Advances in Psychology Volume 45 Neurophysiological & Neuropsychological Aspects of Spatial Neglect

NEUROPHYSIOLOGICAL AND NEUROPSYCHOLOGICAL ASPECTS OF SPATIAL NEGLECT ADVANCES IN PSYCHOLOGY 45 Editors G . E. STELM...

Author: Marc | E Jeannerod

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NEUROPHYSIOLOGICAL AND NEUROPSYCHOLOGICAL ASPECTS OF SPATIAL NEGLECT

ADVANCES

IN PSYCHOLOGY 45 Editors

G . E. STELMACH

P. A . VROON

NORTH-HOLLAND AMSTERDAM . NEW YORK . OXFORD .TOKYO

NEUROPHYSIOLOGICALAND NEUROPSYCHOLOGICAL ASPECTS OF SPATIAL NEGLECT Edited by

Marc JEANNEROD Laboratoire de Neuropsychologie Exptrimentale INSERM U 94 and Universitt Claude Bernard Lyon, France

1987

NORTH-HOLLAND AMSTERDAM. NEW YORK . OXFORD .TOKYO

0 ELSEVIER

SCIENCE PUBLISHERS B.V., 1987

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 permission of the copyright owner.

ISBN: 0 444 70193 1

Publishers:

ELSEVlER SCIENCE PUBLISHERS B.V. P.O. Box 1991 1000BZ Amsterdam The Netherlands Sole distributorsfor the U . S . A .and Canada:

ELSEVIER SCIENCE PUBLISHING COMPANY, INC. 52 Vanderbilt Avenue NewYork, N.Y. 10017 U.S.A. Library of Congress CataloginginPublicationData

Neurophysiological and neuropsychological aspects of spatial neglect. (Advances in psychology ; 4 5 ) Bibliography: p. Includes index. 1. Perception, Disorders o € . 2. Space perception. 3 . Spatial behavior. I. Jeannerod, Marc. 11. Title: Spatial neglect. 111. Series: Advances in psychology (Amsterdam, Netherlands) ; 4 5 . RC382.2.N48 1987 616.8 86-32994 ISBN a-444-10193-1

PRlNTED IN T H E NETHERLANDS

To the memory of Jeffrey D. Holtzman ( I 951 -1985)

This Page Intentionally Left Blank

Vii

NEU ISSUES IN SPATIAL NEGLECT Marc J e a n n e r o d

S p a t i a l n e g l e c t is one of t h e few a r e a s i n Neuropsychology where c l i n i c i a n s , p s y c h o l o g i s t s and a n i m a l e x p r i m e n t e r s have succeeded in a d o p t i n g a common l a n g u a g e , w i t h t h e happy consequence of f r u i t f u l i n t e r a c t i o n s between t h e t h r e e a p p r o a c h e s of t h i s s t r i k i n g phenomenon. I m p o r t a n t new developments have r e s u l t e d , which i n t u r n j u s t i f y t h e p u b l i c a t i o n of t h e p r e s e n t book. To my knowledge, no monograph o r m u l t i a u t h o r e d book f u l l y d e v o t e d t o t h i s problem h a s a p p e a r e d s i n c e W e i n s t e i n a n d F r i e d l a n d ( 1 9 7 7 ) a n d D e R e n z i (1982). Recent a d v a n c e s i n Neuropsychology and Neurophysiology which have i n f l u e n c e d t h e u n d e r s t a n d i n g of s p a t i a l n e g l e c t c a n b e summarized under t h r e e main h e a d i n g s . 1. Space is a c o n s t r u c t . The s p a t i a l f i e l d h a s l o n g been c o n s i d e r e d a s c o e x t e n s i v e t o t h e v i s u a l f i e l d , a m i s c o n c e p t i o n which o r i g i n a t e d in t h e c l i n i c a l u s e of t e s t i n g n e g l e c t by imposing f i x a t i o n of p a t i e n t ' s g a z e on t h e o b s e r v e r w h i l e r e s p o n s e s t o s t i m u l i p r e s e n t e d i n e a c h h a l f - f i e l d were explored. The new e x p e r i m e n t a l paradigms c l e a r l y d i s s o c i a t e t h e r e t i n o t o p i c map f r o m t h e body-centeredmap of s p a c e , w h e r e o b j e c t p o s i t i o n s a r e e n c o d e d w i t h r e s p e c t t o a body r e f e r e n c e . A c t i o n - r e l a t e d s i g n a l s , and n o t o n l y s e n s o r y s i g n a l s , are t h o u g h t t o p a r t i c i p a t e i n t h e c o n s t r u c t i o n o f t h e s p a t i a l m a p . The n a t u r e of t h e s e a c t i o n - r e l a t e d s i g n a l s , t h e b r a i n areas where t h e s p a t i a l map might b e e l a b o r a t e d , t h e way normal o r brain-damaged s u b j e c t s p e r c e i v e t h e i r body c o o r d i n a t e s a r e among t h e l i n e s o f r e s e a r c h s u g g e s t e d b y t h e phenomenon of s p a t i a l n e g l e c t . 2 . I n p u t - o u t p u t t r a n s f o r m a t i o n is n o t d i r e c t . C o n s t r a i n t s on s p a t i a l b e h a v i o r i n h i g h e r v e r t e b r a t e s impose t h a t a map o r r e p r e s e n t a t i o n of e x t r a p e r s o n a l s p a c e must be b u i l t b e f o r e a c t i o n s c a n be p r o p e r l y o r i e n t e d . The n o t i o n of r e p r e s e n t a t i o n , o n c e c o n s i d e r e d a s a u s e f u l metaphor i n c o g n i t i v e p s y c h o l o g y , is p r o g r e s s i v e l y g a i n i n g r e s p e c t a b i l i t y among neuroscientists. Simple p s y c h o l o g i c a l t e c h n i q u e s , l i k e t h e c l a s s i c a l r e a c t i o n time measurement, can b e used € o r p r o b i n g r e p r e s e n t a t i o n s . Specific e x p e r i m e n t a l s i t u a t i o n s , l i k e t h o s e i n v o l v i n g p r e s e n t a t i o n of s t i m u l i l i m i t e d t o one h e m i s p h e r e , o r t h o s e i n v o l v i n g c u e i n g of c e r t a i n a s p e c t s of t h e s t i m u l i , a l l o w p r e c i s e i n f e r e n c e on t h e m e n t a l c o n t e n t . S p a t i a l n e g l e c t o f f e r s a paradigm f o r t e s t i n g t h e r e l a t i o n s h i p of s p a t i a l r e p r e s e n t a t i o n t o b r a i n f u n c t i o n , and s p e c i a l l y t o h e m i s p h e r i c specialization. 3. A t t e n t i o n i s a d i s t r i b u t e d f u n c t i o n . Coupling between i n p u t and o u t p u t is i n f l u e n c e d by a t t e n t i o n a l mechanisms. The c o n c e p t of a t t e n t i o n , however, h a s e v o l u e d from t h a t of a s u p r a o r d i n a t e f u n c t i o n c o n t r o l l e d by a s i n g l e b r a i n c e n t e r , t o t h a t of a s e l e c t i v e mechanism o p e r a t i n g a t t h e

Viii

M.Jeannerod

s e n s o r i m o t o r l e v e l i n e a c h modality. T h i s c o n c e p t u a l change owes much t o animal e x p e r i m e n t s showing d i f f e r e n t n e u r o n a l r e s p o n s e s t o s e n s o r y s t i m u l i whenthese responses a r e g o a l s f o r a c t i o n . Human e x p e r i m e n t s have l e d t o s i m i l a r c o n c l u s i o n s i n showing t h a t i n t e n t i o n t o o r i e n t i s s u f f i c i e n t f o r s h i f t i n g a t t e n t i o n , hence d i s s o c i a t i n g t h e a t t e n t i o n a l component f o r t h e motor component of g o a l - d i r e c t e d r e s p o n s e s . S p a t i a l n e g l e c t may r e p r e s e n t a c r i t i c a l condition f o r assessing hemispheric s p e c i a l i z a t i o n f o r d i r e c t i n g a t t e n t i o n t o v a r i o u s r e g i o n s o f space. T h i s book emphasizes t h e s e new i s s u e s i n normal s u b j e c t s , i n p a t i e n t s

withspatialneglectandinanimalmodels.

References DeRenzi, E. D i s o r d e r s of s p a c e e x p l o r a t i o n and c o g n i t i o n . J. Wiley, N e w York,1982. W e i n s t e i n , E.A. & F r i e d l a n d , R.P. (Eds.). H e m i - i n a t t e n t i o n and hemisphere s p e c i a l i z a t i o n . Advances i n N e u r o l o g y , v o l . 18, 1 9 7 7 , R a v e n P r e s s , N e w York.

Acknowledgements M o s t o f t h e m a n u s c r i p t o f t h i s bookwas d i l i g e n t l y t y p e d byM.RouviSre.

ix

TABLEOFCONTENTS I.

Marc Jeannerod Foreword

11. Charles M. Butter Varieties of attention and disturbances of attention: A neuropsychological analysis 111. Eric A. Roy, Patricia Reuter-Lorentz, Louise G.Roy, Sheme Copland and Morris Moscovitch Unilateral attention deficits and hemispheric asymmetries in the control of attention

25

John L. Bradshaw, Norman C. Nettleton, Jane M.Pierson, Lyn E. Wilson and Gregory Nathan Coordinates of extracorporeal space

41

Marcel Kinsbourne Mechanisms of unilateral neglect

69

Marc Jeannerod and Benjamin Biguer The directional coding of reaching movements. A visuomotor conception of spatial neglect

87

IV

V.

VI.

VII. Kenneth M.Heilman, Dawn Bowers, Edward Valenstein and Robert T. Watson Hemispace and hemispatial neglect VIII.

IX.

X.

115

M. Jane Riddoch and Glyn W. Humphreys Perceptual and action systems in unilateral visual neglect

151

Edoardo Bisiach and Anna Berti Dyschiria. An attempt at its systemic explanation

183

Michael S. Gazzaniga and Elisabetta Ladavas Disturbances in spatial attention following lesion or disconnection of the right parietal lobe

203

Table of Contents

X

XI. Jenni A. Ogden The ‘neglected’ left hemisphere and its contribution to visuospatial neglect

215

Giuseppe Vallar and Daniela Perani The anatomy of spatial neglect in humans

235

XIII. A. David Milner Animal models for the syndrome of spatial neglect

259

XIV. Giacomo Rizzolatti and Rosolino Camarda Neural circuits for spatial attention and unilateral neglect

289

XII.

XV.

Ruthmary K. Deuel Neural dysfunction during hemineglect after cortical damage in two monkey models

315

Author Index

335

xi

LIST OF CONTRIBUTORS

Butter

C.M.

Department of Psychology The U n i v e r s i t y of Michigan Neuroscience Laboratory Building 1103 E a s t Huron Ann Arbor, Michigan 48104-1687

U.S.A. A. Berti

B.

Biguer

E. B i s i a c h

D.

Bowers

J.L.

Bradshaw

U n i v e r s i t a d i Milano I s t i t u t o d i C l i n i c a Neurologica Via F. S f o r z a , N. 35 20122 Milano ITALY L a b o r a t o i r e d e Neuropsychologie ExpBrimentale INSERM U n i t 6 94 16 avenue du Doyen LBpine 69500 Bron FRANCE U n i v e r s i t i d i Milano I s t i t u t o d i C l i n i c a Neurologica Via F. S f o r z a , N. 35 20122 Milano ITALY Department of Neurology J. H i l l i s Miller H e a l t h C e n t e r U n i v e r s i t y of F l o r i d a C o l l e g e of Medicine G a i n e s v i l l e , 32610 U.S.A. Department of Psychology Monash U n i v e r s i t y Clayton V i c t o r i a A u s t r a l i a 3168 AUSTRALI A

R. Camarda

I s t i t u t o d i F i s i o l o g i a Umana U n i v e r s i t l d i Parma F a c o l t a d i Medicina e C h i r u r g i a Via A. Grarnsci, 14 43100 Parma ITALY

S. Copland

Department of Psychology Mount Sina'i H o s p i t a l 600 U n i v e r s i t y Avenue T o r o n t o , O n t a r i o M5G 1x5 CANADA

xii

R.K. Deuel

List of Contributors

Department of Pediatrics and Neurology St. Louis Children's Hospital 400 South Kingshighway Building Saint-Louis, Missouri 63110 U.S.A.

M.S. Gazzaniga

Division of Cognitive Neuroscience Department of Neurology Cornell University Medical College New York, New York, 10021 U.S.A.

K.M. Heilman

Department of Neurology J. Hillis Miller Health Center University of Florida College of Medicine Gainesville, 32610 U.S.A.

G.W. Humphreys

Department of Psychology University of London Birkbeck College Malet Street London, WClE 7HX ENGLAND

M. Jeannerod

Laboratoire de Neuropsychologie ExpCrimentale INSERM Unit6 94 16 avenue du Doyen Ldpine 69500 Bron FRANCE

M. Kinsbourne

Department of Behavioral Neurology Shriver Center 200 Trapelo Road Waltham, Mass. 02554 U.S.A.

E. Ladavas

Department of Psychology University of Bologna Viale Berti Pichat 5 Bologna ITALY

A.D. Milner

University of St. Andrews Psychological Laboratory St. Andrews, Fife KY16 9JU SCOTLAND

M. Moscovitch

Department of Psychology Erindale College University of Toronto Toronto, Ontario M5S 1Al CANADA


G. Nathan

Department of Psychology Monash U n i v e r s i t y Clayton V i c t o r i a A u s t r a l i a 3168 AUSTRALIA

N.C.

Nettleton

Department of Psychology Monash U n i v e r s i t y Clayton V i c t o r i a A u s t r a l i a 3168 AUSTRALIA

J.A.

Ogden

Department of Psychology U n i v e r s i t y of Auckland P r i v a t e Bag Auckland NEW ZEALAND

D. P e r a n i

J.M.

Pierson

I s t i t u t o d i C l i n i c a Neurologica U n i v e r s i t a d i Milano Via p. S f o r z a , N.35 20122 MILAN0 ITALY Department of Psychology Monash U n i v e r s i t y Clayton V i c t o r i a A u s t r a l i a 3168 AUSTRAL I A

P. Reuter-Lorenz

Department of Psychology U n i v e r s i t y of T o r o n t o T o r o n t o , O n t a r i o M5S 1 A l CANADA

M.J.

Department of Psychology U n i v e r s i t y of London Birkbeck C o l l e g e Malet S t r e e t London, W C l E 7HX ENGLAND

Riddoch

G. R i z z o l a t t i

I s t i t u t o d i F i s i o l o g i a Umana U n i v e r s i t l d i Parma F a c o l t l d i Medicina e C h i r u r g i a Via A. Gramsci, 14 43100 Parma ITALY

E.A.

Mount Sinai: H o s p i t a l 600 U n i v e r s i t y Avenue T o r o n t o , O n t a r i o M5G 1x5 CANADA

Roy

Xiii


x iv

L.G.

Roy

Department of K i n e s i o l o g y U n i v e r s i t y of Waterloo Waterloo, O n t a r i o N2L 3Gl CANADA

G. V a l l a r

I s t i t u t o d i C l i n i c a Neurologica U n i v e r s i t a d i Milano Via F. S f o r z a , N.35 20122 MILAN0 ITALY

E. V a l e n s t e i n

Department of Neurology J. H i l l i s M i l l e r H e a l t h C e n t e r U n i v e r s i t y of F l o r i d a C o l l e g e of Medicine G a i n e s v i l l e , 32610 U.S.A.

R.T.

Watson

Department of Neurology J. H i l l i s Miller H e a l t h C e n t e r U n i v e r s i t y of F l o r i d a C o l l e g e of Medicine G a i n e s v i l l e , 32610 U.S.A.

L.E.

Wilson

Department of Psychology Monash U n i v e r s i t y Clayton V i c t o r i a A u s t r a l i a 3168 AUSTRAL1A

Neurophysiological and Neuropsychological Aspects of Spatial Neglect, M.Jeannerod (editor) 0 Elsevier Science Publishers B.V.(North-Holland), 1987

1

VARIETIES OF ATTENTION AND DISTURBANCES OF ATTENTION: A NEUBOPSYCROLOGICAL ANALYSIS C h a r l e s M. B u t t e r

I t i s proposed t h a t an u n d e r s t a n d i n g of n e g l e c t a s a d i s o r d e r of a t t e n t i o n m a y b e f a c i l i t a t e d by an a n a l y s i s of t h e v a r i o u s w a y s i n which a t t e n t i o n i s m a n i f e s t e d , on t h e b a s i s of which a model o f v a r i o u s forms o f a t t e n t i o n c a n be f o r m u l a t e d and r e l a t e d t o b r a i n p r o c e s s e s . F o l l o w i n g W i l l i a m James, r e f l e x a t t e n t i o n i s distinguished from v o l u n t a r y a t t e n t i o n ; t h e former i s stimulus-controlled and involves primarily "bottom-up" p r o c e s s i n g , whereas t h e l a t t e r i s c o n t r o l l e d by t h e c e n t r a l a c t i v a t i o n of s t o r e d r e p r e s e n t a t i o n s (James' " p r e p a r a t i o n from w i t h i n " ) and t h u s i n v o l v e s "top-down" a s well as "bottom-up" processing. Several forms of voluntary attention a r e d i s t i n g u i s h e d on t h e b a s i s of d i f f e r e n c e s i n t h e p r o c e s s e s i n v o l v e d i n c e n t r a l a c t i v a t i o n . A model t h a t i n c o r p o r a t e s r e f l e x and v o l u n t a r y a t t e n t i o n i s p r e s e n t e d ; a c c o r d i n g t o t h e model, mechanisms of v o l u n t a r y a t t e n t i o n are "added on" t o t h o s e o f r e f l e x a t t e n t i o n and c o n t r o l t h e l a t t e r i n a h i e r a r c h i c a l manner. I t i s proposed on t h e b a s i s of v a r i o u s n e u r o p h y s i o l o g i c a l and neurobehavioral findings t h a t d i f f e r e n t neural s t r u c t u r e s c o n t r o l d i f f e r e n t p r o c e s s e s t h a t are i n c o r p o r a t e d i n t h e model. The i m p l i c a t i o n s o f t h e m o d e l f o r t h e a n a l y s i s o f n e g l e c t and o t h e r d i s t u r b a n c e s of a t t e n t i o n a r e d i s c u s s e d .

Introduction I n ordertounderstandhemispatialneglect,itisnecessarytoidentify t h e p r o c e s s e s whose d i s t u r b a n c e r e s u l t s i n t h i s d i s o r d e r and t o u n d e r s t a n d how t h e s e p r o c e s s e s f u n c t i o n i n i n t a c t organisms. With r e g a r d t o t h e f i r s t problem, i t i s g e n e r a l l y , a l t h o u g h n o t u n i v e r s a l l y , a g r e e d t h a t h e m i s p a t i a l n e g l e c t i s due t o a d i s t u r b a n c e o f a t t e n t i o n (e.g., C r i t c h l e y , 1966; Heilman, Watson & V a l e n s t e i n , 1985). A t t e n t i o n , a c c o r d i n g t o t h e s e i n v e s t i g a t o r s and t h o s e who s t u d y i t in i n t a c t a n i m a l s and humans, i s a c e n t r a l p r o c e s s t h a t s e l e c t i v e l y f a c i l i t a t e s p a r t i c u l a r s e n s o r y o r motor p r o c e s s e s (Kahneman, 1973). A t t e n t i o n by t h i s view s e l e c t s o r a m p l i f i e s s e n s o r y m e s s a g e s and/ormotorcommands formovement. A d m i t t e d l y , much of t h e e v i d e n c e f a v o r i n g t h e view t h a t a t t e n t i o n a l d i s o r d e r s a r e responsible f o r hemispatial neglect i s negative. Neglect i n a n i m a l s and humans c a n be d i s s o c i a t e d f r o m b o t h s e n s o r y a n d m o t o r d e f i c i t s . Monkeys w i t h u n i l a t e r a l n e g l e c t show c o r t i c a l evoked p o t e n t i a l s t o e x t e r n a l s t i m u l i from t h e n e g l e c t e d s i d e of s p a c e (Watson, Miller & Heilman, 1977; Nakamura and Mishkin, 1980). F u r t h e r m o r e , n e g l e c t a p p e a r s a f t e r u n i l a t e r a l l e s i o n s of c o r t i c a l p o l y s e n s o r y areas f a r removed from p r i m a r y s e n s o r y o r motor a r e a s of t h e c o r t e x ( B u t t e r , i n p r e s s ) . The f i n d i n g t h a t h e m i s p a t i a l n e g l e c t can be d e m o n s t r a t e d when t h e i g n o r e d s e n s o r y i n p u t i s l a t e r a l i z e d n o t i n p h y s i c a l s p a c e b u t i n mental r e p r e s e n t a t i o n a l s p a c e ( s e e B i s i a c h &

2

C.M. Butter

B e r t i , t h i s volume) a l s o UndeKSCOKeS t h e nonsensory n a t u r e of t h e d e f i c i t . One might a r g u e t h a t a s e l e c t i v e d e f i c i t i n r e s p o n d i n g t o a s t i m u l u s c o n t r a l a t e r a l t o a b r a i n l e s i o n when a n i p s i l a t e r a l s t i m u l u s i s simultaneously presented, i.e., sensory extinction, c o n s t i t u t e s p o s i t i v e e v i d e n c e f o r a n a t t e n t i o n a l loss, e s p e c i a l l y i f t h e r e i s no d e f i c i t i n responding t o t h e same c o n t r a l a t e r a l s t i m u l u s when i t i s p r e s e n t e d a l o n e . However, f i n d i n g s t h a t e x t i n c t i o n can be d e m o n s t r a t e d a f t e r s e n s o r y t r a c t l e s i o n s ( B e n d e r . 1952) l e a d s one t o s u s p e c t t h a t i t may be a consequence of s e n s o r y a s w e l l as attentionaldisturbances. The s c a r c i t y of p o s i t i v e e v i d e n c e f o r an a t t e n t i o n a l l o s s u n d e r l y i n g n e g l e c t i s i n p a r t due t o a p a u c i t y of s t u d i e s i n which p a t i e n t s OK a n i m a l s w i t h n e g l e c t have been t e s t e d i n s i t u a t i o n s w h e r e s p a t i a l a t t e n t i o n h a s been manipulated i n d e p e n d e n t l y of s e n s o r y a n d m o t o r f a c t o r s . A n e x a m p 1 e o f s u c h a s t u d y i s one i n which p a t i e n t s w i t h r i g h t p a r i e t a l l e s i o n s (which are f r e q u e n t l y accompanied by l e f t h e m i s p a t i a l n e g l e c t ) were i m p a i r e d i n d i s e n g a g i n g t h e i r a t t e n t i o n f r o m a v i s u a l cue p r e s e n t e d e i t h e r c e n t r a l l y o r i n t h e r i g h t h e m i f i e l d , i n Order t o d i r e c t i t t o a s t i m u l u s p r e s e n t e d i n t h e l e f t h e m i f i e l d ( P o s n e r , W a l k e r , F r i e d r i c h & R a f a l , 1984). I t h a s been p o i n t e d o u t (e.g., De R e n z i , 1982; B u t t e r , i n p r e s s ) t h a t whereas compensation f o r a r e s t r i c t e d s e n s o r y l o s s i s f r e q u e n t l y o b s e r v e d i n p a t i e n t s , t h e s e compensatory movementsare s t r i k i n g l y a b s e n t i n n e g l e c t . Thus, p a t i e n t s w i t h n e g l e c t a r e s e v e r e l y d e f i c i e n t i n e x p l o r i n g t h e a f f e c t e d h a l f of s p a c e when t h e y a r e i n s t r u c t e d t o s e a r c h f o r a t a r g e t (ChBdru, L e b l a n c & L h e r m i t t e , 1973; Chain, ChBdru, Leblanc & L h e r m i t t e , 1972). T h i s f a i l u r e t o compensate f o r t h e d e f i c i t i s t h e t e l l - t a l e s i g n t h a t a system r e s p o n s i b l e n o t o n l y f o r a t t e n d i n g t o one-half of s p a c e , b u t a l s o f o r e x p l o r i n g and i n v e s t i g a t i n g i t , is d i s t u r b e d . Whereas i t i s g e n e r a l l y a g r e e d t h a t n e g l e c t i s due t o an a t t e n t i o n a l d i s o r d e r , t h e r e have been few a t t e m p t s t o u n d e r s t a n d n e g l e c t i n term s o f t h e v a r i o u s ways i n which a t t e n t i o n i s used i n everyday l i f e and i n hbOKatoKy t a s k s . T h e g o a l s of t h e p r e s e n t c h a p t e r a r e : ( a ) t o d e s c r i b e s e v e r a l w a y s i n which a t t e n t i o n i s used and t o p r e s e n t s i m p l e models of t h e p r o c e s s e s involved i n each use ; ( b ) t o present evidence concerning t h e neural s t r u c t u r e s t h a t may c o n t r o l t h e s e p r o c e s s e s ; and ( c ) t o d i s c u s s t h e implicationsoftheseideas f o r t h e a n a l y s i s of n e g l e c t . A Taxonomy of S e l e c t i v e A t t e n t i o n a l F u n c t i o n s

S e l e c t i v e a t t e n t i o n i s m a n i f e s t e d i n s e v e r a l ways, e a c h of w h i c h h a s a p a r t i c u l a r f u n c t i o n a l s i g n i f i c a n c e . A b a s i c d i s t i n c t i o n i n t h e taxonomy of a t t e n t i o n - " r e f l e x " v s . " v o l u n t a r y " a t t e n t i o n - w a s s u g g e s t e d by W i l l i a m James (1890). R e f l e x a t t e n t i o n is t r i g g e r e d by s t i m u l i w i t h p a r t i c u l a r p h y s i c a l c h a r a c t e r i s t i c s ( d e s c r i b e d below); t h e r e c e n c y and f r e q u e n c y w i t h which t h e s t i m u l u s ( O K similar s t i m u l i ) h a s o c c u r r e d a l s o d e t e r m i n e t h e d e g r e e t o which i t c a p t u r e s o u r a t t e n t i o n . The term " r e f l e x a t t e n t i o n " undeKSC0KeS t h e i n v o l u n t a r y and immediate c h a r a c t e r of t h i s form of a t t e n t i o n , which i s c o n t r o l l e d by s t i m u l i t h a t are sudden, i n t e n s e , of h i g h c o n t r a s t O K moving. James (1890) n o t e d t h a t r e f l e x a t t e n t i o n i s a l s o e l i c i t e d by an " i n s t i n c t i v e s t i m u l u s " , one t h a t "by r e a s o n of i t s n a t u r e . . . a p p e a l s t o some one of o u r normal c o n g e n i t a l impulses" (1890, p . 417). Another i m p o r t a n t c h a r a c t e r i s t i c of stimuli (other than " i n s t i n c t i v e " o n e s ) e l i c i t i n g r e f l e x a t t e n t i o n i s n o v e l t y : s t i m u l i t h a t are f r e q u e n t l y r e p e a t e d o f t e n f a i l t o e l i c i t a t t e n t i o n , even i f t h e y have t h e a p p r o p r i a t e p h y s i c a l c h a r a c t e r i s t i c s . I n a d d i t i o n , t h e d e g r e e t o which a t t e n t i o n i s d i r e c t e d t o a s t i m u l u s may a l s o depend on s t i m u l u s c o m p l e x i t y and i n c o n g r u i t y ( B e r l y n e , 1960). One i m p o r t a n t consequence of r e f l e x a t t e n t i o n d i r e c t e d toward s t i m u l i w i t h t h e s e p r o p e r t i e s i s t h a t t h e s t i m u l i

3

Varieties and disturbances of attention

briefly acquire perceptual salience, o r , in James' terms, "clearness" and "distinction fromother things and subdivision". Reflex attention is frequently accompanied by the orienting reflex (Sokoloff, 1963), which includes behaviors that orient the receptors tothe stimulus source and thus enhance sensory information. Like reflex attention, orienting responses are triggered by novel stimuliand habituate to repeated presentation of the same stimuli. Overt orienting reactions to stimuli are temporarily linked to attentional shifts directed to the same stimuli, so that their salience is increased.Thus, programmingthe eyes to move to a target enhances the target's detection (Remington, 1980; Singer, Zihl and Poppel, 1 9 7 7 ) and its identification, if the target is a pattern (Crovitz and Daves, 1962; Bryden,1961).Overtorientingto stimulimayalso be accompanied by other exploratory-investigatory responses including reaching f o r , g r a s p i n g a n d m a n i p u l a t i n g o b j e c t s . These characteristics of reflex attention to visual stimuli are diagrammatically illustrated in Figure 1. According to this model, stimuli are processed in two separate channels. One channel analyzes specific stimulus features, such as color and borders, that are necessary for recognition. The other channel conveys sensory responses to a mechanism that detects novel stimuli and increases their salience. This system also facilitates alerting systems so that the sensory representation of the stimulus is enhanced. By its connections with the alerting systems, the novelty-salience mechanism also activates the system that organizes exploratory-investigatory behaviors directed to the stimulus. The novelty-salience mechanism also commands orienting movements that direct the sense organs to the stimulus. Furthermore, as shown in Figure 1, it is assumed that a short-term central representation of the stimulus is responsible for both a decrement in stimulus salience and habituation of orienting responseswith repeated stimulus presentations.

INVESTIGATORY SYSTEM

SENSORY FEATURE ORIENTING SYSTEM I

STIMULUS4

4

ORIEFITING

RESPONSES

lATORY IGATORY RESl INSES

Figure 1

Processes involved in reflex attention to novel stimuli. Thick arrows refertocentraleffectsofnovelstimuli.

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Whereas reflex attention is controlled primarily by the physical characteristics and novelty of the eliciting stimuli, it is suggestedhere that voluntary attention involves the central activation of stored representations of stimuli. Unlike reflex attention, which is primarily controlled by bottom-up processing of external stimuli, voluntary attention involves top-down as well as bottom-up processing. Here again, James' description of attentional states (James, 1890) provides clues to the underlying mental processes. A s James observed, the effort to attend, for example, to the peripheral region of a picture is "nothing more or less than the effort to form as clear an idea as possible of what is there portrayed" (1890, p. 438). For James, voluntary attention to a peripheral stimulus is equivalent to "anticipatory preparation from within of the ideational centres concerned with the object to which attention is payed" (1890, p. 434). In keeping with James' analysis, it is proposed that voluntary attention involves both the peripheral and central (James' "preparation from within") activation of permanently stored representations of familiar objects (see Figure 2 ) . These stored representations are activated peripherally by the appropriate (i.e., matching) sensory inputs. Recognition occurs when the appropriate matching sensory input activates its central representation to a critical level. Whereas input fromthe m a t c h i n g s t i m u l u s a l o n e m a y b e s u f f i c i e n t t o a c t i v a t e the stored representation to its recognition threshold, it is assumed that central activation of the representation by an instruction (external or self-generated) or another stored representation increases the liklihood that the recognition threshold is attained. The model also assumes that activation of a stored representation to the recognition threshold enhances the perceptual salience of the stimulus by facilitating its sensory representation. Thus, voluntary attention in this model refers to the central facilitation of recognition and of perceptual salience. The assumption that central activation of a stored representation of a stimulus by an instruction (either to attend to or expect the stimulus) enhances its perceptual salience is supported by the finding that attention to astimulus enhances sensitivity ( d ' ) for that stimulus (e.g., Broadbent and Gregory, 1963; Moray and O'Brien, 1967; Treisman and Geffen, 1967). Furthermore, it is assumed that centrally activated stored representationsin turnactivate the novelty-salience mechanisms that control orienting responses and facilitate the alerting system. In one variety of selective attention, the central facilitation of the stored representation is provided by an instruction, either self-generated or from an external source, to detect the occurrence of a stimulus or to search for it. The instruction in turn may be controlled by or be part of a plan to carry out a sequence of goal-directed actions. Invisual search, an increase in perceptual salience ismanifested as the tendency of the target to "pop out", a phenomenon reported by well-practiced subjects in visual search tasks (e.g., Neisser, 1963). The model predicts that distractors sharing features of the target in a search task would be more likely than distractors not sharing the target's features to activate the target's central representation to its recognition threshold. This inappropriate activation would be expected to result not only in false recognition, but also in inappropriate activation of search mechanisms (e.g., those controlling head and eye movements), thus slowing down search. The predicted increase in search time intaskswhere distractors share features with the target has been experimentally confirmed (Treisman h Gelade, 1980).

If central activation of the stored representation of stimulus by a search instructionenhances recongitionof the stimulus, it should be


5

REPRESENTATION

STIMULUS4

ORIENTING RESPONSES EXPL( INVES' RESF

Figure 2 Processes involved in voluntary attention, involving "top-down", OK central activation of a stored representation. Thick arrows refer to central activating effects; interrupted arrow refers to reduced effect OK lack of effect of meaningful familiar stimuli on the novelty-saliencemechanism. possible to demonstrate an effect of searching for a particular target on its recognition. This prediction was tested in a situation where subjects searched for a visual target in an array of identical distractor items (Goodale & Butter, 1985). Interspersed among the search trials was a probe recognition task in which a pair of stimuli was briefly presented; on these probe trials. the subjects indicated whether the two stimuli were the same or different. When presented with a pair of stimuli both of which were the target item, the subjects made fewer recognition errors than they did when presented with a pair of distractOK items. In addition, as predicted by the model, the subjects made more recognition errors when presented with stimulus pairs consisting of a "hybrid" (a stimulus containing all the features of the target and of the distractor) and the target than they did when presented with stimulus pairs consisting of the same hybrid and a distractor. Furthermore, these effects of the search task on recognition were also found when roles of target and distractor were reversed in a subsequent test: thus, they were independent of the particular features of the target and distractors. Another way in which voluntary attention operates is by selecting a stimulus event for recognition (and perceptual salience) because it has intrinsic, permanent value in contrast to the example described above, in which selection OCCUKS as the r e s u l t o f a t r a n s i e n t i n s t r u c t i o n . E x a m p 1 e s of stimuli of intrinsic value are the sound of one's name, which has attentional value in dichotic listening tasks (Moray, 1959). Treisman (1960) assumed that such stimuli are readily recognized and perceptually

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s a l i e n t b e c a u s e t h e a c t i v a t i o n t h r e s h o l d s of t h e i r c e n t r a l r e p r e s e n t a t i o n s a r e permanently lowered r e l a t i v e t o t h o s e of o t h e r s t i m u l i . A similar assumption may bemade i n t h e p r e s e n t model: A s a c o n s e q u e n c e o f i t s lowered r e c o g n i t i o n t h r e s h o l d , t h e s t o r e d r e p r e s e n t a t i o n of a s t i m u l u s w i t h i n t r i n s i c v a l u e , compared t o t h a t of a s t i m u l u s w i t h o u t i n t r i n s i c v a l u e , i s more l i k e l y t o be r e c o g n i z e d and produce t h e downstream f a c i l i t a t i n g effectsshowninFigure 2 w h e n t h e a p p r o p r i a t e s t i m u l u s i s presented. The above exampleof a t t e n t i o n d i r e c t e d t o o n e ' s name i s a l s o a n e x a m p l e of a t t e n t i o n d i r e c t e d t o s t i m u l i of d e r i v e d o r "remote i n t e r e s t " , t o u s e James' term. A s t i m u l u s may a c q u i r e remote i n t e r e s t because i t h a s been a s s o c i a t e d w i t h a s t i m u l u s of i n t r i n s i c i n t e r e s t , which i t s e l f may be a c q u i r e d o r have a n i n h e r i t e d component, a s i n t h e c a s e of s t i m u l i r e l e v a n t t o b i o l o g i c a l d r i v e s . The model p r e s e n t e d h e r e can accomodate t h i s k i n d of v o l u n t a r y a t t e n t i o n by adding l e a r n e d c o n n e c t i o n s from t h e c e n t r a l r e p r e s e n t a t i o n of t h e s t i m u l u s of remote i n t e r e s t ( " f o o d ' s ready!") t o t h e c e n t r a l r e p r e s e n t a t i o n of t h e s t i m u l u s w i t h which i t h a s been a s s o c i a t e d (food on t h e t a b l e ) , which i n t u r n a c t i v a t e s t h e n o v e l t y - s a l i e n c e mechanism. The r e s u l t i n g a c t i v a t i o n of t h e a l e r t i n g s y s t e m s t h e n f a c i l i t a t e s t h e s e n s o r y r e p r e s e n t a t i o n of t h e s t i m u l u s ( " f o o d ' s ready!") ( s e e F i g u r e 3 ) . T h u s , i n t h i s c a s e , u n l i k e t h e o n e s above, c e n t r a l a c t i v a t i o n of a c e n t r a l r e p r e s e n t a t i o n i s i n d i r e c t l y p r o v i d e d by a n o t h e r central representation.

REPRESENTATION REMOTE INTEREST'

REPRESENTATION

EXPLORATORY INVESTIGATORY

INTRINSK: INTEREST"

SENSORY ALERTING SYSTEM NOVELTY SALIENCE MECHANISM SENSORY FEATURE PROCESSING

I STlMU LUS

Figure 3 P r o c e s s e s i n v o l v e d i n v o l u n t a r y a t t e n t i o n based upon a s t i m u l u s of "remote i n t e r e s t ' ' t h a t h a s become a s s o c i a t e d w i t h a s t i m u l u s of i n t r i n s i c i n t e r e s t . Thick arrows r e f e r t o c e n t r a l a c t i v a t i n g e f f e c t . E f f e c t s of s t i m u l u s of remote i n t e r e s t on t h e n o v e l t y - s a l i e n c e mechanism, a s i n F i g u r e 2 , a r e assumed t o be weak o r a b s e n t . O r i e n t i n g and e x p l o r a t o r y - i n v e s t i g a t o r y r e s p o n s e s are assumed t o b e a c t i v a t e d as i n F i g u r e 2.


I

A form of v o l u n t a r y a t t e n t i o n d i f f e r e n t from t h o s e d e s c r i b e d above i n v o l v e s a t t e n t i o n d i r e c t e d t o a r e p r e s e n t a t i o n i n t h e s t i m u l u s domain i n t h e a b s e n c e of t h e a p p r o p r i a t e s t i m u l u s - i n o t h e r w o r d s , a t t e n t i o n d i r e c t e d t o a m e n t a l image o f a f a m i l i a r s t i m u l u s . I n t h i s c a s e , a n i n s t r u c t i o n , e . g . , t o form an image of an a p p l e , f a c i l i t a t e s t h e a p p r o p r i a t e c e n t r a l r e p r e s e n t a t i o n which t h e n a c t i v a t e s t h e s e n s o r y r e p r e s e n t a t i o n of a n a p p l e ( s e e F i g u r e 4 ) . M o r e o v e r , v i s u a l images c a n b e n o t o n l y p a s s i v e l y i n s p e c t e d ; t h e y can a l s o be m e n t a l l y m a n i p u l a t e d - t h a t i s , t r a n s f o r m e d w i t h r e g a r d t o e g o c e n t r i c v i e w p o i n t ( e . g . , r o t a t e d , b r o u g h t up c l o s e and e n l a r g e d , o r viewed f r o m a l o n g d i s t a n c e ; K o s s l y n , 1980).

I

CENTRAL REPRESENTATION

I

SENSORY REPRESENTATION

I

I

Figure 4 Processes involvedinvoluntaryattentiondirectedtoamentalimagein the absenceof sensoryinput.

F u n c t i o n a l and S t r u c t u r a l O r g a n i z a t i o n of A t t e n t i o n a l Mechanisms : R e f l e x Attention Anatomical and physiologicalfindingspointto t h e d e e p e r l a y e r s o f t h e s u p e r i o r c o l l i c u l u s ( t h o s e below t h e s t r a t u m opticum) and t h e m i d b r a i n r e t i c u l a r f o r m a t i o n a s t h e c r u c i a l b r a i n r e g i o n s f o r n o v e l t y d e t e c t i o n and s a l i e n c e . F u r t h e r m o r e , t h e r e i s abundant e v i d e n c e t h a t t h e a s c e n d i n g and d e s c e n d i n g p r o j e c t i o n s of t h e s e s t r u c t u r e s c o n t r o l r e f l e x a t t e n t i o n by f a c i l i t a t i n g s e n s o r y p r o c e s s i n g , e x p l o r a t o r y - i n v e s t i g a t o r y r e s p o n s e s , and orientingresponses (seeFigure5). S e n s o r y i n p u t from a v a r i e t y of e x t e r o c e p t i v e and p r o p r i o c e p t i v e s o u r c e s c o n v e r g e s on neurons i n t h e d e e p e r l a y e r s of t h e s u p e r i o r c o l l i c u l u s and t h e s u b a j a c e n t m i d b r a i n r e t i c u l a r f o r m a t i o n ( H u e r t a a n d H a r t i n g , 1984). Neurons i n t h e s e two r e g i o n s a r e morphologicallyindistinguishable

8

8

CM.Butter

S3SNOdS3M r10bBllS3ANI AYOlVMOldX3

OM3 1VNldS W31SNIVME 53SNOdS38

Figure 5 Neuralmechanismsof r e f l e x a t t e n t i o n . See t e x t f o r e x p l a n a t i o n .

(Edwards, 1980) and show similar d i s c h a r g e c h a r a c t e r i s t i c s ; t h e y respond b e s t t o t r a n s i e n t . moving s t i m u l i , f r e q u e n t l y i n two o r more m o d a l i t i e s ; t h e y a l s o l a c k s p e c i f i c t r i g g e r f e a t u r e s and h a b i t u a t e t o r e p e a t e d p r e s e n t a t i o n s of t h e same s t i m u l u s (Chalupa, 1984). Within t h e d e e p e r l a y e r s of t h e s u p e r i o r c o l l i c u l u s , r e c e p t i v e f i e l d s of s t i m u l i from d i f f e r e n t e x t e r o c e p t i v e m o d a l i t i e s a r e t o p o g r a p h i c a l l y o r g a n i z e d and i n s p a t i a l r e g i s t e r ( S t e i n , 1984). The motor o r g a n i z a t i o n of t h e d e e p e r t e c t a l l a y e r s , as determined by e l e c t r i c a l r e c o r d i n g s and s t i m u l a t i o n , is a l s o t o p o g r a p h i c and i n r e g i s t e r w i t h t h e s e n s o r y maps ( S t e i n , 1984). Furthermore, t h e d e s c e n d i n g p r o j e c t i o n s of t h e m i d b r a i n and d e e p e r l a y e r s of t h e s u p e r i o r c o l l i c u l u s are similar: thedeepertectallayerssendfibers t o v a r i o u s b r a i n s t e m and s p i n a l r e g i o n s c o n t r o l l i n g movements of t h e e y e s , head, v i b r i s s a e , p i n n a and l i m b s ( H u e r t a and H a r t i n g , 1984: S t e i n , 1984). These f i n d i n g s s u g g e s t t h a t t h e t e c t o - t e g m e n t a l r e g i o n p r o v i d e s t h e sensory-motor i n t e g r a t i v e mechanism f o r commanding a p p r o p r i a t e o r i e n t i n g movements t o n o v e l s t i m u l i . Whereas t h e sensory-motor o r g a n i z a t i o n and d e s c e n d i n g c o n n e c t i o n s of t h e t e c t o t e g m e n t a l r e g i o n p r o v i d e i n f o r m a t i o n a b o u t t h e mechanisms c o n t r o l l i n g o r i e n t i n g movements e l i c i t e d by novel s t i m u l i , t h e a s c e n d i n g p r o j e c t i o n s of t h e s e s t r u c t u r e s shedlightonthemechanismsbywhichreflex a t t e n t i o n produces a l e r t i n g a n d a t t e n t i o n a l e f f e c t s . The m i d b r a i n r e t i c u l a r f o r m a t i o n p r o j e c t s t o a number of d i e n c e p h a l i c r e g i o n s i n c l u d i n g s e v e r a l g r o u p s of t h a l a m i c n u c l e i ; i t p r o j e c t s d e n s e l y t o t h e r e t i c u l a r n u c l e u s and i n t r a l a m i n a r n u c l e i , b u t o n l y l i g h t l y t o t h e p a r a l a m i n a r n u c l e i (Edwards & D e Olmos. 1976; G r a y b i e l , 1977; R o b e r t s o n a n d F e i n e r , 1982), which a r e s i t u a t e d l a t e r a l t o t h e i n t e r n a l m e d u l l a r y lamina ( s e e F i g u r e 5 ) . Anatomical and p h y s i o l o g i c a l f i n d i n g s s t r o n g l y s u g g e s t t h a t


9

r e t i c u l a r n u c l e u s n e u r o n s are i n h i b i t e d by e x c i t a t i o n of t h e m i d b r a i n r e t i c u l a r f o r m a t i o n , t h u s r e l e a s i n g sensory-specificthalamic n e u r o n s from t h e r e c u r r e n t i n h i b i t i o n t h a t r e t i c u l a r n u c l e u s n e u r o n s e x e r t on them. C o n s e q u e n t l y , e x c i t a t i o n of t h e r e t i c u l a r f o r m a t i o n e n h a n c e s t h e f l o w of s e n s o r y i n f o r m a t i o n from thalamus t o c o r t e x ( S i n g e r , 1977; S k i n n e r & Y i n g l i n g , 1977; Y i n g l i n g h S k i n n e r , 1977). The i n t r a l a m i n a r n u c l e i have l o n g been c o n s i d e r e d t h e t h a l a m i c component of a n a s c e n d i n g " a r o u s a l " s y s t e m which p r o j e c t s d i f f u s e l y t o a l l c o r t i c a l a r e a s ( L i n d s l e y , 1960). However, r e c e n t a n a t o m i c a l s t u d i e s employing a n t e r o g r a d e t r a c e r s have shown t h a t o n l y c e r t a i n c o r t i c a l areas r e c e i v e moderate o r dense i n t r a l a m i n a r p r o j e c t i o n s . In t h a t c a t , 1 c e n t r a l i s l a t e r a l i s , p a r a c e n t r a l i s and c e n t r a l i s m e d i a l i s p r o j e c t t o sensory-motor, p r e m o t o r , p a r i e t a l and a n t e r i o r l i m b i c c o r t e x (Kaufman & R o s e n q u i s t , 1 9 8 5 ) a s w e l l a s t o a l l v i s u a l c o r t i c a l a r e a s e x c e p t f o r a r e a 17. The c e n t r e median n u c l e u s and n. p a r a f a s c i c u l a r i s a l s o p r o j e c t t o sensory-motor, premotor and a n t e r i o r l i m b i c s t r u c t u r e s (Royce & Mourey, 1985). S t e r i a d e & Glen (1982) r e p o r t e d t h a t 13% of u n i t s i n n. c e n t r a l i s l a t e r a l i s and p a r a c e n t r a l i s o f c a t s p r o j e c t t o a r e a s 4 , 6 , 8 and 5. Whereas a n t e r o g r a d e t r a c e r s have n o t been used t o s t u d y i n t r a l a m i n a r p r o j e c t i o n s i n monkeys, pathways from i n t r a l a m i n a r n u c l e i t o motor and premotor c o r t e x , f r o n t a l eye f i e l d s , a r e a s 5 and 7 , and c i n g u l a t e c o r t e x have been d e s c r i b e d by t h e r e t r o g r a d e H R P m e t h o d i n m o n k e y s (Herkenham, 1 9 8 6 ) . T h e i n t r a l a m i n a r p r o j e c t i o n t o motor, l i m b i c , p o l y s e n s o r y and v i s u a l a s s o c i a t i o n c o r t e x of c a t and monkey s u g g e s t s t h a t v i a t h i s r o u t e t h e m i d b r a i n r e t i c u l a r f o r m a t i o n a c t i v a t e s c o r t i c a l a r e a s beyond t h e primary s e n s o r y a r e a s and t h u s may f a c i l i t a t e processinginhigher cortical areas. Whereas t h e m i d b r a i n r e t i c u l a r f o r m a t i o n p r o j e c t s o n l y l i g h t l y t o t h e paralaminar n u c l e i , t h e deeper c o l l i c u l a r l a y e r s project heavily t o both t h e i n t r a l a m i n a r a n d p a r a l a m i n a r p o r t i o n of n. m e d i a l i s d o r s a l i s as w e l l a s t o o t h e r p a r a l a m i n a r n u c l e i - t h e m e d i a l p u l v i n a r , n. l a t e r a l i s p o s t e r i o r and n. l a t e r a l i s d o r s a l i s ( H a r t i n g , H u e r t a , F r a n k f u r t e r , S t r o m i n g e r & Royce, 1980) ( s e e F i g u r e 5). In t h e monkey, t h e c o r t i c a l p r o j e c t i o n s of t h e p a r a l a m i n a r n u c l e i , u n l i k e t h o s e of t h e i n t r a l a m i n a r n u c l e i , may b e l i m i t e d t o c e r t a i n c o r t i c a l p o l y s e n s o r y and l i m b i c a r e a s i m p l i c a t e d i n n e g l e c t . P r o l i n e i n j e c t i o n s i n t o m e d i a l p u l v i n a r i n t h e monkey r e s u l t i n l a b e l l e d t e r m i n a l s i n c i n g u l a t e c o r t e x , f r o n t a l e y e f i e l d s , t h e d o r s a l bank of t h e s u p e r i o r t e m p o r a l s u l c u s and a r e a 7 ( B a l e y d i e r 6 Mauguiere, 1 9 8 5 ) , a l l of which a r e d i r e c t l y i n t e r c o n n e c t e d ( B u t t e r , i n p r e s s ) . These c o r t i c a l p r o j e c t i o n s of t h e medial p u l v i n a r have a l s o been r e p o r t e d i n s t u d i e s employing r e t r o g r a d e t r a n s p o r t of HRP ( e . g . , B u r t o n & J o n e s , 1976; K a s d o n b J a c o b s o n , 1978; Barbas & Mesulam, 1981). F u r t h e r m o r e , t h e f r o n t a l e y e f i e l d s and a r e a 7 a l s o r e c e i v e p r o j e c t i o n s fromtwootherparalaminarnuclei - n. m e d i a l i s d o r s a l i s , p a r s m u l t i f o r m i s and t h e m a g n o c e l l u l a r d i v i s i o n o f n. v e n t r a l i s a n t e r i o r (Barbas & Mesulam, 1981). I n a d d i t i o n , a n o t h e r c o r t i c a l p o l y s e n s o r y a r e a i m p l i c a t e d i n s p a t i a l a t t e n t i o n , t h e premotor c o r t e x , a l s o r e c e i v e s a n i n p u t from t h e m e d i a l p u l v i n a r and t h e m a g n o c e l l u l a r d i v i s i o n of n. m e d i a l i s d o r s a l i s ( K i e v i t & Kuypers, 1977). F u r t h e r m o r e , a l l t h e c o r t i c a l p r o j e c t i o n s from t h e m e d i a l p u l v i n a r d e s c r i b e d by B a l e y d i e r and Mauguiere (1985) i n c l u d e t e r m i n a l s i n l a y e r I. L i k e w i s e , i n c a t and r a t , a l l p a r a l a m i n a r thalamicprojectionsinvestigated i n c l u d e t e r m i n a l s i n t h i s l a y e r (Herkenham, 1 9 8 6 ) . T h e s e p a r a l a m i n a r i n p u t s c o u l d d e p o l a r i z e a p i c a l d e n d r i t e s of pyramidal c e l l s , i n c r e a s i n g e x c i t a b i l i t y and t h u s c o u l d f a c i l i t a t e p r o c e s s i n g i n c o r t i c a l a r e a s f a r removed from p r i m a r y s e n s o r y i n p u t . A s B a l e y d i e r a n d M a u g u i S r e (1985) n o t e , t h e r e c i p r o c a l c o n n e c t i o n s t h e y d e s c r i b e d between t h e m e d i a l p u l v i n a r and s e v e r a l c o r t i c a l p o l y s e n s o r y a r e a s p r o v i d e cortico-thalamo-cortical

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pathways that mirror the direct cortico-cortical pathways between these polysensory areas: thus, they may facilitate inter-cortical processing of information. Each of the above-described multiple ascending pathways arising from the midbrain may subserve different functions related t o attentional processes. The midbrain tegmentum may play a role in generalized alerting functions by its inhibitory projection to the reticular nucleus and by its excitatory projections to the intralaminar nuclei. Moreover, the apparent preferential projection of the intralaminar nuclei to sensory-specific and polysensory association cortex may increase "synaptic security" and thus facilitate higher cortical processing in these areas, which are removed from primary sensory-specific input. Whereas these projections may subserve generalized alerting functions, the tectoparalaminar pathway to cortical polysensory areas may play a more specific role in spatially-directed attention(see below). A number of physiological and behavioral findings suggest that the tecto-tegmental structures may subserve simple and crude orienting activity, whereas the cortical polysensory structures may mediate more specific and complex exploratory-investigatory behaviors and the shifts of attention that accompany them. Unilateral lesions of the superior colliculus in the monkey and other mammals produce contralateral hemispatial neglect of stimuli in several modalities (Butter, in press). Discrete unilateral lesions of the midbrain tegmentum also result in unilateral neglect (Watson, Heilman, Miller&King, 1974). Several findings suggest that the colliculus also participates in the control of eye movements and in spatial attentional processes related to eye movements. Units in the superficial layers of the colliculus in monkey show enhanced discharges to visual stimuli when these stimuli are the target of a saccade (Goldberg & Wurtz, 1972; Wurtz & Mohler, 1976). Furthermore, the enhanced sensory response is linked temporally with the discharges of eye movement-related neurons in the deeper collicular layers and is related to the intention to move the eyes rather than to the specific parameters of the eye movement (Wurtz & Mohler, 1976). Monkeys with colliculus lesions are impaired in performing discrimination tasks requiring gaze and attention shifts from the response sites to the stimuli (Butter, 1979); they are also impaired in shifting their gaze to peripheral visual targets (Kurtz & Butter, 1980) evenwhen they are presentedtoo b r i e f l y t o b e f i x a t e d ( K u r t z , Leiby & Butter, 1982). The conclusion that the superior colliculus participates in spatial attention linked to eye movements is supported by the finding that patients with supranuclear palsy are impaired in spatially shifting their attention onlyinthe direction inwhich their eye movements areimpaired(Posner,Cohen&Rafal,1982).

The sensory-motor coordinating mechanism provided by tectotegmental structures may contribute to orienting activity even in the absence of the forebrain. Decerebrate rats (Woods, 1964; Grill & Norgren, 1978) and cats (Bignall & Schramm, 1974) show orienting movements of the head to auditory and tactile stimuli. It has also been reported that a human anencephalic infant showed orientingresponses toauditory stimuli (Brackbill, 1971). Spatialattentional functions of the thalamus have not been studied as thoroughly as those of the tectum. However, two studies have reported disturbances in spatial attention following selective unilateral lesions of thalamic nuclei that provide links in the pathways described previously: n. parafascicularis in monkey (Watson, Miller & Heilman, 1978) and 2 centralis lateralis in cats (Orem,Schlag-Rey&Schlag, 1973).Furthermore, injections of GABA-altering drugs into a region of the pulvinar bordering the medial pulvinar appear to alter spatial shifts of attention in monkeys


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( P e t e r s e n , M o r r i s & Robinson, 1984). H e m i s p a t i a l n e g l e c t h a s been r e p o r t e d i n p a t i e n t s w i t h u n i l a t e r a l t h a l a m i c l e s i o n s ( e . g . , Watson & Heilman, 1979): i n most of t h e s e r e p o r t s , t h e l e s i o n s were e x t e n s i v e and i n v o l v e d a number of t h a l a m i c n u c l e i . However, Cambier, E l g h o z i and S t r u b e (1980) d e s c r i b e d a p a t i e n t w i t h v i s u a l and t a c t i l e n e g l e c t c o n t r a l a t e r a l t o a d i s c r e t e u n i l a t e r a l i s c h e m i c t h a l a m i c l e s i o n t h a t i n v o l v e d two t e c t o r e c i p i e n t n u c l e i - t h e p u l v i n a r and d o r s o m e d i a l n u c l e u s - a s w e l l a s 5 ventralis posterior lateralis. H e m i s p a t i a l n e g l e c t i n monkeys r e s u l t s from u n i l a t e r a l l e s i o n s of c o r t i c a l a r e a s t o which t h e p a r a l a m i n a r n u c l e i p r o j e c t - f r o n t a l eye f i e l d s (Kennard, 1939; Welch & S t u t e v i l l e , 1958; L a t t o & Cowey, 1971a & b; Conway. 1980; Crowne, Yeo & S t e e l e R u s s e l l , 1 9 8 1 ) , S c h i l l e r , True & c i n g u l a t e c o r t e x (Watson, Heilman, C a u t h e n & K i n g , 1 9 7 4 ) , i n f e r i o r p a r i e t a l l o b u l e (Denny-Brown & Chambers, 1958; B a t e s & E t t l i n g e r , 1960; Heilman, Pandya & Geschwind, 1 9 7 0 ) , t h e p o l y s e n s o r y r e g i o n of t h e s u p e r i o r t e m p o r a l s u l c u s (Luh, B u t t e r & B u c h t e l , 1986) and premotor c o r t e x ( R i z z o l a t t i , M a t e l l i & P a v e s i , 1983). S i g n i f i c a n t p r o p o r t i o n s o f s i n g l e u n i t s r e s p o n d i n g t o s t i m u l i i n more t h a n one s e n s o r y m o d a l i t y have been r e p o r t e d i n a l l t h e s e c o r t i c a l a r e a s w i t h t h e e x c e p t i o n of c i n g u l a t e c o r t e x , where u n i t p o l y s e n s o r y p r o p e r t i e s have n o t been r e p o r t e d ( B u t t e r , i n p r e s s ) . E l e c t r o p h y s i o l o g i c a l and b e h a v i o r a l - a b l a t i o n f i n d i n g s a l s o i n d i c a t e t h a t t h e i n f e r i o r p a r i e t a l c o r t e x 1 , f r o n t a l e y e f i e l d and s u p e r i o r t e m p o r a l s u l c u s a r e i n v o l v e d i n o r i e n t i n g movements and e x p l o r a t o r y - m a n i p u l a t i v e manual movements ( B u t t e r , i n p r e s s ) . F u r t h e r m o r e , a l l of t h e s e c o r t i c a l a r e a s a r e d i r e c t l y i n t e r c o n n e c t e d and p r o j e c t back t o t h e d e e p e r l a y e r s of t h e s u p e r i o r c o l l i c u l u s ( B u t t e r , i n p r e s s ) (See Heilman e t a l . and R i z z o l a t t i and Camarda, t h i s volume, f o r a d d i t i o n a l d i s c u s s i o n of t h e r o l e of c o r t i c a l and t h a l a m i c s t r u c t u r e s i n s p a t i a l a t t e n t i o n ) . These f i n d i n g s s u g g e s t t h a t t h e d e e p t e c t a l p r o j e c t i o n s v i a t h e thalamus t o c o r t i c a l p o l y s e n s o r y a r e a s and t h e i r p r o j e c t i o n s back t o t h e d e e p e r t e c t a l l a y e r s may comprise a s y s t e m c o n t r o l l i n g b o t h s p a t i a l s h i f t s of a t t e n t i o n and o r i e n t i n g , e x p l o r a t o r y - i n v e s t i g a t o r y b e h a v i o r s ( B u t t e r , i n p r e s s ) . The f i n d i n g s reviewed above a l s o s u g g e s t t h a t s t r u c t u r e s a t t h e b r a i n s t e m and c o r t i c a l l e v e l s of t h e system may make d i f f e r e n t c o n t r i b u t i o n s t o t h e s e p r o c e s s e s . Whereas t h e s u p e r i o r c o l l i c u l u s a p p e a r s t o c o n t r o l e y e movements and s p a t i a l s h i f t s of a t t e n t i o n l i n k e d o n l y w i t h e y e movements, t h e c o r t i c a l a r e a s i n t h e s y s t e m a p p e a r t o c o n t r o l n o t o n l y eye movements b u t complex s p a t i a l l y - d i r e c t e d manual movements i n v o l v e d i n exploringandmanipulatingobjects. There is e v i d e n c e t h a t d e s c e n d i n g pathways from t h e f o r e b r a i n may b e c r u c i a l f o r h a b i t u a t i o n of i n v e s t i g a t o r y r e s p o n s e s . Monkeys w i t h b i l a t e r a l temporal l o b e c t o m i e s abnormally p e r s i s t i n i n v e s t i g a t i n g o b j e c t s , a phenomenon r e f e r r e d t o a s "hypermetamorphosis" (Kliiver & Bucy, 1939). T h i s " r e l e a s e " of i n v e s t i g a t o r y b e h a v i o r a p p e a r s t o be due t o removal o f t h e amygdala ( W e i s k r a n t z , 1 9 5 6 ) , which a l s o h a s been i m p l i c a t e d i n o b j e c t r e c o g n i t i o n i n monkeys (Mishkin, 1982). Thus, t h e amygdala may c o n t r i b u t e t o t h e h a b i t u a t i o n of i n v e s t i g a t o r y r e s p o n s e s by p a r t i c i p a t i n g i n a mechanism f o r l a b e l i n g o b j e c t s as f a m i l i a r . The f r o n t a l l o b e may a l s o be c r u c i a l f o r h a b i t u a t i o n of o r i e n t i n g r e s p o n s e s : removal of o r b i t a l f r o n t a l c o r t e x , b u t n o t l a t e r a l f r o n t a l c o r t e x , i n monkeys i n t e r f e r e s w i t h h a b i t u a t i o n of t h e d i s t r a c t i n g e f f e c t s of v i s u a l and a u d i t o r y s t i m u l i ( B u t t e r , 1964). F r o n t a l l o b e l e s i o n s have a l s o been r e p o r t e d t o i n t e r f e r e w i t h t h e h a b i t u a t i o n of o r i e n t i n g r e f l e x e s i n humans ( L u r i a & Homskaya, 1970). F u r t h e r m o r e , e l e c t r o p h y s i o l o g i c a l e v i d e n c e t h a t f r o n t a l c o r t e x provides excitatory inputs t o the thalamic r e t i c u l a r nucleus (Yingling & S k i n n e r , 1977) i s c o n s i s t e n t w i t h t h e view t h a t t h i s c o r t i c a l r e g i o n

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s u p p r e s s e s s e n s o r y i n p u t t o t h e c o r t e x . The f i n d i n g s reviewed above, t h e n , s u g g e s t t h a t h a b i t u a t i o n of o r i e n t i n g and i n v e s t i g a t o r y r e s p o n s e s may r e q u i r e more t h a n t h e b u i l d i n g u p o f a s h o r t - t e r m c e n t r a l r e p r e s e n t a t i o n , as shown i n F i g u r e 1; i t may a l s o r e q u i r e p a r a l i m b i c and f r o n t a l r e g i o n s which are a n a t o m i c a l l y l i n k e d w i t h t h e highestvisualcorticalarea (and t h o s e i n o t h e r m o d a l i t i e s ) m e d i a t i n g o b j e c t r e p r e s e n t a t i o n s as d e s c r i b e d below ( J o n e s a n d P o w e l l , 1970). V o l u n t a r y a t t e n t i o n . It was proposed e a r l i e r t h a t n e u r a l m e c h a n l s m s o f v o l u n t a r y a t t e n t i o n a r e added on t o t h e mechanisms c o n t r o l l i n g r e f l e x a t t e n t i o n , a view c o n s i s t e n t w i t h p r e v a i l i n g i d e a s of t h e h i e r a r c h i c a l o r g a n i z a t i o n of t h e c e n t r a l n e r v o u s system. According t o t h e view p r e s e n t e d h e r e , a common f e a t u r e of t h e n e u r a l mechanisms u n d e r l y i n g t h e v a r i o u s m a n i f e s t a t i o n s of v o l u n t a r y a t t e n t i o n is t h e a c t i v a t i o n of c e n t r a l r e p r e s e n t a t i o n s of s t i m u l i t o which a t t e n t i o n is drawn, which i n t u r n f a c i l i t a t e t h e same a l e r t i n g s y s t e m s i n v o l v e d i n r e f l e x a t t e n t i o n . According t o t h i s view, one would e x p e c t t h a t t h e h i g h e s t l e v e l s of t h e sensory-specific association cortex play a c r u c i a l r o l e i n voluntary a t t e n t i o n , f o r t h e s e c o r t i c a l a r e a s a p p e a r t o b e t h e s t o r a g e s i t e s of c e n t r a l r e p r e s e n t a t i o n s of f a m i l i a r o b j e c t s , a t l e a s t i n t h e v i s u a l m o d a l i t y . There is a good d e a l of e v i d e n c e from b e h a v i o r a l - a b l a t i o n (Mishkin, 1982) and e l e c t r o p h y s i o l o g i c a l s t u d i e s ( G r o s s , B r u c e , Desimone, Fleming & G a t t a s s , 1981) t h a t i n f e r i o r temporal c o r t e x ( s e e F i g u r e 6 ) i s t h e h i g h e s t c o r t i c a l l e v e l a t which v i s u a l i n f o r m a t i o n is p r o c e s s e d i n t h e p r i m a t e v i s u a l s y s t e m and is t h e s i t e o f c e n t r a l v i s u a l r e p r e s e n t a t i o n s . It is l i k e l y t h a t t h e o t h e r e x t e r o c e p t i v e s y s t e m s a r e a l s o o r g a n i z e d a s a s e r i e s of p r o c e s s i n g s t a g e s w i t h r e p r e s e n t a t i o n s of meaningful s t i m u l i s t o r e d i n c i r c u i t s a t t h e h i g h e s t l e v e l of s e n s o r y - s p e c i f i c p r o c e s s i n g .

