., Hueting, J., & Merckx, H. (1984). Spatial compatibility w i t h a two-dimensional stimulus arrangement. Bulletin of the Psychonomic Society, 2 2 , 125- 128. Sternberg, S . (1969). of Donders' method.

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Umilt6, C., & Liotti, M. (1987). Egocentric and relative spatial codes in S-R Compatibility. Psychological Research, 49, 81 -90. Umilti, C., & Nicoletti, R. (1985). Attention and coding effects in S-R compatibility due t o irrelevant s p a t i a l cues. In M. I. Posner & 0. S. M. Marin (Eds.), Attention and performance Xi (pp. 457-471). Hillsdale, NJ: Lawrence Erlbaum. Wallace, R. J. (1971). S-R compatibility and t h e idea of a response code. Journal of Experimental Psychology, 88, 354-360. (1972). Spatial S-R compatibility effects involving Wallace, R. J. kinesthetic cues. Journal of Experimental Psychology, 9 3 , 163-168.


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SOME ASPECTS OF SPATIAL STIMULUS-RESPONSE COMPATIBILITY IN ADULTS AND NORMAL CHILDREN E L I S A B E T T A LXDAVAS Dipartimento di Psicologia U n i v e r s i t a di Bologna, I t a l y I n several choice reaction-time ( R T ) t a s k s employing an a r r a y o f stimuli a n d an a r r a y o f responses, c e r t a i n stimulus-response (S-R) p a i r i n g s lead t o f a s t e r RTs t h a n d o o t h e r s . T h e more e f f i c i e n t p a i r i n g s Simon, a r e termed "compatible" a n d t h e less efficient, "incompatible. " Sly, a n d Vilapakkam (1981) d i s t i n g u i s h e d t h r e e t y p e s o f compatibility: symbolic compatibility, spatial compatibility a n d a third t y p e t h a t can b e Symbolic compatibility termed t h e "Simon effect" (Hedge & Marsh, 1975). r e s u l t s f r o m t h e correspondence between t h e v e r b a l labels associated w i t h t h e stimulus a n d with t h e response. F o r example, if t h e r e a r e r e d a n d g r e e n l i g h t s a n d r e d a n d g r e e n keys, responses a r e f a s t e r when t h e t a s k r e q u i r e s t h a t t h e r e d k e y b e pressed in response t o t h e r e d light a n d t h e g r e e n k e y b e pressed in response t o t h e g r e e n light, as compared t o a t a s k i n v o l v i n g t h e opposite S-R p a i r i n g s (Hedge & Marsh, 1975; Simon e t al., 1981; Simon & Sudalaimuthu, 1979).

T h e speed o f response also can b e affected by t h e spatial charact e r i s t i c s o f t h e stimulus a n d response codes, regardless o f w h e t h e r t h e selection o f response is based o n a spatial c u e o r n o t . When t h e selection o f t h e response i s d i r e c t l y based on t h e position o f t h e stimulus, a "spatial compatibility effect" occurs. Thus, if t h e r e a r e t w o light stimuli, one in t h e right visual f i e l d a n d t h e o t h e r in t h e left, a n d t w o response keys, one o n t h e right a n d t h e o t h e r on t h e left, R T i s f a s t e r when t h e t a s k r e q u i r e s t h a t t h e right k e y b e pressed in response t o t h e right light a n d t h e l e f t k e y b e pressed i n response t o t h e l e f t light, as compared t o a t a s k i n v o l v i n g t h e opposite S-R p a i r i n g s (Anzola, Bertoloni, Buchtel, & Rizzolatti, 1977; B r e b n e r , Shephard, & Cairney, 1972). A d i f f e r e n t v a r i e t y o f spatial compatibility e f f e c t is t h e Simon effect, which can b e o b s e r v e d i n R T t a s k s f o r w h i c h t h e position o f t h e stimulus i s i r r e l e v a n t f o r t h e selection o f t h e response a n d y e t has an influence on r e s p o n d i n g speed. T h i s compatibility e f f e c t is apparent when a visual p a t t e r n (e.g., a triangle), p r e s e n t e d sometimes in t h e right a n d sometimes in t h e l e f t v i s u a l field, m u s t b e responded t o by p r e s s i n g t h e right key, a n d another p a t t e r n (e.g., a square), also p r e sented sometimes i n t h e right a n d sometimes in t h e l e f t visual field, must Reactions associated w i t h b e responded t o by p r e s s i n g t h e l e f t k e y . spatially c o r r e s p o n d i n g S-R p a i r i n g s (i.e., stimulus a n d k e y on t h e same side) a r e f a s t e r t h a n reactions associated w i t h non-corresponding S-R p a i r i n g s (i.e., stimulus o n one side a n d k e y o n t h e o t h e r side), even t h o u g h spatial position is i r r e l e v a n t (Simon, 1968; Simon e t al., 1970).

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Spatial compatibility a n d t h e Simon e f f e c t a r e n o t d u e t o t h e basic anatomical c o n n e c t i v i t y o f t h e responding h a n d w i t h t h e hemispheric representation o f t h e t w o v i s u a l f i e l d s (Myers, 1965; S p e r r y , 1964), but t o t h e correspondence between t h e spatial positions o f t h e stimulus a n d o f t h e response. T h e neuroanatomical pathway hypothesis was r u l e d o u t by experiments in w h i c h t h e position o f t h e response h a n d was crossed (Anzola e t al., 1977; Wallace, 1971). When t h e right h a n d was on t h e right side o f t h e b o d y a n d t h e l e f t h a n d on t h e l e f t side ( t h e uncrossed position), t h e right h a n d responded f a s t e r t o stimuli presented t o t h e right v i s u a l f i e l d a n d t h e l e f t h a n d t o stimuli presented t o t h e l e f t visual In contrast, when t h e subjects crossed t h e i r hands so t h a t t h e field. right h a n d was on t h e l e f t side o f t h e b o d y a n d t h e l e f t h a n d on t h e right side ( t h e crossed position), t h e right h a n d was f a s t e r f o r l e f t - s i d e d stimuli a n d t h e l e f t h a n d f o r r i g h t - s i d e d stimuli. T h i s was t r u e f o r b o t h t h e spatial-compatibility e f f e c t (Anzola e t JI., 1977; B r e b n e r e t al., 1972; Nicoletti e t al., 1982) a n d t h e Simon e f f e c t (Simon e t al., 1970; Wallace 1971, 1972). T h e Responding H a n d as a Determining Factor in P r o d u c i n g t h e Spatial C o m p a t i b i l i t y E f f e c t s One i n t e r e s t i n g f i n d i n g o f t h e experiments t h a t used uncrossed a n d crossed positions was t h a t R T s were f a s t e r in t h e uncrossed condition (Anzola e t al., 1977; B e r l u c c h i e t al., 1977; B r e b n e r e t al., 1972; Nicol e t t i e t al., 19821, i r r e s p e c t i v e o f w h e t h e r t h e stimulus-response arrangement was compatible o r incompatible. There are two tentative explanations f o r t h i s observation. T h e f i r s t one, the postural hypothesis, suggests t h a t t h e slower RTs w i t h crossed hands a r e d u e t o an e f f e c t o f arm p o s t u r e on t h e movement speed o f t h e r e s p o n d i n g f i n g e r . T h i s p o s t u r a l e f f e c t c o u l d arise because t h e crossed position is somewhat uncomfortable. A n o t h e r explanation, t h e coding hypothesis (Nicoletti e t al., 1984; Riggio, Gawryszewski, & Umilta, 1986), is in terms o f compatibility between t h e position of t h e response h a n d a n d t h e side o f t h e body w i t h w h i c h t h e h a n d is connected. A c c o r d i n g t o t h e coding hypothesis, t w o spatial codes related t o t h e e f f e c t o r a r e formed, one t h a t describes t h e position in space where t h e h a n d is located (right o r l e f t ) a n d t h e o t h e r t h a t describes t h e side o f t h e b o d y w i t h which t h e h a n d is connected. When a stimulus is presented, i t s spatial code is compared w i t h t h e spatial codes d e s c r i b i n g t h e position o f t h e h a n d a n d t h e side o f t h e b o d y w i t h w h i c h t h e h a n d is connected. T h e overall slowing of R T w i t h t h e hands crossed is a t t r i b u t e d t o a mismatch between t h e codes d e s c r i b i n g t h e positions i n space where t h e h a n d is located (right o r l e f t ) a n d t h e side o f t h e b o d y w i t h w h i c h t h e h a n d is connected. With hands crossed, a conflict would arise between t w o i n t e r n a l representations o f t h e same effector, one locational a n d t h e o t h e r anatomical. Nicoletti, Umilta, a n d Ladavas (1984) rejected t h e p o s t u r a l h y p o t h esis by showing t h a t t h e crossed-hand e f f e c t is absent in a simple R T task, although t h e response h a n d is k e p t in an uncomfortable position, (e.g., on t h e opposite side o f t h e b o d y t o which it is connected). Therefore, t h e y concluded t h a t t h e crossed-hand e f f e c t is b e t t e r explained in terms o f a c o n f l i c t between t h e codes d e s c r i b i n g t h e response h a n d a n d t h e side where t h e response h a n d is located. These

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r e s u l t s can b e considered as i n d i r e c t evidence t h a t t h e anatomical status o f t h e t w o hands is coded. More d i r e c t evidence of S-R compatibility effects based o n a rightl e f t classification of t h e r e s p o n d i n g h a n d comes f r o m a s t u d y by LBdavas a n d Moscovitch (1984). T h i s s t u d y showed t h a t t h e r e s p o n d i n g hand, r a t h e r t h a n t h e response position, determines t h e d i f f e r e n t i a l S-R associations u n d e r l y i n g t h e spatial-compatibility effects. T w o spatial codes can b e used t o describe t h e position o f t h e v i s u a l stimuli: an environmental ( o r physical) frame o f reference a n d an egocentric ( o r retinal) frame o f reference. When t h e head is in t h e normal upright position, t h e t w o f r a n e s o f reference coincide. F o r example, " l e f t " in one frame i s also " l e f t " in t h e o t h e r frame. B u t if t h e subject t i l t s h i s o r h e r head t o t h e r i g h t by 90°, t h e t w o frames o f reference n o l o n g e r coincide because "left" in t h e physical frame o f reference is "down" in t h e egocentric Thus, a subject frame o f reference, a n d so on f o r t h e o t h e r positions. w i t h t h e head t i l t e d can use t w o codes, one environmental a n d t h e o t h e r egocentric, f o r d e s c r i b i n g a n y position in t h e v i s u a l space. LBdavas a n d Moscovitch (1984) demonstrated t h a t when t h e e n v i ronmental a n d egocentric frames o f reference coincide (as when t h e head i s upright) a n d stimuli a n d responses a r e d i s t r i b u t e d along t h e same plane (horizontal o r vertical), t h e n t h e r e l a t i v e positions o f stimuli a n d responses determine S-R compatibility effects. I n contrast, if t h e stimuli a n d responses a r e p e r p e n d i c u l a r t o each o t h e r a n d t h e frames o f reference d o n o t coincide (as when t h e head is tilted), r e s p o n d i n g h a n d replaces response position as t h e r e l e v a n t f a c t o r . This introduces a tendency t o code stimulus position in terms o f l e f t a n d right. As a consequence, subjects adopt t h e frame of reference t h a t enables them t o classify t h e stimuli as l e f t a n d right, regardless o f w h e t h e r t h e y a r e horizontally o r v e r t i c a l l y d i s t r i b u t e d . T h e l e f t - r i g h t stimulus classification is mapped o n t o t h e r e s p o n d i n g hand, t h e r e b y p r o d u c i n g t h e o b s e r v e d compatibility effects. Similar S-R compatibility effects between t h e r e s p o n d i n g hands a n d t h e spatial position o f t h e stimuli were obtained by Lddavas (1987) in a visuomotor choice R T t a s k t h a t i n v o l v e d a p e r p e n d i c u l a r arrangement o f displays a n d controls (e.g., t h e stimuli were displayed v e r t i c a l l y a n d t h e responding hands were placed h o r i z o n t a l l y ) . A l t h o u g h t h i s arrangement does n o t imply a n y o v e r t p r e f e r e n t i a l association between t h e spatial p r o p e r t i e s o f t h e stimuli a n d those of t h e responses, responses o f t h e dominant h a n d (right f o r r i g h t - h a n d e r s a n d l e f t f o r l e f t - h a n d e r s ) were f a s t e r t o t h e u p p e r light a n d those o f t h e non-dominant h a n d were f a s t e r t o t h e lower light. These effects were comparable in magnitude t o t h e t y p i c a l compatibility effects f o u n d w i t h a parallel arrangement o f displays a n d controls. Theories proposed t o explain S-R compatibility effects cannot easily accommodate t h e effects obtained in absence of a n y o v e r t correspondence between t h e spatial p r o p e r t i e s o f t h e stimulus a r r a y a n d those o f t h e response a r r a y . Simon a n d associates (Simon, 1968; Simon e t al., 1970) postulated t h a t spatial S-R compatibility effects a r i s e f r o m a basic t e n dency t o react t o w a r d t h e source o f t h e stimulus, such t h a t a n y reaction o c c u r r i n g in t h a t general d i r e c t i o n i s facilitated, whereas a l t e r n a t i v e reactions in t h e d i r e c t i o n opposite t o t h e p r e f e r r e d one a r e h i n d e r e d . T h e postulated t e n d e n c y t o move t o w a r d t h e source of t h e stimulus m i g h t i n t e r a c t w i t h t h e motor outflow r e q u i r e d f o r k e y - p r e s s i n g . But, g i v e n

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t h e arrangement of displays a n d controls used in Lddavas's experiment (1987), such interaction, if effective, should produce t h e same p a t t e r n o f responses f o r b o t h hands (e.g., it could facilitate t h e response o f b o t h hands to t h e u p p e r light a n d h i n d e r t h e response o f b o t h hands t o t h e lower light). Therefore, t h e p r e f e r e n t i a l associations o f t h e dominant hand w i t h t h e u p p e r stimulus a n d o f t h e non-dominant hand w i t h t h e lower stimulus l i k e l y r e s u l t f r o m o t h e r factors, such as some relation between t h e positional coding o f t h e stimuli a n d t h a t o f t h e hands. Wallace (1971, 1972) proposed a general model o f sensorimotor integ r a t i o n based o n positional coding. According t o Wallace, t h e c o r respondence between t h e position o f the stimuli a n d t h e f e l t (proprioceptive) position o f t h e responding p a r t o f t h e b o d y (e.g., a hand) i s t h e c r u c i a l f a c t o r t h a t facilitates compatible responses. Thus, a stimulus a n d a response a r e considered compatible when t h e i r respective positions can b e matched according t o some spatial code, as suggested by t h e f a c t t h a t spatial-compatibility effects can b e observed even in t h e absence of an o v e r t spatial correspondence between stimulus displays and responses. For example, Nicoletti, Anzola, Luppino, Rizzolatti, and Umiltd (1982) measured choice R T in a situation in which t h e r e were t w o possible stimuli horizontally a r r a n g e d in m e visual f i e l d a n d t w o horizont a l l y disposed response keys on t h e side o f t h e midline opposite t h e visual f i e l d (see also Nicoletti e t al., 1984; Umiltd & Nicoletti, 1985, f o r similar experiments). Because in b o t h t h e stimulus set a n d t h e response set one member could b e classified as right a n d t h e o t h e r as left, independent o f t h e position o f t h e set w i t h t h e respect o f t h e midline, subjects were f a s t e r in responding w i t h t h e right k e y t o t h e right light a n d w i t h t h e l e f t k e y t o t h e l e f t light, as compared t o r i g h t - l e f t a n d l e f t right associations. T h e spatial -codi ng hypot hes is seems t o e x p la i n t h e pr e f e r e nt i a I association between t h e dominant hand w i t h t h e u p p e r stimuli a n d t h e non-dominant hand w i t h t h e lower stimuli b e t t e r than does t h e hypothesis t h a t subjects t e n d t o respond in t h e direction o f t h e stimulus. It is possible t o postulate t h a t t h e t w o hands a r e coded n o t only o n t h e h o r i zontal dimension (i.e., right a n d left), b u t also o n t h e v e r t i c a l dimension, w i t h t h e dominant hand b e i n g assigned a " h i g h e r " position compared t o t h e non-dominant hand. When bimanual activities a r e c a r r i e d o u t u n d e r visual control, t h e o p e r a t i n g dominant hand is most f r e q u e n t l y above t h e h o l d i n g non-dominant hand. Repeated bimanual a c t i v i t y o f t h i s kind may g r a d u a l l y build u p a combined i n t e r n a l representation of t h e t w o hands, whereby t h e image o f t h e p r e f e r r e d hand i s placed above t h a t o f t h e n o n - p r e f e r r e d hand. T h e notion t h a t t h e mental images o f t h e t w o hands may d i f f e r in several respects is s u p p o r t e d by Sekiyama (19821, who showed t h a t t h e right hand is assigned a " h i g h e r " position compared t o t h e l e f t hand. In right-handers, t h e right hand's image may b e generated more easily t h a n t h e l e f t hand's image, because judgments f o r right hands are faster than those f o r l e f t hands. T h e "higher" position o f t h e dominant hand in t h i s mental representation may express i t s e l f i n simple motor behavior, such as t h e c r o s s i n g of t h e extended and pronated forearms. When subjects a r e r e q u i r e d t o cross t h e extended and pronated forearms, r i g h t - h a n d e r s consistently place t h e right hand above t h e l e f t hand, whereas l e f t handers consistently place t h e l e f t hand above t h e right hand.


