Fish Physiology: Sensory Systems and Electric Organs

T Volume V Sensoy System and

Electric Organs

CONTRIBUTORS M. V. L. BENNETI'

0. LOWENSTEIN

J. DIAMOND

F. W. MUNZ

AKE FLOCK

R. W. MURRAY

TOSHIAKI J. HARA

WILLIAM N. TAVOLGA

DAVID INGLE

T. TOMITA

FISH PHYSIOLOGY Edited by W. S. HOAR DEPARTMENT OF ZOOLOGY UNIVERSITY O F BRITISH COLUMBIA VANCOUVER, CANADA

and

D . J. R A N D A L L DEPARTMENT OF ZOOLOGY UNIVERSITY OF BRITISH COLUMBIA VANCOWER, CANADA

Volume V

Sensory Systems and Electric Organs

(23

Academic Press New York and London

1971

COPYRIGHT @ 1971, BY ACADEMIC PRESS, INC. ALL RIGHTS RESERVED NO PART OF THIS BOOK MAY BE REPRODUCED IN ANY FORM, BY PHOTOSTAT, MICROFILM, RETRIEVAL SYSTEM, OR ANY OTHER MEANS, WITHOUT WRIlTEN PERMISSION FROM THE PUBLISHERS.

ACADEMIC PRESS, INC.

111 Fifth Avenue, New York, New York 10003

United Kingdom Edition published by ACADEMIC PRESS. INC. (LONDON) LTD. 24/28 Oval Road, London N W l ID’D

LIBRARY OF CONGRESS CATALOG CARDNUMBER: 76-84233

PRINTED IN THE UNITED STATES OF AMERICA

CONTENTS ix

LISTOF CONTRI~UTORS PREFACE

xi xiii

OF OTHERVOLUMES CONTENTS

1. Vision: Visual Pigments F. W. Munz 1

I. The Eye as an Optical System 11. Visual Pigments References

14 27

2. Vision: Electrophysiology of the Retina

T . Tomita I. 11. 111. IV. V. VI.

Introduction Electroretinogram Response of Single Ganglion Cells Response of Photoreceptors Responses in the Inner Nuclear Layer Retinal Mechanisms of Color Vision References

33 35 40 43 47 51 53

3. Vision: The Experimental Analysis of Visual Behavior

David Ingle I. 11. 111. IV. V. VI.

Introduction Relative Discrimination Weaknesses Configurational Properties of Shapes Perceptual Equivalence and Change in Spatial Position Selective Attention Toward a Unified Outlook on Visual Behavior References V

59 61 64 68 72 74 76

CONTENTS

vi

4. Chemoreception Toshiaki 1. Hara I. 11. 111. IV. V.

Introduction Anatomy of Chemical Sense Organs Behavioral Studies of Chemoreceptive Functions Electrophysiological Studies of Chemoreceptor Responses Biological Aspects of Chemoreception References

79 81 91 94 104 114

5. Temperature Receptors R. W. Murray I. Introduction 11. Thermal Sensitivity of Fishes 111. The Sense Organs Involved IV. Electrophysiology V. Thermal Responses of Other Sense Organs References

121 121 123 125 131 132

6. Sound Production and Detection William N . Tavolga I. 11. 111. IV. V.

Introduction Sound Production Sound Detection Acoustic Communication in Fish Problems and Prospects for the Future References

135 136 162 183 189 192

7. The Labyrinth 0. Lowenstein I. Structure 11. Function References

207 214 236

8. The Lateral Line Organ Mechanoreceptors Ake Flock I. 11. 111. IV. V.

Introduction Structure of the Sense Organ Sensory Excitation in the Hair Cell Transmission at the Sensory Synapse Initiation of Nerve Impulses

241 242 248 255 258

CONTENTS

VI. VII.

The Central Nervous System and Feedback Conclusion References

Vii

259 261 262

9. The Mauthner Cell 1. Diamond I. Introduction The Basic Anatomy of the Mauthner Neuron The Selective Activation of Mauthner Neurons The “Mauthner Reflex” The Spinal Circuitry The Anatomy of the Spinal Circuitry The Precision and Constancy of the Minimum Discrimination Time The Excitation of the Mauthner Cells The Functions of the Mauthner Cells References

11. 111. IV. V. VI. VII. VIII. IX.

265 267 271 278 284 297 310 315 331 344

10. Electric Organs

M . V. L. Bennett I. 11. 111. IV.

Introduction Electric Organs and Electrocytes Neural Control of Electric Organs Conclusions and Prospects References

347 355 460 483 484

11. Electroreception M . V. L. Bennett I. 11. 111. IV. V. VI. VII.

Introduction Distribution of Electroreceptors Tonic Electroreceptors Phasic Electroreceptors Receptor Function in Electroreception Evolution of Electrosensory Systems and Electric Organs Implications for Receptor Function in General References

493 496 503 520 544 561 564 568

AUTHORINDEX

575

SYSTEMATIC INDEX

587

SUBJECTINDEX

594

LIST OF CONTRIBUTORS Numbers in parentheses indicate the pages on which the authors’ contributions begin.

M. V. L. BENNETT(347, 493), Department of Anatomy, Albert Einstein College of Medicine, Yeshiva University, Bronx, N e w York

J . DIAMOND*(265), Department

of Physiology, University College

London, London, England Am FLOCK (241), King Gustaf V Research Institute, and Department of Otolaryngology, Karolinska Sjukhuset, Stockholm, Sweden TOSHIAKI J. H A R A(79), ~ Department of Physiology, Kumumoto University Medical School, Kumumoto, Japan

DAVIDINGLE (59), The Neurophysiological Laboratory, McLean Hospital, Belmont, Massachusetts 0. LOWENSTEIN (207), Department of Zoology and Comparative Physiology, University of Birmingham, Birmingham, England F. W. MUNZ (l), Department of Biology, Uniuersity of Oregon, Eugene, Oregon R. W . MURRAY(121), Department of Zoology and Comparative Physiology, University of Birminghum, Birmingham, England WILLIAMN . TAvoLGh ( 1351, Department of Biology, T h e City College of the City University of New York, New York, New York

T. TOMITA(33), Department of Physiology, Keio University, School of Medicine, Shinjuku-ku, Tokyo, Japan

* Present address: Department of Neurosciences, McMaster University, Hamilton, Ontario, Canada. f Present address: Fisheries Research Board of Canada, Freshwater Institute, Winnipeg, Manitoba, Canada. ix

This Page Intentionally Left Blank

PREFACE Volume V of this treatise is concerned with sensory systems and some aspects of function of the central nervous system. Sensory systems have been extensively studied in fish not only because of a wide general interest in the behavioral and sensory physiology of this group but also because, in many instances, fish are technically suitable for general studies of sensory systems and have certain receptors not present in other groups. Electroreceptors fall into this category; these receptors are unique to fishes, and studies of this system have application to receptor function in general. Electric organs, an effector rather than receptor system, are discussed in this volume because of the functional relationships between electroreception and electric organ discharge. The Mauthner neuron which is another system studied both to increase understanding of neuronal organization in fish and because the Mauthner cell constitutes a useful preparation for studying synaptic function and the integration of activity in neuronal networks in general is discussed in another chapter. Neurophysiology, particularly sensory physiology, is a very active area of biology. The chapters in this volume, perhaps more than in other volumes, can only present a summary of the present state of science in this rapidly expanding and developing field. We hope that this volume reflects some of the excitement and activity in sensory physiology and will be a useful introduction to students in this area of biology. W. S. HOAR D. J. RANDALL

xi

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CONTENTS OF OTHER VOLUMES Volume I The Body Compartments and the Distribution of Electrolytes W. N . Holmes and Edward M . Donaldson The Kidney Cbveland P. Hickman, Jr., and Benjamin F . Trump Salt Secretion Frank P. Conte

The Effects of Salinity on the Eggs and Larvae of Teleosts F. G. T . Holliday Formation of Excretory Products Roy P. Forster and Leon Goldstein Intermediary Metabolism in Fishes P. W. Hochachka Nutrition, Digestion, and Energy Utilization Arthur M . Phillips, Jr. AUTHORINDEX-SYSTEMATIC

INDEX-SUB

JECT INDEX

Volume I1 The Pituitary Gland: Anatomy and Histophysiology J. N . Ball and Bridget 1. Baker The Neurohypophysis A. M . Perks Prolactin (Fish Prolactin or Paralactin) and Growth Hormone 1. N . Ball Thyroid Function and Its Control in Fishes Aubrey Gorbmun xiii

XiV

CONTENTS OF OTHER VOLUMES

The Endocrine Pancreas August E p p b The Adrenocortical Steroids, Adrenocorticotropin and the Corpuscles of Stannius 1. Chester Jones, D. K . 0 . Chan, I . W. Henderson, and J . N . Ball

The Ultimobranchial Glands and Calcium Regulation D. Harold Copp Urophysis and Caudal Neurosecretory System Howard A. Bern AUTHORINDEX-SYSTEMATICINDEX-SUBJECT INDEX Volume I11 Reproduction Willium S. Hoar Hormones and Reproductive Behavior in Fishes N . R. Liley Sex Differentiation Toki-o Yamumoto Development: Eggs and Larvae J . H . S . Blaxber Fish Cell and Tissue Culture Ken Wolf and M . C . Quimby Chromatophores and Pigments Ryozo Fujii Bioluminescence J. A. C . Nicol Poisons and Venoms Findlay E . Russell AUTHORINDEX-SYSTEMATIC INDEX-SUBJECT INDEX Volume IV Anatomy and Physiology of the Central Nervous System Jerald 1. Bernstein

CONTENTS OF OTHER VOLUMES

xv

The Pineal Organ James Clurke Fenwick Autonomic Nervous Systems Graeme Campbell The Circulatory System D. 1. Randall Acid-Base Balance C. Albers Properties of Fish Hemoglobins Austen Riggs Gas Exchange in Fish

D. J. Randall The Regulation of Breathing G . Shelton Air Breathing in Fishes Kiell Johansen

The Swim Bladder as a Hydrostatic Organ Johan B. Steen Hydrostatic Pressure Malcolm S. Gordon Immunology of Fish John E . Cushing AUTHOR INDEX-SYSTEMATIC INDEX-SUBJECT INDEX Volume VI The Effect of Environmental Factors on the Physiology of Fish: An Examination of the Different Categories of Physiological Adaptation F . E. J. Fry Action of the Environment on Biochemical Systems P. W . Hochachka and G. N . Somero Freezing Resistance in Fishes Arthur L. DeVries

xvi

CONTENTS OF OTHER VOLUMES

Learning and Memory Paul Rozin and Henry Gleitman The Ethological Analysis of Fish Behavior G . P . Baerends Biological Rhythms H . 0 . Schwasmnn Orientation and Fish Migration A. D . H a s h Special Techniques D. J . Randall and W. S. Hoar

AUTHORINDEX-SYSTEMATICINDEX-SUBJECT INDEX

1 VISION: VISUAL PIGMENTS F . W.M U N Z

. . . . . . I. The Eye as an Optical System A. Structure of the Eye . . . . . . . B. Image Formation and Accommodation . . . . C. Light and Dark Adaptation . . . . . . D. Specializations for Deep-sea Life . . . . . 11. Visual Pigments . . . . . . . . . A. Photochemistry . . . . . . . . . B. Methods of Study . . . . . . . . C. A Choice of Retinenes: Rhodopsin and Porphyropsin D. A Multiplicity of Opsins . . . . . . E. Pigments of Color Vision . . . . . . References . . . . . . . . . . .

.

. . . . .

.

. . .

. .

. .

. . . .

. . . .

. .

1 1 5 8 12 14 14 15 19 23 25

27

I. THE EYE AS AN OPTICAL SYSTEM

The fascinating and detailed study by Walls (1942) remains a primary source of information on functional anatomy of the eye, but the less well known works of Rochon-Duvigneaud (1943, 1958) should also be consulted. Valuable reviews of the structure and function of fish eyes have been prepared by Brett (1957) and Nicol (1963). A treatment by Prince (1956) contains considerable information on fishes, but its usefulness is reduced by incomplete documentation. The present brief account is intended to give necessary background, together with the results of recent research; wherever specific reference is omitted, please refer to Walls (1942). A. Structure of the Eye The eyes of fishes are constructed along the general vertebrate plan (Fig, 1 ) , They are more or less flattened and have the normal complement 1

2

F. W. MUNZ

&epchoOriM

VFd

lymph space

susmnsorv I I I \

Fig. 1. Diagrammatic vertical section of a typical teleost eye. Not all structures shown are present in every teleost eye; e.g., hyaloid vessels are not present in conjunction with a falciform process. From Walls ( 1942).

of s i x oculomotor muscles. Except in cyclostomes, in which it is fibrous, the sclera is usually reinforced with cartilage. This is often calcified in elasmobranchs; teleosts frequently have one or two scleral ossicles in addition to cartilaginous support. Little refraction occurs at the corneal surface, for its refractive index approximately equals that of water. In air, however, the fish eye is myopic because of the added refraction. Baylor (1967b) has shown that in flying fish the corneal surface is pyramidal, with the lower third covered by a flat face. When the flying fish is airborne, its downward vision should be relatively undistorted. Many fastswimming teleosts have additional streamlining and protective structures, the transparent “adipose eyelids.” These vertical folds, which are really adipose in Mugil, but not in clupeoids or scombroids, overlie the cornea in various configurations. Many sharks have mobile eyelids; in some species there is an active nictitating membrane.

1.

VISUAL PIGMENTS

3

The lens is usually spherical and protrudes through the pupil. In lampreys it is held against the cornea by the vitreous humor; no suspensory ligaments or muscles are attached to the lens. In most teleosts the pupil is immobile, but in many elasmobranchs the iris is capable of extensive contraction. In the absence of corneal refraction, protrusion of the lens through the pupil assures a wide field of view. The eye accommodates by small movements of the lens. In teleosts, it is pulled backward by a retractor muscle; in elasmobranchs, it is pulled forward by a protractor muscle. The lens or cornea of many fishes contains pigments that filter out violet or ultraviolet radiation, probably improving visual acuity (Kennedy and Milkman, 1956; Denton, 1957; Motais, 1957). Little is known about the aqueous or vitreous humors; among different species, the vitreous ranges in consistency from a liquid to a firm gel. Continuous with the peripheral border of the iris is the complex tissue named “choroid” ( alternatively spelled chorioid) . The choroid combines the functions of nourishing the retina and of absorbing stray light or reflecting it by a tapetum lucidum back through the retina (Section I, C ) . In common with other nervous tissue, the retina has a high oxygen consumption (Lindeman, 1943). The innermost part of the choroid, lying just behind the retina, is modified into a choriocapillary structure. In most teleosts, but not in elasmobranchs, the choroid projects through the optic cleft into the posterior chamber of the eye, as the richly vascular, pigmented falciform process (Fig. 1; see Hanyu, 1959). The implication of a nutritive function for this process is strengthened by the occurrence of vitreal blood vessels closely applied to the retinal surface (Fig. 1) only in those species lacking the falciform process (e.g., eels, puffers, and anglerfishes). Most teleosts ( and Amia) have a peculiar, specialized “choroid gland,” which is actually a rete mirabile, located behind the retina. Its structural similarity to the gas gland of teleost swim bladders led Wittenberg and Wittenberg (1962) to measure the oxygen pressure in the eyes of living marine fishes. In the vitreous, immediately in front of the retina, the partial pressure of oxygen was highest (average values 250420 mm Hg) in teleosts with a prominent rete. Teleosts with smaller retia had lower oxygen pressures (20-210 mm); elasmobranchs and teleosts which lack a choroid gland had still lower pressures ( 10-20 mm) . Clearly, active secretion of oxygen (the second case known in animals) is associated with the choroid gland. Those fishes that have lost the pseudobranch invariably lack a choroid gland also. Only a general description of the retina is given here; additional information may be found in the section on visual electrophysiology. Fish retinae are organized according to the ordinary vertebrate plan. Innermost are the various neuronal and glial elements, which are relatively transparent. Light passes through these to the photoreceptor (“visual”) cells.

4

F. W. MUNZ

Outermost, adjacent to the choroid, is the pigment epithelium (Section I, C). The visual cells of lampreys are usually of two morphologically distinct types, but their affinities with rods and cones of other vertebrates are not certain (Walls, 1942). Elasmobranchs and teleosts each depart from the familiar duplex pattern of vertebrates, but in different ways. Most elasmobranchs are thought to have pure-rod retinae, but some sharks ( Mustelus, Lamna, Squutina, Negaprwn, Carcharinus, Sphyrna, and Ginglymostm) and rays (Myliobatis and Dasyatis) are reported to have cones as well as rods (Walls, 1942; Rochon-Duvigneaud, 1943; Gruber et al., 1963; Hamasaki and Gruber, 1965; Tamura and Niwa, 1967). In contrast, teleosts typically have both cones and rods. In addition to the ordinary, single cones, they also have peculiar visual cells called “twin cones.” These differ from the double cones found in most vertebrate groups, in that the “twins” in each cell pair are morphologically similar and fused longitudinally. Certain teleosts, in which a tapetum is well developed, lack single cones (e.g., anchovies; see Tamura, 1957; OConnell, 1963). In many deep-sea fishes, cones are entirely absent (Section I, D). It is probably typical of teleosts that the visual cells are not distributed uniformly over the retina. Commonly there is a specialized temporal “area” in which cones are more numerous. The area may represent a concentration of twin cones alone (as in anchovies, which lack single cones) or of both twin and single cones ( OConnell, 1963). A cone-rich area also occurs in the shark, Mustelus. The fovea is a further retinal specialization for increased visual acuity and consists of a depression or pit in the retina, overlying the area of more numerous (and often smaller) visual cells. Blood vessels are typically absent from the fovea. The occurrence of a fovea is accompanied by the ability to fixate the image of a moving object on this special region by voluntary eye movements. Fixation is normally associated with binocular vision. In the laterally placed eyes of fishes, it is not surprising, therefore, that foveae are usually located near the posterior (temporal) border of the retina. Rods and twin cones are nearly or entirely absent in the fovea, which has concentrations of single cones. The pupil is usually larger than the lens in these eyes; in other words, there is an aphakic space anterior to the lens (Walls, 1942). This reduction of the iris allows light from objects directly ahead of the fish to be focused on the fovea. Retinal images of objects viewed to the side must be degraded, of course, by light passing through the aphakic space. Actually, foveae are rare in teleosts; they have been described in about 20 littoral marine species (Kahmann, 1936; Baron and Verrier, 1951). Foveae have also been reported in freshwater species of Fundulus and Umbridw (Prince, 1956). Temporal foveae occur in the

1.

VISUAL PIGMENTS

5

pure-rod retinae of several genera of deep-sea fishes: Platytroctes and Bathytroctes (Brauer, 1908; the latter may be Sear&, according to Munk, 1966), Bathylugus (Vilter, 1954a,b; Munk, 1966), Scopelosaurus and Searsia ( Marshall, 1966), and Platytroctegen ( Munk, 1966). Because several families are represented, it seems probable that pure-rod foveae have evolved more than once. As in shallow-water teleosts, a prominent aphakic space is correlated with these temporal foveae (Marshall, 1966). One other pure-rod fovea has been reported in the rhynchocephalian, Sphenodon (Walls, 1942), but Vilter (1951) has shown that its visual cells are cones. Therefore, the pure-rod foveae of deep-sea fishes are unique.

B. Image Formation and Accommodation In all groups of fishes the corneal index of refraction is about the same as that of water (1.33)and the ocular humors. Refraction and image formation, therefore, depend almost entirely upon the lens. The lens is spherical, with a very high effective index of refraction (about 1.67). Since the maximum index for any transparent material of biological origin is about 1.53 (Pumphrey, 1961), the fish lens cannot be homogeneous. That its refractive index is highest at the center (1.53) and gradually decreases (to 1.33) toward the outside was confirmed by Pumphrey ( 1961), who showed that the lens has no spherical aberration. This permits unaberrated image formation without stopping down the lens ( f / 0 . 8 in teleosts). Stopping down would be disadvantageous because the lack of corneal refraction means that the lens must protrude through the iris to achieve a wide visual field. This same gradation of refractive index from 1.53 to 1.33occurs in the small lenses of young fish and large lenses of older fish. The lens substance shows concentric discontinuities, suggestive of growth increments; but the way in which the index of refraction of the inner and outer parts could be altered continuously and differentially during growth is not understood. The lens lacks any appreciable chromatic aberration, but the means by which this is achieved is also unknown (Pumphrey, 1961). An almost constant feature of fish eyes is the distance from the center of the lens to the retina divided by the radius of the lens. This ratio is about 2.55 ( Matthiessen’s ratio). Accommodation results from changing the distance between the lens and the retina rather than altering lenticular shape. Lampreys have a unique corneal muscle that inserts on the spectacle covering the eye. When it contracts, the cornea is flattened, pushing the lens closer to the

6

F. W. MUNZ

retina and bringing more distant objects into focus. In elasmobranchs there are smooth muscle fibers in the ciliary body that pull the lens closer to the cornea. (The ciliary body is a continuation of the choroid that separates the anterior and posterior chambers and from which the iris projects. ) Thus, accommodation would bring near objects into focus. This mechanism has not been demonstrated unequivocally in elasmobranchs ( Nicol, 1963). Pumphrey (1961) has stated very clearly what he believes is the method of accommodation in trout and, probably, in other teleosts. When a distant object to the side of the animal is viewed by the unaccommodated eye, its image is focused on the retina (Fig. 2A). Because of the short focal length of the lens, closer objects are imaged only a few microns farther from the lens. Therefore, even quite nearby objects should also be in good focus. During accommodation, the retractor lentis muscle (of ectodermal origin) moves the lens posteriorly, rather than toward the fundus (back) of the eye (Fig. 2A). Laterally placed objects should still be imaged on the retina fairly sharply. The view of objects directly in front of the trout is very different. The retina is not a hemisphere concentric with the lens, but it is somewhat ellipsoidal. For such objects viewed by the unaccommodated eye, the distance from lens to retina is slightly greater than for laterally placed objects. To the front, therefore, the trout is near-sighted (myopic); Pumphrey stated that objects between 10 and 20 cm away would be seen clearly (Fig. 2B). At rest, the eyes are well adjusted for binocular viewing of any prey at close range. The effect of accommodation is to decrease the distance from lens to retina, bringing more distant objects into focus. In this connection, recall the temporal location of fish foveae and specialized retinal areas. Certain data recently obtained by Baylor and Shaw (1962) are relevant to accommodation and Pumphrey’s interpretation. In the alewife, Alosa, the retina is also ellipsoidal rather than spherical. Retinoscopic measurements were made of refractive error in the eyes of living, immersed, marine fishes ( 5 elasmobranch and 17 teleost species). The eyes were all far-sighted (hypermetropic). In the alewife and silversides, Menidia, measurements were made from positions lateral and anterior to the fish. From the anterior position, the average refractive error was 5 (alewife) or 8 (silversides) diopters less than from the lateral position. These differences are consistent with the ellipsoidal shape of the retina and are within the range of accommodation observed by Baylor and Shaw. The difficulty seems to be whether the unaccommodated eye is normal-sighted (emmetropic) as stated by Pumphrey ( 1961) or farsighted. A crucial question is what specific ocular structure is responsible for the retinoscopic reflection. Baylor and Shaw stated that reflection comes from behind the retina; Nicol (1963) has indicated that this point

1.

7

VISUAL PIGMENTS

anterior :

i posterior

(B) Fig. 2. ( A ) Diagrammatic horizontal section of the left eye in relation to the longitudinal axis of the fish‘s body, ts. Position of the lens at rest (solid line) and in full accommodation (broken line). ( B ) Diagrammatic representation of the horizontal visual field of a fish, eyes unaccommodated, showing the area in focus (diagonal lines). The triangular open area in front of the fish lies beyond the “far point”; the circular open area to the sides is within the locus of the “near point” (interrupted posteriorly by the shadow of the fish‘s body). Redrawn after Puniphrey ( 1961).

should be reexamined. At any rate, it now seems clear that fishes are not myopic, at least to the side, as some earlier authors maintained (Walls, 1942; Brett, 1957). Recently, Baylor ( 1967a) has examined nine species of teleost reef fishes. Their behavior suggested that they can accommodate for near vision; using the retinoscopic method, Baylor found that they tend to be emmetropic. When frightened, they often displayed an aggressive posture, in which they turned so as to view the human observer binocularly. In this situation, the eyes became hypermetropic; these observations would seem to fit Pumphrey’s treatment of image formation and accommodation (see Fig. 2 ) . In comparing his most recent results with the earlier findings of Baylor and Shaw (1962), Baylor ( 1967a) stated that “, . . it can be tentatively concluded . . . that reef fishes are

8

F. W. MUNL

better adapted for close-up vision than open-water fishes.” Some of the species examined in the earlier study, however, are benthic (e.g., Prionotus, Paralichthys, and Pseudopleuronectes ) and would presumably gain advantage from emmetropic rather than hypermetropic sight. An abstract of recent work by Bogatyrev (1966) indicates that fishes can focus their eyes for sharp vision over a wide range, from a near point of 5 cm or less out to infinity, in apparent agreement with the conclusions of Pumphrey. This problem merits further study. In addition to accommodation, several other devices can have somewhat the same result, but without requiring any active mechanism (Walls, 1942). (1) A pinhole pupil produces a fairly sharp image regardless of the distances from it to the object and to the retina. When light adapted, such elasmobranchs as Scyliorhinm and Raja have a pupil with a very small aperture. ( 2 ) A “ramp” retina, which is tilted away from the lens, could simultaneously have in focus images of objects located at different distances. In Raja, the upper portion of the retina is farther from the lens than the lower portion. Objects nearby on the ocean bottom could, therefore, be in focus at the same time as distant objects located above the animal. ( 3 ) The fact that the outer segments of the visual cells have considerable length means that objects at various distances would be equally in (or out of! ) focus; presumably this has more to do with increasing sensitivity than with accommodation. (4) Another structural modification is to have the eye permanently set for vision at two particularly useful distances; Walls suggested the analogy of bifocal spectacles. Subdivision of the retina into two parts in the tubular eyes of certain deep-sea fishes is one example (Section I, D ) . Better known is the “four-eyed” fish ( Anubbps; see Walls, 1942; Schwassman and Kruger, 1965). Anableps swims at the surface with its eyes partly out of water. The pupil of the light-adapted eye is divided horizontally by flaps of the iris. Objects in air are imaged on the ventral part of the retina, those in water on the dorsal part. The lens is egg-shaped; compensating for the lack of corneal refraction under water, there is greater curvature at the lens surfaces concerned with aquatic than with aerial vision. The two parts of the retina are specialized for their different functions ( Inouye and Noto, 1962; Schwassman and Kruger, 1965). C. Light and Dark Adaptation

Gnathostome fishes can change the effective light intensity at the receptor level by several means. Pupil movement and photomechanical

1. VISUAL

PIGMENTS

9

changes in the retina will be described, together with the tapetum lucidum. In some species, the tapetum can be occluded by a migration of black pigment, which is the reason for its mention here. The most familiar components of light and dark adaptation-changes in concentration of visual pigment and modifications in neuronal interaction-are described in Chapter 2. With the possible exception of deep-sea forms, elasmobranchs usually have a highly mobile pupil. According to Young (1933a), Nicol (1964), and Kuchnow and Gilbert ( 1967), the pupil closes quite rapidly (2-15 min) after the eye is exposed to light. Dilatation in darkness occurs more slowly, requiring 30 min or more. In elasmobranchs, Young (1933a) found that the irideal sphincter contracts in direct response to illumination and is not under nervous control. The dilatator muscle is innervated by the oculomotor nerve. Anguillu, one of very few teleosts with appreciable mobility of the pupil, has been studied by Seliger ( 1962). Light has a direct effect on excised pieces of iris, causing the sphincter muscle to contract. The action spectrum for this effect has a primary maximum near 500 nm, suggesting that the light may be absorbed by a rhodopsinlike pigment. In darkness the iris relaxes. The iris of Uranoscopus and Lophius has dual innervation, oculomotor to the dilatator and sympathetic to the sphincter muscles (Young, 1931, 1933b). Some flatfishes have a mobile pupillary operculum, but not all (Nicol, 1965a). In the great majority of teleosts, photomechanical (also called “retinomotor”) movements take the place of pupillary control of light intensity at the retinal level. The main processes were reviewed by Walls (1942), Brett ( 1957), and Nicol ( 1963) and need only be summarized. The outermost retinal cell layer is called the “pigment epithelium.” Long processes of the pigment cells extend toward the visual cells and interdigitate with their outer segments. In the dark-adapted eye, melanin granules within these pigment cells are drawn back, away from the visual cells (Fig. 3). Exposure to light is soon followed by migration of pigment granules into the processes. The visual cells have a contractile “myoid region proximal to the outer segment. When the rod-cell myoids change length, the outer segments move counter to the melanin in the pigment epithelium. Under conditions of dark adaptation, the rod outer segments are pulled proximally, toward the focal plane; and the pigment is retracted. In the lightadapted retina, the expanded pigment surrounds and shields the extended outer segments of the rods. The cone outer segments move also, in a manner opposite to the rods. They are never shielded by the pigment epithelium. Perhaps their extension in the dark-adapted retina merely clears the focal plane for the rod outer segments. In Ameiurus ( = Zctalurus), photomechanical movements follow a die1 cycle (Welsh and Osborn, 1937).

10

F. W. MUNZ

Light-adapted

Dark-adapted A

A

P. f

P.

i

V.

t

m. A m. 1

5

r

V

i J

I

0.n.i.

bl

J

Fig. 3. The retina of Clupea drawn semidiagrammatically in the dark- and lightadapted state. Key: bJ., bipolar layer; c., cone; e . , ellipsoid; e l m . , external limiting membrane; ni., length of cone inyoids; o.n.l., outer nuclear layer; o.s., outer segment; p., pigment epithelium; r., rod; and v., visual cell layer. From Blaxter and Jones (1967).

Additional information on photomechanical responses has recently been published by several authors. In young Oncorhynchus, light adaptation is complete in 20-25 min, but dark adaptation takes about an hour (Brett and Ali, 1958). As the salmon grow older, the time for light adap-

1. VISUAL

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11

tation shortens, but the time required for dark adaptation tends to increase (Ali, 1959). Light adaptation is more rapid and complete when Salmo is exposed to high intensities; after a preliminary exposure to bright light, dark adaptation takes longer ( Ali, 1962). Albino Salvelinus, which lack retinal melanin, undergo normal photomechanical migrations of rod and cone outer segments ( Ali, 1964b). Three flatfishes ( Microstomus, Pleuronectes, and Solea) that have immobile pupils have normal photomechanical movement ( Nicol, 1961b, 1965a); flatfishes with mobile pupils have not been investigated. The retina of Carassius shows a persistent circadian rhythm in its photomechanical changes. The cones of fish kept in constant darkness for 3 days continued to shift positions in synchrony with the die1 cycle (John et al., 1967). Goldfish were restrained and anesthetized by Ali (1964a), and then one eye was exposed to a bright light. The rods and cones of the dark-adapted eye did not move, but the retinal melanin expanded partially. Ali suggested that pigment migration may be influenced by hormones. Walls (1942) commented on the confusion surrounding the mechanisms controlling photomechanical movements; the situation is still far from clear. A mirror or tapetum lucidum at the back of the eye is a common device to increase the visual sensitivity of nocturnal animals. Although several morphologically distinct types were described by Walls ( 1942), only three are common in fishes. A retinal tapetum occurs in the pigment epithelium of many freshwater fishes ( cyprinids and percids ) . The epithelial cells contain particles or crystals of the reflective substance guanine. Melanin is present in the same cells and migrates normally, occluding the tapetum in bright light. A nonocclusible tapetum of the same type was said to occur in pelagic deep-sea teleosts (Walls, 1942). This statement was evidently based on the investigation of Brauer ( 1908), but Munk (1966) has not found retinal or choroidal tapeta in any deepsea teleost. Some surface-dwelling marine teleosts have a fibrous, choroidal tapetum which is shiny, like a tendon; this type is not occlusible. Almost all elasmobranchs have a choroidal tapetum lucidum just external to the choriocapillary layer; Myliobatis, a pelagic ray, seems to be the only exception (Denton and Nicol, 1964). The tapetum consists of flattened, often imbricate cells that contain guanine (Denton and Nicol, 1965; Best and Nicol, 1967). The reflecting plates are typically oriented perpendicularly to the light incident at each region of the retina. Reflection from these plates is highly directional (specular), which should minimize blurring of the image (Denton and Nicol, 1964, 1965). In benthic, neritic species (e.g., Scyliorhinus) the fundus has a black ventral area; a tapetum occurs elsewhere and is not occlusible (Nicol, 1961a,

12

F. W. MUNZ

Fig. 4. A sketch showing diagrammatically the geometric arrangements of reflecting cells and melanophore cell processes in the tapetum lucidum of Squalus. Illumination is normal to the surface of the plates. The pigment is shown in three stages of expansion. From Denton and Nicol (1964).

1964). The tapetum is also not occlusible in several deep-sea forms (squaloid sharks, Raja richardsonii, Hydrolagus afinis; Nicol, 1964). Active, pelagic sharks of the neritic zone (e.g., Squalus and Mustelus) have an occlusible tapetum over the entire fundus (Nicol, 1964). Reflectivity of the dark-adapted tapetum of Squulus is about 85%.When the fish is illuminated, choroidal melanophores send pigment out over the reflecting cells in order to conceal them (Fig. 4).The pigment retreats in darkness, this process requiring about 2 hr, in either direction. In Negaprion, the pigment migration is somewhat more rapid (Kuchnow and Gilbert, 1967). The pigment cells of the choroid seem to behave as independent effectors which are sensitive to light ( Nicol, 196%). Pigment migration is decreased by anoxia but is independent in each eye and is unaffected by nerve cutting, excision of endocrine glands, or administration of drugs. Nicol (1964) listed the elasmobranchs known to have occlusible, partly occlusible, and nonocclusible tapeta and suggested that the tapetum is concealed (except in deep-sea forms) either by pupil closure or by migration of pigment in order to avoid displaying eye-shine. The lack of retinal photomechanical movements in elasmobranchs is consistent with the typically pure rod retina. Absence of melanin in the retinal “pigment epithelium” is correlated with the development of a choroidal tapetum, and a mobile pupil allows this basically nocturnal visual system to cope also with higher light intensities.

D. Specializations for Deep-sea Life Among the most fascinating specializations for vision in a particular habitat are those of deep-sea animals. Ever since Brauer’s treatise (1908) revealed the startling diversity of ocular structures in deep-sea fishes,

1. VISUAL

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13

authors have speculated on their functional significance (see review by Marshall, 1954). Certain of these fragile animals can be caught at night at the ocean surface, but their vision has scarcely been studied. Anatomical investigations, however, have continued ( Munk, 1959, 1963, 1964a,b, 1965a,b, 1966; Pearcy et al., 1965). Many ocular features of deep-sea fishes must increase visual sensitivity. In deep-sea elasmobranchs and teleosts the retina typically contains only rods, which may be extremely numerous and have very long outer segments. The one well-established exception is Omosudis, which has an almost pure-cone retina (Munk, 1965b). Some teleosts have their rod outer segments arranged in several distinct layers (Vilter, 1954a; Munk, 1963, 1966; Pearcy et al., 1965). Presumably this arrangement increases sensitivity, perhaps without decreasing resolving power; other suggestions were also discussed by Munk (1963, 1966). Black pigment is often absent from the back of the eye, and a tapetum may be well developed in elasmobranchs (Denton and Nicol, 1964). Weale ( 1955) pointed out that binocular vision may approximately double the monocular sensitivity, the increase being as much or more than that from a tapetum. Binocular vision is well developed in a number of teleost groups, most extravagantly in those with tubular eyes (Brauer, 1908; Walls, 1942; Munk, 1966). The eye is elongate and the lens very large; the iris is absent and the mechanisms of accommodation are rudimentary or absent (Fig. 5A). The main retina is at the bottom of the tube (Matthiessen’s ratio still holds true, however); in effect, the tubular eye is equivalent to the axial part of a much larger conventionally shaped eye. Lightgathering power is increased at the expense of narrowing the visual field. This is usually compensated to some extent by an accessory retina located along the inner wall of the eye where it touches the lens. Presumably, it detects light and motion in a larger visual field. Tubular eyes are often directed upward (e.g., Argyropelecus and Opisthoproctus), but in some species are turned forward (Giganturu and Winteria). This orientation is probably related to the animal’s method of prey-capture or other behavior ( Clarke, 1963). Degeneration of the eyes in fishes from great depths has been examined by Munk (1964a, 1965a). The most interesting of these forms is Zpnops. This benthic animal has a spatulate snout, with transparent bony plates covering the orbital area. Beneath these bones are odd flattened organs that have been described either as photophores (Walls, 1942) or as modified eyes. Although they lack cornea and lens, Munk (1959) has shown unequivocally that they are eyes, with rod cells and optic nerve. Another very peculiar fish ( Bathylychnops) has eyes (Pearcy et al.,

14

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Fig. 5. Specialized eyes of deep-sea fishes. ( A ) Dorsally directed tubular eye of Scopelarchus. Thin arrow points at a piece of the accessory retina located below and medially to the optic nerve (from Munk, 1966, after Brauer, 1908). ( B ) Eye of Bathylychnops as seen in vertical section at right angles to equator of eyeball. Thin arrow points at choroid fissure (from Munk, 1966). Key: ar, accessory retina; c, cornea; i, iris; 1, lens; Id, laterad; lm, lens muscle; Ip, lens-pad; mr, main retina; on, optic nerve; r, retina; s, sclera; sg, secondary globe; sl, scleral lens; and w, window of the diverticulnm-retina. Thick arrows point to corneal borders.