ATORY GATORI NSES

Figure 6 Neuralrnechanismsofvoluntaryattention. See t e x t f o r e x p l a n a t i o n .


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According to the model of voluntary attention presented earlier (see Figure 2 ) , central representations receive peripheral inputs from sensory representations and are activated centrally by plans or instructions; in turn, central representations activate sensory representations ( s o as to enhance the perceptual salience of stimuli and produce mental images), as well as the novelty-salience mechanism. These features of the model of voluntary attention are consistent with the results of anatomical and neurobehavioral studies of the visual system. Inferior temporal cortex, the visual cortical area that is implicated in visual memory, is directly connected with several brain structures that mediate the functions included in the model (see Figure 6). Inferior temporal cortex receives projections from prefrontal cortex (Kuypers etal., 1965; Pandya&Kuypers, 1969; Jones & Powell, 1970; Umitsu h Iwai, 1981), which is implicated in the control of central sets (Mishkin, 1964) and planning of the temporal ordering of behavior (Pribram et al., 1964). Furthermore, Luria (1966) has presented evidence that patients with frontal lobe lesions are impaired inutilizing verbal instructions to control learned motor responses. Inferior temporal cortex also projects to V4 (Kuypers et al., 1965). a higher-level, retinotopically-organized visual cortical area that is part of the prestriate complex and fromwhich it receives dense projections (Desimone, Fleming & Gross, 1980). Anatomical and physiological evidence suggest that V4, together with other prestriate visual areas with which it is interconnected, provide the structure for representations of visual inputs (Van Essen, Maunsell & Bixby, 1981). Thus, via its interconnectionswithV4 inferior temporal cortex receives visual inputs so that recognition can take place and so that it can modulate sensory representations in order to increase their perceptual salience or to provide visual mental images (see Figure 6). In addition, inferior temporal cortex also sends projectionsto the deeper layers of the superior colliculus (Whitlock & Nauta, 1956; Kuypers & Lawrence, 1967; Fries, 1984); via this route it can indirectly activate search mechanisms and the alerting systems withwhich the superior colliculusis connected. Implications of the Present Analysis for Neglect and Other Cognitive Disorders The available evidence with regard to associations and dissociations between neglect and other cognitive disorders will be examined in this section in order to determine whether they are consistent with the analysis presented here. In addition, predictions will be made of expected relationships betweenneglect and other cognitive disorders on the basisof the presentanalysis. This analysis draws a fundamental distinction between reflex and voluntary attention. It assumes that voluntary attention involves stored representations and instructions mediated by neural structures that are located at higher cortical levels and that are superimposed on the mechanisms of reflex attention. Thus, according to this view, cortical lesions destroying structures contributing to voluntary attention may leave mechanisms of reflex attention intact or functioning at a reduced level, so that attention may be still directed to novel salient stimuli. Tests of hemispatial neglect that are commonly used, both in the clinic and experimentally, assess voluntary attention; that is, they all involve tasks including instructions to reporta stimulus or performsome action, suchas copying a drawing or bisecting a line. It is difficult to observe and quantify manifestation of reflex attention such as orienting responses, in part because they habituate rapidly. However, it might be possible to evaluate reflex attention in patients with neglect indirectly, by testing

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the distracting effects of novel stimuli on their performance in cognitive or sensory-motortasks. Whereas hemispatial neglect appears after unilateral lesions involving a variety of brain structures,its associationwithdamage to the inferior parietal lobule (usually on the right side) is by far the one most frequently and consistently encountered (Heilman et al., 1985). As mentioned previously, damage to this cortical region is frequently associated with visuospatial disturbances and disorientation in space. Although the relationship between these impairments and hemispatial neglect is not obligatory (e.g., Ratcliff, 1979; Perenin and Vighetto, 19831, its frequent occurrence suggests a close linkage between spatial attention and spatial perception, perhaps analagous to the one posited earlier between object recognition and attention directed to objects. Furthermore, both spatially-directed attention and spatial perception are both functionally associated with exploratoryandinvestigatorymovements, which also depend upon the same cortical region. The anatomical findings reviewed previously also suggest a close relationship between brainstem mechanisms of spatially-directed attention and the cortical areas involved in visuospatial perception: The paralaminar thalamic nuclei, which receive a direct input from the deeper tectal layers, project selectively to the cortical polysensoryareas, includingthose in the inferiorparietallobe. The failure of some studies to find spatial attentional disorders accompanyingvisuospatial deficits o r d e f i c i t s i n v i s u a l l y - g u i d e d reaching (optic ataxia) may be due, at least in part, to differences in test sensitivity. Whereas most tests of hemispatial neglect (such as line cancellation or copying a drawing) are designed to detect rather gross disturbances and present no problems for normal subjects, many tests of visuospatial disturbances are more difficult for normal subjects, who may commit errors in performing them. Furthermore, it is possible that lesions involving the inferior parietal lobule might initially disrupt the functions of other structures, such as other cortical polysensory areas or the superior colliculus,which are implicated in spatial attention andwith which the inferior parietal region is interconnected. With the passage of time, however, spatial attention may r e t u r n t o n o r m a 1 , w h e r e a s v i s u o s p a t i a l functions may not, because of their dependence on the integrity of the inferiorparietallobe. On the basis of the present analysis of the voluntary attention, one would predict that disorders of different aspects of voluntary attention are dissociable by selective lesions. According to the model presented here, one set of disorders of voluntary attention could arise because stored representations of stimuli are no longer activated centrally by plans or instructions. Consequently, detection and search for these stimuli would be no longer under the control of plans orinstructions. Abreakdownin the central activation of a stored representation could be due either to a disconnection of the stored representation from neural structures generating plans or instructions or to direct damage to the latter neural structures. As mentioned previously, the frontal lobes appear to becrucial for planning and for executing aask-related responses according to verbal instructions. Thus, one might expect that frontal lesions or lesions disconnecting the frontal and temporal lobes would disrupt search and instruction-based detection or imagery, without necessarily producing neglect. One would also expect that the perceptual phenomenon of target "pop-out" and the enhanced recognition of search targets, which according to the model are dependent upon central activation of stored representations, would also be impaired by the same lesions. The model also predicts that lesions resulting in a modality-specific recognition loss


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(i.e., a n a s s o c i a t i v e v i s u a l a g n o s i a ) , which d e s t r o y t h e c e n t r a l r e p r e s e n t a t i o n s of f a m i l i a r o b j e c t s , would a l s o i m p a i r t h e p e r c e p t u a l phenomena (pop-out and enhanced t a r g e t r e c o g n i t i o n , enhanced s a l i e n c e of s t i m u l i w i t h i n t r i n s i c o r remote v a l u e ) t h a t accompany s t i m u l i w i t h "attention-value". One might a l s o e x p e c t t h a t s u b c o r t i c a l l e s i o n s d i s c o n n e c t i n g t h e t e m p o r a l l o b e s from t e c t o - t e g m e n t a l s t r u c t u r e s would impair search f o r a t a r g e t without necessarily a l t e r i n g s p a t i a l a t t e n t i o n evaluatedbystandardtestsofneglect. The model a l s o makes a n o t h e r and r a t h e r d i f f e r e n t k i n d of p r e d i c t i o n , c o n c e r n i n g t h e a c t i v i t y of n e u r o n s i n i n f e r i o r t e m p o r a l c o r t e x , t h e c o r t i c a l r e g i o n which i n monkeys a p p e a r s t o be t h e s i t e of s t o r e d v i s u a l r e p r e s e n t a t i o n s . I t would be e x p e c t e d t h a t i n f e r i o r t e m p o r a l n e u r o n s s e l e c t i v e l y r e s p o n d i n g t o complex s t i m u l i ( s u c h a s a p a r t i c u l a r o b j e c t o r a f a c e ) would a l s o show enhanced d i s c h a r g e s when t h e monkey s e a r c h e s f o r t h e stimulus,evenwhenitisnotinthevisual field. According t o t h e model p r e s e n t e d h e r e , l o s s e s of imagery r e s u l t i n g from p a r t i c u l a r b r a i n l e s i o n s would be a s s o c i a t e d i n a n o b l i g a t o r y manner w i t h l o s s e s of some f u n c t i o n s b u t n o t o t h e r s . Thus, one would e x p e c t t h a t m o d a l i t y - s p e c i f i c r e c o g n i t i o n l o s s e s f o r p a r t i c u l a r s t i m u l i (e.g., f a c e s , p h o t o g r a p h s o f p a r t i c u 1 a r o b j e c t s ) w o u l d beaccompaniedbyselectivelosses i n t h e a b i l i t y t o r e p o r t images of t h e same s t i m u l i , o r p e r f o r m t a s k s t h a t a r e assumed t o r e q u i r e images of t h e s e s t i m u l i (draw o r d e s c r i b e v e r b a l l y from memory). Two r e c e n t r e v i e w s of t h e l i t e r a t u r e on imagery impairments ( F a r a h , 1984; L e v i n e , Warach & F a r a h , 1985) have r e p o r t e d a h i g h i n c i d e n c e of p a t i e n t s showing imagery l o s s f o r v i s u a l l y p r e s e n t e d material who a l s o are s e l e c t i v e l y i m p a i r e d i n r e c o g n i z i n g t h e s a m e m a t e r i a l . I n c o n t r a s t t o t h i s c l o s e a s s o c i a t i o n of i m a g e r y a n d r e c o g n i t i o n l o s s , t h e r e are many r e p o r t s of b r a i n damage r e s u l t i n g i n imagery loss f o r f a m i l i a r v i s u a l s t i m u l i i n t h e a b s e n c e of o b v i o u s h e m i s p a t i a l n e g l e c t ( e . g . , B a s s o , B i s i a c h & L u z z a t t i , 1980). The model p r e s e n t e d h e r e i s c o n s i s t e n t w i t h t h i s k i n d o f d i s s o c i a t i o n , f o r i t d i s t i n g u i s h e s between mechanisms u n d e r l y i n g v i s u o s p a t i a l and s p a t i a l a t t e n t i o n f u n c t i o n s o n t h e one hand and mechanisms of o b j e c t r e c o g n i t i o n , imagery and o b j e c t - d i r e c t e d a t t e n t i o n on t h e o t h e r hand. I n c a s e s of imagery l o s s s u c h as t h e p a t i e n t r e p o r t e d by Basso e t a l . ( 1 9 8 0 ) , i n whom n e g l e c t was a b s e n t , t h e i n v e s t i g a t o r s e v a l u a t e d p r e d o m i n a n t l y imagery of p a r t i c u l a r o b j e c t s . I n c o n t r a s t , imagery based upon body-centered r e l a t i o n s of o b j e c t s o r s p a t i a l r e l a t i o n s between o b j e c t s , would be e x p e c t e d t o accompany n e g l e c t s i n c e i t i s assumed t h a t v i s u o s p a t i a l and s p a t i a l a t t e n t i o n a l f u n c t i o n s a r e c l o s e l y r e l a t e d . Evidence s u p p o r t i n g t h i s view h a s been p r o v i d e d by B i s i a c h , C a p i t a n i , L u z z a t t i 6 P e r a n i ( 1 9 8 1 ) . who r e p o r t e d t h a t p a t i e n t s w i t h l e f t hemineglect due t o right-sided l e s i o n s , u n l i k e those without neglect a f t e r r i g h t - s i d e d l e s i o n s , were i m p a i r e d i n r e p o r t i n g t h e l e f t s i d e of m e n t a l images of f a m i l i a r p l a c e s . I n t h i s c o n t e x t , t h e " l e f t s i d e " i s d e f i n e d i n r e f e r e n c e t o imagined body o r i e n t a t i o n . However, t h e r e are a number of r e p o r t s of d e f i c i t s i n t a s k s r e q u i r i n g "dynamic imagery", t h a t i s , t h e m e n t a l m a n i p u l a t i o n of imagery, i n t h e a b s e n c e o f c l e a r h e m i s p a t i a l neglect. Patients with right posterior lesions a r e deficient i n tasks demanding e s t i m a t i o n of t h e p o s i t i o n of c i t i e s on a n imagined map (Morrow, R a t c l i f f & J o h n s t o n , 1985). "mental r o t a t i o n " of imagined o b j e c t s ( B u t t e r s , B a r t o n h Brody, 1970; Warrington & T a y l o r , 1973; R a t c l i f f , 19791, "mental d i s p l a c e m e n t " of ego-centered v i e w p o i n t ( B u t t e r s e t a l . , 1970) and m e n t a l r e c o n s t r u c t i o n of t h e s p a t i a l f e a t u r e s of a v i s u a l d i s p l a y (Ogden, 1985); t h e s e d e f i c i t s o c c u r r e d i n t h e a b s e n c e of n e g l e c t i n any o r some o f t h e p a t i e n t s t e s t e d . Oneway t o r e s o l v e t h e d i s c r e p a n c y b e t w e e n t h e p r e d i c t i o n s of t h e model and t h e s e r e p o r t s of d i s s o c i a t i o n between dynamic imagery and

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neglect is by appealing to the same factors previously suggested to explain the dissociation between neglect and visuospatial disorders - differences in task difficulty, and the possibly greater distribution of spatial attention mechanisms compared to that of visuospatial mechanisms. The argument based on differences in task difficulty appears to be even more cogent in this case, for in many of the imagerymanipulationtasks employed in the studies cited above, control subjects achieve less than perfect scores. Alternatively, it is possible that a subtle form of internally-directed spatial attention required in imagery manipulation tasks is disturbed in patients who are impaired in imagery manipulation tasks and that this disturbance may be a factor in their deficient performance on these tasks. According to this view, then, one might expect that patients with right-sided lesions, who are impaired in imagery-manipulation tasks but who do not show overt neglect, would be more severely impaired if they were instructed to perform imagery manipulation on the left side of imagined space thanon the right side. SurmaryandConclueions

The analysis of neglect presented here differs from others (e.g., Heilmanet al., 1985; Kinsbourne, 1970; B i s i a c h , L u z z a t t i & P e r a n i , 1979)in that it is based both upon a distinction between reflex and voluntary attention and upon distinctions between different forms of voluntary attention, in all of which, it is suggested, central activation of a stored representation is a common factor. The model based on these distinctions is consistent with structural, physiological and neurobehavioral findings. Furthermore, an initial test of one prediction made by the model (selective recognition enhancement of search targets) yielded results consistent with it. T h e m o d e l d e a l s w i t h v o l u n t a r y attention in terms of concepts, suchas sensory representations, stored representations and plans, that are accepted concepts in cognitive psychology and may be mediated by distinct neural mechanisms. Thus, it has an advantage over models that attempt to explain attention in terms of hypothetical processes such as a "scanner", forwhich there is no known physiological mechanism. Furthermore, bymaking cerebral activation of representations the crucial factor in voluntary attention, higher-level influences acting on sensory and cognitive processes "from above" are restored to the position they formally had in earlier theoretical formulations in psychology, which used terms such as "determining tendency'' and Aufgabe (the mentally represented task) to try to account for the selective, goal-directed aspect of cognitive task performance (Woodworth, 1938). With regard to the puzzling phenomenon of neglect itself, the most general conclusions suggested by the present analysis are that neglect may take various forms and each may result from disturbance of a particular process. Thus, hemispatial neglect may be just one of several forms of neglect, including disturbances in directing attention to objects or images, that only become apparent when patients with particular selective lesions are tested under certain conditions. The question whether these predicted attentional deficits are demonstrable can only be answered by future neuropsychologicalinvestigations. R e f erencea

Baleydier, C. 6 Mauguisre, F. Anatomical evidence for medial pulvinar connections with posterior cingulate cortex, the retrosplenial area


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Woods, J.W. Behavior of chronic decerebrate rats. Journal of Neurophysiology,1 9 6 4 , 7 , 635-644. Woodworth,R.S.ExperimentalPsychology.NewYork:Holt, 1938, chapterXXX. Wurtz. R.H. & Mohler. C.W. Organization of monkey suuerior colliculus: enhanced visual response of superficial layer cells. Journal of Neurophysiology,1 9 7 6 , 2 , 745-762. Yingling, C.D. & Skinner, J.E. Gating of thalamic input to cerebral cortex by nucleus reticularis thalami. Progress in clinicalneurophysiology,

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Footnote 1. Two recent experiments in which the cortex of the inferior parietal lobule was selectively lesioned in monkeys failed to find evidence of hemispatial neglect (Lynch & McLaren, 1984; Watson, Valenstein, Day 6 Heilman, 1985) reported in earlier studies. Thus, it is likely that the inferior parietal cortex, unlike other cortical areas receiving projections from the paralaminar nuclei, may not be involved in spatially directed attention.


Neurophysiological and Neuropsychological Aspects of Spatial Neglect, M.Jeannerod (editor) 0 Elsevier Science Publishers B.V. (North,Holland), 1987

25

UNILATERAL ATTENTION DEFICITS AND AEtiISPRERIC ASYMWTUIES IN THE CONTROL OF ATTeNTION E r i c A . Roy, P a t r i c i a Reuter-Lorenz, Louise G. Roy, S h e r r i e Copland and M o r r i s Moscovitch

Hemispheric d i f f e r e n c e s i n t h e c o n t r o l of a t t e n t i o n a r e t h e f o c u s of t h i s c h a p t e r . D i f f e r e n t i a l h e m i s p h e r i c involvement i n t h e c o n t r o l of a t t e n t i o n may be r e f l e c t e d i n t h e i n c r e a s e d i n c i d e n c e and s e v e r i t y of hemi-inattention associated with right hemisphere damage. C l u e s t o t h e b a s i s of h e m i s p h e r i c asymmetries i n t h e c o n t r o l of a t t e n t i o n a r e , t h e n , sought through c o n s i d e r i n g how components of a t t e n t i o n may be a f f e c t e d i n t h e s e hemia t t e n t i o n a l d e f i c i t s . A l e r t i n g o r a r o u s a 1 , o r i e n t i n g and c a p a c i t y components of a t t e n t i o n a r e e a c h c o n s i d e r e d t h r o u g h r e v i e w i n g workdoneinourlaboratoryandbyothers.

D e f i c i t s i n a t t e n t i o n have been a f o c u s of studyinneuropsychologyfor many y e a r s ( e . g . , Heilman, W a t s o n & V a l e n s t e i n , 1985;Mesulam,1985).Oneof t h e more p u z z l i n g and well-known a t t e n t i o n d e f i c i t s i s u n i l a t e r a l n e g l e c t o r h e m i - i n a t t e n t i o n , a d i s o r d e r i n which p a t i e n t s a p p e a r unaware of and f a i l t o respond t o s t i m u l a t i o n o c c u r i n g c o n t r a l a t e r a l t o t h e damaged hemisphere. L e f t n e g l e c t a s s o c i a t e d w i t h r i g h t hemisphere damage t e n d s t o be more common and s e v e r e t h a n rightneglectassociatedwithlefthemisphere damage. T h i s d i f f e r e n c e i n t h e i n c i d e n c e of l e f t v e r s u s r i g h t n e g l e c t may r e f l e c t d i f f e r e n t i a l h e m i s p h e r i c involvement i n t h e c o n t r o l of a t t e n t i o n . C l u e s t o t h e b a s i s of t h i s d i f f e r e n c e between t h e hemispheres may emerge t h r o u g h c o n s i d e r i n g how t h e components of a t t e n t i o n c o n t r i b u t e t o t h e v a r i e d m a n i f e s t a t i o n s of h e m i - a t t e n t i o n a l d e f i c i t s : a d e f i c i t i n a l e r t i n g o r a r o u s a l (Heilman & Watson, 1 9 7 7 ) , a d e f i c i t i n o r i e n t i n g (Kinsbourne, 1977; P o s n e r , Cohen & R a f a l , 1982), o r i n d i r e c t e d a t t e n t i o n (Mesulam, 1981). T h i s c h a p t e r w i l l c o n s i d e r h e m i s p h e r i c d i f f e r e n c e s i n t h e c o n t r o l of a t t e n t i o n i n view of t h e s e d i f f e r e n t a t t e n t i o n a l p r o c e s s e s . A b r i e f overview of t h e d i s t i n c t i o n s between a r o u s a l / a c t i v a t i o n , c a p a c i t y , and t h e s e l e c t i o n a s p e c t s of a t t e n t i o n b e g i n s t h e d i s c u s s i o n . Evidence c o n c e r n i n g p o s s i b l e h e m i s p h e r i c asymmetries i n t h e s e p r o c e s s e s i s t h e n c o n s i d e r e d t h r o u g h r e v i e w i n g work done i n o u r l a b o r a t o r y and by o t h e r s . S t u d i e s on normal s u b j e c t s and p a t i e n t s w i t h l a t e r a l i z e d b r a i n damage, w i t h and w i t h o u t n e g l e c t , a r e c o n s i d e r e d i n an e f f o r t t o u n d e r s t a n d t h e c o n t r i b u t i o n s of t h e r i g h t and l e f t hemisphere t o v a r i o u s a t t e n t i o n p r o c e s s e s a n d t h e roleoftheseprocessesinneglect. Componentsof Attention I t has l o n g been r e c o g n i z e d t h a t a t t e n t i o n comes i n many v a r i e t i e s (James, 1890). Three b a s i c a s p e c t s of a t t e n t i o n have been d e s c r i b e d o v e r t h e p a s t t w o d e c a d e s . These comprise a r o u s a l , c a p a c i t y , and s e l e c t i o n ( P o s n e r b B o i e s , 1971). Arousal o r a l e r t n e s s i s t h o u g h t t o be c l o s e l y r e l a t e d t o t h e

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E. A. Roy et al.

u n d e r l y i n g l e v e l of p h y s i o l o g i c a l a c t i v a t i o n (Duffy, 1957). A r o u s a l i s r e f l e c t e d i n performance e f f i c i e n c y o r t h e r e a d i n e s s t o t a k e i n and respond t o i n f o r m a t i o n i n t h e environment ( E a s t e r b r o o k , 1959; Kahneman, 1970). V i g i l a n c e t a s k s which r e q u i r e p e r i o d s of s u s t a i n e d a t t e n t i o n h a v e t y p i c a l l y beenused t o study the influenceof a l e r t n e s s oninformationprocessing.The e f f e c t s of warning s i g n a l s on r e a c t i o n time and a c c u r a c y have a l s o been s t u d i e d t o d e t e r m i n e t h e r o l e of p h a s i c changes i n a r o u s a l o n p e r c e p t u a l and response r e a d i n e s s (Posner & B o i e s , 1971). A v a r i a t i o n of t h e s e r e a c t i o n time e x p e r i m e n t s i n v o l v e s p r e s e n t i n g warning s i g n a l s t o one o r t h e o t h e r v i s u a l h e m i f i e l d (hemisphere) (e.g., Heilman & Van den A b e l l , 1979) o r p r e s e n t i n g t h e r e a c t i o n s i g n a l t o one o r t h e o t h e r v i s u a l h e m i f i e l d s (e.g., B e r l u c c h i , 1978), and o b s e r v i n g d i f f e r e n c e s in r e a c t i o n times between t h e two hands o r between s i g n a l s p r e s e n t e d t o t h e two h e m i f i e l d s . These comparisons e n a b l e a s t u d y of h e m i s p h e r i c asymmetries in r e s p o n s e preparation. The n o t i o n of c a p a c i t y o r r e s o u r c e s r e f e r s t o t h e a l l o c a t i o n of m e n t a l energy t o a t a s k which, i n t u r n , i n f l u e n c e s t h e q u a l i t y of performance (Navon&Gopher, 1 9 7 9 ; W i c k e n s , 1 9 8 4 ) . S i n c e t h e p o o l o f a v a i l a b l e r e s o u r c e s is f i n i t e , t h e a b i l i t y t o perform numerous t a s k s a t once i s l i m i t e d . The d e f i c i t i n performance i n c u r r e d by doing two t a s k s s i m u l t a n e o u s l y r a t h e r t h a n s e p a r a t e l y may r e f l e c t t h e c a p a c i t y r e q u i r e m e n t s of a g i v e n t a s k . C a p a c i t y r e q u i r e m e n t s can v a r y due t o p r a c t i c e and as a r e s u l t of t h e n a t u r e anddifficultyofatask. The d i s t i n c t i o n between a u t o m a t i c and c o n t r o l l e d , o r e f f o r t f u l , p r o c e s s i n g h a s been i m p o r t a n t i n a c c o u n t i n g f o r t h e d e c r e a s e d need f o r a t t e n t i o n a l involvement t h a t o c c u r s w i t h e x t e n d e d p r a c t i c e ( S c h n e i d e r & S h i f f r i n , 1976; S c h n e i d e r , D u m a i s & S h i f f r i n , 1984). The t y p e of p r o c e s s i n g a s s o c i a t e d w i t h seemingly e f f o r t l e s s , w e l l - p r a c t i c e d b e h a v i o u r s h a s been c a l l e d a u t o m a t i c . These b e h a v i o u r s can o c c u r i n v o l u n t a r i l y w i t h l i t t l e o r no c o n s c i o u s i n t e r v e n t i o n and r e q u i r e minimal r e s o u r c e s . On t h e o t h e r h a n d , requires r e s o u r c e s and i s controlled processing is e f f o r t f u l , subject-regulated. The s e l e c t i v e p r o p e r t y of a t t e n t i o n is t h a t which d e t e r m i n e s what i n f o r m a t i o n w i l l be p r o c e s s e d r e l a t i v e t o a l l t h e s o u r c e s p r e s e n t . S e l e c t i v e a t t e n t i o n p r o v i d e s a means f o r c h o i c e t o be e x e r c i s e d r e g a r d i n g s e n s o r y e x p e r i e n c e . The s e l e c t i o n of one s t i m u l u s from among many can b e done on t h e b a s i s of any of a number of s t i m u l u s a t t r i b u t e s , s u c h a s c o l o u r , s i z e , and l o c a t i o n (Treisman, 1969; Duncan, 1980). I n t h e s t u d y of u n i l a t e r a l n e g l e c t , t h e p r o c e s s of s e l e c t i n g on t h e b a s i s of s p a t i a l l o c a t i o n h a s been of most i n t e r e s t (Kinsbourne, 1970a,b, 1974; P o s n e r e t a l . , 1982). O r i e n t i n g towards t h e r e l e v a n t l o c a t i o n i s c e n t r a l t o s p a t i a l s e l e c t i o n . O r i e n t i n g may i n v o l v e o v e r t movements of t h e e y e s and head o r j u s t a c o v e r t s h i f t o f a t t e n t i o n ( P o s n e r , 1978). I n t h e following s e c t i o n , we consider evidence f o r hemispheric asymmetries i n e a c h of t h e s e p r o c e s s e s : a r o u s a l / a c t i v a t i o n , c a p a c i t y , and s e l e c t i o n . We f i r s t d i s c u s s a r o u s a l / a c t i v a t i o n p r o c e s s e s . Thenwe c o n s i d e r c a p a c i t y and t h e s p a t i a l a l l o c a t i o n of a t t e n t i o n i n t h e c o n t e x t of v i s u a l s e a r c h . F i n a l l y , we examine work on t h e o r i e n t i n g of a t t e n t i o n , a p r o c e s s important i n s e l e c t i v e a t t e n t i o n ;

Arousal and Activation Heilman and h i s c o l l e a g u e s (Bowers & Heilman, 1980; Heilman & Van den A b e l l , 1979, 1980) have s u g g e s t e d t h a t t h e r e may be d i f f e r e n c e s between t h e hemispheres i n a r o u s a l / a c t i v a t i o n p r o c e s s e s . They p r o p o s e t h a t t h e r i g h t hemisphere is dominant f o r a r o u s a l / a c t i v a t i o n p r o c e s s e s and is c a p a b l e of a c t i v a t i n g ( p r e p a r i n g ) r e s p o n s e s f o r b o t h hands. The l e f t hemisphere,

Unilateral attention deficits and hemispheric asymmetries

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however, i s c a p a b l e of a c t i v a t i n g r e s p o n s e s f o r t h e c o n t r a l a t e r a l r i g h t hand only. I n s t u d i e s by Heilman used t o s u p p o r t t h i s n o t i o n , s u b j e c t s were r e q u i r e d t o r e l e a s e a r e s p o n s e key upon t h e appearance of t h e r e a c t i o n s i g n a l which was preceded by a v i s u a l warning s i g n a l p r e s e n t e d t o one hemisphere o r t h e o t h e r . T h e m a i n f o c u s of t h e s e s t u d i e s w a s on t h e r e l a t i v e e f f e c t s of p r e s e n t i n g t h e warning s i g n a l t o t h e r i g h t o r l e f t hemisphere on r e a c t i o n t i m e . The p r e d i c t i o n was t h a t r e a c t i o n time should b e f a s t e r when t h e warning s i g n a l i s d i r e c t e d t o t h e r i g h t hemisphere t h a n t o t h e l e f t hemisphere s i n c e t h e r i g h t hemisphere was thought t o e n j o y a n a d v a n t a g e i n arousal/activation processes. The r e s u l t s from t h e s e s t u d i e s g e n e r a l l y s u p p o r t e d t h i s p r e d i c t i o n i n t h a t f a s t e r r e a c t i o n t i m e s were observed f o l l o w i n g warning s i g n a l s p r e s e n t e d t o t h e r i g h t hemisphere. I n a d d i t i o n , i t was found t h a t warning s i g n a l s p r e s e n t e d t o t h e r i g h t hemisphere produced r e d u c t i o n s i n r e a c t i o n time f o r b o t h hands r e l a t i v e t o a no warning s i g n a l c o n d i t i o n , w h i l e t h o s e p r e s e n t e d t o t h e l e f t hemisphere reduced r e a c t i o n t i m e s o n l y i n t h e c o n t r a l a t e r a l ( r i g h t ) hand. S t u d i e s i n o u r l a b o r a t o r y (Copland, Note 1) a r e f u r t h e r examining Heilman's p r e d i c t i o n s . A l l t h r e e e x p e r i m e n t s t o b e d i s c u s s e d below i n v o l v e d a t a s k s i m i l a r t o Heilman's i n which s u b j e c t s were r e q u i r e d t o d e p r e s s a r e s p o n s e key upon t h e a p p e a r a n c e of a r e a c t i o n s i g n a l . E a c h t r i a l b e g a n w i t h t h e a p p e a r a n c e of a f i x a t i o n p o i n t followed 200 msec l a t e r by a warning s i g n a l l a t e r a l i z e d t o one v i s u a l f i e l d . A s i n Heilman's s t u d i e s , t h e r e a c t i o n s i g n a l followed t h e warning s i g n a l a t v a r y i n g time i n t e r v a l s . A l s o , a s i n Heilman's s t u d y , a no-warning s i g n a l c o n d i t i o n w a s i n c l u d e d . I n experiment o n e , t h i s n o w a r n i n g c o n d i t i o n w a s embedded i n t h e w a r n e d t r i a l s , w h i l e i n t h e second e x p e r i m e n t , t h e no-warning c o n d i t i o n was run s e p a r a t e l y . A s i m p l e r e a c t i o n time paradigm was used i n which o n l y one hand responded i n a n y o n e b l o c k o f t r i a l s . O v e r a l l , t h e r e s u l t s of t h e s e f i r s t two e x p e r i m e n t s d i d n o t r e p l i c a t e Heilman's f i n d i n g s , t h a t i s , t h e r e w a s no a d v a n t a g e i n r e a c t i o n time f o r t h e r i g h t hemisphere. P o s s i b l y , t h i s l a c k o f s u p p o r t d e r i v e s f r o m t h e f a c t t h a t a s i m p l e r e a c t i o n time t a s k was used i n t h e s e e x p e r i m e n t s , a paradigmwhere t h e s u b j e c t knows i n advance t h e r e q u i r e d r e s p o n s e . Bowers and Heilman (1980) q u a l i f i e d t h e i r t h e o r y of r i g h t hemisphere dominance f o r a c t i v a t i o n by i n d i c a t i n g t h a t "a r e s p o n s e - l i n k e d d e c i s i o n a l p r o c e s s was n e c e s s a r y f o r i n d u c i n g a c t i v a t i o n asymmetries". T h e r e f o r e , a c h o i c e r e a c t i o n time paradigm i n which t h e s u b j e c t does n o t know whichhand w i l l b e u s e d p r i o r t o t h e r e a c t i o n s i g n a l might be more s e n s i t i v e t o a c t i v a t i o n asymmetries. The t h i r d experiment was d e s i g n e d t o examine d i f f e r e n c e s i n r e a c t i o n time betweenwarning s i g n a l c o n d i t i o n s , i n a c h o i c e r e a c t i o n time paradigm. R e s u l t s of t h i s experiment b a s i c a l l y r e p l i c a t e d t h o s e o f t h e f i r s t two e x p e r i m e n t s . There were no d i f f e r e n c e s i n r e a c t i o n time between t h e lateralizedwarning conditions. I n c o n s i d e r i n g t h e e v e n t s o n e a c h t r i a l , t h e l a c k o f a r i g h t hemisphere e f f e c t of t h e l a t e r a l i z e d w a r n i n g s i g n a l may have r e l a t e d t o t h e a p p e a r a n c e of t h e f i x a t i o n d o t a t t h e b e g i n n i n g of e a c h t r i a l . That i s , on e a c h t r i a 1 . a f i x a t i o n d o t appeared 200msec p r i o r t o t h e l a t e r a l i z e d w a r n i n g s i g n a l . The f i x a t i o n d o t may have a c t e d a s a warning s i g n a l , p o s s i b l y washing o u t t h e p r e d i c t e d r e d u c t i o n i n r e a c t i o n time f o l l o w i n g warning s i g n a l s d i r e c t e d t o t h e r i g h t hemisphere. I f t h e a p p e a r a n c e of t h e f i x a t i o n p o i n t w a s a c t i n g t o a l e r t t h e s u b j e c t , t h e n i t would seem r e a s o n a b l e t o remove i t s a l e r t i n g c h a r a c t e r i s t i c s t h e r e b y e n a b l i n g t h e l a t e r a l i z e d warning s i g n a l s t o be t h e s o l e s o u r c e of a c t i v a t i o n . In a new series of e x p e r i m e n t s , t h e n , s e v e r a l m e t h o d o l o g i c a l changes a r e b e i n g made t o r e d u c e t h i s a l e r t i n g e f f e c t o f t h e f i x a t i o n p o i n t . These changes may permit a c l e a r e r e x a m i n a t i o n of t h e

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a c t i v a t i n g e f f e c t o f t h e l a t e r a l i z e d w a r n i n g s i g n a l s on r e a c t i o n time.

V i s u a l Search Kinsbourne (1970a.b) proposed a model of o r i e n t i n n in which a common . .~ r e g u l a t o r y p r i n c i p l e g o v e r n s h e m i s p h e r i c c o n t r o l of a l l d i r e c t i o n a l o r i e n t i n g behaviours. H e e x p l a i n e d t h a t l i k e head and eye movements, a t t e n t i o n s h i f t s a r e d i r e c t e d t o t h e r i g h t o r l e f t by t h e c o n t r a l a t e r a l hemisphere. I n n e g l e c t , t h e d i r e c t i o n a l tendency of t h e i n t a c t hemisphere dominates when i t can no l o n g e r be opposed by t h e damaged hemisphere. T h e r e f o r e , l e f t n e g l e c t is due t o t h e unopposed r i g h t w a r d a t t e n t i o n a l b i a s of t h e i n t a c t l e f t h e m i s p h e r e , w h e r e a s r i g h t n e g l e c t is a n e x p r e s s i o n o f t h e r i g h t h e m i s p h e r e ' s l e f t w a r d b i a s . Kinsbourne (1974, 1977) h a s a l s o proposed t h a t t h e r i g h t w a r d o r i e n t i n g t e n d e n c y o f t h e l e f t hemisphere is e s s e n t i a l l y s t r o n g e r t h a n t h e l e f t w a r d tendency of t h e r i g h t hemisphere. The b a s i c d i f f e r e n c e i n s t r e n g t h is r e f l e c t e d i n t h e asymmetry i n o v e r t o r i e n t i n g in t h e form of head t u r n i n g found i n i n f a n t s ( ( T u r k e w i t z , G o r d o n d B i r c h , 1965; Coryell.1985)and i n t h e d i f f e r e n t i a l incidenceof r i g h t a n d l e f t neglect. Heilman's group has a l s o o f f e r e d an a c c o u n t of n e g l e c t which h a s c o n s i d e r e d t h e a l l o c a t i o n of a t t e n t i o n i n s p a c e . Theirmulti-facetedtheory a d d r e s s e s many a s p e c t s of n e g l e c t phenomenaand c e n t e r s on t h e i d e a t h a t t h e r i g h t hemisphere is dominant f o r a t t e n t i o n , a l t h o u g h t h e y do n o t r e f e r s p e c i f i c a l l y t o s p a t i a l o r i e n t i n g in t h e s e n s e used b y K i n s b o u r n e (1970a,b; 1974; 1 9 7 7 ) a n d P o s n e r ( 1 9 8 0 ; s e e b e l o w ) . O n e a s p e c t o f t h i s d o m i n a n c e i s t h e a b i l i t y of t h e r i g h t hemisphere t o a t t e n d ( 0 r i e n t ) t o b o t h t h e r i g h t a n d l e f t s i d e s of s p a c e , whereas l e f t hemisphere c o n t r o l is c o n t r a l a t e r a l . T h i s p r o p o s a l is based p r i m a r i l y on t h e f i n d i n g t h a t e l e c t r o e n c e p h a l o g r a p h i c measures (EEG) in normal s u b j e c t s i n d i c a t e d r i g h t hemisphere d e s y n c h r o n y t o e i t h e r r i g h t v i s u a l f i e l d (RVF) o r l e f t v i s u a l f i e l d (LVF) warning s i g n a l s . L e f t hemisphere desynchrony followed o n l y RVF warning e v e n t s (Heilman and VanDenAbell, 1980). B i l a t e r a l c o n t r o l c o u l d be o p e r a t i o n a l i z e d as t h e a b i l i t y t o o r i e n t t o t h e r i g h t and l e f t s i d e s of space. A l t e r n a t i v e l y , i t c o u l d imply t h a t t h e r i g h t hemisphere can a t t e n d t o a b r o a d e r r e g i o n of s p a c e a t a g i v e n p o i n t in time whereas t h e l e f t hemisphere a t t e n d s t o a more r e s t r i c t e d r e g i o n . Heilman e x p l a i n s t h a t b i l a t e r a l a t t e n t i o n of t h e r i g h t hemisphere e n a b l e s i t t o compensate f o r t h e loss of a t t e n t i o n a l c o n t r o l when t h e l e f t c a n n o t p r o v i d e t h e same hemisphere is damaged. The l e f t hemisphere compensation a f t e r r i g h t hemisphere damage. Thus, l e f t n e g l e c t i s more common. K i n s b o u r n e ' s i d e a of a s t r o n g e r r i g h t w a r d t h a n l e f t w a r d o r i e n t i n g tendency p r e d i c t s t h a t r i g h t hemisphere damage would l e a d t o a g r e a t e r d i r e c t i o n a l b i a s . I n t h i s s e c t i o n , r e s e a r c h on h e m i s p h e r i c c o n t r o l of s p a t i a l a t t e n t i o n w i l l b e c o n s i d e r e d in l i g h t of t h e s e h y p o t h e s e s . The r o l e of h e m i s p h e r i c mechanisms i n t h e s p a t i a l a l l o c a t i o n of a t t e n t i o n may be r e f l e c t e d i n t h e s e a r c h b e h a v i o u r of p a t i e n t s w i t h l a t e r a l i z e d l e s i o n s . A number of s t u d i e s have examined t h e a b i l i t y of r i g h t hemisphere and l e f t hemisphere p a t i e n t s t o l o c a t e a t a r g e t a m i d s t an a r r a y of diStKaCtOKS. S e a r c h t i m e and a c c u r a c y a s a f u n c t i o n of t a r g e t l o c a t i o n p r o v i d e a n i n d e x o f s e a r c h e f f i c i e n c y i n t h e r i g h t and l e f t s i d e s o f s p a c e . DeRenzi, F a g l i o n i and S c o t t i (1970) found t h a t , on a v i s u a l s e a r c h t a s k , b o t h r i g h t hemisphere and l e f t hemisphere p a t i e n t s took s l i g h t l y l o n g e r t o f i n d c o n t r a l e s i o n a l t h a n i p s i l e s i o n a l t a r g e t s . On a t a c t i l e s e a r c h t a s k , a similar b u t even s t r o n g e r p a t t e r n e m e r g e d . R i g h t hemisphere p a t i e n t s were more s e v e r e l y i m p a i r e d , o f t e n f a i l i n g t o f i n d t h e t a r g e t w i t h i n t h e a l l o t e d time p e r i o d when i t was i n t h e c o n t r a l e s i o n a l f i e l d . T h e a u t h o r s p o i n t o u t t h a t s e a r c h r e q u i r e s t h e i n t e g r a t e d f u n c t i o n i n g of m o t o r i c , a t t e n t i o n a l , and r e p r e s e n t a t i o n a l p r o c e s s e s . Although t h e y


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c o n c l u d e t h a t t h e multimodal n a t u r e of t h e d e f i c i t a r g u e s f o r a d i s o r d e r i n s p a c e r e p r e s e n t a t i o n , t h e i r f i n d i n g s do n o t r u l e o u t t h e p o s s i b i l i t y of a t t e n t i o n a l impairment. I t i s c o n c e i v a b l e t h a t an o r i e n t i n g b i a s consequent t o b r a i n damage c o u l d c o n s t r a i n n o t o n l y t h e s p a t i a l a l l o c a t i o n of a t t e n t i o n , b u t a l s o t h e d i r e c t i o n a l c o n t r o l o f headandlimbmovements. Although s h i f t s i n a t t e n t i o n and eye movements can b e d i s s o c i a t e d , t y p i c a l l y , t h e p o s i t i o n of a t t e n t i o n and g a z e c o i n c i d e s . Thus, one way t o s t u d y a t t e n t i o n a l o r i e n t i n g i s t o examine eye movements. Chedru, Leblanc and L h e r m i t t e (1973) r e c o r d e d t h e eye movement s o € r i g h t hemisphere and l e f t hemisphere p a t i e n t s u s i n g a v i s u a l s e a r c h t a s k . P a t i e n t s showed an o v e r a l l i n c r e a s e i n s e a r c h time i n r e l a t i o n t o c o n t r o l s , and b o t h r i g h t hemisphere and l e f t hemisphere g r o u p s took l o n g e r t o Eind a t a r g e t p o s i t i o n e d c o n t r a l e s i o n a l l y . T h e r e f o r e , d i s o r d e r e d s e a r c h b e h a v i o u r was common t o b o t h groups. The measure which d i s c r i m i n a t e d among p a t i e n t g r o u p s w a s t h e p e r c e n t a g e of t i m e s p e n t e x p l o r i n g t h e r i g h t v e r s u s l e f t s i d e s of t h e d i s p l a y . Eye movement r e c o r d s r e v e a l e d t h a t t h e r i g h t hemisphere p a t i e n t s s p e n t a g r e a t e r p r o p o r t i o n of t h e s e a r c h time i n t h e r i g h t s i d e of t h e d i s p l a y t h a n i n t h e l e f t . L e f t hemisphere p a t i e n t s d i v i d e d t h e i r time equallybetweenthe twohalves. Both Chgdru e t a l . and DeRenzi e t a l . found i n c r e a s e d s e a r c h time f o r c o n t r a l e s i o n a l t a r g e t s r e g a r d l e s s of t h e l a t e r a l i t y o f t h e l e s i o n . H o w e v e r , p a t i e n t s w i t h r i g h t hemisphere damage seemed t o have t h e i r a t t e n t i o n anchored i n t h e i p s i l e s i o n a l s i d e of s p a c e more so t h a n d i d l e f t hemisphere patients. This suggests t h a t while a s p a t i a l l y s e l e c t i v e d e f i c i t i n s e a r c h i n g b e h a v i o u r may o c c u r a f t e r r i g h t o r l e f t hemisphere damage, t h e d e f i c i t a s s o c i a t e d w i t h r i g h t hemisphere damage may be more s e v e r e and may r e f l e c t a d i f f e r e n t u n d e r l y i n g d i s t u r b a n c e t h a n t h e l e f t hemisphere d e f i c i t . These f i n d i n g s , however, do n o t d i s t i n g u i s h between K i n s b o u r n e ' s andHeilman's theories. Some r e c e n t work i n o u r l a b o r a t o r y (Roy, Note 2 ; Roy & Roy, Note 3 ) h a s u t i l i z e d t h e v i s u a l s e a r c h paradigm t o e v a l u a t e E u r t h e r t h e p r o p o s a l t h a t t h e l e f t hemisphere c o n t r o l s a t t e n t i o n p r i m a r i l y i n t h e r i g h t hemispace, w h i l e t h e r i g h t hemisphere d i r e c t s a t t e n t i o n t o both f i e l d s . I n t h i s t a s k , p a t i e n t s were r e q u i r e d t o i n d i c a t e whether a t a r g e t l e t t e r a p p e a r e d on a t e l e v i s i o n m o n i t o r by moving a s m a l l t o g g l e s w i t c h . T h e t a r g e t a p p e a r e d w i t h a p r o b a b i l i t y of .75, was p r e s e n t e d e q u a l l y o f t e n i n t h e l e f t o r r i g h t hemispace, and appeared a l o n e o r i n c o n c e r t w i t h 17 o r 35 d i s t r a c t o r s . According t o Heilman's p r o p o s a l , t h e p r e d i c t i o n w a s t h a t damage t o t h e r i g h t hemisphere s h o u l d be a s s o c i a t e d w i t h i n c r e a s e d s e a r c h time and d e c r e a s e d a c c u r a c y i n b o t h f i e l d s , w h i l e damage t o t h e l e f t hemisphere s h o u l d i m p a i r s e a r c h time and a c c u r a c y i n t h e r i g h t h e m i s p a c e o n l y . The r e s u l t s f o r s e a r c h time seemed t o s u p p o r t t h e s e p r e d i c t i o n s . R i g h t hemisphere p a t i e n t s d e m o n s t r a t e d no s i g n i f i c a n t d i f f e r e n c e s between t h e h e m i s p a t i a l f i e l d s , w h i l e f o r t h e l e f t hemisphere p a t i e n t s , s e a r c h t i m e w a s s i g n i f i c a n t l y l o n g e r i n t h e c o n t r a l e s i o n a l hemispace. Both g r o u p s exhibited longer search times than t h e controls. The f i n d i n g t h a t r i g h t hemisphere p a t i e n t s e x h i b i t e d no d i f f e r e n c e i n s e a r c h time between h e m i s p a t i a l f i e l d s s u g g e s t s t h a t damage t o t h i s hemisphere d e p r e s s e d speed of v i s u a l s e a r c h u n i f o r m l y a c r o s s b o t h h e m i s p a t i a l f i e l d s . S u c h a p a t t e r n m i g h t b e e x p e c t e d i f t h e r i g h t hemisphere were i n v o l v e d i n b i l a t e r a l a t t e n t i o n a l c o n t r o l . The o b s e r v a t i o n t h a t l e f t hemisphere p a t i e n t s e x h i b i t e d i n c r e a s e d s e a r c h time o n l y i n t h e contralesional ( r i g h t ) hemispatial f i e l d i s consistent with the i d e a t h a t l e f t hemisphere a t t e n t i o n a l c o n t r o l i s p r i m a r i l y c o n t r a l a t e r a l . While o u r s e a r c h time d a t a seem t o p r o v i d e some s u p p o r t f o r t h e n o t i o n t h a t t h e r i g h t hemisphere m a y d i r e c t a t t e n t i o n t o b o t h h e m i s p a t i a l f i e l d s , a

E.A. Roy et al. c a r e f u l c o n s i d e r a t i o n of t h e s e d a t a i n c o n j u n c t i o n w i t h t h o s e f o r a c c u r a c y r a i s e s some c o n c e r n s about t h e e x t e n t of s u p p o r t f o r t h i s n o t i o n . One might p r e d i c t t h a t t h e p a t t e r n f o r a c c u r a c y d a t a s h o u l d conform t o t h a t f o r t h e s e a r c h time d a t a . Such a c o n s i s t e n c y i n p a t t e r n was n o t o b s e r v e d , however. Right hemisphere p a t i e n t s were s i g n i f i c a n t l y l e s s a c u r a t e i n r e s p o n s e t o t a r g e t s i n t h e l e f t h e m i s p a t i a l f i e l d w h i l e a t t h e same time e x h i b i t i n g a b i l a t e r a l i n c r e a s e i n s e a r c h time. F o r l e f t h e m i s p h e r i c p a t i e n t s , a c c u r a c y was e q u i v a l e n t a c r o s s h e m i s p a t i a l f i e l d s , whereas s e a r c h time was s i g n i f i c a n t l y longer for t a r g e t s i n the r i g h t hemispatial f i e l d . The unexpected d i s s o c i a t i o n of s e a r c h t i m e and a c c u r a c y measures c o m p l i c a t e t h e i n t e r p r e t a t i o n of t h e s e r e s u l t s and c a u t i o n s a g a i n s t making a t t r i b u t i o n s of h e m i s p h e r i c dominance o r s u p e r i o r i t y i n t h e c o n t r o l of s e a r c h . The d i s s o c i a t i o n a l s o u n d e r s c o r e s t h e importance of u s i n g s e v e r a l measures of t a s k performance i n o r d e r t o d e t e r m i n e how s e a r c h i s conducted. A s i n p r e v i o u s i n v e s t i g a t i o n s of s e a r c h , b o t h p a t i e n t g r o u p s show some s p a t i a l l y s p e c i f i c d e f i c i t . However, t h e s t y l e s o r s t r a t e g i e s of s e a r c h used by t h e two p a t i e n t g r o u p s seem t o d i f f e r . R i g h t hemisphere p a t i e n t s m a i n t a i n a c o n s t a n t s e a r c h time a c r o s s h e m i s p a t i a l f i e l d s a t t h e c o s t of more e r r o r s i n t h e c o n t r a l e s i o n a l f i e l d . L e f t hemisphere p a t i e n t s , on t h e o t h e r hand, m a i n t a i n accuracy s c o r e s a c r o s s h e m i s p a t i a l f i e l d s a t t h e c o s t of i n c r e a s e d s e a r c h time i n t h e contralesionalhemispatial f i e l d . Damage t o one o r t h e o t h e r hemisphere may c a u s e d i f f e r e n t impairments i n a t t e n t i o n a l c o n t r o l which i n t u r n seem t o i n f l u e n c e t h e way t h e s e a r c h t a s k i s c a r r i e d o u t . These f i n d i n g s do n o t c l e a r l y s u p p o r t p r e d i c t i o n s from Heilman's o r K i n s b o u r n e ' s models. To s t u d y t h e c a p a c i t y demands of v i s u a l s e a r c h , two s e a r c h c o n d i t i o n s were u s e d , a s i n g l e f e a t u r e and a c o n j o i n e d f e a t u r e c o n d i t i o n ( s e e Treisman & Gelade, 1980). I n t h e s i n g l e f e a t u r e c o n d i t i o n , t h e t a r g e t d i f f e r e d from t h e d i s t r a c t o r s on a s i n g l e f e a t u r e ( c o l o u r o r l e t t e r s h a p e ) . I n t h e c o n j o i n e d f e a t u r e c o n d i t i o n , t h e t a r g e t d i f f e r e d from t h e d i s t r a c t o r s on t h e two d i m e n s i o n s , s h a r i n g c o l o u r w i t h some d i s t r a c t o r s and shape w i t h t h e others. C o n s i d e r i n g t h e n o t i o n of c a p a c i t y demands i n t h i s t y p e of v i s u a l s e a r c h t a s k , Treisman and Gelade (1980) have shown t h a t t h e c o n j o i n e d feature condition involves a s e r i a l search while the single feature c o n d i t i o n i n v o l v e s a p a r a l l e l s e a r c h . T h i s d i f f e r e n c e i n t h e n a t u r e of s e a r c h i s r e f l e c t e d i n t h e r e l a t i o n s h i p between s e a r c h time and number of d i s t r a c t o r s . I n t h e s i n g l e f e a t u r e c o n d i t i o n , t h e y found t h a t s e a r c h time d i d n o t i n c r e a s e w i t h number of d i s t r a c t o r s , w h i l e t h e r e was a s i g n i f i c a n t i n c r e a s e i n s e a r c h time w i t h t h e number of d i s t r a c t o r s i n t h e c o n j o i n e d f e a t u r e c o n d i t i o n . One c o u l d i n f e r f r o m t h e s e d i f f e r e n c e s i n t h e s e a r c h time f u n c t i o n , t h a t t h e c o n j o i n e d f e a t u r e c o n d i t i o n i n v o l v e d a more a t t e n t i o n demanding s e a r c h p r o c e s s . To a s s e s s whether t h e r e was any d i f f e r e n c e i n a t t e n t i o n demands of v i s u a l s e a r c h f o l l o w i n g l e f t o r r i g h t hemisphere damage, comparisons of t h e s e a r c h time f u n c t i o n s i n t h e c o n j o i n e d a n d s i n g l e f e a t u r e c o n d i t i o n s were made between t h e two brain-damaged g r o u p s and t h e controls. Examination of t h e d a t a r e v e a l e d a t r e n d f o r l e f t hemisphere p a t i e n t s t o e x h i b i t l o n g e r s e a r c h t i m e s o v e r both f e a t u r e c o n d i t i o n s . T h i s f i n d i n g may be due t o t h e f a c t t h a t v e r b a l s t i m u l i w e r e u s e d i n t h i s t a s k . T h e r e w e r e , however, no d i f f e r e n c e s among t h e groups i n t h e s l o p e s of t h e s e a r c h time f u n c t i o n s i n e i t h e r f e a t u r e c o n d i t i o n . A c l o s e r e x a m i n a t i o n of t h e s e a r c h time f u n c t i o n s i n a l l t h r e e g r o u p s r e v e a l e d a s i g n i f i c a n t i n c r e a s e i n s e a r c h time w i t h number of d i s t r a c t o r s i n b o t h t h e s i n g l e and c o n j o i n e d f e a t u r e c o n d i t i o n s , s u g g e s t i n g t h a t a s e r i a l , a t t e n t i o n - d e m a n d i n g s e a r c h was involvedinbothconditions.