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While t h e spatial-compatibility effects r e p o r t e d h e r e a r e most p a r simoniously explained by t h e above hypothesis o f an asymmetry in t h e coding of t h e dominant a n d non-dominant hands along t h e v e r t i c a l dimension, o t h e r f a c t o r s can e x p l a i n such effects. In a r e c e n t study, LBdavas (1988) has shown t h a t in t h e mental representation o f space in r i g h t - h a n d e r s t h e r e i s a p r e f e r e n t i a l l i n k a g e of t h e concepts "right" a n d "above" a n d o f t h e concepts " l e f t " a n d "below, w h i l e in l e f t - h a n d e r s t h i s asymmetrical l i n k a g e is r e v e r s e d . T h e r e s u l t s o f t h i s s t u d y showed t h a t r i g h t - h a n d e r s were f a s t e r in v e r i f y i n g a n d f a l s i f y i n g t h e statements cont a i n i n g t h e terms RIGHT a n d ABOVE as compared t o statements containing t h e terms LEFT a n d BELOW, whereas l e f t - h a n d e r s were f a s t e r in v e r i f y i n g a n d f a l s i f y i n g statements c o n t a i n i n g t h e terms LEFT a n d ABOVE as compared t o statements c o n t a i n i n g t h e terms R I G H T a n d BELOW. In o t h e r words, t h e p o s i t i v e t e r m on t h e horizontal dimension in r i g h t - h a n d e r s a n d l e f t - h a n d e r s is t h a t c o r r e s p o n d i n g t o t h e dominant hand, whereas t h e p o s i t i v e t e r m on t h e v e r t i c a l dimension is independent o f handedness. Therefore, it is possible t o i n t e r p r e t t h e S-R compatibility e f f e c t f o u n d in right handers w i t h p e r p e n d i c u l a r a r r a n g e ment o f stimuli a n d respo7:es as a !ink of,,the concepts "right" a n d "above" a n d t h e concepts l e f t " a n d below, and vice versa f o r lefthanders. T h i s p r e f e r e n t i a l - l i n k i n g i n t e r p r e t a t i o n o f t h e S-R compatibility effects w i t h p e r p e n d i c u l a r arrangement o f stimuli a n d responses seems t o b e less plausible t h a n t h e i n t e r p r e t a t i o n in terms o f an asymmetry in t h e coding o f t h e dominant a n d non-dominant h a n d along t h e v e r t i c a l dimension, because t h e f i r s t i n t e r p r e t a t i o n implies a v e r b a l labeling stage. Most a u t h o r s (Chase & C l a r k , 1971; Sholl & Egeth, 1981) agree t h a t t h e d i f f e r e n t d i s c r i m i n a b i l i t y o f t h e terms r i g h t - l e f t a n d above-below o c c u r s when t h e v e r b a l I!bels a r e used in t h e comparison stage o f processing. I n o t h e r words, above" positions appear t o b e processed f a s t e r t h a n " l e f t " positions o n l y when v e r b a l labels a r e applied t o judgments o f r e l a t i v e locations. I n contrast, t h e d i f f e r e n t d i s c r i m i n a b i l i t y between "right" a n d " l e f t " positions a n d "above" a n d "below" positions disappears when t h e t a s k r e q u i r e s perceptual encoding o f t h e r e l a t i v e location. T h e p r e f e r e n t i a l - l i n k i n g i n t e r p r e t a t i o n cannot explain t h e r e s u l t s obtained by LBdavas (1987), because t h e l i n g u i s t i c d e s c r i p t i o n o f t h e position o f t h e stimuli a n d responses does n o t appear t o p l a y a role in t h e compatibility effects. T h i s is demonstrated by t h e following s t u d y where c h i l d r e n show S-R compatibility effects, a l t h o u g h t h e y have n o t y e t learned t o t e l l right f r o m l e f t a n d t h e r e f o r e cannot use v e r b a l labels t o code t h e positions o f stimuli a n d responses. Spatial S-R C o m p a t i b i l i t y E f f e c t s In Y o u n g C h i l d r e n Wallace (1971, 1972) p o i n t e d o u t t h a t t h e c r u c i a l f a c t o r t h a t facilitates compatible responses is t h e correspondence between t h e code d e s c r i b i n g t h e position o f t h e stimulus a n d t h e code d e s c r i b i n g position o f t h e responding p a r t of t h e body. In t h e case o f symb6lic compatibility, t h e n a t u r e o f t h e codes i s v e r y l i k e l y t o b e verbal, whereas in t h e case o f b o t h spatial compatibility a n d t h e Simon effect, it c o u l d b e e i t h e r spatial o r v e r b a l . In o t h e r words, it is n o t a p p a r e n t w h e t h e r t h e subjects make use o f spatial codes o r o f t h e c o r r e s p o n d i n g v e r b a l descriptions in comparing t h e position o f t h e stimulus w i t h t h a t o f t h e response. A d i r e c t way t o t e s t t h e n a t u r e o f t h e codes i n v o l v e d in spatial compatibility i s t o assess t h e presence o f compatibility effects in

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c h i l d r e n who cannot discriminate between l e f t a n d right and, therefore, cannot use v e r b a l labels t o code t h e positions o f stimuli a n d responses. T h e aim o f t h e p r e s e n t s t u d y was t o assess t h e presence o f spatial comp a t i b i l i t y effects along t h e horizontial a n d v e r t i c a l dimensions a n d t h e a b i l i t y t o discriminate between l e f t a n d right in 4-5 y e a r - o l d chil.dren. Many studies have f o u n d a p r o g r e s s i v e development o f l e f t - r i g h t discrimination s k i l l s t h r o u g h t h e age r a n g e f r o m 6 t o 9 years, w i t h maturity in t h i s discrimination reached o n l y a r o u n d t h e 12th y e a r (Belmont & Birch, 1963; Benton, 1959; Boone & Prescott, 1968). More precisely, Belmont a n d B i r c h (1963), u s i n g t h e Piaget b a t t e r y o f l e f t - r i g h t awareness items, f o u n d t h a t discrimination o f l e f t - r i g h t followed a sequence w h i c h f i r s t includes one's own b o d y p a r t s , t h e n those o f another person f a c i n g t h e subject, a n d f i n a l l y object relations in t h e environment. Studies c o n c e r n i n g t h e motor a b i l i t y o f y o u n g c h i l d r e n in reaching f o r objects have f o u n d a p r e f e r e n c e f o r an ipsilateral o v e r a contralateral response a n d a difficulty in p e r f o r m i n g movements t h a t cross t h e b o d y midline ( B r u n e r , 1969; Kephart, 1971; Provine & Westerman, 1979). These l a t t e r studies suggest t h a t ipsilateral S - R l i n k s a r e p r e s e n t b e f o r e contralateral S-R l i n k s a n d t h a t these effects a r e independent o f t h e a b i l i t y t o discriminate between l e f t a n d right. T h i s suggestion was v e r i f i e d in t h e following s t u d y . S-R C o m p a t i b i l i t y E f f e c t F o r Horizontal Dimension E i g h t subjects, 4 males a n d 4 females, between t h e ages 4 a n d 5 p a r t i c i p a t e d in t h e experiment. T h e y h a d normal o r corrected-to-normal v i s i o n a n d were n a i v e as t o t h e p u r p o s e o f t h e experiment. Handedness was assessed by a s k i n g t h e subjects t o p e r f o r m some actions described in a h a n d p r e f e r e n c e questionnaire (Annet, 1970) w i t h t h e i r p r e f e r r e d hand. T h e subjects were asked t o p e r f o r m s i x actions: T o w r i t e a l e t t e r legibly, t o t h r o w a b a l l t o hit a t a r g e t , t o c u t w i t h scissors, t o h o l d a t o o t h b r u s h while cleaning teeth, t o draw, t o c u t food w i t h k n i f e . T h e h a n d used by t h e subject f o r each a c t i v i t y was recorded. Awareness o f r i g h t - l e f t relations was assessed by a s k i n g t h e s u b j e c t t o make r i g h t - l e f t discriminations in relation (a) t o his own b o d y parts, (b) t o those o f another person, ( c ) t o object relations in t h e e n v i ronment, a n d (d) t o t h e capacity t o o r i e n t himself t o l e f t a n d right in one d i r e c t i o n a n d t h e n r e v e r s e t h e process in t h e opposite d i r e c t i o n . In t h e f i r s t l e f t - r i g h t discrimination, t h e subjects were asked t o show t h e i r right o r l e f t hand, leg, ear, in t h e second one t h e same b o d y p a r t o f a person f a c i n g t h e subject, in t h e third one t h e r e l a t i v e position o f t w o objects displayed along t h e horizontal dimension, a n d f i n a l l y in t h e last one t h e i r capacity t o o r i e n t themselves t o l e f t a n d right u n d e r t h e ins t r u c t i o n s o f t h e experimenter. Each o f t h e f o u r conditions consisted o f s i x items, e q u a l l y d i v i d e d between l e f t a n d right items. Each subject sat w i t h b o t h eyes open i n f r o n t o f a panel d i s p l a y i n g t h r e e l i g h t - e m i t t i n g diodes (LEDs) . T h e distance between t h e eyes a n d t h e m i d p o i n t o f t h e panel was 50 cm. A central, g r e e n LED s e r v e d as a f i x ation point, a n d t w o r e d LEDs s e r v e d as stimuli. T h e y were located o n e i t h e r side, a t a n d above t h e f i x a t i o n p o i n t a t a 45' angle f r o m t h e h o r i zontal plane a n d passing t h r o u g h it. Each LED was 11.5 cm f r o m t h e f i x a t i o n p o i n t a n d subtended a v i s u a l angle o f about 0.57'. When activated, it emitted a pul e of r e d light t h a t lasted 100 ms a n d h a d an Each h a n d h e l d a p l a s t i c c y l i n d e r equipped i n t e n s i t y o f 27.22 cd/m .

1


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w i t h a p u s h b u t t o n on i t s t o p . T h e t w o hands h o l d i n g t h e c y l i n d e r s rested o n t h e desk, w i t h t h e right h a n d in f r o n t o f t h e right light a n d t h e l e f t h a n d in f r o n t o f t h e l e f t light (uncrossed condition), o r t h e right h a n d in f r o n t o f t h e l e f t light a n d t h e l e f t h a n d in f r o n t o f t h e right light (crossed condition). A special-purpose computer f o r t h e presentation o f t h e stimuli a n d t h e r e c o r d i n g a n d analysis o f t h e r e sponses was located in t h e same room, b e h i n d t h e subject. T h e t r i a l s were a r r a n g e d in a quasi-random sequence so t h a t t h e p r o b a b i l i t y o f o c c u r r e n c e o f a l e f t o r right light on each t r i a l was equal; t h e o n l y r e s t r i c t i o n was t h a t no more t h a n t h r e e consecutive t r i a l s c o u l d o c c u r on one side. A w a r n i n g signal was p r o v i d e d by lighting t h e f i x a t i o n LED 1-3 sec p r i o r t o each stimulus presentation. T h e subject was i n s t r u c t e d t o p r e s s one o f t h e k e y s upon t h e appearance o f a g i v e n light. A l l possible combinations between l i g h t s a n d k e y s w e r e tested. Each subject attended t w o experimental sessions f o r each condition (uncrossed a n d crossed condition) t h a t were run o n consecutive days. Each session consisted o f s i x blocks o f 10 p r a c t i c e t r i a l s a n d 60 experimental t r i a l s p e r In t h e t w o b l o c k w i t h a 10-min r e s t p e r i o d between consecutive blocks. conditions o f S-R pairings, one S-R p a i r i n g i n v o l v e d p r e s s i n g t h e right k e y f o r t h e right light a n d t h e l e f t k e y f o r t h e l e f t light; t h e o t h e r S-R p a i r i n g i n v o l v e d p r e s s i n g t h e right k e y t o t h e l e f t light a n d t h e l e f t k e y t o t h e right light. T h e o r d e r o f S-R p a i r i n g conditions was counterbalanced across sessions a n d across subjects. O n l y R T s longer t h a n 100 ms a n d s h o r t e r t h a n 2000 ms were collected. T r i a l s in w h i c h subjects responded i n c o r r e c t l y a n d / o r RTs were o u t s i d e t h e above limits w e r e n o t repeated. T h e reduced v e r s i o n o f t h e h a n d p r e f e r e n c e questionnaire (Annet, 1970) showed t h a t s i x subjects h a d a right h a n d p r e f e r e n c e f o r all t h e manual activities a n d o n l y t w o subjects h a d a l e f t h a n d preference. The r i g h t - l e f t awareness t e s t showed t h a t o n l y f o u r subjects c o u l d make a discrimination between t h e i r own l e f t a n d right p a r t s o f t h e body, a n d n o one subject was able (a) t o discriminate between t h e l e f t a n d right side o f t h e person located in f r o n t of them, (b) t o d i s t i n g u i s h between t h e r e l a t i v e position o f t w o objects aligned along t h e horizontal dimension, a n d (c) t o t u r n t o t h e l e f t o r t o t h e right u n d e r t h e experimenter's instructions. Mean R T was computed across sessions f o r each subject f o r each o f t h e conditions r e s u l t i n g f r o m t h e combinations between t h e side o f t h e stimulus ( l e f t a n d right), t h e r e s p o n d i n g h a n d ( l e f t a n d right), a n d h a n d position (uncrossed a n d crossed positions). T h e means across subjects f o r t h e e i g h t basic conditions in uncrossed a n d crossed conditions a r e shown in Figures 1 a n d 2. A n analysis o f variance u s i n g Condition, Side o f t h e Stimulus, a n d Responding Hand as main f a c t o r s showed a s i g n i f i c a n t e f f e c t of Condition [ F ( l , 7)= 8.49, p < .025], w i t h uncrossed condition b e i n g f a s t e r (860 ms) t h a n crossed condition (993 ms). No o t h e r main e f f e c t was significant. T h e o n l y s i g n i f i c a n t i n t e r a c t i o n was t h a t between condition, side o f t h e stimulus a n d r e s p o n d i n g h a n d [ F ( l , 7) = 26.48, p < .005]. Paired t tests showed t h a t in t h e uncrossed condition, t h e 283 ms advantage o f t h e l e f t o v e r t h e right h a n d in r e s p o n d i n g t o t h e l e f t stimulus was s i g n i f i c a n t [ t ( 7 ) = 3.86, p < .01], as well as t h e 274 ms advantage o f t h e r i g h t o v e r t h e l e f t h a n d in r e s p o n d i n g t o t h e right stimulus [ t ( 7 ) = 3.75,

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I

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- 1000 --E aoo -u)

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F i g u r e 1. Reaction times as a f u n c t i o n of stimulus position ( l e f t and right) and responding hand ( l e f t and r i g h t ) i n uncrossed condition.

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= . 1

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Reaction times as a f u n c t i o n of stimulus position ( l e f t and F i g u r e 2. r i g h t ) and responding hand ( l e f t and r i g h t ) i n crossed condition.


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p < .01]. In t h e crossed condition, t h e 260 ms advantage o f t h e l e f t o v e r t h e right h a n d in r e s p o n d i n g t o t h e right stimulus was s i g n i f i c a n t [t(7)=3.08, p .02], as well as t h e 284 ms advantage o f t h e right o v e r t h e l e f t h a n d in r e a c t i n g t o t h e l e f t stimulus [ t (7) = 2.75, p < . 0 5 ] .

A similar analysis o f variance was c a r r i e d o u t o n t h e a r c - s i n e transformation o f percentage o f e r r o r s . These data a r e g r a p h i c a l l y dep i c t e d in Figures 3 a n d 4. O n l y t h e interaction between Condition, Side o f t h e Stimulus a n d Responding Hand was significant [F(l, 7) = 41.72, p < .001]. Paired t tests showed t h a t in uncrossed condition t h e l e f t h a n d was more accurate t o respond t o t h e l e f t t h a n t h e right stimulus, [t(7) = 2.99, p < .02], whereas t h e advantage o f t h e right h a n d i n r e s p o n d i n g t o t h e right t h a n t o t h e l e f t stimulus was n o t s i g n i f i c a n t [t(7) = 1.721. In t h e crossed condition, t h e right hand, located o n t h e l e f t side o f t h e body, was more accurate t h a n t h e l e f t hand, located o n t h e right side o f t h e body, t o respond t o t h e l e f t stimulus [t(7) = 2.36, p < .05], whereas t h e l e f t h a n d was more accurate t h a n t h e right t o respond t o t h e right stimulus [ t ( 7 ) = 4.66, p < .001]. T h e p r e s e n t s t u d y shows t h a t t h e r e is n o t a simple relation between t h e establishment o f lateral p r e f e r e n c e in h a n d usage a n d t h e development o f t h e a b i l i t y t o discriminate right f r o m l e f t . The results pointed out t h a t in 4 - 5 - y e a r - o l d c h i l d r e n h a n d preference is stabilized, whereas t h e a b i l i t y t o make accurate r i g h t - l e f t identifications on t h e i r own body, o n t h e b o d y o f t h e examiner, a n d among environmental objects i s n o t y e t developed. T h e r e s u l t s obtained on l e f t - r i g h t awareness a r e in agreement w i t h those obtained by Belmont a n d B i r c h (1963). I n t h e i r s t u d y , t h e age o f 7 appears t o b e c r i t i c a l f o r t h e development o f t h e a b i l i t y t o d i s t i n g u i s h l e f t a n d right i n relation t o one's own b o d y p a r t s , although it is f r o m age 10 o n w a r d t h a t all c h i l d r e n respond c o r r e c t l y t o all questions. Moreover, Belmont a n d B i r c h showed t h a t when t h e t a s k r e q u i r e d t o d i s criminate between l e f t a n d right in relation t o objects in t h e e x t e r n a l environment, f u l l y accurate r i g h t - l e f t awareness was n o t stabilized b e f o r e t h e age o f 1 1 . T h e novel f i n d i n g o f t h e p r e s e n t experiment is t h e presence o f S - R compatibility effects, similar t o those showed by adults, in c h i l d r e n who were unable t o discriminate between l e f t a n d r i g h t . T h e r e s u l t s showed that, as i n adults, speed a n d accuracy o f response was f a c i l i t a t e d by t h e correspondence between side o f t h e stimulus a n d side o f t h e response key, r a t h e r t h a n t h e correspondence between t h e side o f t h e stimulus a n d t h e responding hand. In o t h e r words, t h e r i g h t k e y was f a s t e r f o r t h e r i g h t stimulus, a n d t h e l e f t k e y was f a s t e r f o r t h e l e f t stimulus, r e g a r d less o f w h e t h e r t h e right k e y was pressed w i t h t h e right h a n d a n d t h e l e f t k e y was pressed w i t h t h e l e f t h a n d (uncrossed condition) o r t h e right k e y was pressed w i t h t h e l e f t h a n d a n d t h e l e f t k e y was pressed w i t h t h e right h a n d (crossed condition). Furthermore, RTs w e r e f a s t e r i n t h e uncrossed condition t h a n in t h e crossed condition (crossed-hand effect), w i t h t h i s e f f e c t b e i n g g r e a t e r f o r t h e c h i l d r e n t h a n it u s u a l l y i s f o r adults. In children, R T s in t h e uncrossed condition a r e 133 ms f a s t e r t h a n in t h e crossed condition, whereas in a d u l t s t h e advantage o f t h e uncrossed condition o v e r t h e crossed condition t y p i c a l l y is a r o u n d 50 ms .

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HORIZONTAL DIMENSION: UNCROSSED CONDITION

c W

B %

7-6--

5-4--

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Right Hand

-

F i g u r e 3. Percentage o f e r r o r s as a f u n c t i o n o f stimulus position ( l e f t a n d right) a n d r e s p o n d i n g h a n d ( l e f t a n d r i g h t ) i n uncrossed condition.

H0R IZONTAL DIMEN S I0N: CROSSED C0NDIT I0N


Left Hand

I I

Right Hand

Percentage of e r r o r s a s a f u n c t i o n of stimulus position ( l e f t F i g u r e 4. a n d right) a n d responding h a n d ( l e f t a n d right) in crossed condition.

Spatial

S-R Compatibility in Adults and Children

A n o t h e r d i f f e r e n c e t h a t emerges g r e a t e r d i f f e r e n c e between compatible c h i l d r e n t h a n in adults. In adults, R T s f o r compatible S-R p a i r i n g s t h a n f o r c h i l d r e n t h e d i f f e r e n c e is a r o u n d 270 ms.