1965; Munk, 1966) that put to shame the claim of AnabZeps to being called “four-eyed.” The primary globe of each eye is located on the flattened snout and directed dorsally, with a large binocular field. There is a secondary globe (Fig. 5B), which overhangs the jaw and is directed ventrally and slightly caudally. The two globes are continuous, but a flap keeps light from passing between them. The retina of the secondary globe is a diverticulum of the main retina. Although living individuals have been observed, the functions of this unique visual arrangement remain a matter for speculation. 11. VISUAL PIGMENTS

A. Photochemistry

The known visual pigments consist of a protein (called “opsin”), conjugated with a prosthetic group, the aldehyde of vitamin A (“retinene” or “retinal”). When light is absorbed, the retinene is isomerized, initiating a series of chemical rearrangements in the opsin. Excitatory events lead-

1. VISUAL

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15

ing to vision are triggered at some early stage in this sequence. The general processes of visual photochemistry are familiar and require no restatement (see Dartnall, 1957, 1962; Morton and Pitt, 1969; Wald, 1959, 1960). Abrahamson and Ostroy ( 1967) have recentIy reviewed the complex chain of events following illumination and have evaluated current theories about the structure of visual pigments and initiation of vision. These concepts have been derived from studies of the visual pigments of frogs and cattle and are not within the scope of the present review. Interest in the visual pigments of fishes derives primarily from their great diversity, which is discussed below. These differences are based on alterations of both parts of the molecule: a series of species-specific opsins has been described (e.g., Bridges, 1965a; Dartnall and Lythgoe, 1965) and two retinenes are known (Wald, 1959, 1960). Retinene,, the aldehyde of vitamin Al, has one double bond in its ring structure, whereas retinene, (derived from vitamin A,) has two. In principle, any opsin can combine with either retinene; in this way a pair of visual pigments can be formed. A number of these pairs have been described (Bridges, 1965a; Dartnall and Lythgoe, 1965; Munz and Schwanzara, 1967). The nomenclature of visual pigments has been a source of confusion (Dartnall, 1962, p. 389). It is convenient to attach names to the series of visual pigments based on each of the two retinenes. Visual pigments based on retinene, may be called “rhodopsins” and those based on retinene,, “porphyropsins,” without regard to their origin in rods or cones. This usage is operational, for the origin of extracted visual pigments is usually not known with certainty. Some authors would prefer to restrict these names to the visual pigments of rods (e.g., Wald, 1959, 1960)) but this limitation is unworkable at present (see Dartnall, 1962, p. 505). B. Methods of Study 1. PARTIALBLEACHING

The primary method for investigation and identification of visual pigments is known as partial bleaching. It has been developed chiefly by Dartnall, who has described the method in detail (Dartnall, 1962). Retinal extracts usually contain hemoglobin or other light-absorbing substances, in addition to visual pigments. Identification of any one component in the composite absorbance spectrum is made uncertain by the presence of the others ( Fig. 6A, curve 1 ) . Fortunately, visual pigments are photosensitive (i.e., bleached by light), but the usual impurities are not. (In Fig. 6A, note that as the absorbance of the visual pigment decreases in the visible region, a product of bleaching appears in the ultra-

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16

Wavelength (nrn)

Fig. 6. Partial bleaching experiment with a single retinenel pigment extracted from Scutophugus. ( A ) Curve 1, initial absorbance spectrum; curve 2, after 12 min exposure to k 660 nm light; curve 3, after 15 min further exposure to k 660 nm light; curve 4, after 6 min exposure to k 610 nm light. ( B ) Difference spectra; each bleaching operation shown as absorbance loss for the visual pigment and absorbance gain for the products of bleaching. Curves 1-2 and 2-3, results of k 660 nm irradiation; curve 3-4, result of k 610 nm irradiation. From Schwanzara ( 1967).

violet; this will be discussed later.) The absorbance changes that follow exposure to a bleaching light are called ''difference spectra" ( Fig. 6B). Under properly controlled conditions (temperature, pH, etc.), the difference spectrum closely approximates the absorbance spectrum of the pure visual pigment, over a broad wavelength region. It is important that the visual pigment be bleached in stages (Fig. 6) rather than all at once. If only a single visual pigment is present in the extract, the difference spectra are all the same, first to last, whatever the color of the bleaching light (Fig. 6 B ) . Suppose, on the other hand, that the unbleached extract contains a mixture of two visual pigments, both

1.

17

VISUAL PIGMENTS

photosensitive, in addition to stable, light-absorbing impurities ( Fig. 7A, curve 1 ) . If one of the visual pigments is more sensitive to the red light to which the extract is initially exposed it will be bleached preferentially, and the first difference spectrum will represent the absorbance spectrum of the red-sensitive component ( Fig. 7B, curve 1-2). Continued exposure to red or orange light will eventually destroy the remaining red-sensitive pigment, together with some of the second pigment, which is less sensitive to red light. Finally, this second component can also be bleached by an appropriate light and its difference spectrum plotted (Fig. 7B, curve 3-4). This description of the method is somewhat simplified; its successful application requires considerable skill. Mixtures have been

Wavelength (nm)

Fig. 7. Partial bleaching experiment with a retinal extract from Ctenobrycon. Both retinenel and retinenet components present in approximately equal amounts. ( A ) Curve 1, initial absorbance spectrum; curve 2, after 30 min exposure to h 661 nm light; curve 3, after 60 min exposure to h 641 nni light; curve 4, after 6 min exposure to h 610 nm light. ( B ) Difference spectra. Curve 1-2, result of ?, 661 nm irradiation; curve 2-3, result of h 641 nm irradiation; curve 3 4 , result of h 610 nm irradiation. From Schwanzara ( 1967 ).

F. W. MUNZ

500

600

Wavelength (nm)

Fig. 8. Identification of A,,,,, of the visual pigment pair extracted from Ctenohrycon. Curve 1, difference spectrum (curve 3-4 from Fig. 7 B ) ; curve 2, constructed from Dartnall's nomogram assuming A,,:, equal to 503 nm. Curve 3, difference spectrum (curve 1-2 from Fig. 7 B ) ; curve 4, constructed from the nomogram of Munz and Schwanzara, assuming A,,,3x equal to 527 nm. Each curve scaled to 100%at its maximum. From Schwanzara ( 1967 ).

found so often that Dartnall and Lythgoe (1965) regarded partial bleaching as a necessary part of the characterization of any visual pigment. Identification of visual pigments is facilitated by Dartnall's observation (1953) that their absorbance spectra have the same shape when plotted on a scale of frequency rather than wavelength. Any visual pigment may be described therefore by specifying the prosthetic group (retinene, or retinene,) and the wavelength of maximal absorbance (h,,,,,). The absorbance spectrum of any retinene, pigment can be constructed from the nomogram devised by Dartnall (1953). Retinene, pigments have a somewhat broader absorbance spectrum (Bridges, 1967) and require a different nomogram (Munz and Schwanzara, 1967). In favorable cases, appropriate nomogram curves can be fitted to both components in a mixture of visual pigments (Fig. 8). If individuals of a particular species possess mixtures of two known visual pigments (such as a rhodopsin-porphyropsin pair, based on the same opsin) , the proportions of the two can be estimated (Dartnall et al., 1961; Munz and Beatty, 1965). 2. ANALYSISOF RETINENES

Two colorimetric methods are used to analyze retinene. In the CarrPrice reaction, a characteristic blue color appears after an antimony tri-

1. VISUAL

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19

chloride reagent is added to a chloroform extract containing retinene. The blue color quickly fades, necessitating a highly standardized procedure for reproducible results. Water and some other substances can cause turbidity, upsetting the analyses; sometimes acetic anhydride is added to reduce this turbidity. Finally, the spectra of the absorbance bands seem to be variable, A,, reported for the retinene, band varying from 661 to 664 nm, and for retinene, from 705 to 741 nm (Grangaud et al., 1962; Naito and Wilt, 1962; Plack, 1961; Wald, 1939a; Wilt, 1959). The second method uses the results of partial bleaching experiments. The difference spectrum of the product of bleaching (Figs. 6 and 7 ) can be used to characterize retinene, and retinenez (Crescitelli, 1958). The A,, values are less variable than in the antimony trichloride reaction; and the presence of stable, light-absorbing impurities is unimportant. Both methods appear to be reliable in the hands of experienced workers. 3. VISUAL PIGMENTS IN PHOTORECEPTORS

The light absorption of visual pigments is the same, whether they are solubilized in retinal extracts or are in situ in suspensions of the visualcell outer segments (Dartnall, 1961, 1962). When the outer segments are oriented, however, as in the retina, light absorption is quantitatively greater than in randomly oriented suspensions or in extracts (Denton and Wyllie, 1955). A method devised by Denton (Denton and Warren, 1957; Denton and Walker, 1958; Denton, 1959; Denton and Nicol, 1964) uses the intact retina. The light absorption is measured at several wavelengths before and after bleaching the visual pigment. Results are similar to the more precise measurements obtained from retinal extracts but generally lack the essential qualification that homogeneity must be tested by partial bleaching. Nevertheless, they have the advantage of giving information on how much of the light incident on the retina is absorbed there. To anticipate slightly, deep-sea fish retinas can capture a very large fraction of the visual pigment, this may be more of the incident light; at A,, than 90% (Denton and Warren, 1957; Denton, 1959). The intact retina is also used in the method of microspectrophotometry (Section 11, E ) .

C. A Choice of Retinenes: Rhodopsin and Porphyropsin Wald (1936, 1939b) showed that certain freshwater fishes have a different visual pigment from marine fishes. Finding mixtures of both visual systems in some euryhaline species ( Wald, 1941), he proposed an elegant generalization (Wald, 1947, 1958, 1959, 1960) that is widely accepted. According to this view, marine fishes have rhodopsin, the visual pigment based on retinene,. Freshwater fishes have a different prosthetic

20

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group, retinenez; he called their visual pigment “porphyropsin.” Fishes that migrate between the sea and freshwater have mixtures of the two visual pigments, with the pigment which corresponds to the salinity of the spawning habitat predominating in the mixture. Freshwater fish families are not a homogeneous group but have been placed by zoogeographers in three divisions, according to their salinity tolerance and presumed evolutionary history ( Darlington, 1957) . Families in a primary division are restricted to freshwater and “have probably been confined to fresh water so long that their present distributions are the result of dispersal through fresh water, even though their remote ancestors may have lived in the sea” (Darlington, 1957, p. 46). Fishes in a secondary division have greater salinity tolerance and may have dispersed through the ocean. Peripheral freshwater fishes, “although found in fresh water, are somehow closely connected with the sea or have been so recently derived from it that their present distributions may be and often are largely the result of dispersal through the sea” (Darlington, 1957, p. 47). The physiological salt tolerance of a given species, of course, may not fit the probable evolutionary history of the family. The primary division constitutes what should be called freshwater fishes in a strict sense. Two families (Characidae and Cyprinidae) make up a very large proportion of this group. “Euryhaline” is a relative term, which is roughly equivalent to the secondary and peripheral divisions. Many peripheral freshwater fishes migrate between the sea and freshwater, but most secondary species do not. These divisions by zoogeographers seem to provide the most rational background against which to examine the distribution of visual pigments. Wald’s simple pattern does not adequately describe the experimental results obtained by several investigators over the last 20 years; for a competent review, see Schwanzara (1967). The most striking discrepancy is the common occurrence in primary freshwater fishes of rhodopsin, either alone or mixed with porphyropsin (Table I ) . This is evident among the approximately 10 characid and 20 cyprinid species that have been tested. Freshwater catfishes and centrarchids are the most prominent groups having porphyropsin alone. Some species ( mostly cyprinids ) even have rhodopsin alone. Secondary and peripheral freshwater fishes are considered together for their visual pigments have similar distributions. Many species have mixtures and many (notably poeciliids) have rhodopsin alone. It is true that more of the strictly freshwater species have porphyropsin alone and more of the euryhaline species have rhodopsin alone. Perhaps more significant is the unexpected fact that about half of the sample, whether euryhaline or not, has mixtures of the two visual pigments. Marine fishes, at least, seem to be as conservative as predicted

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PIGMENTS

Table I Distribution of Retinenel and Retinenel Pigments in Fishes Number of species” Division

Families

Retinenel

Mixtures

Retinenea

Total

Primary freshwater Secondary and peripheral freshwater Marine

18 20

8 28

28 31

25 3

61 62

27

58

3

2

63

+

(L 184 species 2 subspecies. Included are the published results in which visual pigments have been subjected to a homogeneity test by partial bleaching. Sources: Bridges (1966), Dartnall and Lythgoe (1965), Munz (1964, 1965), and Schwanzara (1967).

(but see Section 11, D ) . The sample of species examined is relatively large and diverse; more than 60 families are represented. A major exception is that very few elasmobranchs have been examined adequately (e.g., Denton and Shaw, 1963; but see Beatty, 1969a, and Crescitelli, 1969). Presumably, most of this largely marine group have retinenel pigments; but a few species are euryhaline or even confined to freshwater ( Potamotrygonidae) . The biological significance of the difference between rhodopsin and porphyropsin remains in doubt. Willmer (1956) suggested that it might be secondary to some role of vitamin A in salt or water balance. Wald (1957, 1958) also doubted that the distribution of rhodopsin and porphyropsin could be attributed to any special visual significance in freshwater and marine environments and proposed some sort of evolutionary recapitulation. Simpson ( 1964), on the other hand, thought that an adaptive significance is extremely probable. The question is not yet settled. So far, this treatment misses a crucial point: in species with mixtures of rhodopsin and porphyropsin, the individuals may actually have a “choice” of retinenes. A migratory lamprey, Petromyxon, undergoes a succession of rhodopsin and porphyropsin that is somehow related to its life cycle ( Wald, 1957); but another, Entosphenus, may have rhodopsin throughout life (Crescitelli, 1956). Dartnall et al. (1961) found that a cyprinid, Scardinius, has more retinene2 pigment in winter than summer. Porphyropsin increased in fish kept in darkness, while daylight caused the rhodopsin to increase. The changes in visual pigment were not influenced by diet and were completely unrelated to salinity, for the fish were maintained in freshwater. These results opened a new approach to the rhodopsin-porphyropsin problem. Another cyprinid, Notemigonus, a poeciliid, Belonesox, and a freshwater gadid, Lota, show the same type

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of seasonal succession of retinenel and retinene, pigments ( Bridges, 1965b; Beatty, 1 9 6 9 ~ ) .Pacific salmon, Oncorhynclzus, undergo somewhat similar changes in their visual pigments during the life cycle (Beatty, 1966). The retinene balance of juvenile salmon is partly controlled by light but is not altered as easily as in the cyprinids. Salinity has no effect on the visual pigments at this stage. Young salmon, even in winter, always have a considerable proportion of rhodopsin. Salmon caught on the high seas (in winter) had rhodopsin alone. Associated with the spawning migration is a nearly complete conversion to the retinene, system. This can start in the ocean, but may be hastened by entry into freshwater. Beatty suggested that sexual maturation may accelerate this process. The sockeye salmon, Oncorhynchus nerka, is best adapted to freshwater for it can complete its life cycle without entering the sea ( landlocked form called “kokanee”). Contrary to expectations, Beatty found that this species, and especially the landlocked form, never has much porphyropsin. The predicted effects of salinity have not actually been demonstrated on the visual pigments of any fishes but probably could be if the right species were used. Further experiments are needed to unravel the nature of environmental, dietary, and hormonal factors that control the proportions of retinene, and retinene, pigments. A clue in this direction was provided by the demonstration that a centrarchid, Lepomis, converts vitamin A, to retinene, in the eye and that thyroxine inhibits the conversion (Naito and Wilt, 1962). Thyroxine has the opposite effect of increasing the proportion of retinene, pigment in the eyes of salmonids; its mechanism of action is not known (Munz and Swanson, 1965; Beatty, 1969b). In a frog, Rana, the immediate precursor of retinene? appears to be retinene,, rather than vitamin A, (Ohtsu et al., 1964). Whether vitamin A, and A, can ever be interconverted directly is not known, but there is sometimes little relation between the forms of vitamin A in the liver and retinene in the eye (e.g., Munz, 1965; Beatty, 1966). The biological significance of retinene, pigments may possibly be related to their frequent occurrence in mixtures with retinene, pigments (Munz, 1965). Proportions of the two pigments can be altered within individual fish ( e.g., Scardinius) in response to environmental light levels. An outstanding feature of freshwater photic environments is their instability, both seasonally and on a geological time scale. An adaptable visual system, therefore, may have selective advantage for some freshwater fishes. Although it is difficult to describe the light in so variable an environment, it is probably richer in long wavelengths (redder) than light in the sea. An increased sensitivity to red light (which would result from a visual system based on retinene,) may be advantageous to many

1. VISUAL

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23

freshwater fishes, but the arguments and uncertainties brought out by Lythgoe ( 1966) could also be applied here (see Section 11, D ) . Schwanzara (1967) found a tendency of surface-feeding freshwater fishes to have retinene, pigments, while bottom dwellers have retinenez pigments. This is consistent with the filtering undergone by sunlight as it penetrates to greater depths. She found that retinene, pigments are more common in tropical fishes than in Temperate Zone species, and the converse; this was also evident within a single primary freshwater family, the Cyprinidae. These trends may be related to some general difference in spectral quality of light in tropical and temperate freshwaters, but comparative data are lacking. At the least, speculation about the possible visual significance of retinenez should provoke experiments that may give insight into this problem in biochemical evolution.

D. A Multiplicity of Opsins The visual pigments of fishes are more conspicuously diverse than those of all other vertebrates combined (Dartnall and Lythgoe, 1965). These differences are referable to a series of species-specific opsins, as well as to the occurrence of retinenee in some fishes. A histogram summarizes the published results (Fig. 9 ) . Only the most abundant visual pigment of each species is presented, except in cases where retinenel-retinene2 pairs have been described. Presumably, these are visual pigments of the rods. The source of less abundant pigments may be either rods or cones but is unknown in most cases. The visual pigments of more than 180 species have been subjected to adequate spectrometric analysis, and the list is growing so rapidly that any table would be obsolete before its publication. The figure should be regarded as a progress report, therefore, and not as the conclusion of a completed survey. Availability affects the choice of species in any survey; as far as possible, however, efforts have been made to sample species from a variety of taxa and of habitats (see Lythgoe, 1966; Schwanzara, 1967). Marine fishes can have any of a series of rhodopsins. The A,, values are not normally distributed about some wavelength such as 500 nm, but there seem to be clusters at several wavelengths as described by Dartnall and Lythgoe (1965). These authors suggested that such a distribution implies a limited number of possible variations in opsin structure. Study of hybrid salmonids, Salvelinus, indicated that a single-factor difference distinguishes the opsins of two species in which the visual pigments have A,,,,, 9 nm apart ( McFarland and Munz, 1965). This first genetic information is at least consistent with the view of Dartnall and Lythgoe. A

24

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470

480

490

500

510

A,,

(nm)

520

530

540

Fig. 9. The distribution of visual pigments in fishes. The histograms give the frequency of occurrence (number of species) vs. pigment x,,. Solid squares represent retinenel pigments; open squares represent retinenez pigments. Half-filled squares are pairs of pigments: retinenel pigments are solid along the bottom; retinene? pigments are open along the bottom. Fishes are grouped by family, according to habitat and phylogeny (see text). Numbers in parentheses indicate the families sampled; the other numbers are the numbers of species. Sources as in Table I. Note: When appropriate, published Amax values of retinenel pigments have been decreased by 1 nm, in accord with a correction of the nomogram (Dartnall, 1967).

similar grouping of both retinene, and retinene, pigments about “preferred positions” was proposed in freshwater fishes by Bridges (1965a, 1966), but the addition of Schwanzara’s data (1967) seems to obscure the relationship that he described. One point made by Bridges (see also Dartnall and Lythgoe, 1965; Munz and Schwanzara, 1967) needs explanation: The ,,A values of paired retinene, and retinene, pigments are correlated. In primary freshwater species there is less diversity of opsins than among marine fishes. The presumed ability of many freshwater fishes to alter the proportions of their retinene, and retinenez pigments (Section 11, C ) may largely eliminate the selective advantage of different opsins (Munz, 1965; Schwanzara, 1967). Thus, the visual system of the individual has a flexibility unavailable to most marine fishes. Attempts have been made to assess the biological significance of the different retinene, pigments of marine fishes. Deep-sea fishes have rhodopsins with A,,,,, values at about 490 nm or less, in evident correlation with the predominant wavelengths in sunlight after it has been filtered

1. VISUAL

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25

by passage through a long column of clear oceanic water (Denton and Warren, 1957; Wald et al., 1957; Munz, 1958a). The generally blue luminescence of deep-sea animals should also favor selection of bluesensitive visual pigments ( Munz, 1958a). There is no obvious correlation between the phylogenetic relationships of fishes and the maxima of their visual pigments. It was tempting, therefore, to generalize from the deepsea fishes and seek ecological correlations instead (Munz, 1958b, 1964, 1965). The predominant colors of sunlight are certainly not the same, as filtered by different seawater types such as oceanic, coastal, and inshore. But the attempt to associate visual pigment maxima with these photic environments has broken down as additional species have been sampled ( Dartnall and Lythgoe, 1965). His own experience as a diver led Lythgoe (1966, 1968) to emphasize that visual contrast, rather than sensitivity, may be the main selective agent. Water acts as a color filter, progressively narrowing the spectrum of transmitted light as well as decreasing its intensity. Light reflected from an object, such as the silvery side of another fish, travels a shorter distance underwater than light scattered back from the water behind it. Background illumination is therefore more monochromatic than light reflected from the object. Its maximum is further from the spectral region dominant in sunlight at the water surface and closer to the wavelength of maximum transmission by the water. This means that the greatest visual contrast between such an object and the background occurs if the visual pigment has its A, at a wavelength removed from the transmission maximum; the object is therefore seen to be brighter than the background. Should the difference be too great, of course, visual sensitivity would be drastically reduced. Lythgoe has shown that the visual pigments of marine fishes from several different photic environments appear to fit the requirements for a balance of visual contrast and relatively high sensitivity. He also pointed out that understanding of these problems is hampered by our lack of definite knowledge of the origin of the various visual pigments in rods or cones and whether the pigments occur in separate receptors or are mixed indiscriminately. In summary, current thinking suggests that the many different visual pigments of fishes probably have been selected for their adaptive advantage in different photic environments.

E. Pigments of Color Vision Of great interest are the mechanisms responsible for color vision, both in ourselves and in other animals, such as teleosts, that can discriminate

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between lights of different wavelengths. How many different visual pigments are involved, one for each primary color? Are they similar to other visual pigments? Does each pigment occur within a different class of receptor cells, or may the visual pigments be mixed together? Direct evidence has recently been obtained by microspectrophotometry of individual retinal cones of fishes; this technically difficult method has also been applied to visual pigments of frogs and primates. Microspectrophotometry was first applied to the cones of carp by Hanaoka and Fujimoto (1957), who reduced a beam of monochromatic light to a diameter of 3 p, small enough to pass through the outer segment. Exposure to bright light changed the absorbance, but the difference spectra obtained in this way were only roughly similar to those of known visual pigments. More sensitive photomultipliers have recently reduced the amount of bleaching caused by absorption of the measuring light (Liebman and Entine, 1964), or the absorbance data have been subjected to computer analysis to compensate for this bleaching (Marks, 1965). These authors have studied the goldfish; they agree that there are three different classes of cones, each possessing a single visual pigment. The data of Liebman and Entine seem more amenable to direct interpretation, but they gave no estimates of A,,,. Marks found that the cone pigments appeared to be generally similar to other known visual pigments, but no good evidence with respect to the product of bleaching has yet been obtained. The approximate A,,,,, values of these pigments are 625 nm ( r e d ) , 530 nm (green), and 455 nm (blue). Each of the pigments occurred in single cones and each in twin cones. In more than 50 single cones, the ratio was approximately 2 red:4 green:l blue. The two members of a twin pair never had the same pigment. Of 30 pairs of twins, 29 were red-green pairs and one was blue-green. No red-blue pairs were found. It is fair to ask, how good is the evidence that goldfish have color vision? Electrophysiological activity in the retinae of goldfish has been found to be consistent with this idea ( MacNichol et al., 1961; Tamura and Niwa, 1967). Potentials have been measured from the inner segments of single cones in the closely related carp by Tomita et al. (1967), who compared their results with those of Marks. Behavioral tests show that goldfish can discriminate between different colors when brightness cues are eliminated (e.g., McCleary and Bernstein, 1959; Muntz and CronlyDillon, 1966) although this capacity has not been investigated systematically (see Yager, 1967). The biochemical and physiological evidence that has been gathered at several different levels should be relevant therefore to the problems of color vision.

1. VISUAL

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PIGMENTS

REFERENCES Abrahamson, E. W., and Ostroy, S. E. (1967). The photochemical and macromolecular aspects of vision. Progr. Bioplays. Mol. B i d . 17, 179-215. Ali, M. A. ( 1959). The ocular structure, retinomotor and photobehavioral responses of juvenile Pacific salmon. Can. J. 2001.37, 965-996. Ali, hl. A. (1962). Influence of light intensity on retinal adaptation in Atlantic salmon (Salmo salar) yearlings. Can. J. Zool. 40, 561-570. Ali, M. A. ( 1964a). Retinomotor responses of the goldfish (Carassius aurutus) to unilateral photic stimulation. Rev. Can. Biol. 23, 45-53. Ali, M. A. (196413). Retina of th6 albino splake (Salvelinus fontinah X S . namaycush). Can. J. Zool. 42, 1158-1160. Baron, J., and Verrier, M.-L. (1951). RCfraction e t cerveau des poissons ?I fovea. Contribution i l’ittude des corrblations organiques. Bull. B i d . France Belg. 85, 105-11 1. Baylor, E. R. (1967a). Vision of Bermuda reef fishes. Nature 214, 306-307. Baylor, E. R. ( 196713). Air and water vision of the Atlantic flying fish, Cypselurus laeterurus. Nature 214, 307-309. Baylor, E. R., and Shaw, E. (1962). Refractive error and vision in fishes. Science 136, 157-158. Beatty, D. D. ( 1966). A study of the succession of visual pigments in Pacific salmon (Oncorhynchus). Can. J. Zool. 44, 429455. Beatty, 1). D. (1969a). Visual pigments of three species of cartilaginous fishes. Nature 222, 285. Beatty, D. D. (1969b). Visual pigment changes in juvenile kokanee salmon in response to thyroid hormones. Vision Res. 9, 855-864. Beatty, D. D. ( 1 9 6 9 ~ ) .Visual pigments of the burbot, Lota lota, and seasonal changes in their relative proportions. Vision Res. 9, 1173-1183. Best, A. C. G., and Nicol, J. A. C. (1967). Reflecting cells of the elasmobranch tapetum lucidum. Contributions Marine Sci., Univ. Texas 12, 172-201. Blaxter, J. H. S., and Jones, M. P. (1967). The development of the retina and retinomotor responses in the herring. J . Marine Biol. Assoc. U . K . 47, 677697. Bogatyrev, P. B. ( 1966). On the visual accommodation of some fish species. (Russian.) Ref. Zh., Biol. No. 21, 156; Biol. Abstr. 48, 9444 ( 1967) (abstr.). Brauer, A. ( 1908). “Wissenschaftliche Ergebnisse der deutschen Tiefsee-Expedition auf dem Dampfer Valdivia 1898-1899,” Vol. 15, Part 2. Fischer, Jena. Brett, J. R. ( 1957). The sense organs: The eye. In “The Physiology of Fishes” ( M. E. Brown, ed.), Vol. 2, pp. 121-154. Academic Press, New York. Brett, J. R., and Ali, M. A. (1958). Some observations on the structure and photomechanical responses of the Pacific salmon retina. J. Fisheries Res. Board Can. 15, 815-829. Bridges, C . D. B. (1965a). The grouping of fish visual pigments about preferred positions in the spectrum. Vision Res. 5, 223-238. Bridges, C. D. B. (1965b). Variability and relationships of fish visual pigments. Vision Res. 5, 239-251. Bridges, C. D. B. ( 1966). Absorption properties, interconversions, and environmental adaptation of pigments from fish photoreceptors. Cold Spring Harbor Symp. Quant. Biol. 30, 317-334.

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Bridges, C. D. B. (1967). Spectroscopic properties of porphyropsins. Vision Res. 7, 349-369. Clarke, W. D. (196‘3). Function of bioluminescence in mesopelagic organisms. Nature 198, 1244-1246. Crescitelli, F. (1956). The nature of the lamprey visual pigment, J. Gen. Physiol. 39, 423435. Crescitelli, F. (1958). The natural history of visual pigments. Ann. N . Y. Acad. Sci. 74, 230-255. Crescitelli, F. (1969). The visual pigment of a chimaeroid fish. Vision Res. 9, 14071414. Darlington, P. J., Jr. ( 1957). “Zoogeography: The Geographical Distribution of Animals.” Wiley, New York. Dartnall, H. J. A. (1953). Interpretation of spectral sensitivity curves. Brit. Med. Bull. 9, 24-30. Dartnall, H. J. A. ( 1957). “The Visual Pigments.” Methuen, London. Dartnall, H. J. A. (1961). Visual pigments before and after extraction from visual cells. Proc. Roy. SOC. B154, 250-266. Dartnall, H. J. A. (1962). The photobiology of visual processes. In “The Eye” ( H . Davson, ed.), Vol. 2, pp. 323-533. Academic Press, New York. Dartnall, H. J. A. (1967). Personal communication. Dartnall, H. J. A., and Lythgoe, J. N. (1965). The spectral clustering of visual pigments. Vision Res. 5, 81-100. Dartnall, H. J. A,, Lander, M. R., and Munz, F. W. (1961). Periodic changes in the visual pigment of a fish. Proc. 3rd Intern. Congr. Photobiol., Copenhagen, 1960 pp. 203-213. Elsevier, Amsterdam. Denton, E. J. (1957). Recherches sur l’absorption de la lumidre par le cristallin des poisons. Bull. Inst. Ocearwg. 1071, 1-10. Denton, E. J. ( 1959). The contributions of the oriented photosensitive and other molecules to the absorption of whole retina. Proc. Roy. SOC. B150, 78-94. Denton, E. J., and Nicol, J. A. C. (1964). The chorioidal tapeta of some cartilaginous fishes (Chondrichthyes). J . Marine Biol. Assoc. U . K . 44,219-258. Denton, E. J., and Nicol, J. A. C. (1965). Direct measurements of the orientation of the reflecting surfaces in the tapetum in Squalus acanthias, and some observations on the tapetum of Acipenser sturio. J. Marine Biol. Assoc. U . K . 45, 739-742. Denton, E. J., and Shaw, T. I. (1963). The visual pigments of some deep-sea elasmobranch. J. Marine Biol. Assoc. U . K . 43, 65-70. Denton, E. J., and Walker, M. A. (1958). The visual pigment of the conger eel. Proc. Roy. SOC. B148, 257-269. Denton, E. J., and Warren, F. J. (1957). The photosensitive pigments in the retinae of deep-sea fish. J. Marine B i d . Assoc. U . K . 36, 651-662. Denton, E. J., and Wyllie, J. H. (1955). Study of the photosensitive pigments in the pink and green rods of the frog. J. Physiol. (London) 127, 81-89. Grangaud, R., Massonet, R., and Moatti, J.-P. (1962). Etude de la vitaniine A et du r&ini.ne des yeux de Gambusia holbrooki Grd. Compt. Rend. S O C . Biol. 155, 2150-2153. Gruber, S. H., Hamasaki, D. H., and Bridges, C. D. B. (1963). Cones in the retina of the lemon shark (Negaprion breuirostris). Vision Res. 3, 397-399. Hamasaki, D. I., and Gruber, S. H. (1965). The photoreceptors of the nurse shark,

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Ginglymostoma cirratum, and the sting-ray, Dasyatis sayi. Bull. Marine Sci. Gulf Caribbean 15, 1051-1059. Hanaoka, T., and Fujimoto, K. (1957). Absorption spectrum of a single cone in carp retina. Japan. J . Physiol. 7, 276-285. Hanyu, I. (1959). On the falciform process, vitreal vessels and other related structures of the teleost eye. I and 11. Bull. Japan. SOC. Sci. Fisheries 25, 595-619. Inouye, K., and Noto, S. (1962). Structure of the retina in Anableps (four-eyed fish). Zool. Mag. ( T o k y o ) 71, 188. John, K. R., Segall, M., and Zawatzky, L. (1967). Retinomotor rhythms in the goldfish, Carassius auratus. Biol. Bull. 132, 200-210. Kahmann, H. ( 1936). Uber das foveale Sehen der Wirbeltieren. Arch. Ophthalmol. 135, 265-276. Kennedy, D., and Milkman, R. D. (1956). Selective light absorption by the lenses of lower vertebrates, and its influence on spectral sensitivity. Biol. Bull. 111, 375-386. Kuchnow, K. P., and Gilbert, P. W. ( 1967). Preliminary in vivo studies on pupillary and tapetal pigment responses in the lemon shark, Negaprion brevirostris. In “Sharks, Skates and Rays” ( P . W. Gilbert, R. F. Mathewson, and D. P. Rall, Johns Hopkins Press, Baltimore, Maryland. eds. ), pp. 46-77. Liebman, P. A., and Entine, G. ( 1964). Sensitive low-light-level microspectrophotometer: Detection of photosensitive pigments of retinal cones. J . Opt. SOC. Am. 54, 1451-1459. Lindeman, V. F. (1943). A comparative study of the oxygen consumption of the vertebrate retina, with especial reference to the nucleo-protoplasmic ratio. Am. J. Physiol. 139, 9-16. Lythgoe, J , N. (1966). Visual pigments and underwater vision. In “Light as an Ecological Factor” (R. Bainbridge, G. C. Evans, and 0. Rackham, eds.), pp. 375-390. Wiley, New York. Lythgoe, J. N. (1968). Visual pigments and visual range underwater. Vision Res. 8, 997-1012. McCleary, R. A., and Bernstein, J. J. (1959). A unique method for control of brightness cues in the study of colour vision in fish. Physiol. Zool. 32, 284-292. McFarland, W. N., and Munz, F. W. (1965). Codominance of visual pigments in hybrid fishes. Science 150, 1055-1057. MacNichol, E. F., Wolbarsht, M. L., and Wagner, H. G. (1961). Electrophysiological evidence for a mechanism of color vision in the goldfish. I n “Light and Life” ( W . D. McElroy and B. Glass, eds.), pp. 795-813. Johns Hopkins Press, Baltimore, Maryland. Marks, W. B. (1965). Visual pigments of single goldfish cones. J . Physiol. (London) 178, 14-32. Marshall, N. B. (1954). “Aspects of Deep Sea Biology.” Hutchinson, London. Marshall, N. B. (1966). Family Scopelosauridae. I n “Fishes of the Western North Atlantic” (G. W. Mead et al., eds.), Sears Found. Marine Res., Mem. No. I, Part 5, pp. 194-204. Yale Univ. Press, New Haven, Connecticut. Morton, R. A., and Pitt, G. A. J. (1969). Aspects of visual pigment research. Adv. Enzymol. 32, 97-171. Motais, R. (1957). Sur I’absorption de la lumibre par le cristallin de quelques poissons de grande profondeur. Bull. Inst. Oceanog. 1074, 1 4 . Munk, 0. (1959). The eyes of Ipnops murrayi Gunther, 1878. Galathea Rept. 3, 79-88.

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Munk, 0. (1963). The eye of Stomias boa ferox Reinhardt. Vidensk. Medd. Dansk Naturh. Foren. 125, 353359. Munk, 0. (1964a). The eyes of three benthic deep-sea fishes caught at great depths. Galathea Rept. 7, 137-149. Munk, 0. (1964b). The eyes of some ceratioid fishes. Appendix: The refraction of light in a spherical lens by K. Steenbury. Dana Rept. 61, 1-15. Munk, 0. (1965a). Ocular degeneration in deep-sea fishes. Galathea Rept. 8, 21-31. Munk, 0. (196513). Omosudis lowei Giinther, 1887, a bathypelagic deep-sea fish with an almost pure-cone retina. Vidensk. Medd. Dansk Naturh. Foren. 128, 341-355. Munk, 0. (1966). Ocular anatomy of some deep-sea teleosts. Dana Rept. 70, 1-62. hluntz, W. R. A., and Cronly-Dillon, J. R. (1966). Colour discrimination in goldfish. Animal Behaviour 14, 351455. Munz, F. W. (1958a). Photosensitive pigments from the retinae of certain deep-sea fishes. J. Physiol. ( L o n d o n ) 140,220-235. Munz, F. W. ( 1958b). The photosensitive retinal pigments of fishes from relatively turbid coastal waters. J. Gen. Physiol. 42, 445-459. Munz, F. W. (1964). The visual pigments of epipelagic and rocky-shore fishes. Vision Res. 4, 441454. 14Un2, F. W. (1965). Adaptation of visual pigments to the photic environment. Ciba Found. Symp., Colour Vision, Physiol. Exptl. Psychol. pp. 2 7 4 5 . Munz, F. W., and Beatty, D. D. (1965). A critical analysis of the visual pigments of salmon and trout. Vision Res. 5, 1-17. Munz, F. W., and Schwanzara, S. A. (1967). A nomogram for retinenez-based visual pigments. ViPion Res. 7, 111-120. Munz, F. W., and Swanson, R. T. (1965). Thyroxine-induced changes in the proportions of visual pigments. Am. Zoologist 5, 683 (abstr.). Naito, K., and Wilt, F. H. (1962). The conversion of vitamin A, to retinener in a fresh-water fish. J. Biol. Chem. 237, 3060-3064. Nicol, J. A . C. (1961a). The tapetum in Scyliorhinw canicula. J. Marine Biol. Assoc. U. K. 41, 271-277. Nicol, J. A . C. (1961b). Photo-mechanical changes in the eyes of fishes. I. Retinomotor changes in Solea solea. J. Marine Biol. Assoc. U . K . 41, 695-698. Nicol, J. A. C. (1963). Some aspects of photoreception and vision in fishes. Aduan. Marine Biol. 1, 171-208. Nicol, J. A. C. (1964). Reflectivity of the chorioidal tapeta of selachians. J . Fisheries Res. Board Can. 21, 1089-1100. Nicol, J. A. C. (1965a). Retinomotor changes in flatfishes. J. Fisheries Res. Board Can. 22, 5 1 3 5 2 0 , Nicol, J. A. C. (1965b). Migration of the chorioidal tapetal pigment in the spur dog Syualus acanthias. J. Marine Biol. Assoc. U . K . 45, 405427. O’Connell, C. P. (1963). The structure of the eye of Sardinops caerulea, Engraulis mordax, and four other pelagic marine teleosts. J. Morphol. 113, 287-329. Ohtsu, K., Naito, K., and Wilt, F. H. (1964). Metabolic basis of visual pigment conversion in metamorphosing Rana catesbeiana. Develop. Biol. 10, 216-232. Pearcy, W. G., Meyer, S. L., and Munk, 0. (1965). A ‘four-eyed’ fish from the deep-sea: Bathylychnops exilis Cohen, 1958. Nature 207, 1260-1262. Plack, P. A. (1961). The colorimetric reaction between vitamin A, aldehyde and antimony trichloride. Biochem. J. 81, 556-561. Prince, J. H. ( 1956). “Comparative Anatomy of the Eye.” Thomas, Springfield, Illinois.