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Taken t o g e t h e r , o u r d a t a s u g g e s t t h a t t h e r e a r e no d i f f e r e n c e s i n t h e e f f e c t s o f v a r y i n g a t t e n t i o n demands o n t h e manner i n which s e a r c h i s c a r r i e d o u t by l e f t and r i g h t hemisphere p a t i e n t s . Damage t o one o r t h e o t h e r hemisphere, t h e n , d o e s n o t a p p e a r t o a l t e r s e l e c t i v e l y t h e r e s o u r c e s available for processinginformation inthistypeofvisua1searchtask.Had damage t o one hemisphere l i m i t e d p r o c e s s i n g r e s o u r c e s , one might have e x p e c t e d a g r e a t e r s l o p e t o t h e s e a r c h time f u n c t i o n p a r t i c u l a r l y i n t h e c o n j o i n e d f e a t u r e c o n d i t i o n i n t h a t brain-damaged group. T h i s f i n d i n g s u g g e s t s t h a t t h e d i f f e r e n c e between p a t i e n t g r o u p s on s e a r c h time and a c c u r a c y measures i s n o t due t o t h e e f f e c t s of l a t e r a l i z e d damage o n r e s o u r c e a v a i l a b i l i t y . O t h e r r e s o u r c e demanding t a s k s , however, need t o be examined b e f o r e one c a n a c c e p t t h i s c o n c l u s i o n w i t h c o n f i d e n c e . The unexpected e v i d e n c e f o r s e r i a l s e a r c h even i n t h e s i n g l e f e a t u r e c o n d i t i o n s u g g e s t s t h a t t h i s t y p e o f a t t e n t i o n - d e m a n d i n g s e a r c h p r o c e s s was employed by a l l t h e p a t i e n t s , e v e n t h e c o n t r o l , non-brain-damaged p a t i e n t s . Such was n o t t h e case i n Treisman and G e l a d e ' s (1980) s t u d y i n t h a t s e r i a l s e a r c h was found o n l y i n t h e c o n j o i n e d c o n d i t i o n . T h i s d i s c r e p a n c y w i t h T r e i s m a n ' s work c o u l d be due t o t h e f a c t t h a t a l l t h e s u b j e c t s used i n t h i s s t u d y were a t l e a s t f o r t y t o f i f t y y e a r s o l d e r t h a n T r e i s m a n ' s s u b j e c t s . P e r h a p s , t h e g e n e r a l slowing a s s o c i a t e d w i t h advancing a g e ( S a l t h o u s e & Somberg, 1982; Smith, 1984) p l a c e s c o n s t r a i n t s o n p a r a l l e l p r o c e s s i n g w h i c h s e l e c t i v e l y a f f e c t s a u t o m a t i c p r o c e s s i n g and f o r c e s t h e u s e of more controlled processing a s reflected i n the s e r i a l s e a r c h p a t t e r n s .

Orienting P o s n e r and h i s c o l l e a g u e s a r e p i o n e e r s i n t h e i n v e s t i g a t i o n of t h e e f f e c t s of l a t e r a l i z e d b r a i n damage on c o v e r t a t t e n t i o n a l s h i f t s . P o s n e r , Walker, F r i e d r i c h and R a f a l (1984) had p a r i e t a l p a t i e n t s perform a d e t e c t i o n t a s k i n which an advance cue i n d i c a t e d which of two l o c a t i o n s would most l i k e l y c o n t a i n a t a r g e t . L i k e n o r m a l s u b j e c t s , p a r i e t a l p a t i e n t s were f a s t e r a t d e t e c t i n g a t a r g e t a t t h e e x p e c t e d l o c a t i o n t h a n a t t h e unexpected l o c a t i o n . T h i s p a t t e r n emerged f o r b o t h t h e i p s i l e s i o n a l and contralesional t a r g e t s , although reaction t i m e t o contralesional t a r g e t s was c o n s i s t e n t l y s l o w e r . T h i s f i n d i n g i n d i c a t e s t h a t t h e s e p a t i e n t s can v o l u n t a r i l y s h i f t t h e i r a t t e n t i o n i n response t o a c u e . The most s t r i k i n g d e f i c i t s emerged when p a t i e n t s were misinformed a b o u t t h e s u b s e q u e n t t a r g e t l o c a t i o n ( i n v a l i d t r i a l s ) . When t h e y e x p e c t e d t h e t a r g e t c o n t r a l e s i o n a l l y and i t o c c u r r e d on t h e o p p o s i t e s i d e , t h e y showed t h e normal i n c r e a s e i n r e a c t i o n time. However, when t h e y moved t h e i r a t t e n t i o n t o t h e i p s i l e s i o n a l o r "good" s i d e and t h e t a r g e t a p p e a r e d o n t h e o p p o s i t e s i d e , i t t o o k t h e p a t i e n t s s i g n i f i c a n t l y l o n g e r t o respond t o t h e t a r g e t . T h i s was t r u e f o r b o t h r i g h t and l e f t p a r i e t a l p a t i e n t s , b u t t h e e f f e c t s were s i g n i f i c a n t l y g r e a t e r f o r r i g h t p a r i e t a l p a t i e n t s . P o s n e r e t a l . proposed t h a t t h e d e f i c i t o n i n v a l i d t r i a l s when t h e t a r g e t i s p r e s e n t e d c o n t r a l e s i o n a l l y i s due t o t h e i n a b i l i t y t o d i s e n g a g e a t t e n t i o n i n o r d e r t o s h i f t c o n t r a l e s i o n a l l y . According t o K i n s b o u r n e ' s a c c o u n t of u n i l a t e r a l n e g l e c t , t h e b i a s of t h e i n t a c t h e m i s p h e r e d o m i n a t e s w h e n i t c a n n o l o n g e r b e opposed by t h e damaged hemisphere. A l s o , t h e r i g h t w a r d o r i e n t i n g b i a s o f t h e l e f t hemisphere i s s t r o n g e r t h a n t h e l e f t w a r d b i a s of t h e r i g h t hemisphere. T h e r e f o r e , p a t i e n t s w i t h r i g h t hemisphere damage s h o u l d have more d i f f i c u l t y d i s e n g a g i n g from t h e r i g h t t h a n l e f t hemisphere p a t i e n t s s h o u l d have d i s e n g a g i n g from t h e l e f t . I n f a c t , t h e f i n d i n g s of P o s n e r e t a l . a r e c o n s i s t e n t w i t h t h i s p r e d i c t i o n . Moving a t t e n t i o n e i t h e r i p s i l e s i o n a l l y o r c o n t r a l e s i o n a l l y d i d n o t seem t o be t h e problem f o r t h e s e p a t i e n t s s i n c e attention s h i f t s i n eitherdirectionwereevidentonvalidtrials.Thus,any d i f f e r e n c e s between r i g h t hemisphere and l e f t hemisphere p a t i e n t s o n t h i s

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t a s k c a n n o t be r e a d i l y accounted f o r by t h e h y p o t h e s i s t h a t t h e r i g h t hemisphere c a n d i r e c t a t t e n t i o n t o e i t h e r s i d e of space. In a s t u d y of p a t i e n t s w i t h r i g h t hemisphere damage, Riddoch and Humphrey (1983) found similar r e s u l t s u s i n g l a t e r a l c u e s o n a l i n e b i s e c t i o n t a s k . P a t i e n t s w i t h l a t e r a l i z e d damage t y p i c a l l y draw t h e i n t e r s e c t towards t h e i p s i l e s i o n a l endpoint r a t h e r than a t t h e midpoint, i n d i c a t i n g t h e i r tendency t o u n d e r e s t i m a t e t h e c o n t r a l e s i o n a l e x t e n t of t h e l i n e . Riddoch and Humphreys p l a c e d s i n g l e l e t t e r c u e s a t e i t h e r t h e r i g h t o r l e f t e n d p o i n t o r b i l a t e r a l l y . P a t i e n t s w i t h l e f t n e g l e c t were asked t o name any cue t h a t t h e y saw and t h e n t o b i s e c t t h e l i n e . I t was found t h a t a u n i l a t e r a l l e f t c u e was c o n s i s t e n t l y named and s i g n i f i c a n t l y reduced t h e amount of n e g l e c t . When t h e c u e s w e r e b i l a t e r a l , s u b j e c t s o f t e n f a i l e d t o name t h e l e f t cue and showed no s i g n i f i c a n t d e c r e a s e i n n e g l e c t r e l a t i v e t o t h e no cue c o n d i t i o n . Neglectwas t h e g r e a t e s t w h e n o n l y t h e r i g h t cuewas present. Three i m p o r t a n t p o i n t s a r e r a i s e d by Riddoch and Humphreys' and P o s n e r ' s r e s u l t s . F i r s t , when c u e d , p a t i e n t s can d e l i b e r a t e l y o r i e n t t h e i r a t t e n t i o n t o b o t h t h e i p s i l e s i o n a l and c o n t r a l e s i o n a l hemispaces. Second, i n t h e p r e s e n c e of a competing i p s i l e s i o n a l s t i m u l u s , t h e tendency f o r c o n t r a l e s i o n a l o r i e n t i n g i s minimized. F u r t h e r m o r e , P o s n e r ' s f i n d i n g s suggest t h a t there i s g r e a t e r d i f f i c u l t y i n disengaging a t t e n t i o n fromthe r i g h t hemispace f o r r i g h t hemisphere p a t i e n t s t h a n d i s e n g a g i n g from t h e l e f t hemispace f o r l e f t hemisphere p a t i e n t s . A p o s s i b l e b a s i s f o r t h e e f f e c t s of l a t e r a l i z e d b r a i n damage on o r i e n t i n g h a s been s u g g e s t e d by o u r own work (Reuter-Lorenz, Note 4 ; Reuter-Lorenz, Moscovitch 6 Kinsbourne, Note 5 ) on t h e h e m i s p h e r i c c o n t r o l of o r i e n t i n g i n normal s u b j e c t s . A t a c h i s t o s c o p i c l i n e b i s e c t i o n t a s k was used t o a s s e s s t h e d i s t r i b u t i o n of a t t e n t i o n i n s p a c e . S u b j e c t s viewed a s e r i e s of b r i e f l y p r e s e n t e d ( l e s s t h a n 120 msec) h o r i z o n t a l l i n e s e a c h of which had a n i n t e r s e c t p o s i t i o n e d a t midpoint o r s l i g h t l y t o t h e l e f t o r r i g h t of c e n t e r . The s u b j e c t ' s t a s k was t o j u d g e whether t h e i n t e r s e c t was l o c a t e d a t t h e midpoint o r t o t h e l e f t o r r i g h t of c e n t e r . The tendency t o u n d e r e s t i m a t e t h e l e f t o r r i g h t e x t e n t s of t h e l i n e was r e f l e c t e d in t h e p a t t e r n o f e r r o r s associatedwithidentifying t h e i n t e r s e c t ' s l o c a t i o n . When l i n e s were p r e s e n t e d u n i l a t e r a l l y , s u b j e c t s c o n s i s t e n t l y u n d e r e s t i m a t e d t h e i p s i l a t e r a l e x t e n t . That i s , when t h e l i n e w a s i n t h e RVF, i t s l e f t e x t e n t was u n d e r e s t i m a t e d , whereas, w h e n i n t h e L V F , t h e r i g h t e x t e n t was u n d e r e s t i m a t e d . T h i s p a t t e r n s u g g e s t s t h a t t h e l e f t hemisphere h a s r i g h t w a r d a t t e n t i o n a l b i a s and t h a t t h e r i g h t hemisphere h a s a l e f t w a r d bias. The same p a t t e r n o f r e s u l t s e m e r g e d i n a f u r t h e r experiment i n w h i c h t h e l i n e s t h e m s e l v e s were p r e s e n t e d f o v e a l l y . The l i n e was f l a n k e d by a box whichwas s l i g h t l y d i s p l a c e d f r o m e i t h e r t h e r i g h t o r 1 e f t e n d p o i n t . O n h a l f t h e t r i a l s , t h e box c o n t a i n e d a d o t a n d o n h a l f , i t d i d n o t . I n o n e c o n d i t i o n , t h e s u b j e c t s were t o l d t o i g n o r e t h e boxes and simply t o r e p o r t where t h e i n t e r s e c t s o c c u r r e d . In a second c o n d i t i o n , t h e y had t o a t t e n d t o t h e u n i l a t e r a l boxes, r e p o r t whether t h e y were empty o r f u l l , and, t h e n , i n d i c a t e t h e i n t e r s e c t p o s i t i o n . S u b j e c t s were i n s t r u c t e d t o keep t h e i r e y e s f i x a t e d c e n t r a l l y . C o n d i t i o n s were blocked s o t h a t t h e box was p r e s e n t e d i n t h e same v i s u a l f i e l d f o r a s e r i e s of t r i a l s . R e g a r d l e s s of whether t h e boxes were a t t e n d e d o r i g n o r e d , t h e y s y s t e m a t i c a l l y b i a s e d a t t e n t i o n . The RVF box produced a r i g h t w a r d b i a s o r r e l a t i v e l e f t n e g l e c t on t h e l i n e b i s e c t i o n t a s k , whereas t h e LVF box produced a l e f t w a r d b i a s o r r i g h t n e g l e c t . These b i a s e s were o p p o s i t e i n d i r e c t i o n b u t e q u i v a l e n t i n magnitude. An i n t e r e s t i n g asymmetry emerged i n f u r t h e r e x p e r i m e n t s when c o n f l i c t i n g o r i e n t i n g demands were produced by t h e viewing c o n d i t i o n s . In


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one s e t of c o n d i t i o n s , s t i m u l i were randomized s o t h a t s u b j e c t s c o u l d n o t a n t i c i p a t e i n which v i s u a l f i e l d t h e box would a p p e a r . I n a n o t h e r , boxes were p r e s e n t e d i n b o t h v i s u a l f i e l d s and s u b j e c t s had t o a t t e n d s e l e c t i v e l y , , t o one o r t h e o t h e r w h i l e m a k i n g t h e b i s e c t i o n judgement. I n both t y p e s of c o n f l i c t s i t u a t i o n , t h e r e were no o v e r a l l d i f f e r e n c e s i n b i s e c t i o n accuracy forRVFversusLVF conditions.However, t h e r i g h t b i a s a s s o c i a t e d w i t h t h e RVF c o n d i t i o n s p r o v e d t o b e r o b u s t , w h e r e a s t h e l e f t w a t - d b i a s a s s o c i a t e d w i t h LVF c o n d i t i o n s was s i g n i f i c a n t l y d i m i n i s h e d i n t h e p r e s e n c e of o r i e n t i n g c o n f l i c t . I n o t h e r words, i n t h e p r e s e n c e of l a t e r a l o r i e n t i n g c o n f l i c t , normal s u b j e c t s showed a s t r o n g e r tendency t o o r i e n t t o t h e r i g h t and n e g l e c t t h e l e f t e x t e n t of t h e l i n e t h a n t o o r i e n t t o t h e l e f t and n e g l e c t t h e r i g h t e x t e n t . T h i s p a t t e r n i s c o n s i s t e n t w i t h K i n s b o u r n e ' s p r o p o s a l t h a t t h e r i g h t w a r d l a t e r a l o r i e n t i n g tendency is s t r o n g e r t h a n t h e l e f t w a r d tendency. F u r t h e r m o r e , t h e s e f i n d i n g s may l e n d s u p p o r t t o one i n t e r p r e t a t i o n of Heilman's n o t i o n of r i g h t hemisphere b i l a t e r a l a t t e n t i o n . D i f f e r e n t i a l o r i e n t i n g s t r e n g t h may u n d e r l i e h e m i s p h e r i c d i f f e r e n c e s i n t h e s p a t i a l a l l o c a t i o n of a t t e n t i o n . A s t r o n g d i r e c t i o n a l b i a s i n a s s o c i a t i o n w i t h l e f t hemisphere c o n t r o l may be r e l a t e d t o a h i g h l y s e l e c t i v e , f o c a l a l l o c a t i o n p o l i c y . A weaker d i r e c t i o n a l o r i e n t i n g b i a s a s s o c i a t e d w i t h t h e r i g h t hemisphere may a l l o w a t t e n t i o n t o be a l l o c a t e d l e s s s e l e c t i v e l y i n s p a c e . A weaker d i r e c t i o n a l b i a s may e n a b l e t h e r i g h t hemisphere t o d i s t r i b u t e a t t e n t i o n o v e r a b r o a d e r s p a t i a l r e g i o n .

Discussion The e v i d e n c e reviewed i n t h i s c h a p t e r s u g g e s t s t h a t t h e r e may be h e m i s p h e r i c d i f f e r e n c e s i n some a s p e c t s of a t t e n t i o n a l c o n t r o l . Our own work w i t h normal s u b j e c t s h a s i n d i c a t e d l i t t l e s u p p o r t f o r t h e p r o p o s a l t h a t t h e r i g h t hemisphere h a s an a d v a n t a g e i n a r o u s a l / a c t i v i t a t i o n p r o c e s s e s . Warning s i g n a l s p r e s e n t e d t o t h e r i g h t hemisphere d i d n o t s e r v e t o d e c r e a s e t h e time t o r e a c t t o t h e r e a c t i o n s i g n a l r e l a t i v e t o warning s i g n a l s p r e s e n t e d t o t h e l e f t hemisphere. The e f f e c t of t h e l a t e r a l i z e d warning s i g n a l s on r e a c t i o n time may have been reduced due t o t h e a l e r t i n g e f f e c t a s s o c i a t e d w i t h t h e a p p e a r a n c e of t h e f i x a t i o n p o i n t . M e t h o d o l o g i c a l changes a r e b e i n g made t o remove t h i s a l e r t i n g e f f e c t s o as t o assess more c l e a r l y t h e e f f e c t o f t h e l a t e r a l i z e d w a r n i n g s i g n a l o n r e a c t i o n time. With r e g a r d t o o r i e n t i n g and s e l e c t i o n p r o c e s s e s , w o r k b y P o s n e r e t a l . (1982, 1984) s u g g e s t s t h a t damage t o t h e p a r i e t a l r e g i o n s of e i t h e r hemispheres l e a d s t o a n impairment i n d i s e n g a g i n g a t t e n t i o n from one location, particularly locations i n the ipsilateralhemispace, i n order t o d i r e c t i t t o a n o t h e r l o c a t i o n . T h i s d i s e n g a g e component of o r i e n t i n g seems t o be more a f f e c t e d by r i g h t p a r i e t a l damage and i s p a r t i c u l a r l y e x e m p l i f i e d i n a t e n d e n c y t o m a i n t a i n o r i e n t a t i o n toward t h e r i g h t . The f i n d i n g s of Reuter-Lorenz e t a l . reviewed above, f i t w e l l w i t h t h i s p a t t e r n . Evidence t h a t e a c h hemisphere d i r e c t s a t t e n t i o n c o n t r a l a t e r a l l y was o b t a i n e d i n normal s u b j e c t s . F u r t h e r m o r e , t h e r i g h t w a r d o r i e n t i n g tendency was found t o be more r o b u s t t h a n t h e l e f t w a r d tendency. These f i n d i n g s s u g g e s t t h a t t h e o r i e n t i n g b e h a v i o r of p a t i e n t s w i t h l a t e r a l i z e d b r a i n damage may r e f l e c t t h e b i a s of t h e i n t a c t hemisphere. As Kinsbourne has proposed, r i g h t hemisphere damage l e a v e s t h e c o n t r a l a t e r a l o r i e n t i n g b i a s of t h e l e f t hemisphere unopposed, whereas l e f t hemisphere damage l e a v e s t h e r i g h t hemisphere unopposed. Thus, t h e i n a b i l i t y t o d i s e n g a g e a t t e n t i o n from t h e i p s i l e s i o n a l focusmay b e d u e t o t h e d o m i n a t i n g i n f l u e n c e of t h e i n t a c t hemisphere. F u r t h e r m o r e , a s t r o n g e r r i g h t w a r d t h a n l e f t w a r d b i a s should l e a d t o g r e a t e r d i f f i c u l t y i n t h e d i s e n g a g e o p e r a t i o n f o r r i g h t t h a n l e f t hemisphere p a t i e n t s , w h i c h i s t h e p a t t e r n f o u n d b y P o s n e r ' s g r o u p . These f i n d i n g s s u g g e s t t h a t t h e h e m i s p h e r e s m a y d i f f e r i n t h e i r c o n t r o l

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of t h e s p a t i a l a l l o c a t i o n of a t t e n t i o n . A weaker l a t e r a l o r i e n t i n g b i a s i n a s s o c i a t i o n w i t h r i g h t hemisphere c o n t r o l may e n a b l e a b r o a d e r s p a t i a l d i s t r i b u t i o n of a t t e n t i o n , whereas a s t r o n g e r o r i e n t i n g b i a s may p e r m i t a f o c a l , h i g h l y s e l e c t i v e a t t e n t i o n a l mode i n a s s o c i a t i o n w i t h l e f t hemisphere c o n t r o l . The s e l e c t i o n a s p e c t s of a t t e n t i o n have a l s o been examined i n t h e c o n t e x t of s e a r c h t a s k s i n p a t i e n t s w i t h l a t e r a l i z e d b r a i n damage. G e n e r a l l y , t h e s e s t u d i e s have shown a g r e a t e r impairment i n terms of more e r r o r s and s l o w e r s e a r c h time i n r i g h t hemisphere p a t i e n t s , p a r t i c u l a r l y t h o s e w i t h v i s u a l f i e l d d e f e c t s . A s t u d y i n o u r l a b o r a t o r y (Roy & Roy, Note 3 ) examined b o t h t h e s e l e c t i o n a n d c a p a c i t y a s p e c t s of a t t e n t i o n i n a v i s u a l s e a r c h t a s k . While t h e r e were no c l e a r d i f f e r e n c e s between t h e l e f t and right-hemisphere p a t i e n t s i n t e r m s of o v e r a l l a c c u r a c y o r s e a r c h t i m e , somewhat d i f f e r e n t s e a r c h p a t t e r n s were observed i n t h e two p a t i e n t groups. R i g h t hemisphere p a t i e n t s seemed t o m a i n t a i n s e a r c h t i m e c o n s t a n t a c r o s s h e m i s p a t i a l f i e l d s a t t h e c o s t of i n c r e a s e d e r r o r s i n t h e c o n t r a l e s i o n a l ( l e f t ) hemispace, w h i l e t h e l e f t hemisphere p a t i e n t s seemed t o o p t f o r m a i n t a i n i n g a c c u r a c y a c r o s s s p a t i a l f i e l d s a t t h e c o s t of i n c r e a s e d time t o f i n d t a r g e t s i n t h e c o n t r a l e s i o n a l ( r i g h t ) hemispace. These d i f f e r i n g s e a r c h p a t t e r n s may r e f l e c t d i f f e r e n t s t r a t e g i e s . The r i g h t hemisphere p a t i e n t s may be f o c u s i n g on s e a r c h t i m e , w h i l e t h e l e f t hemisphere p a t i e n t s may be f o c u s i n g o n a c c u r a c y . The immediate i m p l i c a t i o n s of t h e s e a p p a r e n t l y d i f f e r e n t s t r a t e g i e s f o r u n d e r s t a n d i n g s e a r c h performance i s n o t c l e a r . What i s c l e a r , however, i s t h a t we need t o u s e t a s k s and measures of performance which a f f o r d t h e o p p o r t u n i t y t o i d e n t i f y d i f f e r e n t s t r a t e g i e s i n performance. C o n s i d e r i n g t h e c a p a c i t y a s p e c t of a t t e n t i o n , t h e r e were no c l e a r d i f f e r e n c e s between t h e h e m i s p h e r i c g r o u p s i n t h e e f f e c t s of v a r y i n g c a p a c i t y d e m a n d s o n v i s u a l s e a r c h performance. These f i n d i n g s t h a t have been reviewed p r o v i d e some i n i t i a l c l u e s t o t h e n e u r o b e h a v i o u r a l bases of a t t e n t i o n , p a r t i c u l a r l y a s t o h e m i s p h e r i c asymmetries. Many i s s u e s remain t o be c o n s i d e r e d and examined, however. F i r s t , w h i l e w e have viewed a t t e n t i o n n o t a s a u n i t a r y c o n c e p t , b u t a s one which i n v o l v e s a number of c o m p o n e n t p r o c e s s e s , i t i s i m p o r t a n t t o r e c o g n i z e t h a t t h e s e components t h e m s e l v e s may i n v o l v e s u b p r o c e s s e s of t h e i r own. O r i e n t a t i o n , f o r example, seems t o i n v o l v e a t l e a s t t h r e e a s p e c t s : d i s e n g a g i n g a t t e n t i o n from t h e c u r r e n t f o c u s , moving a t t e n t i o n , and engaging a t t e n t i o n a t a new l o c a t i o n ( P o s n e r & Cohen, 1984). A r o u s a l and a c t i v a t i o n , l i k e w i s e , seem t o i n v o l v e a t l e a s t two a s p e c t s , a s e n s o r y ( i n p u t ) and a motor ( o u t p u t ) component. Given t h i s i d e a of s u b p r o c e s s e s , i t behooves u s t o c a r e f u l l y s t u d y e a c h of t h e s e w i t h a view t o u n d e r s t a n d i n g t h e i r n e u r o b e h a v i o u r a l b a s i s . Posner e t a l . (1982, 1984) have b e g u n t o show t h a t t h e components of o r i e n t i n g may have d i f f e r e n t n e u r a l s u b s t r a t e s . Disengaging a t t e n t i o n , a s w e have s e e n , seems t o depend on p a r i e t a l a r e a s . Moving a t t e n t i o n , on t h e o t h e r hand, seems more dependent on m i d b r a i n and c o l l i c u l a r s t r u c t u r e s . I n t h e same v e i n , g i v e n t h a t t h e r e a p p e a r t o be b o t h s e n s o r y and motor components t o a c t i v a t i o n , i t would be i m p o r t a n t t o d e t e r m i n e , f o r example, whether t h e r i g h t hemisphere a d v a n t a g e f o r a c t i v a t i o n proposed by Heilman i s r e l a t e d t o a n a d v a n t a g e i n p r o c e s s i n g input (sensory a s p e c t s ) o r p r e p a r i n g a response (motoraspects). Another p o i n t h e r e r e l a t e s t o t h e c a p a c i t y component of a t t e n t i o n . One s t u d y (Roy, Note 2 ; Roy& Roy, Note 3 ) c a r r i e d o u t i n o u r l a b o r a t o r y s u g g e s t s t h e r e may n o t be d i f f e r e n c e s between t h e hemispheres i n t h e e f f e c t s of v a r y i n g c a p a c i t y demands, a t l e a s t a s measured by s e a r c h t i m e f u n c t i o n s i n t h e c o n t e x t of v i s u a l s e a r c h . Heilman's argument t h a t t h e r i g h t hemisphere i s c a p a b l e of c o n t r o l l i n g a t t e n t i o n i n b o t h h e m i s p a t i a l f i e l d s , however,

Unilateral atten tion deficits and hemispheric asymmetries

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s u g g e s t s t h a t t h e r i g h t hemisphere may engage i n a p r o c e s s i n g mode which i s less demanding o f r e s o u r c e s a n d , t h u s , e n j o y s t h e c a p a c i t y of d i s t r i b u t i n g t h o s e r e s o u r c e s t o b o t h h e m i s p a t i a l f i e l d s . Using S c h n e i d e r a n d S c h i f f r i n ' s (1977) c o n c e p t s of a u t o m a t i c v e r s u s c o n t r o l l e d p r o c e s s i n g , t h e a d v a n t a g e c o n f e r r e d on t h e r i g h t hemisphere may t h e n a r i s e because t h i s hemisphere i s more c a p a b l e of a u t o m a t i c p r o c e s s i n g t h a n i s t h e l e f t hemisphere. Workwith normals indeed s u g g e s t s t h a t t h e r i g h t hemisphere may be more c a p a b l e of p r o c e s s i n g i n f o r m a t i o n i n p a r a l l e l (Bryden, 1982). T h i s n o t i o n of c a p a c i t y demand c o u l d be f u r t h e r examined u s i n g a d u a l t a s k paradigm i n which patientsmust performasecondarytaskwhileengaginginvisualsearch. A second r e l a t e d i s s u e c o n c e r n s t h e i n t e r f a c e between p s y c h o l o g i c a l and n e u r a l p r o c e s s e s of a t t e n t i o n . I n t h i s c h a p t e r , we have been p a r t i c u l a r l y i n t e r e s t e d i n h e m i s p h e r i c asymmetries i n a t t e n t i o n a l p r o c e s s e s . These h e m i s p h e r i c a s p e c t s form o n l y a s m a l l p a r t of a l a r g e r network of n e u r a l p r o c e s s e s u n d e r l y i n g a t t e n t i o n . A t t e n t i o n l i k e o t h e r a s p e c t s of human b e h a v i o u r c a n be viewed a s i n v o l v i n g a complex s y s t e m of f u n c t i o n s , a s o - c a l l e d f u n c t i o n a l system. T h i s i d e a of a f u n c t i o n a l s y s t e m r e f l e c t s t h e c u r r e n t view of b r a i n - b e h a v i o u r r e l a t i o n s termed f u n c t i o n a l p l u r i p o t e n t i a l i s m ( L u r i a , 1974) and has been a p p l i e d t o p r a x i s (Roy, 1978, 1983) and t o p r o c e s s e s of a t t e n t i o n (Mesulam, 1981). I n t h i s view, a t t e n t i o n i n v o l v e s a number of f u n c t i o n a l components e a c h of which i s s u b s e r v e d by a p a r t i c u l a r b r a i n a r e a . These b r a i n a r e a s , comprised of r e t i c u l a r s t r u c t u r e s , c i n g u l a t e c o r t e x , and p a r i e t a l and f r o n t a l c o r t i c a l r e g i o n s , form a n e u r a l network. Damage t o any of t h e s e a r e a s seems t o d i s r u p t a t t e n t i o n a l p r o c e s s e s i n a c h a r a c t e r i s t i c way depending on which component of a t t e n t i o n h a s been compromised ( s e e Mesulam, 1981). Given t h i s view, a c l e a r e r u n d e r s t a n d i n g of a t t e n t i o n would seem t o depend on p a r a l l e l advances i n p s y c h o l o g i c a l and n e u r o l o g i c a l p e r s p e c t i v e s of a t t e n t i o n . Developing c o n c e p t s of t h e p s y c h o l o g i c a l p r o c e s s e s u n d e r l y i n g a t t e n t i o n may b e mapped on t o n e u r a l s t r u c t u r e s , t h u s f o s t e r i n g a d e s c r i p t i o n of a t t e n t i o n based on a n emerging i n t e r f a c e between b e h a v i o u r a l and n e u r a l p r o c e s s e s . T h i s a p p r o a c h i s e x e m p l i f i e d w e l l i n P o s n e r ' s work ( e . g . , Posner,inpress). A f i n a l c o n s i d e r a t i o n d e a l s w i t h t h e r e l a t i o n s h i p of t h e components of a t t e n t i o n d i s c u s s e d h e r e t o an a c c o u n t of n e g l e c t . The e v i d e n c e reviewed s u g g e s t s t h a t t h e r e may be h e m i s p h e r i c d i f f e r e n c e s i n c e r t a i n a s p e c t s o f a t t e n t i o n a l p r o c e s s e s . Our own work on t h e e f f e c t s of l a t e r a l i z e d warning s i g n a l s on r e a c t i o n time provided no c l e a r i n d i c a t i o n t h a t t h e r i g h t hemisphere i s dominant f o r a c t i v a t i o n i n normal s u b j e c t s . T h i s r e s u l t 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 t h e r i g h t hemisphere h a s t h e a b i l i t y t o assume c o n t r o l of a c t i v a t i o n a l p r o c e s s e s once t h e l e f t hemisphere i s damaged, whereas t h e l e f t hemisphere c a n n o t do s o i n t h e a d v e n t of r i g h t hemisphere damage. Such a n a b i l i t y may e x p l a i n t h e b i l a t e r a l impairment i n s e a r c h time found by Roy and Roy (Note 3 ) a n d , a s o t h e r s have s u g g e s t e d , c o u l d e x p l a i n t h e g r e a t e r i n c r e a s e i n r e a c t i o n time a f t e r r i g h t , as opposed t o l e f t , hemisphere damage. Y e t , how could an a c t i v a t i o n problem o f t h i s k i n d produce t h e s p a t i a l l y s e l e c t i v e ( i . e . , c o n t r a l e s i o n a l ) d i s t u r b a n c e found i n n e g l e c t ? The answer t o t h i s q u e s t i o n i s n o t l i k e l y t o b e a s i m p l e one because i t i n v o l v e s t h e i s s u e of t h e r e l a t i o n s h i p among t h e d i f f e r e n t components of a t t e n t i o n . A d e f i c i t i n d i r e c t i o n a l o r i e n t i n g would p r o v i d e a s t r a i g h t f o r w a r d e x p l a n a t i o n of t h e s p a t i a l f e a t u r e s of hemi-neglect. But can i t a l o n e f u l l y a c c o u n t f o r t h e e p i d e m i o l o g i c a l f a c t of g r e a t e r l e f t t h a n r i g h t n e g l e c t ? According t o P o s n e r ' s r e s u l t s , p a r i e t a l d a m a g e t o e i t h e r h e m i s p h e r e i m p a i r s t h i s d i s e n g a g e o p e r a t i o n a s s o c i a t e d w i t h o r i e n t i n g . The g r e a t e r impairment on t h i s t a s k found i n r i g h t t h a n l e f t hemisphere p a t i e n t s may be r e l a t e d t o

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t h e s t r o n g e r r i g h t w a r d b i a s i n normal s u b j e c t s (Reuter-Lorenz, Note 4 ) and may c o n t r i b u t e t o t h e g r e a t e r i n c i d e n c e of l e f t n e g l e c t ( K i n s b o u r n e , 1974, 1977). However, w h i l e t h e o r i e n t i n g d e f i c i t found by P o s n e r ' s g r o u p was r e l i a b l y associated with p a r i e t a l d a m a g e , i t w a s e v i d e n t i n p a t i e n t s w i t h o r w i t h o u t s i g n s of n e g l e c t . I t i s p o s s i b l e t h a t t h e m a g n i t u d e o f t h e o r i e n t i n g d e f i c i t on t h i s t a s k may c o r r e l a t e w i t h o t h e r i n d i c e s of h e m i - i n a t t e n t i o n ; however, t h i s h a s y e t t o b e e s t a b l i s h e d . I t seems r e a s o n a b l e t o h y p o t h e s i z e t h a t a d i r e c t i o n a l o r i e n t i n g d e f i c i t could form t h e c o r e o f t h e n e g l e c t syndrome a n d , a s s u c h , p r o v i d e t h e b a s i s f o r t h e h e m i s p a t i a l o r u n i l a t e r a l n a t u r e of t h e d i s o r d e r . A s t r o n g e r rightward than leftward a t t e n t i o n a l b i a s could c o n t r i b u t e t o t h e d i f f e r e n t i a l i n c i d e n c e of r i g h t and l e f t n e g l e c t . I f a c t i v a t i o n o r a r o u s a l p r o c e s s e s a r e a l s o d i s t u r b e d , t h e d i f f i c u l t i e s associatedwithanorienting impairment may be e x a c e r b a t e d . H o w e v e r , d i s t u r b a n c e s of a c t i v a t i o n / a r o u s a l a l o n e may be i n s u f f i c i e n t t o produce a u n i l a t e r a l impairment i n s p a t i a l attention. Decrements i n r e s o u r c e s may a l s o be i n s u f f i c i e n t t o produce u n i l a t e r a l a t t e n t i o n impairment. A s n o t e d above, Roy and Roy (Note 3 ) found t h a t t h e magnitude of c o n t r a l e s i o n a l s e a r c h d e f i c i t w a s n o t i n f l u e n c e d by t h e t y p e o r t h e number of d i s t r a c t o r s . I f r e s o u r c e decrements u n d e r l i e n e g l e c t , t h e n i n c r e a s e d c a p a c i t y demands should have e x a c e r b a t e d t h e u n i l a t e r a l s e a r c h d e f i c i t . I n t e r a c t i o n s between t h e c a p a c i t y and s e l e c t i o n a s p e c t s of a t t e n t i o n may emerge i f a h e a v i e r a t t e n t i o n a l l o a d i s imposed a n d / o r t h e t a s k u s e s m a t e r i a l s (e.g., s h a p e s ) which t h e p a t i e n t f i n d s d i f f i c u l t t o i d e n t i f y ( c f . L e i c e s t e r , Sitman, Stoddard & Mohr, 1969). Our f i n d i n g s s u g g e s t t h a t decrements i n a t t e n t i o n a l r e s o u r c e s a l o n e s e e m i n s u f f i c e n t t o produce unilateralattentionaldisturbance. T h i s a n a l y s i s s u g g e s t s t h a t a n impairment i n l a t e r a l o r i e n t i n g m a y be a n e c e s s a r y c o n d i t i o n f o r hemi-attentionaldisturbancesof any k i n d . I n c a s e s where o n l y o r i e n t i n g a s p e c t s of a t t e n t i o n a r e a f f e c t e d . o n l y s u b t l e f e a t u r e s of t h e n e g l e c t syndrome, such a s e x t i n c t i o n , may be e v i d e n t . I n c r e a s i n g l y s e v e r e forms of n e g l e c t may i n v o l v e a d d i t i o n a l impairment i n o t h e r components of a t t e n t i o n ( i . e . , a r o u s a l / a c t i v a t i o n ) i n c o n j u n c t i o n withanunderlyingdeficitinorienting. Our aim i n t h i s paper h a s been t o move toward a more a c c u r a t e c h a r a c t e r i z a t i o n of t h e n a t u r e of t h e a t t e n t i o n a l impairments a s s o c i a t e d w i t h l a t e r a l i z e d c o r t i c a l l e s i o n s i n g e n e r a l and n e g l e c t i n p a r t i c u l a r . I t i s o u r b e l i e f t h a t t h i s t y p e of a p p r o a c h w i l l h e l p t o d e f i n e t h e n a t u r e of h e m i s p h e r i c d i f f e r e n c e s i n a t t e n t i o n and w i l l a i d i n t h e e l a b o r a t i o n of n e u r o b e h a v i o r a l a t t e n t i o n t h e o r y . Moreover, i t may a l l o w f o r t h e development of a taxonomy of h e m i a t t e n t i o n a l d i s t u r b a n c e s which can g u i d e patient c l a s s i f i c a t i o n a n d possibly patient treatment.

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Acknowledgements : P r e p a r a t i o n of t h i s m a n u s c r i p t was funded t h r o u g h g r a n t s t o E. Roy, f r o m t h e N a t u r a l S c i e n c e s a n d E n g i n e e r i n g R e s e a r c h C o u n c i 1 and t h e N a t i o n a l H e a l t h Research Development Program, H e a l t h & W e l f a r e , Canada.


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Footnotes

1. Copland, S. Hemispheric d i f f e r e n c e s i n a t t e n t i o n and r e s p o n s e p r e p a r a t i o n . Unpublished M a s t e r ' s t h e s i s , Department of Psychology, U n i v e r s i t y o f W a t e r l o o , M a y , 1985.

2. Roy, L. A t t e n t i o n d e f i c i t s i n p a t i e n t s w i t h l a t e r a l i z e d b r a i n damage. U n p u b l i s h e d M a s t e r ' s t h e s i s , DepartmentofKinesiology,University o f W a t e r l o o , M a y , 1985. 3. Roy, L. & Roy, E.A. A t t e n t i o n d e f i c i t s i n p a t i e n t s w i t h l a t e r a l i z e d b r a i n damage. P o s t e r p r e s e n t a t i o n a t a n n u a l meeting of North American S o c i e t y f o r P s y c h o l o g y o f S p o r t & P h y s i c a l A c t i v i t y , M a y , 1985. 4 . Reuter-Lorenz, P.A. Hemispheric c o n t r o l of s p a t i a l a t t e n t i o n . Unpublished D o c t o r a l d i s s e r a t i o n , Department of Psychology, U n i v e r s i t y of T o r o n t o , 1986.

5. Reuter-Lorenz, P.A., Moscovitch, M., & K i n s b o u r n e , M. L a t e r a l a t t e n t i o n b i a s i n a v i s u a l l i n e b i s e c t i o n t a s k : S i m i l a r i t i e s between t h e performances of n e g l e c t p a t i e n t s and normal s u b j e c t s . Paper r e a d a t North American C o n f e r e n c e , I n t e r n a t i o n a l N e u r o p s y c h o l o g i c a l S o c i e t y , S a n D i e g o , C a l i f o r n i a , F e b r u a r y , 1985.


Neurophysiological and Neuropsychological Aspects of Spatial Neglect. M.Jeannerod (editor) 0 Elsevier Science Publishers B.V. (North-Holland). 1987

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COORDINATES OF EXTRACORPOBEAL SPACE John L . Bradshaw, Norman C. N e t t l e t o n J a n e M. P i e r s o n , Lyn E . Wilson and Gregory Nathan

U n i l a t e r a l n e g l e c t may be c o n s i d e r e d w i t h r e f e r e n c e t o s e v e r a l s p a t i a l - c o o r d i n a t e s y s t e m s , e . g . , t h o s e r e l a t i n g t o t h e body m i d l i n e , head and r e t i n a , and s e e m s t o i n v o l v e a l t e r a t i o n s i n t h e deployment of a t t e n t i o n . L e f t h e m i n e g l e c t may be more f r e q u e n t o r , p o s s i b l y , merely more s a l i e n t t h a n r i g h t h e m i n e g l e c t , p e r h a p s b e c a u s e of asymmetries i n t h e r e p r e s e n t a t i o n of e x t r a c o r p o r e a l s p a c e . I n normal s u b j e c t s w e have r e p o r t e d a number of a n a l o g o u s phenomena. V i s u a l l y p r e s e n t e d l i n e s o r tactually/kinesthetically presented r o d s were t y p i c a l l y t r a n s e c t e d s l i g h t l y t o t h e l e f t of t h e t r u e m i d p o i n t , a n e f f e c t which c o u l d be manipulated by v a r y i n g t h e s a l i e n c e of t h e two e x t r e m i t i e s . I n a v a r i e t y of t a c t u o m o t o r t a s k s i n v o l v i n g t h e f i n g e r s , t h e p o s i t i o n i n s p a c e t o l e f t o r r i g h t of t h e body was found t o be more i m p o r t a n t t h a n t h e a c t u a l hand ( l e f t o r r i g h t ) employed, which was placed e i t h e r on i t s own s i d e of t h e body o r across themidline t o t h e opposite side. I n v i b r o t a c t i l e r e a c t i o n time e x p e r i m e n t s , performance was a l s o found t o be d e t e r m i n e d by t h e p o s i t i o n of t h e r e s p o n d i n g hand, though hand asymmetries r e p l a c e d hemispace asymmetries under c o n d i t i o n s of s t i m u l u s u n c e r t a i n t y . I n t h e a u d i t o r y m o d a l i t y , i t was t h e p o s i t i o n , r e a l o r p e r c e i v e d ( a s under c o n d i t i o n s of v i s u a l c a p t u r e ) of a sound s o u r c e ( l o u d s p e a k e r ) which d e t e r m i n e d performance asymmetries. I n a l l t h e s e e x p e r i m e n t s asymmetries were l o s t by d i s s o c i a t i n g t h e head and body c o o r d i n a t e s y s t e m s , through t h e maintenance of a 90" head t u r n t o l e f t o r r i g h t , o r by t h e d i s s o c i a t i o n of t h e g r a v i t a t i o n a l and c o r p o r e a l c o o r d i n a t e s y s t e m s , when l y i n g h o r i z o n t a l l l y upon t h e l e f t o r r i g h t s i d e . The hemispheres may map b o t h proximal s e n s o r y (and motor) e v e n t s i n v o l v i n g t h e body s u r f a c e and more d i s t a l e v e n t s o c c u r r i n g o u t i n e x t r a c o r p o r e a l space. T h e s e two r e p r e s e n t a t i o n s may be e x p e r i m e n t a l l y dissociated.

Hemineglect U n i l a t e r a l b r a i n damage may l e a d t o unawareness o r n e g l e c t of s t i m u l i o r e v e n t s i n t h e s i d e of s p a c e o p p o s i t e t o t h e l e s i o n (De R e n z i , 1982; Heilman, Bowers & Watson, 1984; Heilman & V a l e n s t e i n , 1979; Mesulam, 1981, 1983). The syndrome i s u s u a l l y s a i d t o be more common a f t e r r i g h t hemisphere damage, though a m i n o r i t y f i n d no d i f f e r e n c e s i n t h e f r e q u e n c y of t h e d i s o r d e r a f t e r l e s i o n s on e i t h e r s i d e (Ogden, 1985, and s e e h e r review i n t h i s volume). R i g h t hemisphere l e s i o n s , however, do u s u a l l y l e a d t o a more s e v e r e m a n i f e s t a t i o n of t h e c o n d i t i o n (Ogden,

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1985). In severe cases it is as if the left half of the patient's world has ceased to exist, with failure to report, respond to or orient towards stimuli on the side contralateral to the lesion. The patient may shave, groom or dress only the right side, leave food untouched on the left side of the plate, read only words, letters or sentences written on the right side of a page, leave an unusually wide margin on the left, copy only the right side of a drawing, and bisect a visuallypresented horizontal line to the right of the true midpoint, especially if the whole line lies to the left of the midline (Heilman & Valenstein, 1979). There may even be neglect of the left side of a scene which is currently being imagined (Bisiach & Luzzatti, 1978). such that the patient's inability to recall objects depends upon his or her imaginary standpoint. Thus we can delineate the affected "space" with respect to the actual observer (patient). i.e. extrapersonal space, or with respect to the vantage point of an imagined observer during topographical recall, or with respect to a particular component of a larger object (Halsband, Gruhn 6 Ettlinger, in preparation). The lesions causing the syndrome are usually large and either of sudden onset or are rapidly progressive, though the symptoms typically diminish over time. Nevertheless extinction, when all the other above symptoms have finally abated, may remain relatively intractable. Here, a patient capable of adequate response to unilaterally presented stimuli on either side, when given simultaneous bilateral stimulation (visual, auditory or tactual) may fail to report stimuli contralateral to the damaged hemisphere. (With normal subjects, the dichotic right-ear advantage with competing simultaneous stimulation of both ears may be an analogous phenomenon, see e.g. Bradshaw, Burden 6 Nettleton, 1986, for a review.) Hemineglect does not of course depend upon extinction, and unlike right hemineglect, right side extinction is by no means rare (Schwartz, Marchok, Kreinick & Flynn, 1979). Indeed hemineglect may reflect not just a deficit in directed attention, but also a deficit in how space is actually represented (De Renzi, 1982). Moreover unilateral neglect may be considered with reference to more than one spatial coordinate system, e.g. the body midline (the currently preferred viewpoint), head coordinates (e.g. when the head is turned with respect to the body), and even retinal coordinates, with respect to gaze direction (Bisiach, Capitani 6 Porta, 1985; Heilman et al., 1984); all three coordinate systems can be mutually dissociated by appropriate turn conditions. Bisiach et al. (1985, and see also Bradshaw and Pierson, 1985) have concluded that egocentric and extracorporeal space is organized topographically in the brain, in a projection system separate from that of the proximal receptor or body surface, and that there are circumscribed brain areas wherein lesions can result in representational l o s s limited to definite regions of space. Bisiach et al. found that the boundary of the neglected area of the tactile apparatus was influenced both by the sagittal midplane of the trunk, and by line of sight in terms of both head and gaze orientation. They also cite physiological and anatomical evidence of thalamic and premotor cells firing to visual stimuli in a definite region of peripersonal space irrespective of gaze direction, and independently of the retinal In man, the syndrome of coordinates of the proximal stimulus. hemineglect may involve damage to the frontal and cingulate cortex, the inferior parietal lobule, the basal ganglia, thalamus and mesencephalic reticular formation, all of which are richly interconnected and are involved with arousal and attention to meaningful stimuli (Bisiach, Bisiach et al., 1985; Crowne, Comacchia, Sterzi 6 Vallar, 1984;

Coordinates of extracorporeal space 1983;

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Heilman et al., 1983; Mesulam, 1981, 1983; Mishkin, Ungerleider Macko, 1983; Ogden, 1985; Stuss & Benson, 1984). Animal studies (Meredith & Stein, 1985) emphasise the role of the superior colliculus, which receives polysensory information, in controlling orientation of the eyes, pinnae and head. Left sided neglect after right hemisphere (RH) injury is generally held to be more common, severe and longer lasting than right hemineglect Ratcliff, 1982), after left hemisphere (LH) injury (Mesulam, 1981; though according to Ogden (1985) it may be severity rather than frequency of occurrence which characterizes the asymmetry. While right hemineglect may indeed be masked by incapacitating aphasia after LH trauma (De Renzi, 1982), left hemineglect is very often present in purely visual or copying tasks. The phenomenon cannot however be simply due to RH specialization for visuospatial processing, since if these mechanisms were damaged we would not expect the disability to be largely limited to left hemispace. Nor, probably, is hemineglect merely the consequence of the prevalence of the intact hemisphere's contraversive turning tendency, which can no longer be countered by the damaged hemisphere; disordered eye movements (Smith and Latto, 1982) may certainly play a role, though probably more as a consequence than as a cause of the syndrome (though cf. De Renzi, Colombo, Faglioni & Gilbertoni, 1982). A traditional view (see e.g. De Renzi et al., 1982; Geschwind, 1981; Heilman, 1979; Mesulam, 1981) is that while the LH exclusively mediates attention for contralateral (right) hemispace, the intact RH can cope with both sides, even though its dominant tendency might be towards contralateral (left) hemispace. Thus LH damage is unlikely to produce hemineglect, as the intact RH can take over, while RH damage will lead to left hemineglect as the LH cannot compensate. Certainly RH injury is often associated with lowered affect, defective alerting, poorer arousal and reaction time decrements (with either hand), all indicating (Heilman, 1979) that the RH may mediate bilateral as well as contralateral arousal. Ogden (1985) found that right brain damage (leading to left hemineglect) was more often associated with posterior lesions, while left brain damage (leading to equally frequent but milder right hemineglect) usually stemmed from anterior damage. She speculates that a language invasion of posterior left hemisphere processing areas may bring about an impaired ipsilateral representation of (left) hemispace by the left hemisphere. However the LH must also have some residual ipsilateral capacity, since significant neglect is generally absent after right hemispherectomy and forebrain commissurotomy (Plourde & Sperry, 1984, though see below). So can the LH operate in both halves of extrapersonal space, with, in the presence of intact commissures, differential suppression of left side awareness by a focally damaged RH? This model suggests that RH damage disrupts attentional functions of an entire integrated system, interfering with the expression of compensatory abilities in the intact as well as in the damaged hemisphere, since a damaged and nonfunctioning RH retains its dominant and suppressive role over the LH with respect to attentional functions - a mirror-image version of the conventional account of LH language dominance. Heilman et al. (1984) demonstrated the role of the commissures in integrating hemisphere-hand and hemisphere-hemispace mapping, which may be dissociated when the arms cross the midline. Thus when a commissurotomy patient tactually bisects a line with the right hand in left hemispace, because of the disconnection the right hand cannot be influenced by the RH which is &

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critical for mediating attentional-intentional functions in left hemispace, but will Instead be subject to the LH, which of course also subserves sensory-motor processing of the right hand along the conventional anatomical pathways. The attentional-intentional system of the disconnected LH will direct the sensory-motor apparatus towards contralateral right hemispace and so will generate errors to the right of the true midpoint when the right hand tactually bisects in left hemispace. This is exactly what was found, together with errors to the left of the true midpoint whenever the left hand bisected lines in right hemispace. In both cases such errors were far more pronounced when the arms crossed the midline, each limb erring towards its own hemispace, a consequence of the two (disconnected) hemispheres each mapping two incongruent (because the arms were crossed) relationships, the hemispatial and the sensory-motor. In normal subjects Heilman et al. suggest that the tendency of each hemisphere to "intend" towards contralateral hemispace may be counteracted by the other hemisphere, an opposition which is lost when one hemisphere is damaged or the commissures are severed. Line Bisection by Normal Subjects We have already seen that when left-hemineglect patients try to

bisect a line which lies across their midline, they tend to place the transection to the right of the true center (Heilman C Valenstein, 1979) as if a large extent to the left of center is seen as smaller than it Rosenberger (1974) investigated visual line bisection i n really is. normal subjects. Here, rather than actually transecting the lines, the subjects tried to discriminate between lines which were either accurately bisected or asymmetrically transected. No asymmetries were observed, but this could well have been due to the relative coarseness of Rosenberger's difference scale, since when we required subjects actually to bisect lines placed across the midline (Bradshaw, Nettleton. Nathan C Wilson, 1985). we found that they consistently placed the transection slightly to the left of center, a phenomenon which may be labelled left side underestimation (LSU). It is as if such subjects see the extent to the left of center as larger than it really is, possibly because of the greater visuospatial processing power of the right hemisphere (for review, see Bradshaw C Nettleton, 1981, 1983). and so in compensation make the left side slightly smaller to seem equal to the right. Thus when our subjects transected 10 lines, ranging in length from 80 to 170 mm, they produced a highly significant (p < .001) LSU averaging 1.6% of the true half length, an effect shown by 22 of the 24 dextral subjects. Interestingly when on another occasion (unpublished data) we required 5 year old dextral and sinistral children to perform the task, using both the preferred and the nonpreferred hand, dextral children generally behaved like adults, showing a LSU which was slightly larger with the left than the right hand. Sinistral children, however, placed the transection far to the left with the left hand and far to the right with the right hand, as if unwilling even to approach the midline with either hand. V i s u a l Studies

Is it possible that the LSU depends upon subjects maintaining a normal upright posture, with alignment of two possible sets of spatial coordinates, those relating directly to the body midline itself (whatever its posture) and those relating to left/right in terms of normal gravitational coordinates, i.e. with respect to the gravitational vertical and the direction in which a (possibly recumbent) subject may happen to be looking? We therefore asked what are the roles of retinal

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(corporeal) and gravitational coordinates (Bradshaw et al., 1985). These can be dissociated by getting subjects to recline horizontally on one or other side, with stimulus rods horizontal or vertical. Subjects performed a visual rod bisection task; they maintained fixation upon a central ring through which a rod passed, and adjusted the rod extremities (seen in peripheral vision) until they were judged equal. Such an arrangement of course also ensured that the two ends of the rods would project to opposite hemispheres, as scanning was not permitted. A LSU would now incorporate both a hemiretinal-hemispheric and a hemispatial-hemispheric component, and might therefore be larger than that which we had previously obtained (1.6%) when subjects freely scanned the lines before transecting them. We obtained a very large (3.75%) and significant (p < .001) LSU when subjects were upright, an effect which was significantly reduced when subjects lay horizontally, thus dissociating gravitational and retinal coordinates of space. The two systems of spatial coordinates must therefore be in alignment for hemispatial asymmetries to occur. We can now ask whether the clinical phenomenon of left hemineglect is due to a reduced salience or "attention-getting quality" of events to the left of the midline. Conversely, with normal subjects, are extents to the left excessively salient, perhaps because of the relatively We greater visuospatial information-processing capacity of the RH? (Bradshaw, Nathan, Nettleton, Wilson 6 Pierson, in press) varied the salience of the test rods by manipulating the contrast with the Thus we had all black rods, or all background (left or right sides). white rods, on a horizontal background half of which (to the left or to the right of the subject's midline) was of contrasting tone (black background beneath a white rod, or vice versa); the other half was of noncontrasting (i.e. similar, low salience) tone, (white background For control purposes we beneath white rod, or black beneath black). also included a uniform background of high salience - i.e. an all white background beneath a black rod or vice versa. The midpoint of the uniform background, or the black-white juxtaposition of the two sharply demarcated and contrasting backgrounds, lay clearly marked exactly in front of the subject's midline. The subject adjusted the rods, by means of wires attached to their ends, so that the two extremities were judged equidistant from the marked midpoint. We included conditions of fixation upon the midpoint, and free scanning, to verify that LSUs are indeed greater under the former condition. We found this to be the case (p < -025, 2.65% versus 1.62%). The fact that the value for free scanning (where only hemispatial factors can operate) was considerably more than half that of the value for central fixation (where both hemispatial and hemiretinal factors would operate) suggests that hemispatial factors are more important than the traditional anatomical pathway variables (Kimura, 1961, 1967) associated with hemiretinal factors. However when we examined the effect of salience, LSUs proved to be biggest when salience was low on the left and high on the right, intermediate with a uniform background, and smallest when salience was We must therefore low on the right and high on the left (p < .05). conclude that while a manipulation of salience does affect the LSU in normal subjects, the phenomenon is not due to excessive salience of stimuli to the left of the midline, since a reduction in left-side Indeed it is as if fainter salience only serves to increase the LSU. stimuli are somehow seen as bigger, again perhaps because they require more processing resources. We addressed these issues by requiring (normal) subjects to bisect the space between two point sources of

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light, which were either both constantly illuminated (or both simultaneously flashing) with one light dim and the other bright, or one light on longer than another when simultaneously flashing, or both We found flashing alternately with same or different durations. (unpublished data) that faint, brief or flashing stimuli somehow attracted the subjective midpoint when either hand was used, and that the consistent effect of placing the subjective midpoint to the left of true centre was more pronounced when the left hand was used. We next presented subjects with a single vertical target rod in left or right hemispace for 5 seconds. They then adjusted a vertical test rod located in the same or opposite hemispace to equal the remembered length of the target, under conditions of free scanning. We found that the configuration target-right/test-left produced a significantly greater (p < .01) LSU than the opposite configuration. Moreover while left and right target hemispace did not differ from each other, there was a very much greater (p < .001) underestimation for left (compared to right) test hemispace. This indicates that the phenomenon i s not memory dependent (target based) but is related to the instantaneous perception of the test item. Furthermore this experiment suggested that it is truly a case of LSU rather than right side overestimation, as the configuration target-left/test-right did not produce overestimation, only a (nonsignificant) underestimation. We must then conclude that stimuli to the left of the midline are seen as 'bigger' (not more salient) than they 'really' are, again perhaps because the RH has more visuospatial processing capacity, so that compensation in matching tasks causes extents to the left to be made smaller, with the result that, for the subject, both extents appear equal. G o e s t h e t i c Studies It has long been known that a right visual field (RVF) superiority, or a right ear advantage (REA), occurs with lateral presentations of verbal material (Bradshaw 6 Nettleton, 1983; Bryden, 1982); the opposite applies with certain classes of nonverbal or visuospatial stimuli. In the auditory (Kimura, 1961, 1967) and tactual modalities, laterality effects have traditionally been ascribed to the prepotency of the contralateral over the ipsilateral afferent pathways projecting to a hemisphere specialized for a particular mode of processing. However under these experimental circumstances receptor location and hemispace may as we have seen be systematically confounded; hemispace here refers of course to the position in extracorporeal space to left or right of the body midline, wherein stimuli may occur and towards which responses may be initiated, as distinct from ear of entry, hand or visual field. One way to unconfound the two factors is to place either hand in left or right hemispace, by making the arm cross the midline. Bowers and Heilman (1980) required their blindfolded subjects tactually to explore a rod which lay to the left or right of, or across the body midline. They were to point with the forefinger of the left or right hand, held either in its own hemispace, or across the midline, t o the subjective center of the rod. The authors obtained a LSU (when the task was performed across the midline or in the right hemispace) which they termed a 'pseudoneglect', by analogy with the left hemineglect shown by patients suffering from RH trauma, though the direction of the two phenomena are of course opposite. We (Bradshaw, Nettleton, Nathan 6 Wilson, 1983) performed a version of this experiment with left or right hands in their own or opposite hemispaces; subjects tried to subdivide (into halves, quarters OK fifths) a laterally located rod, using only