155

from t h e present study is t h e a n d incompatible responses in a r e between 40 a n d 80 ms f a s t e r incompatible ones, whereas in

T h e r e a r e t w o possible explanations f o r these r e s u l t s . T h e f i r s t i s t h a t t h e crossed-hand e f f e c t i s g r e a t e r in c h i l d r e n t h a n in a d u l t s because t h e mismatch between t h e spatial code d e s c r i b i n g t h e r e s p o n d i n g h a n d a n d t h a t d e s c r i b i n g t h e position o f t h e response h a n d is more difficult t o resolve f o r c h i l d r e n t h a n f o r adults. T h i s explanation also can b e applied t o t h e d i f f e r e n c e between compatible a n d incompatible responses, w i t h c h i l d r e n h a v i n g more d i f f i c u l t y t h a n a d u l t s when t h e r e is 'a lack o f correspondence between t h e code d e s c r i b i n g t h e spatial position o f t h e stimulus a n d t h a t d e s c r i b i n g t h e spatial position o f t h e r e s p o n d i n g hand. T h e second explanation i s t h a t t h e o v e r a l l longer R T s in c h i l d r e n t h a n in a d u l t s may m a g n i f y t h e differences between crossed a n d uncrossed conditions a n d between compatible a n d incompatible responses. T h e most d i r e c t way t o d i s t i n g u i s h between t h e t w o explanations is t o a r t i f i c i a l l y inflate t h e RTs o f adults, f o r instance by r e d u c i n g t h e i n t e n s i t y o f t h e v i s u a l stimuli. In t h i s case, RTs should b e slowed overall, and, if t h e second explanation is correct, t h e differences between conditions should increase t o magnitudes similar t o those shown by c h i l d r e n . T h e f i r s t explanation, t h a t c h i l d r e n have e x t r a difficulty in r e s o l v i n g mismatches between stimulus a n d response codes, seems plausible when we consider t h e d i f f i c u l t y t h a t y o u n g i n f a n t s have in r e a c h i n g across t h e midline ( B r u n e r , 1969). Provine a n d Westerman (1979) have shown t h a t , in i n f a n t s o f 9 t o 20 weeks of age, v i s u a l l y d i r e c t e d h a n d extension a n d reaching develop p r o g r e s s i v e l y f r o m t h e ipsilateral domain t o t h e midline a n d l a t e r t o t h e contralateral domain. Thus, it seems t h a t ipsilaterat S-R l i n k s a r e p r e s e n t b e f o r e contralateral ones, a n d in y o u n g c h i l d r e n S-R p a i r i n g s on t h e same side o f t h e b o d y midline a r e predomin a n t o v e r S-R p a i r i n g s t h a t c r o s s t h e b o d y midline (Wapner E Cirillo, If we consider t h a t t h i s e f f e c t decreases w i t h age (Wapner & 1968). Cirillo, 1968), it is n o t s u p r i s i n g t h a t t h e d i f f e r e n c e between compatible a n d incompatible responses i s g r e a t e r f o r c h i l d r e n t h a n f o r adults. T h e second s t u d y was i n t e n d e d t o assess w h e t h e r spatial compatibility effects in c h i l d r e n a r e confined t o t h e horizontal dimension or o c c u r in t h e v e r t i c a l dimension as well. Several studies (Nicoletti & Umilta, 1984; Simon, Mewaldt, Acosta, & Hu, 1976) have demonstrated in adults above-below compatibility effects similar t o those f o u n d f o r t h e horizontal dimension. T h a t is, RTs a r e s h o r t e r when t h e t o p stimulus r e q u i r e s a response w i t h t h e t o p h a n d t h a n when it r e q u i r e s a response w i t h t h e bottom hand, a n d v i c e versa f o r t h e bottom stimulus.

S-R Compatibility E f f e c t For Vertical Dimension T h i s s t u d y i s essentially a r e p e t i t i o n of t h e p r e v i o u s one t h a t h a d shown t h e spatial compatibility f o r t h e r i g h t h e f t dimension. The only notable d i f f e r e n c e is t h a t t h e l i g h t stimuli a n d t h e response k e y s a r e a r r a n g e d v e r t i c a l l y . Six new subjects, 3 males a n d 3 females, selected as in t h e p r e v i o u s experiment, took p a r t in t h e experiment. T h e apparatus was t h e same as t h a t described f o r p r e v i o u s one, e x c e p t t h a t t h e t w o LEDs were above a n d below t h e f i x a t i o n p o i n t a t a distance o f 11.5 cm. T h e hands were aligned w i t h t h e midsagittal plane o f t h e b o d y a n d placed

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one above t h e t a b l e a n d t h e o t h e r below t h e table. T h e experiment comp r i s e d t w o conditions o f S-R p a i r i n g . In one condition t h e subject was r e q u i r e d t o respond t o t h e t o p light w i t h t h e t o p h a n d a n d t o t h e bottom light w i t h t h e bottom hand, whereas in t h e o t h e r condition t h e assignment was r e v e r s e d so t h a t t h e t o p light corresponded t o t h e bottom h a n d a n d t h e bottom light t o t h e t o p hand. In each condition, t h e responses were g i v e n w i t h t h e right h a n d above a n d t h e l e f t below. Mean R T was computed across sessions f o r each subject f o r each o f t h e conditions r e s u l t i n g f r o m t h e combination between t h e position o f t h e stimulus (above a n d below) a n d t h e position o f t h e c o r r e s p o n d i n g h a n d (above a n d below). These data a r e depicted g r a p h i c a l l y in F i g u r e 5. A n analysis o f variance u s i n g position o f t h e stimulus a n d position of t h e r e s p o n d i n g h a n d as main f a c t o r s showed t h a t o n l y t h e interaction was s i g n i f i c a n t [F(1,5) = 25.16, p < .005]. Paired t - t e s t s showed t h a t t h e 296 ms advantage o f t h e t o p h a n d o v e r t h e bottom h a n d i n responding t o t h e t o p stimulus was s i g n i f i c a n t [ t ( 5 ) = 3.11, p < .05], as well as t h e 367 ms advantage o f t h e bottom h a n d o v e r t h e t o p h a n d in responding t o t h e bottom stimulus [ t ( 5 ) = 4.92, p < .01].

A similar analysis o f variance was c a r r i e d o u t on t h e arc-sine transformation o f percentage o f e r r o r s . These data a r e depicted g r a p h i cally in F i g u r e 6. T h e analysis o f variance showed a s i g n i f i c a n t e f f e c t o f stimulus position [ F ( I , 5) = 8.99, p < .05], d u e t o f e w e r e r r o r s f o r t h e bottom stimulus (7.4) t h a n f o r t h e t o p stimulus (9.2). T h e interaction between stimulus position a n d h a n d position was s i g n i f i c a n t [F(1,5) = 9.77 p < .05] a n d showed t h a t t h e t o p h a n d was more accurate t h a n t h e bottom h a n d in r e s p o n d i n g t o t h e t o p stimulus a n d t h e bottom h a n d was more accurate t h a n t h e t o p h a n d in r e s p o n d i n g t o t h e bottom stimulus ( p < .05 in b o t h comparisons). T h e r e s u l t s o f t h e p r e s e n t s t u d y showed t h a t in c h i l d r e n spatial compatibility is n o t confined t o t h e horizontal dimension but occurs in t h e v e r t i c a l dimension as well. RTs were s h o r t e r when t h e t o p stimulus r e q u i r e d a response w i t h t h e t o p h a n d t h a n when it r e q u i r e d a response w i t h t h e bottom hand, a n d v i c e versa f o r t h e bottom stimulus. As f o r t h e horizontal dimension, t h e d i f f e r e n c e between compatible a n d incomp a t i b l e responses was g r e a t e r f o r c h i l d r e n t h a n f o r a d u l t s . F o r adults, R T s t y p i c a l l y a r e a r o u n d 50 ms f a s t e r f o r compatible S-R p a i r i n g s t h a n f o r incompatible ones, whereas f o r c h i l d r e n t h e d i f f e r e n c e is 331 ms. Conclusions From C o m p a t i b i l i t y Studies With C h i l d r e n T h e p r e s e n t s t u d y showed t h a t (a) 4-5-year-old c h i l d r e n have spatial compatibility effects i n b o t h t h e horizontal a n d t h e v e r t i c a l dimension a n d (b) spatial compatibility effects a r e g r e a t e r f o r c h i l d r e n t h a n f o r adults. These f i n d i n g s allow us t o reject t h e hypothesis t h a t t h e codes i n v o l v e d in spatial compatibility a r e v e r b a l (e.g., t h a t t h e y c o r r e spond t o t h e v e r b a l labels d e s c r i b i n g t h e r e l e v a n t positions o f stimuli a n d responses), because y o u n g c h i l d r e n a r e unable t o a p p l y v e r b a l labels t o t h e positions o f stimuli aligned along t h e horizontal dimension. Therefore, these codes a r e l i k e l y t o b e spatial. In conclusion, t h e compatibility e f f e c t s f o u n d w i t h a p e r p e n d i c u l a r arrangement o f stimuli a n d r e sponses by Lgdavas (1987) in right handers cannot b e i n t e r p r e t e d as


VERTICAL DIMENSION

TOP Stimulus

Bottom Stimulus

1200 3 1000

T

.

157

= n

E

0 ‘3 0

400

m

g

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Bottom Hand

Figure 5. Reaction times as a function of position of the stimulus (top and bottom) and position of t h e responding hand (top and bottom).

VERTICAL DIMENSION

TOP

Stimulus

Bottom Stimulus

Top Hand

= . I

I

Bottom Hacd

Figure 6. Percentage of e r r o r s as a function of position of t h e stimulus (top and bottom) and position of t h e responding hand (top and bottom).

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S-R l i n k s o f t h e v e r b a l codes f o r "right" a n d "above" a n d t h e v e r b a l codes f o r " l e f t " a n d "below." Rather, t h e effects a r e b e t t e r explained as an asymmetry in t h e coding of t h e dominant h a n d a n d non-dominant h a n d along t h e v e r t i c a l dimension, w i t h t h e dominant h a n d b e i n g assigned a "higher" position compared t o t h e non-dominant hand.

Some D e t e r m i n i n g Factors I n P r o d u c i n g S-R Compatibility E f f e c t s Before d i s c u s s i n g t h e relevance o f t h e coding o f t h e anatomical status o f t h e h a n d in d e t e r m i n i n g t h e spatial compatibility effects, it i s necessary t o discuss t h e relevance o f t w o o t h e r f a c t o r s . So f a r i we have considered t h e response set by making reference t o t h e positions o f t h e effectors in general. However, t w o positional features o f t h e response can b e distinguished: one is t h e position o f t h e e f f e c t o r a n d t h e o t h e r is t h e position o f t h e response goal. I n spatial S-R compatibility effects b o t h positional features have been t a k e n i n t o consideration. Brebner et al. (1972) stressed t h e importance of t h e spatial coding o f t h e location o f t h e response goal, whereas Wallace (1971, 1972) emphasized t h e spatial c o d i n g o f t h e location o f t h e e f f e c t o r . Because in all p r e v i o u s studies effectors a n d response goals were confounded, Riggio e t al. (1986) f o u n d an experimental s i t u a t i o n in w h i c h each e f f e c t o r (hand) h a d a response goal ( k e y ) t h a t shared (uncrossed condition) o r did n o t share (crossed conditions) t h e same locational code In t h i s study, t h e subjects operated t w o s t i c k s t h a t o f t h e effectors. were e i t h e r uncrossed o r crossed. I n t h e uncrossed condition, t h e effectors ( t h e t w o hands) a n d t h e response goals ( t h e t w o keys) occupied t h e same location in b o t h absolute ( i . e . , i n relation t o t h e b o d y midline) a n d r e l a t i v e (i.e., in relation t o t h e o t h e r e f f e c t o r o r response goal) terms. In t h e crossed condition, t h e effectors a n d response goals occupied opposite locations, again absolutely o r r e l a t i v e l y . The results were clear in showing t h a t spatial compatibility depended on t h e location o f t h e response goal, whereas t h e location o f t h e e f f e c t o r h a d no influence. In t h e crossed condition, t h e right hand, which was on t h e right side but operated t h e l e f t key, was f a s t e r f o r t h e l e f t t h a n f o r t h e right stimulus, whereas t h e l e f t hand, which was o n t h e l e f t side but operated t h e right key, was f a s t e r f o r t h e right t h a n f o r t h e l e f t stimulus. In o t h e r words, t h e locational coding o f t h e response goal dominated t h a t o f t h e location o f t h e e f f e c t o r . It is s u r p r i s i n g t h a t also in t h i s experiment t h e responses w i t h t h e s t i c k s crossed were slower t h a n those w i t h t h e s t i c k s uncrossed, although t h e hands were always in t h e anatomical uncrossed positions. It seems t h a t t h e r e a r e t h r e e factors t h a t a r e responsible for S-R compatibility effects: (a) t h e spatial coding o f t h e goal, (b) t h e spatial coding o f t h e response effectors, a n d (c) t h e c o d i n g o f t h e anatomical status o f t h e response (i.e., w h i c h h a n d emits t h e response). These f a c t o r s seem t o b e h i e r a r c h i c a l l y o r d e r e d . T h e h i g h e r - l e v e l f a c t o r dominates a n y f a c t o r o f lower r a n k , but a lower-level f a c t o r emerges if t h e h i g h e r - l e v e l f a c t o r s a r e n o t available. A s shown by Riggio e t al. (1986), t h e location o f t h e response goal dominates t h e location o f t h e effector, whereas t h e location o f t h e effector dominates t h e anatomical status o f t h e r e s p o n d i n g h a n d (Anzola e t al., 1977; B r e b n e r e t al., 1972).

Spafial S-R Compatibility in Adults and Children

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However, t h e anatomical status o f t h e r e s p o n d i n g h a n d dominates t h e o t h e r t w o f a c t o r s when t h e spatial coding o f t h e stimulus a n d t h e spatial coding o f t h e response goal and/or t h e location o f response e f fectors belong t o d i f f e r e n t spatial dimensions (i.e., horizontal a n d v e r t i cal). In o t h e r words, t h e r e s p o n d i n g h a n d i s t h e c r u c i a l f a c t o r in det e r m i n i n g t h e S-R compatibility effects, when t h e spatial relation between stimulus locations a n d response locations is considered t o b e n e u t r a l . T h i s was t h e case in a s t u d y by Klapp, Greim, Mendicino, a n d Koenig (19791, in w h i c h t h e spatial position o f t h e stimuli ( l e f t o r right) was u n r e l a t e d t o t h e spatial position o f t h e hands (up o r down). However, since t h e r e was a relationship between t h e t h u m b (right o r l e f t ) a n d stimulus location (right o r l e f t ) , t h e y f o u n d S - R Compatibility e f f e c t s simi l a r t o those obtained between stimulus location a n d response location. LBdavas a n d Moscovitch (1984) a n d LBdavas (1987) f o u n d similar r e s u l t s in an experimental situation in w h i c h t h e stimuli were displayed v e r t i c a l l y (above a n d below) a n d t h e hands h o r i z o n t a l l y ( l e f t a n d right). In a recent study, P r o c t o r a n d Reeve (1985) also f o u n d t h a t t h e spatial locat i o n is n o t t h e o n l y response f e a t u r e t h a t can b e used t o p r o d u c e S-R compatibility effects, but t h a t responses can b e coded also o n t h e basis o f a d i s t i n c t i o n between hands. I n an experimental condition w h e r e t h e hands were overlapped, t h a t is, one above t h e other, a n d symbolic stimuli were assigned t o f i n g e r s such t h a t t h e salient f e a t u r e o f t h e stimuli corresponded t o t h e h a n d d i s t i n c t i o n , subjects used h a n d coding instead o f spatial-location c o d i n g t o p r o d u c e S-R compatibility effects. Therefore, we can conclude t h a t when t h e spatial position o f t h e stimuli a n d t h e spatial position o f t h e response a r e unrelated, t h e r e s p o n d i n g h a n d replaces t h e response goal a n d t h e location o f response e f f e c t o r as a determinant f a c t o r i n p r o d u c i n g S - R compatibility effects. Summary In t h i s chapter, I examine t h e p o s s i b i l i t y t h a t t h e r e a r e t h r e e factors h i e r a r c h i c a l l y o r d e r e d t h a t a r e responsible f o r spatial S - R comp a t i b i l i t y effects: (a) t h e spatial coding o f t h e goal, (b) t h e spatial coding o f t h e response effectors, a n d (c) t h e c o d i n g o f t h e anatomical status o f t h e response (i.e., w h i c h h a n d emits t h e response). T h e location of t h e response goal dominates t h e location o f t h e effector, whereas t h e location of t h e e f f e c t o r dominates t h e anatomical status o f t h e responding h a n d . However, t h e anatomical status o f t h e r e s p o n d i n g h a n d dominates t h e o t h e r t w o f a c t o r s when t h e r e is n o t dimensional o v e r l a p between stimulus locations a n d response locations. A n o t h e r question t h a t has been addressed i n t h e p r e s e n t c h a p t e r is Two t h e n a t u r e o f t h e codes i n v o l v e d in spatial compatibility effects. experiments were described t h a t show t h e presence o f spatial compatibility effects i n c h i l d r e n who cannot discriminate between l e f t a n d right and, therefore, cannot use v e r b a l labels t o code t h e positions o f stimuli a n d responses. T h e r e s u l t s showed t h a t (a) 4-5-year-old c h i l d r e n have spatial compatibility effects in b o t h t h e horizontal a n d t h e v e r t i c a l dimension, a n d (b) spatial compatibility effects a r e g r e a t e r f o r c h i l d r e n than for adults. Therefore, these f i n d i n g s allows u s t o reject t h e hypothesis t h a t t h e codes i n v o l v e d i n spatial compatibility a r e v e r b a l .

E. ldavas

160

Acknowledgement T h i s research was s u p p o r t e d by a CNR g r a n t t o E. LBdavas and by Natural Sciences a n d Engineering Research Council o f Canada G r a n t 48347 t o M o r r i s Moscovitch. I t h a n k Carlo UmiltB f o r h i s h e l p o n t h e discussion of t h e manuscript. References Annet, M. (1970). A classification o f hand preference by analysis. British Journal of Psychology, 61, 303-321. Anzola, G. P . , Bertoloni, G . , Buchtel, H. A., & Rizzolatti, G. (1977). Spatial compatibility a n d anatomical factors in simple a n d choice r e action time. Neuropsychologia, 15, 295-302. Belmont, L., E Birch, H. G. (1963). Lateral dominance a n d r i g h t - l e f t awareness in normal c h i l d r e n . Child Development, 34, 257-270. Benton, A. L. (1959). Right-left discrimination and finger localization. New Y o r k : Hoeber-Harper. Berlucchi, G., Crea, F., D i Stefano, M., & Tassinari, G. (1977). Influence o f spatial stimulus-response compatibility o n reaction time o f ipsilateral a n d contralateral h a n d t o lateralized light stimuli. Journal of Expertmental Psychology: Human Perception and Performance, 3,

505-517. Boone, D. R., & Prescott, T. E. (1968). Development o f l e f t - r i g h t d i s crimination in normal c h i l d r e n . Perceptual and Motor Skills, 26 267-

274. Brebner, J., Shephard, M., & Cairney, P. (1972). Spatial relationships a n d S-R compatibility. Acta Psychologica, 36, 1-15. Bruner, J. (1969). Eye, hand a n d mind. In D. E l k i n d a n d J. H. Flavell (Eds.), Studies in cognitive development: Essays in honor of Jean Plaget. New Y o r k : O x f o r d U n i v e r s i t y Press. Hedge, A., E Marsh, N . W. A. (1975). T h e effect o f i r r e l e v a n t spatial correspondence on two-choice response time. Acta Psychologica, 39,

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(1971). The slow learner in the classroom.

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(1979). Klapp, S. T., Greim, D. M. Mendicino, C . M. E Koenig, R. S. Anatomic a n d environmental dimensions of stimulus-response compatibility: Implication f o r theories o f memory coding. Acta Psychologica, 43, 367-379. Ladavas, E. (1987). Influence of handedness on spatial compatibility effects w i t h perpendicular arrangement o f stimuli a n d responses. Acta Psychologica, 64,13-23.