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Pumphrey, R. J. (1961). Concerning vision. In “The Cell and the Organism. Essays Presented to Sir James Gray” ( J . A. Ramsay and V. B. Wigglesworth, eds.), pp. 19S208. Cambridge Univ. Press, London and New York. Rochon-Duvigneaud, A. ( 1943). “Les yeux et la vision des vertibr6s.” Masson, Paris. Rochon-Duvigneaud, A. ( 1958). L’oeil et la vision. In “Trait6 de Zoologie” (P.-P. Grass&,ed. ), Vol. 13, pp. 1099-1142. Masson, Paris. Schwanzara, S. A. (1967). The visual pigments of freshwater fishes. Vision Res. 7, 121-148. Schwassman, H. O., and Kruger, L. (1965). Experimental analysis of the visual system of the four-eyed fish (Anableps microlepis). Vision Res. 5, 269-281. Seliger, H. H. (1962). Direct action of light in naturally pigmented muscle fibers. I. Action spectrum for contraction in eel iris sphincter. J. Gen. Physiol. 46, 333342. Simpson, G. G. ( 1964). Organisms and molecules in evolution. Science 146, 15351538. Tamura, T. (1957). A study of visual perception in fish, especially on resolving power and accommodation. Bull. Japan. SOC. Sci. Fisheries 22, 536-557. Tamura, T., and Niwa, H. (1967). Spectral sensitivity and color vision of fish as indicated by S-potential. Comp. Biochem. Physiol. 22, 745-754. Toniita, T., Kaneko, A., Murakami, M., and Pautler, E. L. (1967). Spectral response curves of single cones in the carp. Vision Res. 7,519-531. Vilter, V. (1951). Recherches sur les structures fovkales dans la retine du Sphenodon punctatus. Compt. Rend. SOC. Biol. 145, 2 6 2 9 . Vilter, V. ( 1954a). DiffCrenciation fov6ale dans l’appareil visuel d u n Poisson abyssal, 1e Bathylugus benedicti. Compt. Rend. SOC. Biol. 148, 59-63. Vilter, V. (1964b). Relations neuronales dans la fovea A bitonnets du Bathylugus benedicti. Compt. Rend. SOC. Biol. 148, 466-469. Wald, G. (1936). Pigments of the retina. 11. Sea robin, sea bass, and scup. J. Gen. Physiol. 20, 45-56. Wald, G. (1939a). On the distribution of vitamins A, and G.J. Gen. Physiol. 22, 391415. Wald, G. (1939b). The porphyropsin visual system. J. Gen. Physiol. 22, 775-794. Wald, G. (1941). The visual systems of euryhaline fishes. J. Gen. Physiol. 25, 235245. Wald, G. (1947). The chemical evolution of vision. Harvey Lectures 41, 117-160. Wald, G. (1957). The metamorphosis of visual systems in the sea lamprey. J. Gen. Physiol. 40, 901-914. Wald, G. (1958). The significance of vertebrate metamorphosis. Science 128, 14811490. Wald, G. (1959). The photoreceptor process in vision, In “Handbook of Wysiology” (Am. Physiol. SOC., J. Field, ed.), Sect. 1, Vol. 1, pp. 671-692. Williams & Wilkins, Baltimore, Maryland. Wald, G. (1960). The distribution and evolution of visual systems. Comp. Biochem. 1, 311-345. Wald, G., Brown, P. K., and Brown, P. S. (1957). Visual pigments and depths of habitat of marine fishes. Nature 180, 969-971. Walls, G. L. (1942). “The Vertebrate Eye and its Adaptive Radiation.” Cranbrook Inst. Sci., Bloomfield Hills, Michigan. Weale, R. A. ( 1955). Binocular vision and deep-sea fish. Nature 175, 996.

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Welsh, J. H., and Osborn, C. M. (1937). Diurnal changes in the retina of the catfish, Ameiurus nebulosus. ]. Comp. Neurol. 66, 349-359. Willmer, E. N. (1956). In “The Comparative Endocrinology of Vertebrates. Part 11. The Hormonal Control of Water and Salt-Electrolyte Metabolism in Vertebrates” (I. C . Jones and P. Eckstein, eds.), p. 101. Cambridge Univ. Press, London and New York. Wilt, F. H. (1959). The differentiation of visual pigments in metamorphosing larvae of Rana catesbeiana. Develop. Biol. 1, 199-233. Wittenberg, J. B., and Wittenberg, B. A. (1962). Active secretion of oxygen into the eye of fish. Nature 194, 106-107. Yager, D. ( 1967). Behavioral measures and theoretical analysis of spectral sensitivity and spectral saturation in the goldfish Carmsius auratus. Vision Res. 7 , 707-727. Young, J, Z. (1931). The pupillary mechanism of the teleostean fish Uranoscopus scaber. PTOC.Roy. SOC.B107, 464485. Young, J. Z. (1933a). comparative studies on the physiology of the iris. I. Selachians. Proc. Roy. SOC. B112, 228-241. Young, J. Z. (193313). Comparative studies on the physiology of the iris. 11. Urarwscopus and Lophius. PTOC.Roy. SOC. B112,242-249.

VISION: ELECTROPHYSIOLOGY OF THE RETINA T . TOMZTA I. Introduction . . . . . . . , 11. Electroretinogram . . . . . A. Electroretinogram as a Mass Response . . B. Component Analysis of Electroretinogram . C. Localization of Electroretinogram Components 111. Response of Single Ganglion Cells . . . . . . . A. Response Types . . . B. Receptive Field . . . . . . . IV. Response of Photoreceptors . . . . . A. Early and Late Receptor Potential . . . B. Response of Single Photoreceptors . . . . . V. Responses in the Inner Nuclear Layer . . . . . . . . A. S Potential . B. Responses in Other Cell Types . . . . VI. Retinal Mechanisms of Color Vision . . . . . . . References .

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33 35 35 37 39 . 4 0 40 41 43 43 4 4 47 47 50 51 53

I. INTRODUCTION

The retina responds to light and signals its presence and pattern to the brain by way of the optic nerve. Information of the surroundings entering the eye mediated by light is first caught by the photopigments in rods and cones and transmitted to the ganglion cells through a complex neural network as shown in Fig. 1. The recent rapid development of electron microscopic techniques has contributed a great deal to the elucidation of ultrafine structures of retinal cells and synaptic contacts between them (Villegas, 1960; Villegas and Villegas, 1963; Sjostrand, 1961; Yamada and Ishikawa, 1965; Stell, 1965; Dowling and Boycott, 1966; Dowling and Werblin, 1969). The morphology shows that the organization of the neural network is much the same in all the verte33

34

T. TOMITA

Fig. 1. Diagrammatic vertical section of ( A ) teleost retina and ( B ) elasmobranch retina. A1-5, amacrine cells; BC1-5, bipolar cells; C, cone; CF, centrifugal nerve fiber; G1-3, ganglion cells; HC1-3 and HS1-2, horizontal cells; R, rod; RS, radiaI supporting cell ( MiiIler cell); SG, steller ganglion cell; and TC, twin cone. The inner nuclear layer consists mainly of the cell bodies of amacrine, bipolar, horizontal, and Miiller cells. From Detweiler (1943) after Franz (1913).

brates. Electrophysiologically also, it is usual that an observation in one animal form applies to the others. For example, the observation of Adrian and Matthews (1927a) that the optic nerve of the eel responds with a burst of impulse discharge at both onset and cessation of light is now known to be common to other vertebrates. As another example, the result

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ELECTROPHYSIOLOGY OF THE RETINA

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of analysis of the electroretinogram (ERG) into three components by Granit (1933) using the cat retina has now been generalized to all the vertebrates. The ERG represents a sum of electrical activities in individual retinal cells, whereas the optic nerve activity represents their integrated result. These two electrical phenomena were therefore utilized for the study of retinal mechanisms and of information sent to the brain along the optic nerve (Adrian and Matthews, 192%; Granit, 1933). A striking advance was made when the microdissection technique was introduced into this research field by Hartline (1935, 1938); th'IS was rapidly followed by the use of the microelectrode technique by Granit and Svaetichin (1939). These two techniques made it possible to record the impulse discharge in single ganglion cells (or in single optic nerve fibers) in response to a variety of photic stimulations, which differ in intensity, wavelength, area, pattern, and so forth. The microelectrode technique has the additional advantage that recordings can be made from within the retinal tissue. The depth recording of the ERG with microelectrodes advanced intraretinally served for more direct localization of the ERG components into retinal layers and for recording potentials at the electrode tip. Since the first use of this method (Tomita, 1950), it has now developed into the technique of intracellular recording from single retinal cells to be described later. It should be emphasized that many of the recent important contributions in the field of electrophysiology of the vertebrate retina have been accomplished by the application of these microtechniques to fish retinas. Readers who are interested in more background information should refer to Granit (1947, 1962) and Brindley (1960).

11. ELECTRORETINOGRAM

A. Electroretinogram as a Mass Response As early as 1865 Holmgren discovered that a pair of electrodes, one on the cornea and the other on the back of the eye, would record a weak electrical change when the retina is illuminated by light. This response, now generally known as the electroretinogram or ERG, can also be obtained from the eye with its anterior half removed, or from the retina isolated completely from the rest part of the eye. Two ERG recordings from the same eye of the carp but at different adaptation states are illustrated in Fig. 2. The upper tracing is from the eye which has been dark-adapted ( scotopic) and the lower after light adaptation ( photopic),

T. TOMITA

36 b

0.2 rnV

Fig. 2. Electroretinograms from the opened eye (in situ) of the carp, immobilized by Flaxedil under artificial respiration. Upper, dark adapted; lower, light adapted. The moments of onset and cessation of light are signaled by up- and downpips in the tracing above the time marking. The c-wave is not discerned because of recording by a capacitance-resistance (C-R) coupled amplifier with a time constant of 0.5 sec. Courtesy of K. Watanabe and Y. Hashimoto, from their unpublished records.

In both the ERG begins with a cornea-negative deflection (a-wave) followed by a cornea-positive one (b-wave), and another deflection (dwave), which is usually cornea-positive, at the cessation of light. Besides the above three fast waves, a very slow comea-positive rise (c-wave) is recorded if the eye is dark-adapted and a dc amplifier is used for the recording. The similarity in configuration of the ERG between fish and other vertebrates is evident from a comparison of Figs. 2 and 3, the latter showing the ERG'S of the frog (solid lines) in the scotopic and photopic state (Granit and Riddell, 1934). In general, in the scotopic state the band c-wave predominate while in the photopic the a- and d-waves are largest. For the obvious reason that the ERG is a mass response of the retinal cells, it has provided since Holmgren the most important means of objective study of the retina function as a whole. Measurement of the ERG threshold, for instance, made it possible to follow the course of dark adaptation (Hamasaki and Bridges, 1965; Witkovsky, 1968) and to plot the spectral sensitivity curve ( Burkhardt, 1966; Witkovsky, 1968)

2.

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ELECTROPHYSIOLOGY OF T HE RETINA

,.--I-

(a 1

rnV +I

0

+o 8

,.-.

*

.

+0.6 +0.4

+0.2 0-0.2 -0.4

Fig. 3. Analysis of the electroretinogram (frog) into three components: PI, PII, and PIII. ( a ) Dark-adapted and ( b ) light-adapted. Duration of illumination shown by a solid line at the base, 2 sec. From Granit and Riddell ( 1934).

of the retina disconnected from the brain. Kobayashi (1962) made an extensive comparative study of fish ERGS in more than 25 species, marine and freshwater, with special reference to ecological aspects.

B. Component Analysis of Electroretinogram Among several attempts to analyze the ERG into components of simpler configurations, the analysis of Granit into three components (PI, PII, and PIII) is most generally accepted. His original analysis was performed using the cat (Granit, 1933) and frog ( Granit and Riddell, 1934), but the result applies to other vertebrates including fish. Minor differences between species are quantitative rather than qualitative. As seen in Fig. 3, the PI11 is a cornea-negative potential change which appears first following the onset of light. Shortly after the start of PIII,

38

T. TOMITA

this is interrupted by the cornea-positive PII. The a-wave of the ERG thus represents the early onset of PIII, while the b-wave the rise of PII. The PI corresponds to the c-wave in the ERG, and, as earlier described, its amplitude depends on the state of adaptation. The d-wave at the cessation of light is, according to this analysis, either a positive or negative deflection, determined by the amplitude and time course of the off-effects of PI1 and PI11 which are opposite in polarity. In most fishes, the PI11 predominates especially in the photopic state, resulting in a large a-wave and a large cornea-positive d-wave. Originally, the component analysis of the ERG was made by isolation of one component from others using various physical and chemical agents. In general, the PI1 is more susceptible than the PI11 to various agents. One exception is alcohol, which results in a marked increase in the bwave and decrease in the a- and d-wave (Bernhard and Skoglund, 1941). Granit (1947) provides evidence that the PI1 is associated with excitatory processes in the retina and the PI11 with inhibitory. This view is consistent with the common observation that a burst of impulse discharge in the optic nerve occurs with the b-wave, which is the rise of the excitatory PII, and another burst with the d-wave, which is the rebound

2W -0

250t

Distance in retina

Q

E

a

-250t \*/ 1 - 500

\

0

\

Fig. 4. Distribution of slow potential response (local ERG) at the receptor surface of the carp’s isolated retina plotted by scanning a small spot of light (60 ,U in diameter) across the recording site at the point 0. Positive responses upward. From Motokawa et al. ( 1959).

2.

ELECTROPHYSIOLOGY OF THE RETINA

39

following release from the inhibitory PIII. The following observations (Motokawa et al., 1959, 1961) also support Granit. The carp retina, detached from the pigment epithelium and mounted receptor side up on the indifferent electrode, gives a characteristic pattern of local ERG (Fig. 4 ) to a micropipette electrode placed on the distal retinal surface when a spot of light is scanned across the site of recording. When the light spot is on the recording site, a positive or PIII-dominant ERG is obtained, but as the spot is moved away from the recording site, the local ERG becomes negative or PII-dominant. In this PII-dominant zone, ganglion cells responding to light with on discharge outnumber those inhibited during light, whereas in the PIII-dominant zone the relation is just the reverse. Meanwhile, the characteristic pattern of local ERG shown in Fig. 4 was recently analyzed by Murakami and Sasaki (1968a,b) in terms of spatial distribution of ERG components, using the carp retina. Concerning the significance of the ERG in the chain of events evoked by light in the retina, however, much remains to be studied. Hamasaki and Bridges (1965) report, for instance, that a second light flash applied within 5 sec after the first to a dark-adapted retina (elasmobranches) fails to elicit an ERG, but the evoked responses at the optic tectum to these two successive light flashes are about the same.

C. Localization of Electroretinogram Components It is generally agreed that the PI1 originates from some cell type in the inner nuclear layer (Granit, 1947). This has been confirmed by the observation that the polarity of the b-wave is reversed after the recording microelectrode has penetrated through this layer ( Tomita, 1950; Brown and Wiesel, 1961). Recent work on the carp retina with intracellular micropipettes (Kaneko and Hashimoto, 1969) shows that some cells in the inner nuclear layer are depolarized during light while some others are hyperpolarized ( cf. Fig. 12). Although their cell types have not yet been identified, it is possible that those depolarized by light are related to the PII. The PI is localized in the outermost retinal structures (Tomita, 1950; Brown and Wiesel, 1961). This localization is also based on the finding from depth recording of the ERG with penetrating microelectrodes. After the retina is detached from the pigment epithelium, the PI is lost both in the retina proper and in the remaining part of the eye covered by the pigment epithelium. Tomita (1950) assumes on this basis that the PI is a phenomenon associated with some metabolic interaction between the receptors and pigment epithelium cells. Noell (1954) and Brown and Wiesel (1961) consider the pigment epithelium cells as the origin of PI.

40

T. TOMITA

The PI11 is of dual nature as earlier suggested by Granit (1947). The separation of PIII into two subcomponents was achieved by a technique of fractional depth recording of the ERG (Murakami and Kaneko, 1966). The one subcomponent is termed the “distal PIII” and localized in the receptors themselves. The other termed the “proximal PIII” originates in some structures in the inner nuclear layer. The latency of the proximal PI11 is several milliseconds longer than that of the distal PIII. The proximal PIII is more susceptible to various agents than the distal PIII. The proximal PI11 may be a transretinal manifestation either of the S potential (Byzov, 1962) or of the activity of the cell types in the inner nuclear layer that respond to light with hyperpolarization (see Section V ) , or probably of both. One aspect of the PI11 that made Granit hesitate to localize it in the receptors was its polarity. The PI11 has a polarity just opposite to what one predicts from other receptors including invertebrate photoreceptors. While it is general that the receptors, when excited, are depolarized to form an electric field making their distal tips negative, the polarity of PI11 makes them positive instead of negative. According to the recent observations on single rods and cones with intracellular micropipettes, however, the unusual polarity of the electric field around the vertebrate photoreceptors is attributed to their being hyperpolarized by light ( see Section IV, B ) .

111. RESPONSE OF SINGLE GANGLION CELLS

A. Response Types

The response types of single ganglion cells in the fish are the same as in other vertebrates such as the frog (Hartline, 1938) and the cat ( KuHer, 1953). They are either on type giving a burst of impulses when the light is turned on, the o# type giving a burst of impulses when the light turned off, or the o*o# type which responds with a burst of impulses at both onset and cessation of light. Intracellular recording reveals that in the on type the cell is depolarized during illumination, while in the off type the cell is hyperpolarized during illumination and depolarized following the cessation of light. In the on-off type a depolarization occurs at both on and off. It is obvious that the ganglion cell functions according to the general plan of neurons which are under excitatory and inhibitory presynaptic controls.

2.

41

ELECTROPHYSIOLOGY OF THE RETINA

The response type of a given unit is not always fixed but can change under certain conditions as described below.

B. Receptive Field The ganglion cell responds only when light falls within a certain circumscribed area of the retina. This area is termed, according to Hartline ( 1938), the “receptive field” of the ganglion cell. Mapping with a small spot of white light, several types of organization of the receptive field are discerned which are characteristic to individual ganglion cells. The two simple types are everywhere on and everywhere off type, but more common are those which are apparently made up of the combination of these two simple types. Figure 5 illustrates an off center-on periphery type obtained in the goldfish (Wagner et al., 1960). This is identical with one of the two typical types first observed in the cat by Kuffler (1953). The reverse type, that is, the on center-off periphery type, is also frequently encountered in the fish. Some ganglion cells in the fish are color coded, responding differently to different wavelengths of light (Wagner et al., 1960; Motokawa et al., 1960). The unit shown in Fig. 6 responds with an on discharge to green but with off discharge to red. Units of the reverse relation (red on-

0 0 00

0 X=OFF

0= O N 0=ON-OFF

-

0

..

0

0

0 I

I MM

I

Fig. 5. Receptive field of a single goldfish ganglion cell. Solid black area indicates region where only off responses were found. Hatched area indicates region where on-off type responses were found. On responses were found only in the periphery of on-off area. Test stimulus, 1 5 3 , ~in diameter; wavelength, 600 mp; and intensity, 18.0 pW/cmz. From Wagner et al. (1960).

T. TOMITA

I

i .--..I

I

4 50

..

! -- . .

!

600

.. 650

,

-

500

5 50

I It

1

1

I

I

0.5 sec

'

(b)

Fig, 6. Variation of response from a single goldfish ganglion cell with change in wavelength of stimulus, Wavelength of stimulus in millimicrons at upper right hand of each record. The duration of stimulus is indicated by the step in the signal trace at the base of each series. From Wagner et aZ. (1960).

W

OFF

l50p

ON

X.500

\

+

+

.++

-_.

I

i

I rnm

I

I

/

*+ i

I rnm

Fig. 7. Receptive field plots of the separate component responses of the same ganglion cell (goldfish), taken with stimuli of ( a ) 500 mp and ( b ) 650 inp. Stimulating spot, 150 p in diameter. From Wolbarsht et al. ( 1961).

2.

ELECTROPHYSIOLOGY OF THE RETINA

43

green off) are also found. The on and off components constituting the receptive field of color-coded ganglion cells can be separately evoked by choosing adequate wavelengths ( Wolbarsht et al., 1961). Figure 7 shows the result of separation by applying green (500 mp) and red (650 mp) light to such a unit. The sensitivity of the red component is generally higher in the center of the receptive field, but it falls off more sharply in the periphery than that of the green component. According to the recent work of Daw (1967), each of the red and green zones is very often surrounded by a zone of the opposite response type, which is detectable only by the use of annular light patch. In units such as illustrated in Fig. 7, for example, the organization of the receptive field as mapped with annular light patch could be on centeroff periphery for green (500 mp) and off center-on periphery for red (650 mp). Such arrangements explain the psychophysically known phenomenon of the simultaneous color contrast. The size of the periphery is very large, being at least 5 mm in diameter for both red and green.

IV. RESPONSE OF PHOTORECEPTORS

A. Early and Late Receptor Potential As already mentioned, the distal PI11 is the earliest potential in the ERG and is considered from its localization to correspond to the receptor potential identified in mammals by Brown et al. (1965). While this response has the latency of milliseconds and is easily evoked by moderate intensity of light, another type of response which has no substantial latency is elicited by very intense light flashes. This response was termed the “early receptor potential” (early RP) to distinguish it from the conventional receptor potential which was accordingly termed the “late receptor potential” (late RP) . The early RP was observed originally in the monkey (Brown and Murakami, 1964a,b), but later this was found to be universal to other animal forms including vertebrates and invertebrates. It is agreed that the early RP is a potential change associated with some steps of bleaching of the photopigment and that this is not generated by changes in membrane permeability but most likely by the net displacement of electric charge resulting from configuration changes in photopigment molecules by light (Pak and Cone, 1964; Pak, 1965; Brindley and Gardner-Medwin, 1966; Cone, 1967). For a net displacement of electric charge, a certain orientation of pigment molecules is necessary. Arden and Ikeda (1966) and Cone (1967) have provided

44

T. TOMITA

evidence that the early RF' is lost by disorientation of the pigment, although the pigment remains. It is probable that the lamellar and rhabdomeric structures in the vertebrate and invertebrate photoreceptors are related to the pigment orientation. Since most works on the early RP have been performed in animals other than the fish, no further description of this potential will be given. The receptor potential to be discussed below is the late RP.

B. Response of Single Photoreceptors Because of the extremely high density in population of photoreceptors in the retina, it is impossible to isolate single receptor responses without using intracellular micropipette electrodes. For intracellular recording, it is necessary to select retinas having large rods and/or cones. The carp has a mixed retina consisting of large cones (10 p across at the cone inner segment) and very slender rods of only a few microns across which are difficult to impale by micropipettes. Penetration into single carp cones has been successful by using pipettes having a tip diameter of less than 0.1 p, and with the aid of a device which jolts the retina at a high acceleration toward a vertically held, slowly advancing micropipette ( Tomita, 1965). Whenever a resting potential is recorded and a response to light is observed, the jolting is stopped to maintain the intracellular position of the pipette for further observations. With an electrode marking method the recording site has been identified as the cone inner segment (Kaneko and Hashimoto, 1967). The intracellularly recorded response to light is sustained and graded, and it is hyperpolarizing irrespective of the wavelengths of stimulating light. When the spectrum is scanned, the response spectra of single cones such as shown in Fig. 8 are obtained. The figure illustrates three response spectra which are maximally sensitive at different wavelengths. Figure 9 is the result of a statistical analysis, showing three average response spectra along with standard deviation curves. Their peaking wavelengths shown by histograms in Fig. 10 are close to those of absorption spectra of single cones measured in the goldfish with a microspectrophotometer ( Marks and MacNichol, 1963; Marks, 1965). No electrophysiological evidence has been provided to implicate lateral interaction between adjacent photoreceptors. The response of single cones is practically the same in size, being independent of the retinal area illuminated, and when tested with a small light spot the response falls off sharply as the spot moves off the recording site. This suggests that the individual receptors are functionally independent, making a

2.

45

ELECTROPHYSIOLOGY OF THE RETINA 1

400

.

1

500

'

1

600

1

4

700

Fig. 8. Sample recordings of response spectra from single carp cones demonstrating three cone types. Scanning of the spectrum was made in steps of 20 mp with monochromatic light adjusted to equal quanta ( 2 X lo5 photons/$ sec) and with a duration of light of 0.3 sec at each wavelength followed by an intermission of 0.6 sec. A downward deflection indicates negatively. Recording was made with a C-R coupled amplifier having a time constant of 0.5 sec. The spectral scale is given in terms of millimicrons at the top of the figure. A dominant peaking occurs at ( a ) blue, ( b ) green, and ( c ) red. From Tomita et al. ( 1967).

100-

-g a,

u 3 + .-

-

Q

-

E

50a m ,

-

0 c

a a m (

L

-

o , , , , , , , , , , , , , , , , , ,

Fig. 9. The averaged response spectra and standard deviation curves of three cone types (carp). From Tomita et al. (1967).

46

T. TOMITA

Mean = 4 6 2 A15 m p

N = 2 3 (16%)

400

500

600

700

’

G 0

Meon = 5 2 9 2 14 m p N=14 (10%)

0

: o

1

n

400

E

500

1

I

600

700

600

700

1

3

40-

3020-

Meon =611? 2 3 m p

N

105 ( 7 4 % )

10-

0-

,

400

I

500

Wavelength ( m p )

Fig. 10. Histograms of the peaking wavelengths of three cone types ( c a r p ) : ( B ) blue type, ( G ) green type, and ( R ) red type. From Tomita et al. (1967).

strong contrast to responses obtained from layers proximal to the receptors. The hyperpolarizing response of vertebrate photoreceptors has been a puzzle, but the latest experiment of Toyoda et al. (1969), although not in the fish, clearly demonstrates that the membrane conductance of single rods (in Gekko gekko) and cones (in Necturus muculosus) is decreased by light according to the degree of hyperpolarization, which is a function of the intensity of light. They also provide evidence that the vertebrate photoreceptors are kept depolarized in darkness and repolarized in light; the amplitude of response to light is increased by extrinsic hyperpolarizing current and decreased or even reversed by depolarizing current. Thus the vertebrate photoreceptor membrane behaves as if it were “excited

2.

ELECTROPHYSIOLOGY O F THE RETINA

47

in darkness and recovering by light toward the “resting state” in a graded manner. On this basis, the unusual polarity of the distal PI11 as the receptor potential can be accounted for in the following way. The electric field around the receptors in darkness is such as to make their distal tips negative owing to a sink existing at or near their distal tips just as in “excited” receptors. With light the sink disappears or becomes weaker, and this brings the distal tips to a relative positivity. The problem remains to be solved of whether or not the electrical response mediates the flow of information from the distal segment, where light is absorbed, to the proximal terminal, where synaptic transmission to secondary neurons takes place. If it does, the amplitude of intracellularly recorded response per photon absorbed should be large enough to meet the extremely high sensitivity of the visual system (some further discussion is given by Tomita, 1968).

V. RESPONSES IN THE INNER NUCLEAR LAYER

A. S Potential

Svaetichin ( 1953) observed in the fish that intraretinal micropipettes record a resting potential of some 40 mV at a certain depth, and, upon illumination with white light, a 20-30 mV hyperpolarization which is sustained and graded. In the belief that the potential was obtained intracellularly from single cones, he termed it the “cone action potential.” However, the response was later relocalized in structures proximal to the receptors (Tomita, 1957; Tomita et al., 1958, 1959; MacNichol and Svaetichin, 1958; Mitarai, 1958; Oikawa et al., 1959). The response had to be retenned accordingly, but different viewpoints regarding the origin and nature of the response brought about different terminologies. For example, the term “glial membrane potential” (GMP) is one of those, reflecting the viewpoint of Svaetichin and his co-workers ( Svaetichin et al., 1961; Laufer et al., 1961; Mitarai et al., 1961; Fatehchand et al., 1966) that the response is a manifestation of interaction between neurons and glia cells. They include the horizontal cells, Muller cells, and amacrine cells as glia cells. Morphological and physiological studies, however, do not always support their view. Structures typical of synapses have been found between the receptors, horizontal cells, and bipolar cells in the outer plexiform layer (Stell, 1965; Dowling and Boycott, 1966), and between the bipolar, amacrine, and ganglion cells in the inner plexiform layer (Dowling and Boycott, 1966). Dowling and Boycott did not

48

T. TOMITA

work on fish but on primate retinas. Under such circumstances, a simple term “S potential,” abbreviated from Svaetichin’s potential, is now most commonly used. The S potentials are classified into two major types from their response patterns to spectral light ( Svaetichin, 1956; MacNichol and Svaetichin, 1958). Those responding only with hyperpolarization to all wavelengths of light are called the “luminosity type” ( L response) and those in which the response polarity is wavelength-dependent are called the “chromaticity type” ( C response). The C response is further subdivided into Y-B (yellow-blue) type and R-G (red-green) type. Examples of these responses are illustrated in Fig. 11. The Lutianidae and other species collected in water 30 to 70 meters deep gave only an L response that had a peak toward the blue end of the spectrum. Of the shallow water fishes, the mullet, Mugil, gave both the R-G and Y-B responses in addition to the L response, but the Serranidae gave L and a

I

.

,

.

,

.

,

.

Fig. 11. Response spectra of S potentials from retinas belonging to different families. ( 1 ) Lutianidae (achromatic vision), which live deeper than 30 meters, giving only L responses with peak at about 490 mp. ( 2 ) Serranidae (dichromatic vision), giving both L and Y-B responses. ( 3 ) Centropomidae ( dichromatic vision ), giving both L and R-G responses. (4) Mugilidae, giving all three types of response. The fishes used to make records 2-4 were all caught in very shallow water. From MacNichol and Svaetichin (1958).

2.

ELECl'ROPHYSIOLOGY OF THE RETINA

49

Y-B responses while the Centropomidae L and R-G responses. It is evident that the types of responses are dependent upon the fish species used. Tamura and Niwa (1967) extended the exploration to some other species and divided the L response into three subtypes (L,-L,) according to the peaking wavelengths, and the C response into four (C1-C4) according to their spectral response patterns. In spite of the difference in classification, the results are similar to those of MacNichol and Svaetichin. From a number of studies (Mitarai, 1960; MacNichol and Svaetichin, 1958; Yamada and Ishikawa, 1965; Byzov and Trifonov, 1968) the site of recording of the S potential was suggested to be the horizontal cell. Recording is easier from retinas having large horizontal cells. The problem is how and where it originates. The localization of S potential in a horizontal cell might not necessarily mean that the membrane of that cell is responsible for the electrogenesis. The recorded potential could be of a passive nature conducted to the recording site from some other structures. In the discussion below concerning the electrogenesis of S potential, let us confine our attention to the L response, just to make the matter simple. After the theory of glia-neuron interaction had been argued for many years without much agreement, Trifonov (1968) presented a new hypothesis that the S cell, which is the horizontal cell in synaptic contact with photoreceptors, is kept depolarized or facilitated in the dark by transmitter substance continuously released from the receptor terminals, and that light acts to suppress the release of transmitter with the result that the S cell is repolarized or disfacilitated. Related to this hypothesis are the observations of Trifonov and Byzov (1965) on the turtle and Byzov and Trifonov ( 1968) on the carp that the S cell which is hyperpolarized or disfacilitated in the presence of adapting light responds to a transretinally applied sclera-positive electric pulse with a depolarization, the amplitude of which is graded according to the intensity of the stimulating pulse, and, if the intensity is h e d , to the degree of hyperpolarization by light. From the polarity of current pulses effective for eliciting the depolarizing response, Trifonov and Byzov consider the receptor endings as the acting site of the pulses which effect a release of transmitter. The new hypothesis of Trifonov is attractive, particularly when the vertebrate photoreceptors are found to function in a similar way: depolarized in darkness and repolarized in light. From this new viewpoint, some of the properties of the S potential, which appeared not to be the kind of electrical activity usually ascribed to neurons, are now better understood. Although not always clearly demonstrated as in the photoreceptors, the amplitude of S potential tends to increase (decrease) by extrinsically applied hyperpolarizing ( depolarizing) current ( Mura-

50

T. TOMITA

kami and Kaneko, cited in Tomita, 1965), and the conductance of the S cell tends to decrease during response to illumination (Toyoda et al., 1969). These are exactly what are observed in the photoreceptors. It should be noted that in spite of the similarities between the S potential and single photoreceptor response there is one distinct difference in that the S potential has a large area effect or a strong dependence of the response amplitude upon the retinal area illuminated, while the single photoreceptor response has substantially no such effect. A convergence of a great many photoreceptors to S cells is suggested. Lateral electric connections between S cells can multiply the effect, and the electron microscope has proved tight junctions between adjacent horizontal cells. The argument might not be complete without reference to the C response. The depolarization in the C cell at certain bands of spectrum appears to be associated with an increase in the conductance of the cell. Conceivably, the C cell is in a half-facilitated state in the dark, and a further facilitation is caused by certain wavelengths of light, while disfacilitation is caused by other wavelengths.

B. Responses in Other Cell Types The intracellular study of single cells in the inner nuclear layer of the fish retina has become possible only recently by the application of a technique developed for single photoreceptors. Figure 12 illustrates three response types recorded in this layer by Kaneko and Hashimoto (1969). They are on, off, and on-off type, being substantially the same as in single ganglion cells. Some of these cells even respond with impulse spikes superimposed on depolarizing phases of slow potentials, confirming the extracellular observation of Brown and Wiesel (1959) in the cat. More common in the carp, however, are those responding to light with slow membrane potential changes superimposed by nonunitary spikelike or oscillatory fluctuations on the depolarizing phases ( Fig. 12). The organization of the receptive field of these cells also resembles that of the ganglion cells, as demonstrated in the cat by Brown and Wiesel (1959). Some have a receptive field of everywhere on, everywhere off, or everywhere on-off, but others have the concentric receptive field such as on center-off periphery or the reverse. The response in the periphery is more easily detectable by an annular light patch. The size of the receptive field is larger than predicted from the dendritic field of bipolar cells and seems to be comparable with that of the S potential. From these observations, Kaneko and Hashimoto suspect that the S cells might intervene between the photoreceptors and these cells to convey information, at least in the periphery of the receptive field.

51

2 . ELECTROPHYSIOLOGY OF THE RETINA

-

-L*bLLLLt 0 I + I S EC -U-iLLU

Fig. 12. Sample records from units in the inner nuclear layer (carp), ( a ) on type, ( b ) off type, and ( c ) on-off type. From Kaneko and Hashimoto ( 1969).

Localization of these cells in the inner nuclear layer has been established by means of the electrode marking, but the identification of cell types is left to future studies [but see the latest work of Werblin and Dowling ( 1969) and Kaneko ( 1970) 1, VI. RETINAL MECHANISMS OF COLOR VISION

The above description of electrical events at each retinal level involved observations related to the color coding. Accordingly, the main task now will be to collect these scattered data along with some others not yet referred to and to arrange them for a brief summary of the present status of understanding of color vision. The behavioral aspects of color vision will be discussed in Chapter 3, by Ingle. Over a century and a half ago, Young (1802) theorized in man that the color discrimination is mediated by three photoreceptor substances, each maximally sensitive to a different region of the spectrum. Two recent studies on single cones of Cyprinidae, one microspectrophotometric ( Marks, 1965) and the other electrophysiological ( Tomita et al., 1 9 6 7 ) , have shown that Young's trichromatic theory also applies to the fish, although it remains to be studied how far the findings in Cyprinidae can

52

T. TOMITA

be extended to other fish species. Results from these entirely different approaches were substantially the same in differentiating three pigments, each contained in different cone groups. Their maximally sensitive wavelengths are compared in the accompanying tabulation.

462 f 15

529 rt 14

611 k 23

455 f 15

530 f 5

625 f 5

Tomita et al. (1967) (carp) Marks (1965) (goldfish)

Coming to the S potential level, the processes of Hering’s opponent color type (1878) predominate. Hering’s theory, which was derived from psychological observations, states that there are four primary colors which are coupled in mutually antagonistic pairs; red-green and yellow-blue. Apparently, the R-G type and Y-B type of C responses in Fig. 11 substantiate these pairs. A transformation from Young’s type to Hering’s type at a certain level of the visual pathway has been suggested by some pioneer workers such as von Kries ( 1905) and Schrodinger ( 1925). It is now obvious that the site of the transformation is the synaptic network in the outer plexiform layer. Concerning the mechanism of the transformation, however, little is known. It might be that the positive component of a C response is related to one cone type and the negative component to another cone type (Orlov and Maksimova, 1965). On this assumption, the absorption maxima of red and green pigments should be determined from analysis of the R-G type of S potential. The result of work along this line by Naka and Rushton ( 1966a,b,c), however, is not consistent with the result of direct spectrophotometric measurements on single cones. Naka and Rushton find that the red-green units peak at 680 and 540 mp, respectively. Marks found a green pigment with maximum near 540 mp a d a red pigment with maximum near 620 mp. In addition, Naka and Rushton find an L-type unit with maximum at 620 mp. Although they have not yet measured a blue component, if present, this means that in Cyprinidae there are four types of cone pigments. [The analysis of C respase by Witkovsky (1967) also suggests a far-red pigment with maximum at 665 mp.] The significance of four cone pigments instead of three, as suggested from the analysis of S potential, remains to be explained. At the ganglion cell level, the rule of Hering’s type is also obvious (Fig. 6), but the component analysis becomes more and more difficult

2.

ELECTROPHYSIOLOGY OF THE RETINA

53

as we go further away from the receptors. Witkovsky ( 1965), working on single carp ganglion cells, concludes that in the photopic state the three pigments of Marks are just suEcient to account for all spectral response types at the ganglion cell level. MacNichol et al. ( 1961), on the other hand, report a red component maximally sensitive at 650 mp in single goldfish ganglion cells. Approaches to the same problem but using the ERG as the index were made recently by Burkhardt (1966, 1968) and Witkovsky (1968). REFERENCES Adrian, E. D., and Matthews, R. (1927a). The action of light on the eye. I. The discharge of impulses in the optic nerve and its relation to the electric change in the retina. J. Physiol. (London) 63, 378414. Adrian, E. D., and Matthews, R. ( 1927b). The action of light on the eye. 11. The processes involved in retinal excitation. J. Physiol. (London) 64, 279-301. Arden, G. B., and Ikeda, H. (1966). Effects of hereditary degeneration of the retina on the early receptor potential and the corneo-fnndal potential of the rat eye. Vision Res. 6, 171-184. Bernhard, C. G., and Skoglund, C. R. ( 1941). Selective suppression with ethylalcohol of inhibition in the optic nerve and of the negative component PI11 of the electroretinogram. Actu Physiol. Scund. 2, 10-21. Brindley, G. S. (1960). “Physiology of the Retina and Visual Pathway.” Arnold, London. Brindley, G. S., and Cardner-Medwin, A. R. ( 1966). The origin of the early receptor potential of the retina. J . Physiol. (London) 182, 185-194. Brown, K. T., and Murakami, M. (1964a). A new receptor potential of the monkey retina with no detectable latency. Nature 201, 626428. Brown, K. T., and Murakami, M. (1964b). Biphasic form of the early receptor potential of the monkey retina. Nature 204, 739-740. Brown, K. T., and Wiesel, T. N. (1959). Intraretinal recording with micropipette electrodes in the intact cat eye. J. Physiol. (London) 149, 537-562. Brown, K. T., and Wiesel, T. N. ( 1961). Localization of origins of electroretinogram components by intraretinal recording in the intact cat eye. J. Physiol. (London) 158, 257-280. Brown, K. T., Watanabe, K., and Murakami, M. (1965). The early and late receptor potentials of monkey cones and rods, Cold Spring Harbor Symp. Quant. Biol. 30, 457482. Burkhardt, D. A. ( 1966). The goldfish electroretinogram: Relation between photopic spectral sensitivity functions and cone absorption spectra. Vision Res. 6, 517-532. Burkhardt, D. A. (1968). Cone action spectra: Evidence from the goldfish electroretinogram. Vision Res. 8, 839-853. Byzov, A. L. (1962). On the origin and some properties of the PIII-component of the frog electroretinogram. Proc. Intern. Union Physiol. Sci., 22nd Intern. Congr., Leiden, 1962 Vol. 1, Part 1, pp. 473-476. Excerpta Med. Found., Amsterdam. Byzov, A. L., and Trifonov, Yu. A. (1968). The response to electric stimulation of horizontal cells in the carp retina. Vision Res. 8, 817-832. Cone, R. A. ( 1967). Early receptor potential: Photoreversible charge displacement in rhodopsin. Science 155, 1128-1131.