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tactual and kinesthetic information. While we found no evidence of a LSU, the left hand significantly (p < .05) underestimated relative to the right hand. Moreover overall performance (i.e. irrespective of directional effects) was slightly more accurate in left than in right hemispace, though the effect just failed to reach statistical significance. Interestingly, a significant left hand underestimation relative to the right hand also emerged in another tactual experiment in this series where subjects were required to bimanually bisect vertical rods, positioned in left or right hemispace with one hand always crossing the midline. (The rod passed vertically through a laterally located horizontal baseplate and one hand kinesthetically measured and adjusted the extent above the baseplate, while the other operated in the Subjects wore goggles which occluded lateral same way beneath it.) vision while allowing central fixation. Subjects next performed a bimanual integration task, with the rod now lying across the midline and the two arms either each occupying their respective hemispaces or crossing the midline. One hand attempted to reproduce on one side an extent, or ratio ( 1 / 2 , 2 / 3 etc.) of an extent, perceived by the other hand on the other side, by laterally moving the rod through a central piece of short pipe. We found that the extent in left hemispace was consistently (p < . 0 1 ) underestimated by about 1 % . Thus, overall, while neither hand over- or under-estimated relative to its fellow, the extent in left hemispace was consistently under-estimated. However, these effects completely disappeared when the subject performed a version of the task with the head turned 90" to left or right, thus dissociating the coordinates of head and body hemispace. There was therefore no LSU, either with respect to head or body hemispace, whether the rod ran from (To achieve front-to-back passage of a side-to-side or front-to-back. rod through the subject's midline, the whole experiment was performed upon a specially constructed stool with the rod running directly beneath the seated subject; a pipe extended out either between the subject's legs and behind and beneath the coccyx, or on either side of and below the hips. Subjects laterally adjusted the rod, which passed through and was longer than the pipe, so that the protruding extents at either end were felt to be equal in extent.) Thus just as in visual line bisection, alignment of corporeal and gravitational coordinates may be required for asymmetries to appear, so also in the kinesthetic modality alignment of the coordinates of head and body hemispace may be necessary for the development of asymmetries. Moreover early visual experience may also be necessary for the development of a proper sense of extracorporeal space, as LSUs proved to be absent in a group of congenitally blind adults tested under normal orientation conditions (unpublished data). Indeed it has long been claimed that "conventional" asymmetries are reduced in the congenitally blind (Bradshaw, Nettleton 6 Spehr, 1982; Harris, 1980; Hermelin 6 O'Connor, 1971a,b; Karavatos, Kaprinis 6 Tzavaras, 1984; Larsen 6 Hakonsen, 1983) and the long-term profoundly deaf (e.g. Bonvillian, Orlansky 6 Garland, 1982; Boshoven, Cranney 6 Ashton, 1982; Gibson 6 Bryden, McNeil 6 Harvey, 1982; 1984; Neville, Kutas 6 Schmidt, 1982; Weston 6 Weinman, 1983). In conclusion, there may be underestimation of extents "perceived" by the RH, either in terms of left hand or left hemispace performance, the latter being easier to demonstrate when the extents symmetrically cross the subject's midline, rather than exclusively occupying one or other hemispace. Finger S t i u l a t i o n , Spacing and Sequencing T a sks We have seen (above) that in our first kinesthetic experiment, a

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magnitude estimation task was performed rather more accurately when it took place entirely in left hemispace, as compared to right hemispace; this effect was separate from the general phenomenon of a LSU, but, like it, was probably a common manifestation of superior spatial processing by the RH. Blindfolded dextral children (3.5 and 5 years of age) were the subjects for an experiment in which textile patches were brushed across the tip of the forefinger of the left or right hand, located in left or right hemispace (Burden, Bradshaw, Nettleton 6 Wilson, 1985). This target stimulus was followed by a test patch which was either tactually identical to the target, or differed from it (the level of difficulty of the difference discrimination being individually adjusted for each child). The test patch was administered to the same or opposite hand as the target, held in same or opposite hemispace as before. We found no hand asymmetries whatsoever, but obtained a significant (p < ,025) left hemispace superiority for the initial target presentation, though not for the subsequent test hemispace, suggesting in this situation that memory rather than perceptual factors may determine hemispace asymmetries. Either hand performed better when located in its own hemispace than when it crossed the midline, an effect which was much greater for young (p < .025) and female (p < .025) than for older, male children, and may reflect otherwise established differentials in the rates of commissural maturation (Hewitt, 1962; Rakic 6 Yakovlev, 1968; Salamy, 1978; Yakovlev 6 Lecours, 1967). In another task with an essentially similar hand-hemispace design, blindfolded 5 year old boys and girls reproduced static configurations of finger spacings which had been moulded on their hands by the experimenter and then scrambled by clenching the fist. This time there were both hand and hemispace asymmetries, again only at the level of the initial (memory) target and not at the level of the subsequent (perceptual) test. Thus performance was superior for left (compared to right) target hemispace (p < .001), and for left (compared to right) target hand (p < .01). The fact that both hand and hemispace effects emerged may be due to the combination of spatial, kinesthetic and motor components in this task. Once again, either hand was better (p < .05) when operating in its own hemispace, rather than crossing the midline. Finally, in a sequential finger-touching task designed to appeal to the LH, 8 year old dextral and sinistral children were touched, serially, on the fingers of the left or right hand, held in left or right hemispace; they reproduced the target sequence with movements of the fingers of the same or opposite hand held in same or opposite hemispace. Dextrals showed no hemispace asymmetries, though both the right target and the right test hand were superior to the left (in both cases at p < .025), and again either hand performed better in its own hemispace. With sinistrals there were no hand asymmetries, and either (target) hand was better when in right than in left (target) hemispace. We can conclude that hemisphere-= connections predominate in dextral children for this task, and hemisphere-hemispace connections with sinistrals, who otherwise might have been subjected to a dissociation between their preferred hand and the mediating hemisphere. In either case a LH superiority emerged in this active sequential task, with again effects appearing stronger at the level of initial (memorized) target than for the subsequent (perceptual) test stimulus. Vibrotactile Experiments So far we have measured performance accuracy rather than speed while investigating hemispace asymmetries. Reaction times (RTs) to

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vibrotactile stimuli are a convenient way of unconfounding the effects of anatomical connectivity and hemispace while using a speed measure. In our first such experiment (Bradshaw, Nathan, Nettleton, Pierson & Wilson, 1983) subjects depressed a microswitch with the forefinger of the left or right hand, held for a block of trials either in its own or in contralateral (i.e. across the midline) hemispace, as soon as the microswitch was felt to vibrate (250 Hz). Twenty-two out of 24 dextral subjects gave a highly significant (p < .001) right hemispace superiority of 9 msec, the hands not differing. When we repeated the experiment (unpublished data) with 12 male and 12 female strongly sinistral subjects, however, the RSAs became nonsignificant, only 17 subjects showing such an effect. Nevertheless the fact that the left side was not favored with these subjects indicates that the phenomenon of a RSA with dextrals was not simply due in some way to a preference for performing manipulative or motor tasks on the same side of the body as the preferred hand. When next we (Bradshaw et al, 1983, Experiment 2) unconfounded head and body hemispace by requiring subjects to turn the head 90' to left or right, with the stimulated and responding limb held out either in the midline, or to the side of the body, all hemispace asymetries were lost with respect to the body (hand held out from side) and greatly reduced with respect to the head (hand held in the midline). So just as 90" head turns earlier destroyed LSU's in a rod bisection task, so also right-of-body hemispace superiorities were lost and right-of-head hemispace superiorities greatly reduced under similar circumstances in a vibrotactile RT task which dissociated the head and body components. What happens if instead we dissociate gravitational and corporeal coordinates, as in the line bisection task, by getting subjects to perform the task while recumbent on one or other side? By corporeal we mean left and right with respect to the spinal axis irrespective of posture; by gravitational left-right we mean with respect to the gravitational vertical, while the subject is still facing in the direction in which though now recumbent he or she is looking. Thus when a recumbent subject lies on the left side, gravitational left extends out beyond the top of the head, and gravitational right towards the knees. These relationships reverse when the subject lies on the right side. We repeated our first vibrotactile experiment with blocks of trials for hand (left, right), posture (lie on left, right), arm (up, down, or beyond head, between knees), and analyzed the data in terms both of gravitational and corporeal coordinates (Bradshaw & Pierson, 1985). There were no significant hand or hemispace effects whatsoever, indicating, as in the rod and line bisection tasks, that just as a 90" head turn dissociates head and body hemispace and destroys hemispace asymmetries, so also are hemispace asymmetries lost when gravitational and corporeal hemispace are dissociated, when a subject reclines horizontally on one or other side. Are these right side advantages (RSAs) motor or sensory? If one hand is stimulated and the other responds, we can separately assess the independent contributions of left and right sensory hand, left and right motor hand, left and right sensory hemispace and left and right motor -hemispace. Under these circumstances we found that only motor hemispace gave a significant (p < .01) RSA ( 6 msec). So the locus of this effect appears to reside in the preparation of a response, rather than the discrimination of a signal. We investigated further the possible role of stimulus uncertainty by including a low (as well as a high) signal intensity condition. We found that while there was again a RSA ( 4 msec)

50


for motor hemispace (p < .01) with high intensity signals, just as before, this effect disappeared with low intensity stimulation, being replaced by an almost significant 4 msec sensory RSA. A t the same time a hand asymmetry now appeared for the first time, the configuration left-hand-stimulated/right-hand-responding proving significantly (p < .025) faster (9 msec) than the reverse configuration. The responding (rather than the stimulated) hand was probably responsible, as the spatial-compatibility interaction Hand by Motor Hemispace (but not Hand by Sensory Hemispace) reached significance (p < . 0 5 ) . Thus left-hand-stimulated/right-hand-responding was faster in right as compared to left motor hemispace, and right-hand-stimulated/left-handresponding was faster in left as compared to right motor hemispace. Hitherto, we have obtained vibrotactile RSAs with trials to one or other side presented in blocks, rather than randomly. Does the phenomenon depend upon the establishment of position sets, subjects having a greater ability to hold attention to the right (compared to the left) rather than to shift attention to the right, as might occur with randomly alternating side of stimulation and/or response? If this is the case, then randomization of the side of stimulus and response should lead to a loss of effects. Subjects must now divide attention between the two sides of space to obtain maximal performance. If attention is therefore directed to both sides of space, then stimuli on the right cannot benefit from a greater ability to hold attention to this side for an extended period of time. We adopted just such a randomly-alternating paradigm, each hand having its own stimulus and response unit, the hand stimulated being the one to respond. Hands were held either in their own (uncrossed) or opposite (crossed) hemispace. This arrangement (which is of course also yet another approach to the question of stimulus-response uncertainty, as with the low-intensity threshold-level signals) is now in effect a choice RT task, though one of very high stimulus-response spatial compatibility. We found, as expected in an essentially choice task, that RTs considerably increased, and the crossed configuration was now for the first time considerably slower than the uncrossed (p < .001). Of course, this constitutes a hand/hemispace spatial compatibility effect, i.e. each hand is faster More importantly, however, all when located in its own hemispace. hemispace asymmetries were lost, as in the low stimulus intensity condition, and were replaced by a significant (p < .025) right hand superiority (13 msec). Spatial compatibility effects are of course traditionally absent from simple RT tasks, and present in choice RT situations (Bradshaw & Umilta, 1984); conversely, hemispace effects may be absent from choice tasks and present in simple RT tasks, where the spatial coding of limb position is unimportant. If s o , we can ask whether hemispace asymmetries reappear, at the expense of hand asymmetries, if we employ blocks of trials for a given responding hand or side, but randomly alternate, as before, the stimulated hand or side. As the vibrotactile RSA has been shown to be motor rather than sensory, a RSA might be predicted when side of response is blocked. When we performed just such an experiment involving stimulus but n o t response uncertainty, once again there was no hemispace asymmetry, but the right hand was 8 msec faster (p = .05) than the left. Perhaps not surprisingly, when the same hand was stimulated and responded, performance was also faster (18 msec, p < .05) than when opposite hands were stimulated and responded. Moreover there was now no longer any advantage for the uncrossed configuration, so the need to select a response hand may determine whether or not there will be hand/hemispace


51

spatial compatibility effects. Hemispace asymmetries may therefore only occur when there is no uncertainty about the spatial location of the stimulus, and attention can be wholly allocated to one o r other side, both at the level of stimulation and of response. We can conclude from the vibrotactile experiments that hemispace asymmetries will usually occur only when head and body coordinates and corporeal and gravitational coordinates are aligned. Moreover position of an event in extracorporeal space is at least as important as traditional anatomical connectivities (Kimura, 1961, 1967) in determining asymmetries. The cerebral hemispheres appear to code distal sensory-motor events occurring in contralateral hemispace, and not just events on the proximal receptor surfaces and the actions of the effector musculature. Hemispace RSAs occur with simple, not with choice RTs, i.e. where spatial position is unimportant in stimulus-response coding, and only in the absence of stimulus uncertainty, whether in terms of signal intensity, or uncertainty as to side. Thus they only occur when the subject can wholly allocate attention to one or other side, and they reflect a greater ability to hold attention to the right, rather than to shift it to the right. However these attentional aspects necessarily differ from Kinsbourne's ( 1 9 7 3 ) attentional model; he invokes an activational component whereas our account implicates sustained attentional effects. He predicts that asymmetries should occur only when stimuli and responses occur randomly as to side, rather than when side of stimulation and response is blocked. We of course find that the hemispace asymmetries are absent with random side of stimulation and response, and present when lateral stimulation occurs in blocks. Even haud asymmetries, of course, may be attentional; thus with positional uncertainty (where we found hand asymmetries replaced hemispace effects), a good strategy may be to direct attention to the preferred hand. Indeed, we can ask what is the shape of the distribution of extracorporeal hemispace, as may be indexed by a vibrotactile RT map. Is there a privileged position with very short RTs, for processing incoming information? Is it at the true midline, with hemispace asymmetrically distributed about it? Is it offset therefrom perhaps slightly to the right, with hemispace symmetrically disposed about it? We measured vibrotactile RTs with the stimulated and responding limb (left, o r right) placed out from the body either at the midline ( O O ) , 15" to left or right of midline, o r 45" or 9 0 " to left or right of midline, i.e. at seven possible positions. Reaction times for the two hands were found to be identical, confirming the absence of hand asymmetries in the absence of positional uncertainty. There was a significant (p < .01) position effect; by Tukey test the only position which was significantly different from any other (and it was significantly different from all other positions) was 9 0 " left (277 msec). A l l other positions varied between only 266 and 2 7 0 msec, the shortest in fact being 15' right, though none of these other values differed significantly from each other. We can conclude, therefore, that our RSAs should perhaps instead be characterized as a left side disadvantage (LSD), though perhaps only for positions well to the left (and possibly extending behind the body, though this remains to be determined). Indeed, if such hemispace effects can be extended right around the body, rather than characterizing them, at least in the vibrotactile RT paradigm, as a direct consequence of brain asymmetries, related e.g. to language lateralization, we should perhaps instead see them as reflecting differences in our abilities to attend to stimuli (or, perhaps more properly, to initiate responses) in different regions

52


of circumcorporeal space. Auditory Bemispace The most direct test between Kimura's anatomical-Dathwav model and the hemispace approach lies in the auditory modality. She specifically claims (1961, 1967) that the dichotic right ear advantage (REA) is due to the stronger, dominant, contralateral auditory pathways suppressing the weaker ipsilateral routes, so that the right ear has priviledged We (Pierson, Bradshaw & Nettleton, 1983) access to the verbal LH. replaced the traditional earphones (which separately stimulate each ear) with a single, laterally located, loudspeaker. Through this we played competing unilateral stimulation, sequences of aligned pairs of a digit (1, 8 , 9 or 10) and a letter (9, v. k or z ) which were not mutually confusable, one spoken by a male and one by a female as a further aid to discriminability. The stimuli, played from the single loudspeaker on one or other side of the subject, were loud enough to stimulate both ears. Subjects (dextral males, audiometrically screened to eliminate threshold differences between ears) shadowed (i.e. immediately repeated) each stimulus pair, and the vocal naming latencies were measured. Ten out of 12 subjects demonstrated a RSA of 38 msec (p < .005), the two reversals being very small. This finding conclusively demonstrates that competition at the two ears, as with dichotic stimulation, is unnecessary for demonstrating a verbal REA, and that Kimura's structural model is inadequate. We next asked whether it is the real or the perceived direction of a sound source which determines RSAs. We sought to generate "pseudo" RSAs via a ventriloquistic technique which relies upon the phenomenon of visual capture of the apparent direction of an ambiguously located auditory source. We placed two operating loud speakers one directly in front o f , and one behind the subject's midline, one voice from the previous tape being channelled to one loudspeaker and the other voice to the other, thus creating positional uncertainty about the direction of a An inactive dummy loudspeaker placed perceptually composite signal. either on the left or the right of the subject, who believed it to be fully operational, effectively captured the sound source, creating a powerful directional illusion. Eleven out of 12 subjects were faster (p < .005) by 28 msec when the dummy loudspeaker was on the right than on the left side, indicating that it is the perceived, rather than the real, position of a sound source which determines hemispace asymmetries. We next, as in our rod bisection and vibrotactile RT tasks, sought to dissociate the two coordinates of (auditory) hemispace, head and body, by 90" head turns to left or right. When an (active) loudspeaker was placed before or behind the subject's midline, it lay to the left or right of the turned head. (We were careful to ensure that only subjects were used who were able to maintain the requisite 90" head turns.) Likewise when the loudspeaker was to left or right of the body, it lay exactly before or behind the head midline. Under both these circumstances all hemispace asymmetries again completely vanished, and RTs were identical for the averages of the head-turn-right and headturn-left conditions, itself a finding which counts against Kinsbourne's (1973) activational-attentional account. According to him, performance in a verbal task should be superior when the subject orientates to the right. Indeed, we confirmed that the loss of a RSA with 90" head turns is a robust phenomenon by playing competing unilateral stimulation over a proximal earphone, rather than a distal loudspeaker source. We used the same stimulus tape as before, and as a control reincorporated a head-front (no turn) condition. We found that while the overall RTs for


53

head front, right and left did not differ significantly, with head front there was a very large ( 6 4 msec) and significant (p < .OOl) REA, and no significant ear asymmetries for either of the two turn conditions. Subsequently we repeated the earphone version of the experiment (unpublished data) with subjects lying horizontally on left or right sides, to dissociate gravitational and corporeal coordinates of extracorporeal space, as in our line bisection and vibrotactile tasks. Again, ear asymmetries vanished, and it did not matter on which side subjects lay. General Discussion and Suggestions for Further Besearch Over 70 years ago the first studies were reported concerning magnitude estimation in the two halves of the visual field ( s e e Ritter, 1917). Even the effects of body posture were investigated, but the general conclusions, due probably to coarse measurements and inadequate experimental control, were that individuals merely differed greatly. Since then, the field has lain fallow. Some early workers even used terms such as 'left side underestimation', though often in the opposite sense to ours; where we mean that an adjustable extent on one side is set smaller than it should be, perhaps in compensation for an apparent phenomenological magnification at the perceptual level, others seem in the past to have meant that an adjustable extent was presumably seen as smaller than reality, and therefore was set larger in compensation. From our own and other recent studies we can conclude that in rod and line bisection tasks there is a LSU, especially when the extent crosses the midline, and perhaps less so when it lies wholly in one or other hemispace, or is vertically rather than horizontally oriented. The LSU is immediate and perceptual, rather than somehow deriving from a fading memory trace, unlike traditional asymmetries which tend to be stronger with a memory component (Moscovitch, 1979; Nettleton 6 Bradshaw, 1983). However, as we shall shortly argue, attentional as well as sensory components contribute to the effect. The phenomenon, moreover, is probably a true LSU, rather than one involving right side overestimation. Any empirical attempt at dissociating the two possible determinants, LSU or right side overestimation, however, has to circumvent the procedural problem of a central standard or target magnitude being perceived as lying to the left of a variable or comparison extent on the right, or to the right of one on the left. Moreover the LSU is not due to a greater salience of stimuli lying on the left side, since it is greatest with salience *on the left and on the right. Increased underestimation with low salience (background contrast) might be due to a required increase in directed processing capacity (i.e. attention) to low salient, poorly contrasting stimuli, which consequently appear larger. Generally, extents in the two hemispaces may normally be asymmetrically matched because of differences in visuospatial processing capacities of the two hemispheres, itself perhaps a consequence of language pre-empting visuospatial processing space in the LH. However it is not then clear why such visuospatial asymmetries seem to be so affected by choice of hand used for performing the perceptual (1984) and match. Ramos-Brieva, Olivan, Palomares and Vela Schenkenberg, Bradford and Ajax (1980) both incidentally observed that line bisection LSUs were strongest when performed by the left hand. Our 5 year old dextral children performed similarly, while sinistrals of the same age behaved in an even more extreme fashion, placing the transection far to the left when using the left hand and far to the right with the right hand, as if unwilling even to approach the midline

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with either hand. Conversely, with dextral adults adjusting laterallylocated rods in the visual modality, LSUs tended to be stronger when the right hand made the adjustment. The two kinds of task of course differ in important ways; the former involves bisection of centrally located stimuli, while the latter involves magnitude matching with peripherally located stimuli. Otherwise in both cases hand used seemed to influence performance; LeDoux, Wilson and Gazzaniga (1977) claim that asymmetries are stronger at a manipulospatial than at a purely perceptual level. We have found, informally, that when two visual extents equal in magnitude appear side by side, they appear perceptually identical, i.e. there is no LSU in a purely visual task. To what extent may similar mechanisms determine the slight LSU shown by normal subjects, and the typical left side overestimation shown by hemineglect patients? One such dextral patient (NM) whom we examined had two years previously experienced extensive RH damage; since then he had undergone considerable rehabilitatory training. When tested on our kinesthetic rod-bisection task, he gave a consistent LSU, like a normal subject. However he seemed actively to be directing his attention to the left, as shown both by his eye movements (which we monitored, even though the task was purely kinesthetic), and by his performance in our vibrotactile RT task; in the latter he gave a left side advantage rather than the usual RSA. (Indeed, of 6 other hemineglect patients who had undergone little or no rehabilitation training at the time of testing, 5 showed very large RSAs, and one gave a 1 msec LSA, the group as a whole averaging 32 msec RSA, many times the normal magnitude.) However NM, when given the rod bisection task with low salience (contrast) on the right, overestimated the left extent and gave a typical lefthemineglect response. It was as if he were forced by the low salience on the right to direct attention away from the left to the right, thus unmasking a latent left hemineglect. Consequently left hemineglect patients probably do not find stimuli less salient on the left (i.e. a low-level sensory explanation), but may indeed suffer from a leftwards attentional deficit These hemispace asymmetries seem to be different from and stronger than anatomical pathway effects. The traditional anatomicalconnectivity account of lateral asymmetries (Kimura, 1961, 1967) cannot of course cope with our auditory, tactual or vibrotactile findings, which indicate that it is the position in extracorporeal space (real, or even as perceived) which often determines asymmetries, rather than just the proximal locus of receptor stimulation. (For this reason the verbal REA may not be just a special case of the RSA). Indeed our comparison between the effects of free scanning and controlled fixation in the visual rod tasks, and our observation that auditory asymmetries were larger with a unilateral loudspeaker than with a unilateral earphone (whose sound tends to be localized within the ear itself, rather than "out" in space), indicates the major role of hemisphere-hemispace connections. We found left hemispace superiorities for kinesthetic judgments of extent, texture matching and finger spacing, especially where memory rather than perceptual factors predominated. There was a right hemispace superiority for sequential finger-stimulation tasks (in sinistrals, whose preferred hand and mediating hemisphere might otherwise dissociate), for vibrotactile RTs (again, predominantly a motor effect) in the absence of stimulus uncertainty, and for competing auditory stimulation in a verbal shadowing task. The last effect depended as much upon the perceived (but illusory) position of a sound

.


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source as upon its real location. Hemispace asymmetries were always lost o r greatly reduced with 90" head turns which dissociate head and body coordinates, and with the adoption of a horizontal posture which dissociates corporeal and gravitational coordinates. Indeed recent physiological studies (see e.g. Jay & Sparks, 1984) of how the Superior colliculus maps auditory space indicate that individual neurones are tuned to different sound source loci, and are arranged so that their anatomical position is systematically related to a preferred location in space, with the coordinates of the map shifting along the horizontal plane as the (monkey's) eye changes position. Thus the coordinates of the auditory map move systematically with the eyes. In a similar fashion, Bisiach et al. ( 1 9 8 5 ) found with hemineglect patients that the boundary of neglect was influenced both by gaze direction and by the sagittal midplane of the trunk (trunk orientation). One might therefore speculate that the moment-to-moment symptoms of unilateral neglect might change as a function of head t u r n ( 9 0 " left, right) and body posture (horizontally on one OK other side). If s o , there are obvious rehabilitative imp1ications. A l l this means, of course, that OUK own perceived position in space, and the locus of stimulation in extracorporeal space is the result of polysensory, multimodal integration, perhaps as mediated by posterior (parietal) association areas. Thus the brain probably maintains two simultaneous maps, one of the proximal receptor surface which mediates the traditional asymmetries, and the other one for events (sensory and motor) occurring o u t i n extracorporeal space (see also Rizzolatti, Gentilucci & Matelli, 1985). Pit one sense against another, (e.g., as i n OUT dummy-speaker ventriloquism experiment) and the apparent system of spatial coordinates changes. We should therefore be able to mislead subjects in other interesting ways. An upright subject in a tilted room might lose the auditory or vibrotactile RSA, or a tilted subject within a room tilted the same way may regain the RSA characteristic of an upright subject i n a normal environment, such is the strength of visual capture. This phenomenon of course has been extensively explored in the context of interactions between vision and touch (Rock & Harris, 1967), and vision and vestibular input (Lee & Aaronson, 1974), and see also Pick, Warren and Hay ( 1 9 6 9 ) for proprioceptive capture of nonveridical auditory information where the sound source is systematically misperceived through the use of pseudophones. Indeed a subject who has worn laterally - displacing prisms (see e.g. Weiner, Hallet & Funkenstein, 1983) until adaptation is complete, may on their removal sense that a laterally displaced limb is central to the midline, OK vice versa, all of which should affect vibrotactile RT asymmetries, or pointing accuracy. There is in fact a suggestion (Heilman, Bowers 6 Watson, 1983) that normal subjects (with their eyes closed) may tend to deviate slightly to the left when asked to point out with the right hand directly in front of their midline. This may be somewhat analogous to the normal LSU in line bisection. In an incomplete study, we have found the opposite : 12 out of 14 subjects (with their eyes open) deviated to the right, when using their right hand, and very slightly to the left with the left hand. Hartmann ( 1 9 8 3 ) found that subjects trying to locate ambiguous sound sources were biased to the left from the correct azimuth in their errors by about 0.75'. (He also reviewed evidence of Superior localization accuracy in left rather than right space, analogous to some of our own findings in the tactual modality reviewed above). Finally, Hartmann found that bias

56


effects increased enormously if subjects shifted their gaze, suggesting, to us, once again that the auditory map of space is linked to eye position. Again a wealth of possible research is revealed in terms of accuracy of manual pointing to visual targets as a function of head and/or gaze direction with respect to each other and/or to the body. Gaze and head directionality could also be fruitfully varied in the vibrotactile and the competing monaural auditory paradigms. While not strictly involving the technique of pitting one sense against another, the old observations (see Krueger, 1982, for historical review) that skilled machinists employing screwdrivers, seamstresses using needles, and the blind relying upon their canes all project their sensation of pressure or touch to the very tip of the tool, suggest an interesting experiment. If a subject holds one end of a cane, rod or pipe, with either hand, on one side of the body, and the implement crosses the body to the other hemispace for discriminating e.g. texture, in which hemispace is the discrimination better mediated -- the "proximal" one where the hand holding the cane receives the stimulation, or the Is "distal" one to which the sensations are apparently projected? active exploration rather than passive stimulation necessary for sensations to be so projected? Thus White, Saunders, Scadden, Bach-yRita and Collins ( 1 9 7 0 ) developed a tactual prosthesis for the blind, a two-dimensional array of vibrators on the back of the subject, which was driven by a television camera attached to the wearer's head, and which responded to simple "visual" contours to which it was directed by exploratory head turns. Subjects reported that if they actively explored, they rapidly got the feeling that events were outside and even In front of the body, rather than merely proximal sensations received on the back. We have described how hand rather than hemispace asymmetries may appear during kinesthetic judgments of extent when stimuli lie wholly in one or other hemispace, or in vibrotactile RT tasks when there is stimulus uncertainty in terms either of low signal intensity, or random (left-right) location of the next stimulus or response event. Hemispace asymmetries only occur when the subject can wholly allocate attention in advance to one or other side, reflecting a greater ability to hold it to one or other side than to shift it, since they disappear with random stimulus alternation from side to side. Does this mean that we have then to revert to the traditional anatomical-connectivity account of e.g. hemisphere-hand connections to explain these hand asymmetries? Not necessarily, if hand asymmetries may sometimes reflect a strategy of deployment of attention to the preferred hand e.g. under conditions of spatial uncertainty, instead of to a particular hemispace; in the absence of such uncertainty, hand differences typically disappear in our vibrotactile RT tasks, and are once again replaced by hemispace asymmetries. Does this mean that an account invoking hemisphere hemispace mapping should itself be replaced by one invoking differential attentional biases, towards stimuli, responses or both, just as the hemispace model seems partly to have replaced the anatomical pathway account of e.g. hemisphere-hand connectivitles? In the vibrotactile modality, it is as if the (dextral) subject is more ready to initiate a response everywhere to the right of the midline, and perhaps as far to the left as the hands usually operate in normal psychomotor activity, as long as all sensory and motor aspects of the RT task are predictable and free of uncertainty; the disadvantaged region lies far to the left. In the auditory modality, where the RSA seems to reflect LH language lateralization, it is as if the subject is more ready to respond to


51

stimuli tagged as (really or apparently) coming from the right side. While attentional factors may be responsible for the loss of the vibrotactile RSA when side of presentation is random, such findings could also perhaps be at least partly explained in terms of hemispherehemispace relationships. Sensory-motor events successfully tagged ahead of time as coming from the right or left side of space are processed by the contralateral hemisphere. In the case of auditory-verbal and vibrotactile tasks, stimuli tagged as coming from the right are therefore processed by the left hemisphere which may be relatively specialized for these tasks. However when position information is n o t available ahead of time, there is no division of labour between the hemispheres. Both hemispheres now begin stimulus processing, but sensory-motor events regardless of their spatial location are now mediated by the specialized hemisphere which is always first to complete the required processing. We should of course be wary of assuming thac similar mechanisms underlie the vibrotactile and the auditory RSA. In a preliminary study we have failed to find a significant correlation between the magnitudes of the auditory and the vibrotactile RSAs across a sample of left and right handed subjects. The auditory phenomenon may reflect both language lateralization (a true RSA) and attentional processes (a left side disadvantage?), while the vibrotactile phenomenon may only reflect the latter, a differential ability to allocate attentional processes (perhaps predominantly motor) around the azimuth of extracorporeal space. Nevertheless it is certainly true that we cannot simply replace traditional anatomical-pathway maps of the proximal receptor surfaces, i n explaining all manifestations of lateral asymmetry, by alternative hemisphere-hemispatial representations, without including attention somehow i n the scheme of things. This does not however mean that we can equate our view of attention with that proposed by Kinsbourne (1970, 1975). According to Kinsbourne, the levels of activation in the two hemispheres are normally in a reciprocal balance, and this state is associated with a straight-ahead orientation of receptors and attentional tendencies. Eccentric stimulation or, conversely, endogenously generated activation i n the contralateral hemisphere, both of which disturb this balanced equilibrium, will lead to an orientational response i n that direction, t o inhibition of orienting responses in the other hemisphere, and to a biased readiness to accept stimulation and emit responses i n a direction contralateral to the activated hemisphere. Verbal processes activate the LH; visuospatial processes activate the RH and bias attention and the orientation of receptors i n the opposite direction. Asymmetries are not therefore seen as directly due to structural determinants but as a consequence of attentional and activational biases. However many studies (see Bradshaw & Nettleton, 1983) have failed to support Kinsbourne's hypothesis. The hemispatial account we propose here though superficially similar to Kinsbourne's approach, argues that all sensory stimuli come positiontagged, real or apparent, and the hemisphere contralateral to its real or apparent origin is biased towards its processing. However our account does not require Kinsbourne's activational component; he predicts larger verbal RSAs with rightwards head turns, while according to our account, and on the empirical evidence, asymmetries disappear with head turns. Kinsbourne predicts asymmetries should be largest: 1. When the subject knows in advance the verbal (or nonverbal) nature of the next stimulus (yet Geffen, Bradshaw & Nettleton, 1972, found that such advance knowledge made n o difference to the size or

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direction of asymmetries) when subjects cannot predict which side will next be stimulated, since such advance knowledge permits the appropriate hemisphere (left or right) to build up activation, thus washing out asymmetries; however we found that hemispace asymmetries were lost under these circumstances. Indeed the fact that the traditional procedure of randomly presenting visual stimuli to the left or right of fixation (to forestall fixational eyemovements) did not destroy visual field asymmetries is itself further evidence that anatomical pathway asymmetries (cf. Kimura, 1961, 1967) are qualitatively different from hemispatial asymmetries. Why did we not find a vibrotactile RT RSA when stimuli occurred randomly on either side, but nevertheless one limb responded on the same side for a block of trials? If the general vibrotactile RSA is motor rather than sensory, as we have argued earlier, then a constant side of motor responding should lead to a RSA, rather than to a right hand advantage. Conversely, when dissociating vibrotactile sensory and motor hemispace, subjects had to remember to respond with the limb opposite to that receiving stimulation; why did we not lose any (hemispatial) RSAs and instead gain a hand asymmetry? We would argue that in the latter instance there was no uncertainty since subjects knew where the stimuli would occur and were practised at always responding with the limb opposite to the one stimulated. (It is in fact noteworthy that the hemispace asymmetries were small in this experiment.) However with respect to the former case, any uncertainty, about the position of the stimulus or in terms of (low) intensity, may be enough to destroy motor asymmetries. Thus even though it does not help to know the side on which a stimulus will occur, as in a Stroop task it may not be possible to prevent the encoding of such unwanted information, and here its absence, or its unpredictability, may somehow be disruptive. Alternatively, with stimulus unpredictability processing resources may be borrowed from the motor end (cf. Friedman 6 Polson's, 1981, limitedresource-allocation model). This reallocation of resources may therefore lead to the elimination of these motor hemispace asymmetries. Indeed the distinction (Norman 6 Bobrow, 1975) between data limitation and resource limitation is useful in this context; data-limited stimuli (low intensity) do not lead to increased asymmetries. Asymmetries, which probably occur more at a level beyond the stage of immediate input, may increase as a function of competition for resources. Hence competing monaural presentations (e.g. with male and female speakers), rather than monaural presentations with a single speaker, may be needed to generate asymmetries. If so, will the asymmetries increase as a function of an increasing number of additional to-be-ignored speakers? In the visual modality, bilateral presentations generate greatly increased asymmetries (Seitz & McKeever, 1984). Conversely, if the locus of the vibrotactile RSA is motor rather than sensory, do other motor tasks generate large asymmetries? We have already seen that our finger-sequencing task did so (Burden et al., 1985). What happens if movement time (MT) rather than RT is measured in a vibrotactile-hemispace context (cf. Jeannerod, 1984)? Thus instead of a simple unitary response. a complex ballistic sequence of elements may be timed to completion. A concurrent-task paradigm might also be employed, where the effect of a verbal or a visuospatial secondary task is measured upon a primary (tapping) task taking place in one or other hemispace (though cf. McFarland, 1982). Another form of concurrent task where both tasks are hemispatially located would be to combine the 2.

Coordinates of'extracorporeal space

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auditory and the vibrotactile paradigms. Several other experiments remain to be done in the auditory and vibrotactile modalities. Can we generate a LSA in the auditory modality, with e.g. discriminations based on speaker's emotionality (Bryden & Ley, 1983; Scherer, Ladd & Silverman, 1984) or on voice recognition (Landis, Buttet, Assal h Graves, 1982), or, perhaps best of all, on a speech-perception task which appeals to the right hemisphere such as voice onset time (VOT) discriminations (Molfese & Molfese, 1979a,b)? Thus if two tasks, respectively appealing to the left and right hemispheres, can be given to the same subjects, we can avoid the problem (Bradshaw, et al., 1986) of a single test underestimating the true incidence of language lateralization (Lauter, 1982; Sidtis, 1982), as the results of the two tests can be expressed relative to each other rather than merely to an absolute (zero) midpoint. In the vibrotactile and auditory modalities, what is the effect on hemispace asymmetries of manipulating the predictability of side to be stimulated next? This can be achieved in either of two ways. Either stimuli can be presented in blocks to one or other side, with occasional "intrusive" items occurring on the unexpected side, or subjects can be precued (before each trial) as to side next to be stimulated, with occasionally the precue (deliberately) misinforming the subject. While in either case the hemispace asymmetries of interest are those (if any) occurring with the minority (misleading) stimuli, in the former paradigm the ability to hold attention is paramount, in the latter paradigm what is more of interest is the ability to shift attention. In the vibrotactile modality under these circumstances are hemispace asymmetries again replaced by hand asymmetries? Moving on from this issue, what happens if a go/no-go response is employed (i.e. a nonspatial choice of giving or withholding a response) for discriminating high from low (or vice versa) intensity (or frequency) stimuli? Such an experiment should permit us to ascertain whether it is choice RT per se, or spatial uncertainty, which destroys hemispace asymmetries. What happens if spatial choice is now made between the two adjacent fingers on a single limb, or even between two limbs (crossed or uncrossed with respect to each other) but now both on the ~ a m eside of the midline - is there a superiority for the rightmost of the two (cf. Nicoletti, Umilta & Ladavas, 1984)? Indeed we note that according to MacKenzie and Martenuik (1985), in a choice RT task between pairs (all possible pairs) of fingers, the finger that happened to be on the right always proved faster. Can we measure anterior (motor) and posterior (sensory) evoked potentials, simultaneously with vibrotactile RTs, over left and right cortices, in the contexts of: (i) anatomical connectivities (contralateral pathways between limb, sensory or motor, and hemisphere), and (ii) hemisphere-hemispace relationships (i.e. contralateral hemispace, irrespective of whichever limb currently occupies it)? Thus it might be possible, electrophysiologically, to separate out the two brain 'maps', the traditional anatomical-pathway map, and the map of extracorporeal space. It is, of course, likely that the two representations are fairly tightly coupled, since as we have seen, not only can a hemispace (left or right) within which a hand operates determine (hand) performance (Burden et al., 1985), but a hand (left or right) which operates within a given hemispace can itself influence hemispace asymmetries (e.g. LSUs etc. in line and rod bisection). While our findings cannot yet allow us to determine the exact

J. L. Bradskaw el al. mechanisms of the hemineglect syndrome, with our new procedures we can now for the first time try to quantify the syndrome in all its stages, and (e.g. with the vibrotactile technique) determine the relative contributions of (sensory) posterior and motor (anterior) processes, and maybe demonstrate the hitherto-elusive right-hemineglect syndrome. Indeed one subject with minimal left paresis was able to perform the bimanual version (which permits separate evaluation of sensory and motor components); he demonstrated an 89 msec sensory RSA and only a 15 msec motor RSA. Another patient, late in the recovery phase, showed a near normal vibrotactile RSA until he was required to perform with eyes closed; the RSA then increased enormously. (Chedru, 1976, also found that hemineglect patients without visual field defects demonstrated left neglect on a tactual space-exploration task only when their eyes were closed). Properly designed experiments should permit us to separate out sensory, motor and memory components, attentional deficits, stimulus salience, extinction, failures to acknowledge events or objects on one side of the body, or of a display (wherever positioned), or one side of each of a group of objects in a display, together with midline shifts and unilateral distortions of apparent size - all of which may differentially characterize different manifestations of a nonunitary syndrome. Thus a patient may be asked to adjust a rod laterally so that its two ends are equidistant from a central loop or ring through which it passes; the loop or ring may be placed either objectively on the patient's midline, or where he/she says it is (e.g. to the right of the true midline). In the former case do we find a right side underestimation, with objective equality in the latter case? If so a midline shift would appear to be the underlying mechanism. Otherwise, we would have to appeal to perceptual neglect. With the dummy loudspeaker (ventriloquism) paradigm, do patients perform badly when the dummy loudspeaker is to the left and the sound appears to come from the neglected side, or do they do well because the dummy loudspeaker simply is not noticed, and the sound therefore seems to come from the right? If, in nonverbal tasks, the RH normally pays attention to both sides of space, while the left can only cope with contralateral hemispace, in the auditory-verbal paradigm does the LH now pay attention to both sides of space, and the RH only to the contralateral side? Finally, we can ask what is the possible r o l e of vision in these tasks? We have already seen that 90" head turns may destroy asymmetries; conversely evidence is emerging that deviation of gaze (necessarily limited to a few tens of degrees) towards a given hemispace may favor stimulus processing in the contralateral hemisphere (Gross, Franko 6 Lewin, 1978; Honore, 1982; La Torre 6 La Torre, 1981; Lempert 6 Kinsbourne, 1982). Tressoldi (personal communication) found that lateral asymmetries may appear in visual tasks even when stimuli are presented in right or left hemispace, with foveal presentations, the head facing forward but with fixation deviated to left or right. With respect to long term visual experience, Kinsbourne and Lempert (1980) claimed that tactile-kinesthetic information cannot fully compensate for visual experience in the formation of an internalized representation of the human body, when they compared congenitally blind children with normally sighted but blindfolded controls. Brown (cited in Pick, 1980) found that vision (eyes open) may be necessary for an adequate conception of gravitationally defined coordinates. Attneave and Benson (1969) found that subjects, judging the directionality of vibratory stimulation delivered to the hands held in various orientations with respect to the body, used a gravitational frame of reference when their


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eyes were open, and an egocentric (i.e. corporeal) reference frame with eyes closed. Hermelin and O'Connor ( 1 9 7 1 ~ ) found that sighted children with eyes open performed a tactual task by relying upon gravitational coordinates, while blind and blindfolded children relied upon a corporeal frame of reference. Pick (1980) wonders whether the immediate availability of a concept of visual space encourages the use of gravitational over egocentric (corporeal) coordinate systems. Earlier the same author (Rieser & Pick, 1976) had found that subjects made to adopt a horizontal posture used an egocentric frame of reference for stimuli traced directly upon the proximal body surface, but adopted a gravitational reference frame for more distal objects palpated by the hands i n extracorporeal space. As previously noted, we have already seen that blindfolding appeared to reinstate the acute phase of the syndrome in an apparently recovered hemineglect patient. What happens with normals, as well as hemineglect patients, when blindfolded while performing kinesthetic rod bisections, vibrotactile RT tasks, and auditory hemispace tasks? What happens w i t h the congenitally blind? We have already found (see above) that tactual LSUs are not shown by such subjects, and we earlier noted (Bradshaw et al., 1982) with such a group that neither hand nor hemispace asymmetries emerged in a Braille reading task. Do they appear if an "Optacon" transducer is instead employed, presenting vibrotactile patterns of real letter shapes to the forefinger? Does it depend on the (tactual or phonological) similarity of the stimuli (cf. Mousty, Bertelson & Kurrels, 1982)? We have noted an unusually high incidence of nondextrals in our samples of congenitally blind subjects. (Bonvillian et al., 1982, noted the same in the congenitally deaf.) Karavatos et al., (1984) found that the congenitally blind fail to exhibit the usual ear asymmetries, and Gibson and Bryden (1984) review similar effects in the congenitally deaf. We have found (unpublished data) that the congenitally blind fail to give the usual LSU in kinesthetic rod bisection. Finally, if the blind, or normally sighted but blindfolded subjects, are asked to walk in a straight line, are there consistent turning biases or deviations, perhaps to the left, due perhaps to asymmetries in the extrapyramidal motor system (cf. Glick & ROSS, 1981)? In conclusion, the conventional picture of lateral asymmetries reflecting differences in the way information is processed when presented to receptors found on one OK other side of the body, and which project via fixed anatomical pathways largely to one OK other cerebral hemisphere, needs a radical reappraisal. At the very least this picture cannot take into account the dynamic changeable nature of empirically determined asymmetries. The other approach, which should perhaps be seen as complementary and additional to, rather than alternative to the traditional anatomical-connectivity account, appeals to the concept of hemispace. Thus the brain may map both the distribution of stimuli upon the proximal receptor surfaces, and also the occurrence of relatively distal sensory and motor events which occur out beyond the immediate body in extracorporeal space. These two maps may be dissociated by such strategies as crossing the arms across the body midline, turning the head relative to the body and possibly the eyes relative to the head's orientation), and adopting a horizontal posture upon one or other side, to dissociate corporeal and gravitational coordinates. Gardner and Ward (1979) concluded that information about the spatial position (to left or right of the body) is probably more important than which hand actually felt a stimulus. In this context we find curiously prescient the words of Attneave and Benson (1969):

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"Behaviour is much more simply predictable from the external situation, i.e. from 'distal stimuli', than from events at the receptor surface, or 'proximal stimuli'" (p.216). Indeed their very techniques foreshadowed our own -- the effects of head tilt and rotation on judgments of line orientation, and judgments of orientation relative to an internal reference system, whose vertical may or may not correspond to the objective vertical. They even examined the effects of blindfolding, concluding that spatial location is represented primarily in visual terms, even when another modality is under investigation. Other strategies available to us for dissociating the two hypothesized brain maps, strategies whose effects are not predicted by the traditional anatomical-connectivity account, include pitting one sensory modality against the other (e.g. vision and audition, vision and kinesthesis, and, possibly, kinesthesis and audition), and manipulating the effects of uncertainty and attention. Indeed it seems that vibrotactile hemispace effect (and perhaps auditory effects as well) may also reflect attentional biases towards certain regions of space, rather than just being the simple consequence of hemisphere-hemispace relationships.It is within these contexts that we believe future research, both pure and applied (e.g. with respect to the clinical syndrome of unilateral spatial neglect, and to congenital blindness) needs to be directed. References

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PERCEPTUAL AND ACTION SYSTEpls IN UNILAlXRAL VISUAL NEGLECT J a n e Riddoch and Glyn W. Humphreys M.

I n t h i s c h a p t e r w e review t h r e e d i f f e r e n t a c c o u n t s of u n i l a t e r a l n e g l e c t : one m a i n t a i n i n g t h a t n e g l e c t is due t o e a r l y v i s u a l p r o c e s s i n g d e f i c i t s ; one m a i n t a i n i n g t h a t n e g l e c t i s due t o a d i s o r d e r of a n i n t e r n a l r e p r e s e n t a t i o n of s p a c e ; and o n e m a i n t a i n i n g t h a t n e g l e c t is due t o a d i s o r d e r of v i s u a l a t t e n t i o n . An a t t e n t i o n a l view of n e g l e c t i s e l a b o r a t e d i n which n e g l e c t i s a t t r i b u t e d t o a breakdown i n t h e p r o c e s s e s e n a b l i n g s t i m u l i on t h e c o n t r a l a t e r a l s i d e of s p a c e t o a l e s i o n t o " c a p t u r e " v i s u a l a t t e n t i o n . This a t t e n t i o n a l account p r e d i c t s t h a t c o n t r a l a t e r a l s t i m u l i may v a r y a c c o r d i n g t o t h e e a s e w i t h which t h e y " c a p t u r e " t h e a t t e n t i o n of n e g l e c t p a t i e n t s . F u r t h e r , even when " a t t e n t i o n a l c a p t u r e " d o e s n o t o c c u r , a t t e n t i o n may n e v e r t h e l e s s be c o n s c i o u s l y d i r e c t e d t o t h e n e g l e c t e d s i d e . Data s u p p o r t i n g t h i s p o s i t i o n a r e r e p o r t e d from v i s u a l s e a r c h t a s k s where t a r g e t s a r e d e f i n e d e i t h e r by s i n g l e f e a t u r e d i f f e r e n c e s r e l a t i v e t o d i s t r a c t o r s o r by some combination of l o c a l f e a t u r e i n f o r m a t i o n , and where n e g l e c t p a t i e n t s a r e e i t h e r f r e e t o adopt t h e i r own s e a r c h s t r a t e g y o r t h e y a r e cued t o t h e n e g l e c t e d s i d e . N e g l e c t w a s more marked i n t h e combined-feature searches than t h e s i n g l e - f e a t u r e s e a r c h e s , and i t tended t o be reduced by s p a t i a l cueing. I m p l i c a t i o n s f o r u n d e r s t a n d i n g u n i l a t e r a l v i s u a l n e g l e c t and f o r u n d e r s t a n d i n g t h e o p e r a t i o n o f n o r m a l v i s u a l a t t e n t i o n a r e discussed.

I. Introduction T y p i c a l l y , t h e o r i e s of v i s u a l i n f o r m a t i o n p r o c e s s i n g d i s t i n g u i s h two f u n c t i o n a l l y i n d e p e n d e n t p r o c e s s i n g l e v e l s ( e . g . , Duncan, 1980; Hoffman, 1979; Humphreys, 1985; Kahneman & T r e i s m a n , 1983; N e i s s e r , 1967; T r e i s m a n & Gelade, 1980). A t t h e f i r s t , p r e - a t t e n t i v e l e v e l , p r o c e s s i n g is t h o u g h t t o be t h e f a s t and s p a t i a l l y p a r a l l e l . Accounts of t h e i n f o r m a t i o n e x p l i c i t l y r e p r e s e n t e d a t t h i s l e v e l d i f f e r . For i n s t a n c e , some t h e o r i s t s h o l d t h a t o n l y i n f o r m a t i o n a b o u t d i s c r e t e f e a t u r e a t t r i b u t e s such a s c o l o u r , shape and s i z e ( c f . Q u i n l a n & Humphreys, 1984) a r e s e p a r a t e l y s p e c i f i e d ( e . g . , Treisman, 1982; Treisman & Schmidt, 1982: Treisman, Sykes & G e l a d e , 1977) ; o t h e r s hold t h a t a f u l l , i n t e g r a t e d d e s c r i p t i o n of t h e s t i m u l u s i s represented p r e - a t t e n t i v e l y (e.g., Duncan, 1980; Humphreys, 1985). Whatever t h e c a s e , i t is g e n e r a l l y t h o u g h t t h a t a c o n s c i o u s r e p r e s e n t a t i o n of an i n t e g r a t e d s t i m u l u s i s o n l y made a v a i l a b l e a t a second l e v e l of p r o c e s s i n g . The t r a n s i t i o n between t h e two p r o c e s s i n g l e v e l s can i n v o l v e a limited-capacity, a t t e n t i o n a l mechanism, which o p e r a t e s s e r i a l l y on s t i m u l u s i n f o r m a t i o n (so t h a t d e t r i m e n t s a r e e n c o u n t e r e d w h e n m o r e t h a n o n e s t i m u l u s m u s t be a t t e n d e d a t a t i m e ) .

152

M.J. Riddoch and G. W . Humphreys

U n i l a t e r a l v i s u a l n e g l e c t f o l l o w i n g b r a i n damagemaybe d e f i n e d a s t h e f a i l u r e t o a c t t o v i s u a l i n p u t on t h e s i d e of s p a c e c o n t r a l a t e r a l t o t h e s i t e of l e s i o n ( F r i e d l a n d & W e i n s t e i n , 1977; Heilman & V a l e n s t e i n , 1979). The f a i l u r e t o a c t t o c o n t r a l a t e r a l l y p r e s e n t e d s t i m u l i c a n be found i n a range of t a s k s , v a r y i n g from e v e r y d a y t a s k s such a s e a t i n g t h e food from b o t h s i d e s of t h e p l a t e t o m o r e s p e c i f i c t a s k s s u c h a s r e g i s t e r i n g d o u b l e s i m u l t a n e o u s s t i m u l a t i o n . Such a d e f i c i t s u g g e s t s some fundamental impairment i n i n f o r m a t i o n p r o c e s s i n g p r i o r t o t h e a d d r e s s i n g of motor r e s p o n s e s , s u c h a s a n impaired a b i l i t y t o a t t e n d f o c a l l y t o s t i m u l i i n c o n t r a l a t e r a l s p a t i a l regions. I n t h i s c h a p t e r , we w i l l c o n s i d e r v a r i o u s a c c o u n t s of u n i l a t e r a l n e g l e c t and i n p a r t i c u l a r , t h e i d e a t h a t n e g l e c t p a t i e n t s a r e impaired a t a t t e n d i n g f o c a l l y t o i n f o r m a t i o n made a v a i l a b l e by e a r l y ( p r e - a t t e n t i v e ) v i s u a l p r o c e s s e s . New e v i d e n c e c o n s i s t e n t w i t h t h e l a t t e r a c c o u n t w i l l t h e n be put forward and d i s c u s s e d i n t h e l i g h t of c u r r e n t t h e o r i e s of v i s u a l a t t e n t i o n i n b o t h normal s u b j e c t s and i n p a t i e n t s m a n i f e s t i n g u n i l a t e r a l n e g l e c t . The r e l e v a n c e of t h e e v i d e n c e f o r u n d e r s t a n d i n g normal v i s u a l perceptionwill then bediscussed.

I1.AccountsofUnilateralNeglect H i s t o r i c a l l y t h e r e have been many a t t e m p t s t o e x p l a i n t h e mechanisms u n d e r l y i n g u n i l a t e r a l n e g l e c t , and t h e y can be d i v i d e d i n t o t h r e e g e n e r a l c l a s s e s : 1. t h o s e w h i c h a t t r i b u t e t h e p h e n o m e n o n t o e a r l y v i s u a l p r o c e s s i n g d e f i c i t s ; 2. t h o s e which a s c r i b e i t t o a d i s o r d e r of r e p r e s e n t a t i o n a l schema ; and 3. t h o s e which a s c r i b e i t t o d i s o r d e r e d a t t e n t i o n a l o r orientingsystems.

1.NeglectasDuetoEarlyVisualProcessingDeficits There a r e v a r i o u s formsof thisargument.Forinstance, c e r t a i n w r i t e r s s u g g e s t t h a t n e g l e c t is j u s t one m a n i f e s t a t i o n of t h e o v e r a l l symptomatology produced by l a r g e r c e r e b r a l l e s i o n s . I n d e e d , B a t t e r s b y , Bender, P o l l a c k and Kahn ( 1 9 6 5 ) , i n a s t u d y of 132 p a t i e n t s w i t h s p a c e o c c u p y i n g l e s i o n s , found t h o s e w i t h n e g l e c t t o b e hemianopic, t o h a v e m a r k e d d e f i c i t s i n g e n e r a l mental performance, and t o show somato-sensory and motor d e f e c t s . Z a r i t and Kahn (1974) a l s o d e m o n s t r a t e d t h a t t h e d e g r e e of n e g l e c t i n a sample of 89 p a t i e n t s c o r r e l a t e d w i t h t h e d e g r e e of o t h e r impairments. The h i g h e s t n e g l e c t s c o r e s were o b t a i n e d by p a t i e n t s who a l s o

hadvisualfielddefectsandimpairedintellectualcapacities. However, i t i s u n l i k e l y t h a t n e g l e c t can be a t t r i b u t e d t o g e n e r a l mental d e t e r i o r a t i o n . F o r example, Lawson (1962) found no d e g r e e of i n t e l l e c t u a l impairment i n t h e two c a s e s he s t u d i e d ; E t t l i n g e r , W a r r i n g t o n and Zangwill (1957) found i n t e l l e c t u a l impairment i n o n l y one of t h e n i n e cases t h e y examined, and d e m e n t i a w a s s p e c i f i c a l l y e x c l u d ed i n a t l e a s t two of t h e c a s e s r e p o r t e d by McFie, P i e r c y and Z a n g w i l l (1950). Given t h e e q u i v o c a l d a t a on t h e r o l e p l a y e d b y g r o s s o r g a n i c impairment i n t h e g e n e s i s of u n i l a t e r a l n e g l e c t , i t seems r e a s o n a b l e t o assume t h a t t h e magnitudeof c e r e b r a l damage is n o t c a u s a l l y r e l a t e d t o n e g l e c t . A r a t h e r d i f f e r e n t s u g g e s t i o n i s t h a t n e g l e c t is r e l a t e d t o v i s u a l f i e l d d e f e c t s i n p a t i e n t s . Hecaen (1962) and B a t t e r s b y e t a l . (1965) have b o t h demonstrated t h a t n e g l e c t is a s s o c i a t e d w i t h a h i g h i n c i d e n c e of hemianopia ( w i t h 76% and 100% of t h e i r r e s p e c t i v e samples of n e g l e c t p a t i e n t s having a hemianopia r e c o r d e d ) . However, A l b e r t (1973), i n a s t u d y of 30 r i g h t hemisphere damaged p a t i e n t s , found t h a t o n l y 50%of t h o s e w i t h v i s u a l f i e l d d e f e c t s m a n i f e s t e d n e g l e c t w h i l s t o n l y 55% of t h o s e showing n e g l e c t had v i s u a l f i e l d d e f e c t s . I t a p p e a r s t h a t t h e r e is no a b s o l u t e r e l a t i o n s h i p between n e g l e c t a n d v i s u a l f i e l d d e f e c t s .

Perceptual and action systems

153

The above argument i s f u r t h e r s u p p o r t e d b y t h e s t u d i e s o f Chedru(1976) and DeRenzi, F a g l i o n i and S c o t t i ( 1 9 7 0 ) . Chedru p r e s e n t e d 91 b r a i n damaged p a t i e n t s w i t h a t e l e t y p e keyboard. I n i t i a l l y , p a t i e n t s w e r e b l i n d f o l d e d a n d asked t o t a p keys alloverthekeyboardasquicklyas p o s s i b l e . P a t i e n t s t h e n repeated t h e taskwithout the b l i n d f o l d . I n t h e sighted condition,mostkeys were o m i t t e d by t h e r i g h t b r a i n damaged p a t i e n t s w i t h v i s u a l f i e l d d e f e c t s and t h e s e o m i s s i o n s were on t h e l e f t s i d e of t h e keyboard ( d e m o n s t r a t i n g n e g l e c t ) . L e f t hemisphere damaged p a t i e n t s w i t h f i e l d d e f e c t s made fewer o m i s s i o n s on t h e s i d e c o n t r a l a t e r a l t o t h e i r l e s i o n s r e l a t i v e t o t h e r i g h t hemisphere p a t i e n t s . F u r t h e r , i n t h e b l i n d f o l d e d c o n d i t i o n , most n e g l e c t was produced by r i g h t hemisphere damaged p a t i e n t s w i t h o u t f i e l d d e f e c t s . DeRenzi e t a l . ( 1 9 7 0 ) a l s o a t t e m p t e d t o d e m o n s t r a t e n e g l e c t i n a t a s k n o t dependent on v i s u a l cues. They asked p a t i e n t s t o s e a r c h w i t h a f o r e f i n g e r f o r a m a r b l e p l a c e d a t t h e end of one of f o u r arms of a maze. The maze w a s hidden behind a c u r t a i n . They found t h a t , o v e r a l l , b r a i n damaged p a t i e n t s were n o t worse on t h i s t a s k t h a n non-brain damaged c o n t r o l s u b j e c t s . However, w i t h i n t h e b r a i n damaged group, p a t i e n t s w i t h v i s u a l f i e l d d e f e c t s were r e l i a b l y worse t h a n t h o s e w i t h o u t , and p a t i e n t s t o o k m o r e t i m e t o f i n d t h e marble when i t w a s on t h e s i d e of s p a c e c o n t r a l a t e r a l t o t h e l e s i o n s i t e . D i f f e r e n c e s between r i g h t and l e f t hemisphere damaged p a t i e n t s may have been obscured i n t h i s s t u d y because of t h e u s e b y D e R e n z i e t a 1 . of a c u t o f f s e a r c h t i m e , which meant t h a t many p a t i e n t s performed a t t h e f l o o r level. Both Chedru's ( 1 9 7 6 ) a n d D e R e n z i e t a l . ' s (1970) r e s u l t s i n d i c a t e t h a t n e g l e c t can be found i n t a s k s which d o n o t r e q u i r e v i s u a l i n p u t : t h e y t h e r e f o r e s u g g e s t t h a t n e g l e c t cannot be a t t r i b u t e d s o l e l y t o v i s u a l f i e l d d e f e c t s , and. r a t h e r , t h a t p a t i e n t s n e g l e c t some i n t e r n a l r e p r e s e n t a t i o n o f space. Both r e s u l t s a l s o i n d i c a t e d i f f e r e n c e s between d i f f e r e n t classes of p a t i e n t s . Chedru's f i n d i n g is p a r t i c u l a r l y s t r o n g s i n c e d i f f e r e n c e s were found between two p a t i e n t groupswithequalincidenceof f i e l d d e f e c t s ( w i t h n e g l e c t most m a n i f e s t i n t h e r i g h t hemisphere damaged group). T h i s emphasises t h a t f i e l d d e f e c t s p e r se do n o t produce n e g l e c t , and i t a l s o r u l e s o u t t h e i d e a t h a t n e g l e c t w a s produced by poor motor r e s p o n s e s on t h e s i d e of s p a c e c o n t r a l a t e r a l t o t h e l e s i o n s i t e , s i n c e motor d e f e c t s s h o u l d b e e q u a l i n t h e rightandlefthemispheredamagedgroups. A f u r t h e r a c c o u n t of n e g l e c t i n terms of a f a i l u r e i n a t t a i n i n g appropriate stimulus information is the suggestion t h a t p a t i e n t s with n e g l e c t have impaired v i s u a l s c a n n i n g (Denny-Brown h F i s c h e r , 1976). Support f o r t h i s a c c o u n t i s provided by B e l l u z a , Rappaport, Kennethand H a l l ( 1 9 7 9 ) , who found t h a t a l l t h e p a t i e n t s i n t h e i r sample who m a n i f e s t e d n e g l e c t on drawing t a s k s showed abnormal v i s u a l s c a n n i n g p a t t e r n s , a s measured by e y e movement r e c o r d i n g s . Such p a t i e n t s had s h o r t e r f i x a t i o n d u r a t i o n s and s p e n t s i g n i f i c a n t l y less f i x a t i o n t i m e o n t h e i n f o r m a t i v e p a r t s of s t i m u l i . A l s o , A l b e r t (1973) found t h a t 64% of h i s sample of p a t i e n t s w i t h n e g l e c t had oculomotor d e f e c t s . S i m i l a r r e s u l t s have been o b t a i n e d by Chedru, LeBlanc and L h e r m i t t e ( 1 9 7 3 ) . Again, however, t h i s a c c o u n t h a s d i f f i c u l t y a c c o u n t i n g f o r d a t a s u c h a s t h o s e r e p o r t e d by Chedru ( 1 9 7 6 ) . s i n c e i t is n o t c l e a r why a s c a n n i n g d e f i c i t s h o u l d produce n e g l e c t under b l i n d f o l d c o n d i t i o n s . A l s o , n e g l e c t can be found i n v i s u a l p r o c e s s i n g t a s k s where i t i s u n l i k e l y t h a t e y e movements m e d i a t e performance (e.g.. P o s n e r , Cohen 6 R a f a l , 1982; P o s n e r , Walker, F r i e d r i c h 6 R a f a l , 1984). I t d o e s not seem t h a t n e g l e c t isattributabletoascanningdeficit : i n d e e d , i t is q u i t e p o s s i b l e t h a t s c a n n i n g d e f i c i t s may themselves be caused by some o t h e r p r o c e s s i n g problem, s u c h a s a n i n a b i l i t y t o o r i e n t t o s t i m u l i on t h e c o n t r a l a t e r a l s i d e of s p a c e t o t h e l e s i o n , which a f f e c t s b o t h o v e r t and c o v e r t attentionalmechanisms(see s e c t i o n I I 1 ) .