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(1988). Asymmetries in processing horizontal a n d v e r t i c a l LBdavas, E. dimensions. Memory & Cognition, 16, 377-382. (1984). M u s t egocentric a n d e n v i r o n LBdavas, E., & Moscovitch, M. mental frames o f reference b e aligned t o p r o d u c e spatial S-R comp a t i b i l i t y effects? Journal of Experimental Psychology: Human Perception and Performance, 10, 205-215. Myers, R. E. (1965). T h e neocortical commissures a n d interhemispheric transmission o f information. In E. G. E t t l i n g e r , A . V. S. d e Reuch, & R. P o r t e r (Eds.), Function of the corpus callosum. London: Churchill. Nicoletti, R., Anzola, G. P. , Luppino, G . , Rizzolatti, G . , & UmiltB, C. (1982). Spatial compatibility e f f e c t s on t h e same side of t h e body midline. Journal of Experimental Psychology: Human Perception and Performance, 8 , 664-673. Nicoletti, R., & UmiltB, C. (1984). R i g h t - l e f t prevalence in spatial comp a t i b i l i t y . Perception & Psychophysics, 3 5 , 333-343. (1984). Compatibility d u e t o Nicoletti, R., UmiltB, C., & LBdavas E. Acta Psychot h e coding o f t h e r e l a t i v e position of t h e effectors. logica. 5 7 , 133-143. Proctor, R. W., & Reeve, T. G . (1985). Compatibility effects in t h e assignment o f symbolic stimuli t o d i s c r e t e f i n g e r response. Journal of Experimental Psychology: Human Perceptlon and Performance, 1 1 , 623639. C r o s s i n g t h e midline: Provine, R. R. & Westerman, J. A . (1979). Limits o f e a r l y eye-hand behavior. Child Development, 50, 437-441. Riggio, L., Gawryszewski, L. G., E UmiltB, C. (1986). What is crossed in crossed-hand effects? Acta Psychologica, 62, 89-100. Sekiyama, K . (1982). Kinesthetic aspects o f mental representation in t h e identification o f l e f t a n d right h a n d s . Perception & Psychophysics, 32, 89-95. Simon, J. R . (1968). E f f e c t o f ear stimulated o n reaction time a n d Journal of Experimental Psychology, 7 8 , 344-346. movement time. Simon, J. R., Hinrichs, J. V., & C r a f t , J. L. (1970). A u d i t o r y S-R compatibility: Reaction time as a f u n c t i o n o f e a r - h a n d correspondence Journal of Experimental a n d ear- response-location correspondence, Psychology, 86, 97-102. Simon, J. R . , Mewaldt, S . P., Acosta, E., Jr., & Hu, J. M. (1976). Processing a u d i t o r y information : I n t e r a c t i o n of t w o population stereot y p e s . Journal of Applied Psychology, 60, 354-358. Simon,

Sly, P. E., & Vilapakkam, S . (1981). E f f e c t o f comS-R mapping on reactions t o w a r d the stimulus source. Acta Psychologica, 47, 63-81. J.

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Simon, J. R., & Sudalaimuthu, P. (1979). Effects o f S - R mapping a n d Journal of response modality o n performance in a Stroop t a s k . Experimental Psychology: Human Perception and Performance, 5 , 176187. (1964). T h e g r e a t c e r e b r a l commissure. S p e r r y , R. W. American, 210, 1, 42-52.

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h i l t & , C., & Nicoletti, R. (1985). A t t e n t i o n a n d coding effects i n S - R compatibility d u e t o i r r e l e v a n t spatial cues. I n M. I . Posner, & 0. S. M. Marin, (Eds.), Attention ond performance X I (pp. 457-471). Hillsdale, NY: Erlbaum.

S-R compatibility a n d t h e idea of a response Wallace, R. J. (1971). code. Journal of Experimental Psychology, 88, 354-360. Wallace, R. J. (1972). Spatial S-R compatibility effects i n v o l v i n g kinest h e t i c cues. Journal of Experimental Psychology, 93, 163-168. Wapner, S., & Cirillo, L. (1968). Imitation o f a model's h a n d moveChild ments: A g e changes in t r a n p o s i t i o n o f l e f t - r i g h t relations. Development, 3 9 , 887-895.

S TIMULUS-RESPONSE COMPATIfllUW R.W. Proctor and EG. Reeve (Editors 0 Elsevier Science Publishers 6.V. (Jorth-Holland), 1990

163

THE SALIENT-FEATURES CODING PRINCIPLE FOR SPATIALAND SYMBOLIC-COMPATI B I L l T Y EFFECTS

T. GILMOUR REEVE Motor Behavior C e n t e r Auburn University ROBERT W. PROCTOR Department o f Psychological Sciences Purdue University Stimulus-response (S-R) compatibility effects typically are a t t r i b u t e d t o a stage o f human-information processing t h a t is r e f e r r e d t o as e i t h e r S-R translation, response selection, o r decision (e.g., T e i c h n e r & Krebs, 1974). T h e p r i m a r y f u n c t i o n o f t h i s stage i s t o t r a n s l a t e between t h e codes t h a t a r e used t o r e p r e s e n t t h e stimulus a n d response sets. Situations f o r w h i c h S-R t r a n s l a t i o n is minimal a r e r e g a r d e d as compatible, whereas situations f o r w h i c h it is n o t a r e r e g a r d e d as incompatible. With spatial-location stimuli, S-R compatibility is a f u n c t i o n o f t h e e x t e n t t o w h i c h t h e assignment o f stimulus locations t o response locations maintains a d i r e c t relation. F o r example, when t w o l e f t a n d r i g h t spatially a r r a n g e d stimuli a r e assigned t o t h e l e f t a n d right index f i n g e r s , reaction times (RTs) a r e fastest if t h e right stimulus is assigned t o t h e right response a n d t h e l e f t stimulus t o t h e l e f t response. With hands placed i n a normal manner, t h e relation o f stimulus locations t o response locations is confounded w i t h t h e relation of stimulus locations t o t h e l e f t a n d r i g h t hands. However, t h e t w o relations can b e dissociated by c r o s s i n g t h e hands, such t h a t t h e l e f t - i n d e x f i n g e r i s placed a t t h e right response location a n d t h e r i g h t - i n d e x f i n g e r a t t h e l e f t location. Even w i t h t h i s crossed hands placement, t h e R T s s t i l l a r e f a s t e s t when t h e right stimulus is assigned t o t h e right response location a n d t h e l e f t stimulus t o t h e l e f t response location (e.g., Nicoletti, Anzola, Luppino, Rizzolatti, & U m i l t i , 1982). Thus, t h e relation between stimulus locations. a n d response locations is crucial, r a t h e r t h a n t h e relation between stimulus locations a n d t h e hands. T h i s f i n d i n g indicates t h a t t h e compatibility effects r e f l e c t t r a n s l a t i o n between codes t h a t a r e based o n t h e spatial characteristics o f t h e stimulus a n d response sets (see C h a p t e r 3, by U m i l t i & Nicoletti). Spatial coding also i s a p p a r e n t when symbolic stimuli (e.g., letters, colors) a r e assigned t o l e f t a n d right response locations, but t h e stimuli o c c u r i n e i t h e r of t w o locations t h a t a r e i r r e l e v a n t f o r d e t e r m i n i n g t h e c o r r e c t response (see C h a p t e r 2 , by Simon). T h a t is, in such situations, responses t y p i c a l l y a r e f a s t e r when t h e i r r e l e v a n t location o f t h e symbolic stimulus corresponds t o t h e response f o r t h a t stimulus t h a n when it does not. T h i s relation between i r r e l e v a n t stimulus locations a n d response locations holds even when t h e hands a r e crossed (Umilt6 8 Nicoletti, 1985; Wallace, 19711, w h i c h is consistent w i t h t h e r e s u l t s obtained when t h e locations o f t h e stimuli a r e r e l e v a n t t o t h e responses.

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T.G. Reeve and R. W. Proctor

A l t h o u g h S-R compatibility effects were s t u d i e d i n i t i a l l y in relat i v e l y complex t a s k s (e.g., F i t t s & Seeger, 1953; G a r v e y & Knowles, 19541, most o f t h e r e c e n t evidence f o r spatial coding has been r e s t r i c t e d t o two-choice t a s k s similar t o those j u s t described. Moreover, few studies have examined situations f o r w h i c h t h e responses a r e made a t d i s t i n c t spatial locations, but t h e stimuli d o n o t d i f f e r o n e i t h e r a r e l e v a n t o r i r r e l e v a n t spatial-location dimension (i.e., symbolic stimuli a r e presented a t o n l y one location). A n exception o f t h e r e s t r i c t i o n t o t w o choice t a s k s involves studies t h a t have used a movement-precuing procedure, in w h i c h parameters o f limb movements, such as arm, direction, o r extent, a r e specified in advance (Rosenbaum, 1983). The i n t e n t o f t h e movement-precuing studies has been t o evaluate t h e n a t u r e o f response p r e p a r a t i o n t h r o u g h t h e p a t t e r n s o f R T benefits t h a t a r e obtained when v a r i o u s movement parameters, o r combinations o f movement parameters, a r e precued. In such studies, t h e potential f o r S-R compatibility effects must b e evaluated, because t h e effects have t h e i r basis in S-R t r a n s l a t i o n processes r a t h e r t h a n in response-preparation processes. Several studies have demonstrated t h a t S-R compatibility does in f a c t i n f l u e n c e t h e r e s u l t s obtained i n movement-precuing t a s k s (e.g., Goodman 8 Kelso, 1980; Larish, 1986). However, these studies have been concerned p r i m a r i l y w i t h t h e necessary procedures t o c o n t r o l comp a t i b i l i t y effects, r a t h e r t h a n w i t h t h e u n d e r l y i n g n a t u r e o f S-R compatibility (see C h a p t e r 11, by Zelaznik & F r a n z ) . A l t h o u g h i n t e r e s t i n S-R compatibility effects in t h e movementp r e c u i n g t a s k has focused p r i m a r i l y o n c o n t r o l l i n g such effects, t h e t a s k i t s e l f may b e used as a vehicle f o r i n v e s t i g a t i n g t h e n a t u r e o f S - R compatibility. T h a t is, t h e p r e c u i n g t a s k maintains some o f t h e characteristics o f t h e basic two-choice t a s k but i s more complex. Its complexity in terms o f t h e stimulus a n d response sets, t h e number o f S-R alternatives, a n d t h e use o f advance information allows examination o f a b r o a d e r r a n g e o f compatibility issues. Recently, we have conducted systematic investigations o f S-R compatibility in variations o f t h e movement-precuing t a s k t h a t have used b o t h spatial-location a n d symbolic stimuli assigned t o keypress responses. T h e investigations w i t h spatiallocation stimuli have indicated a r o l e f o r spatial coding in t h e movementp r e c u i n g t a s k t h a t is similar t o t h e role t h a t such coding p l a y s in t h e two-choice t a s k . B u t o u r studies also have shown t h a t coding can o c c u r on t h e basis o f o t h e r features, such as t h e d i s t i n c t i o n between hands, when those features a r e salient. Moreover, coding o f stimulus a n d response sets based o n salient features occurs when t h e stimuli a r e symbolic. T h e r e s u l t s o f o u r studies have c o n v e r g e d t o s u p p o r t a basic p r i n c i p l e , t h e salient-features coding p r i n c i p l e , f o r spatial- a n d symboliccompatibility effects. In t h e following sections, we p r e s e n t t h e p r i m a r y evidence f o r t h i s p r i n c i p l e . Coded Representations o f Spatial-Location Stimuli a n d Responses O u r i n i t i a l studies used a v a r i a t i o n o f t h e movement-precuing task, i n t r o d u c e d by M i l l e r (1982), t h a t involves f i n g e r movements r a t h e r t h a n limb movements. T h e t a s k uses f o u r potential keypress responses t h a t a r e made w i t h t h e i n d e x a n d middle f i n g e r s o f each hand. Each t r i a l begins w i t h a w a r n i n g r o w o f f o u r p l u s signs, followed one-half second l a t e r by a p r e c u e t h a t is presented immediately below t h e w a r n i n g row

The Salient-Features Coding Principle

165

(see Table 1). For an u n p r e p a r e d condition, t h e p r e c u e consists o f another r o w o f f o u r p l u s signs. F o r t h e p r e p a r e d conditions, t h e p r e c u e consists o f p l u s signs in o n l y t w o o f t h e f o u r locations. Following a v a r i a b l e p r e c u i n g i n t e r v a l of 0-3000 ms, a t a r g e t stimulus occurs in a third row a t one o f t h e c u e d locations, a n d t h e subject is t o make t h e c o r r e s p o n d i n g response as q u i c k l y as possible. Table 1 Stimulus Displays f o r Each Preparation Condition When t h e T a r g e t Indicated L e f t Middle-Finger Response Finger

LM

LI

RI

RM

+ +

+

+

+

Unprepared Warning Precue Target

+ +

+ +

Warning Precue Target

+

+

+

+

Warning Precue Target

+

+

+ Prepared: Hand

+

Prepared: F i n g e r + +

+ + Prepared: N e i t h e r

Warning Precue Target Note. finger.

+

+

+

+

+

L = l e f t hand;

R = right hand;

M = middle f i n g e r ;

I = index

T h e p r e p a r e d conditions a r e o f t h r e e d i f f e r e n t t y p e s (see Table 1). I n t h e p r e p a r e d : h a n d condition, t h e p r e c u e specifies t w o f i n g e r s on t h e same h a n d (e.g., t h e left-middle a n d l e f t - i n d e x f i n g e r s ) ; in t h e p r e p a r e d : f i n g e r condition, t h e p r e c u e specifies t h e same f i n g e r on d i f f e r e n t hands (e.g., t h e l e f t - i n d e x a n d r i g h t - i n d e x f i n g e r s ) ; a n d in t h e p r e pared: n e i t h e r condition, t h e p r e c u e specifies d i f f e r e n t f i n g e r s on d i f f e r e n t hands (e.g., t h e left-middle a n d r i g h t - i n d e x f i n g e r s ) . When t h e hands a r e placed i n a normal, adjacent manner ( t h e adjacent-hand placement), a p a t t e r n o f d i f f e r e n t i a l p r e c u i n g b e n e f i t s is obtained. This patt e r n i s t h a t R T s a r e fastest f o r t h e p r e p a r e d : h a n d condition a n d slowest f o r t h e p r e p a r e d : n e i t h e r condition, w i t h t h e p r e p a r e d : f i n g e r condition

T.G. Reeve and R. W.

166 b e i n g intermediate Proctor, 1984).

(Miller,

1982;

Proctor

Proctor &

Reeve,

198813;

Reeve

&

O u r studies indicate t h a t t h i s p a t t e r n o f d i f f e r e n t i a l p r e c u i n g benef i t s is an S-R compatibility e f f e c t t h a t reflects how r a p i d l y t h e subject can determine the. p r e c u e d p a i r o f responses. One finding 'consistent w i t h t h i s compatibility i n t e r p r e t a t i o n is t h a t t h e advantage f o r t h e p r e pared: h a n d condition is a p p a r e n t p r i m a r i l y a t s h o r t p r e c u i n g i n t e r v a l s (Reeve & Proctor, 1984). As shown in F i g u r e 1, when t h e i n t e r v a l is extended t o 3 s, all p a i r s o f responses show approximately equivalent benefits r e l a t i v e to t h e u n p r e p a r e d condition (see also Reeve & Proctor, 1988). Thus, c e r t a i n p a i r s o f p r e c u e d responses can b e selected f a s t e r t h a n others, w i t h n o i n h e r e n t differences in t h e p r e p a r a t i o n of t h e diff e r e n t p a i r s once t h e responses have been selected.

As f o r t h e compatibility e f f e c t t h a t occurs w i t h a normal h a n d placement in two-choice tasks, t h e relation between stimulus locations a n d response locations i s confounded w i t h t h e relation between stimulus locations a n d f i n g e r s when o n l y t h e adjacent-hand placement is used. To determine w h e t h e r response locations o r specific f i n g e r s a r e crucial, we used an overlapped-hand placement (Reeve & Proctor, 1984, Experiment 3 ) . Relative t o t h e adjacent placement, t h e overlapped placement dissociates t h e spatial locations o f t h e stimuli a n d responses f r o m t h e p a r t i c u l a r

-

800

Prepared: Hand Prepared :Finger Prepared: Neither Unprepared

5 700

E W 2 Iz 0

I-

2 600 W

a

500

I

0

I

375

I

750

I

1500 PRECUING INTERVALS lMSl

I

3000

F i g u r e 1. Mean R T s f o r u n p r e p a r e d a n d p r e p a r e d conditions as a f u n c t i o n o f p r e c u i n g i n t e r v a l ( f r o m Reeve & Proctor, 1984, Experiment 1 ) .


167

p a i r s o f f i n g e r s t h a t a r e cued. T h a t is, t h e c u e d locations f o r t h e t w o extreme conditions, t h e p r e p a r e d : h a n d a n d p r e p a r e d : n e i t h e r conditions, a r e switched f o r t h e o v e r l a p p e d placement. Thus, if t h e p r e c u i n g advantage arises f r o m t h e relation between stimulus locations a n d t h e f i n g e r s o n t h e l e f t a n d right hands, it s t i l l should o c c u r f o r t h e p r e p a r e d : h a n d condition. However, if t h e advantage is a f u n c t i o n o f t h e relation between stimulus locations a n d response locations, it should o c c u r f o r t h e p r e p a r e d : n e i t h e r condition. As shown in Table 2, t h i s l a t t e r r e s u l t was obtained, t h u s i n d i c a t i n g t h a t t h e advantage r e a l l y is one f o r t h e t w o leftmost a n d t w o r i g h t m o s t spatial locations, regardless o f w h e t h e r t h e f i n g e r s a r e f r o m t h e same o r d i f f e r e n t hands. Table 2 Mean Reaction Times (in ms) f o r t h e Hand Placement X Preparation Condition I n t e r a c t i o n ( f r o m Reeve & Proctor, 1984, Experiment 3)

Hand placement Preparation condition

U n p repa r e d Prepared: Hand Prepared: F i n g e r Prepared: N e i t h e r

Adjacent hands

Overlapped hands

551 502 516 547

771 765 746 721

One problem w i t h i n t e r p r e t i n g t h e r e s u l t s obtained w i t h t h e o v e r lapped placement as s u p p o r t f o r spatial coding i s t h a t t h e responses w i t h t h a t placement a r e considerably slower o v e r a l l t h a n t h e y a r e w i t h t h e adjacent placement. Thus, t h e comparison between t h e p a t t e r n s o f RTs f o r t h e t w o placements p o t e n t i a l l y is confounded w i t h t h e o v e r a l l level o f RTs (Miller, 1985). We have a r g u e d t h a t t h e p a t t e r n s o f d i f f e r e n t i a l p r e c u i n g benefits a r e independent f r o m t h e r e l a t i v e d i f f i c u l t i e s o f e x e c u t i n g responses w i t h t h e adjacent a n d overlapped placements (Reeve & Proctor, 1985). T o evaluate o u r position r e g a r d i n g t h e o v e r a l l levels o f RT, we conducted experiments t h a t used v e r t i c a l stimulus a n d response a r r a n g e ments (Proctor & Reeve, 1986). With t h e v e r t i c a l stimulus arrangement, t h e warning, precue, a n d t a r g e t stimuli a r e p r e s e n t e d in t h r e e columns o f f o u r locations, r a t h e r t h a n in t h r e e rows (see Table 3 ) . T h e responses a r e made on a row o f f o u r response keys, w h i c h i s s i t u a t e d p e r p e n d i c u l a r t o t h e subject. T o place t h e f i n g e r s on these keys, subjects must turn t h e i r hands i n w a r d .