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Daw, N. W. (1967). Goldfish retina: Organization for simultaneous color contrast. Science 158, 942-944. Detweiler, S. R. ( 1943). “Vertebrate Photoreceptors.” Macmillan, New York. Dowling, J. E., and Boycott, B. B. (1966). Organization of the primate retina; electron microscopy. Proc. Roy. SOC.B166, 80-111. Dowling, J. E., and Werblin, F. S. (1969). Organization of retina of the mudpuppy, Necturus maculosus. I. Synaptic structure. J. Neurophysiol. 32, 315-338. Fatehchand, R., Svaetichin, G., Negishi, K., and Drujan, B. (1966). Effects of anoxia and metabolic inhibitors on the S-potential of isolated fish retinas. Vision Res. 6, 271-283. Franz, V. ( 1913). Sehorgan. In “Oppels Lehrbuch der Vergleichenden Mikroskopischen Anatomie der Wirbeltiere.” Fischer, Jena. Granit, R. (1933). The components of the retinal action potential and their relation to the discharge in the optic nerve. J. Physiol. (London) 77, 207-240. Granit, R. ( 1947 ). “Sensory Mechanisms of the Retina.” Oxford Univ. Press, London and New York. (hinit, R. (1962). Neurophysiology of the retina. In “The Eye” ( H . Davson, ed.), Vol. 2, pp. 575-692. Academic Press, New York. Granit, R., and Riddell, L. A. (1934). The electrical responses of light- and darkadapted frog’s eyes to rhythmic and continuous stimuli. J. Physiol. (London) 81, 1-28. Granit, R., and Svaetichin, G. ( 1939). Principles and technique of the electrophysiological analysis of colour reception with the aid of micro-electrodes. Upsalu Lakareforenings Forh. 65, 161-177. IIamasaki, D. I., and Bridges, C. D. B. (1965). Properties of the electroretinogram in three elasmobranch species. Vision Res. 5, 483-496. Hartline, H. K. (1935). Impulses in single optic nerve fibres of the vertebrate retina. Am. J. Physiol. 113, 59P. Hartline, 13. K. (1938). The response of single optic nerve fibers of the vertebrate eye to illumination of the retina. Am. J. Physiol. 121, 400415. Hering, E. ( 1878). “Znr Lehre vom Lichtsinne,” pp. 107-141. Carl Gerold’s Sohn, Vienna. Hohngren, F. (1865). Method att objectivera effecten av ljusintryck p i retina. Upsala Lakareforenings Forh. 1, 177-191. Kaneko, A. ( 1970). Physiological and morphological identification of horizontal, bipolar and amacrine cells in goldfish retina. J. Physiol. 207, 623-633. Kaneko, A., and Hashimoto, H. (1967). Recording site of the single cone response determined by an electrode marking technique. Vision Res. 7 , 847-851. Kaneko, A., and Hashimoto, H. ( 1969). Electrophysiological study of single neurons in the inner nuclear layer of the carp retina. Vision Res. 9, 37-55. Kobayashi, H. (1962). A comparative study on electroretinogram in fish, with special reference to ecological aspects. J. Shimonoseki Col2. Fisheries 3, 407-538. Kuffler, S. W. (1953). Discharge patterns and functional organization of mammalian retina. J. Neurophysiol. 16, 37-68. Laufer, M., Svaetichin, G., Mitarai, G., Fatehchand, R., Vallecalle, E., and Villegas, J. ( 1961). The effect of temperature, carbon dioxide and ammonia on the neuronglia unit. In “The Visual System: Neurophysiology and Psychophysics” ( R . Jung and H. Kornhuber, eds. ), pp. 457-463. Springer, Berlin. LlacNichol, E. F., and Svaetichin, G. (1958). Electric responses from isolated retinas of fishes. Am. J. Ophthalmol. [3] 46, Part 2, 2 6 4 0 .

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MacNichol, E. F., Wolbarsht, M. L., and Wagner, H. G. (1961). Electrophysiological evidence for a mechanism of color vision in the goldfish. I n “Light and Life” ( W . D. McElroy and B. Glass, eds.), pp. 795-816. Johns Hopkins Press, Baltimore, Maryland. Marks, W. B. ( 1965). Visual pigments of single goldfish cones. J. Physiol. ( L o n d o n ) 178, 14-32. Marks, W. B., and MacNichol, E. F. (1963). Difference spectra of single goldfish cones. Federation Proc. 22, 2143 (abstr.). Mitarai, G. (1958). The origin of the so-called cone action potential. Proc. Japan Acad. 34, 299-304. Mitarai, G. ( 1960). Determination of ultramicroelectrode tip position in the retina in relation to S potential. J. Gen. Physiol. 43, Part 2, 95-99. Mitarai, G., Svaetichin, G., Vallecalle, E., Fatehchand, R., Villegas, J., and Laufer, M. ( 1961). Glia-neuron interaction and adaptational mechanisms of the retina. In “The Visual System: Neurophysiology and Psychophysics” ( R. Jung and H. Kornhuber, eds. ), pp. 463-481. Springer, Berlin. Motokawa, K., Oikawa, T., Tasaki, K., and Ogawa, T. (1959). The spatial distribution of electric responses to focal illumination of the carp’s retina. Tohoku J. Exptl. Med. 70, 151-164. Motokawa, K., Yamashita, E., and Ogawa, T. (1960). Studies on receptive fields of single units with colored lights. Tohoku J. Exptl. Med. 71, 261-272. Motokawa, K., Yamashita, E., and Ogawa, T. (1961). Slow potentials and spike activity of retina. J. Neurophysiol. 24, 101-110. Murakami, M., and Kaneko, A. (1966). Differentiation of PI11 subcomponents in cold-blooded vertebrate retinas. Vision Res. 6, 627-636. Murakami, M., and Sasaki, Y. (1968a). Analysis of spatial distribution of the ERG components in the carp retina. Japan. J. Physiol. 18, 326-336. Murakami, M., and Sasaki, Y. (198813). Localization of the ERG components in the carp retina. Japan. J. Physiol. 18, 337-349. Naka, K. I., and Rushton, W. A. H. ( 1966a). S-potentials from colour units in the retina of fish (Cyprinidae). J. Physiol. ( L o n d o n ) 185, 5 3 M 5 5 . Naka, K. I., and Rushton, W. A. H. (1966b)).An attempt to analyse colour reception by electrophysiology. J. Physiol. ( L o n d o n ) 185, 556-586. Naka, K. I., and Rushton, W. A. H. ( 1 9 6 6 ~ )S-potentials . from luminosity units in the retina of fish (Cyprinidae). J. Physiol. ( L o n d o n ) 185, 587-599. Noell, W. K. (1954). The origin of the electroretinogram. Am. J. Ophthulmol. 38, 78-90. Oikawa, T., Ogawa, T., and Motokawa, K. (1959). Origin of so-called cone action potential. J . Neurophysiol. 22, 102-11 1. Orlov, 0. Yu., and Maksiniova, E. hl. (1965). S-potential sources as excitation pools. Vision Res. 5, 573-582. Pak, W. L. (1965). Some properties of the early electrical response in the vertebrate retina. Cold Spring Harbor Symp. Quant. Biol. 30, 493-499. Pak, Mi. L., and Cone, R. A. (1964). Isolation and identification of the initial peak of the early receptor potential. Nature 204, 8364338. Schrodinger, E. ( 1925). Uber das Verhaltnis der Vierfarben- zur Dreifarbentheorie. Sitzber. Akad. Wiss. W i e n , Math.-Naturw. Kl. Abt. Ila, 134, 471490. Sjostrand, F. S. (1961). Electron microscopy of the retina. I n “The Structure of the Eye” (G. K. Smelser, ed.), pp. 1-28. Academic Press, New York.

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Stell, W. K. (1965). Correlation of retinal cytoarchitecture and ultrastructure in golgi preparations. Anat. Record 153, 389-398. Svaetichin, G. (1953). The cone action potential. Acta Physiol. Scand. 29, Suppl. 106, 565-600. Svaetichin, G. (1956). Spectral response curves from single cones. Acta Physiol. Scand. 39, Suppl. 134, 1 7 4 6 . Svaetichin, G., Laufer, M., Mitarai, G., Fatehchand, R., Vallecalle, E., and Villegas, J. (1961). Glial control of neuronal networks and receptors. In “The Visual System: Neurophysiology and Psychophysics” (R. Jung and H. Kornhuber, eds. ), pp. 445-456. Springer, Berlin. Tamura, T., and Niwa, H. (1967). Spectral sensitivity and color vision of fish as indicated by S-potential. Comp. Biochem. Physiol. 22, 745-754. Tomita, T. (1950). Studies on the intraretinal action potential. Part I. Relation between the localization of micropipette in the retina and the shape of the intraretinal action potential. Japan. J . Physiol. 1, 11Cb117. Tomita, T. ( 1957). A study on the origin of intraretinal action potential of the cyprinid fish by means of pencil-type microelectrode. Japan. J . Physiol. 7 , 80-85. Tomita, T. ( 1965). Electrophysiological study of the mechanisms subserving color coding in the fish retina. Cold Spring Harbor Symp. Quunt. Biol. 30, 559566. Tomita, T. (1968). Electrical response of single photoreceptors. Proc. IEEE, 56, 1015-1023. Tomita, T., and Kaneko, A. (1965). An intracellular coaxial microelectrode-its construction and application. Med. Electron. Biol. Eng. 3, 367-376. Tomita, T., Tosaka, T., Watanabe, K., and Sato, Y. (1958). The fish EIRG in response to different types of illumination. Japan. J. Physiol. 8, 41-50. Tomita, T., Murakami, M., Sato, Y., and Hashimoto, Y. (1959). Further study on the origin of the so-called cone action potential (S-potential). Its histological determination. Japan. J. Physiol. 9, 6-8. Tomita, T., Kaneko, A., Murakami, M., and Pautler, E. L. (1967). Spectral response curves of single cones in the carp. Vision Res. 7, 519-531. Toyoda, J., Nosaki, H., and Tomita, T. (1969). Light-induced resistance changes in single photoreceptors of Necturus and Gekko. Vision Res. 9, 453463. Trifonov, Yu. A. (1968). Study of synaptic transmission between photoreceptors and horizontal cells by means of electric stimulation of the retina. ( I n Russian.) Biofizika 13, N5. Trifonov, Yu. A., and Byzov, A. L. (1965). The response of the cells generating S-potential on the current passed through the eye cup of the turtle. ( I n Russian. ) Biofzika 10, 673-680. Villegas, G. M. (1960). Electron microscopic study of the vertebrate retina. J. Gen. Physwl. 43, No. 6, Part 2, 1543. Villegas, G. M., and Villegas, R. (1963). Neuron-glia relationship in the bipolar cell layer of the fish retina. J. Ultrastruct. Res. 8, 89-106. von Kries, J. ( 1905). Die Gesicktsempfindungen. In “Handbuch der Physiologie des Menschen” (W. Nagel, ed.), Vol. 3, pp. 109-282. Viewveg, Braunschweig. Wagner, H. G., MacNichol, E. F., and Wolbarsht, M. L. (1960). The response properties of single ganglion cells in the golash retina. I. Gen. Physiol. 43, 45-62. Werblin, F. S., and Dowling, J. E. (1969). Organization of the retina of the mudpuppy, Necturus maculosus. 11. Intracellular recording. ]. Neurophysiol. 32,

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Witkovsky, P. (1965). The spectral sensitivity of retinal ganglion cells in the carp. Vision Res. 5, 603-614. Witkovsky, P. ( 1967). A comparison of ganglion cell and S-potential response properties in carp retina. J. Neurophysiol. 30, 546-561. Witkovsky, P. (1968). The effect of chromatic adaptation on color sensitivity of the carp electroretinogram. Vision Res. 8, 823-837. Wolbarsht, M. L., Wagner, H. G., and MacNichol, E. F. (1961). Receptive fields of retinal ganglion cells: Extent and spectral sensitivity. I n “The Visual System: Neurophysiology and Psychophysics” (R. Jung and H. Kornhuber, eds.), pp. 170-177. Springer, Berlin. Yamada, E., and Ishikawa, T. (1965). Fine structure of the horizontal cells in some vertebrate retinae. Cold Spring Harbor Symp. Quant. Biol. 30, 383-392. Young, T. (1802). On the theory of light and colours. Phil. Trans. Roy. SOC. London 92, 12-48.

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VISION: T H E EXPERIMENTAL ANALYSIS OF VISUAL BEHAVIOR DAVID INGLE I. Introduction . . . . . . . . . 11. Relative Discrimination Weaknesses . . . . 111. Configurational Properties of Shapes . . . . IV. Perceptual Equivalence and Change in Spatial Position V. Selective Attention . . . . . . . . VI. Toward a Unified Outlook on Visual Behavior . . . . . . . . . . . . . References

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I. INTRODUCTION

This chapter reviews some analytical studies of fish visual behavior, laying the chief emphasis upon discrimination abilities demonstrated in the laboratory. Since many fishes depend heavily upon vision to guide their countless daily decisions, a complete review would ramble far and wide without providing a good account of our present knowledge concerning underlying visual mechanisms. Therefore, those studies were selected that seem most useful in analyzing fundamental visual processes -studies that seem most heuristic for a future liason between psychologists and those biologists who probe the visual system with scalpels and electrodes. This restriction in subject matter is not entirely arbitrary, since the best behavioral analyses have often been performed by those workers who have hoped to dissect visual behavior into manageable component processes. Most of the older European studies of fish vision are regretably ignored in this chapter. With a few exceptions, these workers did not use enough subjects or a rigorous enough experimental design to allow firm conclusions. Their aim was apparently to find in the fish kinds of visual behavior that we more readily attribute to higher vertebrates: visual 59

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constancies, susceptibility to illusions, or the ability to view different pairs of stimuli as having the same mutual relationships. Very often these studies stimulate the scientific imagination, only to leave the reader hungry for solid proof of the inherent claims. Nevertheless, the scholarly reader will be interested in the array of observations and experiments reviewed by Herter ( 1953). The first aim of the analytical approach is a physical description of the essential attributes of visual objects which guide their “recognition” or their “discriminability.” Objects can be distinguished along various dimensions-size, brightness, color, distance, orientation, and motionand psychologists have usually assumed that each dimension has a discrete physiological basis. Neurophysiologists have discovered within the visual pathways of various vertebrate species single neurons that are, in fact, tuned in to specific dimensions of the stimulus: brightness, color, orientation, or motion. Some of the data of perception is filtered out within the retina and along the retinofugal pathways, and other data are provided by internal central processes ( or other sensory modalities) whose function can be tenuously and imprecisely inferred. The present studies offer relevent information in approaching the first question of selectivity of the visual system: What features of the visual array do fish particularly notice? One kind of analysis likely to reflect limits of peripheral visual processing is the measurement of minimum separable acuity. Weiler (1966) obtained threshold acuity values of 5.3 min of visual angle, averaging the performance of three “Oscars,” Astronotus ocellatus, required to discriminate finely spaced dot patterns from a solid gray plaque. Taking the calculations of Brunner (1935), this limiting angle approximates the diameter of single retinal cones. Of course acuity of primates and some avian species measures far less than their cone diameters. Perhaps the fish has not evolved a mechanism for extracting this additional information through “temporal integration” of signals from the moving retinal image. A second area of research where retinal physiology ought to prove useful to the psychologist is that of color vision. The analysis of goldfish cones by Marks (1965) reveals three separate retinal photopigments and seems to imply that this species should possess trichromatic color vision. A logical proof that fish had color vision at all awaited the study of McCleary and Bernstein (1959) who showed that generalization between pairs of colored stimuli (red and green) could not be attributed to brightness cues, after first determining their subjects’ judgement as to the relative brightness of the critical stimuli. More recently, Muntz and Cronly-Dillon (1966) obtained evidence that goldfish color vision was very probably trichromatic: their subjects could distinguish reds, greens,

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and blues from one another where reliance on brightness cues was excluded. Furthermore, Yager (1967, 1968) demonstrated the ability of goldfish to detect additions of monochromatic light of any hue to pure white light. The saturation functions so determined in three subjects approximated quantitative predictions that Yager had derived from an "opponent color theory" model, itself based upon studies of human color vision. These lines of research-acuity and color measurements-off er twin rewards: detailed interspecies comparisons of the psychophysical laws of visual function and fruitful correlation with anatomy, biochemistry, and electrophysiology. These studies help to confirm a faith in the existence of discrete units that underly visual behavior. Yet further complexities have simply been avoided thus far by psychologists: measurement of "color constancy" or the interaction of color with form vision. In the following sections we shall review other dimensions of fish vision for which our image of discrete visual analyzers offers a still tenuous and unfulfilled hypothesis.

11. RELATIVE DISCRIMINATION WEAKNESSES

The search for relative weaknesses in shape recognition that might be attributed to limits of peripheral analysis of the visual image is exemplified by Mackintosh and Sutherland (1963), who found that goldfish took longer to acquire a discrimination between 45" and 135" oblique rectangles than to distinguish equivalent shapes set horizontally and vertically (Fig. 1). Sutherland has argued ( 1968) that the ability to distinguish orientations of contours might require orientation-specific visual units-somewhere within the afferent system-such as the units described by Hubel and Wiesel (1962) in the cat visual cortex. Indeed, Westerman (1965) has reported that elongated receptive fields of units recorded in the goldfish tectum do seem to fall more often along horizontal or vertical axes than along oblique axes. If goldfish have a paucity of units sensitive to oblique contours, they may nevertheless notice the horizontal or vertical extents of oblique edges. In fact, fish trained by Mackintosh and Sutherland to discriminate vertical from horizontal continued during transfer tests to distinguish shapes oriented at 30" from those set at 60" If goldfish pay more attention to horizontal and vertical components, even of such tilted shapes, it would be important to see whether fish initially trained on the 30"/60" problem improve their performance when tested with the 0"/90"pair.

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I '

7 - .......

I 1

Fig. 1. Apparatus used by Mackintosh and Sutherland (1963) to test the discriininability of dark rectangles at various orientations. The fish swims from forward chamber through hole and obtains food pellet from trough a t middle of correct shape. The incorrect shape is baited with a stone.

It must be pointed out that even such clear-cut results as these are subject to alternative interpretations. Although Ingle ( 1971) confirmed the conclusion of Mackintosh and Sutherland that goldfish have special difficulties with oblique lines, the suggestion was made that fish might nevertheless detect oblique lines as well as those at any other orientation. They might not remember them from trial to trial, having no built-in central classification scheme for distinctions among obliques. A critical experiment would measure the fish's ability to detect minimally visible edges (or gratings) set at various orientations. Such studies of acuity, although often necessary to interpret specific discrimination weaknesses with shapes, are conspicuously absent from the history of animal studies. Sutherland's recent report ( 1968) that goldfish tend to notice differences at the tops-rather than the bottoms-of a pair of shapes, raises similar problems of interpretation. A goldfish approaching the middle of a figure (where food is placed) might simply be in a better position to view the top of the shape. It would be important to determine whether goldfish detect isolated spots or colors more readily when placed at the top of a blank disc, baited in the center. At least one study, by Matthews (1964) in the blue acara, suggests that sensitivity to the top of a shape is not always the rule: His fish failed to distinguish a triangle with a

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bowed-out base from a circle but easily discriminated this pie-wedge shape from a triangle. Early workers also noted directional biases in fish vision, but they seldom used enough subjects to allow firm conclusions. Yet, in the light of subsequent knowledge, it is interesting that Hager (1938) found minnows discriminating one vs. two stripes, or three vs. four stripes, more readily when these patterns were oriented vertically. As Trevarthen ( 1968a) has emphasized, such frontally approached stimuli generate expanding retinal images, which could be analyzed by retinal units sensitive to image motion. Two studies (Jacobson and Gaze, 1964; CronlyDillon, 1964) have revealed that goldfish ganglion cells sensitive to direction of motion are most often sensitive to horizontal movement of small objects or edges. This mechanism might apply as well to Meesters' finding (1940) that a single fish trained to distinguish large and small squares transferred best to long vs. short rectangles when these shapes were horizontally oriented. That is to say, Meesters' subject was more sensitive to image-expansion (measuring the distance between the two edges) when the differences lay along the horizontal axis. An observation by Saxena (1966) suggests that the trout may-in distinction-pay more attention to size differences along the vertical axis. Her subjects failed to transfer a size discrimination involving outline squares when either the top or bottom side was removed (as did a subject tested in the same way by Meesters, 1940). Unfortunately, control experiments involving removal of a vertical side were not reported. It might be useful to pin down such possible species differences in shape recognition, since we assume that visual analyzing mechanisms are variously adapted to the ecology or social behavior of the species. The trout, for example, must execute h e distance judgments within the upper sagittal plane preparatory to jumping for an insect and might well profit from a mechanism sensitive to distances along the vertical axis, An electrophysiological study by Jacobson and Gaze (1964) provides further information on directional bias within the goldfish visual system. Retinal units were more often sensitive to nasalward-as opposed to temporalward-movement of spots within the visual fields. This built-in asymmetry might explain the observation by Harden Jones (1963) that several species of fish would follow nasalward but not temporalward rotation of a surrounding striped drum. Furthermore, Ingle (1967) has demonstrated a behavioral correlate of this directional bias using a cardiac-conditioning method (Fig. 2 ) . When a small 2" spot-moving at 12"1sec in the lateral field-served as a conditioned stimulus paired with shock, all subjects responded with a stronger cardiac deceleration during nasalward motion of the stimulus. This stimulus was comparable to that

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I

Nasal bias

I

I

Temporal bias

Fig. 2. Apparatus used by Ingle (1967) for cardiac conditioning of goldfish. Single or multiple dots moving behind window in aquarium serve as conditioned stimuli when paired with shock.

used by Jacobson and Gaze ( 1964). However, when the same spot moved at only 3"/sec ( a velocity not systematically studied by the physiologists), each of the six fish showed significant cardiac slowing only to the temporalward stimulus. This reversal of directional sensitivity obtained by changing velocity implies two separate underlying mechanisms (not necessarily to be identified with two different unit populations). A third group of goldfish showed greater temporalward responses when a multispotted stimulus was used, even where velocity was 12"/sec. Experiments on motion detection may be useful in establishing further behavioralphysiological correlations since parameters can be precisely varied in both kinds of study. Furthermore, a detailed knowledge of motion perception seems necessary for full interpretation of shape recognition sincc all procedures utilize moving shapes or moving retinal images.

111. CONFIGURATIONAL PROPERTIES OF SHAPES

The above experiments on discrimination of orientation or motion of objects belong to a psychology geared to physiological models. Yet, traditional concerns about configurational properties ( largely raised by the

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Gestalt school) still guide behavioral research. Since we ourselves recognize many shapes despite transformations of size, position, rotation, or brightness contrast, we are tempted to ask whether our animal subjects also have ways of categorizing shapes that transcend these transformations. For example, it is commonly assumed that shapes are easily recognized by animals, despite changes in size (Sutherland, 1968). However, the usual method of demonstrating “size invariance” is inadequate since an animal approaching the stimulus (for food reward) experiences considerable variation in size of the associated retinal images of each shape. To overcome this difficulty, Ingle ( 1971) trained goldfish to discriminate 14” wide circles from equal-sized squares that were presented at a fixed distance lateral to one eye during a conditioned avoidance paradigm. When well trained, five fish learned the opposing habit using smaller (7”) shapes. For example, fish avoiding the large circle but not the square now learned to avoid the small square but not the circle. When subjects were retested (without reinforcement) using the original large shapes, they retained the initial habit despite the intervening training. We cannot say that these fish found no resemblance between large and small squares, but rather that they used two different sets of rules in distinguishing the two pairs of shapes. These results do not settle the important problem of size invariance, but they do indicate that size is sometimes an important determinant of the way a fish can classify particular shapes. Studies by Bowman and Sutherland (reported in Sutherland, 1968) indicate that goldfish do not generalize the properties of a square through a 45” rotation to a diamond. Following training on a circle vs. square discrimination, their subjects failed to distinguish a circle from a diamond. However, other discriminations may be transposed through rotations. Both Schulte (1957) and Saxena (1966) found good transposition of a discrimination of stripe number (and width) from vertical to horizontal settings (Fig. 3 ) . This task requires attention to number of line elements rather than to their spatial relationship. Saxena noted that a square vs. X discrimination would generalize to a diamond vs. cross pair. Here the discrimination is not one of orientation but a topological distinction between open shapes vs. intersecting lines. These experiments show that fish can judge two stimuli (e.g., squares and diamonds) either as equivalent or as distinctive, depending upon which basis of comparison our training procedure forces upon them. It is the aim of the generalization test to indicate the kinds of “similarity” that may exist. As Kliiver (1933) has documented in elegant detail, it is difficult to disentangle the actual dimensions of visual analysis from even an assortment of such tests. The assumption that fish possess inherently fewer and more rigid classifications than the higher mammals is plausible but is by no means demonstrated. Ingle ( 1971) has speculated that large-scale “gestalt” features such

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nrm X Fig. 3. Stimuli used by Sutherland (1968), Schulte (1957),and Saxena (1966) for studies of discrimination transfer to patterns rotated by 45’. Generalization between the first two pairs of shapes failed while the other two transposition problems were successful with carp and with trout, respectively.

as “parallelness” of left and right sides can be used to distinguish circles from squares. This argument assumes that fish are less sensitive to parallelness along the oblique axis (as with a diamond) than along horizontal and vertical axes as with a square. Therefore, the diamond, with oppositely bent sides, ought to resemble a circle to some extent. In fact, goldfish trained to avoid 14” circles, discriminated squares more easily than diamonds as “no go” stimuli. It would be useful to know whether diamonds are actually more similar to circles than to squares and whether or not such results would be obtained with smaller stimuli as well. In any case, the notion that fish discriminate parallelness of contours has been demonstrated by a more direct procedure. Fish that learned to avoid a stimulus containing a pair of parallel lines (both vertical or both tilted 30” from vertical) readily learned to discriminate a nonparallel pair (vertical plus 30”) as “no go” but did not learn to withhold response to the second set of parallel lines. These subjects did not rely upon the individual orientation of isolated line segments or else they would more easily discriminate between the two parallel sets of lines, where both parts of one differed from both parts of the other. Rather, the relationship between parts of a figure ( parallelness) outweighed the potentially useful information about individual lines. Such visual abilities reflect high order perceptual processes that seem not to depend upon awareness of those

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peripheral mechanisms required for the final classification (i.e., some measurement of individual line orientation must precede or participate in the determination of parallelness ) . The implications of this argument for a theoretical approach to visual coding is discussed in some detail by Sutherland ( 1968). These discussions of the circle-square discrimination have supposed that curved and straight lines are easily distinguished. Ingle (1971) performed a further test to determine whether the recognition of “curvature” is invariant with changes in orientation (Fig. 4).For example, goldfish trained to avoid a shallow horizontal curve easily learned to withhold response to a horizontal line but were confused by a vertical curved line. Similarly, subjects learned to distinguish vertical curves from vertical straight lines but not from horizontal curves. However, without data on generalization to oblique curves, we cannot yet say that goldfish recognize “curvature” per se. Go

No go

No go

(3 C

@GI

Fig. 4. Stimuli used by Ingle (1969a) in studies of relative discrimination difficulty. In each of four problems (a-d) the fish avoids the stimulus on the left. Successful “no go” discriminations were obtained with each stimulus in the center but not with stimuli on the right.

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IV. PERCEPTUAL EQUIVALENCE AND CHANGE IN SPATIAL POSITION

For all vertebrates the retinofugal fibers project to midbrain or thalamus in a spatially ordered manner preserving at the tectum or in the striate cortex a mapping of visual space. Many workers have, therefore, been interested in measuring the extent of an animal's ability to judge shapes as equivalent when they fall upon disparate parts of the retina or are viewed by different eyes. Studies of the invariance of shape recognition with changes in position provide a certain way of excluding factors of retinal coding from considerations of mechanisms underlying shape perception. Other possible transformations of a shape-size, orientation, and color-do not necessarily exclude mechanisms of equivalence at the retinal level. Cronly-Dillon et al. ( 1966) have demonstrated "intraretinal transfer" in goldfish by an ingenious method. Since the dorsal and ventral brachia of the goldfish optic tract diverge to innervate the upper and lower tectum, respectively (i.e., mediate shape discrimination in the upper and lower halves of the field), cutting one branch restricts retinal input to half of one tectum. Subjects trained to discriminate vertical from horizontal rectangles after one brachium had been cut could retain this discrimination several weeks later after the severed fibers had regenerated and the second (formerly used) branch was then cut. This clearly proves that shape equivalence of at least a simple order exists when different retinal inputs are employed. However, it is likely that overlap of tectal receiving neurons occurs at the horizontal margin of these two optic inputs since retinal receptive fields may be as large as 30"-40" of visual angle. A study by Ingle ( 1963) indicates that pattern-discrimination transfer may occur when two sets of retinal images are each projected to two disparate tectal regions. Using a cardiac-conditioning method, goldfish showed good transfer of a horizontal vs. vertical or horizontal vs. diagonal stripe discrimination from a temporal training position to a nasal testing position 120" rostrally. If we allow for the occasional eye movements, ranging up to 30", the minimal intraretinal distance over which equivalence was demonstrated is a respectable 90". Whether or not more difficult shape discriminations ( e.g., circle vs. square) would transfer over this distance is a more critical, but unanswered, question. Ingle noted, rather surprisingly, that intraretinal transfer totally failed with the stripe discrimination in 12 subjects trained and tested by a conditioned-avoidance method even though a red-green discrimination did transfer from back to front in the same fish. It is not known whether the success with

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cardiac-conditioning simply reflects the advantages of an easier task for the subject (heart-rate changes are not all-or-none as is the choice demanded by avoidance training) or whether a more interesting physiological explanation is available. Since each eye of a fish sends visual information exclusively to the contralateral tectum, we might expect incomplete behavioral equivalence between opposite eyes. Indeed, the first relevant study by Sperry and Clark (1949) showed that interocular transfer of a simple up vs. down problem was poor or absent in the majority of subjects. However, Schulte (1957) obtained high levels of pattern transfer with carp following extensive training sessions. Schulte found clear transfer failure only when his fish were confronted via the untrained eye with distorted or rotated versions of the training stimuli. These subjects could transpose their training experience while using the training eye, however. This delicate ability to judge similarity between different sets of patterns might have been disrupted by changing eye covers prior to transfer tests which, at first, made some fish “neurotic.” This interpretation is made plausible by the demonstration of McCleary (1960) that goldfish with eye covers may fail interocular transfer of a simple discrimination that occurs readily when blinders are not used. Other studies, however, have demonstrated limits of interocular transfer that cannot be attributed either to emotional disruption or to the use of a suboptimal response criterion. For example, Ingle (1965) trained goldfish to discriminate a striped from a random pattern, where the stimuli also differed in color (red or green). When these fish were tested with the same patterns, each appearing in opposite colors, they could resolve the conflict by responding to either color or pattern differences. Subjects that were tested first via the trained eye responded on the basis of the (more discriminable) pattern dderences, while those fish initially tested via the second eye behaved in accord with the color differences. Therefore, one concludes that pattern information transfers less well than color information in the goldfish. This method of comparing the “transferability” of various visual discriminations could provide guidelines for eventual recording from units in the various interhemispheric commissures of the fish brain. Although the aforementioned study clearly proved the relative failure of pattern transfer, other studies (Ingle, 1968a) argue that a total failure can sometimes be obtained. This result might be more encouraging for the physiologist who prefers all-or-none results. Goldfish trained to discriminate vertical stripes from those rotated by 23” failed to show any evidence of transfer, unlike successful controls trained with vertical vs. 52” rotation, although both groups were tested with the same stimuli differing by 38”. Even transfer of a horizontal-vertical stripe discrimi-

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nation could be prevented if the second brain half were “occluded by the response to an irrelevant spotted stimulus presented to the second eye on each trial. Thus, we conclude that transfer may fail when ( a ) the commissural system cannot resolve fine stimulus differences, or ( b ) the second brain half is jammed by induced noise and cannot record input from the commissures. The fact that interocular integration is sometimes incomplete suggests that independent visual learning might proceed simultaneously within opposite halves of the brain. Ingle (1968a) has demonstrated that goldfish can, indeed, acquire opposing discriminations of pattern or of color via opposite eyes; for example, avoiding red not green via the right eye and avoiding green not red via the left. Double learning is easily achieved when stimuli are presented either ( a ) to both eyes on each trial, or ( b ) to one eye at a time, alternating eyes on successive trials, but is more difficult to attain when ( c ) long sequences of monocular trials are confined to one eye at a time. Perhaps the difficulties inherent in the “alternating sessions” method account for the inability of Schulte (1957) or Shapiro ( 1965) to demonstrate interocular double learning with carp or goldfish. However, Schulte and Shapiro both used frontally approached (expanding) stimuli, while Ingle used size-restricted stimuli confined to the lateral field. Perhaps interocular integration is favored in the ( overlapping) binocular field, while dissociation is more easily obtained using lateral stimuli. Finally, we consider an interocular transfer problem that is rather peculiar in its very formulation: How can stimuli appearing via opposite eyes be judged equivalent when the discrimination is based upon differences in the directional orientation of the shapes? Since the fish‘s eyes are set upon opposite sides of the head, there is considerable ambiguity in predicting how one hemisphere communicates distinctions of leftright or back-front to the other. As Fig. 5 illustrates, a leftward (nasalward) arrowhead viewed by the right eye casts an image projected to the tectum in a rostral-pointing direction just as a frontward pointing arrowhead seen in the right lateral field. If the arrowhead in the frontal plane-seen by both eyes at once-is to produce two images that map onto one another via the commissural system, one must conclude that a nasalward direction for one eye is “equivalent” to a temporalward direction as seen via the second eye. But if one coding process is applicable to all parts of the two retinas, this logic forces the absurd prediction that an object seen in front of the fish via one eye is more similar to a trailing than to a leading stimulus on the other side. Ingle (1967, 1968b) has shown that both mechanisms coexist and are called forth by different kinds of experimental stimuli. Fish trained on

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Fig. 5. Illustration of left-right mode of interocular transfer obtained with mirror image arrowhead discrimination. Fish trained to avoid forward-pointing stimulus on right (but not backward arrowhead) will avoid backward stimulus but not forward version, when tested via left eye. If these equivalent stimuli were moved into the frontal binocular field, they would come into register.

mirror image discriminations of objects seen in Fig. 6 can show two separate modes of interocular transfer. Subjects discriminating either ( a ) the 135" vs. 45" oblique lines or ( b ) the mirror image arrowheads showed a left-right mode of directional equivalence in that stimuli projecting in the same direction on the frontal plane are taken as equivalent during interocular transfer tests. For discriminations ( c ) and ( d ) , a front-back equivalence is obtained: Similar stimuli project in the same direction on the lateral plane that parallels the sagittal plane of the body. The two pairs of stimuli classified by the left-right mode subtended 8"-1O0 of visual angle, while pairs ( c ) and ( d ) were 15" and 22" in size. Possibly the stimuli of larger size are more easily broken down into two partsthe front and back units-so that the brain can describe the stimulus in

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Group I

Group 2

Group 3

Group 4

Fig. 6. Stimuli used for mirror discriminations by Ingle ( 1967). Fish in Groups 1 and 2 showed left-right interocular equivalence (as illustrated in Fig. 5 ) , while just the opposite relationships were obtained during transfer tests in Groups 3 and 4.

terms of spatial position relative to the body (red in front plus green in back). This is undoubtedly true for the red-plus-green squares since fish trained to discriminate mirror image pairs continue to discriminate well when the back squares are removed from each stimulus. Where local sign seems critical for coding the larger pair of shapes, the smaller stimuli would seem to be taken in as units with a visual direction somehow assigned. As argued elsewhere ( Ingle, 1967, 1968b) , this second kind of transfer cannot be based upon any point-to-point mapping between the two optic tecta, but it must involve a shape recognition process that is based upon a nonspatial code at this level of the brain. This distinction between mechanisms of ( a ) positional labeling and ( b ) shape recognition has been considered as analogous with “orienting vs. identifying” modes of vision, which have different neural substrates in mammals ( Schneider, 1967). Furthermore, both Trevarthen (196813) and Held (1968) make similar distinctions between dissociable visual processes described in cat, monkey, and man himself. Although mechanisms of orienting toward and identifying objects are doubtless more complex among mammals than among fishes, it is important to recognize a fundamental dualism within vision that may have appeared with the first vertebrate. V. SELECTIVE ATTENTION

The human observer takes for granted the ability to glance quickly over a complex visual scene, ready to take in those details that he is

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prepared to see. Observations of fish locomotion and associated eye movement also suggest selective visual operations during behavior. It is even possible that “internalized” selection occurs when the fish rests in place, so that only selected features of the visual array (or selected portions of the visual array) have full access to central processes. If this were so, it would impose still another barrier between the psychologist and the neurophysiologist who is usually restricted to an anesthetized or paralyzed preparation. While there is little conclusive evidence at the behavioral side for selective attention, the issue is so important that a review of the few relevant facts may be of heuristic value. Some fish move one eye at a time in response to a moving object, while the other eye stares indifferently into space. One suspects, without direct evidence, that the redirected eye must be the more sensitive at that moment, in connection with the motor dominance of one hemisphere. Visual behavior toward objects appearing within the binocular field of a moving fish presents the same issue. Harris (1965) has shown for the dogfish that eye movements are asymmetrical during the sideward waggling of the head: The eye toward which the head swings is momentarily stabilized by compensatory reflexes in reference to a locus of points 3 feet to the side of the fish. Trevarthen (1968a) showed that compensatory eye movements in a spinalized goldfish are likewise asymmetrical. It seems that a locomoting fish must either accept double vision within the binocular field or attend selectively to information arriving the stabilized eye. We do know that fish can use both eyes simultaneously during a discrimination task when stimuli appear within the lateral fields. Ingle ( 1968a) trained goldfish to compare stimuli seen via opposite eyes (i.e., to avoid an unlike pair such as horizontal plus vertical stripes) while treating a horizontal or a vertical pair as “no go” stimuli. Another experiment in this series showed that goldfish do not ignore monocular information even where it is irrelevant and where it would be to their advantage. Subjects learning a vertical-horizontal stripe discrimination via one eye also viewed the horizontal stimulus via the opposite eye on each trial. On critical “attention” trials, the “no go” horizontal pair was changed by inserting a novel red-on-white horizontal striped stimulus on one side or the other. Subjects were disinhibited by the red color more often via the irrelevant eye which contradicts the hypothesis that they would suppress visual processes on the nondiscriminating side. Attention might be shifted to particular dimensions of analysis (size, orientation, brightness, etc.) as well as to selected regions of the visual field, as Mackintosh (1965a) has suggested on the basis of experiments with rats. Hemmings (1966) has tested one prediction of this model, using tropical fish: that a difficult discrimination will be more efficiently

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learned by pretraining subjects on an easier version of the problem in order to reward them for “switching in the appropriate analyzer.” Indeed, fish pretrained on a circle-triangle discrimination were able to solve an otherwise impossible circle-square discrimination. A second pretraining series on circle vs. larger square, emphasizing the irrelevant dimension of size, did not facilitate learning of the difficult circle-square problem. However, a study by Sutherlands group (Mackintosh et al., 1966) failed to show evidence of selective attention: Overtrained goldfish reversed a shape discrimination more slowly than controls, unlike rats who reverse more quickly under similar conditions ( Mackintosh, 1965b). Other tests will be required to decide whether fish have evolved this interesting facet of behavior. A final answer will be useful to the physiologist who wonders how radically the visual coding processes of retina and tectum may be altered by centrifugal influences.