154

M.J. Riddoch and G. W.Humphreys

Two o t h e r p r o p o s a l s s h o u l d a l s o be mentioned i n t h i s s e c t i o n the p e r c e p t u a l r i v a l r y h y p o t h e s i s and t h e i n t e r - h e m i s p h e r i c i n h i b i t i o n h y p o t h e s i s . Both of t h e l a t t e r h y p o t h e s e s h o l d t h a t n e g l e c t o c c u r s b e c a u s e p a t i e n t s do n o t r e c e i v e a p p r o p r i a t e s t i m u l u s i n f o r m a t i o n , though b o t h s u g g e s t some c e n t r a l p r o c e s s i n g d e f i c i t a s t h e c a u s e ( r a t h e r t h a n a p e r i p h e r a l d e f i c i t such a s a v i s u a l f i e l d d e f e c t ) . The p e r c e p t u a l r i v a l r y h y p o t h e s i s (Denny-Brown, Meyer & H o r e n s t e i n , 1 9 5 2 ) a t t r i b u t e s n e g l e c t t o a n i n a b i l i t y t o s y n t h e s i z e m u l t i p l e s e n s o r y i n p u t on t h e s i d e of t h e body c o n t r a l a t e r a l t o t h e l e s i o n . The s u g g e s t i o n i s t h a t t h i s f a i l u r e t o s y n t h e s i z e s e n s o r y i n p u t p r e v e n t s i n f o r m a t i o n from b e i n g p a s s e d on t o h i g h e r - l e v e l r e c o g n i t i o n p r o c e s s e s . The i n t e r - h e m i s p h e r i c i n h i b i t i o n h y p o t h e s i s h o l d s t h a t n e g l e c t o c c u r s because s t i m u l i on t h e non-neglected s i d e of s p a c e i n h i b i t t h e p r o c e s s i n g of n e g l e c t - s i d e s t i m u l i ( e . g . , B i r c h , Belmont & Karp, 1 9 6 7 ) , and t h a t t h i s d i s r u p t i o n t o p r o c e s s i n g p r e v e n t s recognition fromtaking place. I n a d e t a i l e d s i n g l e c a s e s t u d y of a p a t i e n t w i t h r i g h t hemisphere damage, Denny-Brown e t a l . (1952) found t h a t e l e m e n t a r y i n p u t p r o p e r t i e s s u c h a s t h e p r e s e n c e o r a b s e n c e of t o u c h o r a p i n - p r i c k o n t h e n e g l e c t e d s i d e c o u l d be p e r c e i v e d , but t h a t t h e p e r c e p t i o n of d o u b l e s t i m u l a t i o n was d e f i c i e n t . A l s o , t h e i r p a t i e n t was a b l e t o i d e n t i f y s i n g l e o b j e c t s p r e s e n t e d on t h e l e f t s i d e of s p a c e b u t , when p r e s e n t e d w i t h p i c t u r e s of s e v e r a l o b j e c t s jumbled t o g e t h e r , t h e p a t i e n t c o r r e c t l y named o n l y t h o s e on t h e r i g h t . When t h e p i c t u r e was i n v e r t e d , t h e p a t i e n t t h e n i d e n t i f i e d t h o s e i t e m s on t h e r i g h t ( i . e . , which had p r e v i o u s l y been on t h e l e f t ) b u t n o t t h o s e on t h e l e f t ( i . e . , which had p r e v i o u s l y been on t h e r i g h t ) . Composite p i c t u r e s w i t h o b j e c t s more w i d e l y spaced l e d t o improved performance f o r left-side objects. To a c c o u n t f o r t h e poor i d e n t i f i c a t i o n of l e f t - s i d e s t i m u l i shown by t h e i r p a t i e n t when p r e s e n t e d w i t h s t i m u l i i n b o t h t h e l e f t and t h e r i g h t v i s u a l f i e l d s , Denny-Brown e t a l . a r g u e t h a t t h e f i x a t i o n of n e g l e c t p a t i e n t s i s s h i f t e d t o t h e r i g h t when t h e r i g h t and l e f t s i d e s t i m u l i a r e b o t h p r e s e n t , w i t h a consequent r e d u c t i o n i n v i s u a l p e r c e p t i o n t o t h e l e f t . However, n e g l e c t p a t i e n t s do not s i m p l y f a i l t o r e c o g n i s e s t i m u l i on t h e a f f e c t e d s i d e , t h e y t y p i c a l l y r e a c t a s i f t h e s t i m u l i do n o t e x i s t . The argument t h a t n e g l e c t e d s t i m u l i a r e e q u i v a l e n t t o s t i m u l i p r e s e n t e d i n t h e v i s u a l p e r i p h e r y i s not s u f f i c i e n t . A l s o , t h e p r o p o s a l f a i l s t o accomodate t h e f a c t t h a t n e g l e c t can be m a n i f e s t i n m o d a l i t i e s o t h e r t h a n v i s i o n (e.g., Chedru, 1976). A c c o r d i n g l y , Denny-Brown e t a l . f u r t h e r p o s i t t h a t p e r c e p t i o n r e s u l t s from t h e s y n t h e s i s of s t i m u l u s p r o p e r t i e s v i a a summation p r o c e s s and t h a t n e g l e c t p a t i e n t s e f f e c t i v e l y f a i l t o s y n t h e s i z e more t h a n a few s t i m u l u s p r o p e r t i e s . We may t h i n k of t h i s a s some form of c e n t r a l c a p a c i t y l i m i t a t i o n i n p r o c e s s i n g m u l t i p l e s t i m u l i . The l i m i t a t i o n i s most m a n i f e s t when s t i m u l i a r e p r e s e n t e d on b o t h s i d e s of s p a c e , s i n c e t h e n c a p a c i t y i s devoted s o l e l y t o t h e s i d e i p s i l a t e r a l t o t h e l e s i o n so producingneglectof the c o n t r a l a t e r a l side. The l a s t s u g g e s t i o n i s q u i t e c l o s e t o t h e inter-hemispheric inhibition argument t h a t n e g l e c t i s due t o i n h i b i t i o n of t h e p r o c e s s i n g of s t i m u l i p r e s e n t e d on t h e a f f e c t e d s i d e of space by non-neglected s t i m u l i . F o r i n s t a n c e , B i r c h e t a l . (1967) s u g g e s t t h a t t h e damaged nervous s y s t e m i s c h a r a c t e r i s e d by i n c r e a s e d i n e r t i a , as shown by t h e i n c r e a s e d time needed t o p r o c e s s i n f o r m a t i o n and by i n c r e a s e s i n t h e time r e q u i r e d t o r e c o v e r from t h e e f f e c t s of p r e c e e d i n g s t i m u l a t i o n . Thus, when t h e p a t i e n t i s g i v e n d o u b l e s i m u l t a n e o u s s t i m u l a t i o n , t h e r e is i n t e r f e r e n c e from t h e more r a p i d l y f u n c t i o n i n g i n t a c t d i v i s i o n s of t h e n e r v o u s s y s t e m on t h e damaged p o r t i o n s . T h i s i n t e r f e r e n c e c a n l e a d t o a n i n a b i l i t y t o r e g i s t e r s t i m u l i on t h e a f f e c t e d s i d e under d o u b l e s i m u l t a n e o u s s t i m u l a t i o n c o n d i t i o n s .


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Using a d o u b l e s i m u l t a n e o u s s t i m u l a t i o n p r o c e d u r e , B i r c h e t a l . (1967) d e m o n s t r a t e d i n 19 p a t i e n t s w i t h r i g h t hemisphere damage t h a t 80% of r e s p o n s e s showed e x t i n c t i o n ( i . e . , t h e s t i m u l i were n o t r e p o r t e d ) on t h e a f f e c t e d s i d e w h i l s t o n l y 8%of r e s p o n s e s t o s t i m u l i on t h e i n t a c t s i d e were s o a f f e c t e d . A l s o , w i t h s u c c e s s i v e i n c r e a s e s i n t h e t i m e i n t e r v a l s between t h e p a i r e d s t i m u l i , B i r c h e t a l . found d e c r e a s e s i n t h e p r o p o r t i o n of e x t i n c t i o n s o c c u r r i n g on t h e n e g l e c t e d s i d e and r e l a t i v e i n c r e a s e s i n t h e p r o p o r t i o n of e x t i n c t i o n s t o s t i m u l i on t h e i n t a c t s i d e ( a l t h o u g h t h e o v e r a l l numbers of e x t i n c t i o n s d e c r e a s e d ) . They propose t h a t t h e l a t t e r r e s u l t s o c c u r r e d b e c a u s e e x t r a t i m e between i n p u t s g i v e s more o p p o r t u n i t y f o r s t i m u l i on t h e a f f e c t e d s i d e t o be p r o c e s s e d , and b e c a u s e e a r l y p r o c e s s i n g of s t i m u l i o n t h e a f f e c t e d s i d e can i n t u r n i n h i b i t i n f o r m a t i o n processingonthe intact side. Both t h e p e r c e p t u a l r i v a l r y and t h e i n t e r - h e m i s p h e r i c i n h i b i t i o n h y p o t h e s e s presume t h a t n e g l e c t p a t i e n t s f a i l t o p r o c e s s s t i m u l i on t h e a f f e c t e d s i d e a p p r o p r i a t e l y , so t h a t l a t e r p r o c e s s e s do n o t r e c e i v e t h e c o r r e c t i n p u t t o e n a b l e r e c o g n i t i o n o r even d e t e c t i o n t o o c c u r . However, t h e r e a r e i n d i c a t i o n s t h a t t h i s is not t h e c a s e , o r a t l e a s t t h a t t h e E a i l u r e t o report neglected s t i m u l i d o e s n o t a c c u r a t e l y i n d i c a t e t h e d e g r e e t o w h i c h t h e y a r e p r o c e s s e d . One of t h e most d r a s t i c i n d i c a t i o n s of a d i s s o c i a t i o n between t h e p r o c e s s i n g of n e g l e c t e d s t i m u l i and t h e a b i l i t y of p a t i e n t s t o u s e t h e p r o c e s s e d i n f o r m a t i o n f o r i d e n t i E i c a t i o n p u r p o s e s comes from t h e work of Volpe, LeDoux and Gazzaniga (1979). T h e i r p a t i e n t s were s e l e c t e d because t h e y showed e x t i n c t i o n t o d o u b l e s i m u l t a n e o u s s t i m u l a t i o n . Such p a t i e n t s were t a c h i s t o s c o p i c a l l y p r e s e n t e d w i t h e i t h e r s i n g l e words o r pictures t o either the 1 e f t o r r i g h t v i s u a l f i e l d . A l l t h e patientsnamed the s t i m u l i w i t h h i g h a c c u r a c y ( c f . Denny-Brown e t a l . , 1952). S u b s e q u e n t l y , words o r p i c t u r e s were p r e s e n t e d s i m u l t a n e o u s l y and b i l a t e r a l l y t o t h e l e f t and r i g h t of f i x a t i o n f o r 150 msec. The p a t i e n t s had t o j u d g e w h e t h e r o r n o t t h e two s t i m u l i were "same" o r " d i f f e r e n t " . F o r "same" matches t h e s t i m u l i were i d e n t i c a l ; f o r " d i f f e r e n t " t r i a l s , d i f f e r e n t i t e m s were d i s p l a y e d which s h a r e d no o b v i o u s r e l a t i o n s h i p . Such matches were performed w i t h h i g h a c c u r a c y . The p a t i e n t s were t h e n p r e s e n t e d w i t h t h e s t i m u l i used i n t h e matching t a s k and t h e y were asked t o name them. On t r i a l s where t h e s t i m u l i were i d e n t i c a l , naming t h e i t e m t n t h e n e g l e c t e d f i e l d c o u l d be deduced Erom t h e i r naming i n t h e non-neglected f i e l d . However, t h i s was n o t p o s s i b l e o n " d i f f e r e n t " t r i a l s . Over p a t i e n t s , v e r y few of t h e n e g l e c t - s i d e s t i m u l i c o u l d be named i n t h e s i m u l t a n e o u s p r e s e n t a t i o n c o n d i t i o n , d e s p i t e t h e h i g h l e v e l of performance i n t h e matching t a s k . T h i s s u g g e s t s t h a t t h e n e g l e c t e d s t i m u l i were a t l e a s t p r o c e s s e d t o l e v e l s s u p p o r t i n g t h e "same-dif f e r e n t " judgements used by Volpe e t a l . ( 1 9 7 9 ) , b u t t h a t t h i s d o e s n o t e n s u r e c o n s c i o u s i d e n t i f i c a t i o n . I n d e e d , two p a t i e n t s i n t h e s t u d y r e p o r t e d t h a t t h e y c o u l d n o t even d e t e c t t h e p r e s e n c e of n e g l e c t e d s t i m u l i . Now, w h i l s t t h e r e l i a b i l i t y of s u c h i n t r o s p e c t i v e r e p o r t s may be q u e s t i o n e d ( e . g . , M e r i k l e , 1 9 8 2 ) , t h e d a t a n e v e r t h e l e s s i n d i c a t e t h a t n e g l e c t e d s t i m u l i were p r o c e s s e d t o somewhat h i g h e r l e v e l s t h a n would a p p e a r t o be t h e c a s e from t h e p a t i e n t s ' i d e n t i f i c a t i o n r e s p o n s e s . The r e s u l t i s d i f f i c u l t t o accomodate i f there is someearly processingdeEicit f o r n e g l e c t e d s t i m u l i . A f i n a l p o i n t i s t h a t t h e p e r c e p t u a l r i v a l r y and t h e i n t e r - h e m i s p h e r i c i n h i b i t i o n hypotheses e s s e n t i a l l y attempt t o explain e x t i n c t i o n t o double s i m u l t a n e o u s s t i m u l a t i o n . However, t h e r e l a t i o n s between n e g l e c t and e x t i n c t i o n remain Ear from c l e a r . F o r i n s t a n c e , t h e r e i s commonly some s e n s o r y l o s s on t h e s i d e of t h e body c o n t r a l a t e r a l t o t h e s i t e of a l e s i o n , f o l l o w i n g b r a i n damage : c o n s e q u e n t l y , many p a t i e n t s may e x h i b t t some e x t i n c t i o n t o d o u b l e s i m u l t a n e o u s s t i m u l a t i o n , n o t j u s t t h o s e showing o t h e r s i g n s of n e g l e c t , e s p e c i a l l y w h e n s e n s o r y t h r e s h o l d s o n t h e a f f e c t e d and t h e

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u n a f f e c t e d s i d e s have n o t been e q u a t e d ( c f - B i r c h e t a l . , 1967: see a l s o Riddoch & Humphreys, 1983). F u l l a c c o u n t s of n e g l e c t w i l l need t o e x p l a i n itsoccurrence inallcircumstances.

2.NeglectasaDisorderofRepresentationalSchera I f u n i l a t e r a l n e g l e c t cannot be accounted f o r i n terms of low l e v e l p r o c e s s i n g d e f i c i t s , then c o n s i d e r a t i o n of h i g h e r - l e v e l p r o c e s s e s becomes n e c e s s a r y . Now, n e g l e c t i s c l a s s i c a l l y a s s o c i a t e d w i t h l e s i o n s t o t h e p a r i e t a l r e g i o n of t h e r i g h t hemisphere i n man ( e . g . , B r a i n , 1941; C r i t c h l e y , 1966; McFie e t a l . , 1950; though s e e Mesulam, 1981). B i s i a c h and h i s a s s o c i a t e s (e.g., B i s i a c h & L u z z a t i , 1978) have a r g u e d t h a t t h e p o s t e r i o r p a r i e t a l c o r t e x c o n t a i n s a n e l a b o r a t e s p a t i a l r e p r e s e n t a t i o n of t h e e x t e r n a l world ( s e e a l s o Lynch, 1980). I t is a r g u e d t h a t u n i l a t e r a l damage t o t h i s a r e a of t h e c o r t e x c a u s e s a u n i l a t e r a l l o s s of t h i s s p a t i a l r e p r e s e n t a t i o n , a n d h e n c e n e g l e c t of t h a t a r e a of s p a c e . Evidence f o r t h i s i d e a comes from two p a t i e n t s d e s c r i b e d by B i s i a c h and L u z z a t i (1978). Both of t h e s e p a t i e n t s had s u f f e r e d r i g h t c e r e b r a l damage r e s u l t i n g i n b o t h n e g l e c t and l e f t hemianopia. Both p a t i e n t s were a s k e d t o d e s c r i b e t h e P i a z z a d e l h o m o in M i l a n , a p l a c e v e r y f a m i l a r t o them. They were a s k e d t o d e s c r i b e t h e s q u a r e f r o m d i f f e r e n t d i r e c t i o n s ; f i r s t , l o o k i n g a t t h e c a t h e d r a l , and t h e n l o o k i n g from t h e c a t h e d r a l On e a c h o c c a s i o n , and f o r b o t h p a t i e n t s , l e f t - s i d e d e t a i l s were l a r g e l y o m i t t e d from t h e d e s c r i p t i o n s . Such d a t a cannot be e x p l a i n e d i n terms of s e n s o r y i n p u t d e f i c i t s , a s t h e d e s c r i p t i o n s d i d n o t depend on a c t u a l s t i m u l a t i o n from t h e scene. T h i s s t u d y was expanded by B i s i a c h , C a p i t a n i L u z z a t i and P e r a n i (1981). Three main groups of r i g h t hemisphere damaged p a t i e n t s were s t u d i e d : 1 2 p a t i e n t s w i t h o u t hemianopia o r n e g l e c t (H-, N-) ; 10 p a t i e n t s w i t h hemianopia but w i t h o u t n e g l e c t (H', N-) ; and 1 3 p a t i e n t s w i t h hemianopia and n e g l e c t ( j u d g e d from perEormance o n a l e t t e r c a n c e l l a t i o n t a s k ; H+, N+). F o l l o w i n g f r e e v e r b a l d e s c r i p t i o n s of t h e P i a z z a d e l Duomo a l l t h e p a t i e n t s w e r e a s k e d t o d e s c r i b e f i r s t t h e r i g h t and t h e n t h e l e f t s i d e o f t h e s q u a r e a c c o r d i n g t o a f i r s t p e r s p e c t i v e , and t h e l e f t and r i g h t s i d e s from a n o t h e r p e r s p e c t i v e . Group H+, N'was found t o d i f f e r s i g n i f i c a n t l y f rom t h e o t h e r p a t i e n t s i n t h a t more d e t a i l was o m i t t e d from t h e l e f t t h a n t h e r i g h t , f o r both p e r s p e c t i v e s . F u r t h e r , t h e d i f f e r e n c e between t h e f r e e and t h e cued c o n d i t i o n s of t h e t a s k w a s o n l y s i g n i f i c a n t f o r t h i s group, w i t h performance on l e f t - s i d e d e t a i l s being much improved i n t h e cued c o n d i t i o n . B i s i a c h e t a l . (1981) s u g g e s t t h a t t h e i r d a t a confirm t h e o b s e r v a t i o n s made by B i s i a c h and L u z z a t i (1978). They propose t h a t i m a g i n a l s p a c e i s t o p o g r a p h i c a l l y s t r u c t u r e d a c r o s s t h e hemispheres i n a n analogue of e x t e r n a l s p a c e . I f one hemisphere i s damaged, t h e n t h a t s i d e of i m a g i n a l s p a c e w i l l n o t be r e p o r t e d i n imagery t a s k s . An a l t e r n a t i v e e x p l a n a t i o n of B i s i a c h ' s f i n d i n g s is t h a t n e g l e c t p a t i e n t s a r e impaired a t s c a n n i n g ( c o v e r t 1 y ) o n e h a l f of a n i n t e r n a l s p a t i a l r e p r e s e n t a t i o n . T h i s s c a n n i n g a c c o u n t n e a t l y e x p l a i n s why c u e i n g improved t h e amount of d e t a i l r e c a l l e d on t h e l e f t s i d e of b o t h p e r s p e c t i v e s of t h e imagined s q u a r e i n t h e H+,N+ g r o u p i n B i s i a c h e t a l . (1981). Two f u r t h e r e x p e r i m e n t s d e s i g n e d t o e x p l o r e t h e r e p r e s e n t a t i o n argument a r e d e s c r i b e d by B i s i a c h , L u z z a t i and P e r a n i (1979). N i n e t e e n p a t i e n t s w i t h r i g h t c e r e b r a l damage t o o k p a r t . I n a f i r s t e x p e r i m e n t . t h e p a t i e n t s were r e q u i r e d t o d e t e c t d i f f e r e n c e s o c c u r r i n g w i t h i n p a i r s of s u c c e s s i v e l y p r e s e n t e d p a t t e r n s . The b r a i n damaged group was r e l i a b l y impaired a t d e t e c t i n g d i f f e r e n c e s on t h e l e f t s i d e compared w i t h t h e r i g h t


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s i d e of t h e p a t t e r n s , and compared w i t h non-brain damaged c o n t r o l s u b j e c t s . I n a second e x p e r i m e n t t h e p a t t e r n s moved behind a s t a t i o n a r y v e r t i c a l s l i t , w i t h e a c h p a i r of s t i m u l i b e i n g shown i n b o t h a l e f t w a r d and a r i g h t w a r d motion. Again s u b j e c t s were asked t o d e t e c t d i f f e r e n c e s o c c u r r i n g w i t h i n e a c h p a i r of p a t t e r n s . In o r d e r t o perform t h i s t a s k , t h e p a t i e n t s must r e - c o n s t r u c t t h e forms by temporal i n t e g r a t i o n of t h e s u c c e s s i v e e p i s o d e s of t h e s t i m u l u s . In t h i s i n s t a n c e , p a t i e n t s were impaired a t d e t e c t i n g d i f f e r e n c e s on t h e l e f t s i d e of t h e p a t t e r n s , i r r e s p e c t i v e of whether t h e l e f t s i d e w a s t h e l e a d i n g o r t h e t r a i l i n g e d g e . Morgan, F i n d l a y and Watt (1982) have d i s c u s s e d t h e p r o c e s s e s i n v o l v e d i n t h e v i s u a l p e r c e p t i o n of s h a p e s moving behind narrow a p e r t u r e s w i t h normal o b s e r v e r s . When t h e r e is a r e l a t i v e l y f a s t s t i m u l u s v e l o c i t y ( e . g . , above 8 " / s e c ) , t h e o b s e r v e r may u n c o n s c i o u s l y f o l l o w t h e moving f i g u r e w i t h h i s o r h e r e y e s so t h a t s u c c e s s i v e i m p r e s s i o n s a r e formed o n d i f f e r e n t p a r t s of t h e r e t i n a d u r i n g t h e motion of t h e shape. T h i s i s termed r e t i n a l p a i n t i n g . However, p u r s u i t eye movements a r e n o t n e c e s s a r y f o r shape p e r c e p t i o n t o o c c u r . When o b s e r v e r s f i x a t e a s t a t i o n a r y s p o t t o t h e s i d e of t h e a p e r t u r e , r e l a t i v e l y s l o w l y moving s h a p e s (e.g., less t h a n 3 " / s e c ) can be seen, due t o t h e temporal i n t e g r a t i o n of shape i n f o r m a t i o n ( c f . Morgan e t a l . , 1982). I n B i s i a c h e t a l . ' s s t u d y , t h e s t i m u l u s speed was r e l a t i v e l y slow and t h e p e r c e p t i o n of shape by t h e p a t i e n t s was p r o b a b l y n o t t a k i n g p l a c e by r e t i n a l p a i n t i n g . Oculomotor d i s t u r b a n c e s s h o u l d n o t t h e r e f o r e a f f e c t t h e t a s k . B i s i a c h e t a l . ' ~f i n d i n g t h a t t h e e f f e c t of t h e d i r e c t i o n o f movement was n e g l i g i b l e , i s a l s o c o n s i s t e n t w i t h t h e argument t h a t oculomotor d i s t u r b a n c e s were n o t r e s p o n s i b l e f o r a l t e r e d performance. However, i t i s n o t p o s s i b l e t o c o n c l u d e from t h e s t u d y t h a t t h e p a t i e n t s had i m p a i r e d r e p r e s e n t a t i o n s of s p a c e o r w h e t h e r t h e y w e r e impaired a t s c a n n i n g t h e r e p r e s e n t a t i o n . T o a s s e s s t h e l a t t e r p o s s i b i l i t y , p a t i e n t s s h o u l d be cued t o a t t e n d t o e i t h e r t h e l e f t o r r i g h t s i d e s o f t h e p a t t e r n s ; s u c h c u e i n g may o v e r r i d e ( o r a t l e a s t a m e l i o r a t e ) a n a t t e n t i o n a l d e f i c i t , w h i l s t i t s h o u l d n o t a f f e c t performance i f t h e r e p r e s e n t a t i o n o f t h e n e g l e c t e d s i d e o f s p a c e i s i t s e l f i m p a i r e d ( c f . B i s i a c h e t a l . , 1981). I n summary, t h e r e p r e s e n t a t i o n h y p o t h e s i s does n o t o f f e r a ready e x p l a n a t i o n f o r why c u e i n g s h o u l d improve n e g l e c t ; i n d e e d , t h e e f f e c t s of c u e i n g i n d i c a t e some a t t e n t i o n a l component i n u n i l a t e r a l n e g l e c t . However, t h e h y p o t h e s i s does have some a d v a n t a g e s o v e r t h e s e n s o r y d e f i c i t n o t i o n , s i n c e i t assumes t h a t an i n t e r n a l r e p r e s e n t a t i o n of s p a c e i s d i s r u p t e d ; i t can t h u s a c c o u n t f o r u n i l a t e r a l n e g l e c t i n a v a r i e t y of t a s k s which a r e s u b s e r v e d by t h e same s p a t i a l r e p r e s e n t a t i o n ( e . g . , i n t h e t a c t i l e a s w e l l a s the visual modality).

3,NeglectasaDisorderofAttention S e v e r a l d i f f e r e n t t h e o r i e s a r e subsumed under t h i s heading. T h e s e a r e : K i n s b o u r n e ' s a t t e n t i o n h y p o t h e s i s ; Heilman's u n i l a t e r a l a k i n e s i a hypothesis : Posner's covertorientinghypothesis. K i n s b o u r n e ' s a t t e n t i o n a l h y p o t h e s i s : Kinsbourne (1978) a t t e m p t s t o e x p l a i n n e g l e c t i n terms of h i s a t t e n t i o n a l account of i n t e r - h e m i s p h e r i c a c t i v i t y ( K i n s b o u r n e , 1970). He a r g u e s t h a t a c t i v a t i o n of one hemisphere c a u s e s i n h i b i t i o n of p o t e n t i a l l y homologous f u n c t i o n s i n t h e o t h e r . The consequence of within-hemisphere a c t i v a t i o n l e a d s t o a p e r c e p t u a l b i a s toward t h e c o n t r a l a t e r a l s i d e of s p a c e and t r a n s h e m i s p h e r e i n h i b i t i o n . F u r t h e r , t h e r e i s t h o u g h t t o be a n i n n a t e p r o p e n s i t y t o a t t e n d more t o t h e r i g h t t h a n t o t h e l e f t (Caplan & K i n s b o u r n e , 1976; T u r k e w i t z , Gordon & B i r c h , 1965). To a c c o u n t f o r t h i s , Kinsbourne s u g g e s t s t h a t " i n t h e l e f t h a l f of t h e b r a i n a r e l o c a t e d n o t o n l y t h e f a c u l t y t h a t t u r n s a t t e n t i o n t o

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t h e r i g h t b u t a l s o t h e v e r b a l p r o c e s s o r . When t h e v e r b a l p r o c e s s o r i s a c t i v a t e d because a p e r s o n i s a n t i c i p a t i n g s p e a k i n g , l i s t e n i n g t o s p e e c h o r speaking, then t h a t verbal a c t i v a t i o n overlaps t h e adjacent r i g h t turning c o n t r o l c e n t r e and t h e v e r b a l a c t i v i t y b i a s e s a t t e n t i o n t o t h e r i g h t . The l e f t t u r n i n g c e n t r e , i n t h e r i g h t h e m i s p h e r e , i s f a r removed f r o m t h e s i t e of a c t i v a t i o n and i s overpowered by i t s a c t i v a t e d opponent" ( K i n s b o u r n e , 1978, page 9 ) . While t h i s n o t i o n g a i n s s u p p o r t from t h e predominance of l e f t s i d e d n e g l e c t , i t i s open t o c r i t i c i s m . I f n e g l e c t i s due t o an imbalance of o r i e n t i n g t e n d e n c i e s r e s u l t i n g from i n c r e a s e d ( d i s i n h i b i t e d ) a c t i v i t y i n t h e i n t a c t hemisphere, t h e n t h e performance of t h e r i g h t hemisphere p a t i e n t s on r i g h t s i d e s t i m u l i should be a t l e a s t normal, i f n o t b e t t e r t h a n normal (Heilman & Watson, 1977). However, Heilman and Watson (1977) c i t e e v i d e n c e where p a t i e n t s w i t h l e f t s i d e d n e g l e c t made more e r r o r s on t h e non-neglected ( i p s i l a t e r a l ) s i d e t h a n p a t i e n t s w i t h l e f t hemisphere l e s i o n s made on t h e i r l e f t ( i p s i l a t e r a l ) s i d e . They a l s o s u g g e s t t h a t i f Kinsbourne's hypothesis i s c o r r e c t , then b i l a t e r a l l e s i o n s should a m e l i o r a t e n e g l e c t . Against t h i s , S e g a r r a and Angelo (1970) d e m o n s t r a t e t h a t b i l a t e r a l c i n g u l a t e o r mesencephalic l e s i o n s produce b i l a t e r a l n e g l e c t . C o n t r a r y t o Heilman and Watson's s u g g e s t i o n , though, i t remains p o s s i b l e f o r Kinsbourne's account t o a c c o m m o d a t e b i l a t e r a l n e g 1 e c t . T h i s i s because a r i g h t hemisphere l e s i o n would r e s u l t i n o r i e n t i n g t o t h e r i g h t and n e g l e c t of t h e l e f t , w h i l e a l e f t hemisphere l e s i o n would produce o r i e n t i n g t o t h e l e f t and n e g l e c t of t h e r i g h t . E i t h e r a b i l a t e r a l l e s i o n would c a u s e b i l a t e r a l n e g l e c t , o r t h e o p p o s i t e o r i e n t i n g t e n d e n c i e s w i l l b a l a n c e and m i t i g a t e any n e g l e c t . Heilman's u n i l a t e r a l a k i n e s i a h y p o t h e s i s : Heilman and h i s a s s o c i a t e s s u g g e s t t h a t n e g l e c t i s due t o d e c r e a s e d a c t i v a t i o n of t h e a r o u s a l s y s t e m s of t h e damaged hemisphere ; i n p a r t i c u l a r ( t h o u g h n o t e x c l u s i v e l y ) a s a r e s u l t of l e s i o n s i n t h e c o r t i c o - l i m b i c r e t i c u l a r a c t i v a t i n g l o o p (Heilman & V a l e n s t e i n , 1972; Watson, Heilman, Cauthen & King, 1973). The e f f e c t of such a u n i l a t e r a l d e c r e a s e i n a r o u s a l i s t h o u g h t t o b e t h e s e l e c t i v e l o s s of t h e o r i e n t i n g response t o t h e c o n t r a l a t e r a l s i d e of s p a c e , s i n c e i t i s assumed t h a t o r i e n t i n g r e s p o n s e s t o t h e o p p o s i t e s i d e of s p a c e a r e r e p r e s e n t e d i n e a c h c e r e b r a l hemisphere, s o t h a t t h e damaged hemisphere i s rendered a k i n e t i c . To e x p l a i n t h e predominance of n e g l e c t f o l l o w i n g r i g h t r a t h e r t h a n l e f t hemisphere l e s i o n s ( s e e above, Chedru, 1976; a l s o McFie & Z a n g w i l l , 1960; B r a i n , 1941; C r i t c h l e y , 1966; Oxbury, Campbell & Oxbury, 1974; though F r i e d r i c h , Walker & P o s n e r , 1985, and Riddoch, 1982, p r e s e n t some c o n t r a r y e v i d e n c e ) Heilman and Van d e r Abel (1980) f u r t h e r assume t h a t t h e l e f t hemisphere only c o n t r o l s o r i e n t i n g t o t h e r i g h t s i d e of s p a c e w h i l e t h e r i g h t hemisphere c o n t r o l s o r i e n t i n g t o b o t h s i d e s . The e f f e c t of a l e f t hemisphere l e s i o n on o r i e n t i n g can be compensated f o r by t h e r i g h t hemisphere ; however, a r i g h t hemisphere l e s i o n m a y l e a v e s u b j e c t s w i t h o n l y a right-side (left-hemispheremediated) o r i e n t i n g response. I f t h e a r o u s a l h y p o t h e s i s i s c o r r e c t , i t s h o u l d be p o s s i b l e t o d e m o n s t r a t e e l e c t r o p h y s i o l o g i c a l changes i n s u b j e c t s w i t h n e g l e c t . W a t s o n , Andriola and Heilman (1977) recorded e l e c t r o e n c e p h a l o g r a m s ( E E G s ) f o r 2 3 p a t i e n t s w i t h u n i l a t e r a l n e g l e c t and compared them w i t h 20 s u b j e c t s w i t h a p h a s i a w i t h o u t n e g l e c t . Of t h e p a t i e n t s w i t h n e g l e c t , 2 2 d e m o n s t r a t e d d i f f u s e i s p i l a t e r a l slow waves, w h i l e o n l y 7 of t h e 20 a p h a s i c p a t i e n t s showed i p s i l a t e r a l EEG slowing. Such a r e s u l t s u g g e s t s a u n i l a t e r a l decreaseinarousalinneglectpatients. The a r o u s a l h y p o t h e s i s can be used t o e x p l a i n o t h e r d a t a . For i n s t a n c e , c o n s i d e r B i r c h e t a l . ' s (1967) f i n d i n g t h a t e x t i n c t i o n t o d o u b l e


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s i m u l t a n e o u s s t i m u l a t i o n o c c u r s i n n e g l e c t p a t i e n t s and t h a t i t d e c r e a s e s a s t h e i n t e r v a l between t h e p r e s e n t a t i o n of s t i m u l i t o t h e n e g l e c t e d a n d t h e non-neglected s i d e s l e n g t h e n s ( s e e a b o v e ) . Now, under c o n d i t i o n s of d o u b l e simultaneous s t i m u l a t i o n , sensory i n f o r m a t i o n is t r a n s m i t t e d t o both c e r e b r a l hemispheres. I f t h e r e i s d e c r e a s e d a r o u s a l i n one hemisphere, performance w i l l come t o be dominated by t h e r e a c t i o n of t h e i n t a c t hemisphere, s o c a u s i n g e x t i n c t i o n o n t h e n e g l e c t e d s i d e . I n c r e a s i n g t h e i n t e r v a l between t h e s t i m u l i might a l s o a l l o w any s t i m u l i p r o j e c t e d t o t h e i m p a i r e d hemisphere t o be p r o c e s s e d a g a i n s t a lower l e v e l of background a r o u s a l ( n o i s e ) , so m i t i g a t i n g t h e e f f e c t of r e l a t i v e u n d e r - a r o u s a l i n t h a t hemisphere which i s e n c o u n t e r e d under d o u b l e s i m u l t a n e o u s s t i m u l a t i o n conditions. One f u r t h e r consequence of Heilman' s view t h a t t h e damaged hemisphere i n n e g l e c t p a t i e n t s i s a k i n e t i c due t o u n d e r - a r o u s a l i s t h a t t h e d e g r e e of n e g l e c t m a n i f e s t s h o u l d n o t be s t r o n g l y a f f e c t e d by i n s t r u c t i n g s u b j e c t s t o a t t e n d t o t h e n e g l e c t e d s i d e , s i n c e , a t l e a s t when t h e r e a r e competing s t i m u l i p r e s e n t on t h e non-neglected s i d e , t h e damaged hemisphere s h o u l d s t i l l be under-aroused r e l a t i v e t o t h e i n t a c t hemisphere. Heilman and V a l e n s t e i n ( 1979) examined t h i s p o s s i b i l i t y i n a l i n e b i s e c t i o n t a s k . They p r e s e n t e d n e g l e c t p a t i e n t s w i t h a s e r i e s of l i n e s , w i t h e a c h l i n e h a v i n g a l e t t e r a t e i t h e r end. On h a l f t h e t r i a l s , t h e p a t i e n t s were cued t o l o o k a t t h e l e f t end of t h e l i n e s and t o r e p o r t t h e l e t t e r a t t h a t end ; on t h e o t h e r t r i a l s p a t i e n t s were cued t o l o o k t o t h e r i g h t end of t h e l i n e s and t o r e p o r t t h e l e t t e r t h e r e . P a t i e n t s t h e n had t o draw a l i n e b i s e c t i n g t h e m i d d l e of t h e t a r g e t l i n e , and n e g l e c t was measured i n t e r m s of t h e magnitude of e r r o r (away from t h e n e g l e c t e d s i d e ) . H e i l m a n a n d V a l e n s t e i n (1979) found t h a t t h e cues had no e f f e c t on performance. Heilman and V a l e n s t e i n ' s f a i l u r e t o Eind a n e f f e c t of c u e i n g i s c o n t r a r y t o t h e f i n d i n g of r e l i a b l e c u e i n g e f f e c t s i n imagery t a s k s i n n e g l e c t p a t i e n t s ( B i s i a c h e t a l . , 1 9 8 1 ) . A l s o Riddoch andHumphreys ( 1 9 8 3 ) , i n a s i m i l a r t a s k t o t h a t used by Heilman and V a l e n s t e i n , have r e p o r t e d a n e f f e c t of c u e i n g , w i t h n e g l e c t b e i n g l e s s when t h e p a t i e n t s were cued t o r e p o r t t h e l e f t - s i d e l e t t e r t h a n when t h e y were cued t o r e p o r t t h e r i g h t - s i d e l e t t e r . Riddoch and Humphreys' r e s u l t s u g g e s t s t h a t t h e damaged hemisphere i n n e g l e c t p a t i e n t s s h o u l d not be thought of a s a k i n e t i c , s i n c e t h e d e g r e e of n e g l e c t can be i n f l u e n c e d by i n s t r u c t i n g s u b j e c t s t o o r i e n t t o t h e neglected s i d e of space, keeping a l l o t h e r t a s k c o n s t r a i n t s constant. One r e a s o n f o r t h e d i f f e r e n t r e s u l t s may be s u b j e c t s e l e c t i o n . Heilman and V a l e n s t e i n ' s (1979) p a t i e n t s were s e l e c t e d on t h e b a s i s of e x t i n c t i o n t o d o u b l e s i m u l t a n e o u s s t i m u l a t i o n , which i s a r a t h e r broad i n d i c a t o r of n e g l e c t ; Riddoch and Humphreys (1983) s e l e c t e d p a t i e n t s on t h e b a s i s of t h e i r showing n e g l e c t i n drawing t a s k s , which may o n l y be m a n i f e s t i n p a t i e n t s w i t h more s e v e r e n e g l e c t . C e r t a i n l y , t h e d e g r e e of n e g l e c t i n t h e l i n e b i s e c t i o n t a s k was more marked i n Riddoch and Humphreys' p a t i e n t s t h a n i n t h o s e of Heilman and V a l e n s t e i n , and i t seems p o s s i b l e t h a t t h e d e g r e e of n e g l e c t maybe more l a b i l e i n t h e f o r m e r c a s e s . A l t e r n a t i v e l y , i t may b e t h a t d i f f e r e n t p a t i e n t s m a n i f e s t neglect f o r d i f f e r e n t reasons. Posner's covert orientinghypothesis : A t h i r d a t t e n t i o n a l explanation h a s r e c e n t l y been proposed by P o s n e r and h i s a s s o c i a t e s ( P o s n e r & R a f a l , i n p r e s s ; P o s n e r e t a l . , 1982; P o s n e r e t a l . , 1984). According t o t h i s h y p o t h e s i s , n e g l e c t may be a t t r i b u t e d t o a n i n a b i l i t y t o s c a n a n i n t e r n a l s p a t i a l r e p r e s e n t a t i o n because of a d e f e c t i n v i s u a l a t t e n t i o n . T h i s i n a b i l i t y may e x i s t i n d e p e n d e n t l y of an impairment i n s u s t a i n i n g t h e i n t e r n a l r e p r e s e n t a t i o n o f t h e neglected s i d e o f space (see s e c t i o n 2 a b o v e ; B i s i a c h & L u z z a t i , 1978).

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A t t e m p t s t o a s s e s s some of t h e p r o p e r t i e s o f v i s u a l a t t e n t i o n h a v e b e e n made by examining t h e e f f e c t s of p r e c u e s i n s i m p l e r e a c t i o n time (RT) t a s k s . T y p i c a l l y , t h e s u b j e c t s h a s t o f i x a t e c e n t r a l l y and he o r she i s r e q u i r e d t o p r e s s a key i f a t a r g e t s t i m u l u s ( a l i g h t , a n a s t e r i s k e t c . ) i s p r e s e n t e d e i t h e r t o t h e r i g h t o r t o t h e l e f t of f i x a t i o n . P r i o r t o t h e a p p e a r a n c e of t h e t a r g e t , a cue i s p r e s e n t e d ; t h i s may be a c e n t r a l arrow i n d i c a t i n g t h e l i k e l y l o c a t i o n of t h e t a r g e t on a g i v e n t r i a l , o r i t c a n be a l i g h t b r i e f l y i l l u m i n a t e d on t h e s i d e of f i x a t i o n where t h e t a r g e t i s l i k e l y t o a p p e a r . On t h e m a j o r i t y of t r i a l s , t h e cue i n d i c a t e s t h e t a r g e t l o c a t i o n ( i . e . , i t i s v a l i d ) ; however, on a m i n o r i t y of t r i a l s , t h e cue is i n c o r r e c t and i t i n d i c a t e s a l o c a t i o n on t h e o p p o s i t e s i d e of f i x a t i o n t o where t h e t a r g e t a p p e a r s ( i . e . , i t i s i n v a l i d ) . Performance w i t h v a l i d and w i t h i n v a l i d c u e s can be compared w i t h performance when t h e cue i s n e u t r a l and conveys no i n f o r m a t i o n about t h e l i k e l y l o c a t i o n of t h e t a r g e t ( i . e . , when t h e cue i s a c e n t r a l c r o s s ) . RTs a r e f a s t e r ( J o n i d e s , 1981; P o s n e r 1980) and d i s c r i m i n a t i o n i s more a c c u r a t e ( B a s h i n s k i 6 Bacharach, 1980) when t h e cue i s v a l i d r e l a t i v e t o when i t i s n e u t r a l ( i . e . , performance is f a c i l i t a t e d ) ; and R T s are s l o w e r and d i s c r i m i n a t i o n p o o r e r ( e . g . , B a s h i n s k i & Bacharach, 1980; P o s n e r , 1980; P o s n e r , Snyder & Davison, 1980) w i t h a n i n v a l i d cue ( i . e . , performance is i n h i b i t e d ) . The f a c i l i t a t e d performance w i t h a v a l i d cue i s a t t r i b u t e d t o t h e s h i f t i n g o f a t t e n t i o n t o t h e t a r g e t ' s l o c a t i o n . Normally, a t t e n t i o n a l a c t s may be mediated e i t h e r by s h i f t s of t h e e y e s ( a n o v e r t mechanism) o r by s h i f t s i n some form of i n t e r n a l a t t e n t i o n a l p r o c e s s ( a c o v e r t mechanism); f o r i n s t a n c e , we might c o n c e p t u a l i s e t h a t c o v e r t a t t e n t i o n o p e r a t e s by a p r e - a c t i v a t i o n of t h e p r o c e s s i n g pathways f o r s t i i n u l i f a l l i n g a t t h e a t t e n d e d l o c a t i o n ( e . g . , P o s n e r , 1978, 1980).The o v e r t and c o v e r t a t t e n t i o n a l mechanisms c a n be s e p a r a t e d , w i t h s i m p l e RT performance b e i n g f a c i l i t a t e d by a v a l i d l o c a t i o n cue e v e n where t r i a l s on which eye movements a r e made a r e d i s g a r d e d o r where t h e s t i m u l i a r e p r e s e n t e d t o o b r i e f l y f o r e y e movements t o o c c u r (e.g., J o n i d e s , 1981; P o s n e r , N i s s e n & Ogden, 1978). T h e r e is a l s o a t y p i c a l t i m e c o u r s e f o r p r o c e s s i n g f a c i l i t a t i o n , w i t h RT d e c r e a s i n g ( i . e . , w i t h t h e r e b e i n g more f a c i l i t a t i o n ) as t h e i n t e r v a l between t h e cue and t h e a p p e a r a n c e of t h e t a r g e t i n c r e a s e s , up t o some o p t i m a l i n t e r v a l depending on t h e d i s t a n c e between t h e t a r g e t and f i x a t i o n ( e . g . , e s t i m a t e s i n d i c a t e t h a t a t t e n t i o n may be s h i f t e d a t a c o n s t a n t v e l o c i t y of about 8 msec/deg. of v i s u a l a n g l e ; Shulman, Remington & McLean. 1979; T s a l , 1983). N e v e r t h e l e s s , some f a c i l i t a t i o n g e n e r a l l y o c c u r s even when t h e t a r g e t i s p r e s e n t e d l e s s t h a n 200 msec f o l l o w i n g t h e c u e , which a g a i n emphasises t h a t t h e b e n e f i t s from c o v e r t s h i f t s of a t t e n t i o n can be s e p a r a t e d from o v e r t a t t e n t i o n a l p r o c e s s e s s u c h a s eyemovements ( e . g . , R e m i n g t o n . 1980). I n t e r e s t i n g l y , t h e s h i f t i n g of c o v e r t a t t e n t i o n can i t s e l f be dependent on e i t h e r of two s e p a r a t e mechanisms. T h i s i s b e s t i l l u s t r a t e d by a n experiment r e p o r t e d by P o s n e r e t a l . (1982). They p r e s e n t e d a p e r i p h e r a l cue ( s i g n a l l e d by t h e b r i g h t e n i n g of a box of 10 deg. t o t h e l e f t o r r i g h t of f i x a t i o n ) p r i o r t o t h e p r e s e n t a t i o n of a t a r g e t l i g h t . However, t h e cue was only v a l i d on 20% of t h e t r i a l s , so s u b j e c t s were i n s t r u c t e d t o e x p e c t t h e t a r g e t i n t h e o p p o s i t e v i s u a l f i e l d t o where t h e cue appeared. The i n t e r v a l between t h e cue and t h e t a r g e t was v a r i e d . With s h o r t i n t e r v a l s ( l e s s t h a n 100 msec), RTs were q u i c k e s t t o t a r g e t s a p p e a r i n g on t h e s i d e of t h e cue. With l o n g e r i n t e r v a l s ( o v e r 150 msec), R T s were q u i c k e s t t o t a r g e t s a p p e a r i n g on t h e o p p o s i t e s i d e t o t h e cue. With t h e s h o r t i n t e r v a l t h e n , t h e p e r i p h e r a l cue a p p e a r e d t o " c a p t u r e " a t t e n t i o n , so t h a t a t t e n t i o n was o b l i g a t o r i l y d r a w n t o t h i s cueeventhoughsubjectswere i n s t r u c t e d t o s h i f t a t t e n t i o n t o t h e o p p o s i t e s i d e of f i x a t i o n . With l o n g e r i n t e r v a l s , a t t e n t i o n a p p e a r s dependent on some more c e n t r a l c o n t r o l p r o c e s s ( P o s n e r & Cohen, 1984).


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P o s n e r et a l . (1982) and P o s n e r e t a l . (1984) have examined t h e performance of p a t i e n t s w i t h u n i l a t e r a l n e g l e c t i n t h e p r e - c u e i n g p r o c e d u r e . F o r i n s t a n c e , P o s n e r e t a l . (1984) t e s t e d 1 3 p a t i e n t s w i t h p a r i e t a l l e s i o n s , 7 w i t h l e f t - s i d e l e s i o n s and 6 w i t h r i g h t - s i d e l e s i o n s , 5 of whom showed some c l i n i c a l s i g n s of n e g l e c t ( v a r y i n g from e x t i n c t i o n t o double simultaneous s t i m u l a t i o n t o i n a t t e n t i o n t o a l l s t i m u l i on the n e g l e c t e d s i d e ) . Such c l i n i c a l s y m p t o m s o f n e g l e c t w e r e a l s o p r e s e n t i n 3 o f t h e l e f t p a r i e t a l p a t i e n t s . P a t i e n t s made a s i m p l e RT r e s p o n s e t o a p e r i p h e r a l t a r g e t which was preceded by a p e r i p h e r a l cue. The cue was v a l i d o n 80% of t h e t r i a l s . They found t h a t , when t h e cue w a s v a l i d , t h e r e were r e l a t i v e l y small d i f f e r e n c e s i n R T s t o t a r g e t s p r e s e n t e d b o t h c o n t r a l a t e r a l l y and i p s i l a t e r a l l y t o t h e s i t e of t h e l e s i o n , f o r a l l p a t i e n t s . However, when t h e cue was i n v a l i d ( i . e . , when t h e t a r g e t was p r e s e n t e d o n t h e o p p o s i t e s i d e of f i x a t i o n t o t h e c u e ) , t h e r e were marked d i f f e r e n c e s i n performance f o r c o n t r a l a t e r a l and f o r i s p i l a t e r a l t a r g e t s , w i t h RTs t h e n b e i n g slow f o r c o n t r a l a t e r a l s t i m u l i . T h i s d i f f i c u l t y i n d e t e c t i n g c o n t r a l a t e r a l s t i m u l i g i v e n a n i n v a l i d cue t o t h e i p s i l a t e r a l s i d e was most pronounced w i t h t h e r i g h t hemisphere damaged p a t i e n t s , and i t was p a r t i c u l a r l y s e v e r e f o r t h e p a t i e n t w i t h t h e m o s t marked n e g l e c t . P o s n e r e t a l ' s r e s u l t is of i n t e r e s t , s i n c e i t d e m o n s t r a t e s t h a t a t l e a s t some p a t i e n t s w i t h u n i l a t e r a l n e g l e c t can s h i f t a t t e n t i o n t o t h e n e g l e c t e d s i d e of s p a c e ( c o n t r a r y t o Heilman's a c c o u n t ; s e e a b o v e ) , and t h a t s u c h p a t i e n t s a r e p r i m a r i l y i m p a i r e d o n l y when a t t e n t i o n h a s been drawn t o t h e non-neglected s i d e . P o s n e r e t a l . (1984) d i s t i n g u i s h t h r e e component p r o c e s s e s i n c o v e r t v i s u a l a t t e n t i o n : t h e a b i l i t y t o engage a t t e n t i o n t o a t a r g e t ; t h e a b i l i t y t o d i s e n g a g e a t t e n t i o n from a t a r g e t ; and t h e a b i l i t y t o move a t t e n t i o n from one t a r g e t t o a n o t h e r . Now, s i n c e t h e r e w a s l i t t l e d i f f e r e n c e b o t h i n a b s o l u t e R T s and i n t h e t i m e c o u r s e of t h e r e s p o n s e s t o v a l i d l y cued i p s i l a t e r a l and c o n t r a l a t e r a l t a r g e t s f o r t h e i r p a r i e t a l p a t i e n t s , i t a p p e a r s t h a t s u c h p a t i e n t s c a n move and engage a t t e n t i o n t o c o n t r a l a t e r a l s t i m u l i r e l a t i v e l y normally. P o s n e r e t a l . t h e r e f o r e s u g g e s t t h a t t h e d i f f i c u l t y w i t h i n v a l i d l y cued c o n t r a l a t e r a l s t i m u l i o c c u r s b e c a u s e s u c h p a t i e n t s e x p e r i e n c e d i f f i c u l t y i n d i s e n g a g i n g a t t e n t i o n from s t i m u l i presented i n t h e i s p i l a t e r a l f i e l d l . The s u g g e s t i o n , t h e n , i s t h a t v i s u a l n e g l e c t can stem from a s e l e c t i v e impairment t o one component of c o v e r t a t t e n t i o n . S i n c e some impairments were found w i t h b o t h r i g h t and l e f t hemisphere damaged p a t i e n t s , i t a p p e a r s t h a t t h e p r o c e s s e s i n v o l v e d in a t t e n d i n g t o v i s u a l s t i m u l i a r e l a t e r a l l y o r g a n i z e d , w i t h e a c h hemisphere c o n t r o l l i n g a t t e n t i o n t o t h e c o n t r a l a t e r a l s i d e of s p a c e . Thus, damage t o t h o s e a r e a s i n t h e r i g h t and l e f t c e r e b r a l hemispheres c o n t r o l l i n g a t t e n t i o n w i l l produce some d i f f i c u l t i e s i n a t t e n d i n g t o t h e c o n t r a l a t e r a l s i d e ( s e e a l s o F r i e d r i c h , Walker a n d P o s n e r , 1985). T h i s i s n o t t o s t a t e , however, t h a t t h e r e i s complete symmetry of r e p r e s e n t a t i o n of a t t e n t i o n a l r e s p o n s e s i n t h e two hemispheres ; i n d e e d , t h e e v i d e n c e i n d i c a t e s t h a t t h i s i s n o t so (e.g., see P o s n e r e t a l . , 1984). F o r i n s t a n c e , i t may be t h a t w h i l s t t h e l e f t hemisphere only c o n t r o l s a t t e n t i o n a l s h i f t s t o t h e c o n t r a l a t e r a l ( r i g h t ) s i d e of s p a c e , t h e r i g h t hemisphere may have some c o n t r o l o v e r s h i f t s t o b o t h s i d e s ; a c c o r d i n g l y , t h e d i f f i c u l t y i n s h i f t i n g a t t e n t i o n t o t h e r i g h t s i d e of s p a c e f o l l o w i n g a l e f t hemisphere l e s i o n may be m i t i g a t e d by i s p i l a t e r a l s h i f t s made by t h e r i g h t hemisphere ( s e e a l s o H e i l m a n & V a n d e r A b e l , 1980). P o s n e r e t a l . ' s (1984) argument t h a t n e g l e c t p a t i e n t s h a v e p a r t i c u l a r problems i n d i s e n g a g i n g a t t e n t i o n from s t i m u l i p r e s e n t e d t o t h e i r non-neglected s i d e can accommodate much of t h e d a t a w e have so f a r