168

Table 3 A n Example o f t h e V e r t i c a l Stimulus a n d Response Arrangements, With T a r g e t I n d i c a t i n g t h e T o p Response Location

Stimulus arrangement Warning

Precue

+ +

Hand placement

Target

+

Adjacent LM LI RM RI

Overlapped LM RM LI RI

L = l e f t hand; R = right hand; M = middle f i n g e r , I = i n d e x Note. finger. T h e response arrangements a r e indicated f o r t h e l e f t h a n d b e i n g assigned t o t h e t o p position. With t h e right h a n d on top, t h e h a n d assignments a r e r e v e r s e d . F o r t h e adjacent placement w i t h t h e v e r t i c a l arrangement, t h e t w o f i n g e r s f r o m one h a n d a r e placed on t h e t o p t w o response locations, a n d t h e t w o f i n g e r s f r o m t h e o t h e r h a n d a r e placed o n t h e bottom t w o locations (see Table 3). F o r t h e o v e r l a p p e d placement, t h e f i n g e r s f r o m t h e t w o hands a r e alternated. However, because one h a n d is n o t p h y s i c a l l y o n t o p o f t h e other, as is t h e case w i t h t h e horizontal response a r r a n g e ment, t h e f i n g e r movements a r e n o t o b s t r u c t e d . With t h e v e r t i c a l arrangement, no o v e r a l l d i f f e r e n c e i n R T e x i s t e d between t h e t w o h a n d placements (see F i g u r e 2 ) . Thus, t h e goal of eliminating t h e problem o f o v e r a l l R T differences between t h e adjacent a n d overlapped placements was met. Yet, t h e p a t t e r n o f p r e c u i n g benefits s t i l l followed t h e spatial locations. T h a t is, as indicated by t h e c r o s s i n g o f t h e filled-symbol f u n c t i o n s in F i g u r e 2 , t h e maximum b e n e f i t was obtained when e i t h e r t h e t w o top-most o r t w o bottom-most locations were precued, regardless o f w h e t h e r t h e f i n g e r s were f r o m t h e same o r d i f f e r e n t hands. Thus, w i t h b o t h horizontal a n d v e r t i c a l arrangements, t h e spatial locations o f stimuli a n d responses a r e c r u c i a l t o t h e p a t t e r n s o f r e l a t i v e p r e c u i n g benefits. A d d i t i o n a l evidence f o r spatial coding is apparent in a comparison o f t h e p r e c u i n g p a t t e r n s obtained w i t h t h e horizontal a n d v e r t i c a l arrangements. With t h e horizontal stimulus a n d response arrangements, t h e p r e p a r e d : f i n g e r condition corresponds t o t h e t w o innermost a n d t w o outermost locations, regardless o f w h e t h e r t h e h a n d placement is adjacent o r overlapped. Thus, even t h o u g h t h e r e l a t i v e benefits f o r t h e p r e p a r e d : h a n d a n d p r e p a r e d : n e i t h e r conditions a r e switched f o r t h e t w o h a n d placements, t h e p r e p a r e d : f i n g e r condition is intermediate f o r b o t h placements (Reeve & Proctor, 1984). I n contrast, w i t h t h e v e r t i c a l stimu l u s a n d response arrangements, t h e prepared: n e i t h e r condition c o r r e sponds t o t h e t w o innermost a n d t w o outermost locations. With t h i s arrangement, t h e p r e p a r e d : n e i t h e r condition shows intermediate p r e c u i n g


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Figure 2. Mean RTs w i t h t h e v e r t i c a l stimulus a n d response arrangements as a f u n c t i o n o f h a n d placement a n d p r e p a r a t i o n condition ( f r o m Proctor & Reeve, 1986, Experiment 1). b e n e f i t s for b o t h t h e adjacent a n d overlapped h a n d placements ( P r o c t o r & Reeve, 1986). Therefore, t h e r e s u l t s indicate t h a t p r e c u i n g t h e t w o innermost o r t w o outermost locations produces an intermediate benefit, regardless o f t h e specific p a i r s o f f i n g e r s t h a t a r e cued. A l t h o u g h spatial coding of stimulus and response sets predominates, some f l e x i b i l i t y e x i s t s in t h a t t h e d i s t i n c t i o n between t h e hands can b e used in t h e coding o f t h e response set. Hand coding was a p p a r e n t in t h e experiments t h a t used v e r t i c a l stimulus a n d response sets. T h a t is, although t h e r e l a t i v e p r e c u i n g b e n e f i t s in those experiments w e r e p r i m a r ily a f u n c t i o n o f spatial locations, t h e top-bottom advantage was l a r g e r f o r t h e adjacent placement t h a n f o r t h e o v e r l a p p e d placement. In other words, t h e advantage was l a r g e r when t h e t w o c u e d f i n g e r s were f r o m t h e same h a n d (see F i g u r e 2 ) . T h e l a r g e r top-bottom advantage f o r t h e adjacent-hand placement l i k e l y i s d u e t o t h e relation between t h e h a n d d i s t i n c t i o n a n d t h e t o p bottom spatial d i s t i n c t i o n . T h e h a n d d i s t i n c t i o n c o u l d inhibit t h e t o p bottom advantage w i t h t h e overlapped placement because t h e h a n d d i s t i n c t i o n i s inconsistent w i t h t h e top-bottom d i s t i n c t i o n . A l t e r n a t i v e l y , t h e h a n d d i s t i n c t i o n c o u l d f a c i l i t a t e t h e top-bottom advantage w i t h t h e adjac e n t placement because t h e t w o d i s t i n c t i o n s a r e consistent. We conducted

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t w o experiments t o d i s t i n g u i s h between these alternatives ( P r o c t o r & Reeve, 1986). In one experiment, a single-hand placement was used, in w h i c h subjects responded w i t h t h e f o u r f i n g e r s o f t h e i r right hand; t h e f i n g e r s were aligned v e r t i c a l l y as f o r t h e o t h e r h a n d placements. This single-hand placement showed a top-bottom advantage o f similar magnitude t o t h a t obtained w i t h t h e overlapped placement, t h u s indicatirig t h a t t h e h a n d d i s t i n c t i o n did n o t h a v e an i n h i b i t o r y e f f e c t f o r t h e l a t t e r placement. In t h e o t h e r experiment, an adjacent-side placement was used f o r some subjects. These subjects placed b o t h hands t o t h e right side o f t h e body, in a normal, adjacent manner. T h e logic o f t h i s placement was t h a t t h e v e r t i c a l correspondence between stimulus a n d response locations was maintained, but t h e hands n o longer h a d t o b e t u r n e d i n w a r d . T h i s adjacent-side placement y i e l d e d a smaller top-bottom advantage t h a n did t h e adjacent placement w i t h t h e hands t u r n e d i n w a r d . T h i s finding indicates t h a t t h e h a n d d i s t i n c t i o n h a d a f a c i l i t a t o r y e f f e c t f o r t h e adjacent placement. O u r i n t e r p r e t a t i o n o f t h i s finding is t h a t turning t h e hands i n w a r d makes t h e h a n d d i s t i n c t i o n more salient. T h i s saliency o f t h e hands r e s u l t s in an added p r e c u i n g b e n e f i t when it corresponds t o t h e salient top-bottom spatial f e a t u r e o f t h e stimulus a n d response sets.

Because b o t h t h e horizontal a n d v e r t i c a l arrangements t h a t we have used in t h e p r e c u i n g studies a r e linear, an i n t e r e s t i n g issue is how a stimulus set in one orientation is t r a n s l a t e d t o a response set in t h e o t h e r . We r e c e n t l y conducted an experiment in w h i c h t h e orientations o f t h e stimulus a n d response arrangements were manipulated factorially, w i t h o n l y adjacent h a n d placements b e i n g used (Proctor & Reeve, 1988a). In all situations, a p r e c u i n g advantage was obtained when e i t h e r t h e t w o leftmost (bottom-most) o r t w o r i g h t m o s t ( t o p m o s t ) locations were cued. A mismatch in stimulus- a n d response-set orientations added o n l y a constant t o RTs, w i t h o u t c h a n g i n g t h e p a t t e r n o f p r e c u i n g benefits t h a t was obtained. Thus, an additional orientation t r a n s l a t i o n was r e q u i r e d when t h e orientations o f t h e stimulus a n d response sets d i f f e r e d , but t h i s t r a n s l a t i o n was independent o f t h e spatial coding t h a t produces t h e relative precuing benefits. In summary, t h e r e s u l t s f r o m t h e s p a t i a l - p r e c u i n g t a s k s a r e cons i s t e n t w i t h those f r o m t h e two-choice reaction t a s k s in showing t h e stimu l u s a n d response locations t o b e important. T h e coding o f t h e stimulus a n d response sets occurs on t h e basis o f salient features o f t h e linear arrangements, w i t h t h e predominant f e a t u r e b e i n g t h e l e f t - r i g h t spatial locations. Additionally, t h e d i s t i n c t i o n between t h e t w o hands can b e used f o r coding. Hand c o d i n g i s e v i d e n t when the h a n d d i s t i n c t i o n is made more salient by turning t h e hands inward, but o n l y when t h e h a n d d i s t i n c t i o n coincides w i t h t h e salient spatial f e a t u r e .

Coded Representations o f Symbolic Stimuli a n d Spatial- Location Responses

Discussions o f S-R compatibility t y p i c a l l y s t r e s s t h e relation between t h e spatial locations o f stimuli a n d responses. Thus, most studies o f S-R compatibility t h a t have used symbolic stimuli (such as color patches o r l e t t e r s ) a n d keypress responses have presented t h e stimuli in d i f f e r e n t locations a n d examined t h e influence of t h e i r r e l e v a n t location d i s t i n c t i o n (e.g., Simon, 1969). A s a consequence, the

The Salient-Features Coding Principle p o s s i b i l i t y t h a t some assignments of symbolic stimuli responses may b e more compatible t h a n o t h e r s has t e n d e d looked (see F i t t s & Biederman, 1965, f o r an exception). have obtained evidence f o r such compatibility e f f e c t s f r o m have used two-dimensional, symbolic stimuli.

171 to keypress t o be overHowever, we studies t h a t

I n one experiment (Proctor & Reeve, 19851, t h e stimuli w e r e consonant-vowel p a i r s c o n s t r u c t e d f r o m t h e consonants B a n d M a n d t h e vowels E a n d 0, w i t h t h e l e f t - t o - r i g h t assignment o f stimuli t o response f i n g e r s b e i n g BE, 80, ME, a n d MO (see also Miller, 1982). In t h i s experiment, t h e p r e c u e consisted o f p r e s e n t i n g one letter, w i t h t h e second l e t t e r t h e n presented a f t e r a v a r i a b l e p r e c u i n g i n t e r v a l . A s in t h e p r e v i o u s l y described s p a t i a l - p r e c u i n g experiments, an R T advantage f o r t h e p r e p a r e d : h a n d condition was obtained when t h e hands were adjacent (see Figure 3 ) . T h a t is, responses were f a s t e r when t h e consonant, which precues f i n g e r s f r o m e i t h e r t h e l e f t o r right h a n d ( w h i c h a r e o n t h e t w o leftmost a n d t w o r i g h t m o s t response locations, respectively), was presented f i r s t . Moreover, when t h e hands were overlapped, allowing t h e consonant s t i l l t o cue t h e t w o leftmost o r t w o r i g h t m o s t locations, but w i t h t h e cued f i n g e r s now b e i n g on d i f f e r e n t hands, a c o n s o n a n t - f i r s t advantage s t i l l was obtained. Thus, as w i t h t h e spatial-precuing tasks, spatial locations a r e implicated as b e i n g c r u c i a l i n t h e c o d i n g o f stimuli t o responses in t h i s symbolic-precuing t a s k . The differential precuing benefits obtained w i t h t h e consonant-vowel stimuli seem t o b e S-R compatibility effects of t h e t y p e obtained w i t h spatial-location stimuli.

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F i g u r e 3. Mean RTs w i t h t h e BEBOMEMO assignment as a f u n c t i o n o f l e t t e r presented f i r s t a n d h a n d placement ( f r o m Proctor E Reeve, 1985, Experiment 1 ) . Evidence for spatial coding also was apparent when b o t h features of two-dimensional, symbolic stimuli were p r e s e n t e d simultaneously, a n d t h e relation t o spatial locations was v a r i e d by manipulating S-R assignments. In these experiments (Proctor & Reeve, 19851, t h e stimuli were composed f r o m t w o l e t t e r identities (0, Z) o f t w o sizes (large, small). Pilot experiments showed t h a t l e t t e r i d e n t i t y was more salient t h a n size. T h a t

172


is, a l e t t e r discrimination could b e made more r a p i d l y t h a n a size d i s crimination. U n l i k e t h e experiments described t o t h i s point, t h e e x p e r i ments u s i n g t h e letter/size stimuli did n o t i n v o l v e an o v e r t precue. T h e subject's t a s k simply was t o i d e n t i f y t h e stimulus that o c c u r r e d by making t h e assigned keypress response. T h e manipulation o f importance i n v o l v e d t h e assignments o f stimuli t o responses. For one assignment, l e t t e r i d e n t i t y distinguished between t h e t w o t h e OozZ assignment, leftmost a n d t w o rightmost response locations, whereas f o r t h e o t h e r With the assignment, t h e OzoZ assignment, it did n o t (see F i g u r e 4). adjacent-hand placement, t h e results were similar t o those o f earlier experiments. Responses were f a s t e r w i t h t h e OozZ assignment t h a n w i t h t h e OzoZ assignment, i n d i c a t i n g an advantage when t h e salient stimulus feature, l e t t e r identity, d i s t i n g u i s h e d between t h e t w o leftmost a n d t w o However, with the rightmost response locations (see F i g u r e 5). overlapped placement, t h e RTs were equivalent f o r t h e OozZ and OzoZ assignments.

oozz ozoz

oozz o z o z

F i g u r e 4. T h e OozZ and OzoZ assignments f o r t h e adjacent a n d o v e r lapped placements.


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F i g u r e 5. Mean RTs f o r t h e OozZ a n d OzoZ assignments as a f u n c t i o n o f h a n d placement ( P r o c t o r & Reeve, 1985, Experiment 2 ) . A c r u c i a l d i f f e r e n c e exists between t h e adjacent a n d overlapped With t h e overlapped placement, placements f o r t h e OzoZ assignment. l e t t e r i d e n t i t y distinguishes between responses on t h e t w o hands. Howe v e r , w i t h t h e adjacent placement, l e t t e r i d e n t i t y does n o t d i s t i n g u i s h between e i t h e r t h e hands o r t h e t w o leftmost a n d t w o r i g h t m o s t locations. Because responses a r e no slower f o r t h e OzoZ assignment t h a n f o r t h e OozZ assignment when hands a r e overlapped, h a n d coding a p p a r e n t l y is used w i t h t h e OzoZ assignment To t e s t t h i s account, we i n c l u d e d a h a n d placement t h a t removed t h e d i s t i n c t i o n between hands. For this placement, subjects responded w i t h t h e f o u r f i n g e r s o n a single hand. With t h e single-hand placement, t h e 0022 assignment again showed a l a r g e advantage o v e r t h e OzoZ assignment (see F i g u r e 6). Our conclusion, then, is t h a t S-R compatibility is greatest when t h e r e is a consistent relation between t h e salient stimulus f e a t u r e ( l e t t e r i d e n t i t y ) a n d e i t h e r t h e salient l e f t - r i g h t f e a t u r e o f t h e response arrangement o r the hand distinction. Additional s u p p o r t f o r h a n d coding was obtained in another p r e c u ing experiment t h a t used a d i f f e r e n t mapping o f t h e consonant-vowel stimulus set described p r e v i o u s l y ( P r o c t o r & Reeve, 1985). Because M i l l e r (1982) h a d f o u n d a c o n s o n a n t - f i r s t advantage when t h e l e f t - t o - r i g h t assignment was BE, BO, ME, a n d MO, we h a d i n i t i a l l y evaluated o n l y t h a t assignment. T h e a l t e r n a t i v e assignment, f o r w h i c h Miller h a d n o t f o u n d a c o n s o n a n t - f i r s t advantage, was BE, ME, 6 0 , a n d MO. Miller concluded that, w i t h t h i s BEMEBOMO assignment, t h e c o n s o n a n t - f i r s t advantage was n o t obtained because t h e consonants specified d i f f e r e n t f i n g e r s on d i f f e r e n t hands. However, w i t h an o v e r l a p p e d - h a n d placement, t h e consonants


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F i g u r e 6. Mean RTs f o r t h e OozZ a n d OzoZ assignments as a f u n c t i o n o f h a n d placement ( P r o c t o r & Reeve, 1985, Experiment 3 ) . would c o r r e s p o n d t o t h e h a n d d i s t i n c t i o n b u t n o t t o t h e l e f t - r i g h t spatial d i s t i n c t i o n . Thus, if h a n d coding can b e used independently f r o m spatial coding, it should b e e v i d e n t when t h e BEMEBOMO assignment is used w i t h t h e overlapped placement. T h r e e aspects o f t h e r e s u l t s obtained w i t h t h e BEMEBOMO assignment a r e o f importance. F i r s t , in c o n t r a s t t o o u r o t h e r experiments, R T s did n o t d i f f e r r e l i a b l y f o r t h e adjacent- a n d overlapped-hand placements, w i t h o n l y a t e n d e n c y f o r responses t o b e f a s t e r w i t h t h e f o r m e r placement (compare t h e o v e r a l l levels o f t h e f u n c t i o n s in F i g u r e 7 ) . This relative lack o f a d i f f e r e n c e was p r e d i c t e d because t h e salient consonant f e a t u r e o f t h e stimulus set corresponds w i t h t h e l e f t - r i g h t h a n d d i s t i n c t i o n f o r t h e o v e r l a p p e d placement, but w i t h n e i t h e r t h e l e f t - r i g h t h a n d n o r t h e spatial d i s t i n c t i o n f o r t h e adjacent placement. I n fact, t h e BEMEBOMO assignment is comparable, i n terms o f t h e relation o f t h e salient stimulus f e a t u r e t o t h e response set, w i t h t h e OzoZ assignment used f o r t h e Similarly, t h e BEBOMEMO l e t t e r / s i z e stimuli in o t h e r experiments. assignment described p r e v i o u s l y is comparable w i t h t h e OozZ assignment. Comparison o f t h e r e s u l t s obtained w i t h t h e BEBOMEMO a n d BEMEBOMO assignments shows t h a t t h e p a t t e r n of r e s u l t s is equivalent t o t h a t f o r t h e OozZ a n d OzoZ assignments, p r o v i d i n g f u r t h e r evidence t h a t coding can o c c u r o n t h e basis o f t h e h a n d d i s t i n c t i o n . Second, a c o n s o n a n t - f i r s t advantage was obtained when t h e BEMEBOMO assignment was used w i t h t h e overlapped-placement (see t h e right panel o f F i g u r e 7 ) . T h i s advantage was o f a similar magnitude t o t h a t obtained when t h e BEBOMEMO assignment was used p r e v i o u s l y w i t h t h e adjacent placement (see t h e l e f t panel o f F i g u r e 3 ) . Thus, t h e c o n s o n a n t - f i r s t advantage can b e obtained when t h e salient consonant f e a t u r e o f t h e stimulus set corresponds e i t h e r t o t h e l e f t - r i g h t spatial d i s t i n c t i o n o r t o t h e h a n d d i s t i n c t i o n o f t h e response set.


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LETTER PRESENTED FIRST F i g u r e 7. Mean RTs w i t h t h e BEMEBOMO assignment as a f u n c t i o n o f l e t t e r presented f i r s t a n d h a n d placement ( f r o m Proctor & Reeve, 1985, Experiment 4 ) . T h i r d , a c o n s o n a n t - f i r s t advantage also was obtained when t h e BEMEBOMO assignment was used w i t h t h e adjacent-hand placement, although t h e advantage t e n d e d t o b e smaller in magnitude. This advantage was unexpected because Miller (1982) h a d n o t obtained it a n d because t h e r e was no correspondence between t h e salient stimulus f e a t u r e a n d a salient response f e a t u r e . However, t h e c o n s o n a n t - f i r s t advantage t h a t we obtained f o r t h e BEMEBOMO assignment w i t h t h e adjacent placement was s i g n i f i c a n t l y smaller t h a n t h a t obtained p r e v i o u s l y f o r t h e BEBOMEMO assignment w i t h t h a t placement. Thus, t h e r e s u l t s indicate t h a t p r e c u i n g benefits a r e obtained when t h e p r e c u e i s t h e salient f e a t u r e o f t h e stimulus set, but t h e benefits a r e g r e a t e r when t h i s salient stimulus f e a t u r e corresponds w i t h a salient f e a t u r e of t h e response set. In summary, t h e r e s u l t s f r o m t h e studies t h a t used symbolic stimuli show p a t t e r n s similar t o those obtained w i t h spatial-location stimuli. Responses a r e fastest when t h e salient f e a t u r e o f t h e stimulus set corresponds t o t h e salient l e f t - r i g h t f e a t u r e o f t h e response set. Moreover, a correspondence w i t h t h e h a n d f o r responding i s used when t h e salient stimulus f e a t u r e distinguishes between t h e hands but n o t between t h e t w o leftmost a n d t w o r i g h t m o s t response locations.