VI. TOWARD A UNIFIED OUTLOOK ON VISUAL BEHAVIOR

Aspects of fish behavior which have not yet been analyzed in a sufficiently rigorous manner have been deliberately excluded, although it is clear that vision has been designed for ecological and social functions and not for playing games with psychologists. Analysis of visual mechanisms might be pursued by experiments that describe optimal stimuli for guiding natural movements or eliciting consummatory responses. Briefly summarized below are some hypotheses designed to link observed behavior toward artificial stimuli with the uses of vision in the real world. A more complete discussion of this material has been presented elsewhere ( Ingle, 196813, 1971). The study of motion detection by goldfish (Ingle, 1967, 1968b) reveals two or three independent processes that have been hypothetically identified with aspects of natural visual behavior. The fast-moving single spot is taken as a prototype of predator motion or the motion of a rival’s markings used to direct aggressive attack (an eye or a body spot). The higher probability of catching forward rather than backward moving prey might explain the specialization for nasalward directions with a fast-moving spot. Furthermore, it has been noticed that aggressive mouthbreeders, Tilupiu, seldom, if ever, attack a smaller fish moving in the opposite direction, but they readily chase a rival who moves past in a head-to-head orientation. The slow-moving spot, seen best while moving backward, might correspond to a prey object being pursued; the ternporalward velocity would inform the pursuer how rapidly the gap was being closed and help set

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the timing of the snap, Mouthbreeders do require continuous motion feedback while pursuing food since, under a stroboscopic light at 3 Hz, they badly misjudge the rate of fall and snap too high. The multispotted stimulus might also be an analog of temporally moving features within the visual array through which the fish swims. Here, as well, visual feedback informs the fish of his own progress (in a moving stream, the visual cue to swimming rate would be the most reliable one). Rate of temporalward motion also provides information on distances of objects or surfaces ahead or to the side. In this context, the dominance of horizontal or vertical axes, revealed by shape recognition experiments, makes good sense: These particular axes of image translation are produced by fish that themselves locomote within horizontal or sagittal planes. The interesting ability of goldfish to notice “parallelness” might also serve to discriminate the tilt of surfaces (such as inclining rocks). If a retinal image displacement resulting from body movement maintained the orientation of a contour in the retinal image (i.e., displaced, but still parallel), the fish might infer that the contour is confined to a plane perpendicular to the horizontal plane through which it swims. Nonparallel displacements, on the contrary, represent contours or surfaces that incline toward, or slope away from, the organism. The broad category “curvature” can be seen in this light: A curving retinal displacement must correspond to an object moving independently of the fish. Perhaps as various shapes may have physionomic connotations to man (Werner, 1940), curved lines suggest something “animate” to the fish. These teleological speculations cannot pass for explanations of visual behavior, but they can direct the psychologist toward new methods of measuring specific visual abilities. As yet, we know almost nothing about the sensitivity of fish to the various transformations of the optic array through which they swim. The method of “false feedback used to analyze human sensorimotor abilities could be used with many fish (as Sperry, 1950, achieved by eye rotation). Already, some physiological evidence indicates that concern for differential patterns of motion seen by fish is likely to be fruitful in research. Jacobson (1968) has described directionally sensitive ganglion cells in the goldfish retina that can be suppressed by moving a second object outside of the receptive field in a direction opposite to that of the stimulating object. Discrimination between objects that move together and those that converge may utilize these peripheral intraretinal inhibitions to code differential movement. It is worth adding that the frog-who cannot obtain such parallax information-does not show the kind of direction-specific cross inhibition that Jacobson has revealed in the fish. Perhaps in gaining physiological hints as to the mechanism of spatial vision in higher mammals, the active fish will prove a better model than statuesque amphibians and reptiles,

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REFERENCES Brunner, G. (1935). Uber die Sehscharfe der Elritze (Phoxinus laevis) bei verscheidenen Helligeiten. 2. Vergleich. Physiol. 21, 296-316. Cronly-Dillon, J. R. (1964). Units sensitive to direction of motion in goldfish optic tectum. Nature 203, 214-215. Cronly-Dillon, J. R. Sutherland, N. S., and Wolfe, J. (1966). Intraretinal transfer of a learned visual shape discrimination in goldfish after section and regeneration of the optic nerve branchia. Exptl. Neurol. 15, 455-462. Hager, H. J. ( 1938). Untersuchungen uber das optische Differenzierungsvemogen der Fische. 2. Vergleich. Physiol. 26, 282-302. Harden Jones, R. R. (1963). The reaction of fish to moving backgrounds. J. Exptl. Biol. 40, 437-446. Harris, A. J. (1965). Eye movements of the dogfish, Squalus acanthi- L. J. Exptl. Biol. 43, 107-130. Held, R. ( 1968). Dissociation of visual functions by deprivation and rearrangement. Psychol. Forsch. 31, 338-348. Hemmings, G. (1966). The effect of pretraining in the circle/square discrimination situation. Animal Behaviour 14, 212-216. Herter, K. ( 1953) . “Die Fischdressuren und ihre sinnesphysiologischen Grundlagen.” Akademie Verlag, Berlin. Hubel, D. H., and Wiesel, T. N. ( 1962). Receptive fields, binocular interaction and functional architecture in the cat’s visual system. J. Physiol. (London) 160, 106-154. Ingle, D. (1963). Limits of visual transfer in goldfish. Ph.D. Thesis, University of. Chicago. Ingle, D. (1965). Interocular transfer in goldfish: Color easier than pattern. Science 149, 1000-1002. Ingle, D. (1967). Two visual mechanisms underlying the behavior of fish. Psychol. Forsch. 31, 44-51. Ingle, D. ( 1968a). Interocular integration of visual learning by goldfish. Brain, Behavior Evolution 1, 58-85. Ingle, D. (196813). Spatial dimensions of vision in fish. In “The Central Nervous System and Fish Behavior” (D. Ingle, ed.), pp. 51-60. Univ. of Chicago Press, Chicago, Illinois. Ingle, D. ( 1971). Analyses of shape discrimination abilities in goldfish. Brain, Behavior Evolution ( in press ) . Jacobson, M. (1968). Physiology of fish vision. In “The Central Nervous System and Fish Behavior” ( D . Ingle, ed.), pp. 17-24. Univ. of Chicago Press, Chicago, Illinois. Jacobson, M., and Gaze, R. M. (1964). Types of visual response from single units in the optic tectum and optic nerve of the goldfish. Quart. J. Exptl. Physiol. 49, 199-209. Kliiver, H. ( 1933). “Behavioral Mechanisms in Monkeys.” Univ. of Chicago Press, Chicago, Illinois (reprinted in Phoenix Science Series, Univ. of Chicago Press, Chicago, Illinois, 1961). McCleary, R. A. (1960). Type of response as a factor in interocular transfer in the fish. J . Comp. Physiol. Psychol. 53, 311-321. McCleary, R. A., and Bernstein, J. J. (1959). A unique method for control of brightness cues in the study of color vision in fish. Physwl. Zool. 32, 284-292.

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Mackintosh, N. J. ( 1965a). Selective attention in animal discrimination learning. Psychol. Bull. 64, 124-150. Mackintosh, N. J. ( 196513). Overtraining, extinction and reversal in rats and chicks. J. Comp. Physiol. Psychol. 59, 31-36. Mackintosh, N. J., and Sutherland, N. S. (1963). Visual discrimination by goldfish: The orientation of rectangles. Animal Behauior 11, 135-141. Mackintosh, N. J., Mackintosh, J., Safriel-Jorne, O., and Sutherland, N. S. (1966). Overtraining, reversal and extinction in the goldfish. Animal Behauior 14, 314318. Marks, W. B. (1965). Visual pigments of single goldfish cones. J. PhysioZ. (London ) 178, 14-32. Matthews, W. A. (1964). Shape discrimination in tropical fish. Animal Behauiour 12, 111-115. Meesters, A. ( 1940). Uber die Organization des Gesichtsfeldes der Fische. Zeitschr. Tierpsychol. 4, 84-149. Muntz, W. R. A., and Cronly-Dillon, J. R. (1966). Color discrimination in goldfish. Animal Behavior 14, 351-355. Saxena, A. ( 1966 ) . Lernkapazitat, Gedachtnis und Transpositionsvermogen bei Forellen. Zool. Jahrb., Abt. Allgem. Zool. Physiol. Tiere 69, 63-94. Schneider, J. E. (1967). Contrasting visuomotor functions of tectum and cortex in the Golden Hamster. Psychol. Forsch. 31, 52-62. Schulte, A. ( 1957). Transfer- und Transpositionversuche mit monokulardressierten Fischen. Z. Vergleich. Physiol. 38, 432-476. Shapiro, S. M. (1965). Interocular transfer of pattern discrimination in the goldfish. Am. J. Psychol. 78, 21-38. Sperry, R. W. (1950). Neural basis of the spontaneous optokinetic response produced by visual inversion. J. C o m p . Physiol. Psychol. 43, 4 8 2 4 8 9 . Sperry, R. W., and Clark, E. (1949). Interocular transfer of visual discrimination habits in a teleost fish, Physiol. 2001.22, 372-378. Sutherland, N. S. (1968). Shape discrimination in the goldfish. I n “The Central Nervous System and Fish Behavior” ( D . Ingle, ed.), pp. 35-50. Univ. of Chicago Press, Chicago, Illinois. Trevarthen, C. B. (1968a). Vision in fish: The origins of the visual frame for action in vertebrates. I n “The Central Nervous System and Fish Behavior” ( D . Ingle, ed.), pp. 61-96. Univ. of Chicago Press, Chicago, Illinois. Trevarthen, C. B. (1968b). Two mechanisms of vision in primates. Psychol. Forsch. 31, 299-337. Weiler, I. J. (1966). Restoration of visual acuity after optic nerve regeneration in Astronotus ocellatus. Exptl. Neurol. 15, 377-386. Werner, H. ( 1940). “Comparative Psychology of Mental Development.” Harper, New York. Westerman, R. A. (1965). Specificity in regeneration of optic and olfactory pathways in teleost fish. I n “Studies in Physiology” (D. R. Curtis and A. K. McIntyre, eds.). Springer, New York. Yager, D. ( 1967). Behavioral measures and theoretical analysis of spectral saturation in the goldfish, Carassius auratus. Vision Res. 7, 707-727. Yager, D. (1968). Behavioral analysis of color sensitivities in goldfish. I n “The Central Nervous System and Fish Behavior” ( D . Ingle, ed.), pp. 25-34. Univ. of Chicago Press, Chicago, Illinois.

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CHEMORECEPTION TOSHIAKl 1. H A M I . Introduction . . . . . . . . . . . . . I1. Anatomy of Chemical Sense Organs A Olfactory Organ . . . . . . . . B . Gustatory Organ . . . . . . . . . . . . . . C Free Nerve Endings . . D Morphology of Brain and Feeding Habits . I11. Behavioral Studies of Chemoreceptive Functions . A . Olfactory Sense . . . . . . . . B Gustatory Sense . . . . . . . . IV . Electrophysiological Studies of Chemoreceptor Responses A Olfactory System . . . . . . . . B. Gustatory Receptors . . . . . . . V. Biological Aspects of Chemoreception . . . . A. Chemical Perception of Foods . . . . . B. Reproductive Behavior and Chemical Senses . . C . Discrimination of Body Odors and Schooling . . D Alarm Substances . . . . . . . E . Repellents . . . . . . . . . F. Orientation by Chemical Senses . . . . References . . . . . . . . . . .

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I INTRODUCTION

Chemoreception plays an important and indispensable role in the behavior of fishes. It is involved in the procurement of food. recognition of sex. discrimination between individuals of the same or different species. in defense against predators. in parental behavior. in orientation. and in many other ways. Primarily through conditioning techniques. studies of 79

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chemoreception in fishes have followed two principal directions: (1) the investigation of ecuity and range of the chemical sense organs, and ( 2 ) the investigation of the biological signifkance of chemoreception in the behavior and life cycles of fishes. Chemoreceptive functions in fishes have been well reviewed by Hasler (1957), Teichmann (1962b), and by Kleerekoper (1969). During the last decade there has been a marked increase in the number of studies dealing with the chemical sense in fishes. Electron microscopy has been applied to the fine structure of the chemoreceptor organs; recent electrophysiological investigations of the chemosensory system have added greatly to the understanding of the mechanism underlying chemoreception in fishes. Chemoreception by animals shows a wide range of sensitivities. On the basis of location and structure as well as the central innervation, chemical reception has been divided into three categories: olfaction or smell, gustation or taste, and common or general chemical sense. These sensory modalities overlap somewhat: Some substances elicit responses from both types of receptors. Practically, in terrestrial animals, those receptors which have high sensitivity and specificity, and which are “distance chemical receptors” are distinguished as olfactory, those receptors of moderate sensitivity and stimulated by dilute solutions are gustatory or “contact chemical receptors,” and those receptors which are relatively insensitive and nondiscriminating are considered common chemical sense. In fishes, smell and taste are both mediated by dilute aqueous solutions so that the distinction is made anatomically and physiologically. Olfactory organs are innervated by the &st cranial nerve which contains the axonal extensions of the primary receptor cells to the olfactory bulb. Through the olfactory tracts bulbar activity is related to the rest of the nervous system. Those fishes with well-developed olfactory capacities are called “macrosmatic”; those in which the capacities are less acute are called “microsmatic.” Taste is mediated through the taste buds with the secondary sensory cells. Taste buds lie not only in the mouth and pharynx but also in the gill cavity, on the gill arches, on appendages such as barbels and fins, and also, in some fishes, on all external surfaces of the body. The taste buds are innervated by the VIIth, IXth, and Xth cranial nerves which terminate in much enlarged vagal lobes. Common chemical sense, which was originally named by Parker (1912), is also located on exposed body surfaces of fishes. These are free nerve endings supplied by the spinal nerves. In contrast to the two senses mentioned above, the general chemical sense is relatively low in sensitivity.

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11. ANATOMY OF CHEMICAL SENSE ORGANS

A. Olfactory Organ Extreme variations are found in the morphology of the peripheral olfactory organs depending largely on the degree of development of the olfactory system. In the sharks and rays where the chemical senses are highly important ecologically, the paired olfactory pits or sacs are usually situated at varying distances from the oral opening on the ventral side of the snout. The opening of each pit is divided into two parts by a fold of skin: anterior inlet and posterior outlet; in some instances the latter leads to the mouth. As the fish swims through the water and as it takes water into the mouth to breathe, a current passes through the olfactory sacs. Thus, in the sharks and rays, the olfactory organs are influenced directly by the respiratory current. In the teleost fishes the paired olfactory pits are usually on the dorsal side of the head, somewhat removed from the mouth (Fig. 1).The eels

f

or

on

Fig. 1. Position and internal structure of the nose in the minnow, P h o x i n ~ s phoxinzis ( A ) and eel, AnguiZZu anguillu ( B ) . an, Anterior naris; pn, posterior naris; f, skin flap; and or, olfactory rosette. From H. Teichmann, Umschau Wiss. Tech. 62, 588-591, Frankfurt, Germany, 1962.

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and morays (Anguilliformes), with the most acute demonstrated sense of smell, have large and elongate olfactory pits, extending from the tip of the snout to the orbit of the eye. In contrast, certain puffers (Tetraodontidae ) which are highly visually oriented reef fishes, have completely lost the nasal sacs. Various patterns of intermediate anatomical development can also be seen reflecting the relative role of olfaction in different fishes. Each nasal pit generally has two openings which are separated by an area of skin (Fig. 1).Olfactory currents of water enter the anterior and leave through posterior openings-either passively through the locomotion of the fish in the water, or actively by ciliary action within the pits or by the action of muscles associated with the jaws or gills or by some combinations of these methods (Burne, 1909; Pipping, 1926; Teichmann, 1954). In the lungfish the external nostrils are true anterior nares, whereas the internal nares open into the mouth in a manner corresponding to the choanae of higher vertebrates. Lining the nasal sacs is the olfactory epithelium, which is generally raised from the floor of the organ into a complicated series of folds to make rosettelike arrangements ( Fig. 1).The olfactory folds vary greatly in direction and number. Through these folds the total area of the sensory epithelium is greatly increased. Burne ( 1909) distinguished oval (in most fishes), round (in Esox) and elongate ( Anguillu) olfactory rosettes. Species with elongate rosettes have the most numerous lamellae, which are set at right angles to the longitudinal axis of the nasal sacs; such rosettes can be generally correlated with an acute sense of smell (macrosmatic). Species with round rosettes, on the other hand, normally have only a few lamellar folds and usually show no or minimal behavioral responses to olfactory stimulation (microsmatic). Species with oval rosettes are most common and intermediate between the other two. There have been several attempts to relate the total area of the olfactory epithelia in different species to their particular olfactory sensitivities. Measurements of the surface area of the olfactory epithelia of eleven species of freshwater teleosts (Teichmann, 1954) made it clear that species with round rosettes had the smallest area of olfactory epithelium (Esox, about 0.2%of the whole body surface; Gasterosteus, 0.4%)) and that the broadest olfactory epithelium was not found in fishes with elongate rosettes but in species with oval rosettes (Gobio, 3.6%;Phoxinus, 1.9%).In species with the elongate rosettes, the olfactory epithelium was found to be 1.4%of the whole body surface in Anguillu and 1.3%in Lota hta. However, there is no simple relation between the area of the olfactory epithelium and the number of receptors it contains. It is therefore doubtful whether any simple relation exists between the area of the olfactory epithelium and sensitivity to odors.

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The anatomy of the nose has also been related to the ecological habits of the fishes (Pfeifler, 1963b, 1964, 1965; Teichmann, 1954). On the basis of development of the nose and eye, Teichmann (1954) classified the fishes mentioned above into three groups: ( 1 ) species in which the eye and nose are well developed (Plzoxinus and Gobio ); ( 2 ) species in which the eye is better developed than the nose (Esox and Gusterosteus); and ( 3 ) species in which the nose is exceptionally well developed compared with the eye ( Anguilla and Lotu). The definitions of these groups correspond well with the morphological classification made by Bume ( 1909) : group 1 includes the fish with oval olfactory rosettes; group 2 includes most of the round rosetted species; and group 3 the elongate rosetted species. Olfactory epithelia in fishes generally occurs in isolated sensory areas separated by columnar ciliated cell areas (indifferent epithelium). Three types of arrangements of the sensory cells in the olfactory epithelium have been observed (Holl, 1965): ( 1 ) continuous except for the dorsal parts of the olfactory folds (Anguilla and ZctuZuurus); ( 2 ) separated in large areas between the folds ( E s o x ) ;and ( 3 ) dispersed in small islands, in which the sense cells are arranged somewhat like taste buds (Phoxinus and C yprinus) .

Fig. 2. Diagram of fine structure of the olfactory epithelium of the eel, Anguillu anguille. 1, Receptor cells; 2, supporting cells; 3, ciliated cells; 4, basal cells; 5, goblet cells; 6, club-shaped secretory cells; and 7, olfactory knob with sensory hairs. From Holl ( 1965).

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The olfactory epithelial system consists of receptor cells, supporting or sustentacular cells, and basal cells ( Fig. 2). In some species ( AnguiZZu, MyxocephaZus, etc. ) , large flasklike cells have also been observed; these are probably mucous gland cells (Holl, 1965). In burbot (Lota Zota), furthermore, glial cells have been observed to penetrate into the epithelium from the connective tissue surrounding small bundles of receptor cell axons (Gemne and Doving, 1969). The individual olfactory receptor cells of fishes are similar to those of other vertebrates in general appearance although there is a great variation in details even within a particular olfactory organ. The receptor cell, which is a bipolar primary neuron, sends a slender cylindrical process or dendrite toward the surface of the epithelium. The process terminates in a minute swelling (olfactory knob) which bears a variable number of cilia. Although the receptor cells are not distributed uniformly in all olfactory folds, average numbers of 4 8 x 104/mm2are estimated for the receptor cells in several species (Holl, 196.5). The fine axons arise from the basal pole of the receptor cells. They pass through the basement membrane, become grouped in the submucosa, and form the olfactory nerve fasciculi, which run posteriorly to end in the olfactory bulb. Recent electron microscopic observations have characterized the fine structural organization of the olfactory receptor cells in teleost fishes. Trujillo-Cen6z ( 1961) first studied the ultrastructure of olfactory neurons in fully developed embryos of two cyprinodont species Cnesterodon and Fitzroyia. He found that the olfactory sensory hairs consist of an undetermined number of long cilia which project into the lumen of the olfactory pit and that the dendrite of the olfactory neuron contains profiles of small tubules, aligned parallel to its length. Near the basement membrane of the epithelium, groups of axons, which, as mentioned, arise from the pole of the receptor cells opposite to the distal process, are encased in the surface of the sustentacular cells. Although Trujillo-Cen6z failed to demonstrate in the sensory hairs of receptor cells any common structural pattern which could be related to the chemoreceptor mechanism, he considered these structures as devices serving to enlarge the “active surface” of the cell increasing in this way the effectiveness of the whole receptive system. In earlier reports, Jagodowski ( 1901) working with pike, Esox, and Hopkins (1926) studying Stenesthes claimed that there is a single long apical process on each olfactory receptor instead of the usual group of cilia. To resolve this question, Bannister (1965) compared olfactory receptor cells in two teleosts, a minnow, Phoxinus, and the three-spined stickleback, Gasterosteus. Phoxinus, like all the Cyprinidae, has a welldeveloped olfactory organ. Gasterosteus by contrast is a microsmatic

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ciliated

epithelial cell

receptor ending

-

Receptor with microvilli

Fig. 3. Diagrammatic reconstruction of the olfactory surface of the minnow, Phoxinus. From Bannister ( 1965).

species. Bannister found at least two distinct receptor cells, distinguished by the forms of their distal tips (Fig. 3 ) . The first category consists of the ciliated olfactory receptors, each of which has a ring of 4-6 cilia upon its convex distal end. The ultrastructure of the cilia is generally identical with that of all kinocilia in other organisms; the usual “nine-plus-two’’ pattern of fibrils is present (Bannister, 1965). The second type of receptor endings has been found only in Phoxinus and not previously described in any vertebrate; it bears neither cilia nor microvilli but extends simply as a naked rod from the epithelial surface. Internally, this latter type of receptor resembles the ciliated one in many respects; however, it contains three or more longitudinally oriented bundles of fibers. It may correspond to the single olfactory process of Jagodowski ( 1901) and Hopkins ( 1926). The significance of the presence of more than one type of receptor ending in one species of fish is not clear but suggests a basis for peripheral olfactory discrimination. Confirming the findings of previous investigators regarding the general arrangement of the receptor cells, Wilson and Westerman (1967) have shown certain distinct differences in the receptor cells of the European carp, Carassius carassius. In addition to typical cilia, they noted exceptional ciliary formations on the receptor cells. In these ciliary aggregation,

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the fibrils are grouped together in clusters instead of forming individual cilia and are enveloped by a single limiting membrane. Further, Wilson and Westerman ( 1967) discovered a conspicuous well-differentiated new cell type, the foliaceous cell, which has not been previously described in olfactory mucosa. These foliaceous cells, enclosing prominent leaflike organelles, communicate with the external environment through a small stoma. Their complicated morphology together with their close association with the multifilamentous cilia and the myelinated nerve fibers observed in the stroma suggest that they may be receptor cells; however, further work will be required to provide additional information on their role and neural connection. It is not clear which part of the receptor cell is involved in the initial events of olfactory stimulation. However, it is reasonable to assume that the receptor site lies on the membranes of the olfactory cilia since these are the first points of contact with odorous molecules; moreover, the olfactory knob does not protrude beyond the limiting surface of the mucosa as in other vertebrates. If this is the point of excitation, number, length, and motility of the cilia would be signscant in enlarging the active receptor area and increasing the chance of contact between cilia and molecules. The theory that the olfactory cilia are the locus where electrical excitation in the olfactory organ is initiated by contact with odorous substances has been suggested by Reese (1965), who investigated the fine specialization of olfactory epithelium of the frog. Motile olfactory cilia were first described in Stenesthes by Hopkins (1926). However, the motility of the olfactory cilia in Cyprinus, Esox, and Lampetru has recently been observed ( Vinnikov, 1965; Kleerekoper, 1969). Jagodowski (1901) found only one extremely long, thick cilium on each of the olfactory receptor cells of pike; and Bannister (1965) described a receptor cell with only one rod-shaped process in the minnow. Further works will be needed to determine whether any simple relation exists between the characteristics of the cilia and sensitivity to odors. The supporting cells are polygonal columnar epithelial cells which lie between the receptor cells. They bear a small number of irregular microvilli and have prominent oval nuclei situated basally. The cytoplasm immediately beneath the limiting surface is relatively scant with numerous electron-dense vesicles. In the deeper parts of the cells an endoplasmic reticulum and mitochondria are present as much as the receptor cells. Although the significance of the supporting cells in olfactory perception is not understood, it is likely that they have a significance beyond mere mechanical support. In addition to the olfactory cells mentioned above, free nerve endings of the trigeminal nerve occur in the epithelium (Jagodowski, 1901). Al-

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though the functional significance of the trigeminal nerve is not understood, the concept of a central regulatory control over olfactory afferent inputs has been suggested in rabbits ( Stone et ul., 1968). The olfactory mucosa appears yellow to dark brown or black. Some authors assume that the pigment responsible for this color has a function in olfaction (cf. Moulton and Beidler, 1967; Moncrieff, 1967). However, there is no report on the occurrence of pigments in the olfactory epithelium of fishes. It is, therefore, difficult to assess the role of the olfactory pigment in the function of the olfactory membrane until we know more about pigment location, distribution, and biochemical properties. B. Gustatory Organ

Taste buds of elasmobranchs are restricted to the mouth and pharynx. In teleosts they are also located on the gill rakers and gill arches, on appendages such as barbels and/or fins, and also, in some fishes, on the entire surface of the body (Herrick, 1904). In the roof of the mouth, taste buds are densely packed to form the palatal organ, which is innervated by the palatine nerve in the cyprinids. Based on the histology of the taste buds on the gill rakers and gill arches of 24 species of teleosts, Iwai (1964) concluded that the taste buds are more conspicuous in branchial regions of freshwater fishes than in marine fishes. In some species of hake, Urophycis, and sea robin, Prionotus, taste buds are on the specialized pectoral fin rays, which are often modified into feelers (Scharrer et al., 1947; Bardach and Case, 1965; Bardach, 1967). Taste buds are normally innervated by branches of the n. facialis (VII), n. glossophuryngeus (IX), and n. vugus (X), which terminate in enlarged vagal lobes. Taste buds on the modified fin rays of the hakes are also innervated by spinal nerves. The vertebrate taste bud is typically composed of elongated sensory cells arranged like segments of an orange ( Fig. 4).Recent electron microscopic examination of fish taste buds has revealed the existence of three different cell types: receptor cells, supporting cells, and basal cells ( Trujillo-Cen6z7 1961; Cordier, 1964; Desgranges, 1965; Hirata, 1966). There are also transitional or intermediate forms of cells which may become either sensory or supporting cells. The receptor cell of Corydorar is pear-shaped and occasionally bears few thin and short microvilli ( Trujillo-Cen6z, 1961). Desgranges ( 1965) described various types of microvilli at the apexes of taste receptor cells in the barbels of Ameium; these suggest different functional stages. Hirata (1966) also observed the receptor cells with single or two apical processes of different appearance

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Fig. 4. Cross section through taste bud from the barbel of the sturgeon, Acipeiiser fulvescens. From Bardach (1967).

(spindle, elongated rods, and stalks) in the terminal taste buds on the barbels of freshwater fishes ( Cyprinus, Purusilurus, and Cobitis). The apical region of the receptor cells contain numerous electron-dense tubules sometimes arranged along the cell membrane; there are also abundant vesicles. Some of these vesicles are concentrated at the particular site of cell contact with nerve elements resembling a synaptic contact in the central nervous system ( Hirata, 1966). The supporting cells are provided with a few regular microvilli at their free surfaces. Light and electron microscopy have demonstrated a large number of nerve fibers concentrated beneath the basal pore of the taste buds. These fibers, after entering the bud and forming an intragemmal plexus, terminate on the surface of the receptor cells by mean of a clublike swelling containing mitochondria and a few vesicles. No such specialized contact between neural elements and the receptor cells has been shown in mammalian taste buds. Aside from the taste buds, specialized epidermal “spindle” cells were found on the head and body of minnows and various teleost fishes; in some cases (e.g., sea robins, Triglidue) these occur on the tentacular fin rays (Whitear, 1965; Bardach, 1967). The hypothesis that these cells are

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chemosensory is based on their structural resemblance to the sensory cells of the taste buds.

C. Free Nerve Endings There are numerous free nerve endings in the skin of fishes; these endings are presumed to serve the so-called common chemical sense. The free fin rays of the gourami, Trichoguster, and hake, Urophycis, possess taste buds which are innervated by cranial and spinal nerves, whereas those of sea robins, Prionotus, lack taste buds and the epithelium is richly innervated only by spinal nerves (Scharrer et al., 1947; Bardach and Case, 1965). In behavioral experiments, Scharrer et al. (1947) could not find a clear differentiation between the two; both showed similar positive reactions in response to chemical stimulation. Both the external taste buds and the free nerve endings are sometimes regarded as taste receptors-referred to as taste bud type and free nerve ending type, respectively ( see Bardach, 1967). Electrophysiological evidence indicates that there are two types of nerve discharges (fast- and slow-adapting) following chemical stimulation of hake fin rays; only the fast-adapting type is seen in sea robin fin rays. These findings suggest that the slow-adapting discharges are characteristic of taste buds while fast-adapting discharges may originate from the free nerve endings around taste buds (Bardach and Case, 1965). Reactions following stimulation of the common chemical sense are usually negative or defense reaction. However, there are exceptions and the biological significance of the common chemical sense remains unexplained; some even deny its existence (von Buddenbrock, 1952).

D. Morphology of Brain and Feeding Habits Developmentally, the teleost forebrain is basically different from that of elasmobranchs and the higher vertebrates ( Aronson, 1963). Nevertheless, the morphology of the olfactory bulb and centers are essentially similar throughout the vertebrates although there are minor variations, especially in relative size and position of the different areas. Olfactory nerve fibers (filu olfuctoriu) arise in the nasal mucosa terminate in the olfactory bulb, where they make a special synaptic contact with the bulbar neurons in the glomerulus. The fibers themselves do not branch until they terminate. The length of these olfactory nerves varies greatly in different fishes. They may have either short olfactory

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nerves and a long olfactory tract, pedunculated ( Carassius and lctalurus), or long olfactory nerves and a short olfactory tract, sessile (Anguilla and Esox). In the young goldfish the olfactory bulb is closely located to the olfactory lobe while an increase in the length of the olfactory tract may occur during the growth of the fish (Uchihashi, 1953; Schnitzlein, 1964). Olfactory nerve fibers of the pike ranged from 0.1 to 0.4 p in diameter and showed no obvious size difference from other vertebrates (Gasser, 1956). The average number of axons in the central portion of the olfactory nerve was estimated to be 29/pz in burbots (Gemne and DBving, 1969). The microscopic structure of the olfactory bulb is also essentially similar in all vertebrates. In fishes the olfactory bulb is poorly differentiated and the lamination is not so distinct as in higher vertebrates. The dominant feature of the bulb is the synaptic contact between the olfactory nerve fibers and dendrites of bulbar secondary neurons, mitral and tufted cells. Single mitral cells of fishes often have several dendrites ending in different glomeruli (Allison, 1953). It is significant that the axon of a receptor cell does not terminate in more than one glomerulus, and that each glomerulus receives impulses only from a limited group of several olfactory receptor cells; this is remarkably different from the mammalian mitral cells in which only a single main dendrite ends in each glomerulus. Furthermore, coincident with the lower degree of segregation of glomerular transmission, intrabulbar associational systems such as recurrent collateral also seem to be less elaborately developed in fishes (Allison, 1953; Fig. 5). Information from the olfactory bulbs is conveyed through two main fiber pathways, the lateral and medial olfactory tracts to the basal telencephalic areas. The medial bundle is thicker than the lateral one; both are subdivided into two small bundles. The two main fiber bundles contain myelinated nerve fibers with diameters less than 6 . 5 but ~ ~ in the lateral portion of the medial olfactory tract of Lota Zota the majority of the fibers are smaller than 0.5 p ( DBving and Gemne, 1965). The number of these fibers larger than 0.5 p is estimated to be about lo4. The ratio between receptor cells (primary neurons ) and the myelinated fibers in the tract (secondary neurons) is therefore 1OOO: 1. This is the same order of the convergence ratio between receptor cells and mitral cells in the rabbit (Allison and Warwick, 1949). Some fibers in the medial olfactory tract run directly to the hypothalamus while some cross in the anterior commissure. Ascending or centrifugal nerve fibers running to the olfactory bulbs have also been described (Sheldon, 1912). Recently, Westerman and Wilson (1968) have reported the presence of numerous synapses of two types, axo-axonal and axo-glial, within the medial olfactory tract of the carp, Carassius carassius.

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Fig. 5. Diagrammatic illustration of the essential difference in glomerular transmission between fishes ( a ) and mammals ( b ) : Com. ant., anterior commissure; and Tr. olf., olfactory tract. From Allison (1953).

The morphology of the brain has been related to the ecological habits of fishes ( Uchihashi, 1953; Schnitzlein, 1964). According to the morphological development of the brain, fishes may be classified as forebrain-, mesencephalon-, or medulla oblongata-developed groups. Those species in which the forebrain is well developed (Anguilla) have also a large medulla oblongata and are usually nocturnal; this indicates a dominance of olfactory and gustatory functions. Species in which the facial and vagal lobes are conspicuously developed (catfish and carp) show gustatory feeding behavior.