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c o n s i d e r e d . For i n s t a n c e , e x t i n c t i o n t o double s i m u l t a n e o u s s t i m u l a t i o n i s a t t r i b u t e d t o a t t e n t i o n b e i n g h e l d by t h e i s p i l a t e r a l s t i m u l u s , p r e v e n t i n g i t s s w i t c h i n g t o t h e c o n t r a l a t e r a l s i d e . A l s o , f o r r i g h t hemisphere p a t i e n t s , i n c r e a s i n g t h e time between t h e l e f t and r i g h t s i d e s t i m u l i l e s s e n s e x t i n c t i o n s i n c e t h e r e w i l l t h e n be l e s s of a tendency t o a t t e n d t o t h e r i g h t s i d e ( c f . B i r c h e t a l . , 1961). S i m i l a r l y , i n m e n t a l i m a g e r y t a s k s , a t t e n t i o n may be engaged by s t i m u l i o n t h e r i g h t s i d e o f t h e i n t e r n a l s p a t i a l representation. with the r e s u l t being t h a t p a t i e n t s then f i n d i t d i f f i c u l t t o s h i f t a t t e n t i o n t o t h e n e g l e c t e d s i d e ( c f . B i s i a c h & L u z z a t i , 1978). P e r h a p s m o s t i m p o r t a n t l y , t h e h y p o t h e s i s a l s o g i v e s a ready e x p l a n a t i o n f o r why c u e i n g c a n a m e l i o r a t e n e g l e c t ( P o s n e r e t a l . 1982 ; R i d d o c h & H u m p h r e y s , 1983). We have d i s c u s s e d p r o p o s a l s t h a t n e g l e c t i s due e i t h e r t o i m p a i r e d sensory processes or t o impaired input t o higher-order recognition p r o c e s s e s , t h a t n e g l e c t i s due t o impaired r e p r e s e n t a t i o n of t h e s i d e of space c o n t r a l a t e r a l t o t h e s i t e of a l e s i o n , and t h a t n e g l e c t i s due t o some form of impairment t o v i s u a l a t t e n t i o n . Our s u g g e s t i o n i s t h a t , a t l e a s t i n some c a s e s , n e g l e c t i s caused b y a n a t t e n t i o n a l i m p a i r m e n t . P e r h a p s t h e m o s t convincing e v i d e n c e f o r t h e l a t t e r s u g g e s t i o n i s t h e f i n d i n g , n o w s h o w n i n a number of l a b o r a t o r i e s , t h a t i n s t r u c t i n g p a t i e n t s t o a t t e n d t o t h e n e g l e c t e d s i d e of s p a c e l e s s o n s n e g l e c t ( B i s i a c h e t a l . , 1981; P o s n e r e t a l . , 1982, 1984 ; Riddoch & Humphreys, 1983; see a l s o D i l l e r & Weinberg, 1977; Weinberg, D i l l e r , Gordon, Gerstman, Lieberman, L a k i n , Hodges & E z r a c h i , 1977, 1979). Without e x p l i c i t i n s t r u c t i o n s t o s h i f t a t t e n t i o n c o n t r a l a t e r a l l y , however, p a t i e n t s w i t h n e g l e c t a r e poor a t d i s e n g a g i n g a t t e n t i o n from i s p i l a t e r a l s t i m u l i . S i n c e s u c h e f f e c t s a r e found predominantly f o l l o w i n g l e s i o n s t o t h e r i g h t hemisphere, i t a p p e a r s t h a t t h e r e may be some a s y m m e t r y i n t h e o r g a n i z a t i o n o f a t t e n t i o n a l m e c h a n i s m s i n t h e l e f t and r i g h t c e r e b r a l hemispheres (Heilman & Van d e r Abel, 1980; P o s n e r e t a l . , 1984). 1II.SomePurtherOuestions We have argued t h a t v i s u a l n e g l e c t i s c a u s e d b y a p r o b l e m i n d i s e n g a g i n g a t t e n t i o n from s t i m u l i p r e s e n t e d on t h e s i d e of s p a c e i p s i l a t e r a l t o t h e l e s i o n s i t e . I t may a l s o be p o s s i b l e t o f r a c t i o n a t e t h e d i f f i c u l t i e s i n d i s e n g a g i n g a t t e n t i o n f u r t h e r . For i n s t a n c e , i t could be t h a t i p s i l a t e r a l s t i m u l i e f f e c t a s t r o n g e r "hold" on a t t e n t i o n i n n e g l e c t p a t i e n t s t h a n i n normal s u b j e c t s , o r t h a t c o n t r a l a t e r a l s t i m u l i f a i l t o " c a p t u r e " a t t e n t i o n i n n e g l e c t p a t i e n t s . I f thereissomebreakdowninanobligatoryattentional mechanism ( i . e . , i n a t t e n t i o n a l c a p t u r e ) . p a t i e n t s may be poor a t d e t e c t i n g c o n t r a l a t e r a l t a r g e t s once a t t e n t i o n i s s h i f t e d t o t h e i p s i l a t e r a l s i d e s i n c e a t t e n t i o n may need t o be c a p t u r e d by such t a r g e t s f o r d e t e c t i o n t o occur. N e v e r t h e l e s s , such p a t i e n t s s e e m a b l e t o e f f e c t c o n t r o l l e d s h i f t s o f a t t e n t i o n ; t h u s t h e r e a r e o n l y minor d i f f e r e n c e s i n r e s p o n s e s t o i p s i l a t e r a l and c o n t r a l a t e r a l s t i m u l i w i t h v a l i d c u e i n g and n e g l e c t i s d e c r e a s e d by e x p l i c i t i n s t r u c t i o n s t o s h i f t a t t e n t i o n ( P o s n e r e t a l . , 1984 ; Riddoch&Humphreys, 1983). Some e v i d e n c e t h a t n e g l e c t stems f r o m a l o s s of a t t e n t i o n a l c a p t u r e by c o n t r a l a t e r a l s t i m u l i r a t h e r t h a n i n c r e a s e d a t t e n t i o n a l hold by i p s i l a t e r a l s t i m u l i h a s been r e p o r t e d by Riddoch and Humphreys (1983). A s noted e a r l i e r , Riddoch and Humphreys examined t h e e f f e c t of c u e i n g on l i n e b i s e c t i o n by n e g l e c t p a t i e n t s . I n o n e experiment ( E x p e r i m e n t 2 ) , a group of 5 r i g h t hemisphere damaged p a t i e n t s w i t h u n i l a t e r a l n e g l e c t were p r e s e n t e d w i t h l i n e s which had l e t t e r s a t e i t h e r end and, p r i o r t o l i n e b i s e c t i o n , t h e p a t i e n t s were asked e i t h e r t o r e p o r t o n l y t h e l e f t c u e , o n l y t h e r i g h t cue o r b o t h cues. Riddoch and Humphreys found marked n e g l e c t i n t h e r e p o r t


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r i g h t - c u e c o n d i t i o n r e l a t i v e t o when no l e t t e r cues were p r o v i d e d , and reduced n e g l e c t i n t h e r e p o r t l e f t - c u e o n l y and t h e r e p o r t both-cues c o n d i t i o n s . T h i s r e s u l t s u g g e s t s t h a t n e g l e c t can be reduced by c u e i n g p a t i e n t s t o t h e c o n t r a l a t e r a l s i d e ( i n t h i s c a s e , t h e l e f t ) e v e n when t h e y have f i r s t had t o r e p o r t a r i g h t - s i d e cue ( i . e . , i n t h e r e p o r t both-cues c o n d i t i o n ) . I t d i d n o t seem t o be t h e c a s e t h a t t h e r i g h t - s i d e cue "held" a t t e n t i o n , but simply t h a t the p a t i e n t s f a i l e d t o s h i f t a t t e n t i o n c o n t r a l a t e r a l l y u n l e s s e x p l i c i t l y i n s t r u c t e d t o do s o , c o n s i s t e n t w i t h a breakdown i n a t t e n t i o n a l c a p t u r e . I f t h e l a t t e r h y p o t h e s i s is c o r r e c t , w e may go on t o a s k w h e t h e r some s t i m u l u s p r o p e r t i e s draw a t t e n t i o n t o t h e n e g l e c t e d s i d e b e t t e r t h a n others.Wemayalso questionthe f a t e o f neglectedstimuli. We have a l r e a d y d i s c u s s e d one p i e c e of e v i d e n c e r e l e v a n t t o t h e q u e s t i o n of t h e f a t e of n e g l e c t e d s t i m u l i , namely Volpe e t a l . ' s (1979) f i n d i n g t h a t n e g l e c t p a t i e n t s can u s e c o n t r a l a t e r a l l y p r e s e n t e d s t i m u l i t o perform "same-different'' v i s u a l matching t a s k s even when t h e y a r e u n a b l e t o i d e n t i f y , o r perhaps even t o d e t e c t , s u c h s t i m u l i . Thus t h e f a i l u r e t o i d e n t i f y n e g l e c t e d s t i m u l i s h o u l d n o t be t a k e n t o i n d i c a t e t h e i r l a c k of processing. Another f i n d i n g which h i n t s t h a t t h e r e may be r a t h e r more p r o c e s s i n g of n e g l e c t e d s t i m u l i t h a n i s a p p a r e n t from t h e i r d i r e c t r e p o r t is a g a i n c o n t a i n e d i n t h e d a t a of Riddoch and Humphreys (1983). We have a l r e a d y n o t e d t h a t in t h i s s t u d y n e g l e c t p a t i e n t s were g i v e n a l i n e b i s e c t i o n t a s k under a v a r i e t y of c u e i n g c o n d i t i o n s . F o r o u r p r e s e n t p u r p o s e s , t h e two r e l e v a n t c o n d i t i o n s were when t h e l i n e s were f l a n k e d by two l e t t e r s b u t t h e p a t i e n t s o n l y had t o r e p o r t t h e r i g h t - s i d e l e t t e r ( d u a l c u e , r e p o r t r i g h t ) , and when o n l y a r i g h t - s i d e l e t t e r was p r e s e n t which p a t i e n t s had t o r e p o r t p r i o r t o b i s e c t i n g t h e l i n e . N e g l e c t was s l i g h t l y b u t r e l i a b l y more s e v e r e i n t h e r i g h t s i n g l e - l e t t e r condition than i n the dual cue, report r i g h t condition. T h a t i s , t h e p r e s e n c e of t h e l e f t - s i d e cue i n t h e d u a l cue c o n d i t i o n seemed t o a f f e c t performance even though p a t i e n t s w e r e n o t i n s t r u c t e d t o r e p o r t it. T h i s f i n d i n g s u g g e s t s t h a t a t l e a s t some p r o c e s s i n g of t h e l e f t - s i d e cue occurred, leading t o a m i l d a t t e n u a t i o n o f neglect r e l a t i v e t o w h e n p a t i e n t s o r i e n t e d t o a single right-side cue. The q u e s t i o n i s , w h a t k i n d s o f p r o c e s s i n g a r e n e g l e c t e d s t i m u l i s u b j e c t t o ? I t seems c l e a r t h a t t h i s q u e s t i o n can o n l y be a d d r e s s e d i n d i r e c t l y , s i n c e , by d e f i n i t i o n , p a t i e n t s a r e poor a t d i r e c t l y r e p o r t i n g s t i m u l i on t h e n e g l e c t e d s i d e . R e c e n t l y , we have a t t e m p t e d t o a s s e s s something a b o u t t h e k i n d s of p r o c e s s i n g c a r r i e d o u t upon n e g l e c t e d s t i m u l i by a s s e s s i n g v i s u a l s e a r c h f u n c t i o n s i n t h e n e g l e c t e d and non-neglected f i e l d s of p a t i e n t s w i t h u n i l a t e r a l neglect. In a v i s u a l search task, a subject is presented with a d i s p l a y c o m p r i s i n g a v a r y i n g number of i t e m s and he o r she i s a s k e d t o d e c i d e w h e t h e r a p r e - d e s i g n a t e d t a r g e t i t e m i s p r e s e n t o r a b s e n t . Measures a r e t a k e n of t h e t i m e t a k e n and t h e number of errors made f o r e a c h t y p e of d e c i s i o n a s a f u n c t i o n of t h e number o r d i s t r a c t o r i t e m s p r e s e n t ( t h e d i s p l a y s i z e ) , and i n f e r e n c e s a b o u t t h e n a t u r e of t h e u n d e r l y i n g s e a r c h p r o c e s s a r e made from t h e form of t h e s e a r c h p e r f o r m a n c e - d i s p l a y s i z e f u n c t i o n . For i n s t a n c e , i f e i t h e r t h e number of e r r o r s o r t h e R T s t o d e c i d e t h a t a t a r g e t is present i n c r e a s e l i n e a r l y a s a f u n c t i o n o f t h e d i s p l a y s i z e , t h e n t h e r e a r e grounds f o r s u g g e s t i n g t h a t s u b j e c t s a r e s e a r c h i n g t h e d i s p l a y s e r i a l l y , w i t h each item being t r e a t e d a s a s e p a r a t e perceptual o b j e c t ( e . g . , Treisman & G e l a d e , 1980). When s u c h a s e r i a l s e a r c h i s conducted, t h e r a t e of s e a r c h is g i v e n by t h e s l o p e of t h e RT-display s i z e f u n c t i o n . S e a r c h r a t e on t h e t a r g e t - p r e s e n t t r i a l s s h o u l d be a b o u t h a l f t h a t on t h e t a r g e t - a b s e n t t r i a l s , s i n c e on a v e r a g e s u b j e c t s w i l l o n l y need t o s e a r c h h a l f t h e d i s p l a y i t e m s when t h e t a r g e t i s p r e s e n t , whereas t h e y n e e d

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t o s e a r c h a l l t h e d i s p l a y items when i t is a b s e n t . I n c o n t r a s t , n o n - l i n e a r r e l a t i o n s h i p s between s e a r c h performance and d i s p l a y s i z e , and i n p a r t i c u l a r , f l a t performance-display s i z e f u n c t i o n s , are i n d i c a t i v e of a search process taking place i n p a r a l l e l across t h e display elements (Treisman & Gelade, 1980)2. One s i m p l e i s s u e w e might a d d r e s s , t h e n , i s w h e t h e r n e g l e c t p a t i e n t s demonstrate s i m i l a r search functions f o r s t i m u l i presented i n t h e i r n e g l e c t e d and t h e i r non-neglected f i e l d s . T h i s i s s u e i s r e l e v a n t f o r u n d e r s t a n d i n g t h e n a t u r e of n e g l e c t . I f p a t i e n t s have d e f i c i e n t s e n s o r y p r o c e s s i n g of t h e n e g l e c t e d s i d e of s p a c e , t h e s e a r c h f u n c t i o n s o n t h e n e g l e c t e d and t h e non-neglected s i d e s ought t o d i f f e r q u i t e d r a s t i c a l l y . I n p a r t i c u l a r , t h e r e s h o u l d be l i t t l e e v i d e n c e f o r t h e p a r a l l e l p r o c e s s i n g of d i s p l a y e l e m e n t s i n t h e n e g l e c t e d f i e l d , even when t h e d i s c r i m i n a t i o n s i n t h e non-neglected f i e l d a r e based on p a r a l l e l p r o c e s s i n g . The p r e d i c t i o n s made by t h e t h e o r y t h a t n e g l e c t p a t i e n t s have i m p a i r e d r e p r e s e n t a t i o n s of space a r e l e s s c l e a r . However, i f we suppose t h a t n e g l e c t p a t i e n t s a r e s i m p l y u n a b l e t o r e p r e s e n t t h e p o s i t i o n s of d i s p l a y i t e m s on t h e s i d e of space c o n t r a l a t e r a l t o t h e l e s i o n s i t e , t h e n d e t e c t i o n s of n e g l e c t - s i d e s t i m u l i which r e q u i r e a c c u r a t e s p a t i a l l o c a l i z a t i o n s h o u l d be e x t r e m e l y d i f f i c u l t . A t t e n t i o n a l t h e o r i e s assume t h a t p r e - a t t e n t i v e p r o c e s s i n g i n n e g l e c t p a t i e n t s may be r e l a t i v e l y i n t a c t . The e f f e c t s of poor a t t e n t i o n a l - c a p t u r e by n e g l e c t - s i d e s t i m u l i , t h e n , w i l l depend on t h e r o l e of a t t e n t i o n i n v i s i o n . To f o r m u l a t e more d e t a i l e d p r e d i c t i o n s , w e need t o considermorepreciseaccountsof t h e f u n c t i o n o f v i s u a l a t t e n t i o n .

1V.AccountsofAttentioninVision Although t h e r e i s a common d i s t i n c t i o n made between p r e - a t t e n t i o n a l and a t t e n t i o n a l p r o c e s s i n g i n v i s i o n , t h e r e a r e d i f f e r e n t v i e w s a b o u t t h e k i n d s of i n f o r m a t i o n which may be r e p r e s e n t e d p r e - a t t e n t i v e l y ( s e e s e c t i o n

I). One p o s i t i o n h a s been put forward by Treisman and h e r c o l l e a g u e s (Treisman, 1982, 1984; Treisman & Gelade, 1980; Treisman & Schmidt, 1982; Treisman e t a l . , 1977), who have argued t h a t p r e - a t t e n t i v e v i s i o n i s concerned w i t h t h e p a r a l l e l a n a l y s i s of v i s u a l i n p u t i n t e r m s o f i n d e p e n d e n t maps f o r e a c h s e p a r a t e v i s u a l f e a t u r e ( e . g . , c o l o u r , l o c a l s h a p e , s i z e ) . Within e a c h o f t h e s e m a p s , t h e r e maybe some g r o u p i n g o f t h e f e a t u r e s s o t h a t a r e a s of homogeneity and h e t e r o g e n e i t y a r e made e x p l i c i t . I n t h e c o l o u r domain, such g r o u p i n g p r o c e s s e s w i l l s p e c i f y whether t h e r e is a homogeneous a r e a of c o l o u r ; i n t h e form domain, g r o u p i n g may o p e r a t e on t h e b a s i s of s i m p l e i n t e r - d e p e n d e n c i e s between l o c a l e l e m e n t s ( s u c h as t h e i r r e l a t i v e c o n t r a s t and o r i e n t a t i o n ) or on t h e coding of a few h i g h e r - o r d e r f e a t u r e s (termed " t e x t o n s " by J u l e s z , 1980, 1981). which code i n f o r m a t i o n a b o u t l o c a l d i s p l a y p r o p e r t i e s s u c h a s c o l i n e a r i t y , c l o s u r e , c o r n e r and t h e number of l i n e e n d s o r " t e r m i n a t o r s " ( s e e C a e l l i , J u l e s z 6 G i l b e r t , 1978; J u l e s z , 1980, 1981; J u l e s z & C a e l l i , 1979). However, i n f o r m a t i o n a b o u t s p e c i f i c c o m b i n a t i o n s of f e a t u r e s i s not a v a i l a b l e p r e - a t t e n t i v e l y , and a t t e n t i o n may be r e q u i r e d t o a c c u r a t e l y combine t h e s e p a r a t e f e a t u r e s of a g i v e n o b j e c t . T h u s , i n v i s u a l s e a r c h t a s k s where t a r g e t s a r e d e f i n e d o n t h e b a s i s of a s i n g l e f e a t u r e ( s u c h a s b e i n g a p a r t i c u l a r c o l o u r , o r i e n t a t i o n o r c o n t a i n i n g a p a r t i c u l a r l o c a l form e l e m e n t ) , r e s p o n s e s may be d e t e r m i n e d b y p a r a l l e l pre-attentive processing. I n c o n t r a s t , responses i n v i s u a l search t a s k s w i t h t a r g e t s d e f i n e d by a p a r t i c u l a r combination of f e a t u r e s w i l l be dependent on t h e a p p l i c a t i o n of f o c a l a t t e n t i o n t o e a c h d i s p l a y i t e m ( i n o r d e r t o combine t h e s e p a r a t e f e a t u r e c o d e d a t e a c h l o c a t i o n ) , so g e n e r a t i n g l i n e a r s e a r c h f u n c t i o n s ( e . g . , Treisman & G e l a d e , 1980). Now, a c c o r d i n g t o t h i s feature-integration position, p a t i e n t s w h o a r e u n a b l e t o a t t e n d t o o n e


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s i d e o f space may n e v e r t h e l e s s be a b l e t o make d i s c r i m i n a t i o n s t o n e g l e c t e d s t i m u l i based on t h e k i n d s o f i n f o r m a t i o n made a v a i l a b l e by p r e - a t t e n t i v e v i s i o n ( i . e . , s i n g l e element c o d i n g s o r grouped i n f o r m a t i o n w i t h i n a s i n g l e f e a t u r e domain). On t h e o t h e r hand, d i s c r i m i n a t i o n s t o n e g l e c t - s i d e s t i m u l i based on s p e c i f i c c o m b i n a t i o n s of f e a t u r e s w i l l be e x t r e m e l y d i f f i c u l t 3 * 4 . A r a t h e r d i f f e r e n t v i e w is t h a t information about the s p e c i f i c c o m b i n a t i o n s of f e a t u r e s is r e p r e s e n t e d p r e - a t t e n t i v e l y , but t h a t a t t e n t i o n must be engaged by t h i s i n f o r m a t i o n f o r a c t i o n t o be d i r e c t e d t o t h e s t i m u l u s ( c f . A l l p o r t , T i p p e r & Chmiel, 1985) and e v e n f o r t h e assignment of a unique d i s c r i m i n a t o r y r e s p o n s e t o i t (termed " p e r c e p t u a l i d e n t i f i c a t i o n " by Humphreys, 1985). According t o t h i s view, a t t e n t i o n may need t o be engaged even when s i n g l e f e a t u r e d i s c r i m i n a t i o n s a r e r e q u i r e d ( s e e Duncan, 1985, f o r e v i d e n c e o n t h i s p o i n t ) , though t h e e a s e w i t h which a t t e n t i o n i s drawn t o a g i v e n f e a t u r e w i l l depend on i t s d i s c r i m i n a b i l i t y . I t f o l l o w s t h a t t h e e f f i c i e n c y of v i s u a l s e a r c h w i l l be d e t e r m i n e d by t h e r e l a t i v e d i s c r i m i n a b i l i t y of t h e t a r g e t t o i t s d i s t r a c t o r s and n o t by q u a l i t a t i v e d i f f e r e n c e s between s i n g l e f e a t u r e and combined-feature targets : parallel search functions w i l l a r i s e with relatively easy d i s c r i m i n a t i o n s , s e r i a l s e a r c h f u n c t i o n s w i l l a r i s e w i t h more d i f f i c u l t d i s c r i m i n a t i o n s (Duncan, 1 9 8 5 ) . We have s u g g e s t e d e a r l i e r that u n i l a t e r a l n e g l e c t may o c c u r b e c a u s e s t i m u l i o n t h e n e g l e c t e d s i d e of s p a c e f a i l t o engage v i s u a l a t t e n t i o n . N o w , i f a t t e n t i o n i s necessary f o r perceptual i d e n t i f i c a t i o n , neglect p a t i e n t s may o f t e n f a i l t o d e t e c t b o t h s i n g l e and c o m b i n e d - f e a t u r e t a r g e t s p r e s e n t e d t o t h e n e g l e c t e d s i d e of s p a c e , though s o m e d i f f e r e n c e s between t h e a b s o l u t e numbers of c o r r e c t d e t e c t i o n s may o c c u r depending on t h e r e l a t i v e d i s c r i m i n a b i l i t y of t a r g e t s and d i s t r a c t o r s . More i m p o r t a n t l y , when c o r r e c t d e t e c t i o n s o c c u r , t h e s e a r c h f u n c t i o n s f o r b o t h s i n g l e and combined-feature t a r g e t s s h o u l d be similar f o r n e g l e c t and f o r non-neglect s i d e s t i m u l i , s i n c e b o t h s i n g l e a n d combined-feature i n f o r m a t i o n i s t h o u g h t t o be made a v a i l a b l e by p r e - a t t e n t i v e v i s i o n .

V.

S o l e Data

Wehave i n v e s t i g a t e d t h e above p r e d i c t i o n s i n a s e r i e s o f v i s u a l s e a r c h t a s k s w i t h n e g l e c t p a t i e n t s , which have examined s e a r c h f o r t a r g e t s d e f i n e d by a v a r i e t y of s i n g l e f e a t u r e d i f f e r e n c e s r e l a t i v e t o d i s t r a c t o r s ( s u c h a s c l o s u r e , c o l o u r , o r i e n t a t i o n and s i z e ) and f o r t a r g e t s d e f i n e d by c o m b i n a t i o n s of f e a t u r e s . We w i s h t o h i g h l i g h t h e r e t h e t y p e s of v i s u a l s e a r c h f u n c t i o n s found w i t h one s a l i e n t s i n g l e f e a t u r e t a r g e t and w i t h one combined-feature t a r g e t , s i n c e i t i s t h e c o n t r a s t between t h e s e two t y p e s of t a r g e t which i s c r i t i c a l t o t h e d i f f e r e n t a c c o u n t s of n e g l e c t . I n t h e s i n g l e f e a t u r e c o n d i t i o n , t h e t a r g e t was a r e d c i r c l e and t h e d i s t r a c t o r s w e r e a l l g r e e n c i r c l e s ; i n t h e combined-feature c o n d i t i o n , t h e t a r g e t w a s a n i n v e r t e d "T" and t h e d i s t r a c t o r s were a l l u p r i g h t "T"s. I n t h e l a t t e r c o n d i t i o n , t a r g e t s and d i s t r a c t o r s d i f f e r o n l y i n t h e s p a t i a l arrangement of t h e i r component l i n e s , so t h a t t a r g e t s a r e d e f i n e d o n l y by t h e p a r t i c u l a r combinationof componentfeatures. G e n e r a l method T h r e e p a t i e n t s (one male, two f e m a l e ) p a r t i c i p a t e d i n t h e e x p e r i m e n t s , H.C., I.J. andA.S. ( a g e s 75, 6 8 a n d 7 6 y e a r s respectively).Allthepatients were r i g h t handed pre-morbidly, and e a c h s u f f e r e d damage t o t h e p a r i e t a l a r e a of t h e r i g h t c e r e b r a l hemisphere c o n s e q u e n t on a s t r o k e . H.C. and I.J. had d e n s e h e m i p l e g i a s and A.S. had a more mild h e m i p a r e s i s . A l l t h e p a t i e n t s were c a t e g o r i z e d a s m a n i f e s t i n g u n i l a t e r a l l e f t - s i d e d n e g l e c t i n drawing t a s k s . I.J. and A.S. b o t h had a homonymous l e f t hemianopia ; H.C. had i n t a c t

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M.J. Riddoch and

C. W.Huinphreys

v i s u a l f i e l d s . T e s t i n g f o r e a c h of t h e p a t i e n t s w a s commenced 4 m o n t h s p o s t l e s i o n and c o n t i n u e d f o r a f u r t h e r 4 months, d u r i n g which time t h e i r c o n d i t i o n remained r e l a t i v e l y s t a b l e . T e s t s f o r t h e e f f e c t s of p r a c t i c e o n t h e i r v i s u a l s e a r c h performance f a i l e d t o r e v e a l any s t r o n g improvements t n any s i n g l e c o n d i t i o n o v e r t h e t e s t p e r i o d . I n any one s e s s i o n , e a c h p a t i e n t performed a t l e a s t two d i f f e r e n t v i s u a l s e a r c h t a s k s drawn a t random from t h e f u l l range of s e a r c h e s examined. F o r any one c o n d i t i o n w i t h i n a s e s s i o n , t h e p a t i e n t was p r e s e n t e d w i t h a s e r i e s of 160 c a r d s , 80 w i t h t h e t a r g e t p r e s e n t and 80 w i t h t h e t a r g e t a b s e n t . Of t h e t a r g e t - p r e s e n t c a r d s , 20 had t h e t a r g e t i n t h e l e f t upper q u a d r a n t , 20 i n t h e l e f t lower q u a d r a n t , 20 i n t h e r i g h t upper q u a d r a n t and 20 i n t h e r i g h t lower q u a d r a n t . Within e a c h q u a d r a n t , t h e t a r g e t was p o s i t i o n e d i n t h e upper l e f t r e g i o n 4 t i m e s , t h e r i g h t upper r e g i o n 4 times, t h e l e f t lower r e g i o n 4 t i m e s , t h e r i g h t lower r e g i o n 4 t i m e s and t h e m i d d l e r e g i o n 4 t i m e s . D i s t r a c t o r i t e m s were randomly a r r a n g e d o n t h e c a r d s . The numbers of d i s t r a c t o r s w a s m a n i p u l a t e d . F o r t y c a r d s h a d 4 i t e m s per q u a d r a n t (16 p e r c a r d ) , 40 had 6 i t e m s p e r q u a d r a n t (24 p e r c a r d ) , 40 had 8 i t e m s p e r q u a d r a n t (32 p e r c a r d ) and 40 had 10 p e r q u a d r a n t (40 p e r c a r d ) . The s t i m u l i were p r e s e n t e d on c a r d s which were 12.6 cm ( a b o u t 14 d e g 24 min) wide and 10 cm ( a b o u t 11 deg 26 min) i n h e i g h t . The c a r d s f o r each c o n d i t i o n were p r e s e n t e d randomly t o t h e p a t i e n t who was asked t o d e c i d e whether t h e t a r g e t was p r e s e n t o r a b s e n t a s q u i c k l y a s p o s s i b l e w i t h o u t making e r r o r s . Each c a r d was s i n g l y p r e s e n t e d and p o s i t i o n e d i n t h e p a t i e n t ' s m i d l i n e , and e a c h c o n t a i n e d a c e n t r a l numeral which t h e p a t i e n t was i n s t r u c t e d t o name a l o u d p r i o r t o commencing t h e s e a r c h . RTs were r e c o r d e d u s i n g a d i g i t a l timer, and t i m i n g was s t a r t e d a t t h e naming of t h e c e n t r a l numeral and i t was completed by t h e p a t i e n t g i v i n g a v e r b a l "yes" ( t a r g e t p r e s e n t ) o r "no" ( t a r g e t a b s e n t ) r e s p o n s e . W i t h i n each c o n d i t i o n , t h e numbers of t r i a l s c a r r i e d o u t by e a c h p a t i e n t d i f f e r e d a c c o r d i n g t o t h e numbers of e r r o r s e a c h made, and a g i v e n c o n d i t i o n was a d m i n i s t e r e d o v e r a number of s e s s i o n s u n t i l e a c h p a t i e n t c o n t r i b u t e d a t l e a s t 7 RT d a t a p o i n t s a t e a c h d i s p l a y s i z e t o s t i m u l i p r e s e n t e d e i t h e r contralaterallyoripsilaterallytothe s i t e o f the lesion. Experiment 1 : Colour s e a r c h I n Experiment 1, s u b j e c t s s e a r c h e d f o r a r e d c i r c l e a g a i n s t a background of g r e e n d i s t r a c t o r c i r c l e s . Each c i r c l e had a d i a m e t e r of 8 mm. Each p a t i e n t c a r r i e d o u t a t o t a l of 320 t r i a l s , 40 t a r g e t - a b s e n t t r i a l s a t e a c h d i s p l a y s i z e and 20 t a r g e t - p r e s e n t t r i a l s f o r e a c h d i s p l a y - s i z e and v i s u a l f i e l d combination. Mean RTs and numbers of c o r r e c t r e s p o n s e s f o r e a c h d e c i s i o n ( t a r g e t p r e s e n t and a b s e n t ) a t e a c h d i s p l a y s i z e ( 4 , 6 , 8 and 10 p e r q u a d r a n t ) a r e g i v e n i n T a b l e 1. T r e a t i n g e a c h p a t i e n t a s a s i n g l e c a s e s t u d y , t h e r e were no r e l i a b l e e f f e c t s of d i s p l a y s i z e o n e i t h e r t h e RT o r t h e e r r o r d a t a f o r b0thH.C. and I.J.. T h e r e was a marginal e f f e c t of d i s p l a y s i z e on A.S.'s RTs, though even i n t h i s c a s e RTs d i d n o t i n c r e a s e l i n e a r l y a s a f u n c t i o n of t h e d i s p l a y s i z e . Thus, e a c h p a t i e n t a p p e a r s t o have been a b l e t o d e t e c t t h e c o l o u r - d e f i n e d t a r g e t on t h e b a s i s of a p a r a l l e l s e a r c h of t h e d i s p l a y . N e v e r t h e l e s s , e a c h p a t i e n t a l s o d e m o n s t r a t e d n e g l e c t of t h e l e f t s i d e of t h e d i s p l a y s , s i n c e each made more e r r o r s t o l e f t t h a n t o r i g h t - s i d e s t i m u l i . For H.C. andA.S. t h e r e were no r e l i a b l e d i f f e r e n c e s between c o r r e c t d e t e c t i o n l a t e n c i e s f o r r i g h t and l e f t s i d e t a r g e t s and b o t h were f a s t e r t h a n a b s e n t r e s p o n s e s ; f o r I.J., c o r r e c t d e t e c t i o n s were s l o w e r t o l e f t - s i d e t h a n t o r i g h t - s i d e t a r g e t s , t h o u g h a b s e n t r e s p o n s e s w e r e slower a g a i n . These d a t a i n d i c a t e t h a t n e g l e c t p a t i e n t s can m a n i f e s t p a r a l l e l


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p r o c e s s i n g of s t i m u l i on t h e n e g l e c t e d s i d e of s p a c e , a t l e a s t when t h e t a s k o n l y r e q u i r e s t h e d e t e c t i o n of a s i n g l e d i s j u n c t i v e f e a t u r e ( s u c h a s a c o l o u r change r e l a t i v e t o a homogeneous background). The r e s u l t i s c o n s i s t e n t w i t h t h e i d e a t h a t such p a t i e n t s can have r e l a t i v e l y i n t a c t pre-attentive v i s u a l processing; indeed, the l a t e n c i e s f o r c o r r e c t t a r g e t d e t e c t i o n s f o r H.C. andA.S. s h o w e d n o e f f e c t o f f i e l d o f p r e s e n t a t i o n . I t i s of i n t e r e s t , t h e n , t h a t d e s p i t e d e m o n s t r a t i n g p a r a l l e l p r o c e s s i n g i n b o t h v i s u a l f i e l d s , t h e p a t i e n t s a l s o showed n e g l e c t i n t h a t t h e y missed many l e f t - s i d e t a r g e t s . The l a t t e r r e s u l t i s c o n t r a r y t o t h e argument t h a t r e s p o n s e s can be made d i r e c t l y o n t h e b a s i s of p r e - a t t e n t i v e p a r a l l e l p r o c e s s i n g ( c f . Treisman & G e l a d e , 1980) ; r a t h e r , i t a p p e a r s t h a t a t t e n t i o n needs t o be drawn t o t h e i n f o r m a t i o n s p e c i f i e d by p r e - a t t e n t i v e v i s i o n f o r i t s d e t e c t i o n t o o c c u r ( c f . D u n c a n , 1985). I n n e g l e c t p a t i e n t s , a t t e n t i o n i s not r e l i a b l y c a p t u r e d by c o n t r a l a t e r a l s t i m u l i even when t h e s e s t i m u l i a r e d e f i n e d by a s i n g l e , s a l i e n t d i s j u n c t i v e f e a t u r e w h i c h can b e d i s c r i m i n a t e d by means of a p a r a l l e l s e a r c h p r o c e s s ; c o n s e q u e n t l y , t a r g e t s a r e missed when t h e y a r e p r e s e n t e d c o n t r a l a t e r a l l y t o t h e s i t e of l e s i o n . P a t i e n t I.J. a l s o showed a l a t e n c y i n c r e a s e o n c o r r e c t c o n t r a l a t e r a l t a r g e t d e t e c t i o n s , which f u r t h e r s u g g e s t s t h a t , a t l e a s t i n t h i s p a t i e n t , a t t e n t i o n was f i r s t drawn t o i p s i l a t e r a l s t i m u l i and i t was o n l y s u b s e q u e n t l y s h i f t e d t o t h e c o n t r a l a t e r a l s i d e of s p a c e . Experiment 2 : I n v e r t e d T s e a r c h I n Experiment 2 , s u b j e c t s s e a r c h e d f o r a n i n v e r t e d T t a r g e t a g a i n s t a background of u p r i g h t T d i s t r a c t o r s . Each T was 8 mm h i g h by 8 mm wide. H.C. performedatotalof 6 4 0 t r i a l s , I.J. 480trialsandA.S. 3 2 0 t r i a l s . The mean c o r r e c t R T s and numbers of c o r r e c t r e s p o n s e s f o r e a c h d e c i s i o n a t e a c h d i s p l a y s i z e by e a c h p a t i e n t a r e p r e s e n t e d i n T a b l e 2. F o r a l l t h r e e p a t i e n t s , t h e r e were r e l i a b l e e f f e c t s o n RTs of b o t h t h e d i s p l a y s i z e and t h e c o n d i t i o n s ( t a r g e t - l e f t , t a r g e t - r i g h t and t a r g e t a b s e n t ) . T h e r e were no r e l i a b l e i n t e r a c t i o n s . A l l t h e p a t i e n t s were s l o w e r t o d e t e c t l e f t t h a n r i g h t - s i d e t a r g e t s , and, f o r H.C. and I.J., t h e RTs t o l e f t - s i d e t a r g e t s were a s slow a s t h o s e on t a r g e t - a b s e n t t r i a l s . A l s o , f o r a l l t h e p a t i e n t s t h e r e were r e l i a b l e l i n e a r i n c r e a s e s i n R T s a s a f u n c t i o n of thedisplay size. The e r r o r d a t a t e n d e d t o f o l l o w t h e same p a t t e r n a s t h e RT d a t a . A l l t h e p a t i e n t s made more e r r o r s t o l e f t t h a n t o r i g h t - s i d e s t i m u l i , and f o r H.C. and I . J . , e r r o r s t e n d e d t o i n c r e a s e a s a f u n c t i o n o f t h e d i s p l a y s i z e . I n t h i s experiment, a l l t h e p a t i e n t s demonstrated s e r i a l v i s u a l s e a r c h , w i t h R T s i n c r e a s i n g l i n e a r l y a s t h e d i s p l a y s i z e i n c r e a s e d . The s e a r c h r a t e s tended t o be f a s t e r f o r r i g h t - f i e l d t a r g e t s t h a n f o r l e f t - f i e l d t a r g e t s , though t h e d i s p l a y s i z e X c o n d i t i o n i n t e r a c t i o n s were a l l non-signif i c a n t . The r e s u l t s s u g g e s t t h a t a s e r i a l s e a r c h is n e c e s s a r y t o d e t e c t a t a r g e t d e f i n e d by t h e p r e s e n t c o m b i n a t i o n of h o r i z o n t a l and v e r t i c a l f e a t u r e s ( c f . Experiment 1 ) 5 , and t h a t t h e n e g l e c t p a t i e n t s h e r e tended t o s e a r c h t h e r i g h t f i e l d p r i o r t o t h e l e f t f i e l d . Absent r e s p o n s e s were a c c u r a t e and a t l e a s t a s slow a s l e f t - s i d e t a r g e t p r e s e n t r e s p o n s e s . Now, g i v e n t h e l a r g e numbers of e r r o r s made t o l e f t - s i d e t a r g e t s , i t would a p p e a r t h a t p a t i e n t s o f t e n f a i l e d t o s e a r c h t h e l e f t s i d e s of t h e d i s p l a y s . On s u c h t r i a l s , a b s e n t r e s p o n s e s may be based o n some i n t e r n a l d e a d l i n e (which d i f f e r e d between p a t i e n t s ; c0mpareH.C. w i t h I . J . ) . Such a d e a d l i n e may a l s o come i n t o p l a y on t h e t r i a l s where s u b j e c t s d i d s e a r c h t h e l e f t f i e l d , p r o d u c i n g some f a s t a b s e n t r e s p o n s e s c l o s e t o some of t h e l a t e n c i e s for left-side t a r g e t d e t e c t i o n s ( p a r t i c u l a r 1 y f o r H . C . a n d 1 . J . ) . The c o n t r a s t between t h e s e r i a l s e a r c h f u n c t i o n s found h e r e and t h e p a r a l l e l search functions f o r t h e colour-defined t a r g e t s inExperiment 1 i s

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c o n s i s t e n t w i t h Treisman' s argument t h a t t h e combination of f e a t u r e s needed t o d i s t i n g u i s h a n i n v e r t e d T from u p r i g h t T ' s demands t h e o p e r a t i o n of a s e r i a l a t t e n t i o n - d e m a n d i n g p r o c e s s , w h i l s t s i n g l e f e a t u r e t a r g e t s can be d e t e c t e d d i r e c t l y from p a r a l l e l p r e - a t t e n t i v e p r o c e s s i n g ( e . g . , Treisman & Gelade, 1980; though see Experiment 1 ) . The c o n t r a s t i s a l s o c o n s i s t e n t w i t h the idea t h a t the colour-defined t a r g e t captures a t t e n t i o n m o r e e a s i l y t h a n a t a r g e t d e f i n e d by a combination of f e a t u r e s , so t h a t s t e e p e r s e a r c h f u n c t i o n s a r e o b t a i n e d i n t h e combined-feature c o n d i t i o n ( e . g . , Duncan, 1985). Whichever p o s i t i o n is a d o p t e d , t h e f a c t t h a t t h e p a t i e n t s c o u l d d e t e c t a t l e a s t some l e f t - s i d e combined-feature t a r g e t s h e r e s u g g e s t s t h a t the p a t i e n t s a r e a b l e t o represent d e t a i l e d information about t h e s p a t i a l p o s i t i o n s of l o c a l form e l e m e n t s o n t h e n e g l e c t e d s i d e of s p a c e w h e n t h e form e l e m e n t s a r e a t t e n d e d ( c o n t r a r y t o t h e r e p r e s e n t a t i o n a l schema a c c o u n t ; see section 11). Experiment3 : T h e e f f e c t s o f cueing I n a f i n a l e x p e r i m e n t , w e e x a m i n e d t h e e f f e c t s of c u e i n g o n n e g l e c t f o r s i n g l e - f e a t u r e ( c o l o u r ) and combined-feature ( i n v e r t e d T ) t a r g e t s . P r i o r t o commencing t h e s e a r c h on e a c h t r i a l , t h e p a t i e n t s were cued t o r e p o r t a l e t t e r p o s i t i o n e d t o t h e l e f t s i d e of t h e d i s p l a y . Now, i f n e g l e c t p a t i e n t s f a i l t o a t t e n d t o t h e s i d e of s p a c e c o n t r a l a t e r a l t o t h e l e s i o n b e c a u s e of u n i l a t e r a l a k i n e s i a , o r because of a f a i l u r e t o r e p r e s e n t t h a t s i d e of s p a c e i n t e r n a l l y , t h e n c u e i n g them t o t h e n e g l e c t e d s i d e w h i l s t k e e p i n g o t h e r display c h a r a c t e r i s t i c s c o n s t a n t should n o t reduce n e g l e c t . However, n e g l e c t s h o u l d be reduced i f p a t i e n t s n e g l e c t t h e c o n t r a l a t e r a l s i d e of space because s t i m u l i on t h a t s i d e f a i l t o c a p t u r e a t t e n t i o n , even though t h e p r o c e s s e s i n v o l v e d i n t h e c o n s c i o u s c o n t r o l of a t t e n t i o n remain i n t a c t . Experiment 3a : Cueing c o l o u r s e a r c h . The same c a r d s a s used i n Experiment 1 were employed, e x c e p t t h a t t h e s e c a r d s w e r e m o u n t e d o n a n o t h e r c a r d s o t h a t a b o r d e r 7 . 6 mm wide by 10 cm h i g h o c c u r r e d o n t h e l e f t s i d e of e a c h o r i g i n a l card. I n t h e c e n t r e o f t h i s b o r d e r w a s a c a p i t a l l e t t e r . A t t h e b e g i n n i n g of e a c h t r i a l , t h e p a t i e n t was i n s t r u c t e d t o read t h e l e t t e r on t h e l e f t of t h e c a r d aloud p r i o r t o commencing t h e s e a r c h . Timing began when t h e l e t t e r was readaloud. A l l the patientsundertook320trials. Mean c o r r e c t RTs and numbers of c o r r e c t r e s p o n s e s a r e p r e s e n t e d i n Table 3. F o r a l l t h e p a t i e n t s , t h e r e were r e l i a b l e d i f f e r e n c e s i n RTs i n t h e t h r e e c o n d i t i o n s , p r e s e n t - l e f t , p r e s e n t - r i g h t and a b s e n t . Also, f o r H.C. and f o r A.S. t h e r e were r e l i a b l e main e f f e c t s of d i s p l a y s i z e , a l t h o u g h i n b o t h c a s e s t h e l i n e a r component of t h e d i s p l a y s i z e e f f e c t was n o t s i g n i f i c a n t . T h e r e were no i n t e r a c t i o n s . F u r t h e r a n a l y s i s of t h e c o n d i t i o n s e f f e c t showed t h a t f o r a l l p a t i e n t s a b s e n t r e s p o n s e s were s l o w e r t h a n p r e s e n t r e s p o n s e s , b u t t h a t t h e r e were no d i f f e r e n c e s i n R T s t o l e f t and t o r i g h t - s i d e t a r g e t s . There was a similar l a c k of an e f f e c t of v i s u a l f i e l d i n the e r r o r d a t a f o r each p a t i e n t . I t appears then t h a t cueing t h e p a t i e n t s t o reportaletterontheleftsideof the displays eliminatedneglect. When w e compare t h e performance of e a c h p a t i e n t i n t h e c o l o u r - s e a r c h t a s k w i t h and w i t h o u t c u e i n g (Experiments 1 and 3 a ) , w e f i n d t h a t a l l t h e p a t i e n t s were more l i k e l y t o d e t e c t l e f t - s i d e t a r g e t s under t h e c u e i n g c o n d i t i o n s . However, RTs t o l e f t - s i d e t a r g e t s were n o t s e l e c t i v e l y f a c i l i t a t e d by c u e i n g ; i n f a c t , t h e o n l y s e l e c t i v e e f f e c t of c u e i n g on c o r r e c t d e t e c t i o n l a t e n c i e s was a n i n c r e a s e i n t h e r i g h t - s i d e RTs f o r I.J.. The l a t t e r r e s u l t s u g g e s t s t h a t c u e i n g h e l p e d t o o v e r r i d e 1 . J . l ~t e n d e n c y t o attend f i r s t t o t h e right sidesof thedisplays. The f i n d i n g t h a t c u e i n g had a f a c i l i t a t o r y e f f e c t on performance i s


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c o n t r a r y t o b o t h t h e u n i l a t e r a l a k i n e s i a a c c o u n t of u n i l a t e r a l n e g l e c t and t o t h e r e p r e s e n t a t i o n a l schema a c c o u n t . Most i n t e r e s t i n g l y , t h e c o n t r a s t between t h e f a c i l i t a t i o n of t a r g e t d e t e c t i o n s and t h e l a c k of f a c i l i t a t i o n in r e s p o n s e l a t e n c i e s i n d i c a t e s t h a t c u e i n g b e n e f i t t e d a r a t h e r l a t e detection process r a t h e r than t a r g e t processing, since a processingbenefit would presumably s p e e d c o r r e c t d e t e c t i o n s . I t seems t h a t t h e i n f o r m a t i o n s p e c i f i e d by p r e - a t t e n t i v e v i s u a l p r o c e s s i n g ( e . g . , t h e p r e s e n c e of a t a r g e t a g a i n s t a homogeneous background) cannot be used d i r e c t l y f o r r e s p o n s e p u r p o s e s , and t h a t a t t e n t i o n needs t o be drawn t o t h e i n f o r m a t i o n b e f o r e a d i s c r i m i n a t o r y r e s p o n s e can be based on i t . F o r n e g l e c t p a t i e n t s , t h e r e i s a breakdown i n t h e p r o c e s s e s which e n a b l e p r e - a t t e n t i v e i n f o r m a t i o n t o c a p t u r e a t t e n t i o n ; n e v e r t h e l e s s , such p a t i e n t s remain a b l e t o s h i f t a t t e n t i o n c o n s c i o u s l y , and so t h e y d e t e c t t h e i n f o r m a t i o n s p e c i f i e d p r e - a t t e n t i v e l y w h e n cued t o t h e c o n t r a l a t e r a l s i d e of space. Experiment 3b : Cueing i n v e r t e d T s e a r c h We have shown t h a t c u e i n g p a t i e n t s t o t h e c o n t r a l a t e r a l s i d e of s p a c e can r e s o l v e n e g l e c t f o r s i n g l e d i s j u n c t i v e f e a t u r e t a r g e t s . We have a l s o i n v e s t i g a t e d whether c u e i n g c a n r e d u c e n e g l e c t f o r combined-feature t a r g e t s , such as a n i n v e r t e d T r e l a t i v e t o a background of u p r i g h t T ' s . The c u e i n g p r o c e d u r e was t h e same a s f o r Experiment 3a. H.C. performed 640 t r i a l s i n t o t a l , I.J. andA.S. performed 320 t r i a l s . T a b l e 4 shows t h e mean c o r r e c t R T s and t h e numbers of c o r r e c t r e s p o n s e s f o r each patient. A l l t h e p a t i e n t s showed s t r o n g e f f e c t s of b o t h t h e c o n d i t i o n and t h e d i s p l a y s i z e on RTs, and t h e r e was a r e l i a b l e c o n d i t i o n X d i s p l a y s i z e Averaging o v e r t h e c o n d i t i o n s , I.J. and A.S. i n t e r a c t i o n f o r H.C.. demonstrated r e l i a b l e l i n e a r s e a r c h f u n c t i o n s . For L . J . , a b s e n t r e s p o n s e s were s l o w e r t h a n p r e s e n t r e s p o n s e s b u t t h e r e was no d i f f e r e n c e between absent l e f t - s i d e and r i g h t - s i d e t a r g e t p r e s e n t r e s p o n s e s . F o r A . S . , r e s p o n s e s were a l s o s l o w e r t h a n p r e s e n t r e s p o n s e s ; however, l e f t - s i d e p r e s e n t r e s p o n s e s were a l s o s l o w e r t h a n r i g h t - s i d e p r e s e n t r e s p o n s e s . H.C. d e m o n s t r a t e d r e l i a b l e l i n e a r s e a r c h f u n c t i o n s onboththe left-side p r e s e n t and o n a b s e n t t r i a l s , b u t r i g h t - s i d e p r e s e n t r e s p o n s e s d e p a r t e d from l i n e a r i t y . I n s p e c t i o n of t h e e r r o r d a t a r e v e a l s t h a t t h e n o n - l i n e a r RT-display s i z e s e a r c h f u n c t i o n shown by H.C. f o r r i g h t - s i d e t a r g e t s was p r o b a b l y due t o a speed-accuracy t r a d e - o f f , s i n c e he made more m i s s e s a t t h e l a r g e r d i s p l a y s i z e s f o r r i g h t - s i d e s t i m u l i . A l l t h e p a t i e n t s missed more l e f t than right-sidestimuli. Comparisons of performance h e r e r e l a t i v e t o when no c u e i n g o c c u r r e d (Experiment 2 ) i n d i c a t e s e l e c t i v e l y f a s t e r and more a c c u r a t e l e f t - f i e l d t a r g e t d e t e c t i o n s under t h e c u e i n g c o n d i t i o n f o r I.J. and A.S.. Thus, f o r t h e s e p a t i e n t s , c u e i n g t o t h e l e f t s i d e of t h e d i s p l a y s l e s s e n e d n e g l e c t . I n c o n t r a s t , H . C . ' s performance was n o t f a c i l i t a t e d by c u e i n g ; t h e r e were no r e l i a b l e differences i n e i t h e r h i s response l a t e n c i e s o r accuracy t o l e f t - s i d e s t i m u l i i n Experiment 3a r e l a t i v e t o Experiment 2. Our s u g g e s t i o n i s t h a t t h e s e d i f f e r e n t e f f e c t s of c u e i n g w i t h d i f f e r e n t p a t i e n t s a r o s e because of t h e s t r a t e g i e s t h e p a t i e n t s a d o p t e d i n t h e t a s k s . When cued t o r e p o r t t h e l e f t - s i d e l e t t e r p r i o r t o t h e s e a r c h , H.C. t e n d e d t o s h i f t a t t e n t i o n d i r e c t l y t o t h e cue and t h e n t o r e v e r t t o h i s normal s t r a t e g y of s e a r c h i n g t h e r i g h t - s i d e of t h e d i s p l a y s . However, when cued t o t h e l e f t , I.J. and A.S. tended t o conduct a s e r i a l s e a r c h of t h e l e f t h e m i f i e l d . F o r H.C., t h e n , l e f t - s i d e cues f a c i l i t a t e performance by a l l o w i n g t a r g e t s c l e a r l y s p e c i f i e d by p r e - a t t e n t i v e p r o c e s s i n g t o be d e t e c t e d ( e . g . , w i t h t h e c o l o u r - d e f i n e d t a r g e t s i n E x p e r i m e n t s 1 and 3 a ) ; however, s i n c e a l e f t - s i d e s e r i a l search is not induced, h i s d e t e c t i o n o f t a r g e t s n o t e a s i l y

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segmented from t h e i r background p r e - a t t e n t i v e l y i s n o t f a c i l i t a t e d . I.J. and A . S . do a d o p t a s e r i a l l e f t - s i d e s e a r c h under t h e c u e i n g c o n d i t i o n s . T h e i r d e t e c t i o n of i n v e r t e d T t a r g e t s i s t h e r e f o r e f a c i l i t a t e d , s i n c e t h e d e t e c t i o n o f s u c h t a r g e t s is dependentona s e r i a l search process.

VI.ConclusionsandSpecu1ations I n t h i s c h a p t e r , w e h a v e a t t e m p t e d t o o u t l i n e someof t h e m a j o r a c c o u n t s of u n i l a t e r a l v i s u a l n e-g l e c t , and w e have p r e s e n t e d d a t a which w e b e l i e v e throws f u r t h e r l i g h t on some of t h e p r o c e s s i n g d i f f i c u l t i e s e n c o u n t e r e d by n e g l e c t p a t i e n t s . We have proposed t h a t n e g l e c t may b e s t be a c c o u n t e d f o r i n terms of a breakdown i n t h e p r o c e s s e s whereby v i s u a l s t i m u l i c a p t u r e v i s u a l a t t e n t i o n . T h e datawehave presented support t h i s account. T h r e e p a t i e n t s w i t h r i g h t p a r i e t a l damage have been shown t o m a n i f e s t n e g l e c t i n v i s u a l s e a r c h t a s k s demanding t h e d i s c r i m i n a t i o n of t a r g e t s d e f i n e d b o t h by a s i n g l e d i s j u n c t i v e f e a t u r e and by a c o m b i n a t i o n of f e a t u r e s . N e g l e c t of s i n g l e - f e a t u r e t a r g e t s i s m a n i f e s t e d i n t h e a c c u r a c y of r e s p o n s e , w h i l e t h e r e i s l i t t l e d i E f e r e n c e i n t h e s e a r c h r a t e s f o r t h o s e s t i m u l i i n t h e n e g l e c t e d and t h e non-neglected f i e l d s which a r e c o r r e c t l y d e t e c t e d . A l s o , t h e s e a r c h i n b o t h f i e l d s a p p e a r s t o be s p a t i a l l y p a r a l l e l . From t h i s e v i d e n c e , i t would a p p e a r t h a t p r e - a t t e n t i v e p r o c e s s i n g of t h e n e g l e c t e d f i e l d may be r e l a t i v e l y i n t a c t i n s u c h p a t i e n t s . I t i s i m p o r t a n t t o n o t e t h a t t h e f i n d i n g s s u g g e s t t h a t d i s c r i m i n a t o r y r e s p o n s e s cannot be f o r m u l a t e d d i r e c t l y from p r e - a t t e n t i v e i n f o r m a t i o n , o t h e r w i s e no n e g l e c t would o c c u r ; r a t h e r , a t t e n t i o n must be drawn t o s u c h i n f o r m a t i o n . N e g l e c t o c c u r s because of a b r e a k d o w n i n t h e l a t t e r a t t e n t i o n a l - c a p t u r e p r o c e s s . I n s e a r c h e s f o r combined-feature t a r g e t s , n e g l e c t is m a n i f e s t b o t h i n r e s p o n s e l a t e n c i e s and r e s p o n s e a c c u r a c y . The d e t e c t i o n of the combined-feature t a r g e t s h e r e depended on a s e r i a l s e a r c h p r o c e s s i n b o t h h e m i f i e l d s . Slow and i n a c c u r a t e r e s p o n s e s t o l e f t - s i d e t a r g e t s would t h e n o c c u r i f n e g l e c t p a t i e n t s s e a r c h t h e i p s i l a t e r a l s i d e of s p a c e p r i o r t o t h e s i d e c o n t r a l a t e r a l t o t h e s i d e of l e s i o n , and i f t h e y o f t e n f a i l t o s e a r c h the contralateral side. I n t e r e s t i n g l y , f o r two p a t i e n t s (I.J. and A . S . ) , n e g l e c t i n b o t h t h e s i n g l e and t h e combined-feature s e a r c h e s was reduced by c u e i n g t h e p a t i e n t s t o a t t e n d t o t h e l e f t s i d e of t h e d i s p l a y s . F o r a t h i r d p a t i e n t , H.C., c u e i n g f a c i l i t a t e d s i n g l e - f e a t u r e but n o t combined-feature s e a r c h . The d i f f e r e n t e f f e c t s of c u e i n g seem t o depend on t h e s t r a t e g y a d o p t e d by t h e p a t i e n t when i n s t r u c t e d t o s h i f t a t t e n t i o n i n t h e combined-feature t a s k . I.J. and A . S . b o t h conducted s e r i a l s e a r c h e s of t h e l e f t h e m i f i e l d when g i v e n t h e c u e i n g i n s t r u c t i o n ; H.C. appeared t o s h i f t a t t e n t i o n d i r e c t l y t o t h e l e f t - s i d e cue w i t h o u t s e a r c h i n g e a c h i n d i v i d u a l l e f t - s i d e d i s p l a y member. S i n c e combined-feature i n f o r m a t i o n i s n o t c l e a r l y s p e c i f i e d p r e - a t t e n t i v e l y ( i f i t is s p e c i f i e d a t a l l ) , t h e d e t e c t i o n of such t a r g e t s r e q u i r e s a s e r i a l s e a r c h p r o c e s s . Consequently, c u e i n g o n l y f a c i l i t a t e d I.J. and A . S . i n t h i s c o n d i t i o n . Cueing reduced n e g l e c t f o r a l l p a t i e n t s i n t h e s i n g l e - f e a t u r e t a s k , though, presumably because s i n g l e - f e a t u r e i n f o r m a t i o n i s c l e a r l y specified pre-attentively. The d i f f e r e n t i a l s u c c e s s of c u e i n g f o r d i f f e r e n t p a t i e n t s and d i f f e r e n t t a s k s may a l s o go some way t o e x p l a i n i n g some of t h e i n c o n s i s t e n t e f f e c t s of c u e i n g on n e g l e c t which have been p r e v i o u s l y r e p o r t e d . For i n s t a n c e , t h e r e s u l t s of c u e i n g w i l l depend on whether p a t i e n t s a r e r e l i a n t on i n f o r m a t i o n s p e c i f i e d p r e - a t t e n t i v e l y o r whether t h e y s e r i a l l y a t t e n d t o t h e n e g l e c t e d hemif i e l d , and on whether t h e t a s k c a n be performed on t h e b a s i s of i n f o r m a t i o n made e x p l i c i t p r e - a t t e n t i v e l y . The e f f e c t s of t h e d i f f e r e n t p r o c e s s i n g s t r a t e g i e s a d o p t e d by p a t i e n t s under c u e i n g c o n d i t i o n s need t o be examined more t h o rough1 y

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The p r e s e n t d a t a have i m p l i c a t i o n s f o r u n d e r s t a n d i n g u n i l a t e r a l n e g l e c t and f o r u n d e r s t a n d i n g t h e r o l e of a t t e n t i o n i n n o r m a l v i s i o n . F i r s t , c o n s i d e r t h e i m p l i c a t i o n s f o r u n d e r s t a n d i n g u n i l a t e r a l n e g l e c t . Our f i n d i n g s both s u p p o r t t h e kind of attentionalargumentweoutlinedaboveand t h e y go a g a i n s t a number of o t h e r i n t e r p r e t a t i o n s . F o r i n s t a n c e , t h e p a t i e n t s w e have d i s c u s s e d a l l a p p e a r t o have r e l a t i v e l y i n t a c t p r e - a t t e n t i v e processing, a t l e a s t f o r colour. It is d i f f i c u l t t o argue t h e r e f o r e t h a t t h e p a t i e n t s had i m p a i r e d e a r l y v i s u a l p r o c e s s e s . F u r t h e r , one p a t i e n t (H.C.) had no v i s u a l f i e l d d e f e c t s , so t h e r e s u l t s c a n n o t be a t t r i b u t e d t o s e l e c t i v e e f f e c t s of hemianopia. I n a d d i t i o n , w h i l s t n e g l e c t tended t o be more s e v e r e i n t h e combined-feature t h a n t h e s i n g l e - f e a t u r e c o n d i t i o n ( e . g , c o n s i d e r t h e p r o p o r t i o n s of c o r r e c t d e t e c t i o n s made ; s e e T a b l e s 1 and 2 ) , i t was by no means t h e c a s e t h a t t h e p a t i e n t s were u n a b l e t o d i s c r i m i n a t e any combined-feature t a r g e t s i n t h e n e g l e c t e d h e m i f i e l d . T h i s s u g g e s t s t h a t s u c h p a t i e n t s a r e a b l e t o r e p r e s e n t t h e s p a t i a l l o c a t i o n s of form e l e m e n t s p r e s e n t e d c o n t r a l a t e r a l l y t o t h e l e s i o n s i t e . F i n a l l y , t h e r e l i a b l e e f f e c t s of c u e i n g which were found, w i t h n e g l e c t even b e i n g e l i m i n a t e d i n t h e s i n g l e - f e a t u r e c o n d i t i o n , c o n t r a d i c t t h e view t h a t t h e damaged hemisphere of n e g l e c t p a t i e n t s is underaroused and a k i n e t i c . S i n c e t h e d i s p l a y c h a r a c t e r i s t i c s were k e p t c o n s t a n t under t h e c u e i n g c o n d i t i o n s , t h e damaged hemisphere s h o u l d remain u n d e r a r o u s e d r e l a t i v e t o t h e a r o u s a l l e v e l of t h e i n t a c t hemisphere, and n e g l e c t s h o u l d s t i l l o c c u r . The u n i l a t e r a l a k i n e s i a argument a l s o has d i f f i c u l t y e x p l a i n i n g why d i f f e r e n t e f f e c t s of c u e i n g s h o u l d have been found i n t h e s i n g l e and t h e combined-feature t a s k s . One i m p l i c a t i o n f o r u n d e r s t a n d i n g t h e r o l e of a t t e n t i o n i n normal v i s i o n i s t h a t a t t e n t i o n does n o t s e e m t o b e r e q u i r e d s o l e l y t o c o m b i n e l o c a l f e a t u r e e l e m e n t s ( c f . Treisman, 1982; Treisman & G e l a d e , 19801, s i n c e t h e p a t i e n t s m a n i f e s t e d n e g l e c t w i t h b o t h s i n g l e and combined-feature t a r g e t s : a t t e n t i o n needs t o be engaged by a l l s t i m u l i (even s i n g l e f e a t u r e s ) f o r d i s c r i m i n a t o r y r e s p o n s e s t o be made. The r e s u l t i s c o n s i s t e n t w i t h o t h e r d a t a r e c e n t l y r e p o r t e d by Q u i n l a n and Humphreys ( 1 9 8 4 ) . They i n v e s t i g a t e d v i s u a l s e a r c h i n normal s u b j e c t s f o r 2 s i n g l e - f e a t u r e t a r g e t s , w i t h r e s p o n s e s c o n t i n g e n t on t h e d e t e c t i o n of b o t h f e a t u r e s i n t h e d i s p l a y . They found t h a t s e a r c h r a t e s i n t h e b o t h c o n d i t i o n s were t h e same a s t h o s e found when s u b j e c t s had o n l y t o d e t e c t 1 s i n g l e - f e a t u r e t a r g e t , b u t t h a t t h e r e was a c o n s t a n t increment t o t h e r e s p o n s e l a t e n c i e s ( i . e . , t h e r e was a n e f f e c t on t h e i n t e r c e p t b u t n o t t h e s l o p e s of t h e RT-display s i z e f u n c t i o n s ) . The l a t t e r r e s u l t suggests t h a t subjects processed the 2 f e a t u r e s i n p a r a l l e l , but t h a t t h e y needed t o a t t e n d s e r i a l l y t o t h e i n f o r m a t i o n s p e c i f i e d by t h e p a r a l l e l p r o c e s s i n g b e f o r e r e s p o n d i n g ; t h e s w i t c h i n g of a t t e n t i o n between t h e 2 f e a t u r e - d e f i n e d t a r g e t s produced t h e c o n s t a n t RT i n c r e m e n t a c r o s s t h e d i s p l a y s i z e s . Thus a t t e n t i o n seems t o be needed i f we a r e t o a c t on t h e b a s i s of i n f o r m a t i o n s p e c i f i e d by p r e - a t t e n t i o n a l v i s u a l p r o c e s s e s . A l s o , e v i d e n c e f o r t h e o p e r a t i o n of a p a r a l l e l s e a r c h p r o c e s s s h o u l d n o t be t a k e n a s e v i d e n c e t h a t i n f o r m a t i o n may be made a v a i l a b l e f o r a r e s p o n s e w i t h o u t a t t e n t i o n ( c f . TreismanbKahneman, 1985). F u r t h e r work i s needed i f w e a r e t o u n d e r s t a n d i n more d e t a i l t h e k i n d s of i n f o r m a t i o n s p e c i f i e d by p r e - a t t e n t i v e v i s i o n . The p r e s e n t c o n t r a s t between t h e s e r i a l s e a r c h f u n c t i o n s f o r t h e combined-feature t a r g e t s and the p a r a l l e l functions f o r t h e single-feature t a r g e t s is consistent with e a c h of two r a t h e r d i f f e r e n t views of a t t e n t i o n . I t c o u l d be t h a t t h e p r e - a t t e n t i o n a l p r o c e s s e s do n o t s p e c i f y combined-feature i n f o r m a t i o n , SO t h a t a s e r i a l s e a r c h i s t h e n demanded (Treisman 6 G e l a d e , 1980) ; a l t e r n a t i v e l y , a f u l l d e s c r i p t i o n of t h e s t i m u l u s may be s p e c i f i e d p r e - a t t e n t i v e l y b u t a c t i o n may n o t be e f f e c t e d on t h e b a s i s of t h e

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d e s c r i p t i o n u n l e s s i t i s a t t e n d e d ( c f . Duncan, 1980, 1 9 8 5 ) . A c c o r d i n g t o t h e l a t t e r a c c o u n t , combined-feature s e a r c h e s are r e l a t i v e l y d i f f i c u l t n o t because t h e i n f o r m a t i o n i s u n s p e c i f i e d p r e - a t t e n t i v e l y but because i t i s r e l a t i v e l y d i f f i c u l t t o d i s c r i m i n a t e ( p e r h a p s because l o c a t i o n i n f o r m a t i o n i s n o t a c c u r a t e l y coded o r because I t is r a p i d l y l o s t ) . T o s e p a r a t e t h e above a c c o u n t s , w e need t o examine whether n e g l e c t p a t i e n t s a r e s e n s i t i v e t o h i g h e r - o r d e r p r o p e r t i e s of s t i m u l i p r o j e c t e d t o t h e n e g l e c t e d h e m i f i e l d , such a s t h e i r meaning. F o r t h e p r e s e n t , w e a r e a b l e t o m a i n t a i n t h a t e a r l y p r e - a t t e n t i v e p r o c e s s i n g i s r e l a t i v e l y i n t a c t i n a t l e a s t some n e g l e c t p a t i e n t s , and t h a t n e g l e c t stems from an i n a b i l i t y t o t r a n s l a t e t h e i n f o r m a t i o n s p e c i f i e d p r e - a t t e n t i v e l y i n t o a form which w i l l s u p p o r t a c t i o n . Such a d e f i c i t c o u l d stem from a number of c a u s e s , such a s damage t o c e l l s m e d i a t i n g s h i f t s of c o v e r t a t t e n t i o n t o s t i m u l i ( e . g . , Wurtz, Goldberg & R o b i n s o n , 1982), f r o m d a m a g e t o c e l l s i n v o l v e d i n t h e t r a n s l a t i o n of i n f o r m a t i o n s p e c i f y i n g boundary r e g i o n s i n v i s u a l s c e n e s i n t o a v i s i b l e 1985, form s u i t a b l e f o r t h e d i r e c t i o n o f action(e.g.,Grossberg&Mingolla, i n p r e s s ) , o r from c e l l s m e d i a t i n g t h e t r a n s l a t i o n of i n f o r m a t i o n about "where" a n o b j e c t is t o i n f o r m a t i o n about "what" t h e o b j e c t i s ( c f . U n g e r l e i d e r 6 Mishkin, 1982). These p o s s i b i l i t i e s a r e n o t m u t u a l l y e x c l u s i v e , and i t may be t h a t t r a n s l a t i o n from boundary i n f o r m a t i o n t o v i s i b l e form i n f o r m a t i o n o r from i n f o r m a t i o n about "where" o b j e c t s a r e t o "what" t h e y a r e , a r e mediated by c o v e r t v i s u a l a t t e n t i o n .