Practice a n d T r a n s f e r Effects Translation processes customarily a r e assumed t o p l a y a decreasing Teichner E role in performance as subjects become p r a c t i c e d (e.g., Krebs, 1974). Thus, if t h e d i f f e r e n t i a l p a t t e r n s o f RTs w i t h t h e spatiallocation a n d symbolic stimuli r e f l e c t t r a n s l a t i o n processes, t h e p a t t e r n s should disappear w i t h practice. We i n i t i a l l y tested t h i s p r e d i c t i o n w i t h t h e spatial-precuing t a s k (Proctor & Reeve, 1988b). Subjects received t h r e e sessions o f 310 t r i a l s , w i t h e i t h e r t h e adjacent o r overlapped-hand placements. T h e r e s u l t s o f session 1 replicated b o t h t h e o v e r a l l slower RTs f o r t h e overlapped placement a n d t h e location-determined p a t t e r n o f d i f f e r e n t i a l p r e c u i n g benefits. I n t h e third session, RTs s t i l l were


176

slower f o r t h e o v e r l a p p e d placement, a l t h o u g h t h e magnitude o f t h e More d i f f e r e n c e was less t h a n in t h e f i r s t session (see F i g u r e 8). importantly, in t h e third session, a l l p r e c u i n g conditions w i t h i n each o f t h e h a n d placements showed equivalent p r e c u i n g benefits r e l a t i v e t o t h e In o t h e r words, t h e l e f t - r i g h t u n p r e p a r e d condition (see F i g u r e 8). advantage t h a t is. obtained in a single session i s n o longer e v i d e n t following p r a c t i c e .

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2

3

SESSION F i g u r e 8. Mean R T s as a f u n c t i o n p r e p a r a t i o n condition, h a n d placement, a n d session (from P r o c t o r & Reeve, 1988b, Experiment 1 ) . To evaluate f u r t h e r t h e n a t u r e o f t h e p r a c t i c e effects, experiments In w e r e conducted t h a t used t r a n s f e r designs (Proctor 8 Reeve, 198813). these experiments, subjects p a r t i c i p a t e d in t h r e e sessions u s i n g e i t h e r In t h e adjacent o r overlapped placement, as in t h e p r e v i o u s experiment. a f o u r t h session, h a l f of t h e subjects were t r a n s f e r r e d t o t h e a l t e r n a t i v e h a n d placement. T h i s t r a n s f e r maintains t h e spatial correspondence between stimuli a n d response locations f r o m t h e e a r l i e r sessions, but a l t e r s t h e assignment o f f i n g e r s t o response locations. Thus, if p r a c t i c e r e s u l t s in t h e acquisition o f procedures t h a t d i r e c t l y link t h e stimuli t o


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f i n g e r s , t h e n t h e p a t t e r n o f d i f f e r e n t i a l p r e c u i n g b e n e f i t s should b e reinstated in t h e t r a n s f e r session. A l t e r n a t i v e l y , t h e p a t t e r n should n o t reappear if p r a c t i c e r e s u l t s in more e f f i c i e n t spatial codings o f t h e stimulus a n d response sets. T h e r e s u l t s obtained when subjects t r a n s f e r r e d f r o m t h e overlapped t o t h e adjacent placement were unambiguous. The pattern of differential p r e c u i n g b e n e f i t s was reinstated, w i t h t h e magnitude b e i n g comparable t o t h a t obtained in an i n i t i a l session. T h e p a t t e r n also t e n d e d t o b e reinstated when subjects t r a n s f e r r e d f r o m t h e adjacent t o t h e overlapped placement, but t h e magnitude o f t h e differences was less t h a n in an i n i t i a l session a n d was nonsignificant. However, an additional experiment i n w h i c h subjects t r a n s f e r r e d f r o m t h e adjacent placement t o a- crossedhands placement (in w h i c h t h e l e f t - t o - r i g h t o r d e r i n g of f i n g e r s was right index, r i g h t middle, l e f t middle, a n d l e f t i n d e x ) showed a complete reinstatement. A p p a r e n t l y , subjects did r e v e r t t o t r a n s l a t i v e c o d i n g operations when t r a n s f e r r e d t o t h e overlapped placement, but t h e y i n c o r p o r a t e d t h e h a n d f e a t u r e i n t o t h e coding o f t h e response set t o b e n e f i t t h e normally less compatible conditions. Thus, o u r conclusion i s t h a t t h e salient-features coding used t o t r a n s l a t e between stimuli a n d responses i s no longer used when procedures t h a t d i r e c t l y relate stimuli t o response f i n g e r s have been acquired. However, when t h e assignment of f i n g e r s t o locations i s changed, a n d t h e a c q u i r e d procedures a r e n o longer applicable, subjects r e v e r t t o coded representations. More recently, we have b e g u n i n v e s t i g a t i n g p r a c t i c e effects w i t h t h e symbolic stimulus sets. In an i n i t i a l experiment, we have examined t h e change i n t h e R T s f o r subjects who used t h e OozZ assignment o r t h e OzoZ assignment o f t h e l e t t e r / s i z e stimuli. In t h e f i r s t session, R T s f o r t h e subjects w i t h t h e OozZ assignment were approximately 90 ms f a s t e r t h a n those w i t h t h e OzoZ assignment, r e p l i c a t i n g t h e d i f f e r e n c e obtained p r e v i o u s l y ( P r o c t o r & Reeve, 1985). However, by t h e third session, t h e d i f f e r e n c e was eliminated, b e i n g o n l y a n o n s i g n i f i c a n t 2 ms. T h e p r a c t i c e e f f e c t w i t h t h e symbolic stimuli again suggests t h a t t h e i n i t i a l d i f f e r e n c e between t h e OozZ a n d OzoZ assignments reflects t r a n s l a t i o n processes t h a t p l a y a decreasing role as subjects become p r a c t i c e d . In summary, t h e r e s u l t s f r o m these studies demonstrate t h e role o f t r a n s l a t i o n processes diminishes w i t h practice. The results from the t r a n s f e r studies indicate t h a t subjects r e v e r t t o t h e t r a n s l a t i o n processes when t h e assignment o f f i n g e r s t o locations is changed. Thus, when t h e relation between stimuli a n d responses is novel, t r a n s l a t i o n processes based on coded representations a r e used t o p e r f o r m t h e choice-reaction task. Salient-Features C o d i n g P r i n c i p l e O u r research p r o g r a m p r o v i d e s a coherent p i c t u r e o f S-R compatibility effects t h a t incorporates spatial-location stimuli, symbolic stimuli, o v e r t precues, a n d stimulus-response assignments. A t short p r e c u i n g intervals, t h e r e l a t i v e p r e c u i n g benefits f o r t h e d i f f e r e n t p a i r s o f responses a r e a f u n c t i o n o f S - R t r a n s l a t i o n processes, o r in o t h e r words, o f how f a s t t h e responses indicated by t h e c u e can b e determined. Moreover, similar t r a n s l a t i o n processes appear t o account f o r assignment effects when two-dimensional symbolic stimuli a r e assigned t o keypress responses. In a l l situations, t h e t r a n s l a t i o n processes operate

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according t o a salient-features c o d i n g p r i n c i p l e . This principle is that t h e stimulus a n d response sets a r e coded i n terms o f t h e salient features o f each, w i t h response determination o c c u r r i n g most r a p i d l y when t h e salient features o f t h e r e s p e c t i v e sets correspond. With spatial-location stimuli, spatial c o d i n g predominates. The c o d i n g is based on a h i e r a r c h y of salience f o r t h e spatial locations o f t h e patterns. T h e most salient f e a t u r e f o r linear a r r a y s is location r e l a t i v e t o center. Regardless o f w h e t h e r t h e linear a r r a y s a r e o r i e n t e d h o r i z o n t a l l y o r vertically, o r o f w h e t h e r t h e orientations o f t h e stimulus a n d response arrangements a r e consistent o r inconsistent, precues t h a t indicate t h e p a i r s o f locations t o e i t h e r side o f c e n t e r a r e most effective. T h e t w o i n n e r a n d t w o o u t e r locations a r e o f intermediate salience, w i t h t h e a l t e r n a t i n g locations b e i n g less salient. Also, coding can b e based o n t h e d i s t i n c t i o n between t h e hands, when t h e salience o f t h e h a n d d i s t i n c t i o n is increased a n d is consistent w i t h t h e salient spatial distinction. With two-dimensional, symbolic stimuli, t h e coding is in terms o f salient features o f t h e stimulus set a n d t h e correspondence w i t h t h e salient f e a t u r e o f t h e response set. Responses a r e fastest when t h e salient stimulus f e a t u r e discriminates t h e t w o leftmost a n d t w o r i g h t m o s t response locations o f a horizontal response arrangement. Similarly, a p r e c u e consisting o f t h e salient f e a t u r e o f t h e stimulus set i s most beneficial w i t h such an assignment. Hand coding i s used when t h e salient f e a t u r e o f t h e symbolic stimulus corresponds t o t h e d i s t i n c t i o n between t h e t w o hands but n o t t o t h e l e f t - r i g h t spatial d i s t i n c t i o n . T h e r e s u l t s obtained by u s w i t h spatial-location a n d symbolic stimuli in four-choice t a s k s a r e g e n e r a l l y consistent w i t h t h e r e s u l t s obtained in two-choice t a s k s . In b o t h situations, t h e coding is predominantly spatial, a l t h o u g h evidence f o r h a n d coding is a p p a r e n t in c e r t a i n circumstances (e.g., Ladavas & Moscovitch, 1984). Moreover, o u r studies p r o v i d e an i m p o r t a n t extension o f t h e two-choice f i n d i n g s t o more complex situations a n d indicate how compatibility effects can arise f r o m stimulus a n d response sets t h a t v a r y on seemingly u n r e l a t e d dimensions. A coherent theoretical framework f o r S - R compatibility effects has y e t t o emerge f r o m t h e e x t e n s i v e research on t h e topic. T h e salientfeatures c o d i n g p r i n c i p l e p r o v i d e s a framework t h a t p o t e n t i a l l y could s e r v e t h i s o r g a n i z i n g role. If so, t h e p r i n c i p l e represents a s i g n i f i c a n t advance in o u r knowledge r e g a r d i n g t h e fundamental c o g n i t i v e processes t h a t u n d e r l i e compatibility effects. Acknowledgement Preparation o f t h i s c h a p t e r was s u p p o r t e d in p a r t by g r a n t AFOSR88-0002 f r o m t h e A i r Force O f f i c e of Scientific Research.


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References (1965). S-R compatibility a n d information Fitts, P. M., & Biederman, I . reduction. Journal of Experimental Psychology, 69, 408-412. (1953). S-R compatibility: Spatial c h a r Fitts, P. M., & Seeger, C . M. Journal of Experlmental acteristics o f stimulus a n d response codes. Psychology, 46, 199-210. Garvey, W. D., & Knowles, W. B. (1954). Response time p a t t e r n s assoJournal of Experlciated w i t h v a r i o u s d i s p l a y - c o n t r o l relationships. mental Psychology, 47, 315-322. Goodman, D., E Kelso, J. A. S . (1980). A r e movements p r e p a r e d in parts? Not u n d e r compatible (naturalized) conditions. Journal of Experimental Psychology: General, 109, 475-495. LBdavas, E., & Moscovitch, M. (1984). M u s t egocentric a n d e n v i r o n mental frames o f reference b e a l i g n e d t o p r o d u c e spatial S-R compatibility effects? Journal of Experimental Psychology: Human Perception and Performance, 10, 205-215. Larish, D. D . (1986). I n f l u e n c e o f stimulus-response t r a n s l a t i o n o n response programming: Examining t h e relationship o f arm, direction, a n d e x t e n t o f movement. Acta Psychologica, 61, 53-70. Miller, J. (1982). Discrete v e r s u s continuous models of human informat i o n processing: In search o f p a r t i a l o u t p u t . Journal of Experimental Psychology: Human Perception and Performance, 8 , 273-296. (1985). A h a n d advantage in p r e p a r a t i o n o f simple keypress Miller, J. Journal of Experiresponses: Reply t o Reeve a n d Proctor (1984). mental Psychology: Human Perception and Performance, 1 1 , 221 -233. Nicoletti, R . , Anzola, G. P. Luppino, G . , Rizzolatti, G., & Umilta, C . (1982). Spatial compatibility effects on t h e same side o f t h e bodymidl ine. Journal of Experimental Psychology: Human Perception and Performance, 8 , 664-673. Proctor, R. W., & Reeve, T. G . (1985). Compatibility effects in t h e assignment o f symbolic stimuli t o d i s c r e t e f i n g e r responses. Journal of Experimental Psychology: Human Perception and Performance, 7 1 , 623-639. Salient-feature c o d i n g operaProctor, R. W., & Reeve, T. G . (1986). Journal of Experimental Psychology: t i o n s in spatial p r e c u i n g t a s k s . Human Perception and Performance, 12, 277-285. (1988a, A u g u s t ) . Correspondence of Proctor, R. W., & Reeve, T. G . stimulus and response set orientations i n preculng tasks. Poster p r e sented a t t h e 96th annual convention o f t h e American Psychological Association, Atlanta, GA. T h e acquisition o f task-speProctor, R. W., & Reeve, T . G . (1988b). c i f i c p r o d u c t i o n s a n d modification o f declarative representations in s p a t i a l - p r e c u i n g t a s k s . Journal of Experimental Psychology: General, 117, 182-196.

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Reeve, T . G., & Proctor, R . W. (1984). On t h e advance p r e p a r a t i o n of d i s c r e t e f i n g e r responses. Journal of Experimental Psychology: Human Perception and Performance, 10, 541 -553. (1985). Non-motoric t r a n s l a t i o n p r o Reeve, T. G . , & Proctor, R . W. cesses in t h e p r e p a r a t i o n o f d i s c r e t e f i n g e r responses: A r e b u t t a l o f Journal of Experimental Psychology: Human Miller's (1985) analysis. Perception and Performance, 1 1 , 234-240. Reeve, T . G . , & Proctor, R . W. (1988). Determinants o f two-choice reaction-time p a t t e r n s f o r same-hand a n d d i f f e r e n t - h a n d f i n g e r p a i r i n g s . Journal of Motor Behavior, 20, 317-340. Rosenbaum, D. A . (1983). T h e movement p r e c u i n g technique: AssumpI n R. A. Magill (Ed.), Memory tions, applications, a n d extensions. and control of action (pp. 231-274). Amsterdam: N o r t h Holland. Simon, J. R . (1969). Reactions t o w a r d t h e source o f stimulation. nal of Experimental Psychology, 81, 174-176.

Jour-

U m i l t i , C., & Nicoletti, R . (1985). A t t e n t i o n a n d c o d i n g effects in S-R compatibility d u e t o i r r e l e v a n t spatial cues. In M. I . Posner t 0. S . M. M a r i n (Eds.), Attention and performance X I (pp. 457-471). Hillsdale, NJ: Lawrence Erlbaum. Wallace, R . J. (1971). S-R compatibility a n d t h e idea of a response code. Journal of Experimental Psychology, 88, 354-360.

P A R T Ill PSYCHOPHYSIOLOGICAL INDICES A N D NEUROPHYSIOLOGICAL MECHANISMS T h e f o u r chapters o f P a r t I l l examine t h e relation between stimulus-response compatibility a n d psychophysiological measures o f b r a i n a c t i v i t y , as well as t h e neurophysiological mechanisms t h a t may u n d e r l i e compatibility effects. C h a p t e r 7, by Bashore, examines t h e role o f s t imuIus - response cornp a t ibiI it y in meas u res o f interhemis ph e r ic t r a nsmis sion time. It also p r o v i d e s a t u t o r i a l on t h e use o f event-related b r a i n potentials a n d discusses t h e ways in w h i c h s u c h potentials illuminate t h e n a t u r e o f compatibility effects. Chapter 8, by Ragot, describes r e c e n t studies in w h i c h situations t h a t p r o d u c e similar compatibility effects on reaction time y i e l d disparate effects on e v e n t - r e l a t e d potentials. The c h a p t e r discusses when a n d w h y e v e n t - r e l a t e d potentials can b e used t o discriminate t h e bases f o r compatibility effects. C h a p t e r 9, by B r e b n e r , examines t h e effects on performance o f v a r i o u s recoding processes a n d attempts t o relate p a r t i c u l a r electrophysiological a c t i v i t y t o such p r o cesses. C h a p t e r 10, by Verfaellie, Bowers, a n d Heilman, compares spatial coding a n d attentional hypotheses f r o m a neuropsychological perspective.


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STIMULUS-RESPONSE COMPATI 6IL lT Y VIEWED FROM A COGNITIVE PSYCHOPHYSIOLOGICAL PERSPECTIVE THEODORE R . BASHORE Department o f P s y c h i a t r y T h e Medical College o f Pennsylvania a t EPPl T h e f i r s t systematic studies o f t h e relation between t h e elements o f a stimulus d i s p l a y and t h e responses r e q u i r e d t o them were u n d e r t a k e n in t h e e a r l y 1950s (e.g., F i t t s & Seeger, 1953). T h e aim o f much o f t h i s w o r k was t o solve problems c o n f r o n t i n g t h e human engineer in designing stimulus-display/control-device configurations t h a t optimize t h e operator's performance. These configurations were conceptualized as v a r y i n g along a dimension o f stimulus-response ( S - R ) compatibility. Operationally, t h e degree o f S-R compatibility was defined on t h e basis o f response latency and accuracy. Configurations that p r o d u c e d t h e fastest response times and lowest e r r o r rates were defined as compatible, whereas configurations t h a t produced t h e slowest response times a n d h i g h e s t e r r o r rates were defined as incompatible. These differences in compatibility were i n t e r p r e t e d in information theoretic terms (i.e., i n b i t s o f information t r a n s m i t t e d o v e r t h e i n p u t channel). T h e pioneering research in t h e f i e l d was done by Paul F i t t s a n d h i s colleagues. I n t h e f i r s t p a p e r f r o m t h e group, F i t t s a n d Seeger (1953) defined a compatible response as one in which ". t h e ensemble o f stimulus a n d response combinations comprising t h e t a s k results,, in a high r a t e o f a information t r a n s f e r " (p. 199). T h e y made use o f . . . t h e idea o f a hypothetical process o f information transformation o r recoding in t h e course o f perceptual-motor a c t i v i t y . . ." which assumed t h a t ". . . t h e degree o f compatibility is a t a maximum when recoding processes a r e a t a minimum" (p. 199). It is important t o note t h a t F i t t s a n d Seeger regarded S - R compatibility effects as r e f l e c t i n g t h e degree ,,of correspondence between a stimulus set a n d a response set. Thus, . . . effect i v e performance depends t o a l a r g e e x t e n t upon t h e u n i q u e charact e r i s t i c s o f S - R ensembles r a t h e r t h a n on specific aspects o f p a r t i c u l a r stimulus or response sets" (p. 201).

. .