111. BEHAVIORAL STUDIES OF CHEMORECEPTIVE FUNCTIONS

A. Olfactory Sense

Accurate learning experiments in fish were first made by Strieck ( 1924). Minnows, Phoxinus phorinus, were trained to discriminate' pure odorous (coumarin, skatol, and muscone) and gustatory (glucose, acetic acid, and quinine) substances. These substances were readily detected by minnows. However, trained fishes were unable to discriminate odorous substances after the forebrain was removed although they could still

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perceive taste substances. This clearly demonstrates distinct olfaction and taste receptor functions in fish. Neurath (1949), using essentially similar training technique, determined the threshold values for P-phenylethyl in the minnow Phorinus. and eugenol (6.0 x alcohol (4.3x Similar threshold values for P-phenylethyl alcohol were also determined for the minnow by Teichmann (1959). Teichmann also found that the rainbow trout Salmo irideus detected P-phenylethyl alcohol at dilutions of 1 x this is nearly the same order of sensitivity as in humans. Juvenile sockeye salmon ( Oncorhynchus nerka) detected eugenol at concentrations higher than 1.8 x lo-? (Tarrant, 1966). Hasler and Wisby (1950) used a conditioning technique for the biological assay of pollutants. Upon completion of the training period, blinded bluntnose minnows, Hyborhynchus notutus, discriminated between phenol and p-chlorophenol at dilutions of less than 5 x le4. Similarly, coho salmon fry, Oncorhynchus kisutch, could easily detect (Hasler, 1957). morpholine in concentrations as low as 1 X Using a specially designed conditioning technique, Teichmann ( 1959) trained young European eels, Anguilla anguillu, to detect p-phenylethyl alcohol, Z-menthol, citral, eugenol, and ionon; he established the thresholds for each. Calculations indicate that the limits for detecting a chem( j?-phenylethyl alcohol) to 2 x ical range from dilutions of 3.5 x (ionon). The lowest threshold obtained in the eel is some 100 to 80 times lower than those of the minnow and rainbow trout. This can be compared favorably with the olfactory sensitivity of a terrestrial macrosmatic animal such as the dog. Further, the threshold of the eel for Pphenylethyl alcohol showed seasonal fluctuations; it was lowest in late winter and in midsummer, while about 60 times higher in late fall and in early winter. Such a decrease in the olfactory sensitivity might be explained by central and hormonal regulation ( Teichmann, 1959). Roaches, Leuciscus rutilus, were trained to induce a fright reaction in response to benzol derivatives (Marcstrom, 1959). Thus fishes responded to benzol at dilutions of 2.0 x l W M and to phenol at 9.5 X M . That these responses were olfactory was shown by the failure of responses when the noses were destroyed. The thresholds of the fright reaction to mononitrobenzol and lesolsine were five times higher than those of intact fishes. Recently, Miesner and von Baumgarten (1966) conducted an interesting study of olfactory perception and memory in the goldfish Carassius auratus. Fishes were trained to recognize coumarin, and their swimming movements were registered by means of specially designed technique. Coumarin could be detected at dilutions of less than 1 X Besides the normal positive reaction to the stimulus, the fish exhibited several charac-

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teristic behaviors such as circular and zigzag movements during which the direction of the odor gradient might be searched. Changes of water turbulence at the olfactory mucosa could help to overcome the adaptation of the olfactory receptor cells. When amylacetate was applied instead of coumarin, the fishes generally behaved in a positive way. Furthermore, when coumarin and amylacetate were given simultaneously at the opposite sides of the fish tank, the fish preferred coumarin, to which they were originally trained (primary differentiation). B. Gustatory Sense

As mentioned above, the argument that taste is a different sensory function from olfaction is based on the training of blinded minnows to discriminate certain taste substances even after extirpation of the olfactory lobes, while conditioning for odorous substances was only possible in intact fishes ( Strieck, 1924). Trudel (1929) compared the sensitivity of the minnows for various taste substances-especially saccharides and synthesized sweet substances. Fructose, glucose, galactose, mannose, mannite, arabinose, maltose, lactose, melezitose, raffinose, and two sweet substances, saccharin and dulcin, were all perceived by the minnows as essentially the same quality as sucrose. Trudel also reported that fructose was detected as the sweetest of all and that the relative threshold of the minnows for quinine (as low as 0.0025%)was higher than that of humans ( 0.003%). Krinner (1935) was the first to provide accurate thresholds for sucrose M ) in minnows using his careful trainM ) and salt ( 4 x (2x ing techniques. These thresholds were 512 and 184 times lower, respectively, than those of humans for sucrose and salt. Removal of olfactory lobes caused no change in these thresholds, thus verifying that a true gustatory sense was involved. Recently, by developing Krinner's training technique, Glaser ( 1966) has carefully compared taste sensitivity of the minnow Phoxinus plzoxinus, stickleback Gasterosteus aculeatus, South American salmon Hemigrammus caudovittatus, and Mexican blind cave fish Anoptichthys jordani. The time necessary to learn their task for a given taste substance (e.g., sucrose) differed greatly in various species; in Anoptichthys training was unsuccessful. Differences were also found within a species for various taste substances; for instance, minnows could be most easily trained for sucrose; this was followed by acetic acid, sodium chloride, and quinine. Of four basic taste substances, the reaction time for NaCl was longest in the minnow and that for sucrose in Gasterosteus, Phoxinus, and Hemigram-

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53.0 sec, respectively. No direct correlation between chemical structure and sensitivity was ascertained. According to Humbach (1960), taste sensitivity of Anoptichthys for four basic taste qualities was several thousands times better than that of Phorinus.

mus was 17.8, 19.1, and

IV. ELECTROPHYSIOLOGICAL STUDIES OF CHEMORECEPTOR RESPONSES

A. Olfactory System Electrophysiological studies of the chemoreceptive functions of fishes have not been extensive-partly because of small size of the structure and partly because fish live in water. Adrian and Ludwig (1938) who initiated analysis of the olfactory system in fishes with electrophysiological techniques recorded continuous impulse discharges in the olfactory tract of the catfish, carp, and tench. Such a resting discharge with low frequency and amplitude shifted to a maximum after a latency of 0.5-5 sec in response to mechanical as well as to chemical stimulation of the olfactory sac. The response gradually decreased during stimulation ( adaptation) and was then followed by a refractory period lasting 5-20 sec during which the organ was insensitive to a second stimulus. There has been a marked increase in the number of electrophysiological studies of fish olfactory system in the last few years. 1. MUCOSALPOTENTIALS Resting potentials of the olfactory epithelium were recorded in some teleost fishes with glass microelectrodes (Shibuya, 1960). They were 12.4 mV in Anguilla japonica, 8.7 mV in Misgurnus, 8.6 mV in Parasilurus, and 7.6 mV in Channa, Cyprinus, and Entosphenus. Slow negative potentials were induced in the mucosa during stimulation with odorous fluids such as butyric acid or extract of silkworm pupae (Fig. 6). The shape of the potentials were different in different species of fishes. Generally, however, the potentials showed a fast-rising phase with a slower exponential fall. The potential increased in time with increasing stimulus durations. In Parasilums and Anguilla, it had a short duration (0.4-0.7 sec) with a rapid decline. In the eel it always had a duration of about 0.4 sec regardless of stimulus duration ( Fig. 6e). Corresponding potentials were simultaneously recorded in the olfactory nerves. In addition to the “on-response” (appearing at the onset of stimulation), a distinct shift in potential appeared when stimulus ceased (“off-

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1

0

200pv

I

I

b

200pv

I

I

d

e

I

, -

200pv

200pv

I_

0.5 sec

Fig. 6. Slow potentials induced in the olfactory mucosa when stimulated with extract of silkworm pupae. a, Channa argus; b, carp; c, lamprey; d, catfish; and e, eel. From Shibuya (1960).

response”). The on and off responses occurred singly or successively ( on-off response). Simultaneous recordings with closely positioned microand macroelectrodes showed different types of response. Three different types of response have also been related to the existence of three functionally distinct types of receptors (Shibuya, 1960). Since there has been much argument concerning whether the slow mucosal response is a true generator potential (see Ottoson, 1963; Shibuya, 1964; Ottoson and Shepherd, 1967; Moulton and Tucker, 1964; Takagi, 1967), more precise analyses of the receptor response by recording activity in single receptor cells are required. In this connection, it is noteworthy that there are at least the five chemoreceptors responding to certain kinds of sugars and amino acids ( Adler, 1969), since the olfactory responses to these chemicals can be elicited in receptors and bulbs of some salmonid fish. It has also been suggested that animal receptors have evolved from single-celled flagellated organisms ( Vinnikov, 1965). 2. OLFACTORY NERVEACTIVITY

Information initiated in olfactory receptor sites is relayed through the olfactory nerve to the higher centers in the brain. However, since all

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sensory messages reach the neural centers in the form of impulses, recording activity in the primary olfactory neurons has some advantages from the technical point of view. Recording of the electrical activity of single olfactory nerve fibers may be difficult because of their extremely fine dimensions. Gasser (1956) investigated the properties of the isolated olfactory nerve of the pike Esox estor in a combined electrophysiological and electron microscopic study, He showed that the action potential evoked by electrical stimulation of the olfactory nerve consists of only one single wave of 30 msec duration with a conduction velocity of 0.2 meterslsec. Such a slow conduction velocity of the olfactory nerve has been reported in other animal species ( Ottoson, 1963).

3. ELECTRICAL ACTIVITYOF

THE

OLFACTORY BULB

Spontaneous electrical activities (EEG) have been recorded with bipolar electrodes from the surface of the olfactory bulb of the goldfish Carassius uurutus (Oshima and Gorbman, 1966a, 1968; Hara, 196713; Hara and Gorbman, 1967); EEG's were consistent and characteristic (frequency, 14-16 Hz; amplitude, 70-100 pV) . Infusion of chemical solutions into the nasal cavity induced specific synchronous wave patterns of high amplitude (150-200 pV) in the EEG. Maximal duration of the induced response was %6 sec, despite the continued presence of the chemical stimulant in the olfactory sac (Hara and Gorbman, 1967). The response to NaCl increased in magnitude linearly with increasing concentration within the range tested (1-5 x 1e2 M ) . Qualitatively similar patterns of electrical response were obtained with KCl, LiCl, CaCl,, NaH,PO,, Na,HPO,, Naz-oxalate, and Nag-citrate. No response was induced by choline-chloride at equivalent molar concentrations. Although the nature of the induced response following chemical stimulation of the olfactory sac is still unknown, it seems possible that the responses result from a relatively nonspecific excitation of the olfactory receptor cells. The highly synchronous activity induced in the mammalian and amphibian olfactory bulb by odorous stimulation seems to be analogous. A characteristic potential was also evoked in the olfactory bulb when single electrical stimuli were given to the olfactory mucosa instead of chemical stimuli. The potential at the surface of the bulb recorded through bipolar electrodes, was a biphasic wave with a delay of about 20 msec, a maximum amplitude of about 1.2 mV, and a duration of about 50 msec. The potential consisted of three components with distinctly different properties. The first component lacked a true refractory period and summated to a sustained potential with repeated stimulation. The

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second component had a comparatively long (about 30 msec) refractory period and would not follow repeated stimulation at a frequency higher than about l/sec without undergoing a reduction in amplitude. The third component ( positive-directed afterpotentials ) disappeared after section of the olfactory tract. It has therefore been concluded that the first component is of synaptic origin, the second component represents the activity in the second-order neurons, and that the third component is centrifugal origin. When microelectrodes were inserted into the olfactory bulb, spontaneous unitary activities were recorded in different layers of the olfactory bulb of the goldfish. Oshima and Gorbman ( 1966b) observed three different types of discharge patterns of single cells in the glomerular and subglomerular layers of the olfactory bulb. These spontaneous and evoked unitary activities in response to chemical stimulation were easily influenced by treating with thyroxine and steroid hormones. In the layers about 300-400 p under the surface, large biphasic action potentials were frequently detected ( Hara, 1967a). The discharge frequently varied from cell to cell, but most cells of this type had an average discharge frequency of about 2-6 impulses/sec. Probably much of the activity in this layer is derived from the mitral cells. Different individual neurons responded in different ways to chemical stimulation of the olfactory cavity. The variety of observed responses to 5 x M NaCl from different individual cells included the following: (1)inhibition (decrease in the firing rate) during or after the period of stimulation, ( 2 ) excitation (increase in firing rate) which could outlast for a few seconds the duration of the stimuli, ( 3 ) excitation during stimulus followed by a short inhibition when the stimulus ceased, ( 4 ) excitation at the beginning of the stimulus and inhibition afterward, ( 5 ) a short inhibition at the very beginning of the stimulation followed by excitation, and ( 6 ) no response. More than 60%of the neurons tested were of types (1) and ( 3 ) ,which represent opposite patterns of response. Similar spontaneous activity from single units in the olfactory bulb was recorded with microelectrodes in the burbot Lota lota (Dkiving, 1966a,b). 4. ELECTRICAL ACTIVITY OF THE OLFACTORY TRACTAND CENTRAL REGULATORY SYSTEM Olfactory information filtered in the first relay station, the olfactory bulb, is transferred through the olfactory tract to higher nervous centers. In some teleosts the olfactory tract runs as a long nerve bundle (see Section 11,D ) between the bulb and telencephalon. Such a unique anatomical feature of the olfactory system provides a convenient preparation for elec-

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trophysiological analysis of the messages conveyed in the olfactory tract. As already stated above, the first electrophysiological investigation of this kind was that by Adrian and Ludwig (1938). Similar findings were reported for catfish by Boudreau (1962) who recorded summated activity of the olfactory tract and showed that the increases in tract activity were produced by various chemicals in extremely dilute concentrations. Acetic acid and butyric alcohol remained effective at concentrations of lo-’’ M and M , respectively; at higher concentrations they depressed activity. Electrical properties of the olfactory tract were studied by analyzing compound action potential induced by electrical stimulation in the burbot Lota lota and in some species of the orders Anacanthini and Ostariophysi (Doving and Gemne, 1965; Doving, 1967). The compound action potential of the olfactory tract had three components with different conduction velocities ranging from 0.25 to 5.5 meters/sec at 10°C. Generally, the third component which showed the lowest conduction velocity was present in the medial bundle of the tract. This component, probably nonmyelinated, might be responsible for connecting the olfactory system with the hypophysis ( Kandel, 1964; Jasinski et al., 1966, 1967). Furthermore, the activity of single fibers in the olfactory tract was influenced by stimulating the olfactory epithelium with various chemical solutions in the burbot when efferent inflow was eliminated by sectioning the olfactory tract. Most of the chemicals evoked both excitatory and inhibitory types of response in different individual units. Slow rate of adaptation of the activity of the secondary neurons to continuous stimulation was observed; this confirmed earlier findings in catfish by Adrian and Ludwig (1938). Similar effects on the spontaneous firing of single fibers in the olfactory tract to afferent olfactory stimulation were obtained by Nanba et al. (1966) in Abramis and Carmsius. Recently, Westerman and Wilson (1968) reported the conduction velocity averaging 0.6 meters/sec in the lateral olfactory tract of the carp. The demonstration that afferent nerve impulses in the olfactory system can be directly controlled by influences originating in the central nervous system has been one of the most interesting developments in physiology of the olfactory system. The action is mediated by way of the “centrifugal” fiber system, part of which has long been known anatomically ( Sheldon, 1912; Allison, 1953; Kappers et al., 1960; Aronson, 1963). Section of the ipsilateral olfactory tract caused a marked augmentation of the induced response of the olfactory bulb to NaCl infusion (Hara and Gorbman, 1967; Fig. 7). In the preparation sectioned at the midbrain-hindbrain level ( ceroeau isole‘), cutting the olfactory tract eliminated the similar NaC1-induced bulbar response ( Oshima and Gorbman, 1966a). Bulbar potential waves evoked by electrical stimulation of the

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Fig. 7. Modification of the bulbar response induced by infusion of NaCl solution into the nasal cavity after sectioning the olfactory tract: ( a ) Before tract section and ( b ) after tract section. Calibration, 50 $V and 1 sec. From Hara and Gorbman (1967).

olfactory epithelium were also affected by olfactory tract section; the positive afterpotential almost disappeared and consequently the potential wave became monophasic (Hara and Gorbman, 1967; Fig. 8 ) . These results also verify that the rapid adaptation seen in the olfactory bulb of the intact animal is probably of central origin. Furthermore, electrical stimuli applied to the opposite olfactory bulb or to the anterior commissure, or strong chemical stimuli given to the opposite nostril depressed both intrinsic and afferent-induced electrical activity of the bulb. These findings indicate that olfactory bulbar responses to afferent stimuli can be modulated by influences from the other bulb and from more posterior parts of the brain (Hara and Gorbman, 1967). Spontaneously firing secondary neurons recorded in the olfactory bulb and tract were shown to be similarly regulated by electrical stimulation of the ipsilateral and contralateral olfactory tract ( Doving, 196613; Diiving and Gemne, 1966; Doving and Hyvarinen, 1969; Hara, 1967a).

Fig. 8. Electrically evoked bulbar potential before ( b ) and after ( a ) tract section. Calibration, 0.5 mV and 20 msec. From Hara and Gorbman (1967).

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Central regulation of afferent transmission within the nervous system may be a common process in neural integration. Particularly in fishes, such a mechanism of central regulation, including interbulbar connections, may play an important role in the orientation toward or away from a stimulus source by simultaneous bilateral equating of stimulation.

B. Gustatory Receptors 1. PALATAL ORGANOF CARP

Unlike the olfactory system, taste receptors of fishes are scattered widely over the body surface. For this reason electrophysiological studies on taste function in fishes have mainly involved the recording of potential discharges from the nerve fibers innervating taste buds. Hoagland (1933) first recorded electrical responses from the facial nerve innervating the taste buds on the barbels of the catfish exposed to various taste solutions. Subsequently a more thorough investigation of the electrical responses of the taste receptors was undertaken by Konishi and Zotterman (1961a,b) in carp. Integrated electrical responses to various taste substances were recorded from the glossopharyngeal, facial, and branchial nerves innervating the palatal organ, barbels, and gill rakers. The findings suggest that the palatal organ plays the principal role in gustation in the carp although certain differences exist between Swedish and Japanese carps (Konishi and Zotterman, 1961a,b, 1963). Swedish carp showed a large gustatory response to sucrose (0.5 M ) and acetic acid (0.005M , pH 3.8) and a weak response to quinine (0.01 M ) , while Japanese carp show low sensitivity to sucrose and high sensitivity to quinine. Responses to human saliva were much larger than those to NaCl (0.5 M ) . Single taste fibers from the glossopharyngeal nerve could be qualitatively classified into seven groups according to their response patterns to four basic taste substances and saliva. Acetic acid (0.005M ) stimulated all taste fibers, except for salt fibers which were specifically responsive only to NaCI. The fibers responsive to human saliva were also stimulated by sucrose. Extract of silkworm pupa also induced a marked gustatory response. The final gustatory active compound could not be identified either in saliva or in extract of silkworm pupae. Lytic agents produced an irreversible depression of taste responses. Thus, treatment with 0.3%sodium cholate depressed the response to sucrose while treatment with 0.005%digitonin immediately reduced the responsiveness of the taste receptors. Furthermore, the taste responses of the carp to sucrose, dextrose, levulose, and glycine were analyzed (Hidaka and Yokota, 1967). Re-

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sponses to sucrose, levulose, and glycine increased asymptotically with increasing concentrations, while that for dextrose was sigmoid. Inhibitory interactions may occur when two chemicals are presented to the taste receptors simultaneously or successively. Competitive actions were observed among dextrose, sucrose, and levulose. Mercuric chloride ( 1c4 M) reversibly blocked the taste responses to dextrose, sucrose, and levulose, but not to glycine and NaCl. These results suggest the existence of at least two different kinds of receptors in the palatal organ of carp; one is commonly responsive to all four substances and the other only to glycine. The existence of palatal chemoreceptors responding specifically to dilute solutions (O.OOS0.0005 M for NaC1) of salts with monovalent cations (Konishi and Niwa, 1964; Konishi, 1967) and of various organic compounds (Konishi and Hidaka, 1969) has been demonstrated in carp. Strong responses were produced by various chemicals with polyvalent anions such as Na-citrate, Na2HP0,, Na,Fe( CH) 6 , tetramethylammonium-C1, choline-C1, Na-glutamate, glucose, and glycine. The response decreased with increasing concentration, then increased again at much higher concentrations. In general, the higher the valency of the anion of the compounds, the larger the responses induced. Applications of distilled water, immediately after stimulation with a salt solution at concentrations (O.OOl-O.05A4 for NaCl) where responses were depressed, elicited a marked integrated response (distilled water effect). The effect has been ascribed to the activity of the same receptor as in the response to dilute salt solutions. By analyzing the effects of acid, alkali, and dye salts, a hypothesis which explains underlying mechanism in terms of an interfacial electrokinetic process has been presented ( Konishi, 1967). Such responses to dilute solutions are not restricted to freshwater fishes. Similar responses were also observed in the facial nerves innervating the upper lip of sea catfish Plotosus anguillaris (Konishi and Hidaka, 1967). The biological significance of the response to dilute solution and distilled water is unknown. Similar effects of highly diluted solutions and distilled water on the olfactory bulbar responses have often been observed (Hara, unpublished data). The palatal chemoreceptors of the carp were found to be highly sensitive to carbon dioxide. No detectable responses to oxygen, nitrogen or air were obtained ( Konishi et al., 1969). The responses were confirmed to be independent of pH of the solutions applied. Avoidance behaviors to CO, and/or pH were studied in Atlantic salmon parr, minnow, and roach, in connection with water pollution (Hoglund, 1961; Hoglund and Hardig, 1969). The removal of olfactory tissues and the sectioning of the nerves innervating the lateral line organs did not essentially change the reactions of the fish.

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2. BARBELSOF CATFISH Barbels play an important role in food taking in catfish. Application of taste substances induced a train of spikes in the facial nerves innervating the nasal barbels of the catfish, Ameiurus melas (Tateda, 1961). By stimulating with equivalent molar concentration of various chlorides, the order of magnitude of responses was KC1 > NH,C1 > NaCl > Lick thresholds for these salts were about 0.1 M . Solutions of NaC1, NaN03, NaBr, and Na,-citrate in equivalent normal concentration elicited responses of similar amplitude. Hydrochloric acid induced a large initial burst of impulses, which declined rapidly in 10-20 sec, at concentrations of 0.0005-0.001 hl. Taste responses of isolated barbels of the catfish, Parasilurus asotus, were also analyzed by recording potential discharges from single nerve fibers (Tateda, 1964). The majority of single fibers responded well to hydrochloric acid and salt but not to sucrose and quinine. No simple classification of the taste fibers could be obtained. In this connection, it must be pointed out that one single taste fiber may innervate several taste receptor cells of different natures. Treatment with KC1 and CaC12, although in themselves stimulating, caused a depressive effect on the receptors and resulted in irreversible changes in the taste responses at higher concentrations of 0.25-1.0 M . By short application of urethane ( 13%for 20 sec) and cocaine (0.25%for 30 sec), taste responses of the barbels to HC1 and NH,Cl were reversibly abolished. Furthermore, from the analysis of stimulating effects of hydrochloric acid and several organic acids (formic, acetic, propionic, and butyric) on the catfish barbels, it was shown that the responses to organic acids were greater than those to hydrochloric acid at an equal hydrogen ion concentration and that the responses increased with increase in molecular size of acids tested (Tateda, 1966). In sea catfish Plotosus anguillaris single taste fibers, responsive to both NaCl and quinine, were most commonly found (Konishi et al., 1966). Sucrose did not produce any appreciable response. It is surprising that distilled water gave no positive response in most fibers despite the fact that the receptors were rinsed with seawater between applications of test solutions. Many fibers specifically responded to natural gustatory substances such as extract of a marine worm and blood sera of other animals. The active components of these stimulants are not yet known. Electrical response to cysteine of the taste fibers of the barbels of yellow bullhead ( Ictalurus natalis) was demonstrated to be impaired after exposure to low concentrations (0.5 ppm) of detergents. Histological examination revealed erosion of the taste buds. Affected fish did not fully recover after 6 weeks in detergent-free water (Bardach et al., 196.5). Recently, Mann (1969) reported that the deposition of phenolic sub-

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stances and oils, released from aqueous flora, and their impairing effect on taste and smell in carp was intensified by the presence of detergents in water. 3. MODIFIEDFINSAND OTHERFORMS OF MARINE FISHES

It is important to ascertain whether or not there are differences between external and oral chemoreceptors and to compare the taste sense of marine and freshwater fishes. A series of observations have been carried out with appendages of several species: (1) those whose barbels have cranial innervation only (bullheads, lctalurus) , ( 2 ) those in which fins have both cranial and spinal innervation ( tomcods, Microgadus tomcod), and ( 3 ) those with modified fins innervated only by spinal nerves (sea robins, Prionotus carolinus) (Bardach and Case, 1965; Fujiya and Bardach, 1966; Bardach et al., 196713). Acetic acid produced responses in most nerve fibers of the barbel of the bullhead at a concentration of 0.0008 M , while in tomcod and sea robin about ten times the concentration was needed to evoke a response. This, however, does not imply that marine fishes are less sensitive to acids; on the contrary, it means that marine fishes are less able than freshwater fishes to perceive acids at the same concentrations. Responses to acids depend on the presence of free hydrogen ions in the solutions, and the above two acid solutions produce about the same amount of hydrogen ions. Upon application of NaCI, nerve discharges from both marine and freshwater fishes increased only at concentrations higher than those of their natural ecvironments. Reduction of nerve discharges in tomcods and sea robins was observed at concentrations between 0.4 and 0.3 M NaCl. Complete inhibition occurred when freshwater was applied; this was followed by a transient augmentation of nerve discharges. Responses to choline-chloride were similar to those to NaCl in three species. Varying numbers of fibers responded to quinine hydrochloride. Only a few fibers (less than 10%)responded to sucrose, and no response was observed in sea robins where the fins have not taste buds but only spinal nerve endings. All three preparations responded well to flesh extracts. An attempt has been made to extract the active component. Of various amino acids which were identified in the extracts, only cysteine elicited strong responses; some of the fibers responded selectively only to cysteine. More precise investigation will be required to resolve a question as to whether or not there are taste qualities other than the four basic ones in fishes. Katsuki et al. ( 1969) studied electrophysiological response to chemical stimulation of pit organs of the nurse shark, Ginglymostoma cirratum.

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They responded readily to NaCl and KCI solutions (1M ) . Divalent cations such as Ca and Mg were inhibitory. Responses to acid, sugar, and quinine were either slight or inhibitory.

V. BIOLOGICAL ASPECTS OF CHEMORECEPTION

A. Chemical Perception of Foods There are numerous observations of the searching behavior for foods, and it has been recognized that all senses (e.g., optic, amustic, and chemical) are involved. Clues which initiate searching behavior depend on the species, history, and schooling tendencies as well as environmental conditions; some fishes rely more on chemical senses and others more on optic or acoustic senses. The importance of chemical senses, especially sense of smell, has been demonstrated in the following observations. Parker (1910) placed five normal catfish, Ameiurtu, in an aquarium in which were hung two wads of cheesecloth, one of which contained minced earthworm. The wad containing the worms was seized and tugged eleven times by the fishes, while the wad without worms failed to excite any noticeable reaction in the course of an hour. Fishes whose barbels were removed but had normal olfactory organs reacted in the same manner as intact fish; fishes with cut olfactory tracts never seized the wad containing worms. These findings indicate that the olfactory apparatus of the catfish is useful in sensing food at a distance; catfish truly scent their foods. Barbels are also very valuable to Ameiurus in procuring foods only by coming into direct contact with it (Olmsted, 1918). The eyes of the killifish, FunduZus heteroclitus, in strong contrast with the catfish, play an important role in the initial stages of procuring food but actual swallowing of the food depends on other sense, probably olfactory. Fishes whose olfactory tracts were cut or whose anterior olfactory apertures were stitched never discriminated between two packets of clothone with dogfish meat hidden in it and the other without the food (Parker, 1911). The importance of sense of smell in finding foods has long been demonstrated in Selachians. Dogfish, Mustelus canis, failed to recognize and determine the location of food substances such as crab meat when the olfactory capsules were occluded with cotton. Food recognition was regained when the cottom was removed; plugging of one nostril did not seriously affect that ability ( Sheldon, 1911). A similar directive influence of the sense of smell in the dogfish was observed by Parker ( 1914). Normal dogfish found the foods by making combined right- and left-handed

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movements; this resulted in continuous and characteristic courses in the form of figure eight. Dogfish with one nostril occluded found the food by predominant movements toward the side of the open nostril. Their movements were essentially circular in such a way that the open nostril is toward the center of the circle. Parker explained these movements on the basis of the osmotropotaxis as in the circus movements of invertebrates. Gilbert et al. (1964) reported that electrical potential recorded from the forebrain reflected olfactory processes in three species of sharks ( N e gaprion, Sphyrw, and Ginglymostom) . Both the amplitude and frequency of the potential clearly increased during chemical stimulation of the olfactory sac with extracts of crabs and tuna. Blinded bluntnose minnows, Hyborhynchus notatus, were able to discriminate odors of aquatic plants at extreme dilutions. They also could discriminate between odors of certain aquatic invertebrates ( Hasler, 1957). These findings support the view that aquatic plants may play an important role in the life of fish. Juvenile sockeye salmon, Oncorhynchus nerka, responded by evoking exploratory and feeding behavior to aqueous extracts of foods to which they had been previously conditioned but failed to respond to similar foods which had not previously been in their diet ( McBride e t al., 1962). Responses were characterized by breaking up of the school, increased swimming speeds, and swimming into lighted areas. Fish probably became conditioned to some component( s ) of their foods, but they might be attracted by extracts of foods that they had not previously ingested. As already mentioned, such a complicated feeding behavior is accomplished by successively separated individual behaviors. For instance, once a fish has found food by visual, mechanical, or chemical clues, it may still have to test before eating it. Various environmental stimuli are integrated in the central nervous system, probably hypothalamus, and subsequently organized into a unified expression of the behavior. Electrical stimulation of olfactory areas in the central nervous system of goldfish elicited stereotyped feeding activity indistinguishable from normally induced behavior ( Grimm, 1960). Stimulation of vagal lobes produced no feeding arousal. It is suggested that the olfactory rather than the peripheral gustatory system plays the predominant role in the arousal of feeding activity. Further studies of the central integration of feeding behavior are to be encouraged. B. Reproductive Behavior and Chemical Senses In contrast to work with visual cues, the role of chemical senses in reproductive behavior has been poorly investigated. The role of chemical

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factors was described in the courtship behavior of the catfish Ameiurm by Breder ( 1935), and of gobiid fish Bathygobius by Tavolga (1956). Bathygobius males showed courtship behavior in response to the introduction of a small amount of the water in which a gravid female had been placed for a few minutes. Of various internal body fluids tested only the ovarian fluid elicited a courtship response at extreme dilutions. Whether the stimulating ovarian substance is secreted by the eggs or the ovary has not been determined. The male detected the rapidly diffusing substance by the olfactory sense, since anosmic males whose nostrils were plugged failed to respond to any amount of the courtship-stimulating substance ( Tavolga, 1956). Furthermore, it was demonstrated that anosmic males tended to show fighting behavior and that castrated males never exhibited such behavior (Tavolga, 1956). These results may indicate that the olfactory stimulus by the ovarian substance not only elicits courtship but inhibits fighting. It is also possible that the male sex hormone affects the sensitivity of the olfactory organs in some ways. In this connection, it has been shown that afferent-evoked bulbar electrical responses produced by NaCl infusion into the olfactory sac and by electrical stimulation of the olfactory epithelium in goldfish were markedly influenced by administrations of sex hormones (Hara, 196713; Oshima and Gorbman, 1968, 1969). C. Discrimination of Body Odors and Schooling

Wrede (1932) apparently was the first to demonstrate that a minnow has an individual body odor recognizable by other members of the same species. GO, (1941) later trained the blinded minnow to discriminate the body odor of the catfish Ameiurus and proved that minnows could no longer discriminate the odor after the olfactory lobes were removed. The minnows could be trained to discriminate the body odors of 15 different species of fish from eight families; they could also recognize odors of different individuals of their own species. Furthermore, they could specifically discriminate olfactorily between two different species of frogs ( R a m esculenta and R. temporaria) and between two species of salamanders ( Triturus and Salamandra) , but they could not discriminate between two individuals within the same amphibian species. Yellow bullheads, Ictalurus notatus, also recognized individuals of their own species by means of intraspecific chemical stimuli (Todd et al., 1967). Through conditioning, blinded bullheads were able to discriminate between the odors of two donor fish, but they lost this ability when deprived of their sense of smell. The main source of the intraspecific chemical stimuli in-

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volved in recognition was the integumentary mucus. A change in status after fighting was chemically communicated to other bullheads. Oshima et al. (1969b) reported that chinook salmon (Oncorhynchus tshawytschu) showed olfactory bulbar responses to the water, which had previously evoked only a slight response, by placing other individuals of the same species in it. Keeping coho salmon under the same conditions did not make the water as stimulatory. It has been suggested that olfactory communication plays some part in maintaining the coherence of schools. Keenleyside (1955) found that blinded rudd, Scardinius erythrophthalmus, could detect and preferred water that had contained other rudd but showed no such preference after their olfactory epithelium was destroyed. Stevens (1959) observed in two pelagic schooling fishes, Hepsitia stipes and Bathystoma rimator, that during the day schools were maintained visually; the fish were more active and did not swim in schools at night; and their behavior was clearly directed toward the investigation and sampling of potential food. It seems reasonable to assume, therefore, that schools can be guided to their foods from long distances by olfactory perception. Finally, it may be expected that fishes swim up concentration gradients of substances excreted by their food organisms and that schools of the same species may find each other by this means (Stevens, 1959). Recently, Hemmings (1966a) studied the schooling behavior of the roach, Rutilus rutilus, in relation to the role of olfactory and visual senses; he suggested that school structure is maintained by balanced attractive and repulsive forces, the attraction modalities involved are vision by day and olfaction by night, and the repulsion modality is the lateral line sense. D. Alarm Substances

Von Frisch (1938) discovered that a school of minnows, Phoxinus laevis, showed a strong fright reaction when an injured minnow was introduced into the school. Since then alarm substances, which communicate the presence of danger and which are produced by members of the same species, have been extensively studied by von Frisch and his students. In a series of observations it was shown that the alarm substance is released only from injured skin. Dead minnows with undamaged skin were ineffective. Pieces of their stomach, gut, liver, spleen, and muscle were equally ineffective. Although absolute values of the potency of the extract could not be obtained, a solution of 0.1 g of fresh skin in 100 ml of water was sufficient to induce fright reaction in aquaria of 25-150 liters

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capacity, Little is known concerning the chemistry of the alarm substance. It was suggested that the alarm substance was a pterine and close to isoxanthopterine ( Huttel, 1941; Huttel and Sprengling, 1943). These authors called it ichthiopterine. There is evidence, however, that ichthiopterine is not identical with alarm substance (Schutz, 1956; Pfeiffer, 1967). It was also pointed out that alarm substance was not volatile although very soluble in water. Several amino acids and amines perceived by parasitic Petromyzon marinus were chemically separated from the body odor of brook and brown trout (Kleerekoper and Mogensen, 1959, 1963). From histological observation, on the other hand, Pfeiffer (1960) showed that the alarm substance is produced in specialized epidermal cells (alarm substance cells), which do not open onto the surface but only release their contents when the skin is injured. These cells are found in all species which show a fright reaction. The fright reaction appears at a certain stage of the growth of fishes regardless of their prior experience and does not develop until some time after the alarm substance is formed in the skin (Schutz, 1956; Pfeiffer, 1983a). A predator odor capable of eliciting a fright reaction in local prey species was found in three North American predatory fishes (Lepomis macrochirus, Mkropterus punctulutus, and Esox niger) and in two South American fishes ( Astronotus ocellatus and Cichhoma sezlerum) . No damage to predator skin was necessary to release the substance which caused the alarm response (Reed, 1!369). Only the olfactory sense is involved in the detection of the alarm substance. Minnows never responded to the alarm substance after the olfactory nerve and olfactory bulb were removed (von Frisch, 1941). Schutz (1956) observed that if the excited movements of fish showing the fright reaction could be seen by other minnows, even though separated by glass walls, a typical reaction could be visually transferred without the presence of the alarm substance. Thus, under certain conditions, the movements associated with the fright reaction could initiate a reaction in fish which had not been exposed to the alarm substance. From observations with 60 species of Ostariophysi and 91 species (44 families) of non-Ostariophysi ( von Frisch, 1941; Schutz, 1956; Pfeiffer, 1962, 1963a, 1967)) it may be concluded that the fright reaction exists only in the Ostariophysi and is associated with this group of fish and not with any particular habitat or type of social behavior. The reaction is certainly absent in some species in Serrasalminae and Mylinae. The Mexican blind cave fish, Anoptichythys jordani, does not react to its own skin extract, although the fish does have the alarm substance. Furthermore, although the fright reaction is species specific, a marked interspecific reaction has been observed (Pfeiffer, 1963a). However, the

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strongest response was always obtained with the alarm substance from the same species. Interspecific reactions bear a definite relation to the taxonomic position of the species ( Pfeiffer, 1963a). Skinner et al. (1962) isolated the alarm substance of the top smelt Atherinops afinis by extraction with methanol or ether from suffocated top smelt. These extracts, when introduced into an aquarium containing top smelt, induced a strong alarm reaction in the fish. Reaction was species-specific. Later, however, this was denied by Rosenblatt and Losey ( 1967). Two functions, that is, warning against predators and prevention of intraspecific predation, have been postulated to attribute to the alarm substance, though these have been questioned recently ( Verheijen, 1962).

E. Repellents Brett and MacKinnon (1952) found that striking reduction in the rate of upstream migration of adult sockeye Oncorhynchus nerka, coho 0. kisutch, and spring 0. tschawytscha salmon occurred when human hands were rinsed in the salmon ladder. Later this phenomenon was demonstrated in all five species of migrating Pacific salmon. Of various substances tested only dilute water rinses of mammalian skins had distinct repellent action (Brett and MacKinnon, 1954; Alderdice et al., 1954). Upon detecting the repellent odors, salmon swam excitedly moving in a circle in the enclosed area, exhibiting an alarm reaction. Chemical analysis of the properties of a repellent from human skin indicated that only L-serine elicited a strong repellent action at extremely high dilution (8 x lO-'O); however, the effects were neither so dramatic nor for so long a duration as the response obtained by a hand rise (Idler et al., 1956, 1961). Recently it has been observed that rainbow trout respond to handdipped water and to L-serine at concentrations as low as M by recording electroencephalographic responses from the olfactory bulb (T. J. Hara, unpublished data).

F. Orientation by Chemical Senses Numerous investigations have attested the high acuity of the chemosensory organs in most fishes. Olfactory cues are important in the orientation of migrating fish (eels, Anguilla anguilla, and salmon, Oncorhynchus sp.) and in the localization of spawning grounds; gustatory cues are important in the location of distant chemical stimuli (bullheads,

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Ictalurus) . Many theories have been proposed to explain the mechanisms of migration of anadromous salmon to their spawning grounds (Jones, 1968). It has been suggested that the orientation is mediated through olfaction, and reference has been made to a home-stream substance; some investigators have opposed these views (Ramsay and Hasler, 1961). Since the problems pertaining to the orientation of migrating salmon are dealt with in Volume VI (also see Hara, 1970), only current topics will be mentioned here. Spontaneous electrical potentials ( EEG ) were recorded from several different parts of the brains of adult spawning salmon (Oncorhynchus tschawytscha and 0. kisutch) when they arrived at their home pond (Hara et al., 1965). Activities in the olfactory bulb and in the posterior cerebellum consistently had a much higher amplitude than those of other parts; amplitudes of potentials in the optic lobes were especially low; in some fish the optic lobes were electrically “silent.” In contrast to adult salmon, young salmon exhibited high activities in the olfactory bulb and in the optic lobe. Cerebellar electrical activity in these younger fish was not yet developed. Spontaneous electrical potential records were also made from brains of adult, nonmigratory rainbow trout, Salmo gairdnerri, and of goldfish, Carassius auratus. In both of these nonmigratory species, EEG activity in the olfactory bulbs was relatively low while that of the optic lobes was much higher than in adult spawning salmon. Infusion of “home water” into the nasal cavity of migrating adult salmon produced a clear stimulation in EEG patterns recorded from the olfactory bulb; various natural waters from other nearby sources produced virtually no response ( Fig. 9 ) . These findings suggest that olfaction is an important factor in guidance during the final phases of homeward migration of salmon and that such olfactory discrimination occurs at the level of either the olfactory bulbs or the olfactory epithelium. However, there remains the possibility that certain nonspecific odorous substances, such as foods, were merely in higher concentration in the home water than in any other water used. Apparent specificity of the response to home water was confirmed by treating salmon from three different termini of migration with a series of natural waters (Ueda et al., 1967; Oshima et al., 1 9 6 9 ~ )The . high amplitude EEG response of characteristic pattern recorded from the olfactory bulb by infusion with home water is specific; little or no response can be evoked by water from spawning sites of other groups of breeding salmon (Table I). This implies that each spawning area has its own specific stimulant or specific combination of stimulants recognized and responded to by the anadromous salmon. Furthermore, weaker but definite responses could be evoked by waters (1)traversed by the salmon migrating toward the spawning site, ( 2 ) from the bypassed branch of a

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B

E

' I

J

Fig. 9. Effects of infusion of different waters into the nasal cavity upon EEG activity in the olfactory bulb of an adult spawning salmon: A, Distilled water; B, tap water; C, home water; D-F, waters from three different lakes. Lines below each of the records indicate duration of stimuli. Calibration, 50 p V and 1 sec. From Hara et al. (1965). Copyright 1965 by the American Association for the Advancement of Science.

stream near the spawning site, and ( 3 ) from a point upstream of the spawning site. It is suggested that on returning from the sea, the adult salmon retrace a trail of stimulatory factors, presumably the ones to which they were imprinted as young fish on their seaward migration (see Hara, Table I Specificity of Response to Home Water in Fishes from Three Spawning Systems" Water from

Fish from Fisheries pond (Chinook) Issaquah Creek (silver) Soos Creek (silver)

Fisheries pondb

100 (4) 39.1 f 1 4 . 5 (3) 0 (4)

Issaquah Creekb

0 (4) 100 (3) 0 (4)

soos Creekb 2 . 3 f 3 . 1 (4) 6 . 4 f 3 . 5 (3) 100 (4)

From Ueda et al. (1967). The magnitude of response is represented as a percentage of that to home water [mean k S.E. (number of animals)]. a

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1970). However, this question requires further study before a definitive statement can be made. Intracranial injection of antimetabolites, puromycin, actinomycin D, or cycloheximide in the homing chinook salmon markedly inhibited 01factory bulbar discrimination between home water and other natural waters ( Oshima et al., 1969a). Earlier, Rappoport and Daginawala ( 1968) showed that olfactory stimulation with morpholine induced an increase in brain nuclear RNA and a change in base ratios in both intact and split-brain preparations in isolated heads of marine catfish, Gabichthys felis. Thus, there seems to be agreement that RNA synthesis is part of memory-establishing mechanism. By monitoring and three-dimensional photographic techniques, Kleerekoper ( 1967a,b) analyzed certain aspects of orientation through olfaction in some marine ( Scyliorhinus stellaris, Mustelus mustelus, and Diplodus sargus) and freshwater ( Zctalurms nebulosus and Lepomis gibbosus) fishes. Single specimens were placed in an experimental tank, which was divided into 16 partial compartments by radially placed walls. Direction of the fishes’ movement, left- and right-hand turns, speed of locomotion, the frequency of entries into any one compartment, the frequency of pathways, and the diurnal distribution of total activity were continuously and automatically recorded with or without odorous stimulation. None of the fishes studied moved randomly under experimental conditions devoid of directional cues. The radius of the curve in changing direction was fixed within relatively narrow limits and seemed to be species-specific; left- and right-hand turns were not evenly distributed. Such a locomotor behavior resulted in a nonrandom pattern of relatively rigid parameters. Introduction of an odor without directional cues caused a drastic change in these parameters; the radius of the curve in changing direction decreased and the ratio of left-handed to right-handed turns was greatly changed. When an attractant odor was introduced unidirectionally, none of the species studied could locate the source, unless the odor was associated with a differential in the rate of water flow. It is, therefore, suggested that an attractant odor releases rheotactic response so that the localization of the source of the odor takes place through rheotaxis rather than through osmotropotaxis. Similarly, Hemmings (196613) made an analysis of the mechanism of orientation of roach Rutilus rutilus in an odor gradient. The swimming speed of the fish was not directly related to odor concentration but was low when fish swam into decreasing concentration and much higher in the opposite direction; turning was more frequent at the high concentration end of the gradient. These findings cannot be explained simply in terms of ortho- or klinokinesis. Hemmings suggested that orientation of

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this fish is by spatial and temporal comparison of intensities, that speed is related to the rate of change in stimulation, and that variation in turning serves to maintain fish at high odor concentrations. As is evident from Fig. 7, however, there occurred an immediate adaptation in the olfactory bulbar response in salmon by infusing the home water. It is impossible to assume that there exists such large odor gradients as to overcome the adaptation. Hence, it is probable that the orientation of these fish is not by osmotropotaxis but by “trial and error.” Creutzberg (1958, 1959) reported that the elvers of eels, Anguilla vulgaris, most probably used the tidal streams for their migration; they are transported in the direction of the inland waters by the flood tide and go to the bottom during the ebb in order not to be carried back to sea. From the evidence obtained by preference experiments for the waters, it is further suggested that elvers are guided from the open sea to the inland water by olfactory cues and like adult salmon are thus guided to their parent stream. Eels showed a clear preference for natural freshwater originating from inland sources. The water lost its effectiveness when passed through a charcoal filter. Recently, Miles (1968) reported that elvers of the American eel Anguilla rostrata show a stronger positive rheotaxis to freshwater than to saltwater. By the various treatment of the water he also demonstrated that the attractive components dissolved are biodegradable, heat stable, and nonvolatile; this seems to differ from the substance that attracts salmon to their home stream (Fagerlund et al., 1963; also see Volume VI, and Hara, 1970). Furthermore, the presence of adult eels in the water made it more attractive, while the presence of elvers made it less attractive. However, differences exist between the salmon and the eel; knowledge of the cues seems to be acquired by imprinting in the salmon, whereas it is most likely to be inborn in the elvers. Recent studies of homing by tagging showed no clear evidence that European eels (Anguilla anguilla) utilize olfactory cues to locate their home area after transplantation over long distances up to a few hundred kilometers off the coast (Tesch, 1967, 1970; Deelder and Tesch, 1970). Bullheads, Zctalurus nebulosus and 1. natalis, have an extremely welldeveloped taste sense. Bardach et al. (1967a) have established that taste alone can guide these fish to chemical clues. Bullheads seem to exhibit true gradient searching in the absence of a current b y spatial and temporal comparison of concentrations. Deprivation of the sense of smell did not impair their searching ability, but unilateral deprivation of taste receptors caused pronounced circling toward the intact side. Detailed electrophysiological analysis of neural activity of the barbels in response to chemical stimuli will greatly help to understand the mechanism under-

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5 TEMPERATURE RECEPTORS R. W. MURRAY I. Introduction . . . . . . . 11. Thermal Sensitivity of Fishes . . . 111. The Sense Organs Involved . . . IV. Electrophysiology . . . . . . A. Teleosts . . . . . . . B. Elasmobranchs . . . . . . V. Thermal Responses of Other Sense Organs References . . . . . . . .