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P o s n e r , M . I . , S n y d e r , C.R.R. S D a v i s o n , B . J . A t t e n t i o n and t h e d e t e c t i o n o f s i g n a l s . J o u r n a l of E x p e r i m e n t a l Psychology : G e n e r a l , 1980, 109, 160-1 74. Posner, M . I . , Walker, J . A . , F r i e d r i c h , F . J . 6 R a f a l , R.D. E f f e c t s of p a r i e t a l i n j u r y on c o v e r t o r i e n t i n g of a t t e n t i o n . The J o u r n a l of N e u r o s c i e n c e , 1984 , A , 1863-1874. Q u i n l a n , P . T . 6 Humphreys, G.W. S e a r c h i n g f o r f e a t u r e s and s e a r c h i n g f o r c o n j u n c t i o n s . P a p e r p r e s e n t e d t o t h e E x p e r i m e n t a l Psychology S o c i e t y , London, J a n u a r y , 1984. Remington, R.W. A t t e n t i o n and s a c c a d i c e y e movements. J o u r n a l s of E x p e r i m e n t a l Psychology : Human P e r c e p t i o n and P e r f o r m a n c e , 1 9 8 0 , 2 , 726-744. Riddoch, M.J. U n i l a t e r a l n e g l e c t i n t h e s t r o k e p a t i e n t : I m p l i c a t i o n s f o r t h e p h y s i o t h e r a p i s t . In P r o c e e d i n g s of t h e X I t h World Congress of P h y s i o t h e r a p y , Stockholm, 1982. Riddoch, M . J . 6 Humphreys, G.W. The e f € e c t of c u e i n g on u n i l a t e r a l n e g l e c t . .~ N e u r o p s y c h o l o g i a , 1983,2-l, 589-599. Segarra, J.M. & Angelo, J.N. P r e s e n t a t i o n I . In A.C. Benton ( E d . ) , B e h a v i o u r a l Changes i n C e r e b r o v a s c u l a r D i s e a s e . New York : H a r p e r , 1970. Shulman, G . L . , Remington, R.W. 6 McLean, J.P. Moving a t t e n t i o n t h r o u g h v i s u a l s p a c e . J o u r n a l of E x p e r i m e n t a l Psychology : Human P e r c e p t i o n and Performance , 1979,>, 522-526. Treisman, A . P e r c e p t u a l g r o u p i n g and a t t e n t i o n i n v i s u a l s e a r c h f o r f e a t u r e s and f o r o b j e c t s . J o u r n a l of E x p e r i m e n t a l Psychology : Human P e r c e p t i o n and Performance, 1982,8, 194-214. T r e i s m a n , A . P r o p e r t i e s , p a r t s and o b j e c t s . In K. B o f f , L. Kaufman & J . Thomas ( E d s . ) , Handbook of P e r c e p t i o n and Human Performance. N e w York: Wiley, 1984. Treisman, A . 6 G e l a d e , G. A f e a t u r e - i n t e g r a t i o n t h e o r y of a t t e n t i o n . C o g n i t i v e P s y c h o l o g y , 1 9 8 0 , E , 97-136. T r e i s m a n , A. & Kahneman, D. Addendum : A r e p l y t o Duncan. In M . I . P o s n e r and O.S.M. Marin ( E d s . ) , A t t e n t i o n 6 Performance X I . H i l l s d a l e , N . J . : Erlbaum, 1985. Treisman, A . & S c h m i d t , H. I l l u s o r y c o n j u n c t i o n s i n t h e p e r c e p t i o n of o b j e c t s . C o g n i t i v e Psychology, 1982, 2. 107-141. Treisman, A . , Sykes, M. & G e l a d e , G. S e l e c t i v e a t t e n t i o n and s t i m u l u s i n t e g r a t i o n . In S. D o r n i c ( E d . ) , A t t e n t i o n 6 Performance VI. H i l l s d a l e . N . J . : Erlbaum. 1977. T s a l , Y. Movements of a t t e n t i o n a c r o s s t h e v i s u a l f i e l d . J o u r n a l of E x p e r i m e n t a l Psychology : Human P e r c e p t i o n and P e r f o r m a n c e , 1983,2, 52 3- 5 30. T u r k e w i t z , G., Gordon, E.W. & B i r c h , H . G . Head t u r n i n g in t h e h u m a n n e o n a t e : Spontaneous p a t t e r n s . J o u r n a l o f G e n e t i c P s y c h o l o g y , 1965,107, 143. U n g e r l e i d e r , L.G. & Mishkin, M. Two c o r t i c a l v i s u a l systems. 1 n D . J . I n g l e , M.A. Goodale 6R.J.W. M a n s f i e l d ( E d s . ) , A n a l y s i s o f V i s u a l B e h a v i o u r . Cambridge,Mass. : M I T P r e s s . Volpe, B.T., LeDoux, J . E . 6 Gazzaniga, M.S. I n f o r m a t i o n p r o c e s s i n g in a n " e x t i n g u i s h e d " v i s u a 1 f i e l d . N a t u r e , 1979,282, 722-724. Watson, R.T.. A n d r i o l a , M & Heilman, K.M. The EEG i n n e g l e c t . J o u r n a l of 343-348. N e u r o l o g i c a l S c i e n c e , 1977,*, Watson, R.T., Heilman, K.M., Cauthen, J . C . & King. K.A. N e g l e c t a f t e r cingulectomy. Neurology, 1973 2,1003-1007. Watson, R.T., Heilman, K.M., M i l l e r , B.D. & King, F.A. N e g l e c t a f t e r m e s e n c e p h a l i c r e t i c u l a r f o r m a t i o n l e s i o n s . Neurology, 1974, 3 , 294-298.

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Weinberg, J . , D i l l e r , L . , Gordon, W.A., Gerstman, L . J . , Lieberman, A., L a k i n , P . . Hodges, G. & E z r a c h i , 0 . V i s u a l s c a n n i n g t r a i n i n g e f f e c t o n r e a d i n g - r e l a t e d t a s k s i n a c q u i r e d r i g h t b r a i n damage. A r c h i v e s of P h y s i c a l M e d i c i n e a n d R e h a b i l i t a t i o n , 1977,1_8, 479-486. Gerstman, L . J . , Lieberman, A . , Weinberg; J . , D i l l e r , L., Gordon, W.A., L a k i n , P . , Hodges, G . & E z r a c h i , 0. T r a i n i n g s e n s o r y awareness and s p a t i a l o r g a n i z a t i o n i n p e o p l e w i t h r i g h t b r a i n damage. A r c h i v e s of P h y s i c a l M e d i c i n e a n d R e h a b i l i t a t i o n , 1979.60, 491-196. W U K t Z , R.H., Goldberg, M.E. & Robinson, D.L. B r a i n mechanisms of v i s u a l a t t e n t i o n . S c i e n t i f i c A m e r i c a n , 1 9 8 2 , 2 4 6 , 124-135. Z a r i t , S.H. & K a h n , R.L.Impairmentandadaptationinchronicdisabi1ities : s p a t i a l i n a t t e n t i o n . J o u r n a l of Nervous and Mental D i s e a s e , 1974, 159, 63-72. -

Acknowledgements : The work r e p o r t e d i n t h i s c h a p t e r w a s s u p p o r t e d by a g r a n t from t h e C h e s t , H e a r t and S t r o k e A s s o c i a t i o n t o b o t h a u t h o r s . We wish t o t h a n k t h e P h y s i o t h e r a p y Department a t B a r n e t G e n e r a l H o s p i t a l and D K A . Wilson f o r f a c i l i t a t i n g t h e t e s t i n g and f o r e n a b l i n g o u r a c c e s s t o p a t i e n t s t o O C C U K , and Cathy P r i c e and P h i l i p Q u i n l a n f o r c a r r y i n g o u t t h e a n a l y s e s of t h e d a t a . We e s p e c i a l l y t h a n k t h e p a t i e n t s f o r t h e i r kind co-operation.


177

Footnotes

1. P o s n e r e t a l . ' s (1984) argument t h a t one component of c o v e r t a t t e n t i o n i s d i s r u p t e d i n n e g l e c t i s s u p p o r t e d by o t h e r e v i d e n c e i n d i c a t i n g t h e s e l e c t i v e impairment of o t h e r component p r o c e s s e s . For i n s t a n c e , mid-brain l e s i o n s a p p e a r t o a f f e c t t h e t i m e t a k e n t o s h i f t a t t e n t i o n between l o c a t i o n s ( P o s n e r e t a l . , 1 9 8 2 ) , s u g g e s t i n g a s p e c i f i c d i s r u p t i o n t o t h e "move" o p e r a t i o n . 2. Non-linear e f f e c t s of d i s p l a y s i z e on s e a r c h performance may be a t t r i b u t e d t o a p a r a l l e l s e a r c h p r o c e s s which e i t h e r h a s some c a p a c i t y l i m i t a t i o n s o r which i s c o n s t r a i n e d by l a t e r a l masking.

3. An a n a l o g y may be made h e r e w i t h t h e performance of normal s u b j e c t s when f o c a l a t t e n t i o n t o a v i s u a l d i s p l a y i s p r e v e n t e d , where s u b j e c t s may r e p o r t i n c o r r e c t c o m b i n a t i o n s of s e p a r a b l e f e a t u r e s ( " i l l u s o r y c o n j u n c t i o n s " ; s e e Treisman & S c h m i d t , 1982). Such r e p o r t s c a n be i n t e r p r e t e d a s i n d i c a t i n g t h a t s i n g l e f e a t u r e i n f o r m a t i o n i s made a v a i l a b l e p r e - a t t e n t i v e l y but t h a t , w i t h o u t f o c a l a t t e n t i o n , t h e f e a t u r e s a r e combined a t random. 4 . Normally, d i s c r i m i n a t i o n s r e q u i r i n g t h e combination of f e a t u r e s i n a v i s u a l s e a r c h t a s k a r e e n s u r e d by u s i n g d i s t r a c t o r s which c o n t a i n some but n o t a l l of t h e c r i t i c a l f e a t u r e s d e f i n i n g t h e t a r g e t . The c o r r e c t combination of such f e a t u r e s w i l l t h e r e f o r e demand t h e i r a c c u r a t e l o c a l i z a t i o n , s i n c e o t h e r w i s e i l l u s o r y c o n j u n c t i o n s may a r i s e . The p r e d i c t i o n s of t h e f e a t u r e - i n t e g r a t i o n view, t h e n , a p p r o x i m a t e t h o s e made by a r e p r e s e n t a t i o n a l a c c o u n t of n e g l e c t , and b o t h p r e d i c t t h a t s e a r c h f o r combined-feature target sonthe n e g l e c t e d s i d e o f s p a c e w i l l b e v e r y impaired.

5. Humphreys, Riddoch and Q u i n l a n (1985) have r e c e n t l y r e p o r t e d t h a t n o n - l i n e a r s e a r c h f u n c t i o n s can o b t a i n in a t a s k where s u b j e c t s must d i s c r i m i n a t e a n i n v e r t e d T from a background of homogeneous u p r i g h t T d i s t r a c t o r s . Whether o r n o t linearsearchfunctionsoccur i n t h i s t a s k seems t o depend on t h e s i z e of t h e l o c a l e l e m e n t s and on t h e i r r e l a t i v e p o s i t i o n s i n t h e v i s u a l f i e l d . Presumably, t h e p o s i t i o n a n d s i z e o f t h e l o c a l e l e m e n t s d i d not e n a b l e p a r a l l e l ( n o n - l i n e a r ) p r o c e s s i n g t o o b t a i n here.

No correct

RT

A.S. -

No correct

RT

I.J. -

RT No correct

H.C.

2.8

1.43 16/20

14/20

1.91 17/20

11/20

1.87 12/20

6

1.79 18/20

9/20

3.01

10/20

2.51

8

1.88 17/20

15 13/20

1.90 11/20

10

20/20

1.46

2 0 /20

2.39

1.81 19/20

4

1.70 20/20

1.37 20/20

2.54 20/20

2.45

1.82 17/20

8

20/20

1.93 18/20

6

right f i e l d

left field

2.06 10/20

4

Target present,

Target present,

20/20

1.60

2.62 20/20

2.10 16/20

10

Mean correct R T s ( s e c s ) and numbers of correct responses f o r each p a t i e n t i n

TABLE 1

RT

RT No c o r r e c t A.S. RT No c o r r e c t

I.J. -

No c o r r e c t

H.C. -

7.46 10130 4.63 16/20

3.94 14/20 3.78 13/20

3.70 7/40

8

8.04 16/30

3.67 9/40

6

5.96 15/30

2.74 9/40

4

Target present, left field

4.37 30130 2.14 20/20

1.88 20/20 5.20 16/20

2.67 29/40

6

3.19 30130

2.59 37/40

4

8.63 7/30

4.77 8/40

10

2.49 20/20

4.43 28/30

2.80 27/40

8

Target present, right f i e l d

2.67 20/20

4.07 28/30

3.31 25/40

10

Mean c o r r e c t R T s ( s e c s ) and numbers of c o r r e c t r e s p o n s e s f o r e a c h p a t i e n t i n E 2

TABLE 2

No c o r r e c t

RT

No c o r r e c t

1.54 20/20

3.50 20/20

I.J. RT 2 . 8 1

A.S. -

1.32 16/20

RT No c o r r e c t

H.C.

4

1.67 18/20

16/20

3.41

1.23 16/20

6

1.91 20/20

20/20

3.57

1.51 16/20

8

Target p r e s e n t , l e f t field

1.81 20/20

3.34 20/20

1.80 16/20

10

1.33 20/20

20/20

2.90

1.62 17/20

4

1.68 20/20

20/20

2.91

1.55 15/20

6

1.48 20/20

20/20

3.06

1.62 16/20

8

Target p r e s e n t , right f i e l d

1.62 20/20

20/20

3.38

1.64 15/20

10

Mean c o r r e c t RTs ( s e c s ) and numbers of c o r r e c t responses f o r each p a t i e n t i n Experi

TABLE 3

RT

RT No c o r r e c t

A.S. -

No c o r r e c t

I.J. RT

No correct

H.C. -

2.61 19/20

5.01 15/20

2.45 12/40

4

2.53 19/20

5.39 13/20

3.50 9/40

6

3.06 20/20

4.17 11/20

4.39 10/40

8

Target present, left field

2.86 13/20

5.43 11/20

4.39 10/40

10

1.92 20/20

4.09 11/20

2.70 37/40

4

2.11 20/20

5.92 20/20

2.60 34/40

6

2.24 20/20

5.04 17/20

2.60 26/40

8

Target present, right f i e l d

2.76 20/20

6.19 18/20

3.28 29/40

10

Mean c o r r e c t R T s ( s e c s ) and numbers of c o r r e c t r e s p o n s e s f o r e a c h p a t i e n t i n Exp search.

TABLE 4


Neurophysiological and Neuropsychological Aspects of Spatial Neglect, M.Jeannerod (editor) 0 Elsevier Science Publishers B.V. (North-Holland),1987

183

DYSCHIRIA. AN ATTEMPT AT ITS SYSTEMIC EXPLANATION Edoardo R i s i a c h and Anna Berti

I t is argued, i n t h i s chapter, t h a t u n i l a t e r a l neglect i s s t i l l i n need of a s y s t e m i c e x p l a n a t i o n . The working h y p o t h e s i s i s t h e n s u g g e s t e d t h a t u n i l a t e r a l n e g l e c t h a s remained c o n c e p t u a l l y i s o l a t e d w i t h i n a syndrome of which i t may c o n s t i t u t e t h e most common, but n o t t h e e x c l u s i v e n o r even t h e n e c e s s a r y component. A t h e o r e t i c a l n e u r a l model of an a n a l o g s u b s e r v i n g b o t h s e n s o r y a n d mental r e p r e s e n t a t i o n i n t h e v i s u o - s p a t i a l domain i s o u t l i n e d . Depending on r e l a t i v e l y minor changes i n i t s l o c a t i o n w i t h i n t h e same f u n c t i o n a l u n i t , a c i r c u m s c r i b e d f a i l u r e of t h e a n a l o g may i n v o l v e n e g l e c t o r m i s r e p r e s e n t a t i o n of one s i d e of s p a c e , t h u s l e n d i n g t h e o r e t i c a l s u p p o r t t o o u r working h y p o t h e s i s . The t e r m hemispatial d y s c h i r i a is proposed f o r t h e syndrome of n e g l e c t - m i s r e p r e s e n t a t i o n a s a l a t e t r i b u t e t o H. Z i n g e r l e . who many y e a r s a g o , i n one of h i s p a p e r s which w a s a f o r e r u n n e r of t h i s s u b j e c t , e m p l o y e d t h i s term.

Introduction Looking back o v e r n e a r l y one c e n t u r y of i n q u i r y i n t o t h e complex c o n d i t i o n which u s u a l l y g o e s by t h e ( a r g u a b l y u n d e r i n c l u s i v e ) name of u n i l a t e r a l n e g l e c t of s p a c e , i t may be n o t i c e d t h a t -- a l m o s t w i t h o u t e x c e p t i o n -- t h i s c o n d i t i o n h a s o n l y been a d d r e s s e d under what might be c a l l e d a d e - i n t e r p r e t a t i v e approach. The t e r m d e - i n t e r p r e t a t i v e i s h e r e u s e d , i n h o p e f u l b u t uncommitted c o r r e s p o n d e n c e w i t h Haugeland (1978). t o d e n o t e t h e s h i f t i n scope which o c c u r s when a n o p e r a t i n g s y s t e m i s not e x p l a i n e d a s a whole but i n t e r m s of i t s s i n g l e components, under t h e assumption t h a t t h e g e n e r a l p a t t e r n b e i n g c a r r i e d o u t by t h e f o r m e r i s opaque t o t h e l a t t e r . S p a c e - r e l a t e d b e h a v i o u r may a c c o r d i n g l y be i n t e r p r e t e d i n t e r m s of s e p a r a t e s u b r o u t i n e s f o r s e n s o r y i n t e g r a t i o n , r e c e p t o r o r i e n t a t i o n , a t t e n t i o n a l s h i f t and s t e e r i n g of motor a c t i v i t y , w i t h l i t t l e o r no concern f o r t h e c o m p l e x i t y and meaning of t h e t o t a l program.Thus, l o c a l d i s f u n c t i o n s maybe h e l d r e s p o n s i b l e f o r f a i l u r e i n t h e e x e c u t i o n of t h e l a t t e r , w h i l e t h e s y s t e m i c d i s o r d e r i s l o s t t o view. Such h a v i n g been t h e p r e v a i l i n g a p p r o a c h t o u n i l a t e r a l n e g l e c t , t h e r e g r e t t a b l e consequences which have ensued a r e t h r e e f o l d : f i r s t , wrong i n t e r p r e t a t i o n s have been f o s t e r e d ; second, t h e c o g n i t i v e a s p e c t s of t h i s c o n d i t i o n have been o b s c u r e d and, t h i r d , t h e c o n t r i b u t i o n which t h e s t u d y of u n i l a t e r a l n e g l e c t can g i v e t o t h e r e a p p r a i s a l of t h e o r i e s a b o u t t h e a c t i v i t y of t h e b r a i n a s a whole h a s been d e l a y e d . S e n s o r y h y p o t h e s e s a s e x p l a n a t i o n s of u n i l a t e r a l n e g l e c t a r e well-known ( s e e Heilman 6 Watson, 1977, f o r a review) and do n o t r e q u i r e f u r t h e r a s s e s s m e n t , s i n c e t h e i r v a l i d i t y a g a i n s t which undermining arguments had a l r e a d y been a n t i c i p a t e d by Z i n g e r l e (1913), h a s been

184

E. Bisiach and A . Berti

d e f i n i t i v e l y r e f u t e d by t h e d e m o n s t r a t i o n of n e g l e c t phenomena i n t h e r e p r e s e n t a t i o n a l domain. These phenomena w i l l p r e s e n t l y be reviewed. T h i s of c o u r s e , does n o t mean t h a t s e n s o r y d i s o r d e r s have no r o l e t o p l a y i n s h a p i n g t h e n e g l e c t syndrome. For example, i t h a s been found t h a t n e g l e c t p a t i e n t s w i t h c o n v e n t i o n a l l y diagnosed hemianopia show more s e v e r e n e g l e c t of t h e a f f e c t e d hemispace under v i s u a l c o n t r o l , t h a n i n a c o n d i t i o n i n which t h e l a t t e r i s p r e v e n t e d by a b l i n d f o l d (Chedru, 1976). C o n t r a r y t o t h e s e n s o r y h y p o t h e s e s , t h e r e h a s never beenanunambiguous f o r m u l a t i o n of an oculomotor h y p o t h e s i s . T h i s s o r t of e x p l a n a t i o n , i n d e e d , has been c r i t i c i z e d by some w r i t e r s a l t h o u g h i t h a s n e v e r been f a c t u a l l y upheld. The paper by S c h o t t , J e a n n e r o d and Zahin ( 1 9 6 6 ) , t o which s u c h a h y p o t h e s i s h a s been connected (e.g. D e Renzi, 1982, p. 1 0 6 ) , though l a y i n g s p e c i a l emphasis on oculomotor d i s o r d e r s i n t h e d e s c r i p t i o n of t h e syndrome, i s f a r from c l a i m i n g t h e i r unique r o l e i n t h e c a u s a t i o n of n e g l e c t ; t h e s e a u t h o r s , t o t h e c o n t r a r y , even s u g g e s t t h a t i t may be i n c o r r e c t t o t a l k of n e g l e c t o r i n a t t e n t i o n w i t h r e f e r e n c e t o a s p a c e which i s l o s t i n t h e s u b j e c t ' s r e p r e s e n t a t i o n , t h u s h i n t i n g a t t h e n e c e s s i t y of a much more s o p h i s t i c a t e d i n t e r p r e t a t i o n . I t i s , however, a w e l l e s t a b l i s h e d f a c t t h a t c l e a r m a n i f e s t a t i o n s of u n i l a t e r a l n e g l e c t can be f o u n d , d i s s o c i a t e d from c l i n i c a l l y d e t e c t a b l e oculomotor impairment (HCcaen, P e n f i e l d , B e r t r a n d & Malmo, 1956; Hbcaen, 1962; G a i n o t t i , 1968; B i s i a c h , L u z z a t t i & P e r a n i , 1979). Thus, t h e f r e q u e n t a s s o c i a t i o n of t h e two d i s o r d e r s s h o u l d e i t h e r depend on t h e a n a t o m i c a l c o n t i g u i t y of t h e l e s i o n s r e s p e c t i v e l y r e s p o n s i b l e f o r e a c h of them, o r on t h e d i s f u n c t i o n of a common l i n k , a disfunctionwhichmightexpress i t s e l f d i s p r o p o r t i o n a t e l y o n e i t h e r v e r s a n t . One p o i n t of i n t e r e s t , however, i s t h e f a c t -- t o which w e s h a l l r e t u r n l a t e r -- t h a t c o n j u g a t e gaze d e f e c t s t o w a r d t h e s i d e c o n t r a l a t e r a l t o t h e l e s i o n , o c c u r more f r e q u e n t l y a f t e r right-hemisphere i n j u r y (De R e n z i , Colombo, F a g l i o n i & G i b e r t o n i , 1 9 8 2 ) , which i s s u g g e s t i v e of a f u n c t i o n a l connectionbetween these d e f e c t s a n d u n i l a t e r a l neglect. A t t e n t i o n a l hypotheses a r e most prone t o b o i l away a s q u e s t i o n - b e g g i n g r e d e s c r i p t i o n s of n e g l e c t phenomena and r e s t a t e m e n t s of e x p e r i m e n t a l r e s u l t s . I n o r d e r f o r t h e e x p l a n a t i o n s t o be g e n u i n e , t h e y must s p e c i f y a n u n d e r l y i n g model of t h e mechanismwhich, under p a t h o l o g i c a l c o n d i t i o n s , may g i v e r i s e t o h e m i n e g l e c t . T h i s r e q u i s i t e i s met by t h e s u g g e s t i o n s of Kinsbourne (1970, 1977) and of Heilmanand h i s a s s o c i a t e s (Heilman&VanDen A b e l l , 1980; Heilman, Watson & V a l e n s t e i n , 1985). According t o K i n s b o u r n e , i n t h e nervous system t h e r e i s a c o m p e t i t i o n between two opposed v e c t o r s of l a t e r a l a t t e n t i o n , each p o i n t i n g t o t h e s i d e c o n t r a l a t e r a l t o t h e hemisphere of o r i g i n . R e l a t i v e h y p e r a c t i v a t i o n of one hemisphere, caused by s e l e c t i v e engagement i n p r o c e s s i n g i n f o r m a t i o n o r r e s u l t i n g from impairment of t h e o p p o s i t e hemisphere, enhances i t s o w n v e c t o r . The g r e a t e r i n c i d e n c e and s e v e r i t y of l e f t r a t h e r t h a n r i g h t n e g l e c t might be due t o t h e f a c t t h a t , c e t e r i s p a r i b u s , t h e l e f t-hemisphere v e c t o r i s p h y s i o l o g i c a l l y dominant i n r i g h t - h a n d e r s , and t h a t t e s t i n g p r o c e d u r e s o f t e n i n v o l v e l e f t-hemisphere a c t i v a t i o n . To a c c o u n t f o r h e m i s p h e r i c asymmetries i n t h e o c c u r r e n c e of n e g l e c t , Heilman and a s s o c i a t e s have i n s t e a d s u g g e s t e d t h a t t h e r i g h t hemisphere c o n t r o l s a t t e n t i o n o v e r t h e e n t i r e s p a c e , whereas t h e c o n t r o l of t h e l e f t i s l i m i t e d t o t h e r i g h t hemispace. W e w i l l n o t proceed t o e v a l u a t e t h e a t t e n t i o n a l i n t e r p r e t a t i o n per s e p r i o r t o c o n s i d e r i n g a f u r t h e r c o n c e p t i o n , w h i c h sees u n i l a t e r a l n e g l e c t a s a b a s i c a l l y representational disorder. This is not intended a s a t a c t i c t o c r e a t e s u s p e n s e ; w e w i s h t o a v o i d a r g u i n g pro and con t h e same i d e a s d e f i n e d w i t h d i f f e r i n g terms or metaphors. To p r e v e n t a m b i g u i t i e s , w e s h a l l h e r e a f t e r employ t h e t e r m s of "sensory" and "mental" r e p r e s e n t a t i o n t o r e f e r , r e s p e c t i v e l y and w i t h o u t any f u r t h e r i m p l i c a t i o n , t o t h e p a t t e r n of

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c o n s t r a i n t s imposed upon n e u r a l a c t i v i t i e s by c u r r e n t s e n s o r y s t i m u l a t i o n and t o p a t t e r n s of a c t i v i t y o r i g i n a t i n g from w i t h i n t h e nervous system i t s e l f , mimicking a s p e c t s of t h e f o r m e r i n any s e n s o r y domain and t h e r e b y r e c a l l i n g p a s t ( o r s i m u l a t i n g p o s s i b l e ) s t a t e s of a f f a i r s i n t h e world. Whenever a d j e c t i v a t i o n i s o m i t t e d , r e f e r e n c e w i l l be made t o something s h a r e d by b o t h k i n d s of r e p r e s e n t a t i o n .

ANeglectedClassiconNeglect The i d e a t h a t u n i l a t e r a l n e g l e c t i s a m a n i f e s t a t i o n of a more g e n e r a l r e p r e s e n t a t i o n a l d i s o r d e r i s l o n g s t a n d i n g . I t had been c l e a r l y e x p r e s s e d even b e f o r e t h e f u l l i n d i v i d u a t i o n of t h e syndrome had become, much t o B a b i n s k i ' s c r e d i t , an e s t a b l i s h e d f a c t i n c l i n i c a l n e u r o l o g y . I n 1914 and 1918, B a b i n s k i s u b m i t t e d a p a r t i a l o u t l i n e of t h e syndrome under c o n s i d e r a t i o n , which was d e s t i n e d t o remain famous, t o t h e French N e u r o l o g i c a l S o c i e t y . A t a d i s t a n c e of many y e a r s , two a s p e c t s of t h o s e e v e n t s may s t r i k e u s a s b e i n g s i n g u l a r . F i r s t , t h e b y s t a n d e r s (among whom were Souques, D e j e r i n e , P i e r r e - M a r i e , Meige & Claude) r e a c t e d somewhat t e p i d l y t o B a b i n s k i ' s p r e s e n t a t i o n of what was t o become one of t h e major i s s u e s i n neuropsychology. Second, n o t w i t h s t a n d i n g t h e a u r a of n o v e l t y i n t i m a t e d by t h e r e c o r d s of t h e two s e s s i o n s , t h e syndrome had i n f a c t a l r e a d y been d e l i n e a t e d . Even assuming t h a t t h e r e c e n t , n o t e w o r t h y c o n t r i b u t i o n of Z i n g e r l e , p u b l i s h e d i n t h e 1913 i s s u e of M o n a t s s c h r i f t f u r P s y c h i a t r i e und N e u r o l o g i e , could n o t have reached t h e French n e u r o l o g i c a l community owing t o t h e p o l i t i c o - m i l i t a r y v i c i s s i t u d e s of t h e moment, i t seems u n l i k e l y t h a t e a r l i e r s t u d i e s ( q u o t e d by Z i n g e r l e h i m s e l f ) had n o t enjoyed s u f f i c i e n t l y wide c i r c u l a t i o n i n F r a n c e . I n t h a t p a p e r , Z i n g e r l e d e s c r i b e d two p a t i e n t s (Cases 2 and 3) who had c o m p l e t e l y l o s t " t h e n o t i o n of one s i d e " . Both p a t i e n t s had a r i g h t - h e m i s p h e r e l e s i o n : Case 2 a s u s p e c t e d hemorrage i n t h e p o s t e r i o r limb of t h e i n t e r n a l c a p s u l e ; Case 3 a n a u t o p t i c a l l y a s c e r t a i n e d s m a l l a b s c e s s i n t h e f r o n t a l l o b e w i t h d i f f u s e meningeal p h l o g o s i s . Case 2 had a l e f t h e m i p l e g i a w i t h h e m i a n a e s t h e s i a and hemianopia. He would not t a l k about h i s p a r a l y s i s . Whenever h i s a t t e n t i o n was drawn t o h i s l e f t s i d e , a f t e r a momentary g l a n c e a t h i s l e f t l i m b s , he would pay no f u r t h e r a t t e n t i o n t o them and would s t a r t t o t a l k a b o u t a n o t h e r s u b j e c t . Although acknowledging t h a t a l l p e o p l e have a r i g h t and a l e f t s i d e , he d i d n o t seem t o a p p l y t h a t n o t i o n t o h i m s e l f . H e would a f f i r m t h a t a woman was l y i n g on h i s l e f t s i d e ; hewould u t t e r w i t t y r e m a r k s a b o u t t h i s and sometimes c a r e s s h i s l e f t arm. He would become p e r p l e x e d and s i l e n t whenever t h e c o n v e r s a t i o n touched upon t h e l e f t h a l f of h i s body; even a t t e m p t s t o evoke memories of i t were u n s u c c e s s f u l . No o t h e r i n t e l l e c t u a l d i s o r d e r s were found d u r i n g t h e c o u r s e of t h e i l l n e s s . About 1 4 days a f t e r t h e o n s e t , d e l u s i o n a l phenomena had d i s p e r s e d ; t h e p a t i e n t remembered t h e i m p r e s s i o n of a s t r a n g e woman a t h i s s i d e and wondered a t i t , b u t was o n l y a b l e t o g i v e a poor r e p o r t of h i s e a r l i e r s t a t e . Commenting o n t h i s c a s e , Z i n g e r l e acknowledged Anton's p r i o r c l a i m s t o h a v i n g drawn a t t e n t i o n t o t h i s p e c u l i a r syndrome and u n d e r l i n e d t h a t i t was f a r from b e i n g a f i x e d concomitance of somatosensory d i s o r d e r s due t o b r a i n l e s i o n , which could n o t , t h e r e f o r e , be a s c r i b e d any s u b s t a n t i a l c a u s a t i v e r o l e . On t h e o t h e r hand, Z i n g e r l e remarked t h a t t h i s p a t i e n t , though having l o s t somatosensory i n f o r m a t i o n from t h e l e f t s i d e of h i s body, could s t i l l p e r c e i v e i t t h r o u g h o t h e r s e n s e s ( s i g h t and t o u c h ) of t h e unimpaired s i d e ; t h e s e p e r c e p t s , however, had no b e a r i n g on h i s c o r p o r e a l Ego. Moreover, t h e p a t i e n t n o t o n l y l a c k e d s e n s a t i o n s from t h e l e f t s i d e of h i s body and a w a r e n e s s of h i s p a r a l y s i s : l i k e A n t o n ' s p a t i e n t he had a l s o l o s t t h e memory of t h i s s i d e . Case 3 never made u s e of h i s l e f t limbs though t h e y were n o t p a r a l y z e d .

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Figure 1 Hermann Z i n g e r l e (1870-1935)

He was a l e r t and co-operated i n n e u r o l o g i c e x a m i n a t i o n s e x c e p t when r e f e r e n c e was made t o h i s l e f t s i d e , i n w h i c h c a s e he seemed s u d d e n l y d e a f a n d would make no a t t e m p t t o u n d e r s t a n d t h e examiner, whose r e q u e s t seemed r a t h e r t o annoy him. About t h i s c a s e Z i n g e r l e remarked t h a t s i n c e t h e p a t i e n t was n o t p a r a l y z e d and h i s s e n s i b i l i t y was r e t a i n e d a t l e a s t in p a r t on t h e l e f t h a l f of h i s body, one had t o h y p o t h e s i z e a loss of r e p r e s e n t a t i o n of t h e l a t t e r o r a d i s o r d e r e d p r o c e s s i n g of s e n s o r y i n f o r m a t i o n needed f o r t h e s p a t i a l p e r c e p t i o n of t h e body. He a l s o a d m i t t e d t h a t i n t h i s c a s e t h e s e v e r e n e g l e c t of t h e l e f t s i d e might s u g g e s t a d i s o r d e r of a t t e n t i o n , a h y p o t h e s i s he had n o t e n v i s a g e d when d i s c u s s i n g Case 2, presumably on account of t h e f a c t t h a t p a t i e n t 2 c o u l d g i v e a t t e n t i o n t o h i s l e f t l i m b s , though m a n i f e s t i n g d e l u s i o n a l i d e a s about them.

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Z i n g e r l e ' s m e r i t i s n o t c o n f i n e d t o t h e k e e n n e s s of h i s c l i n i c a l r e c o r d s . He o f f e r e d a n i n t e r p r e t a t i o n of t h e d i s o r d e r which is s t i l l up-to-date and a f f o r d e d a glimpse of t h e r e l e v a n c e i t would have f o r t h e neurologyof cognition. The l o s s of s e n s o r y i n f o r m a t i o n may be placed a l o n g s i d e t h e i m p o s s i b i l i t y of r e p r o d u c t i o n of e a r l i e r memories (from a l l s e n s o r y m o d a l i t i e s ) i n t h e framework of a d i s o r d e r e d a w a r e n e s s of t h e It is p a r t i c u l a r l y important, not l a t e r a l i t y of o n e ' s own body, o n l y t h a t t h e m a n i f e s t r e c a l l cannot be awakened by c o r r e s p o n d i n g sensoryinformation,butalsothatthedefectcannotbeamendedthrough s i g h t and t o u c h and t h e p a t i e n t does n o t r e c o g n i z e t h e l e f t h a l f of h i s body, he does n o t s e e i t a s h i s p r o p e r t y I n h i s awareness, the body's r e p r e s e n t a t i o n i s a p. .p e a r e n t l y c o n f i n e d t o t h e r i g- h t h a l f (Zingerle's i t a l i c s ) .

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Thus, a c c o r d i n g t o Z i n g e r l e , t h e b a s i c e x p l a n a t i o n o f t h e s y n d r o m e l i e s i n d i s o r d e r e d r e p r e s e n t a t i o n . He added t h a t t h i s e x p l a n a t i o n must be s u f f i c i e n t l y f l e x i b l e i n o r d e r t o a c c o u n t f o r t h e v a r i a b i l i t y of t h e syndrome. I n d e e d , whereas some patientsaretotallyobliviousof o n e s i d e o f t h e s p a c e , o t h e r s , though unaware of t h e i r p a r a l y s i s , do n o t i g n o r e t h e l e f t h a l f of t h e i r body. A s a l r e a d y remarked by Anton, s e n s o r y i n f o r m a t i o n i n t h e s e p a t i e n t s c o u l d undergo f a u l t y c e n t r a l p r o c e s s i n g , so t h a t i n s t e a d of n e g l e c t , t h e y d i s p l a y d e l u s i o n a l b e l i e f s c o n c e r n i n g t h e l e f t h a l f of t h e i r body. I t is not easy t o f i n d a s a t i s f a c t o r y e x p l a n a t i o n f o r t h e s i n g u l a r s t a t e of o b l i v i o n which f o r so many y e a r s h a s d e p r i v e d neuropsychology of such a n i m p o r t a n t c o n t r i b u t i o n and h a s l e f t u n c h a l l e n g e d t h e t e n d e n c y whereby ( s t r i c t 0 s e n s u c o n s i d e r e d ) u n i l a t e r a l n e g l e c t has remained c o n c e p t u a l l y i s o l a t e d w i t h i n a syndrome of which i t may indeed c o n s t i t u t e t h e most common, but n o t t h e e x c l u s i v e n o r even t h e n e c e s s a r y component. I n d e e d , l e s i o n s which i n v o l v e ( i n most instances) the r i g h t temporoparieto-occipital carrefour or functionally related brain structures (see V a l l a r & P e r a n i , t h i s volume) do not always r e s u l t i n mere n e g l e c t : sometimes t h e p a t i e n t s do a t t e n d t o t h e l e f t h a l f of s p a c e , though t h i s r e v e a l s a p a t h o l o g i c a l s t a t e of a g e n e r i c a l l y n e g a t i v e kind. They maymourn o v e r t h e i r p a r a l y z e d limbs w h i l e a b s u r d l y d e c l a r i n g t h e m s e l v e s r e a d y t o engage i n a c t i v i t i e s from which t h e y a r e now o b v i o u s l y p r e c l u d e d . Well-known are t h e s o - c a l l e d somato-paraphrenic phenomena (Gerstmann, 1942), whereby a p a t i e n t c o n t e n d s , e . g . , t h a t h i s l e f t arm b e l o n g s t o a n o t h e r p e r s o n i n s p i t e of t h e f a c t t h a t he can l o o k a t i t , t o u c h i t and sometimes even move i t d e l i b e r a t e l y , o r can r e c o g n i z e a r i n g on one of t h e f i n g e r s of h i s l e f t hand a s h i s . S t i l l more, t h e p a t i e n t s ' a t t e n t i o n a p p e a r s i n some i n s t a n c e s i n v i n c i b l y a t t r a c t e d towards t h e a l i e n l i m b s , f o r which t h e y may show a s p i r i t e d d i s l i k e and even p e r s e c u t o r y i m p u l s e s ( C r t i c h l e y , 1953). The pleomorphism of t h e s e m a n i f e s t a t i o n s could e x p l a i n t h e d e l a y , t h e slow c o u r s e and t h e r e s i d u a l u n c e r t a i n t y of t h e i r f r a m i n g i n t o a u n i t a r y complex. I n d e e d , t h e l a c k i n c u r r e n t l i t e r a t u r e of a t e r m s u i t a b l e f o r comprehensive d e n o t a t i o n makes a n e n d u r i n g abeyance i n t h e s e t t l e m e n t of t h e d i s o r d e r s under c o n s i d e r a t i o n i n t o a u n i t a r y syndrome e v i d e n t . Y e t , i f under t h e working a s s u m p t i o n of a s i n g l e Grundstoerung i t makes s e n s e t o c o l l e c t t h e d i v e r s e phenomena a t i s s u e (whether d e f e c t i v e -- n e g l e c t -- o r p r o d u c t i v e -- somato-paraphrenia and a n a l o g o u s d i s o r d e r s r e l a t e d t o e x t r a p e r s o n a l s p a c e ) i n t o a u n i t a r y c l u s t e r , i t is a l s o c o n v e n i e n t t o d e s i g n a t e them c o l l e c t i v e l y w i t h a s i n g l e term, which ought t o be b o t h

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c o n s i s t e n t a t t h e d e s c r i p t i v e l e v e l and uncommitted a t t h e i n t e r p r e t a t i v e l e v e l . There is indeed no need t o c o i n a new word, s i n c e t h e term ' d y s c h i r i a ' (Greek c h e i r , c h e i r o s : 'hand' a n d , by e x t e n s i o n , ' s i d e ' ) , employed by Z i n g e r l e (Note l ) , meets b o t h r e q u i r e m e n t s and p e r f e c t l y c a p t u r e s t h e concept of a l i e n a t i o n of one s i d e of space which, however m a n i f e s t e d , i s t h e e s s e n t i a l f e a t u r e o f t h e syndrome. I t may be t h a t on account of t h e heavy i d e o l o g i c a l c o n d i t i o n i n g due t o Behaviourism, and more p r e c i s e l y due t o t h e a v e r s i o n f o r p o s t u l a t e d e n t i t i e s such a s r e p r e s e n t a t i o n s , an even b e t t e r c i r c u l a t i o n of Z i n g e r l e ' s paper would n o t have e x e r t e d s t r o n g t h e o r e t i c a l i n f l u e n c e o v e r s e v e r a l decades. I t is a f a c t (and p e r h a p s a s i g n i f i c a n t one) t h a t t h e s t u d y of d y s c h i r i a went through a l o n g p e r i o d of r e l a t i v e s t a g n a t i o n , u n t i l Geschwind ( 1 9 6 5 ) . K i n s b o u r n e (1970) a n d H e i l m a n a n d a s s o c i a t e s ( s e e H e i l m a n e t a l . , 1985) reawakened i n t e r e s t i n t h i s a r e a by d e m o n s t r a t i n g t h a t i t could r e p a y f r e s h a n d v e r s a t i l e i n q u i r y .

NeglectPhenomenaintheRepresentationalDomain In r e c e n t y e a r s anumber of e x p e r i m e n t a l c o n t r i b u t i o n s h a v e r e s u m e d t h e r e p r e s e n t a t i o n a l t h e o r y of d y s c h i r i a . P a t i e n t s w i t h n e g l e c t f o r t h e l e f t s i d e of s p a c e can make c o r r e c t s a m e - d i f f e r e n t judgments of p a i r s of v i s u a l s t i m u l i w h e n e v e r t h e d i f f e r e n c e (e.g. i n shape) l i e s i n t h e u n a f f e c t e d r i g h t hemispace, whereas t h e y commit e r r o r s when t h e d i f f e r e n c e , a s i n F i g u r e Za, l i e s i n t h e n e g l e c t e d l e f t hemispace. T h i s i s obvious enough. However t h e same outcome was o b t a i n e d ( B i s i a c h e t a l . , 1979) i n a c o n d i t i o n i n which t h e p a t t e r n s , i n s t e a d of b e i n g s t a t i o n a r y and exposed t o f u l l v i s i o n , were moved from l e f t t o r i g h t o r i n t h e o p p o s i t e d i r e c t i o n behind a c e n t r a l v e r t i c a l s l i t ( F i g u r e 2b) so t h a t

a

b

Figure 2 Patterns employedtodemonstrate representational neglect.

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t h e p a t i e n t s had t h e o p p o r t u n i t y of f o c u s s i n g a t t e n t i o n on a l l d e t a i l s i n s u c c e s s i o n but t h e a p p r e h e n s i o n of t h e whole s h a p e r e q u i r e d , a s i n e x p e r i m e n t s s u c h a s t h o s e of P a r k s (1965) and Hochberg ( 1 9 6 8 ) , a p r o c e s s of m e n t a l , r e p r e s e n t a t i o n a l r e c o n s t r u c t i o n . T h u s , t h e mental s p a c e i n which t h e s e v i s u a l r e p r e s e n t a t i o n s were u n f o l d e d t u r n e d o u t i t s e l f t o be n e g l e c t e d i n i t s l e f t half e x a c t l y a s t h e o u t e r space. A s i m i l a r experiment w i t h somewhat d i f f e r e n t p a t t e r n s (Ogden, 1985) has r e c e n t l y confirmed t h e s e results. The same phenomenon was e n c o u n t e r e d w i t h mental r e p r e s e n t a t i o n s drawn from long-term memory. P a t i e n t s w i t h l e f t n e g l e c t were a s k e d t o g i v e a d e s c r i p t i o n of t h e c a t h e d r a l s q u a r e i n M i l a n , w i t h which t h e y were q u i t e f a m i l i a r , w h i l e imagining t h e c a t h e d r a l t o be i n f r o n t of them ( B i s i a c h 6 L u z z a t t i , 1978; B i s i a c h , C a p i t a n i , L u z z a t t i 6 P e r a n i , 1981). A v a r i a b l e number of d e t a i l s , some of which f a i r l y s a l i e n t , were o m i t t e d from t h e l e f t h a l f of t h e v i e w . When a d e s c r i p t i o n o f t h e sameviewwas r e q u e s t e d , b u t from t h e o p p o s i t e v a n t a g e p o i n t , p r e v i o u s l y o m i t t e d l e f t - s i d e d e t a i l s were r e p o r t e d once t r a n s p o s e d t o t h e r i g h t , whereas r i g h t - s i d e d e t a i l s w h i c h t h e p a t i e n t had r e p o r t e d a moment b e f o r e , were o m i t t e d once t r a n s p o s e d t o t h e l e f t . Q u i t e comparable r e c o r d s had p r e v i o u s l y been made by Messerli ( p e r s o n a l communication, F e b r u a r y 1984) by a s k i n g n e g l e c t p a t i e n t s t o g i v e a mental d e s c r i p t i o n of t h e P l a c e Neuve i n Geneva, d u r i n g r o u t i n e c l i n i c a l t e s t i n g . A r e l a t e d and e v e n m o r e t e l l i n g o b s e r v a t i o n h a s m o r e r e c e n t l y b e e n reported (Raxter & Warrington, 1983), r e l a t i v e t o a p a t i e n t w i t h l e f t n e g l e c t who m i s s p e l l e d t h e l e f t h a l f of words, b o t h f o r w a r d s and backwards, a s i f r e a d i n g l e t t e r a f t e r l e t t e r from words w r i t t e n on a n imaginary d i s p l a y , of which t h e l e f t h a l f was h i d d e n t o t h e m i n d ' s eye. I t i s e s p e c i a l l y s i g n i f i c a n t ( a s i t w i l l be a r g u e d l a t e r ) t h a t t h i s p a t i e n t was q u i t e aware of h i s d i s o r d e r and t h a t he himself " d e s c r i b e d a t t e m p t i n g t o s p e l l l i k e r e a d i n g o f f an image i n which t h e l e t t e r s on t h e r i g h t s i d e were c l e a r e r t h a n t h o s e on t h e l e f t " ! T h i s s i n g u l a r phenomenon might i n f a c t be f a r from e x c e p t i o n a l , s i n c e i t has s u b s e q u e n t l y been o b s e r v e d i n a n o t h e r n e g l e c t p a t i e n t (Gazzaniga & B a r b u t , p e r s o n a l communication, J u n e 1985).

Implications Relative to the Structure and Levels of Neural Representation

of Space T h e r e seem t o be two r a d i c a l l y d i s t i n c t ways i n which t h e b r a i n may a c c o m p l i s h t h e t a s k of p r e s e r v i n g s p a t i a l p r o p e r t i e s o f i n f o r m a t i o n t h r o u g h o u t p e r c e p t i o n , t h o u g h t and a c t i o n ; t h a t i s p e r f o r m i n g t h e f u n c t i o n w e c o n s i d e r when w e t a l k of space r e p r e s e n t a t i o n . One way i s t h a t of some mechanism o p e r a t i n g a s a s p a t i a l a n a l o g ; t h e o t h e r is based upon a p r o p o s i t i o n a l encoding of s p a t i a l r e l a t i o n s . I t would exceed t h e s c o p e of t h i s c h a p t e r t o d w e l l o n a n d a t t e m p t a f u l l e l u c i d a t i o n o f t h e s e two c o n c e p t s s i n c e t h e work of Shepard e t a l . ( e . g . S h e p a r d , 1 9 7 5 ) , Kosslyn e t a l . (e.g. K o s s l y n , 1980) and P y l y s h i n (e.g. Pylyshyn, 1980, 1 9 8 1 ) , a l o n g w i t h a w e a l t h of c o r o l l a r y w r i t i n g s a r g u i n g pro and c o n t r a i m a g i n a l and i m a g e l e s s t h o u g h t -- o r a g n o s t i c (e.g. Anderson, 1978) -- p r o v i d e s v a s t pabulum t o whomever were tempted by t h e e n t e r p r i s e . What seems p l a i n , anyhow, i s t h a t bothmodes e x i s t and i n t e r p l a y i n t h e nervous system. T h i s i s obvious i f , a t one extreme of t h e dichotomy, one t h i n k s of t h e mapping of v i s u a l space on t h e s u r f a c e of t h e r e t i n a and of t h e more o r l e s s r e a d j u s t e d r e t i n o t o p i c arrangement of n e u r a l s t r u c t u r e s more and more remote from t h e p e r i p h e r y up t o t h e s o - c a l l e d a s s o c i a t i o n c o r t e x , and g r a n t s t h a t a n a t o m i c a l c o n t i g u i t y c o r r e s p o n d s , i n t h e s e s t r u c t u r e s , t o f u n c t i o n a l c o n t i g u i t y . Consider f o r i n s t a n c e t h e p r o j e c t i o n o f a chess-board o n t h e r e t i n a : s p a t i a l r e l a t i o n s among d i f f e r e n t l o c i of t h e chess-board a r e r e p r e s e n t e d i n a p i c t u r e - l i k e f a s h i o n by c o r r e s p o n d i n g

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r e l a t i o n s among d i f f e r e n t l o c i of t h e r e t i n a ; n o t , of c o u r s e , by v i r t u e of t h e a c t u a l s p a t i a l p o s i t i o n of t h e l a t t e r b u t t h r o u g h a p p r o p r i a t e c o n n e c t i v i t y which might p r e s e r v e i t s f u n c t i o n -- a t t h e c o s t of c o n s i d e r a b l e w i r i n g entanglement -- even i f , owing t o a whim of n a t u r e , d i o p t r i c media were such as t o s c a t t e r t h e images of e x t e r n a l o b j e c t s i n a d i s o r d e r l y way o v e r t h e whole r e c e p t o r s u r f a c e . T h i s not b e i n g t h e c a s e , a s p a t i a l l y c i r c u m s c r i b e d d e s t r u c t i o n of t h e r e t i n a o r of a r e t i n o t o p i c a l l y o r g a n i z e d s t r u c t u r e such a s primary v i s u a l c o r t e x , e n t a i l s a s p a t i a l l y circumscribed sensory scotoma which demonstrates that neural r e p r e s e n t a t i o n of t h e e x t e r n a l w o r l d , a t t h e s e l e v e l s , depends on a n a n a l o g d e v i c e , l i k e a s u n - d i a l ( a n exemplary a n a l o g ) which i f d e s t r o y e d i n one q u a d r a n t can no l o n g e r r e p r e s e n t t h e c o r r e s p o n d i n g a r c of t i m e , w h i l e r e g u l a r l y f u n c t i o n i n g f o r t h e remainder. A t t h e o t h e r extreme, however, t h e s p a t i a l l o c a t i o n s of i n d i v i d u a l s q u a r e s of t h e chess-board may be r e p r e s e n t e d i n a much more a b s t r a c t way by means of symbols made up by one l e t t e r and one d i g i t ; and t h e o p e r a t i o n s performed on such symbols r e l y upon a n e u r a l mechanism, t h e p r o p e r t i e s of which a r e remote from t h o s e of t h e above c o n s i d e r e d a n a l o g and more and more a k i n t o t h e s y s t e m s u b s e r v i n g t h e r e p r e s e n t a t i o n of s p a t i a l r e l a t i o n s i n any n a t u r a l l a n g u a g e , t h a t i s t o t h e p r o p o s i t i o n a l mode of r e p r e s e n t a t i o n p a r e x c e l l e n c e . Any t r a c e of a n a l o g p r o c e s s i n g h a s h e r e d i s a p p e a r e d and no s e l e c t i v e d i s o r d e r -of language would e n t a i l a d i s o r d e r e d r e p r e s e n t a t i o n o f a c i r c u m s c r i b e d a r e a of space. Now, i f b o t h modes of r e D r e s e n t a t i o n o D e r a t e and c o - o n e r a t e i n t h e nervous system, why s h o u l d t h e r e be any d e b a t e about them ? The c o n t r o v e r s y seems t o r e l a t e t o t h e i r comparative a c c e s s i b i l i t y t o t h e i l l - d e f i n e d c a t e g o r y of c o g n i t i o n . N e e d l e s s t o s a y t h a t i f s u c h a c a t e g o r y i s s i m p l y d e f i n e d a c c o r d i n g t o t h e p r o p o s i t i o n a l q u a l i t y of r e p r e s e n t a t i o n s a c t i v e a t i t s l e v e l , t h e n analogue p r o c e s s i n g i s ope l e g i s r e l e g a t e d t o a n o t h e r (more o r l e s s t a c i t l y u n d e r r a t e d ) domain of nervous a c t i v i t y . What K o s s l y n h a s endeavoured t o a r g u e f o r a l o n g time and what w e o u r s e l v e s have r e p e a t e d l y claimed ( B i s i a c h e t a l . , 1979; B i s i a c h , B e r t i & V a l l a r , 1985a; B i s i a c h , 1985; B i s i a c h , M e r e g a l l i & B e r t i , 1985c; Bisiach,Perani,Papagno,Vallar& Berti, f o r t h c o m i n g ) is t h a t n o n - p r o p o s i t i o n a l modes of r e p r e s e n t a t i o n p l a y a c e n t r a l r o l e i n c r u c i a l a s p e c t s of c o g n i t i o n s u c h a s b e l i e f - f i x a t i o n and t h e complex of s e l f - m o n i t o r i n g a c t i v i t i e s which c o n s t i t u t e what we o r d i n a r i l y d e s i g n a t e by t h e term ' a w a r e n e s s ' . F u r t h e r s c r u t i n y o f e v i d e n c e from i n v e s t i g a t i o n of u n i l a t e r a l n e g l e c t and r e l a t e d d i s o r d e r s and of argumentsadvanced on i t s b a s i s w i l l e l u c i d a t e t h e p o i n t . F i r s t of a l l . it needs be observed t h a t t h e a n a l o g mode of p r o c e s s i n g of s p a t i a l r e l a t i o n s is not c o n f i n e d t o p e r c e p t u a l a c t i v i t y : a s s t a t e d i n t h e f o r e g o i n g s e c t i o n , i n d e e d , i t i s s h a r e d by r e p r e s e n t a t i o n a l a c t i v i t i e s s u c h a s t h o s e i n v o l v e d i n r e c o n s t i t u t i n g t h e shape of a n o b j e c t moving behind a s l i t , i n imagining and d e s c r i b i n g a view and e v e n , a t l e a s t i n some o c c a s i o n s , i n word-spelling. In f a c t , s p a t i a l l y c i r c u m s c r i b e d d e s t r u c t i o n of b r a i n s t r u c t u r e s may n o t o n l y produce s p a t i a l l y c i r c u m s c r i b e d s e n s o r y losses but also spatially circumscribed scotomata in mental r e p r e s e n t a t i o n s . T h i s s u g g e s t s t h a t t h e same a n a l o g may a t a c e r t a i n s t a g e s u b s e r v e p e r c e p t i o n as w e l l a s m e n t a l r e p r e s e n t a t i o n ; which f i t s w i t h t h e a n a t o m i c a l argument s u g g e s t i n g t h a t t o p o l o g i c a l l y o r g a n i z e d c o r t e x beyond primary a r e a s , g i v e n i t s c o n s i d e r a b l e e x t e n t , is much more l i k e l y t o be implied b o t h i n p e r c e p t i o n and i n m e n t a l r e p r e s e n t a t i o n (Merzenich & Kaas, 1980). To c o u n t e r one of t h e arguments advanced a g a i n s t t h e t h e s i s of imaginal ( i . e . a n a l o g ) v i s u o s p a t i a l mental r e p r e s e n t a t i o n (Pylyshyn, 1981), i t is w o r t h r e a f f i r m i n g ( c f B i s i a c h e t a l . , 1985a) t h a t t h e appearance of a m e n t a l scotoma i n n e g l e c t p a t i e n t s cannot be i n t e r p r e t e d a s c o n s t r u e d by t h e p a t i e n t s t h e m s e l v e s i n t h e a c t of i m a g i n i n g how t h e y would

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perceive t h e represented o b j e c t given t h e i r sensory loss. Indeed, u n i l a t e r a l n e g l e c t may be p r e s e n t i n p a t i e n t s w i t h f r o n t a l l e s i o n s which do not i n v o l v e v i s u a l pathways, and when i t i s a s s o c i a t e d w i t h t r u e hemianopia t h e p a t i e n t s , as a r u l e , a r e q u i t e unaware of t h e l a t t e r . T h u s , b o t h p e r c e p t i o n and m e n t a l r e p r e s e n t a t i o n seem t o r e q u i r e a common s u b s t r a t u m on which i n f o r m a t i o n o r i g i n a t i n g e i t h e r from w i t h o u t ( p e r c e p t i o n ) of from w i t h i n t h e n e r v o u s s y s t e m i t s e l f (mental r e p r e s e n t a t i o n ) is l a i d down by means of an a n a l o g p r o c e d u r e r e i n s t a t i n g t h e s p a t i a l p r o p e r t i e s of s e n s o r y a r r a y s . An o v e r s i m p l i f i e d , two-dimensional model of t h i s p r o c e s s i n g s t a g e i s p r e s e n t e d i n F i g u r e 3 ( s e e B i s i a c h e t a l . , 1985c, f o r f u r t h e r d e t a i l s ) . I n w h a t f o l l o w s , t h e t e r m s ' c e l l assembly' w i l l r e f e r t o long-term f u n c t i o n a l g r o u p i n g s of cross-connectedneuronswithina l a r g e r n e t (Hebb, 1949) a s w e l l a s t o v e r y s h o r t - t e r m g r o u p i n g s (Goddard, 1980; von d e r Malsburg, 1981; C r i c k , 1984) such a s t h o s e which r a p i d l y form and d e c a y i n consequence of c u r r e n t s e n s o r y s t i m u l a t i o n .