Although an e a r l y goal o f compatibility research was t o articulate t h e mechanisms o f information processing t h a t mediate S-R compatibility effects ( F i t t s & Deininger, 1954; F i t t s & Seeger, 19531, manipulations o f S-R compatibility have been used generally t o t e s t a n d r e f i n e models o f human information processing, r a t h e r t h a n t o f u r t h e r o u r u n d e r s t a n d i n g o f t h e processes u n d e r l y i n g S-R compatibility. T h i s may r e f l e c t a widely held belief t h a t S-R compatibility effects a r e mediated a t t h e response translation stage o f mental processing (see T e i c h n e r & Krebs, 1974, f o r example) a n d t h a t no f u r t h e r delineation is e i t h e r necessary o r possible. I n t h i s chapter, I a r g u e t h a t t h e concept o f S-R compatibility can b e r e f i n e d u s i n g c o g n i t i v e psychophysiological measures. I demonstrate how these measures n o t o n l y can help d i f f e r e n t i a t e t h e neuroanatomical f r o m t h e c o g n i t i v e constituents o f a reaction, b u t also can articulate t h e l a t t e r w i t h more precision t h a n can behavioral measures alone. One

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problem t h a t lends i t s e l f r a t h e r nicely t o such a n analysis is estimating t h e time r e q u i r e d t o convey information f r o m one cerebral hemisphere t o the other. T h e experimental procedures used t o a r r i v e a t estimates o f these transmission rates have revealed a dissociation between t h e simple reaction a n d more complex reactions, which suggests t h e utility o f electrophysiological measures in delineating t h e reaction processes t h a t u n d e r l i e S-R compatibility. Hence, my discussion w i l l begin w i t h a review o f t h e research t h a t has estimated interhemispheric transmission time. Because o f m y predilections, t h e b u l k o f t h i s c h a p t e r w i l l b e devoted t o reviewing t h e role o f S-R compatibility in visuo-motor reactions. Reaction Time Estimates

of Interhemispheric Transmission Time

T h e i n t e n t o f these studies is t o d e r i v e an estimate o f t h e time r e q u i r e d t o t r a n s m i t information f r o m one cerebral hemisphere t o t h e o t h e r v i a t h e commissural pathways connecting them (see review in Bashore, 1981). T h i s w o r k i s g u i d e d by t h e organizational characterist i c s o f t h e nervous system: Sensory i n p u t i s mediated p r i m a r i l y in t h e cerebral hemisphere opposite t h e stimulated side o f t h e body (via p a t h ways t h a t cross t h e neural axis during t h e i r ascent) a n d commands t o execute hand a n d f i n g e r movements o r i g i n a t e f r o m t h e hemisphere oppos i t e t h e responding d i s t a l limb ( v i a pathways t h a t cross t h e n e u r a l axis during t h e i r descent). T h e w o r k i s also motivated by t h e assumption t h a t t h e transmission o f simple sensory information and t h e execution o f uncomplicated manual responses, when b o t h c o g n i t i v e a n d response demands a r e minimal, a r e mediated o v e r f i x e d a n d reasonably well-isolated neuroanatomical pathways. T h e l a t t e r has led t o t h e use o f a s t r a i g h t f o r w a r d procedure, t h e subtraction method, t o estimate interhemispheric transmission time ( I H T T ) in behavioral studies. T h e experimental paradigm developed t o d e r i v e t h e estimate involves t h e presentation o f an u n p a t t e r n e d stimulus (e.g., p o i n t source o f light) i n t o t h e visual h a l f - f i e l d and t h e execution o f a simple, f i x e d manual response t o it (e.g., depression o f a response b u t t o n w i t h t h e right index f i n g e r ) . T h e reasoning, f i r s t enunciated by Poffenberger (1912), is t h a t if b o t h reception of t h e stimulus i n p u t a n d execution o f t h e motor o u t p u t occur w i t h i n t h e same hemisphere, t h e n reaction times (RTs) should b e s h o r t e r t h a n when cross-commissural communication is r e q u i r e d t o mediate t h e response f r o m t h e o t h e r hemisphere. T h e d i f f e r e n c e between t h e t w o response latencies i s t h o u g h t t o r e f l e c t t h e time r e q u i r e d t o convey information to t h e opposite hemisphere, f r o m which t h e motor command i s t h e n i n i t i a t e d (i.e., I H T T ) . Simple Reaction-Time Tasks Reliable estimates o f I H T T have o n l y been p r o d u c e d u s i n g simple R T tasks (Bashore, 1981). In these tasks, t h e subject makes an uncomplicated manual response t o t h e h a l f - f i e l d presentation o f an unpatterned, salient stimulus t h a t occurs on every, trial, w i t h o u t exception. The d u r a t i o n o f t h e stimulus must b e s u f f i c i e n t l y b r i e f (less t h a n 200 ms) t o minimize t h e likelihood o f saccadic eye movements. Presentation o f t h i s imperative stimulus i s usually preceded by a w a r n i n g stimulus (e.g., a

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tone) t h a t is t h e n followed by a c e n t r a l f i x a t i o n stimulus (e.g., a cross). T h e d u r a t i o n o f t h e f i x a t i o n stimulus v a r i e s randomly f r o m 1 t o 3 s a n d allows t h e subject t o p r e p a r e t h e response. Random f i x a t i o n d u r a t i o n s a r e used t o e n s u r e against lateral s h i f t s in gaze timed t o t h e onset o f t h e t a r g e t stimulus a n d t o reduce t h e likelihood o f a n t i c i p a t o r y responses. Immediately following t h e o f f s e t o f t h e f i x a t i o n stimulus, t h e t a r g e t stimulus is p r e s e n t e d i n t o t h e v i s u a l h a l f - f i e l d . T h e subject is t y p i c a l l y r e q u i r e d t o respond w i t h a f i n g e r on one h a n d during a n y block o f trials. However, t h e subject may also make a bimanual f i n g e r response (DiStefano, Morelli, Marzi, & Berlucchi, 1980; Jeeves, 1969; Jeeves & Dixon, 1970). T h e imperative stimulus is p r e s e n t e d e i t h e r pseudo-randomly in t h e l e f t o r right v i s u a l h a l f - f i e l d during a b l o c k o f t r i a l s (i.e., w i t h c o n s t r a i n t s o n t h e number o f consecutive presentations in - t h e same half-field; DiStefano e t al., 1980; Lines & Milner, 1983; McKeever & Hoff, 1979; Milner & Lines, 19821, o r in t h e same v i s u a l h a l f - f i e l d f o r t h e e n t i r e b l o c k o f t r i a l s (Anzola, Bertoloni, Buchtel, & Rizzolatti, 1977; Berlucchi, Heron, Hyman, Rizzolatti, & UmiltB, 1971; Berlucchi, Crea, DiStefano, & Tassinari, 1977; Jeeves, 1969; Jeeves E Dixon, 1970; Poffenberger, 1912; Smith, 1938). T h e f a s t e s t response latencies a r e p r o d u c e d in t h e simple reaction by t h e response h a n d ipsilateral t o t h e v i s u a l h a l f - f i e l d stimulated, w i t h e i t h e r blocked o r randomized h a l f - f i e l d presentations (Anzola e t al., 1977; Berlucchi e t al., 1971, 1977; DiStefano e t al., 1980; Jeeves, 1969; Jeeves & Dixon, 1970; Lines & Milner, 1983; McKeever & Hoff, 1979; M i l n e r & Lines, 1982; Poffenberger, 1912). Thus, t h e h a n d c o n t r o l l e d by t h e hemisphere t h a t is activated i n i t i a l l y by t h e sensory input has t h e s h o r t e s t response latency. T h i s ipsilateral response h a n d advantage is t h o u g h t t o r e f l e c t intrahemispheric processing o f t h e sensory input a n d motor o u t p u t t h a t obviates t h e need f o r interhemispheric communication t o execute t h e motor response. T h e advantage is r e p o r t e d t o r a n g e f r o m 1 t o 6 ms a n d p r o v i d e s t h e behavioral estimate o f I H T T . Poffenberger's (1912) estimates, however, a r e t h e longest (5.6 ms, 6.0 ms), w i t h contemporary estimates f a l l i n g t y p i c a l l y between 2 a n d 3 ms f o r unimanual responses a n d 1 t o 2 ms f o r bimanual responses. O f i n t e r e s t f o r o u r purposes is t h e consistent observation t h a t spat i a l compatibility does n o t a l t e r t h e response-hand s u p e r i o r i t y in a simple reaction (Anzola e t al., 1977; B e r l u c c h i e t al., 1977). T h a t is, t h e h a n d controlled by t h e hemisphere t h a t receives t h e i n i t i a l sensory input p r o duces t h e fastest RT, w h e t h e r t h e response device is positioned on t h e same side o f t h e b o d y as t h e source o f stimulation o r on t h e opposite side ( s o t h a t t h e h a n d is crossed o v e r t h e body's midline). T h i s suggests t h a t simple R T estimates o f I H T T r e p r e s e n t t h e transmission o f information o v e r fixed, p r i m a r y neuroanatomical pathways whose activation may n o t b e influenced by h i g h e r o r d e r c o g n i t i v e ( i . e . , c e n t r a l ) variables, such as S R compatibility. A similar inference is suggested by t h e observation t h a t attentional biases induced by p r e s e n t i n g t h e stimulus in t h e same h a l f f i e l d during a block o f t r i a l s d o n o t a l t e r t h e estimate o f I H T T in t h e simple reaction, whereas t h e y do i n more complicated d i s j u n c t i v e reactions (see discussion below; Ledlow, Swanson, & Kinsbourne, 1978a,b). Stimulus-Detection Tasks U n l i k e t h e simple R T task, in w h i c h an imperative stimulus is presented a n d a response is executed on e v e r y t r i a l , a stimulus-detection

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task has blank ("catch") trials inserted on which t h e response is withheld. This procedure was designed t o reduce t h e likelihood of anticipatory responding and, as such, t o obtain a p u r e r measure of t h e rate of t h e simple reaction (Woodworth, 1938). However, insertion of these "catch" t r i a l s transforms t h e task. It is no longer a simple RT task, but a "go/no go" task 'in which a stimulus must be detected before a response is executed. I n t h e simple RT task, stimulus detection is not actually required because t h e subject knows t h a t on e v e r y t r i a l a stimulus will be presented as soon as t h e fixation stimulus leaves t h e central visual field; t h e subject need only prepare and execute t h e response. In a stimulus-detection task, the subject must, i n addition, detect t h e imperative stimulus following t h e fixation stimulus and withhold a response if none is detected. Thus, detection and motor-execution processes are added. This added task complexity is reflected i n longer RTs and greater variability i n estimates of IHTT, ranging from 222 t o 307 ms and from 5.0 t o 28.5 ms, respectively (Kleinman, Carron, Cloninger, & Halvachs, 1976; Moscovitch & Smith, 1979; Smith & Moscovitch, 1979; Swanson & Ledlow cited in Swanson, Ledlow, & Kinsbourne, 1978). Unfortunately, t h e relevant parametric analyses t o assess t h e effects of variables l i k e S-R comp a t i b i l i t y on performance i n t h i s task and the concomitant influence on estimates of I H T T have not been done. Two-Choice Reaction-Time Tasks

A two-choice RT task designed t o estimate I H T T requires t h e subject t o discriminate between two stimuli, each of which signals a d i f f e r e n t response. As in t h e simple RT and stimulus-detection tasks, simple unpatterned stimuli and uncomplicated manual responses are used. The subject may be required t o make a unimanual digit selection (e.g., press one response button w i t h t h e index f i n g e r on t h e r i g h t hand t o t h e t a r get stimulus when it i s presented i n t h e l e f t visual half field and press another button w i t h t h e middle f i n g e r on t h e r i g h t hand if t h e stimulus is shown i n t h e r i g h t visual half field), o r t o make a response-hand decision (e.g., press a response button w i t h t h e index f i n g e r on t h e left hand t o l e f t visual half-field stimulation and press another response button w i t h t h e index f i n g e r on t h e r i g h t hand t o r i g h t visual half-field stimulation). Thus, a two-choice RT task d i f f e r s from a stimulus-detection task i n t h a t stimulus location is mapped t o response selection. Further, unlike the stimulus-detection task, t h e choice task requires a response on every trial. Estimates of I H T T t h a t derive from two-choice RT tasks are generally longer than those derived from t h e simpler tasks. Bradshaw and Perriment (1970) reported an I H T T of 20 ms favoring t h e response hand ipsilateral t o t h e visual half-field stimulated, and Harvey (1978) estimated the I H T T t o be 25 ms. However, with t h e introduction of location d i s crimination and response selection, the influence of spatial compatibility assumes t h e plenary role in determining response latencies and, as a result, most of t h e choice RT research has centered on identifying t h i s effect, rather than on estimating I H T T .

S-R Compatibility and I H T T The task most often used in choice RT analyses of S-R compatibility effects on interhemispheric transmission requires a manual response by


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t h e l e f t o r right h a n d t o a stimulus presented in t h e l e f t o r right v i s u a l half-field. T h e subject's hands a r e positioned e i t h e r a t t h e side o f t h e b o d y o r a r e crossed o v e r t h e b o d y ' s midline. U n d e r b o t h conditions, responses a r e always fastest when t h e response h a n d a n d stimulus a r e on t h e same side o f t h e body, w h e t h e r t h e hands a r e crossed o r uncrossed ( i . e . , t h e y a r e spatially compatible; Anzola e t al., 1977; Brebner, 1973; Brebner, Shephard, & Cairney, 1972; Wallace, 1971, 1972). This reflects, o f course, t h e e f f e c t o f spatial compatibility on t h e response a n d suggests, in c o n t r a s t t o t h e simple reaction, t h a t t h e influence o f t h i s variable on t h e f i n a l response o u t p u t is s t r o n g e r t h a n is t h e i n f l u e n c e o f t h e u n d e r l y i n g anatomical pathways. Studies designed t o assess t h e impact o f S-R compatibility o n estimates o f I H T T have been restricted, however, t o manipulations o f spatial relations. T h e r e a r e o t h e r forms o f S-R compatibility t h a t influence processing latency as well. In t h e n e x t section, I d e s c r i b e some t y p e s o f S - R compatibility t h a t a r e p e r t i n e n t t o t h i s discussion. Varieties of S-R C o m p a t i b i l i t y

S - R compatibility can b e g r o u p e d b r o a d l y i n t o t w o classes, spatial a n d symbolic (Simon, Sly, & Vilapakkam, 1981). T h e f o r m e r is based on t h e locational correspondence between t h e positions o f t h e imperative stimuli a n d o f t h e response devices, r e l a t i v e t o t h e b o d y ' s midline. Thus, when response h a n d selection decisions a r e made, a compatible c o n f i g u r a t i o n involves responding w i t h t h e h a n d placed on a response device positioned on t h e same side o f t h e b o d y as t h e source o f stimulation, a n d an incompatible arrangement r e q u i r e s r e s p o n d i n g w i t h t h e h a n d placed on a response device positioned on t h e side o f t h e b o d y opposite t h e source o f stimulation. As I discussed i n t h e p r e c e d i n g section, t h i s response advantage is a p p a r e n t w h e t h e r t h e compatible response is made by a h a n d placed in i t s normal anatomical position o r by a h a n d crossed o v e r t h e b o d y ' s midline (Anzola e t al., 1977; B r e b n e r , 1973; B r e b n e r e t al, 1972; Wallace, 1971, 1972; see also L i d a v a s & Moscovitch, 1984; Nicoletti, U m i l t i , & L i d a v a s , 1984; U m i l t i & Nicoletti, 1985). These same response device-stimulus location relations d o n o t obtain, however, when t h e head is t i l t e d 90O i n t h e horizontal plane; r a t h e r , t h e response handstimulus location correspondence is most i n f l u e n t i a l in d e t e r m i n i n g compatibility effects ( L i d a v a s & Moscovitch, 1984).

A directional advantage similar t o t h a t seen when response h a n d selection is r e q u i r e d is e v i d e n t when unimanual digit selection choices a r e made, whether t h e h a n d is i n t h e crossed o r uncrossed position (Bradshaw & Perriment, 1970; Harvey, 1978). T h a t is, t h e r e l a t i v e positions o f t h e d i g i t s a n d o f t h e sources of stimulation comprise t h e compatib l e arrangement, i r r e s p e c t i v e o f t h e side o f t h e b o d y o n which t h e h a n d is placed. Compatibility effects a r e also o b s e r v e d when t h e stimuli sign a l i n g a l e f t o r r i g h t response a r e presented in one v i s u a l hemifield a n d each response device is in a d i f f e r e n t lateral hemispace o r b o t h devices a r e in t h e same hemispace, w i t h hands crossed o r uncrossed a n d t h e head o r i e n t e d s t r a i g h t ahead o r r o t a t e d s l i g h t l y to t h e side (Nicoletti e t al, 1982, 1984; U m i l t i 8 Nicoletti, 1985). A s in t h e case o f t h e compatibility effects seen w i t h unimanual digit selection, t h e r e l a t i v e positions of t h e response devices a n d t h e imperative stimuli establish t h e compatible relations.

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Locational compatibility effects a r e manifest as well when t h e c o r r e spondence is between v i s u a l stimuli shown above o r below f i x a t i o n in t h e horizontal plane a n d up o r down response requirements ( L i d a v a s & Moscovitch, 1984; Proctor & Reeve, 1986; Umilt6 & Nicoletti, 1984). A compatible response i n t h i s c o n f i g u r a t i o n involves p r e s s i n g b u t t o n s in t h e v e r t i c a l plane a t t h e subject's midline t o a stimulus shown above o r below f i x a t i o n in t h e c e n t r a l v i s u a l f i e l d . These v e r t i c a l compatibility relations can b e d i s r u p t e d , however, by p r e s e n t i n g t h e stimuli above o r below t h e horizontal plane in t h e l e f t o r right hemifield, a n d r e q u i r i n g e i t h e r a unimanual response on a device positioned a t t h e midline (Cotton, Tzeng, & H a r d y c k , 1977, 1980) o r a response h a n d selection on devices in separate hemispaces positioned above a n d below one another (Umilta & Nicoletti, 1984). T h e subject is i n s t r u c t e d t o p r e s s t h e t o p b u t t o n t o t h e u p p e r stimulus a n d t h e bottom b u t t o n t o t h e lower stimulus, w i t h no mention made o f t h e l e f t - r i g h t d i s t i n c t i o n . Compatible responses when unimanual responses a r e made i n v o l v e p r e s s i n g t h e t o p o r bottom b u t t o n o f a response device w i t h t h e l e f t index f i n g e r i n response t o a f l a s h o f l i g h t in t h e u p p e r p o r t i o n o f t h e l e f t v i s u a l h a l f - f i e l d o r lower p o r t i o n o f t h e right v i s u a l half-field, respectively; a n d incompatible responses i n v o l v e p r e s s i n g t h e t o p o r b o t tom b u t t o n w i t h t h e l e f t index f i n g e r i n response t o t h e flash in t h e u p p e r right o r lower l e f t v i s u a l half-field, respectively. T h e lateral c o r respondence i s r e v e r s e d when t h e right h a n d is responding. When response h a n d decisions a r e made a n d t h e response devices a r e in separ a t e hemispaces, t h e l e f t - r i g h t d i s t i n c t i o n s determine t h e d i r e c t i o n o f t h e compatibility e f f e c t and, indeed, an incompatible above-below c o n f i g u r a t i o n can p r o d u c e a s h o r t e r response latency t h a n i t s compatible c o u n t e r p a r t (UmiltB & Nicoletti, 1984). Symbolic compatibility has a number o f v a r i a n t s as well. It can b e divided into three types. I n one, t h e c r i t i c a l element i s t h e correspondence between t h e v e r b a l label f o r t h e stimulus a n d t h e response r e q u i r e d t o it. In t h i s case, f o r example, a compatible response would i n v o l v e p r e s s i n g a r e d b u t t o n t o a r e d light o r a g r e e n b u t t o n t o a g r e e n light, a n d an incompatible response would i n v o l v e t h e opposite stimulusresponse device mapping (see C h a p t e r 2, by Simon, in t h i s volume). A n o t h e r t y p e o f symbolic compatibility involves t h e correspondence between a directional l i n g u i s t i c label (e.g., LEFT) o r symbol (e.g., -->) a n d t h e r e q u i r e d response t o it ( l e f t o r right directional response, respectively, f o r a compatible response; a n d t h e opposite directional movement f o r an incompatible response; Bashore & Osman, 1987; Magliero, Bashore, Coles, & Donchin, 1984; M c C a r t h y & Donchin, 1981). And, another v a r i a n t o f symbolic compatibility comprises t h e correspondence between t h e negation o f a l i n g u i s t i c label ( e . g . , NOT LEFT) o r symbol (e.g., -/->) a n d t h e r e q u i r e d response t o it (right o r left, respectively, f o r a compatible response; a n d t h e converse mapping f o r an incompatible response (Whitaker,1980, 1982). T h e r e l a t i v e prepotence o f spatial compatibility o v e r symbolic comp a t i b i l i t y is revealed in what has come t o b e called t h e "Simon effect" (Hedge & Marsh, 1975) a f t e r J. R. Simon, who f i r s t i d e n t i f i e d t h i s e f f e c t a n d whose research has been l a r g e l y responsible f o r a r t i c u l a t i n g i t s parameters (see C h a p t e r 2 , by Simon, in t h i s volume). U n d e r circumstances i n w h i c h a symbolic cue (e.g., t h e w o r d LEFT) is presented i n t o t h e lateral hemispace (e.g., l e f t o r r i g h t ear) a n d t h e subject must p r e s s a response button t o t h e l e f t o f t h e body's midline w i t h the l e f t index