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I. INTRODUCTION

Because of the all-pervading nature of heat and its influence on chemical reactions and physical processes there are many kinds of effect of “temperature” on animals, including variations in metabolic processes, temperature optima, acclimation processes, and so on, which do not come within the scope of this chapter. Even among sense organs there are a number whose responses may be different at different temperatures, or which may respond to changes of temperature, but which are not thermoreceptors in the strict sense of of receptors responding to thermal stimuli and initiating or controlling behavior relevant to the thermal properties of the environment. Clearly behavioral rather than electrophysiological experiments are needed to identify thermoreceptors thus defined, but the latter techniques provide the different evidence required for the understanding of the receptor mechanisms.

11. THERMAL SENSITIVITY OF FISHES

Fishes, or at least bony fishes, often behave as if they were aware of environmental temperatures, and this sensitivity has been well established 121

122

R. W. MURRAY

experimentally (Table I). Thresholds have been found as low as 0.03"C (Bull, 1936) in experiments in which the fish were trained to swim up a long sloping trough against a water stream when the temperature of that water was suddenly increased. An increase in temperature can be distinguished from a decrease (Dijkgraaf, 1940), and although it is normally changes of temperature rather than absolute levels that are important, the rate of change found in other experiments (Bardach and Bjorklund, 1957) was so slow that, to the sensory physiologist, tonic rather than phasic receptor mechanisms would seem to be involved. In these experiments the conditioned stimulus was the slow warming of a well-stirred tank, and food was added when the temperature had risen 2°C. When fully trained, the majority of fishes showed characteristic and identifiable food-seeking behavior within %l min of the onset of the Table I Sensitivity of Fishes to Temperatiire Change in Conditioning Experiments Threshold Species

("C)

Reference

Ulennius pholis, Centronotus gunnellus, Cottus bubalis, Gadus mei,langus, Liparis montagui, Onos muslela, Zoarces viviparus

+0.03

Bull (1936)

Colt us scorpius, Crenilabws melops, Cyclopterus lumpus, Gadus callarias. Gobius jiavescens, Platichthys jlesiis, Pleiironectes platessa, Raniceps ranintis, Spinachia v ulgaris

+0.05

Bull (1936)

Gadus virens .Yer.ophis 1umbricifot mis

+O.OG +0.07

Bull (1936) Bull (1036)

Phozinus laevis, Awiiurus nebulosus

E

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i

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2 rnsec

Fig. 26. Insensitivity of a large receptor to maintained stimuli. From the mormyrid Gnathonemus. Same receptor, recording, and display as in Fig. 25; several superimposed sweeps in each record. ( A ) Threshold stimulation using a short pulse. ( B ) Superposition of the same threshold stimulus on a maintained anodal stimulus about 10 times stronger had no effect on the externally recorded response or nerve impulse. ( C ) A similar result was obtained when the test stimulus was superimposed on a large cathodal stimulus. Slow potential changes during the prolonged pulses were probably a result of capacity of the skin and neighboring receptors. From Bennett ( 1967).

of the outer faces and a specific capacitance of about 1 pF/cm2 which is typical of plasma membranes in general (Bennett, 1967). There is evidence-that transmission between receptor cells and nerve fiber is mediated electrically. The delay between excitation of the receptor and initiation of an impulse in the innervating fiber is very short, about 0.2-0.3 msec (Bennett, 1967; Steinbach and Bennett, 1971). This delay is shorter than the synaptic delay at chemically transmitting synapses at the same temperature, but it is quite reasonable for electrically mediated transmission (Bennett et al., 1967a). If the innervating fiber is antidromically stimulated, a small positive-negative potential is recorded outside the receptor. Occasionally this potential excites the full-sized external impulse, which arises near the positive peak of the externally recorded antidromic response. In most known cases, electrically mediated transmission occurs at low

532

M. V. L. BENNETT

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0.2 msec

Fig. 27. Absence of net current flow during externally recorded responses at a large receptor. From the mormyrid Gnathonemus. ( A ) A spontaneous impulse (several superimposed sweeps to show base line). The areas under 6rst and second phases are equal to within 10% as measured by planimeter. ( B ) When the receptor is oscillating, areas under the positive and negative phases are also equal to within 10%. The horizontal line shows the zero level of potential obtained by placing the electrode on the nearby skin. Voltage calibration in (A), 2 mV. From Bennett ( 1967).

resistance junctions between cells where current flows as diagramed in Fig. 24E (Bennett, 1968a). This arrangement means that if the nerve impulse generates a positive-going potential outside the receptor, there is simultaneously depolarization of the nonjunctional part of inner face of the receptor cell. If, on the other hand, transmission is chemically mediated, there is likely to be a gap between receptor cell and nerve fiber. A current producing an external positivity tends to hyperpolarize the inner faces of the receptor cells ( Fig. 24D). Excitation of the receptor cell by antidromic activity of the nerve, if it occurs, should be delayed until after the external positivity. Thus, the phase at which the full-sized antidromic spikes occur supports the hypothesis of electrically mediated transmission at the receptor synapse. At gymnotid phasic receptors, where there is convincing evidence of chemically mediated transmission, the receptor cell response to antidromic excitation of the nerve occurs as would be expected from the relationships in Fig. 24D. A further indication of electrically mediated transmission is that the threshold for nerve impulses from large receptors is virtually unaffected by high magnesium solutions ( Steinbach and Bennett, 1971). In contrast, chemically mediated transmission at medium receptors is profoundly depressed ( Section IVYC ) .

11. ELECXRORECEPTION

533

Ultrastructural studies of the large receptors are in progress and should provide further evidence as to mode of synaptic transmission. Available data indicate that small branches of the nerve form some receptor synapses typical of those where transmission is chemically mediated (Derbin and Szabo, 1968). Morphological evidence of electrically mediated transmission has not been found but could have been missed. It should be recalled that morphological features associated with chemically mediated transmission often occur at electrically transmitting synapses, even when there is little or no chemically mediated component in the PSPs ( Bennett et al., 1967a,b). Transmission at least at one other receptor synapse appears to be electrically mediated. At calyx synapses of the vestibular system the cuplike shape of the nerve terminal would greatly impede postsynaptic currents through chemically sensitive membrane on the inner surface of the calyx, and on this basis alone electrically mediated transmission appears likely ( Bennett, 1964). Recently, close appositions between receptor cell and afferent nerve have been found that resemble electrotonic synapses at other sites (Spoendlin, 1966; see Figs. 71 and 72 in Chapter 10, this volume).

B. Phasic Receptors of Gymnotids The phasic receptors of gymnotids generally contain 10 or more receptor cells in a single receptor cavity (Fig. 20). The cells protrude into the cavity, and the lumenal surface is further increased by microvilli (Fig. 22). All the cells are innervated by a single nerve fiber, and in some species (Eigenmunnia and Sterrwpygus) a group of receptors lying close together are also innervated by the same fiber. The response of a phasic receptor in the gymnotid Gymnotus is shown in Fig. 28. The first trace shows stimulating current, the second trace shows the response recorded outside the receptor, and the third trace shows the impulses in the innervating nerve. All but the largest stimulus (Fig. 28H) evoke damped oscillations outside the receptor. These responses occur at onset and termination of both anodal and cathodal stimuli. As the stimulus strength is increased, the responses to onset of anodal and termination of cathodal stimuli gradedly increase in amplitude and finally become spikelike (except in Fig. BH, see below). These are responses where the stimuli depolarize the inner face of the receptor cells. The responses to termination of anodal and onset of cathodal stimuli increase in amplitude as stimulus strength is increased, but they are progressively delayed. These are responses where the stimuli hyperpolarize the inner face of the receptor cells and presumably are analogous to

M. V. L. BENNETT

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Fig. 28. Responses of a phasic receptor of the gymnotid Gymnotus. Upper trace: Current applied at receptor opening. Middle trace: potential at the receptor opening. Lower trace: recording from the afFerent fiber. Slower sweep speed in (I-L), Description in text. From Bennett ( 1967).

anode break responses. In Figs. 28D and H there is apparently no response at these times. Other experiments demonstrate that stimuli causing a large hyperpolarization of the inner face of the receptor cells put them into an unresponsive state for a period of several hundred milliseconds (Bennett, 1967). The nature of the unresponsiveness is unknown, but its termination is signaled by a burst of impulses in the nerve and often an impulse-like response external to the receptor. The initiation of the unresponsive state explains the absence of responses to termination of the stimuli in Figs. 28D and H. Aside from these, there is good correspondence between the externally recorded responses to onset of anodal and termination of cathodal stimuli and between the responses to termination of cathodal and onset of anodal stimuli. Provided the stimuli are sufficiently strong and long lasting, there are multiple discharges in the nerve at both onset and termination of anodal and cathodal stimuli. For long-lasting stimuli of equal amplitude, the numbers of impulses correspond for onset of anodal and termination of cathodal stimuli and vice versa (Figs. 281-L). The correspondence does not hold for the shorter stimuli in Figs. 28A-C and E-G, presumably be-

535

11. ELECTRORECEPTION

cause the system had been left somewhat depressed by the responses to the onset of the stimuli. The externally recorded response of phasic receptors of Gymnotus shows no excitability change during maintained stimuli as illustrated in Fig. 29, an experiment identical in procedure to that of Fig. 26. This property also holds for the nerve discharge if the testing stimulus is given long enough after the onset of the maintained stimulus. Otherwise the nerve discharge evoked by the test stimulus is reduced as in Fig. 29B. In the gymnotids it is difficult to establish the absence of net current flow during a response because there are ordinarily no spontaneous impulses external to the receptors. The damped oscillations such as those following the stimuli in Figs. 28B and C do appear to be symmetrical around the line representing passive decay of the applied voltage, and in

(C)

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5 0 msec

Fig. 29. Insensitivity of a phasic receptor to maintained stimuli. From the gymnotid Gymnotus, recording and display as in Fig. 28. ( A ) Response to a brief test stimulus well above threshold, ( B ) When the test stimulus is superimposed on a strong anodal stimulus about 60 msec after its onset, the externally recorded response is unchanged, but there is a slight reduction in the number of afferent impulses ascribable to depression following the response to onset of the stimulus. ( C ) When the test stimulus is superimposed on a strong cathodal stimulus, both externally recorded and nerve response are unchanged, From Bennett ( 1967).

536

M. V. L. BENNETT

other experiments similar results are obtained with oscillations following much smaller and briefer stimuli (Fig. 32). All-or-none impulses can be evoked at the end of the unresponsive state induced by strong cathodal stimuli, and these responses show little net current flow. It can be concluded that these phasic receptors also exhibit the properties indicating the presence of a series capacity. As in the mormyrids the morphological relationships suggest that the capacity is in the outer faces of the receptor cells. The polarity of the externally recorded responses is consistent with a conventional polarity of responsiveness in the inner faces. Although the tendency to oscillate is unusually great, similar graded subthreshold oscillations are predicted by the Hodgkin-Huxley equations (Mauro et al., 1970). In gymnotid phasic receptors the response in the nerve is poorly correlated with the response recorded outside the receptor. There may be oscillations outside the receptor with no nerve discharge, and the nerve discharge can outlast the externally recorded oscillations. Also, when the external resistance is increased by removing overlying water, the small and brief nerve impulse can be recorded external to the receptor in the same way as at tonic receptors (Fig. 15; Bennett, 1967). These data by themselves suggest that the externally recorded oscillations are generated by the receptor cells and that transmission to the nerve is chemically mediated. Confirmation of chemically mediated transmission was obtained by experiments similar to those of Fig. 15 where it was found that strong cathodal stimuli do not block responses initiated by brief anodal stimuli and that there is a synaptic delay. Furthermore, the external response at phasic receptors can be obtained at least one month following denervation which is after externally recorded nerve impulses have disappeared at both phasic and tonic receptors (Bennett, 1967). Finally, tetrodotoxin blocks the nerve responses, but not the externally recorded oscillations ( Zipser, 1971).

C. Medium Receptors of Morrnyrids The medium receptors are morphologically more complex than other electroreceptors. There is an outer and an inner receptor cavity connected by a small channel (Fig. 21). The outer cavity is, as in other phasic receptors, covered over by loose epithelial tissue and the outer layer of epidermis but not by the layers of flattened epidermal cells which are closely apposed to the wall of the outer receptor cavity. Receptor cells occur in both cavities. In the outer cavity the cells are embedded in its wall; in the inner cavity the cells protrude into the cavity and are covered by microvilli on their lumenal surfaces. Three nerve fibers innervate each

11. ELECTRORECEPTION

537

receptor, but whether particular fibers end on one or both kinds of receptor cells has not been determined. The thresholds of medium receptors can be quite high, 10 mV or more, but in the range of stimulation provided by the electric organ (Section V, B). The response of a medium receptor is illustrated in Fig. 30. The middle traces show the external potential recorded at low gain. The receptor response is generally small compared to the stimulus required to evoke it, and in order to observe the response more clearly it may be convenient to use a bridge circuit that subtracts from the response a voltage step proportional to stimulus strength. The upper traces of Fig. 30 were recorded in this way. The lower traces show the response in the innervating fiber. Stimulating current and responses recorded externally by means of a bridge circuit are shown for another receptor in Fig. 31.

Fig. 30. Responses of a medium receptor from the mormyrid Gnathonernus. Upper trace: high gain recording at the receptor using a bridge circuit; rectangular current pulses were applied (input resistance of the receptor and skin, 1.4 MO). Middle trace: low gain recording from another electrode at the receptor opening. Lower trace: recording from the afferent nerve fiber. ( A-D ) Responses to increasing strengths of anodal stimuli. (E-H) Responses to increasing cathodal stimuli. In ( H ) four nerve impulses were evoked in the nerve, but the last occurred after the end of the sweep. The response at the termination of anodal and onset of cathodal stimuli is indicated by an arrow in ( Dj and ( H j. From Bennett ( 1967).

538

M. V. L. BENNETT

Fig. 31. Responses recorded external to a medium receptor of the mormyrid Gnathonemu. Upper trace: current applied at the receptor opening. Lower trace:

responses externally recorded by a bridge circuit. ( A ) Anodal stimulation. ( B ) Cathodal stimulation. Several superimposed sweeps at different stimulus strengths in each record. From Bennett ( 1965).

The externally recorded responses are graded in amplitude and increase with increasing strength of stimulation. At the onset of an anodal stimulus and termination of a long-lasting cathodal stimulus, the response is diphasic initially positive. There are corresponding impulses in the afferent fiber that decrease in latency and increase in number as stimulus strength is increased. Up to four impulses are present in the responses of Fig. 30. Somewhat larger numbers have been observed from other receptors. There is a small and brief external response at the onset of cathodal stimuli and termination of anodal stimuli. In some but not all afferent fibers there are impulses corresponding to this phase of stimulation (Bennett, 1965). These responses are of higher threshold than the other responses, and they also decrease in latency and increase in number as stimulus strength is increased. As would be expected, the voltage threshold for afferent impulses increases as stimulus duration is decreased. Surprisingly the threshold for sufficiently brief stimuli is lower for cathodal than for anodal stimuli (Bennett, 1965). The responses of medium receptors show equivalence of onset and termination of long-lasting stimuli of opposite polarity (Figs. 30 and 31). Also, their excitability is unchanged during maintained stimuli, These properties indicate the presence of a blocking capacity as at the other phasic receptors. It is difficult to determine whether externally recorded responses involve net current flow because of their small size and high threshold.

11. ELECTROFECEPTION

539

It is tempting to ascribe the two kinds of external response to the two kinds of receptor cell. The responses at onset of anodal and termination of cathodal stimuli probably represent either delayed rectification or graded regenerative responses of ordinary polarity in the inner faces of one kind of receptor cell. The responses at onset of cathodal and termination of anodal stimuli are unusual in that their latency changes little as stimulus strength is increased. They do not appear to be anode break responses and may represent increases in resistance during hyperpolarization similar to those observed in a number of other cells (Bennett and Grundfest, 1966). The brevity of these responses as compared to the others suggests that the blocking capacity in series with the generating membrane is different for the two response types. This observation is consistent with the responses being generated in the two kinds of receptor cell and with location of the blocking capacity in the outer faces of the cells. The briefer response would be most likely to arise in the outer receptor cells, the external surface of which is smaller and presumably of lower capacity. Nerve fibers that innervate both classes of receptor would then respond to either direction of change in stimulating voltage; those innervating only the inner receptors would respond only to onset of anodal and termination of cathodal stimuli. The relatively small outer face of the outer receptor cells would presumably not be important because of the low sensitivity of these cells. The lack of correspondence between external response and afferent impulses suggests that transmission from receptor cell to innervating nerve is chemically mediated as at the phasic receptors of gymnotids. This inference is supported by the fine structure of the receptor synapses (Barets and Szabo, 1964) and by the presence of a synaptic delay of about 1msec (Steinbach and Bennett, 1971). A limited amount of pharmacological data has been obtained from medium receptors ( Steinbach and Bennett, 1971). High magnesium solutions reduce transmitter secretion at neuromuscular junctions and interneuronal synapses, presumably by competing with Ca2+at some stage of the release process (cf. Katz, 1969). These solutions greatly increase the stimulus strength required to evoke afferent impulses from medium receptors. Nerve responses are not eliminated completely; at least a single impulse always remains. A tentative explanation is that Mg2+can, to a small extent, substitute for Ca2+in the release process. Transmission at medium receptors is not cholinergic; curare, atropine, and acetycholine have no effect. The levorotatory form of glutamate has been implicated as a transmitter at a number of synapses (Beranek and Miller, 1968; Usherwood and Machili, 1968). The medium receptors are excited by L-glutamate and by the related L-aspartate, but they are not affected by D-glutamate. In procedures to date the concentrations

540

M. V. L. BENNETT

required are large, but the data are nevertheless suggestive of glutaminergic transmission.

D. Phasic Receptors in Gymnarchus Two types of tuberous receptor are found in Gymnarchus. These may correspond to large and medium receptors of mormyrids, but they have not yet been adequately studied physiologically. In each receptor type there is the peculiarity that the receptor cavity invaginates into the receptor cell. However, numerous microvilli increase the lumenal surface far beyond that of the other surface (Mullinger, 1969), and action of the lumenal face as a blocking capacity is not unreasonable. One type of tuberous receptor (Szabo’s type B, Szabo, 1965) contains a single receptor cell but occurs in clusters innervated by a single fiber. The other kind (Szabo’s type C) contains one large and one small receptor cell innervated by the same fiber, which may also innervate several neighboring receptors of the same kind. It is tempting to homologize the latter receptors with the medium receptors of mormyrids (Szabo’s type B) because of the presence of two kinds of receptor cell. However, this type of receptor is supposed to generate the spontaneous potentials on the skin that resemble the potentials external to large receptors of mormyrids ( Szabo, 1962). The correspondence between receptor types in mormyrids and Gymnarchus requires further study.

E. Amplification and Oscillation at Phasic Receptors The external responses at phasic receptors of gymnotids show an interesting correlation with the organ discharge of the particular species. The period of evoked oscillations at the receptors is similar to the duration of each organ discharge (Bennett, 1967). The correlation holds whether organ pulses are emitted at high or low frequency. The responsiveness of the receptors appears to represent tuned amplification of the main frequency components present in the discharge. Most phasic receptors in the eel appear not to have regenerative electrical responses (Bennett, 1967). Perhaps the large voltages produced by even the weakly electric organ do not require the amplification present in phasic receptor cells of other gymnotids. Given a regenerative responsiveness like that of phasic receptor cells, it would not be surprising if maintained oscillations could be obtained under suitable conditions. In fact, in a number of gymnotids, phasic re-

541

11. ELECTRORECEPTION

ceptors begin to oscillate continually if the skin is allowed to dry (Figs. 32E and F). These oscillations cease immediately when the skin is wet; they result from reduced electrical loading of the receptor and are not a result of injury through dessication, The effect of loading is illustrated in Fig. 32 where a low resistance, saline-filled electrode over the receptor is connected to the indifferent electrode by a variable resistance. If the loading is small ( a high resistance connection to the indifferent electrode) a brief stimulus evokes an oscillatory potential that is only slowly damped (Figs. 32A and C). If the loading is substantially increased, the oscillation is much more rapidly damped; it is also slightly reduced in amplitude (Figs. 32B and D ) . Spontaneous activity of large receptors of mormyrids ordinarily is not so great as to obscure threshold measurements. If, however, the skin is allowed to dry the receptors will often oscillate at high frequencies (Figs. 33A and F). Depending on the species and receptor, the frequency can be in the range of 1OO0, 2000, or even 3000 impulses/sec (Harder, 1968).

-15

20msec

,

> rllllllUllrllllllg ~

50msec

Fig. 32. “Ringing” of phasic receptors of the gymnotid Gymnotus. ( A-D) A large electrode (upper trace ) filled with aquarium water in agar and a smaller Ringer-agar electrode (lower trace) were placed over the receptor, the water was drained away, and the skin was allowed to dry. Stimulating currents were applied through the large electrode using a bridge circuit. (A, C ) The large electrode was connected to the stimulator by means of a 10 M a resistance. A cathodal stimulus of approximately 4 mV, measured by the small electrode, evoked a slowly damped oscillation. Well after the stimulus, there appeared to be little net current flow during the oscillation, which was nearly symmetrical around the base line. (B, D ) The large electrode was connected to the indifferent electrode by a resistance of 100 kn. The same voltage stimulus as measured by the small electrode evoked a smaller and more rapidly damped oscillation. The change from ( A ) to ( B ) was immediately reversed by changing the shunt resistance to its former value. Same sweep speed in ( A ) and ( B ) and in ( C ) and ( D ) . Lower trace gain is the same in ( A-D). Upper trace gain is higher in ( B ) and ( D ) than in ( A ) and ( C ) . ( E-F) A different experiment, in which the receptor was oscillating continually. Periods in which the nerve impulses tended to follow the oscillation one-to-one alternated with periods in which impulses failed altogether. From Bennett ( 1967).

542

M. V. L. BENNETT

(B)

J+

0

102

~

2msec

Fig. 33. Effect of shunting on spontaneous activity of a large receptor. From the morniyrid Gnathonemus. Recording by means of a small, Ringer-filled electrode. A large (about 1 mm diam) electrode filled with aquarium water, with a resistance of about 50 kf? cm is also placed over the receptor. This electrode is connected to the indifferent electrode by various resistances. ( A-F) From left to right: sample records from the small electrode, value of shunting resistance, and rate of spontaneous impulses. The sequence of changes in shunting resistance is from ( A-F). From Bennett ( 1967 )

.

Again, the oscillations result from reduced loading, for if the load is varied as in the experiment of Fig. 32, the rate of spontaneous activity is changed accordingly (Figs. 33B-E). As would be expected, the effect of loading is immediate (within 1 msec), which may be conveniently shown by using a high-speed relay to connect and disconnect the loading resistance ( Bennett, 1967). During maintained oscillations the nerve discharges cease to follow the receptor cell frequency (Figs. 32E and F ) and may cease altogether. The receptor thus becomes insensitive to stimuli other than the very strong ones required to cause a significant pause in the maintained oscil-

11.

ELECTRORECEPTION

543

lations. Although oscillations have been recorded from some mormyrid large receptors immersed in water (Harder, 1968), it is to be expected that these responses are infrequent under normal conditions and represent malfunctioning of the system. An important factor in the oscillations of submerged receptors is the conductivity of the water. The rate of spontaneous discharge of receptors submerged in low conductivity water is substantially reduced when a small quantity of salts is added (M. V. L. Bennett, unpublished data). It would be of interest in this respect to know the conductivity of the natural waters in which mormyrids are found. The input-output relationship of synaptic membrane of phasic receptor cells differs from that of tonic receptor cells in that there is little resting release of transmitter. It is difficult to be confident of the shape of the potential-secretion relationship because of active processes in the receptor cells. It is likely but uncertain that the secretory membrane of phasic receptors does have an input-output relationship which has a much greater slope than that at known interneuronal and neuromuscular synapses. At some phasic receptors the number of impulses in response to a stimulus rises sharply after a certain threshold is reached and then saturates ( Hagiwara and Morita, 1963; Szabo and Hagiwara, 1967). Since these receptors are activated by the electric organ and presumably operate in electrolocation, it makes biological sense for them to have a high threshold that lies in the range of stimuli provided by the organ and to be very sensitive to changes above this threshold, that is, to have a low incremental threshold once absolute threshold has been reached. These receptors would then be best able to signal small influences of objects in distorting the field of the electric organ. It is difficult to be confident of the amplitudes of the membrane responses of the receptor cells. The maximum values of the externally recorded responses are 10-20 mV when the skin is dry, but significant loading may still be present. Nonetheless, on the reasonable assumption that permeability changes underlie the responses, a small voltage must produce a fairly large change in permeability, and an impulse of 10-20 mV amplitude might be sufficient to move a sensitive receptor cell through the entire cycle of permeability changes associated with the response. Since loading blocks receptor oscillations, it probably reduces the amplitude of the transmembrane potentials, and at large receptors of mormyrids there is marked shortening of the externally recorded response (Fig. 3 3 ) . The large receptors of mormyrids and ampullae of Lorenzini are apparently comparable in terms of the energy required to stimulate, about

544

M. V. L. BENNETT

W sec. The threshold currents are similar for the two kinds of receptor, about 1@l0 A. The threshold voltage is about 100-fold greater for the mormyrid receptors, but the duration of stimulus required is at least 100-fold shorter (Bennett, 1965; Murray, 1967). The mormyrid receptors do not appear to be limited by electrical noise of the Johnson type under normal conditions ( Bennett, 1965), and the ampullae of Lorenzini should be even farther from this limitation. The mormyrid large receptors in particular are remarkable in that they are poised very close to threshold, generally without excessive spontaneous activity. Other nerve cells presumably can be held as close to threshold for short times, but subthreshold responses and accommodation would be expected to follow within a few milliseconds. V. RECEPTOR FUNCTION IN ELECTRORECEPTION

A. Accessory Structures and Receptor Responses to Potential Gradients in the Environment As the middle ear and lens are important to their sense organs, so are accessory structures important to electroreceptors. It has already been suggested that current is channeled through the receptor cells by the flattened cells in the skin and receptor canal and by the circumferential tight junctions between receptor and supporting cells. The morphological relationships as well as our general knowledge of synaptic transmission indicate that the receptors should be sensitive to the potential across the skin and insensitive to the gradients along it. Experimental verification of this point is shown in Fig. 34. In this experiment one recording electrode is placed external to a tonic receptor and a microelectrode is pushed through the skin to record the potential beneath the receptor. When stimuli are applied externally fairly close to the opening, the potential inside the fish is small and the positioning of the internal electrode is not very critical. When a stimulus is applied directly over the receptor, the sensitivity is greatest; the minimum current is required to generate a given potential across the skin and to evoke a given response in the nerve ( Fig. 34D ) . When the stimulating electrode is moved 2 mm away either rostrally (Fig. 34A), dorsally (Fig. 34B), ventrally (Fig. 34C), or caudally, a considerably larger current is required to produce the same potential across the skin, but the corresponding neural response is very nearly the same. (There is some slowing of the time course of the potential in Figs. 34A-C compared to Fig. 34D because of skin capacity and separation of the electrodes.)

11,

545

ELECTRORECEPTION

-+ I r

IP

Fig. 34. Sensitivity to potentials across the skin and independence of tangential gradients: a tonic receptor of the gymnotid Gymnotus. Upper trace: recording from an afferent fiber. Middle traces starting from the same base line: potential at the receptor opening and potential recorded by a microelectrode pushed through the outer layers of nearby skin; the former potential is the larger in each record. The potential across the skin is given by the difference between the two. Lower trace: stimulating current through a separate electrode. The stimulating electrode is 2 mm anterior in ( A ) , 2 mm dorsal in ( B ) , and 2 mm ventral in ( C ) . The same current pulse produces about the same potential change across the receptor and the same nerve response. ( D ) Stimulation over the receptor. A much smaller current than in ( A-C) is required to produce the same potential across the skin, which evokes nearly the same nerve response. The potential across the skin is less slowed by the capacity of the skin than in (A-C) because the recording and stimulating electrodes are closer together. Lower current gain in (A-C). From Bennett (1967).

An important aspect of this experiment is that it contradicts the hypothesis of Lissmann and Machin (1958) that the receptors are sensitive to the second spatial derivative of the potential over the surface of the skin. The ingenious feature of their hypothesis is that it predicts that the sensitivity to applied stimuli should be the same as that to distortions of the electric organ discharge, which is in fact approximately true. Since different receptors are apparently involved in the two modes of operation, the agreement may be fortuitous. In mormyrids the skin resistivity is about 50 kn cm2 which is some hundred times greater than that of goldfish or hatchetfish (the latter is a South American freshwater fish). The skin resistivity is much smaller in gymnotids, 1-3 kn cm', but it is still greater than in the other freshwater fish (Bennett, 1965, 1967). It is not clear to what extent these values are affected by current passing through tonic receptors in the area of measurement.

546

M. V. L. BENNETT

The resistive barrier is localized to the skin surface as shown in Fig. 35, which also illustrates the importance of the skin in determining the potentials across the receptor cells. Electrodes are placed as in Fig. 34D, and anodal and cathodal stimuli are applied that accelerate and retard the nerve discharge (Figs. 35A and B ) . Then the current electrode is moved 2 mm caudally and pressed against a superficial scratch in the skin; the polarity of sensitivity is now reversed, although the electrode does not penetrate the pigment layer of the dermis (Figs. 35C and D ) . The same trans-epidermal potential causes the same responses as in Figs. 35A and B, but the current required to produce the potential is greatly increased as well as altered in sign. Moving the electrode away from the skin a short distance reestablishes the same polarity as in Figs. 35A and B, but the sensitivity remains low. Evidently the current passes through the receptor cells in opposite directions depending on whether the electrode is just internal or external to a resistive barrier localized in the skin. Similar experiments indicate that the electroreceptors of catfish are sensitive to the potential across the skin and insensitve to the gradient along it ( M . V. L. Bennett, unpublished data; Roth, 1969). When a fish is placed in a uniform potential gradient, the sensitivity of a particular receptor is greatly dependent on accessory structures. It turns out that the maximum potential difference across the fish is a more

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Fig. 35. Insulating characteristic of the skin. Same receptor and display as in Fig. 34. (A, B ) Anodal and cathodal stimulation at the receptor opening; the external potential is the larger. (C, D ) The stimulating electrode is pressed against a superficial scratch in the skin 2 mm caudal to the receptor. A cathodal stimulus evokes acceleration and an anodal stimulus evokes deceleration; the potential internal to the receptor is the larger. Potentials across the skin associated with a given neural response are the same as in ( A ) and ( B ) . From Bennett ( 1967).

11. ELECTRORECEPTION

547

meaningful measure of stimulus strength than is the potential gradient, that is, voltage per unit length along the fish. Because of accessory structures, an applied voltage gradient multiplied by the diameter of a receptor cell gives a very great underestimate of the actual potential across the receptors. The simplest example is provided by the skate, the skin of which is of relatively low resistance. When the fish is placed in a uniform field, the field is not greatly distorted within the fish, although the body is of somewhat higher resistivity than seawater (Murray, 1967). As noted above, canals of the ampullae of Lorenzini have walls of very high resistance, and the space constants are quite long compared to the lengths ( Waltman, 1966). As far as dc potentials are concerned, the potential in the ampulla itself must be very close to that at the external opening of the canal. The receptor thus detects the difference in potential between the canal opening and the serosal side of the receptor cell. The receptor is like a voltmeter with its terminals at these points. If one assumes a uniform field, the stimulus to the receptor is given by the field (in volts per unit length) times the distance between receptor cells and canal opening times the cosine of the angle between the field and the axis of the canal. The stimulus is greatest if the canal is oriented in the direction of the field; the stimulus is zero if the canal is oriented perpendicular to the field. The fish obtains directional information by having many receptors with canals oriented in different directions (Fig. 6 ) . Canals are of different lengths, and receptors with long canals are sensitive to smaller uniform gradients but provide less spatial resolution. Receptors with long canals are also less sensitive to higher frequencies because of the capacity of the canal walls (Waltman, 1966). There may be a sacrifice of frequency response for sensitivity in these receptors, which could represent a useful adaptation to overcome noise. However, response of the receptor cells drops off at high frequencies in any case (Murray, 1965) and the more important limiting factor is undetermined. When uniform gradients are applied to mormyrids and gymnotids the situation is more complex. The skin is of high resistance and the inside of the fish is of much lower resistance than the external medium. The voltage gradients within the fish are thus much smaller than those in the external medium, although they have not been determined in detail. The individual receptors detect the difference between the external and the internal potential, where the latter is some average of the potential over the whole fish. In a uniform gradient the stimulus to the receptors depends on the maximum voltage across the animal. If the skin resistance is uniformly high, about one-half the maximum voltage across the animal is developed across the skin at either end (or side) in the direc-

548

M. V. L. BENNETT

tion of the field. For longitudinally oriented stimuli the receptors at the rostral and caudal ends of the fish are most affected. There is a smaller potential across the skin near the center of the fish, and receptors in this region are less affected. Since current enters the fish at one end and leaves at the other, the polarity of stimulation is opposite at the two ends. Near the center of the fish there is a point or rather a line around the fish where current is parallel to the body surface and there is no potential across the skin. Most fish are considerably longer than they are wide, and the maximum voltage across the animal is greater for longitudinally oriented fields than for transversely oriented fields. Thus, it would be expected that the fish's sensitivity is considerably greater for a field oriented in the longitudinal direction. The effect of receptor location on sensitivity to longitudinally oriented stimuli is illustrated for large receptors of a mormyrid in Fig. 36. There is no consistent variation along the body of thresholds to locally applied stimuli. However, longitudinally oriented stimuli excite receptors at the ends of the fish at much lower thresholds than they excite the receptors in the center of the body. Moreover, there is a sharply localized transition region anterior to which the receptors respond to onset of head-positive stimuli and caudal to which the receptors respond to onset of headnegative stimuli. These data indicate that when head-positive stimuli are applied, current enters the fish anterior to the transition region and exits posterior to it. The lower sensitivity near the center of the body indicates that a much smaller fraction of the applied voltage is developed across the skin in this region. The greater sensitivity of the receptors in the head region as compared to the most caudal ones may mean that the skin is of higher resistance rostrally; alternatively, the most caudal receptors are located well rostral to the tail (Fig. l ) , which also could account for their lower sensitivity. In freshwater electric fish the arrangement of the receptors and accessory structures probably represents a compromise between the requirements of detecting weak fields of extrinsic origin, of establishing the field distribution, and of setting up and measuring the field of the fish's own organ. The distribution of skin capacity may be important in receptor responses, particularly since tonic and phasic receptors are sensitive to different frequency ranges. Accurate measurements must be obtained to distinguish the resistance and capacity of the receptors from those of surrounding skin, and more complete studies of regional differences are required. The flattened epidermal cells of mormyrids ( F in Fig. 10) are present only in the receptor regions; the skin resistance is not much lower on the sides of the fish (Bennett, 1965),but presumably the capacity is greater. In Torpedo the skin resistance over the electric organs is lower than that over the remainder of the body, which tends to maximize cur-

11. ELECTRORECEPTION

549 Distance (cm)

I

O

Head-tail stimulation

Dorsal Ventral

..... . 0

0

0

00

Fig. 36. Responses of receptors to longitudinally oriented potential gradients. The diagram of the fish is at the same scale as the abscissa and shows the approximat6 positions of the receptors on the longitudinal axis of the fish; the stippling indicates the regions in which the receptors are located. In the graph points from the receptors in dorsal and ventral regions are indicated by filled and open circles, respectively. The points below the horizontal dashed line are voltage thresholds for stimulation by an electrode at the receptor opening. The points above the dashed line are thresholds for head-tail stimulation measured as the potential difference between head and tail. These thresholds have been normalized by multiplying by the ratio of 0.4 mV t o the threshold for local stimulation of that receptor. The vertical dashed line separates receptors that respond at make of head-positive stimuli from those that respond at make of head-negative stimuli. From Bennett (1965).

rent in the external medium (Bennett et al., 1961). Analogous differences may be found in weakly electric fish. The effects of body openings should be evaluated but may not be very important. The accessory organs of Steatogenys make the inside of the mouth positive with respect to the outside even when the mouth is open. The skin of freshwater catfish is probably of high resistance similar to the situation in freshwater weakly electric fish; the canals of the small pit organs are quite short; and a high skin resistance would be required to achieve a high receptor sensitivity. Although the skin lacks the layers of flattened cells found in the weakly electric fish (Wachtel and Szamier, 1969), the resistance could still be high because a single epithelial layer can have a very high resistance; however, the capacitance would be expected to be large (Waltman, 1966). Probably a high skin capacity would not be important to catfish because of lack of receptor sensitivity to high frequencies. In contrast to freshwater catfish the ampullary receptors of the marine

550

M. V. L. BENNETT

catfish Plotosus have long canals like the receptors of marine elasmobranchs ( Gkrard, 1947). Conversely ampullae of Lorenzini of the freshwater stingray have very short canals (R. B. Szamier and M. V. L. Bennett, unpublished data). It seems likely that the difference in canal lengths is an adaptation to the relative resistivities of fresh- and saltwater. The tissue of a marine fish is of higher resistivity than the water surrounding it, and even if its surface resistance is low the tissue will not “load’ the current pathway through the seawater surrounding the fish. In contrast the tissue of a freshwater fish is of considerably lower resistivity than is the water in its environment. If there were not a high surface resistance, the resistance of the environment would be large compared to the resistance of the fish and the voltage across the animal would be reduced. Because of its high surface resistance the freshwater electric fish presents less of a load to the environment. Actually, there seems to be some impedance matching in freshwater electric fish, at least as far as dc voltages are concerned. These fish do load their environment to some extent, as they should for maximum power dissipation across the receptors.