Figure 3 Model of 2-dimensionalvisuo-spatial processing.

P e r c e p t i o n of t h e h o r i z o n t a l l y e l o n g a t e d p a t t e r n MN r e q u i r e s , i n O U K model, t r a n s f e r of i n f o r m a t i o n from a s e n s o r y t r a n s d u c e r ( l a y e r I) t o a s p a t i a l p r o c e s s o r c o n s i s t i n g of a n e u r a l n e t in which a p p r 0 p r i a t . e c e l l s u b a s s e m b l i e s ( l a y e r 11) a r e d i r e c t l y r e c r u i t e d by s i n g l e s u b p a t t e r n s (M,

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N) and i n d i r e c t l y r e c r u i t e d by a c t i v e s u b a s s e m b l i e s a t t h e same l e v e l . I n d i r e c t r e c r u i t m e n t may c o n t r i b u t e t o t h e p e r c e p t i o n of a p a r t i a l l y occluded s t i m u l u s . C e l l s u b a s s e m b l i e s u n r e l a t e d t o a c t u a l p o r t i o n s of t h e incoming s t i m u l a t i o n ( l a y e r 111) b u t s p a t i a l l y c o r r e s p o n d i n g t o t h e " v e r i d i c a l " s u b a s s e m b l i e s of l a y e r I1 may a l s o be r e c r u i t e d by t h e l a t t e r and, through pathways o m i t t e d i n t h e f i g u r e , by a u t o c h t h o n o u s , s e l f - o r g a n i z i n g a c t i v i t y of t h e system. I n normal p e r c e p t i o n c r o s s - t a l k i s prevented by i n h i b i t i o n of p a r a s i t i c l a y e r 111 s u b a s s e m b l i e s from s p a t i a l l y matching s u b a s s e m b l i e s of l a y e r 11, so t h a t t h e o u t p u t of t h e p r o c e s s o r ( l a y e r IV) c o n s t i t u t e s t h e a d e q u a t e c o d i f i c a t i o n of t h e p e r c e i v e d o b j e c t , ready f o r whatever f u r t h e r p r o c e s s i n g through h i g h e r o r d e r c e l l a s s e m b l i e s and whatever mode of r e p r e s e n t a t i o n ( a n a l o g o r p r o p o s i t i o n a l ) might be r e q u i r e d . L a y e r 111, on t h e o t h e r hand, s u b s e r v e s m e n t a l r e p r e s e n t a t i o n a l a c t i v i t i e s s u c h a s v i s u a l imagery, t o which -- i n normal c o n d i t i o n s and by v i r t u e of t h e damping a c t i o n from l a y e r 11 -- no b e l i e f of r e a l i t y i s f i x e d . Complete i n a c t i v a t i o n o f one h a l f of t h e a n a l o g ( F i g u r e 4 ) e n t a i l s

IV

Figure 4 Disorder underlying u n i l a t e r a l neglect.

u n i l a t e r a l n e g l e c t of t h e c o r r e s p o n d i n g h a l f of s p a c e . I n a c t i v a t i o n l i m i t e d t o one h a l f of l a y e r I1 would u n i n h i b i t t h e a c t i v i t y i n t h e c o r r e s p o n d i n g h a l f of l a y e r 111 ( F i g u r e 5 ) a n d r e l e a s e n o n - v e r i d i c a l r e p r e s e n t a t i o n o f one h a l f of s p a c e r e s u l t i n g i n phenomena of p a t h o l o g i c a l c o m p l e t i o n a n d / o r i n e r r o n e o u s b e l i e f s such a s t h o s e e x e m p l i f i e d by a n o s o g n o s i c and somato-paraphrenic phenomena o r by a n a l o g o u s d e l u s i o n s r e l a t i v e t o one h a l f of e x t r a p e r s o n a l space.

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IV

111

II

Figure 5 D i s o r d e r u n d e r l y i n g p a t h o l o g i c a l completion. ~t i s i m p o r t a n t t o n o t e t h a t l a y e r I1 i s not d i r e c t l y a c c e s s e d by t h e s e n s o r y t r a n s d u c e r ; t a k i n g v i s u a l m o d a l i t y as t h e most f a m i l i a r paradigm, i n f o r m a t i o n c a r r i e d by t h e s e n s o r y t r a n s d u c e r i s i n f a c t assumed t o undergo d i s j u n c t p r o c e s s i n g of s e p a r a t e f e a t u r e s such a s c o l o u r , s h a p e and movement, c a r r i e d o u t i n p a r a l l e l by s p e c i a l i z e d c o r t i c a l a r e a s b e f o r e r e a c h i n g l a y e r 11. F u r t h e r m o r e , t h e network of t h i s l a y e r a c t s as a r e t i n o i d (Trehub, 1 9 7 7 ) on which o v e r w r i t t e n images from t e m p o r o - s p a t i a l l y s e p a r a t e e y e - f i x a t i o n s a r e d i s e n t a n g l e d and reassembled conforming t o c o r o l l a r y i n f o r m a t i o n r e l a t i v e t o e x p l o r a t o r y eye- and head-movements. The same is t r u e f o r t h e u n f o l d i n g of images s u c h as t h o s e g e n e r a t e d by moving o b j e c t s behind a s t a t i o n a r y s l i t , which might imply c o m p u t a t i o n s o v e r ( n o t n e c e s s a r i l y e x e c u t e d ) oculomotor programs. A c o r r e s p o n d i n g r e t i n o i d c o n s t i t u t e d by endogenously r e c r u i t e d c e l l a s s e m b l i e s , s u c h a s t h o s e a r t i f i c a l l y diagrammed as a s e p a r a t e l a y e r of t h e same f u n c t i o n a l u n i t i n o u r model ( l a y e r 111) might a c h i e v e t h e s y n t h e s i s of a c o m p o s i t e m e n t a l r e p r e s e n t a t i o n . To what e x t e n t programs f o r eye- and head-moements a r e i n v o l v e d h e r e i s f a r from c l e a r . The f a c t t h a t , even i n p a t i e n t s w i t h s e v e r e l y d e p r e s s e d v i g i l a n c e , i p s i l e s i o n a l eye d e v i a t i o n i s more marked a f t e r r i g h t t h a n a f t e r l e f t a c u t e b r a i n damage (De RenzI e t a l . , 1982) i s s u g g e s t i v e of some i m p l i c a t i o n o f t h e oculomotor f u n c t i o n , a t i t s h i g h e s t l e v e l s of o r g a n i z a t i o n , i n t h e b u i l d i n g of complex m e n t a l images which may

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undergo t h e abovementionedlocaldisarrangement i n a s y n d r o m e - - d y s c h i r i a -- which i s i t s e l f mainly r e l a t e d t o r i g h t hemishpere l e s i o n . However, t h i s i m p l i c a t i o n does n o t seem t o be v e r y c l o s e , s i n c e -- a p a r t from t h e a l r e a d y mentioned p o s s i b i l i t y of d i s s o c i a t i o n between n e g l e c t and oculomotor d i s o r d e r s -- i t h a s been argued t h a t s h i f t i n g s of a t t e n t i o n may be q u i t e independent of eye-movements(Posner,Pea h V o l p e , 1982). I t is a l s o w o r t h o b s e r v i n g t h a t t h e v e r y c o n c e p t of c e l l assembly i m p l i e s t h a t t h e n e u r a l n e t of l a y e r s I1 and 111 i s f a r from b e i n g a n a i v e s t r u c t u r e . Even e x c l u s i v e l y s e n s o r y - d r i v e n a c t i v i t y d e v e l o p s h e r e t h r o u g h s y n a p s e s tuned by p r i o r e x p o s u r e t o t h e environment. S o , t h i s n e u r a l n e t i s n o t a p a s s i v e s t r u c t u r e on which s e n s o r y and m e n t a l r e p r e s e n t a t i o n s a r e t e m p o r a r i l y laid-down from a s e n s o r y b u f f e r o r from a s e p a r a t e long-term s t o r e ; i t i s s u g g e s t e d t o c o n t a i n , b e s i d e s t h e " s u r f a c e " image c u r r e n t l y e n t e r t a i n e d and i n t h e form of p o s s i b l e p a t t e r n s of n e u r a l a c t i v i t y a p t t o be t r i g g e r e d by a p p r o p r i a t e i n p u t , a l l k i n d s of "deep" o r " q u i e s c e n t " r e p r e s e n t a t i o n s a v a i l a b l e t o a n i n d i v i d u a l nervous s y s t e m d e p e n d i n g o n p a s t and p r e s e n t c o n t i n g e n c i e s ( s e e van d e r M a l s b u r g , 1981, f o r a d e t a i l e d model of dynamic network s t r u c t u r e s s u c h a s t h e one e n v i s a g e d h e r e ) . The c o m b i n a t o r i a l expanse of s y n a p t i c a c t i v i t y i n t h i s network i s h e l d t o be such a s t o make r e a s o n a b l e i t s s u g g e s t e d a b i l i t y t o s u b s e r v e a n a l o g r e p r e s e n t a t i o n a l p r o c e s s e s i n a l l t h e i r complexity. Two o t h e r i s s u e s c o n c e r n i n g t h e s p a t i a l p r o c e s s o r o u t l i n e d i n o u r m o d e l remain t o be c o n s i d e r e d . The dynamics of t h e l a t t e r , i n d e e d , must be s u c h as t o account f o r two i m p o r t a n t d a t a . One i s t h e f a c t t h a t i n n e g l e c t p a t i e n t s a l e f t / r i g h t s p a t i a l a n i s o t r o p y may a l s o be o b s e r v e d i n t h e hemispace i p s i l a t e r a l t o t h e l e s i o n . T h i s phenomenon i s known t o c l i n i c a l n e u r o l o g i s t s i n t h e g u i s e of p a r a d o x i c a l e x t i n c t i o n of t h e l e f t m o s t ( n e a r e r t o t h e f o v e a ) of two s i m u l t a n e o u s v i s u a l s t i m u l i d e l i v e r e d i n t h e r i g h t f i e l d of r i g h t - p a r i e t a l p a t i e n t s and has been d e m o n s t r a t e d by v a r i o u s e x p e r i m e n t a l p r o c e d u r e s b o t h i n a n i m a l s (La M o t t e & Acuna, 1978) and i n man ( C o r i n & Bender, 1972; Kinsbourne, 1977; Altman, Balonov h D e g l i n , 1979; B i s i a c h , C o r n a c c h i a , S t e r z i 6 V a l l a r , 1984; Gazzaniga 6. L a d a v a s , t h i s volume). The o t h e r is t h e f a c t t h a t h e m i n e g l e c t seems t o r e l a t e t o a t l e a s t two d i s t i n c t f r a m e s of s p a t i a l r e f e r e n c e : one r e t i n o t o p i c and one p o s s i b l y connected w i t h t h e m i d - s a g i t t a l p l a n e of t h e p a t i e n t l s body (Heilman h V a l e n s t e i n , 1979; B i s i a c h , C a p i t a n i & P o r t a , 1985b; Gazzaniga h Ladavas, t h i s volume). P e r h a p s , t h e b e s t d e m o n s t r a t i o n t h a t t h e d i s o r d e r i s n o t o n l y framed i n terms of r e t i n o t o p i c c o o r d i n a t e s is t h e f a c t t h a t i n t a s k s r e q u i r i n g t h e s c a n n i n g of a v i s u a l a r r a y , e.g. i n a c a n c e l l a t i o n t a s k , p a t i e n t s w i t h l e f t n e g l e c t a l m o s t always s t a r t from t h e r i g h t m o s t i t e m and proceed l e f t w a r d s u n t i l t h e y s t o p , t h u s n e g l e c t i n g i t e m s l o c a t e d i n t h e same r e t i n a l p o s i t i o n w h i c h , d u r i n g f o v e a t i o n o f t h e e a r l i e r s c a n n e d i t e m s , was occupied by non-neglected items ( a f a c t , i n c i d e n t a l l y , which seems t o p r o v i d e t h e most e l e m e n t a r y c o n f u t a t i o n of s e n s o r y e x p l a n a t i o n s of unilateral neglect). An i n t e r p r e t a t i o n of t h e f i r s t phenomenon may be advanced i n t e r m s of r e c e p t i v e f i e l d s of c e l l s c o n t r i b u t i n g t o t h e c o n s t i t u t i o n of n e u r a l n e t s implementing a n a l o g s p a t i a l p r o c e s s o r s . I n d e e d , i t h a s been found t h a t i n t h e p o s t a r c u a t e c o r t e x of monkeys, where one s u c h a n a l o g might be l o c a t e d , 20% of t h e n e u r o n s have e x c l u s i v e l y c o n t r a l a t e r a l , 2% i p s i l a t e r a l and 69% b i l a t e r a l f i e l d s ( R i z z o l a t t i , S c a n d o l a r a , M a t e l l i 6 G e n t i l u c c i , 1981). B i l a t e r a l f i e l d s a r e always a s t r i d e t h e m i d l i n e , and v a r y i n h o r i z o n t a l e x t e n s i o n , so t h a t i n i t s most l a t e r a l a r e a s s p a c e r e p r e s e n t a t i o n r e l i e s a l m o s t e x c l u s i v e l y on t h e c o n t r a l a t e r a l c o r t e x , w h i l e p r o c e e d i n g towards t h e m i d l i n e i t shows a p r o g r e s s i v e t r e n d towards b e i n g e q u a l l y s h a r e d between t h e two hemispheres. " I n t h e c a s e of l e s i o n of one hemisphere t h e

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whole v i s u a l f i e l d w i l l be a f f e c t e d b u t w i t h a g r a d i e n t of s e v e r i t y g o i n g from a maximum i n t h e extreme c o n t r a l a t e r a l h e m i f i e l d t o a minimum i n t h e extreme i p s i l a t e r a l hemif i e l d " ( R i z z o l a t t i , G e n t i l u c c i & M a t e l l i , 1985). I n man, an a s y m m e t r i c a l i p s i l a t e r a l c o n t r i b u t i o n t o s p a c e r e p r e s e n t a t i o n i n a n a l o g p r o c e s s o r s might c a u s e t h e d i s p r o p o r t i o n a t e i n c i d e n c e of hemineglect f o l l o w i n g l e f t and r i g h t hemisphere l e s i o n . An a l t e r n a t i v e e x p l a n a t i o n o f t h e same phenomenonmight assume t h a t s e n s o r y r e p r e s e n t a t i o n of a p a t t e r n of e c c e n t r i c s t i m u l a t i o n i n t h e v i s u a l f i e l d , does not remain anchored t o t h e r e t i n a l c o o r d i n a t e s of t h e proximal s t i m u l u s , b u t i t s c e n t r o i d , a s i t were, i s f o v e a t e d f o r f u r t h e r p r o c e s s i n g by t h e m i n d ' s eye t h r o u g h a t r a n s l a t i o n o v e r t h e s u r f a c e of a r e t i n o i d s t r u c t u r e ; f o r m a l model of t h i s p r o c e s s h a s been provided by Trehub (1977). I n p a t i e n t s w i t h l e f t h e m i n e g l e c t , t h e l e f t h a l f of s t i m u l u s c o n f i g u r a t i o n , wherever l o c a t e d i n t h e o u t e r s p a c e , would t h u s f a l l i n t o t h e d i s o r d e r e d h a l f of a s t r u c t u r e s u p p o r t i n g e g o c e n t r t c space r e p r e s e n t a t i o n so t h a t i t s p r o c e s s i n g r e l a t i v e t o t h a t of t h e r i g h t h a l f i s t o a v a r i a b l e e x t e n t impaired. A s f o r t h e second i s s u e , i t seems n e c e s s a r y t o i n f e r t h a t e a c h p o i n t of t h e a n a l o g f o r s p a c e r e p r e s e n t a t i o n -- whatever t h e r e l a t i v e c o n t r i b u t i o n of e a c h hemisphere t o i t s i m p l e m e n t a t i o n -- i s double-indexed i n t e r m s of r e t i n a l and body c o o r d i n a t e s , o r t h a t s p a c e r e p r e s e n t a t i o n a r t i c u l a t e s i n a m a n i f o l d of a n a l o g s w i t h d i f f e r e n t frames of r e f e r e n c e . I t i s w o r t h n o t i n g h e r e t h a t t h e a c t u a l e x i s t e n c e of more t h a n one s p a t i a l a n a l o g i s s u g g e s t e d by i n d i c a t i o n s of d o u b l e d i s s o c i a t i o n between n e g l e c t f o r p e r s o n a l o r p e r i p e r s o n a l space and n e g l e c t f o r e x t r a p e r s o n a l s p a c e found i n monkeys ( R i z z o l a t t i e t a l . , 1985) a s w e l l a s i n m a n ( B i s i a c h , P e r a n i , V a l l a r & B e r t i , forthcoming).

SpatialAnalogsandCognitiveProcesses In o u r model, r e p r e s e n t a t i o n by s p a t i a l a n a l o g i s f a r from b e i n g a stupid picture-in-the-head c r i t i c a l l y s u r v e y e d and i n t e r p r e t e d by a n i n t e l l i g e n t mind's e y e . The f i r i n g of c e l l a s s e m b l i e s of l a y e r s I1 and I11 must be r e g a r d e d a s a n i n t r i n s e c a l l y c o g n i t i v e a c t i v i t y , s i n c e no h i e r a r c h i c a l l y h i g h e r form of s e n s o r y and mental r e p r e s e n t a t i o n seems t o e x i s t and, a s w i l l p r e s e n t l y be a r g u e d , i t shows d i r e c t involvement i n c o n s c i o u s n e s s . Whatever r e s i d u e s of a n c e s t r a l a v e r s i o n might be r a i s e d a g a i n s t t h e n o t i o n of an e n t i t y where c o g n i t i v e p r o p e r t i e s merge w i t h e x t e n s i o n a l p r o p e r t i e s , t h i s is t h e c r u c i a l a s p e c t of O U T model. Both l a c k of r e p r e s e n t a t i o n and c o n t e n t f u l m i s r e p r e s e n t a t i o n of one h a l f of s p a c e such a s c o n j o i n t l y o r s e p a r a t e l y o b s e r v a b l e i n d y s c h i r i a a r i s e from i n t r i n s i c changes i n t h e s p a t i a l a n a l o g , not from f a i l u r e of a superimposed s c a n n i n g d e v i c e which we have no f a c t u a l r e a s o n t o h y p o t h e s i z e ( B i s i a c h e t a l . , 1985c; B i s i a c h & B e r t i , 1985) and whichwould i m p l y a n i n f i n i t e r e g r e s s of s p a c e r e p r e s e n t a t i o n s . I t is worth s t r e s s i n g t h i s point i n order t o d i s p e l t h e b e l i e f t h a t a n a l o g t h e o r i e s u n a v o i d a b l y s h a r e t h e "assumption t h a t images a r e e s s e n t i a l l y i n n e r o b j e c t s a t which t h e homunculus can look" ( N e i s s e r , 1978). Due t o o u r n a t u r a l d i s i n c l i n a t i o n t o l o o k c r i t i c a l l y beyond t h e i r f a c e v a l u e , metaphors a r e o f t e n q u i t e m i s l e a d i n g and i t i s admonishing t h a t under t h e v e r y t e r m s of " a t t e n t i o n a l s e a r c h l i g h t " C r i c k (1984) h a s p r o v i d e d a n e u r a l model which n i c e l y f i t s w i t h o u r h y p o t h e s i s i d e n t i f y i n g t h e m i n d ' s l a y e r s I1 and 111. eye w i t h t h e p a t t e r n of a c t i v i t i e s o c c u r r i n g i n , not I n C r i c k ' s model, " i n t e n s i f i c a t i o n " of p a r t of a r e p r e s e n t a t i o n C u r r e n t l y e n t e r t a i n e d Ln t h e b r a i n may be s e l f - o p e r a t e d by two p o s i t i v e f e e d b a c k loops: one i n t r a t h a l a m i c , i n v o l v i n g r e l a y and r e t i c u l a r n e u r o n s and b a s i c a l l y sensory-driven; t h e o t h e r cortico-thalamic, g r a f t e d i n t o t h e f o r m e r and a c c e p t i n g top-down i n f l u e n c e s . Such a k i n d of a u t o g e n o u s ,

over,

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s e l e c t i v e m o d u l a t i o n of i n f o r m a t i o n i n an a n a l o g r e p r e s e n t a t i o n a l network might c o n s t i t u t e t h e n e u r a l i m p l e m e n t a t i o n of n o n - p r o p o s i t i o n a l t h o u g h t processes. In o u r model, s h i f t s of f o c u s i n mental s p a c e r e f l e c t m i g r a t i o n of a c t i v i t y o v e r t h e s u r f a c e of s p a t i a l a n a l o g s ; t h u s , t o some e x t e n t , i n t e r p r e t a t i o n s of d y s c h i r i a may c o n v e n i e n t l y be p h r a s e d i n terms of a t t e n t i o n a s l o n g a s n e g l e c t phenomena a r e c o n s i d e r e d , even i n t h e domain of mental r e p r e s e n t a t i o n . T h i s is however no l o n g e r t r u e w i t h r e f e r e n c e t o m i s r e p r e s e n t a t i o n s which may sometimes be i n v o l v e d i n f o r e g r o u n d c o g n i t i v e a c t i v i t y . F u r t h e r m o r e , i n t e r p r e t a t i o n s i n t e r m s of a t t e n t i o n f a i l t o a c c o u n t f o r phenomena s u c h a s t h e i n t r o s p e c t i v e r e p o r t of B a x t e r and W a r r i n g t o n ' s p a t i e n t and might miss t h e s u b t l e r p o i n t t h a t what a p p e a r s a s a s h i f t of a t t e n t i o n m i g h t m e a n a r e c o i l f r o m d a u n t i n g r e p r e s e n t a t i o n s . C o g n i t i v e p r o c e s s e s e n t e r i n g a way of p r o p o s i t i o n a l r a t h e r t h a n of a n a l o g p r o c e s s i n g , and t h e r e f o r e n o t p r i m a r i l y a f f e c t e d by t h e d i s o r d e r u n d e r l y i n g d y s c h i r i a , may not o n l y be t o t a l l y u n a b l e t o compensate t h e l a t t e r b u t may themselves be e n t r a p p e d and m i s l e a d , a s we have a r g u e d on t h e ground of c l i n i c a l and of e x p e r i m e n t a l e v i d e n c e ( B i s i a c h , B e r t i & V a l l a r , 1985; B i s i a c h , M e r e g a l l i S B e r t i , 1985); which i s f a r from b e i n g a n i n d i c a t i o n o f c o g n i t i v e supremacy. By f a r t h e most c r i t i c a l , t h e r e remain t h e i m p l i c a t i o n s of d y s c h i r i a f o r t h e o r i e s of c o n s c i o u s n e s s , meant a s s e l f - m o n i t o r i n g and t h e h i g h e s t form of s e l f - c o n t r o l of nervous a c t i v i t i e s . In s p i t e of i t s i m p o r t a n c e , o r r a t h e r because of i t , t h i s a s p e c t w i l l o n l y be i n t r o d u c e d h e r e s u c c i n t l y ( s e e B i s i a c h , 1985, f o r amplerdiscussionandatentativeelucidationof t h e t e r m o f consciousnessasemployedinthis c h a p t e r ) . C l i n i c a l and e x p e r i m e n t a l e v i d e n c e ( e . g . B i s i a c h e t a l . , 1985a. c ; B i s i a c h , P e r a n i , Papagno, V a l l a r 6 B e r t i , f o r t h c o m i n g ) shows t h a t t h e r e p r e s e n t a t i o n a l scotoma m a n i f e s t i n g i t s e l f i n t h e m a n i f o l d t r a i t s of u n i l a t e r a l n e g l e c t and t h e f a l s e b e l i e f s which a r e f i x e d t o r e p r e s e n t a t i o n s i s s u e d , i n o u r model, from t h e d i s o r d e r e d h a l f of t h e s p a t i a l a n a l o g as a consequence o f some c i r c u m s c r i b e d b r a i n l e s i o n s e n c r o a c h u n c o n t r o l l e d o n c o g n i t i v e p r o c e s s e s . In s e v e r e c a s e s , any e f f o r t t o b e s e t t h e p a t i e n t and f o r c e him t o admit and c r i t i c a l l y e v a l u a t e h i s p a t h o l o g i c a l c o n d i t i o n is doomed t o f a i l u r e : e i t h e r t h e p a t i e n t e l u d e s t h e problem a l t o g e t h e r , o r he c u t s s h o r t and s h e l t e r s h i s c o g n i t i v e d i s o r d e r by arguments of which a c o n f u t a t i o n would be i n v a i n . An i l l u s t r a t i v e i n s t a n c e of a n u n s u c c e s s f u l i n t e r v i e w c a r r i e d o n l o n g e r t h a n u s u a l w i t h a p a t i e n t who had l e f t n e g l e c t a n d a n o s o g n o s i a f o r l e f t h e m i p l e g i a , maybe found i n B i s i a c h e t a 1 . ( 1 9 8 5 ~ ) . The consequences of t h i s p e r t u r b a t i o n a r e however local and c o g n i t i v e a c t i v i t y o u t s i d e t h e c o r r u p t e d v e i n may remain t o t a l l y u n a f f e c t e d . T h e s e f a c t s s u g g e s t that r e l a t i v e l y p e r i p h e r a l l e v e l s of nervous a c t i v i t y s u c h a s t h o s e of a n a l o g mechanisms of r e p r e s e n t a t i o n may p l a y a c r u c i a l r o l e i n t h e g e n e r a t i o n of t h o s e e v e n t s whereby s e n s o r y and m e n t a l r e p r e s e n t a t i o n s a c q u i r e t h e p r o p e r t y of b e i n g c o n s c i o u s . We know from b l i n d - s i g h t (Weiskrantz, W a r r i n g t o n , S a n d e r s S M a r s h a l l , 1974) t h a t c o n s i d e r a b l e p r o c e s s i n g of i n f o r m a t i o n d e l i v e r e d by s e n s o r y t r a n s d u c e r s can t a k e p l a c e w i t h o u t any c o n s c i o u s c o r r e l a t e . We a l s o know t h a t i n t e r r u p t i o n of t h e f l o w of s e n s o r y i n f o r m a t i o n up t o a c e r t a i n s t a g e beyond p r i m a r y s e n s o r y a r e a s is monitored by t h e b r a i n . On t h e o t h e r hand, w e know ( a t l e a s t s i n c e A n t o n ' s 1899 c e l e b r a t e d p a p e r ) t h a t f a i l u r e of a more c e n t r a l s t r u c t u r e ( t o which t h e s p a t i a l a n a l o g of our model r e f e r s ) n o t o n l y c u t s a d e f i n i t e amount of i n f o r m a t i o n o u t of c o n s c i o u s n e s s , b u t r e n d e r s , a s a r u l e , t h e d e f e c t i t s e l f unconscious and may r e l e a s e u n c r i t i c i z e d p a t h o l o g i c a l r e p r e s e n t a t i o n s . T h i s seems t o e n t a i l t h a t c o n s c i o u s n e s s is i n h e r e n t i n t h e r e p r e s e n t a t i o n a l a c t i v i t y of t h e s e a n a l o g s t r u c t u r e s , b o t h as r e f e r r i n g t o t h e m o n i t o r i n g of

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t h e s e a c t i v i t i e s and a s r e f e r r i n g t o t h e i r c o n t r o l . Without t a k i n g i n t o c o n s i d e r a t i o n t h e d i s o r d e r e d a p p a r a t u s which i s r e s p o n s i b l e f o r anosognosia r e l a t i v e t o some forms of d y s p h a s i a , and which i s s t i l l t o o o b s c u r e t o be d i s c u s s e d h e r e , no f u r t h e r mechanisms f o r c o n s c i o u s n e s s seem t o e x i s t , e i t h e r i n t h e form of a u n i t i z e d , h i e r a r c h i c a l l y superimposed component of t h e c o g n i t i v e machinery o r emerging from t h e whole of c o g n i t i v e a c t i v i t i e s of t h e b r a i n . A s f o r t h e l a t t e r , what we have s o f a r l e a r n t from d y s c h i r i a c h a l l e n g e s t h e h y p o t h e s i s of an i s o t r o p i c s t r u c t u r e of t h o u g h t , whereby a t t h i s l e v e l each p r o c e s s has f r e e a c c e s s t o any o t h e r p r o c e s s , b o t h i n t h e s e n s e of an u n l i m i t e d range of i n f o r m a t i o n a n d i n t h a t of a n u n l i m i t e d f i e l d of c o n t r o l ( F o d o r , 1983). T h i s h y p o t h e s i s i s undermined by phenomena such a s t h o s e a l r e a d y p o i n t e d o u t by Z i n g e r l e w i t h r e f e r e n c e t o h i s Case 2 , who w h i l e knowing t h a t a l l human b o d i e s have a l e f t and a r i g h t s i d e , could n o t a p p l y t h a t n o t i o n t o h i m s e l f . Phenomena of t h i s kind s u g g e s t t h a t t h o u g h t i t s e l f may have a t e x t u r e imposed by p r o c e s s o r s o r g a n i z e d i n t h e form of a n a l o g s and may b r e a k a l o n g t h e l i n e s of t h i s t e x t u r e , w i t h consequences which c a n n o t be amended by any kind of p r o p o s i t i o n a l a c t i v i t y .

Conclusions We have t r i e d t o s k e t c h a s y s t e m i c a n a l y s i s of d i s o r d e r s which have s o f a r been r e l a t i v e l y n e g l e c t e d n o t w i t h s t a n d i n g t h e w e i g h t t h e y may have i n t h e advancement of o u r i n s i g h t i n t o c o g n i t i o n . We a r e s t i l l f a r from having a n e n t i r e l y connected s t o r y , however, and o u r r e p r e s e n t a t i o n a l e x p l a n a t i o n of t h e s e d i s o r d e r s , a s w e l l a s o u r i n f e r e n c e s about c o g n i t i v e a c t i v i t i e s , l a r g e l y r e s t on t h e a s s u m p t i o n t h a t n e g l e c t and m i s r e p r e s e n t a t i o n of one s i d e of s p a c e e x p r e s s d i s f u n c t i o n of t h e same f u n c t i o n a l u n i t . We have o u t l i n e d a model of a u n i t of t h i s k i n d , where a c i r c u m s c r i b e d f a i l u r e of t h e mechanism can indeed g e n e r a t e , a c c o r d i n g t o r e l a t i v e l y m i n o r changes i n i t s l o c a t i o n , n e g l e c t o r m i s r e p r e s e n t a t i o n ; y e t , however p l a u s i b l e , t h i s o f f e r s no proof t h a t what r e a l l y happens i n t h e nervous system f o l l o w s t h e s e lines. Our s u g g e s t i o n h a s a n o t a b l e a n t e c e d e n t i n Z i n g e r l e ' s paper. To-day, a r e p r e s e n t a t i o n a l e x p l a n a t i o n of t h e d i s o r d e r s c o n s i d e r e d i n t h i s c h a p t e r a l s o b e n e f i t s from t h e f a c t t h a t , u n l i k e i n Z i n g e r l e ' s t i m e s , 'representation' i s no l o n g e r a m e r e l y p s y c h o l o g i c a l c o n s t r u c t . N e u r o p h y s i o l o g i s t s have g i v e n t h i s c o n c e p t a p h y s i c a l f o u n d a t i o n t h r o u g h d i s c o v e r i e s c o n c e r n i n g t h e a c t i v i t y of networks which a r e s u g g e s t e d t o p r o v i d e a n e u r a l model of e g o c e n t r i c s p a c e (e.g. M o u n t c a s t l e , 1981). Indeed, it i s i n t h e l i g h t of recent neurophysiological and n e u r o p s y c h o l o g i c a l advances t h a t Z i n g e r l e ' s c o n t r i b u t i o n shows i t s f u l l s i g n i f i c a n c e . S e n t t o p r e s s i n t h e t w i l i g h t of an epoch (whose d i s s o l u t i o n , due t o a c u r i o u s c o i n c i d e n c e , was d e l i m i t e d t o time by t h e two s p e e c h e s made by B a b i n s k i i n F r a n c e ) t h e paper of t h e Graz n e u r o l o g i s t would seem t o have s h a r e d t h e d e s t i n y of t h e Austro-Hungarian empire. I t s i m p o r t a n t t h e o r e t i c a l message had no r e v e r b e r a t i o n . I n t h e p r e s e n t c h a p t e r , t h e r e s t o r a t i o n of t h e t e r m ' d y s c h i r i a ' h a s been proposed on two grounds. I t s a b i l i t y t o c o v e r t h e f u l l range of phenomena of a l t e r e d c o g n i t i o n of a h a l f - s p a c e seems, a t t h e v e r y l e a s t , h e u r i s t i c a l l y h e l p u l i n d e f i n i n g a f i e l d which s t i l l l a c k s t h e a p p r o p r i a t e u n i t y t o be f u l l y r e c o g n i z e d as a c r u c i a l node i n t h e i n v e s t i g a t i o n of c o g n i t i v e b r a i n - a c t i v i t i e s . F u r t h e r m o r e , r e a d o p t i o n of t h i s term would be a w e l l - d e s e r v e d t r i b u t e t o t h e A u s t r i a n n e u r o l o g i s t and would compensate f o r so many y e a r s of n e g l e c t . Although t h i s p r o p o s a l may e n c o u n t e r r e s i s t a n c e r a n g i n g from p e r s o n a l t h e o r e t i c a l s t a n c e s t o s i m p l e i n d i f f e r e n c e , i t i s t o be hoped t h a t i t w i l l a t l e a s t e n j o y t h e s e r i o u s c o n s i d e r a t i o n of t h o s e who a t t r i b u t e a c e r t a i n v a l u e t o our c u l t u r a l i n h e r i t a n c e .

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I

w,

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20 1

A c k n w l e d g e i e o t s : The p r e p a r a t i o n of t h i s c h a p t e r was s u p p o r t e d by a g r a n t from t h e M i n i s t e r 0 d e l l a P u b b l i c a I s t r u z i o n e t o t h e f i r s t a u t h o r . We g r a t e f u l l y acknowledge t h e h e l p of F r a n c e s A n o l e r s o n , who c a r e f u l l y reviewed t h e E n g l i s h .

Footnote 1. The term ' d y s c h i r i a ' was t a k e n from J o n e s , who i s l i k e l y t o have coined i t . I n 1910, under t h e t i t l e of "Die P a t h o l o g i e d e r D y s c h i r i e " , E. J o n e s , a T o r o n t o U n i v e r s i t y p r o f e s s o r , had d e s c r i b e d t w o p a t i e n t s s u f f e r i n g from " h y s t e r i a " and " h y s t e r o n e u r a s t h e n y " r e s p e c t i v e l y . T h e i r symptoms were v e r y complex b u t not w i t h o u t s i m i l a r i t y t o Z i n g e r l e ' s o r g a n i c c a s e s . The n e u r o t i c n a t u r e of s u c h symptoms might however be q u e s t i o n e d , s i n c e t h e f i r s t p a t i e n t had developed them f o l l o w i n g a r a i l w a y a c c i d e n t , p o s s i b l y involving head trauma, whereas the second suffered from "hystero-epylept ic'lseizures.


Neurophysiological and Neuropsychological Aspects of Spatial Neglect, M.Jeannerod (editor) 0 Elsevier Science Publishers B.V. (North-Holland),1987

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DISTURBANCES I N SPATIAL ATTENTION FOLLOWING LESION OR DISCONNECTION OF THE R I m PARIETAL LOBE* Michael S . Gazzaniga and E l i s a b e t t a Ladavas

A s e r i e s of s t u d i e s a r e r e p o r t e d t h a t s u g g e s t t h e p a r i e t a l l o b e of humans i s i n v o l v e d i n t h e e s t a b l i s h m e n t of s p a t i a l r e f e r e n t s i n g r a v i t a t i o n a l s p a c e . T h i s f u n c t i o n goes beyond t h e more e l e m e n t a r y f u n c t i o n of p r o c e s s i n g r e t i n o t o p i c i n f o r m a t i o n and most l i k e l y p l a y s a n i m p o r t a n t r o l e i n g o v e r n i n g a wide range of l o c o m o t o r a c t i v i t i e s . I t is a l s o suggested t h a t e a c h p a r i e t a l l o b e is a c t i v e i n t h i s process.

Introduction I t has become i n c r e a s i n g l y c l e a r o v e r t h e p a s t s e v e r a l y e a r s t h a t damage t o t h e p a r i e t a l l o b e i n p r i m a t e s produces d i s t u r b a n c e s i n a t t e n t i o n a l s y s t e m s , p a r t i c u l a r l y i n man ( f o r review, s e e De R e n z i , 1982). The d r a m a t i c c l i n i c a l d e f i c i t s s e e n f o l l o w i n g p a r i e t a l l o b e damage were o r i g i n a l l y c h a r a c t e r i z e d i n terms of a n impairment i n t h e p r o c e s s i n g of b a s i c s e n s o r y i n f o r m a t i o n (Denny-Brown, Meyer and H o r e n s t e i n , 1952). Recent o b s e r v a t i o n s s u g g e s t t h a t t h e d e f i c i t s a r e more c l o s e l y r e l a t e d t o d i s t u r b a n c e s i n t h e c a p a c i t y t o s w i t c h a t t e n t i o n from one s p a t i a l l o c a t i o n t o a n o t h e r ( P o s n e r , Cohen and R a f a l , 1982; Riddoch and Humphrey, 1983). Thus, i n a n i n i t i a l o b s e r v a t i o n by P o s n e r and c o l l e g u e s ( P o s n e r , Walker, F r i e d r i c h and R a f a l , 1 9 8 4 ) , i t was shown t h a t p a t i e n t s w i t h u n i l a t e r a l p a r i e t a l d i s e a s e were d e l a y e d i n s h i f t i n g t h e i r a t t e n t i o n from a p o i n t i n t h e i n t a c t v i s u a l f i e l d o v e r t o a p o i n t i n t h e impaired v i s u a l f i e l d . T h i s kind of o b s e r v a t i o n h a s now been confirmed and extended by o t h e r s (Baynes, Holtzman a n d V o l p e , 1986). These d a t a were c o n s i s t e n t w i t h t h e view t h a t t h e p a r i e t a l l o b e i s f u n c t i o n a l i n e s t a b l i s h i n g and m a i n t a i n i n g s p a t i a l maps t h a t a p p e a r t o be r e t i n o t o p i c a l l y o r g a n i z e d . I n o t h e r words, t h e c o o r d i n a t e s of t h e a t t e n t i o n a l s p a t i a l maps, a s d e t e c t e d t h r o u g h priming s t u d i e s , were i n r e g i s t e r with the primary v i s u a l map as established through retino-geniculo-striate projections. This interpretation is consistent w i t h a v a r i e t y of n e u r o p h y s i o l o g i c a l a n a l y s e s of p a r i e t a l l o b e f u n c t i o n i n t h e monkey (Yin and M o u n t c a s t l e , 1977). R e s u l t s from a n i m a l s t u d i e s i n d i c a t e t h a t t h e f a c i l i t a t e d r e s p o n s e s d e t e c t e d i n a r e a 7 r e c o r d i n g s was p a r t of a n e u r a l c e l l system t h a t was r e t i n o t o p i c a l l y o r g a n i z e d . The c e l l s responded most v i g o r o u s l y p r i o r t o eye and hand movements t o p a r t i c u l a r p o i n t s i n r e t i n o t o p i c c o o r d i n a t e s . On t h e o t h e r hand, t h e r e have b e e n o t h e r r e p o r t s s u g g e s t i n g some p a r i e t a l c e l l s respond o n l y when t h e animal h a s d i r e c t e d i t s gaze t o a p a r t i c u l a r l o c a t i o n i n s p a c e (Lynn, 1966; Andersen, E s s i c k & S i e g e l , 1985). T h i s would s u g g e s t t h a t a t l e a s t some of t h e c e l l s wereorganized i n t e r m s o f r e f e r e n t s i n a c t u a l physical space. Data f r o m r e s e a r c h o n h u m a n s have a l s o b e e n a c c u m u l a t i n g i n r e c e n t *Dr. J e f f r e y D. Holtzman was f i r s t i n v i t e d t o w r i t e t h i s review. Upon h i s d e a t h , w e g l a d l y p r e p a r e d t h i s c h a p t e r i n h i s honor.

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y e a r s and i n d i c a t e t h a t t h e p a r i e t a l l o b e i s i n v o l v e d i n e s t a b l i s h i n g r e f e r e n c e p o i n t s i n c o n c e p t u a l space. I n an i n g e n i o u s s e t of o b s e r v a t i o n s , B i s i a c h and L u z z a t i ( 1 9 7 8 ) demonstrated t h a t p a t i e n t s w i t h r i g h t p a r i e t a l d i s e a s e were u n a b l e t o d e s c r i b e i n f o r m a t i o n a b o u t t h e l e f t h a l f of an imagined s c e n e when "viewed" from a p a r t i c u l a r v a n t a g e p o i n t . When s u b s e q u e n t l y a s l e d t o d e s c r i b e t h e same s c e n e from t h e o p p o s i t e v a n t a g e p o i n t , t h e y were immediately a b l e t o d e s c r i b e t h e p r e v i o u s l y n e g l e c t e d a s p e c t b u t f a i l e d t o d e s c r i b e t h e view p r e v i o u s l y r e p o r t e d b e c a u s e i t now f e l l i n t o t h e l e f t , i n t e r n a l l y n e g l e c t e d space. T h i s o b s e r v a t i o n s u g g e s t s t h a t t h e p a r i e t a l l o b e may be o r g a n i z e d i n s u c h a way t h a t i t f u n c t i o n s t o e s t a b l i s h and m a i n t a i n r e f e r e n c e p o i n t s i n b o t h imagined and r e a l p h y s i c a l s p a c e , i n d e p e n d e n t of t h e r e t i n o t o p i c maps of primary s e n s o r y c o r t e x . I n t h i s c h a p t e r w e examine a s e r i e s of q u a n t i t a t i v e s t u d i e s i n p a t i e n t s w i t h r i g h t p a r i e t a l d i s e a s e a s w e l l a s p a t i e n t s who have undergone c a l l o s a l s e c t i o n . These s t u d i e s a r e c o n s i s t e n t w i t h t h i s i n t e r p r e t a t i o n of p a r i e t a l l o b e f u n c t i o n . We w i l l a l s o s u g g e s t t h a t t h e f u n c t i o n o f t h e p a r i e t a l l o b e t o e s t a b l i s h p o i n t s of r e f e r e n c e i n p h y s i c a l s p a c e i s n o t a s p e c i a l i z e d f u n c t i o n of t h e r i g h t hemisphere. We w i l l a r g u e t h a t , even i f e a c h hemisphere can a t t e n d t o any p o s i t i o n i n t h e p h y s i c a l s p a c e , t h e r i g h t hemisphere i s s p e c i a l i z e d i n d i r e c t i n g o v e r t a t t e n t i o n t o l e f t s p a c e , and t h e l e f t hemisphere t o r i g h t s p a c e .

Deficit on Distribution of Spatial Attention following Right Parietal Damage There a r e two c l a s s i c frames of r e f e r e n c e f o r d e s c r i b i n g t h e p o s i t i o n of v i s u a l s t i m u l i r e l a t e d t o an o b s e r v e r . There i s a r e t i n o t o p i c frame of r e f e r e n c e and a p h y s i c a l frame of r e f e r e n c e . The r e t i n o t o p i c frame of reference i f t y p i c a l l y defined according t o the retinotopic coordinates, e.g., t h e p o s i t i o n of t h e s t i m u l u s on t h e r e t i n a . The p h y s i c a l o r e n v i r o n m e n t a l frame of r e f e r e n c e i s d e f i n e d a c c o r d i n g t o g r a v i t a t i o n a l coordinates. The e x p e r i m e n t s d e s c r i b e d h e r e a t t e m p t t o a s c e r t a i n w h i c h of t h e s e two frames of r e f e r e n c e i s d i s r u p t e d f o l l o w i n g r i g h t p a r i e t a l d i s e a s e . We t a k e a s o u r p o i n t of d e p a r t u r e thewell-documented f i n d i n g t h a t p a t i e n t s w i t h t h e e x t i n c t i o n produced by r i g h t p a r i e t a l d i s e a s e a r e s l o w e r t o respond t o simple s t i m u l i p r e s e n t e d i n t h e l e f t a s opposed t o t h e r i g h t v i s u a l f i e l d . When t h e head i s i n t h e normal u p r i g h t p o s i t i o n a n d t h e e y e s a r e l o o k i n g s t r a i g h t f o r w a r d , t h e s e two frames of r e f e r e n c e c o i n c i d e , i . e . , f o r t h e s t i m u l i p r e s e n t e d i n t h i s t e s t c o n d i t i o n , w h a t i s l e f t a n d r i g h t i n o n e frame of r e f e r e n c e i s l e f t and r i g h t i n t h e o t h e r ( S e e F i g u r e l ) . H o w e v e r , w h e n t h e head i s t i l t e d by 90 d e g r e e s , t h e two frames of r e f e r e n c e no l o n g e r c o i n c i d e . What a r e l e f t and r i g h t i n t h e p h y s i c a l frame of r e f e r e n c e a r e left-down and l e f t - u p (head r i g h t - t i l t e d ) and r i g h t - u p and right-down (head l e f t - t i t l e d ) i n t h e r e t i n a l frame. As a consequence, w i t h t h e head t i l t e d one can u s e e i t h e r t h e r e t i n o t o p i c o r g r a v i t a t i o n a l code f o r d e s c r i b i n g any p o s i t i o n i n v i s u a l space. When t h e head i s t i l t e d , i f t h e attentionaldeficitassociated w i t h r i g h t p a r i e t a l l o b e damage i s r e l a t e d t o t h e r e t i n o t o p i c frame of r e f e r e n c e , no d i f f e r e n c e s i n r e a c t i o n timewould b e e x p e c t e d b e t w e e n t h e t w o s t i m u l i s i n c e i n r e t i n o t o p i c c o o r d i n a t e s t h e y b o t h f a l l i n t h e same v i s u a l f i e l d and one i s above t h e o t h e r . I n c o n t r a s t , i f t h e d e f i c i t i s r e l a t e d s t r i c t l y t o t h e u s e of p h y s i c a l frame of r e f e r e n c e , we would e x p e c t s l o w e r reaction times t o the g r a v i t a t i o n a l l e f t than t o the g r a v i t a t i o n a l r i g h t stimulus F i v e p a t i e n t s w i t h CT confirmed u n i l a t e r a l v a s c u l a r l e s i o n s i n v o l v i n g t h e r i g h t p a r i e t a l l o b e , who e x t i n g u i s h e d t h e l e f t s t i m u l u s under d o u b l e s i m u l t a n e o u s v i s u a l s t i m u l a t i o n , p a r t i c i p a t e d i n t h i s s t u d y (Ladavas,

.

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RETl NOTOPIC vs GRAVITATIONAL

L, TILT-RVF

R, TILT-LVF

Figure 1 Shows e x p e r i m e n t a l s t r a t e g y f o r d i s s o c i a t i n g r e t i n o t o p i c f a c t o r s from gravitational factors in assessing d e f i c i t s i n s p a t i a l orientation. With t h e head t i l t e d t o t h e l e f t b o t h s t i m u l u s l o c a t i o n s f a l l i n t o t h e r i g h t v i s u a l f i e l d and w i t h t h e head t i l t e d t o t h e r i g h t , t h e y b o t h f a l l i n t o the l e f t v i s u a l f i e l d . I n both conditions l e f t i n physical space remains c o n s t a n t .

1 9 8 6 ) . T h e v i s u a l d i s p l a y c o n s i s t e d o f t w o squareboxeslocated 10.0degrees t o e i t h e r s i d e of a f i x a t i o n p o i n t , and 9 d e g r e e s above t h e p l a n e of f i x a t t o n . The p a t i e n t was i n s t r u c t e d t o t i l t t h e head t o t h e l e f t o r t h e r i g h t by 9 0 d e g r e e s a n d t o f i x a t e a t t h e c e n t r a l s t i m u l u s . T h e y w e r e t h e n r e q u e s t e d t o p u s h w i t h t h e i n d e x f i n g e r of t h e r i g h t hand t h e r e s p o n s e b u t t o n when t h e s t i m u l u s ("x") appeared i n one of t h e two boxes. The s t i m u l u s was d i s p l a y e d f o r 150 msec. The r e s p o n s i v e n e s s i n t h e two s p a t i a l p o s i t i o n s was t e s t e d i n f o u r e x p e r i m e n t a l s e s s i o n s , two f o r e a c h h e a d - t i l t e d c o n d i t i o n . Each e x p e r i m e n t a l s e s s i o n c o n s i s t e d of 30 p r a c t i c e t r i a l s and 100 experimental t r i a l s . The r e s u l t s show t h a t , i n d e p e n d e n t of t h e v i s u a l f i e l d s t i m u l a t e d , t h e stimuluswhich occupied a r e l a t i v e l e f t p o s i t i o n y i e l d e d l o n g e r R T s t h a n t h e one on t h e r i g h t ( F i g u r e 2 ) . T h u s , evenwhen t h e two s t i m u l i a p p e a r e d i n t h e r i g h t v i s u a l f i e l d (RVF), s u b j e c t s responded more s l o w l y t o t h e t a r g e t i n the gravitational l e f t visual field.Furthermore, the responselatenciesto LVF s t i m u l i w e r e 1 o n g e r t h a n t h o s e t o R V F s t i m u l i .

206

M.S. Gazzaniga and E. Ladavas

DEFICITS I N PHYSICAL SPACE

Figure 2 R i g h t p a r i e t a l l e s i o n e d p a t i e n t s a r e s l o w e r t o respond t o t h e l e f t w a r d moststimulusnomatterwhichvisual f i e l d i s s t i m u l a t e d .

Taken t o g e t h e r , t h e p r e s e n t f i n d i n g s s u g g e s t t h a t t h e reduced a b i l i t y t o s h i f t a t t e n t i o n from one s p a t i a l l o c a t i o n t o a n o t h e r i s t i e d t o g r a v i t a t i o n a l c o o r d i n a t e s , i n a d d i t i o n t o r e t i n o t o p i c c o o r d i n a t e s . In s h o r t , these s t u d i e s argue f o r i n t e r p r e t i n g p a r i e t a l lobe function a s c e n t e r e d on d i r e c t i n g a t t e n t i o n i n terms of a c t u a l p h y s i c a l s p a c e i n a d d i t i o n t o r e t i n o t o p i c space. O t h e r s t u d i e s have now been c a r r i e d o u t t o examine t h e g r a v i t a t i o n a l e f f e c t i n a more n a t u r a l head/body p o s i t i o n , such a s t h e u p r i g h t p o s i t i o n , and t o f u r t h e r c h a r a c t e r i z e t h e n a t u r e of t h i s d e f i c i t i n t h e p r o c e s s i n g of g r a v i t a t i o n a l i n f o r m a t i o n . In a f o l l o w up s t u d y , Ladavas ( 1 9 8 4 ) , examined whether O K n o t t h e d e f i c i t was d i s p l a y e d on a continuum a l o n g a l e f t - r i g h t dimension i n p h y s i c a l space. F u r t h e r m o r e , t h e experiment was d e s i g n e d t o i n v e s t i g a t e whether t h e d e f i c i t r e f l e c t s a n i n c a p a c i t y t o d i s e n g a g e a t t e n t i o n f r o m t h e r i g h t m o s t s t i m u l u s in r e a l space. Visuomotor r e a c t i o n t i m e s t o t h r e e stimulihorizontallyaligned above t h e f i x a t i o n m a r k w e r e s t u d i e d . The s t i m u l i w e r e l o c a t e d t o t h e l e f t , t o t h e r i g h t , and d i r e c t l y above t h e f i x a t i o n mark. I f t h e f o c u s of a t t e n t i o n i s caught by t h e s t i m u l u s i p s i l a t e r a l t o t h e l e s i o n , and t h e a t t e n t i o n a l d e f i c i t is a s s o c i a t e d w i t h t h e gravitationalcoordinates, a f a c i l i t a t i o n o f

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processing e f f i c i e n c y a t the rightmost location plus a gradual increaseof RTs t o t h e two s t i m u l i l o c a t e d t o t h e l e f t of t h e i p s i l a t e r a l s t i m u l u s would be e x p e c t e d . I f , on t h e o t h e r hand, t h e d e f i c i t i s o n l y r e l a t e d t o t h e r e t i n o t o p i c c o o r d i n a t e s , we would e x p e c t a f a c i l i t a t i o n of p r o c e s s i n g e f f i c i e n c y a t t h e m i d l i n e p o s i t i o n i n comparison t o t h e o t h e r l o c a t i o n s , s i n c e c e n t r a l s t i m u l i occupy a l e s s e c c e n t r i c p o s i t i o n on t h e r e t i n a . We would a l s o e x p e c t a d e c r e a s e o f R T s toLVF s t i m u l i . Sixneurological subjects selectedaccordingtothesame c r i t e r i a a s i n t h e f i r s t e x p e r i m e n t s , and s i x p a t i e n t s w i t h o u t n e u r o l o g i c a l o r p s y c h i a t r i c d i s o r d e r s , matched a c c o r d i n g t o a g e , p a r t i c i p a t e d i n t h i s e x p e r i m e n t . The v i s u a l d i s p l a y c o n s i s t e d of t h r e e s q u a r e boxes l o c a t e d 21.5 d e g r e e s t o t h e l e f t , 15.8 d e g r e e s above, and 21.5 d e g r e e s t o t h e r i g h t of t h e f i x a t i o n p o i n t , a l l 15.8 d e g r e e s above t h e p l a n e of f i x a t i o n . The p a t i e n t was i n s t r u c t e d t o l o o k a t t h e c e n t r a l f i x a t i o n p o i n t and t o push t h e r e s p o n s e b u t t o n w i t h t h e i n d e x f i n g e r of t h e r i g h t hand when t h e s t i m u l u s ("x") appeared i n one of t h e t h r e e b o x e s . T h e s t i m u l u s w a s d i s p l y a e d f o r 150msec. The r e s p o n s i v e n e s s i n t h e t h r e e s p a t i a l p o s i t i o n s w a s t e s t e d i n t h r e e s e s s i o n s . Each s e s s i o n c o n s i s t e d of 30 p r a c t i c e t r i a l s and 90 e x p e r i m e n t a l trials. The r e s u l t s o b t a i n e d f o r b o t h p a t i e n t s w i t h e x t i n c t i o n syndrome and t h o s e i n t h e c o n t r o l g r o u p a r e d e p i c t e d i n Figure 3. A l l p a t i e n t s w i t h r i g h t p a r i e t a l l o b e i n j u r y showed a g r a d u a l i n c r e a s e of RTs t o t h o s e s t i m u l i l o c a t e d t o t h e l e f t of t h e r i g h t m o s t s t i m u l u s . They r e s p o n d e d s i g n i f i c a n t l y more q u i c k l y t o t h e r i g h t s t i m u l u s t h a n t o t h e c e n t r a l and l e f t s t i m u l i . Moreover, t h e i r r e s p o n s e l a t e n c i e s t o t h e c e n t r a l s t i m u l u s were s i g n i f i c a n t l y f a s t e r t h a n r e s p o n s e l a t e n c i e s t o t h e l e f t s t i m u l u s (p

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