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f i n g e r (i.e., t h e symbolic stimulus a n d t h e location o f t h e response device/response h a n d a r e anatomically compatible), t h e presentation o f t h e stimulus i n t o t h e spatially incompatible hemispace (in t h i s case, t h e right ear) p r o l o n g s t h e response latency. T h a t is, t h e spatial o r directional cue slows t h e response t o t h e imperative symbolic stimulus even t h o u g h it is i r r e l e v a n t . T h i s e f f e c t obtains w h e t h e r a bimanual response h a n d selection o r a unimanual directional movement (e.g., move a l e v e r t o t h e l e f t o r right w i t h t h e right hand) is r e q u i r e d . A l t h o u g h t h e Simon e f f e c t suggests t h a t spatial relations between t h e stimulus a n d response a r e more compelling t h a n t h e symbolic c o n t e n t of a stimulus a n d t h e response it signals in d e t e r m i n i n g response latencies, i n i t i a l characterizations o f t h e r e l a t i v e c o n t r i b u t i o n s of each t o t h e ultimate processing time have been made in t h e research o f Reeve a n d Proctor (1984) a n d Proctor a n d Reeve (1985, 1986). Their work is b e g i n n i n g t o a r t i c u l a t e t h e p r o p e r t i e s of stimulus a n d response sets t h a t i n t e r a c t t o determine v a r y i n g degrees o f S-R compatibility. It has revealed t h a t t h e most compatible configurations a r e determined by t h e mapping of c e r t a i n features o f a stimulus set o n t o c e r t a i n features o f a response set, w h e t h e r o r n o t a p r e c u e r e d u c i n g t h e response choices is presented. T h u s f a r t h e response features t h e y have i d e n t i f i e d i n c l u d e spatial components, l e f t - r i g h t positioning o f t h e response b u t t o n s ( r e l a t i v e t o t h e b o d y ' s midline) a n d response h a n d decisions, a n d symbolic components, consonant ( v s . vowel) a n d l e t t e r i d e n t i t y ( v s . size) distinctions. Reeve a n d Proctor's f i n d i n g s obtained w i t h spatial-location stimuli a r e consistent w i t h much p r e v i o u s w o r k in showing t h e dominant r o l e o f t h e spatial components (see t h e I H T T section o f t h i s c h a p t e r a n d t h e coding chapters i n t h i s volume). Most important, however, is t h e i r demonstration w i t h symbolic stimuli t h a t t h e p a r t i c u l a r features o f t h e response set i n t e r a c t w i t h features o f t h e stimulus t o p r o d u c e compatibility effects. F o r example, when t h e stimulus set consists o f t h e stimuli 0 , 0, z, a n d Z, a n d t h e response set consists o f t h e index a n d middle f i n g e r s o f each hand, responses a r e f a s t e r w i t h an assignment f o r w h i c h l e t t e r i d e n t i t y distinguishes between t h e t w o leftmost a n d t w o rightmost responses t h a n w i t h an assignment f o r w h i c h it does not. However, when t h e f i n g e r s f r o m t h e t w o hands a r e alternated, response latencies w i t h an assignment f o r w h i c h l e t t e r i d e n t i t y distinguishes between responses o n t h e t w o hands a r e equal t o those w i t h t h e assignment f o r w h i c h i d e n t i t y distinguishes t h e t w o leftmost a n d t w o rightmost responses. Thus, w i t h t h e same assignments of stimuli t o response locations, d i f f e r e n t r e s u l t s a r e obtained when t h e f i n g e r s a r e alternated. In t h e f o r m e r case, t h e assignment f o r w h i c h t h e salient l e t t e r - i d e n t i t y f e a t u r e i s consistent w i t h t h e l e f t - r i g h t spatial component o f t h e response set shows a b e n e f i t i n r e s p o n d i n g . B u t in t h e l a t t e r case, both t h a t assignment a n d t h e assignment f o r w h i c h t h e l e t t e r i d e n t i t y f e a t u r e distinguishes between t h e hands show equivalent benefits, p r o v i d i n g evidence t h a t e i t h e r t h e spatial component o r t h e anatomical component of t h e response set can b e used if it coincides w i t h t h e l e t t e r - i d e n t i t y f e a t u r e o f t h e stimulus set. A t t h i s point, discussion w i l l t u r n t o ERPs a n d t o t h e i r role i n t h e analysis o f mental chronometry in general a n d S-R compatibility in p a r t i c ular.

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Event-Related B r a i n Potentials Data Collection and Basic Description Cognitive psychophysiologists w i t h interests i n mental chronometry study two classes o f dependent variables, traditional behavioral measures, response latency (i.e., RT) and accuracy, and measures of b r a i n electrical a c t i v i t y recorded a t t h e scalp, commonly r e f e r r e d t o as event-related b r a i n potentials (ERPs). ERPs are recorded by a f f i x i n g electrodes on t h e subject's scalp, t y p i c a l l y a t placement sites used f o r both clinical and research purposes (10-20 o r International Electrode Placement System, Jasper, 1958; see Figure 1 ) . These potentials represent positive o r negative deflections (called components) i n the voltage of t h e on-going electroencephalographic (EEG) signal t h a t are produced by t h e presentation of a stimulus t o which t h e subject either sits passively o r makes a decision and initiates a response. However, t h e r e is a t y p e of movementrelated activity, t h e readiness potential (RP), t h a t is evident when a subject makes a series of spontaneous (i.e., unsignaled), repetitive movements (e.g., f i n g e r tapping) o r responds t o t h e presentation of an imperative stimulus in simple o r more complicated R T tasks. This scalp a c t i v i t y i s presumed t o reflect t h e activation of cortical motor neurons t h a t mediate t h e movement (see discussion in Bashore, McCarthy, Heffley, T h e electrical signals emanating from t h e Clapman, & Donchin, 1982). b r a i n are analog. However, i n most research applications these signals are not sampled continuously, b u t are digitized a t rates v a r y i n g typically from 100 t o lo00 Hz (i.e., e v e r y 1-10 ms). The digitized a c t i v i t y is stored on magnetic tape and all of t h e analyses on t h i s a c t i v i t y are done o f f - Iine

.

Components of t h e ERP t h a t are elicited by stimulus events are labeled on t h e basis of t h e i r electrical p o l a r i t y and t h e minimum latency at which t h e i r peak amplitude i s reached (Donchin, Ritter, & McCallum, 1978; Picton & Stuss, 1980; see Figure 2). For example, N140 refers t o a negative change i n voltage whose peak is achieved at about 140 ms a f t e r presentation of a stimulus. Analyses are done most often on t h e amplitude, latency, and scalp distribution (i.e., relative amplitudes a t d i f f e r e n t scalp sites) of these potentials. Because the voltages of ERPs are small relative t o those of t h e EEG, these signals are obscured by t h e background EEG and are most evident i n a signal t h a t represents t h e average of a large number of repetitions of t h e stimulus. However, multivariate techniques, such as discriminant functions and lag cross-correlations, have allowed t h e detection of larger ERP components on single t r i a l s w i t h a high degree of success (Horst & Donchin, 1980; Squires F. Donchin, 1976). T o perform these single t r i a l analyses, however, t h e digitized signal must be f i l t e r e d off-line t o extract t h e signal from the noise (Farwell, Martinerie, Bashore, 8 Rapp, i n preparation; Ruchkin 8 Glaser, 1978).


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F i g u r e 1. T h e placement sites used in t h e I n t e r n a t i o n a l (10-20) Electrode Placement System. Recording sites o v e r t h e l e f t hemisphere a r e designated by o d d numbers a n d o v e r t h e right hemisphere by even numbers. T h e l e t t e r p r e c e d i n g t h e number r e f e r s t o t h e p a r t of t h e cerebral c o r t e x o v e r w h i c h t h e electrode is placed (F=frontal; Czcentral; Pzparietal; O=occipital; a n d T=temporal), F p l a n d Fp2 designate sites on t h e forehead a n d A l / A 2 i d e n t i f y mastoid sites commonly used as references f o r scalp r e c o r d i n g s . T h i s d r a w i n g is f r o m H a r n e r & Sannit (1974). It is r e p r i n t e d w i t h t h e p u b l i s h e r ' s permission.

T.R. Bashore

192

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HUMAN SEP RMN SmI&i

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Figure 2. Schematic representations o f ERPs as t h e y would b e evoked in t h e a u d i t o r y (labeled AEP), v i s u a l (VEP), a n d somatosensory (SEP) modalities f r o m a normal, awake a d u l t u n d e r conditions of passive stimulation (i.e., t h e subject is n o t p e r f o r m i n g a n y specific t a s k ) . Components a r e n o t necessarily l a r g e s t a t t h e sites shown in these drawings. N in t h e SEP ERP r e f e r s t o an electrbde located o n t h e p o s t e r i o r p a r t of t h e neck, a n d RMN indicates right median n e r v e . These d r a w i n g s were t a k e n from Goff, Allison, & Vaughan (19781, a n d a r e r e p r i n t e d w i t h t h e a u t h o r s ' a n d p u b l i s h e r ' s permission.


193

Exogenous a n d Endogenous Components of t h e ERP Exogenous components. T h e earliest components o f t h e ERP a r e o b l i g a t o r y electrical responses t o stimulation recorded t y p i c a l l y in circumstances designed t o assess t h e i n t e g r i t y o f p r i m a r y a f f e r e n t pathways (i.e., f o r c l i n i c a l . evaluations) o r t o characterize f a c t o r s t h a t influence transmission rates in these systems (e.g., age, b o d y temperature). T h e i r p r o d u c t i o n is dependent on e x t e r n a l stimulation, a n d t h e amplitudes a n d latencies of these signals v a r y l a r g e l y as a f u n c t i o n o f t h e p h y s i c a l p r o p e r t i e s o f t h e e l i c i t i n g stimulus (e.g., an increase in t h e i n t e n s i t y o f t h e stimulus w i l l p r o d u c e an increase in t h e amplitude a n d decrease in t h e latency o f t h e component t o asymptote). Hence, these components a r e o f t e n r e f e r r e d t o as "exogenous" o r evoked potentials (EPs). Moreover, t h e scalp s i t e w h e r e t h e amplitude o f t h e EP i s l a r g e s t a n d latency s h o r t a v i s u a l stimulus e s t is determined by t h e modality stimulated (e.g., evokes t h e most r o b u s t response o v e r o c c i p i t a l - - p r i m a r y v i s u a l c o r t e x - r e c o r d i n g sites). Consequently, electrode placement i s determined by t h e Examples of sensory modality in w h i c h t h e stimulus is delivered. exogenous potentials e l i c i t e d u n d e r passive stimulation conditions in t h e auditory, visual, a n d somatosensory systems a r e shown in F i g u r e 2. Endogenous components. When decisions must b e made a b o u t t h e stimuli presented (e.g., c o u n t t h e high p i t c h e d tones, i g n o r e t h e low p i t c h e d tones), exogenous components a r e manifested w i t h i n t h e f i r s t 100150 ms following t h e stimulus. A t longer latencies, a series o f components, r e f e r r e d t o as endogenous, is e v i d e n t . T h e amplitude, latency, a n d scalp d i s t r i b u t i o n o f these components a r e r e l a t i v e l y i n s e n s i t i v e to Instead, t h e amplichanges in t h e p h y s i c a l p r o p e r t i e s of t h e stimulus. t u d e a n d latency o f these l a t e r components a r e v e r y sensitive t o changes in t h e psychological demands o f t h e task, a n d t h e i r scalp d i s t r i b u t i o n i s r e l a t i v e l y constant across t a s k s a n d sensory modalities. Thus, these components a r e believed t o b e generated by n e u r a l events associated w i t h h i g h e r o r d e r processing o f t h e ,?timulus i n p u t a n d response o u t p u t . Hence, t h e y a r e r e f e r r e d t o as endogenous" components, examples of which a r e t h e N200 a n d P300 discussed below. Because t h e scalp d i s t r i b u t i o n of these components is n o t tightly coupled t o t h e sensory modality in w h i c h stimulation occurs, c o g n i t i v e psychophysiological investigations o f mental processing t y p i c a l l y u t i l i z e a l a r g e r a r r a y o f electrode placements t h a n d o investigations o f p r i m a r y a f f e r e n t systems. Electrodes a r e usually placed a t locations along t h e midline o f t h e scalp ( r e f e r r e d t o as Fz [frontal], Cz [central], Pz [parietal], a n d Oz [occipital]), a n d a t a v a r i e t y o f lateral r e c o r d i n g sites (e.g., temporal sites T3, T5; c e n t r a l sites C3, C4; parietal sites P3, P4; see F i g u r e 1 ) .

It is i m p o r t a n t t o note t h a t t h e n e u r a l o r i g i n s o f t h e e a r l y evoked potentials a r e characterized f a r b e t t e r t h a n a r e those of t h e l a t e endogeIndeed, it i s n o t nous components (Goff, Allison, 8 Vaughan, 1978). u n f a i r t o assert t h a t t h e sources o f t h e late potentials a r e u n k n o w n It i s (Wood, McCarthy, Squires, Vaughan, Woods, & McCallum, 1984). f a i r t o claim, however, t h a t t h e elements o f mental processing i n d e x e d by a v a r i e t y o f these ERPS a r e becoming i n c r e a s i n g l y well-characterized (Donchin, Karis, Bashore, Coles, & Gratton, 1986; Picton & Stuss, 1980; R i t t e r , Simson, & Vaughan, 1983). Hence, we can u t i l i z e these measures in tandem w i t h behavioral measures t o h e l p a r t i c u l a t e t h e s t r u c t u r e a n d Before d i s c u s s i n g this, however, I shall t i m i n g of c o g n i t i v e processing. review t h e electrophysiological studies of IHTT t o see how t h e y i n f o r m us. I shall t h e n discuss t h e endogenous components of t h e ERP, u s i n g

194

T.R. Bashore

(Note t h a t f o r purposes t h e P300 component of t h e ERP as t h e example. of t h i s discussion I have not differentiated what many r e f e r to as the middle latency potentials [100-200 ms poststimulus], thought t o have both exogenous and endogenous properties .) This tutorial provides t h e necessary backdrop f o r t h e discussion t h a t follows of ERP estimates o f I H T T . Electrophysiological Estimates of I H T T A small number of ERP studies have been done t o estimate I H T T . The same logic f o r estimating t h i s value t h a t motivates behavioral studies drives electrophysiological studies. The assumption is t h a t activation o f t h e primary cortical area opposite t h e source of stimulation will be manifest i n t h e latency of a component of the ERP. This information will then be conveyed t o t h e homologous receptive area in the other hemisphere where t h e same component will be evident b u t a t a longer latency, and t h e difference in component latency will provide t h e estimate of I H T T . The earliest such studies were n o t influenced by t h e behavioral work; rather, estimates were derived under passive stimulation conditions. Latency differences between contralateral and ipsilateral scalp sites i n t h e generation o f an evoked response t o monaural clicks were reported t o be about 2 ms (Wolpaw & Penry, 1977); t o tactile stimulation of t h e f i n g e r from 3 t o 8 ms (Salamy, 1978); and t o visual stimulation from 8 t o 33 ms (Andreassi, Okamura, & Stern, 1975). Rugg and associates have completed a series of studies t h a t is s t r o n g l y influenced by t h e earlier behavioral work (Lines, Rugg, & Milner, 1984; Rugg, Lines, & Milner, 1984, 1985). They have utilized simple and go/no go choice reactions t o derive estimates of I H T T from both behavioral and electrophysiological measures. The choice task required a left- o r right-index f i n g e r response (with t h e response b u t tons placed t o t h e l e f t and right of t h e body's midline) t o a l e f t o r right visual half-field presentation of t h e target o r go stimulus (determined by duration), whose probability of occurrence was .24. In one s t u d y (Rugg et a!., 1984), spatial compatibility was varied. I n all of t h e other studies, subjects were required only t o make compatible responses. ERPs were recorded from standard lateral central (C3, C4) and occipital (01, 02) sites, a t a standard midline site (Pz), and at two non-standard sites lateral t o t h e standard occipital placements ( r e f e r r e d t o as LO1 and LO2 by t h e authors). T h e i r work has focused primarily on t h e N160 component and has revealed t h a t t h e latency of t h i s component is always shorter a t contralateral (i.e., directly stimulated) than at ipsilateral occipital scalp sites. This directional asymmetry is apparent consistently a t central sites, but i t s magnitude i s smaller than t h a t observed a t occipital sites and is not always significant. Lines e t al. (1984, Experiment 2 ) reported t h a t the N160 evident i n a simple reaction had shorter latencies and larger amplitudes a t central and occipital sites over t h e directly stimulated hemisphere than at the homologous sites over t h e other hemisphere. They also found t h a t these effects varied as a function of t h e intensity of t h e light stimulus a t occipital but not a t central sites ( b r i g h t flashes produced shorter latenThe estimated IHTT, cies and larger amplitudes than did dim flashes). collapsed across electrode sites and stimulus intensity, is 7.9 ms f o r l e f t visual half-field stimulation and 11.1 ms f o r right visual half-field stimulation (my calculations). Estimates of I H T T varied, however, a t d i f f e r e n t scalp sites; collapsed across visual half-field of stimulation and stimulus

A Psychophysiological Perspective intensity,

195

t h e estimate is 11.73 ms a t t h e s t a n d a r d occipital sites (01,

0 2 ) , 14.23 ms a t t h e n o n - s t a n d a r d occipital sites (LO1, L02), a n d 2.58 ms a t t h e c e n t r a l sites (C3, C4) (my calculations). Stimulus i n t e n s i t y altered t h e estimate o f I H T T a t t h e occipital sites but n o t a t t h e c e n t r a l sites; a t LO sites it was 17.6 ms w i t h dim stimuli a n d 10.9 ms w i t h bright stimuli, a n d a t 0 sites it was 15.1 ms a n d 8.4 ms f o r dim a n d bright stimuli, respectively. Amplitude effects paralleled these latency effects; t h e l a r g e s t amplitude N160 was apparent o v e r t h e contralateral hemisphere a n d it was l a r g e s t t o t h e b r i g h t stimulus. T h e R T estimate o f I H T T , 1.8 ms, r e p o r t e d by Lines e t al. is consistent w i t h p r e v i o u s studies. A l t h o u g h R T s were f a s t e r t o t h e bright stimuli, t h e estimate o f I H T T did n o t d i f f e r as a f u n c t i o n of stimulus i n t e n s i t y .

A similar p a t t e r n of asymmetries was revealed in t h e i r analyses o f t h e choice reaction (Lines e t a l . , 1984, Experiment 1; R u g g e t al., 1984, 1985; see F i g u r e 3). Thus, t h e estimate o f I H T T was smaller a t c e n t r a l

Bright LVF

Dim RVF

LVF

RVF

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