B. Receptor Responses to Electric Organ Discharges Many of the studies of afferent impulses evoked by organ discharge have not involved identification of receptor type, although it is often possible to infer from the response what kind of receptor was being stimulated. Because tonic receptors have low sensitivity to higher frequency (sinusoidal) stimuli, they do not respond to the electric organ discharge in species in which the discharge has little low frequency dc component (Bennett, 1970; Bullock and Chichibu, 1965; Roth, 1967; Suga, 1967a; Szabo, 1970). It appears then that in most electric fish tonic receptors act largely passively and detect low frequency signals of extrinsic origin. What are probably tonic receptors in the eel do respond to discharges of the weak electric organ (Hagiwara et al., 1965a); these pulses are monophasic and therefore have a significant low frequency component. The tonic receptors could probably also be activated by the electric organ discharge in the gymnotid species of Hypopoinus that has a monophasic discharge and in the mormyrid Mormyrw. The rostra1 accessory organs of several gymnotids also emit monophasic pulses. There may be some stimulation of tonic receptors by muscle action potentials and dc potentials such as those resulting from injuries of skin or muscle (Bennett, 1967; Bullock and Chichibu, 1965). In the dogfish, ampullae of Lorenzini apparently respond to potentials associated with the animal’s own res-

11.

ELECTRORECEPTION

551

piratory movements ( Dijkgraaf and Kalmijn, 1966). Conceivably, changes in these responses could signal the presence of objects. In many afferent fibers the presence of insulators and conductors alters the number of impulses evoked by the electric organ discharge (Bullock and Chichibu, 1965; Hagiwara and Morita, 1963; Hagiwara et al., 1965a,b; Szabo and Fessard, 1965). Except for the stimulation of tonic receptors noted above, all of these responses are presumably from phasic receptors, as has been confirmed by direct identification in Gymnotus (Suga, 1967a) and mormyrids (Roth, 1967; Szabo and Hagiwara, 1967). Although both medium and large receptors of mormyrids are activated by organ discharge, changes in afferent responses have been fully reported only for medium receptors. Because of the great sensitivity of large receptors, they may be maximally activated by the organ and transmit negligible information about distortions of its field. If so, their primary function may be detection of weak high frequency signals from other mormyrids, which is perhaps of significance in communication ( cf. Moller, 1970). The effects of objects on afferent discharges, presumably from a phasic receptor, are illustrated for the gymnotid Steatogenys in Fig. 37. On the left are shown oscilloscope records with histograms of the number of impulses after each organ discharge when a silver plate was in the indicated positions. On the right are graphs of mean number of impulses in two fibers for successive positions of paraffin blocks as well as silver plates. The monopolar threshold measurements indicate that for the upper graph, at least, the receptor lies at about the point where the minimum response is obtained with the silver plate and the maximum response is obtained with the paraffin block. A variety of coding mechanisms have been described whereby changes in afferent impulses evoked by organ discharge may signal distortions of the organ fields (Bullock and Chichibu, 1965; Hagiwara and Morita, 1963; Hagiwara et al., 196%; Szabo and Hagiawara, 1967). In fish emitting discharges at a low frequency, a fiber may carry several impulses following each discharge; changes in the field may alter the number of impulses per discharge as in Fig. 37. Latency may or may not be changed concurrently. Particularly in fish emitting discharges at high frequencies, the impulses may fail to follow the organ discharge frequency one to one, and changes in the field may alter the probability of a single impulse’s occurrence as well as its latency. Latency changes are very pronounced in discharges from some medium receptors of the mormyrid Gnathonemus. Other medium receptors show less change in latency but greater changes in number of spikes. Phasic receptors continue to give an altered response in the presence of a stationary stimulating object, although there may be some adaptation. The continued responding has sometimes been

552

M. V. L. BENNETT

I Threshold

10 2 3 4 5 6

F E O C B A

7 8 91011 121314 cm

F E D C B A

cm

Fig. 37. Afferent volleys evoked by electric organ discharges are altered by the presence of conductors and insulators. Left: Oscilloscope records of afferent impulses, organ discharges appear as a small deflection preceding each nerve volley. The diagram indicates the center positions of a 1.5 by 0.5 cm silver plate for corresponding oscilloscope records. Histograms by each record show the distribution of number of impulses in 100 successive volleys. Right: Graphs of mean number of impulses in volleys evoked by organ discharge as affected by a silver plate or paraffin block. Same fiber on the left as in lower graph. The open circles in the upper graph are thresholds for monopolar stimulation and indicate that this receptor lay at a point nearly corresponding to 7 cm along the abscissa. From Hagiwara and Morita ( 1963).

called tonic (Hagiwara et al., 1965b), although it is not owing to a maintained electric stimulus but rather to repeated stimulation of the receptor by an organ discharge of altered size. There results are important in that they establish the possibility that electroreceptors operate in electrolocation. Unfortunately, little has been done to determine how the presence of insulators and conductors changes the potentials across the skin resulting from the organ discharge. Further investigations along these lines are required to correlate the responses to objects with the responses of the receptors to local stimulation. C. Central Projections of Electroreceptor Activity

A noteworthy characteristic of fish with electrosensory systems is the large size of the cerebellum compared to that of other fish. In Fig. 38 are

11, ELECTRORECEPTION

553

diagrams of the brains of several fish with and without electrosensory systems. Neuroanatomy of the cerebellum of fish has recently been reviewed by Nieuwenhuys ( 1967) and Schnitzlein and Faucette ( 1969). The trout, Salmo fario, may be taken as a generalized teleost (Fig. 38A). The buffalo fish Curpiodes is a sucker, a bottom feeding relative of the carp. It has a very developed gustatory system and correspondingly the vagal lobes are very enlarged, but otherwise its brain resembles that of the trout (Figs. 38B and C). The squirrelfish has very large eyes, and its optic tecta are quite enlarged (Fig. 38D). The stargazer has an electric organ but is not known to have electroreceptors. Its brain is quite similar to that of the trout except for the smaller cerebellum and the very large oculomotor nerves that innervate the electric organ (Fig. 38E). The cerebellum is also implicated in the control of movement, and the small size in the stargazer correlates with its sedentary mode of life. It generally lies buried in the sand with only its small eyes protruding and waits until prey comes near enough to gulp down. The elasmobranchs are divided into two main lines, the sharks and the batoids including skates, electric rays, stingrays, and guitarfish. A wide but similar range of cerebellar development is seen within each line ( Ariens-Kappers et ul., 1936; Schnitzlein and Faucette, 1969). The brain of a ray that has a simple cerebellum is shown in Fig. 38F. This cerebellum is still considerably larger than that of the trout. In the guitarfish and stingray, the corpus is highly convoluted and greatly enlarged. No data are available for correlation of electrosensory function with cerebellar enlargement within the elasmobranchs. There is some tendency for more elaborate cerebella to be found in more actively swimming species, but whether this increased development represents increased sensory or control requirements is not yet clear. The cerebellum of the catfish Amiurus is greatly enlarged compared to that of the teleosts without an electrosensory system (Fig. 38G). The cerebellum of gymnotids is similarly enlarged ( Fig. 38H). The extreme case of enlargment of the cerebellum is in the mormyrids in which it constitutes most of the brain (Figs. 381 and J ) . The cerebellum in mormyrids is extraordinarily specialized cytologically ( Nieuwenhuys and Nicholson, 1969a,b; Kaiserman-Abramoff and Palay, 1969). Just as increase in optic tecta or vagal lobes implies a role in increased visual or gustatory sensitivity, hypertrophy of the cerebellum in fish with electrosensory systems implies a role in electroreception. There are corresponding increases in the afferent pathways from lateral line inputs to at least the valvulae of the cerebellum (Berkelbach van der Sprenkel, 1915; Nieuwenhuys and Nicholson, 1969a; Szabo, 1967). There is limited physiological evidence for involvement of the cerebel-

554

M. V. L. BENNETT

L

Fig. 38. Brains of fish with and without electrosensory systems. ( A ) Trout, Salrno fario (from Nieuwenhuys, 1967); ( B , C ) dorsal and lateral views, buffalo fish, Carpiodes (from Herrick, 1905); ( D ) squirrelfish Holocentrus (from Meader, 1934); ( E ) stargazer Astroscopus. ( F ) thornback ray, Platyrhinoides triseriata (from Nicholson et al., 1969); ( G ) catfish Leptops (from Herrick, 1905); ( H ) electric eel Ekxtrophorus (from Couceiro and Fessard, 1953); ( I ) mormyrid Gnathonernus (from Nieuwenhuys and Nicholson, 1969a); and ( J ) mormyrid Mormyrops (from Stendall, 1914). Abbreviations: C, cerebellum; L, lateral line nerve; 01, olfactory bulb; Op, optic tectum; V, valvula of the cerebellum; VL, vagal lobe; and 111, oculomotor nerve.

lum in electroreception. Volleys from electroreceptors have been observed to project to the lateral line lobes, midbrain, and valvulae of the cerebellum in mormyrids (Bennett and Steinbach, 1969; B. Zipser and M. V. L. Bennett, unpublished data). Morphological studies indicate the cited order is the actual pathway, but projections of cerebellar outputs have not yet been identified (Nieuwenhuys and Nicholson, 1969a). Physio-

11. ELECTRORECEPTION

555

logical data establish projections of electroreceptors to the eminentia granulosa in gymnotids (Enger and Szabo, 1965) and to the lateral line lobes in gymnotids and elasmobranchs (Enger and Szabo, 1965; Nicholson et al., 1969). Single units in the lateral line lobe of gymnotids can signal the presence of objects by an alteration in the number of spikes following an electric organ discharge (Enger and Szabo, 1965). No responses to electrosensory inputs have been found in the corpus cerebelli of elasmobranchs, and the suggestion has been made that this part of the cerebellum is involved in control of movement rather than electroreception ( Nicholson et al., 1969). An interesting aspect of the mormyrid electrosensory system is that the neural command signal that fires the electric organ also projects to many parts of the cerebellum and hind brain. This signal presumably prepares the afferent pathways for incoming signals from the electroreceptors. Both inhibitory and facilitatory effects on central responses to afferent volleys can be observed when electroreceptors are activated in specific time relation to the command signal to excite the organ (Bennett and Steinbach, 1969; B. Zipser and M. V. L. Bennett, unpublished data). These effects are independent of actual organ discharge and occur in the curarized animal. They indicate that the fish “knows” when it is going to discharge its electric organ. At least in principle, the fish can measure the latency of the receptor response. In a passive sensory system only differences in latency between receptors can be determined.

D. Behavioral Responses to Voltage Gradients and Conductance Changes

A number of different kinds of behavioral responses that apparently involve electroreception have been observed in electric fish. Many gymnotids move away from a metal rod that is brought close to them, whereas they tend to ignore visually similar insulators. Individuals of more aggressive species such as Gymnotus and Electrophorus may attack metal objects, A number of catfish and sharks also respond to metal rods, much more strongly than to insulators (M. V. L. Bennett, unpublished data; Dijkgraaf, 1963, 1968). In electric fish the differential responsiveness may involve active electroreception, and as already noted alterations in the nerve volleys evoked by organ discharge can be produced by metallic objects (Fig. 37). In addition inhomogeneities in metals cause eddy currents in surrounding water, and these fields can be adequate to stimulate tonic receptors as has been observed in gymnotids and dogfish (Fig. 39). Presumably the latter mode of action is

556

M. V. L. BENNETT

0 . 5 sec

Fig. 39. Response of a tonic receptor to a metallic rod. From the gymnotid Gymnotus. Recording from the afferent fiber in a curarized animal. ( A ) Resting discharge. ( B ) Responses to passing a submerged screwdriver back and forth over the receptor. The discharge is alternately retarded and accelerated. ( C ) The response to slowly placing the tip over the receptor is a gradual block of discharge. Upon removal there is a long-lasting burst of impulses. The changes in base line in ( B ) and ( C ) result from ac coupling of the amplifier. From Bennett ( 1967).

responsible for the detection of metals by animals with passive electrosensory systems. (Because of eddy currents the finding of responses to metals or of discrimination between metallic objects and insulators does not demonstrate the existence of an active electrosensory system.) Lissmann (1958) noted an interesting correlation between the occurrence of weakly electric organs and mode of swimming. All weakly electric fish tend to swim with the body held relatively rigid. In gymnotids propulsion for ordinary swimming is provided by the anal fin. The dorsal fin of Gymnarchus serves a similar function (see Fig. 1of Chapter 10, this volume). Most of the body musculature does not act in ordinary swimming and is probably used primarily for escape movements. The suggestion is that the body is held rigid to simplify the functioning of the electrolocation system, just as in many animals the eye can be rotated around all three axes to stabilize the retinal image. A further behavioral feature of freshwater weakly electric fish is their tendency to back into a strange environment and to investigate it by moving the tail around. Although the electroreceptors are concentrated in the head region, the external field strength is greatest just outside the tail, and sensitivity may be reasonably great using the backward approach. It would also seem faster to escape from an unpleasant situation by swimming forward rather than by backing out and turning around.

11. ELECTRORECEPTION

557

Many gymnotids caught in the field have partially regenerated tails. This common history of injury could be a result of their peculiar mode of investigation. It is also true that the tip of the tail is the most salient part of an electric fish to another fish that is detecting it electrically, and when electric fish fight in aquaria, the tails are often the primary target of attack. Several kinds of unconditional responses in addition to avoidance of metallic rods have been used in exploration of electric sensitivity. For example, heart rate in the skate or dogfish may accelerate when a weak electric stimulus is presented ( Dijkgraaf and Kalmijn, 1966). These fish will also burrow into sand over electrodes emitting signals similar to the respiratory potentials of flatfish on which they feed, a dramatic demonstration of a function of passive electrosensory systems. Mormyrids and variable frequency gymnotids accelerate their organ discharge rate when weak electric stimuli are given or when the conductance is altered between a pair of electrodes at head and tail (Bennett, 1965; Bennett and Grundfest, 1959; Enger and Szabo, 1965; Hagiwara et al., 1965a; Larimer and MacDonald, 1968; Moller, 1970; Szabo and Fessard, 1965). In the second, fifth, and seventh of these examples the case for active electroreception is strong because the response to a conductance change occurs only if an organ discharge occurs during the change; no response is obtained to changes restricted to periods between discharges. An interesting form of unconditional response can be evoked in higher frequency fish ( Watanabe and Takeda, 1963; Larimer and MacDonald, 1968; Bullock, 1970). If the animal is presented with a stimulus close in frequency to its own discharge, it will shift its discharge frequency away from the interfering frequency. The experimenter can slowly “chase” the fish‘s frequency up or down within a certain range; beyond this range the fish will fairly rapidly change its frequency to the other side of that of the interfering signal. The response appears to represent avoidance of “jamming” of the sensory system by the closely neighboring frequency. A related kind of response is evoked in Gymnotus and Gymnurchus. A weak electric stimulus near the fish‘s own frequency can cause the fish to cease discharging entirely for a period of a fraction of a second up to minutes (references in Section 111 of Chapter 10, this volume). The longer cessations may represent a kind of hiding in these fish which are highly aggressive toward other electric fish including members of their own species. Similar cessations can be evoked in mormyrids by weak electric stimuli over a fairly wide range of frequencies (Moller, 1970). There are interactions between mormyrids evidenced by other changes in discharge rate that must be mediated by the electrosensory system

558

M. V. L. BENNETT

( Black-Cleworth, 1970; Mohres, 1957; Moller, 1970). In Gymnotus cessations appear to be involved in aggressive interactions between individuals ( Black-Cleworth, 1970; Westby and Box, 1970). Recent field studies show that changes in organ discharge frequency are involved in courtship behavior of several gymnotids ( C. Hopkins, unpublished data). Positive reinforcement by feeding has been used by Lissmann and Machin to establish sensitivity to conductivity differences and electric stimuli in Gymnarchus (1958) and to electric stimuli in the catfish Clarias ( 1963). Avoidance conditioning in which acceleration of organ discharge is the conditioned response has been carried out using a conductance change as the conditional stimulus and strong electric shock as the aversive stimulus (Bennett, 1968b; cf. Mandriota et al., 1968). A few experiments indicate that as would be expected the components of the electrosensory system mediate the behavioral responses to electric stimuli. In gymnotids section of the posterior branch of the anterior lateral line that innervates the electroreceptors in the posterior of the body abolishes the response to conductors placed posteriorly, but conductors near the head region are still avoided (Bennett and Grundfest, 1959). Unilateral section of this nerve reduces sensitivity on the affected side and causes all responses to be toward that side, indicating that the responses that do occur are mediated by the intact contralateral nerve. Section of the posterior lateral line nerves, which contain only mechanoreceptive fibers, has no effect on the avoidance response. Similar results have been obtained by cutting the appropriate nerves in catfish and dogfish ( Dijkgraaf, 1968; Dijkgraaf and Kalmijn, 1963). Simple energy considerations indicate that the electric organ discharge is necessary for the detection of conductance changes. As noted above, electric fish do not give the unconditional response of acceleration of discharge rate if a conductance change is restricted to the periods between organ discharges.

E. Thresholds for Receptor and Behavioral Responses Granted that the electroreceptors mediate the behavioral responses, it may then be asked whether the observed behavioral thresholds can be explained in terms of the receptor thresholds. The comparison is difficult to make because of differences in the methods by which the two types of experiment have been carried out. Nonetheless, from considerations of receptor thresholds and the effects of accessory structures, a preliminary evaluation can be made (Table 11). In the skate the threshold field for cardiac acceleration is about 0.01

559

11. ELECTRORECEPTION Table I1 Behavioral and Receptor Thresholds"

Behavioral response

Fish Skates, dogfish, sharks Gymnarchus Mormyrids Catfish Clarias K ryptopterus Gymnotids

Threshold gradient (rV/cm)

Computed Measured receptor receptor stimulus threshold (PV) (PV)

Heart rate acceleration Feeding Change in organ discharge rate

0.01 0.15 50

4

Feeding

1

10

Jamming avoidance

3-100

15-500

0.2

500

2 1006 20w

100 100

References in text. Mormyrid tonic receptors. Large receptors.

pV/cm, and the minimum receptor threshold is about 2 pV (Dijkgraaf and Kalmijn, 1966; Murray, 1967). Since the longest ampullary canals were probably 20 cm or more in length in the behavioral experiments, an applied field of 0.01 pV/cm would result in a stimulus to these receptors of 0.2 pV or more. The Gymnurchus studied by Machin and Lissmann (1960 ) was sensitive to fields of about 0.15 pV/cm, and a slightly higher threshold was calculated for distortions of fields produced by objects. Since the animal was about 50 cm in length, the field strength would result in a receptor stimulus of about 4 pV. The thresholds of receptors in Gymnurchus have not been well studied, but the required sensitivity is only 10-20 times greater than is observed in small (tonic) receptors in mormyrids. In Gymnarchus the threshold for high frequency rectangular pulses is given by the dc component of the stimuli. This observation indicates that the phasic receptors are no more sensitive to the ac component of this form of stimulus than the tonic receptors are to the dc component ( granted that these classes of receptor exist). The catfish Clarias has been shown to be sensitive to a gradient of about 1 pV/cm in training using food reinforcement (Lissmann and Machin, 1963). For a 20-cm animal this would correspond to a receptor stimulus of about 10 pV assuming that the skin is of high resistance. Receptor thresholds 5-10 times this value have been observed in the transparent catfish Kryptopterus ( Bennett, 1971a,b). In mormyrids, threshold gradients along the entire fish for electric pulses to evoke changes in organ discharge frequency are about 0.05

560

M. V. L. BENNETT

mV/cm (Moller, 1970). These gradients would give a receptor stimulus of about 0.5 mV in a 20-cm long fish which is above threshold for most large receptors ( Bennett, 1965, 1967). Thresholds for locally applied stimuli to evoke an acceleration of organ discharge are similar to the thresholds of large receptors (Harder et al., 1967). The jamming avoidance response of gymnotids has a sensitivity of 3100 pV/cm (Watanabe and Takeda, 1963). In these experiments the receptor stimulus would have been about five times the applied voltage per centimeter. The larger values easily exceed the absolute thresholds of the receptors measured directly ( Bennett, 1967). The relevant parameter in the jamming avoidance response is the incremental threshold of the receptors when the applied stimulus is superimposed on the stimulus provided by the organ discharge. This property has not been studied in fish emitting discharges at a fairly constant frequency. In Gyrnnotus incremental thresholds are not much different from absolute thresholds ( Suga, 1967a). However, in phasic receptors of Hypopornus, the stimulus required to evoke a single spike is much greater than the increase in stimulus required to add a spike to a train evoked by a moderately suprathreshold stimulus (Hagiwara and Morita, 1963). Correspondingly, the animal responds to a weaker applied pulse when it sums with the electric organ discharge than when it is presented between discharges (Bullock, 1970; Larimer and MacDonald, 1968). In the mormyrid Gnathonemus the behavioral responses to electric pulses have a threshold at least 10-fold higher when the pulses are superimposed on the organ discharge compared to when they are given between discharges ( Moller, 1970). Although for some medium receptors incremental thresholds are lower than absolute thresholds (Szabo and Hagiwara, 1967), the large receptors no doubt mediate the lower threshold behavioral responses, and their sensitivity to changes in the organ discharge is not yet clear. The “significance” to the fish may also differ in that pulses between organ discharges could represent another electric fish while pulses superimposed on organ discharges could represent resistance changes. The conclusion at this time is that there is a factor of approximately ten between observed behavioral and receptor thresholds. Direct measurements of the stimuli applied to the receptors under the conditions of the behavioral experiments are still required and may alter this factor somewhat. At least part of the discrepancy must lie in the difficulties in electrophysiological experiments of finding the lowest threshold receptors and of maintaining the animal in optimal condition. A further aspect of the discrepancy may be correlation by the central nervous system of inputs from a number of receptors. In the presence of

11. ELECTRORECEPTION

561

spontaneous activity the central nervous system may also discriminate changes in the responses of a single receptor more sensitively than does the experimenter. Although it can be accepted that the electroreceptors are sensitive to electric fields and thereby provide perceptually useful information, there is a possibility that these receptors also convey other kinds of information to the central nervous system. Phasic receptors are quite insensitive to mechanical, thermal, or other modes of stimulation, but tonic receptors may be rather sensitive to temperature and salinity changes and also have some sensitivity to mechanical stimulation (Murray, 1967; Suga, 1967a; Szabo, 1970). The effects of salinity changes are ascribable to changes in diffusion potentials (Murray, 1967). In the ampullae of Lorenzini the location deep in the body would reduce sensitivity to thermal and mechanical stimuli. The sensitivity to pressure differences between inside and outside of the canal is not very great but might become significant during rapid swimming. Pressure changes with depth would have little effect being communicated equally to inner and outer faces of the receptor cells. It is an open question whether activation of electroreceptors by nonelectric stimuli is interpreted by the animal as electric stimuli in the same way that we see lights when our eyes are electrically stimulated. Extensive conditioning experiments would be required to resolve this question,

VI. EVOLUTION OF ELECTROSENSORY SYSTEMS AND ELECTRIC ORGANS

In a chapter of The Origin of Species entitled “Difficulties of the Theory,” Darwin raised the problem of evolution of electric organs. It was difficult to see by what small intermediate steps the strongly electric organs of the eel and Torpedo could have arisen, when the early stages could have no value in defense or offense. Furthermore, the highly specialized but weakly electric organ of the ray could have no value in these functions. He wrote, “. . . it would be extremely bold to maintain that no serviceable transitions are possible by which these organs might have been gradually developed.” Only relatively recently did Lissmann (1958) provide evidence for a transition stage by demonstration of the electrosensory system of the weakly electric fish. The explanation became that early stages of the weakly electric organs were selected for their electrosensory function. Once a weakly electric organ reached a certain size, it then began to be adaptive in defensive-offensive actions. In view

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of the ineffectiveness of electric organs in offensive roles it seems likely that the defensive functions were more important in initial stages of conversion from purely electrosensory to defensiveoffensive organs. In agreement with Lissmann’s hypothesis the eel retains a weakly electric organ and has many weakly electric relatives. Some torpedinids also have a weakly electric organ, and some of the little known benthic forms are blind and may use their organs in an active electrosensory system. No evidence for weakly electric organs has been found in catfish, and the intermediate stages leading to the electric catfish are presumably lost. Evidence for active or passive electroreception is lacking for the stargazers. Lissmann further suggested, although less explicitly, that electroreceptors and electric organs developed concurrently in the evolution of the weakly electric fish. As lateral line receptors became modified to detect electric signals, muscles became modified to generate electric signals. In light of subsequent knowledge it seems more reasonable to believe that passive electrosensory systems were evolved initially and that electric organs were added subsequently. Most elasmobranches have ampullae of Lorenzini indicating that the primitive forms giving rise to rajids and torpedinids had passive electrosensory systems. Many catfish have electrosensory systems so that the hypothesized weakly electric ancestor of the electric catfish probably originally had a passive electrosensory system. All known mormyrids and gymnotids have electric organs, but the presence of the tonic electroreceptors is consistent with a prior stage possessing a passive electrosensory system. It does seem possible that the phasic receptors in these groups developed concurrently with the electric organs and they are certainly adapted to each other in different fish (Section IV, D ) . The similarities between receptors of mormyrids and Gymnarchus suggest that Gymnurchus already had an electric organ when it diverged from the mormyrid group. No data concerning electroreceptors are available from the stargazers, nor is the cerebellum at all enlarged unlike known electrosensory fish (Section V, C ) . This group remains a possible anomaly. In the initial stages of evolution of passive electrosensory systems, lateral line receptors presumably lost mechanical sensitivity and developed electrical sensitivity. This transformation probably required little more than an alteration of the outer face of the receptor cell. It would be expected that the inner face would already have developed a high electrical sensitivity for detection of sinall microphonic potentials generated by the outer face (see Section VII). The arrangement of the receptor cells in an epithelium is similar in all lateral line receptors and their homologs, and the major change leading to tonic electroreceptors may

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well be increased skin resistance, at least in freshwater fish. Further insight into these questions should be gained when membrane properties of receptor cells of ordinary lateral line organs are determined. Mechanoreceptors in weakly electric fish are 20 (Roth, 1968) to 400 times (Suga, 1967b) less sensitive to electric stimuli than are the electroreceptors. This difference is considerably less than the difference between electric and nonelectric fish ( Machin and Lissmann, 1960). Probably the increased skin resistance in freshwater fish with electroreceptors increases the electric sensitivity of the mechanoreceptors as well. Many electric fish live in turbid waters or, if they live in clear waters, are nocturnal. The electrosensory system which is basically quite short range would appear to be most useful under these conditions of restricted use of vision. Some electric fish have quite reduced eyes and the retinae of the electric eel apparently degenerates with age (see discussion by Keynes following de Oliveira Castro, 1961). But a number of electric fish have what appear to be quite good eyes, and many elasmobranchs at least are diurnal. In addition to the parallel evolution of electric organs, electroreceptors, and a peculiar mode of swimming, long snouts have been developed in both mormyrids and gymnotids (and probably more than once in each group; see Figs. 16 and 60 in Chapter 10). The snouts are certainly used in poking about muddy bottoms, but are most likely a later development than the electrosensory systems. It is not clear that development of electrosensory systems was always in compensation for poor visual conditions, although it seems likely to have been so a number of groups (Lissmann, 1958, 1961). In any case, development of lateral line organs with electric sensitivity has not led to any great decline of the ordinary lateral line system, which like the electrosensory systems should be particularly useful under poor visual conditions (Dijkgraaf, 1963). It would be useful to know more about electric fields present in the ordinary fish's environment and the extent to which these stimulate ordinary lateral line receptors. Muscle potentials up to tens of millivolts can be produced by synchronously contracting muscles. Examples are sonic muscles where the fundamental frequency is set by the frequency of muscle contraction and axial muscles when excited by Mauthner fibers (unpublished, cf. Lissmann, 1958). These potentials conceivably could activate mechanoreceptors electrically. Many catfish have sonic muscles and their innervation is similar to that of the electric organ of the electric catfish, which lacks a sonic muscle. Perhaps this organ was modified from a sonic muscle, a suggestion which could be strengthened by further morphological and embryological study of the catfish group (see Johnels, 1956).

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A final question that has been raised by the evolution of electrosensory systems is their absence in invertebrates and aquatic amphibia (Grundfest and Bennett, 1961). There are too few of the latter to make their difference from the fishes significant, and perhaps also too few invertebrates of sufficient size to make an electrosensory system useful. The cutaneous receptors of invertebrates are generally peripherally located neurons without separate receptor cells, and receptors of this kind may not well adapt to an electroreceptive function. VII. IMPLICATIONS FOR RECEPTOR FUNCTION IN GENERAL

The electroreceptor can be looked upon as a model of a secondary receptor, that is, one where the primary transduction is done by a receptor cell which then transmits synaptically to an afferent fiber. Functionally, a receptor cell can be divided into three parts: (1) the outer face which is passive in electroreceptors; ( 2 ) the "sides," which can be passive in tonic electroreceptors or electrically excitable in phasic electroreceptors; and ( 3 ) the presynaptic membrane, which secretes transmitter if the synapse is chemically transmitting (Fig. 40). The sides and presynaptic membrane comprise the inner face of electroreceptors (and some others); this face is separated from the outer face by a circumferential tight junction with neighboring cells. The three functional parts of a receptor cell correspond to dendritic (receptive), axonal (conductive), and terminal (secretory) portions of a generalized nerve cell ( Grundfest, 1966). In electroreceptors of freshwater fish, the outer face is unaffected by the stimulus, but the stimulus can be modified by the outer face, either

T

SUPPORTING C f l l

OUTER FACE: PASSIVE, M K H A N O SENSITIVE, CHEMOSENSITIVE OR PHOTOSENSITIVE

PI

SIDE: PASSIVE OR ELKTRKALLY EXCITABLE

PRESYNAPTIC MEMBRANE: SECETORY OR FORMING A N ELKTROTONIC SYNAPSE

Fig. 40. Diagram of a generalized receptor cell.

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as a series capacity distorts it or as a resting potential changes its dc level. The sides of the receptor cell may have no active influence on the (electrical) stimulus as in tonic receptors or they may alter it by responding as in most phasic receptors. The response may be regenerative, either graded or all-or-none, or it may be only degenerative, as produced by delayed rectification. The stimulus thus transformed (not really transduced) is transmitted electrotonically to excite the nerve, or it causes the presynaptic membrane to increase or decrease the release of transmitter. There may or may not be sufficient resting secretion to cause tonic activity in the nerve. The operation of other receptor cells is probably analogous. There is considerable evidence that the outer faces of mechanoreceptor cells generate a potential by changing their resistance and/or potential in response to a deformation (cf. Davis, 1965; Furukawa and Ishii, 1967), and carotid body receptors may operatc similarly (Eyzaguirre et al., 1965, 1970). This potential change is (electronically) conducted to the secretory face of the receptor cell and acts on the presynaptic membrane to cause release of transmitter ( Furukawa and Ishii, 1967). In one kind of vestibular receptor, morphological evidence indicates that the receptor potential is directly transmitted to the postsynaptic nerve by low resistance pathways ( Spoendlin, 1966). There are little data except in electroreceptors indicating whether receptor responses are modified by electrically excitable membrane in the sides of receptor cells. The occurrence of such membrane in one kind of receptor cell suggests that it will be found in others as well. Of course, axons of primary sensory neurons must conduct action potentials, if the axon is long compared to its space constant. What have been termed the outer face, sides, and presynaptic membrane may not be spatially segregated. Depending on how the stimulus can reach the cell and the mode of innervation, the three idealized regions could be intermixed with each other to a greater or lesser extent. In most electroreceptors the sides and presynaptic membrane are probably comingled in the inner face. Nonethcless, one can distinguish three separate functions and it is probable that the membranes mediating them are distinguishable, at least on the molecular level. The restriction of function to particular regions is only required where membranes of different function must be oriented differently with respect to the cell's environment. Generally, exteroceptors will have an outer face that is indeed directed outward, but sides and presynaptic membrane can be intermixed. Because the receptor cell is so short, it can be considered analogous to a presynaptic terminal. In this respect it is interesting that the elec-

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trically excitable responses of receptor cells of electroreceptors are insensitive to tetrodotoxin (Zipser, 1971). In the same species, and in most other forms, the ordinary Na channels mediating propagated spikes are blocked by tetrodotoxin. However, tetrodotoxin resistant spikes can be obtained in prcsynaptic terminals of the squid giant synapse and apparently of the neuromuscular junction as well (Katz and Miledi, 1969a,b). These neuronal responses are dependent on divalent ions and presumably mediate the normal Ca influx required for transmitter secretion. Although the ionic dependence of the receptor cell responses has yet to be studied, it is tempting to suppose that their spike generating mechanism represents evolution of an increased number of electrically excitable channels that carry divalent ions. Smaller numbers of these channels would presumably be present in receptor cells lacking an obvious electrical response where they would mediate transmitter release. Divalent ions certainly affect transmitter release in electroreceptors, although there is not the clear-cut antagonism between Ca and Mg that has been observed at a number of interneuronal synapses ( Steinbach and Bennett, 1971). An interesting question about receptor synapses is whether transmitter is released in small packets or quanta as has been so elegantly documented for the neuromuscular junction ( see Katz, 1969). The presence of presynaptic vesicles at receptor synapses suggests that release is quantized, and a certain amount of indirect confirmatory evidence involving analysis of intervals between afferent impulses has been obtained for lateral line receptors ( Harris and Flock, 1967). The irregularity of intervals between impulses from tonic receptors could be a result of quanta1 variations in the PSP, and intracellular recording from nerve terminals should provide more direct evidence ( Fig. 18). It is likely that an input-output relationship similar to that of tonic receptors will be found in other receptor cells, where more or less linearly graded information about stimulus amplitude is transmitted centripetally. The absolute sensitivity is also likely to be similar to that in electroreceptors, at least in the most sensitive receptors for a given modality. In all the sensitive receptors there should be strong selection pressure for evolution of a presynaptic face that can detect very small signals generated by the outer face. The cochlear microphonic is estimated to have a value of 0.01 pV at threshold for hearing (Machin and Lissmann, 1960). The potcntial across the inner face of the hair cells is undoubtedly much larger since the receptor should have evolved so that most of the voltage drop is developed across the inner face. A hundred- to a thousandfold larger potential across the secretory membrane is not unreasonable and brings the actual potential across the secretory face into the range where elec-

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troreceptors are capable of operating. The cochlear microphonic would not cause much electrical interference between cells since the external current from a single receptor is divided into the low resistance of the external path around the cells and the parallel paths provided by all the other cells. Intracellular recording from receptor cells of lateral line receptors of an amphibian indicates that a quite small microphonic, as low as 100 pV across the hair cell inner membrane, can be associated with a stimulus well suprathreshold for excitation of the postsynaptic fiber (Harris et al., 1970). This amplitude of potential would be adequate to excite tonic electroreceptors of freshwater fish. Thus, it appears that at least the most sensitive mechanoreceptors of the acoustico-lateralis system are of similar sensitivity to electroreceptors in terms of the potentials generated across their secretory faces. (An unanswered question is the mechanism whereby high frequency sinusoidal microphonics still cause release of transmitter. Possibly the secretory membrane of tonic electroreceptors would respond to high frequencies, but the potential across it at these frequencies is limited by its large time constant. ) The sensitive input-output relationship of electroreceptor synapses may be found in some neurons as well as receptor cells. In short axon cells such as the bipolar cells of the retina (Dowling and Boycott, 1965) and at reciprocal synapses such as those of the olfactory bulb (Rall et d, 1966), the propagated action potential probably is not required to transmit potentials between receptive and output parts of the cells. Slow and small PSPs generated at one site could cause release of transmitter by secretory membrane without intervention of large regenerative responses. In this connection it is interesting that the bipolar cell synapses have presynaptic ribbons like those found otherwise only in receptor cells. The experimental advantage of the tonic electroreceptors of freshwater fish is that they are specialized to detect low frequency voltages across the skin. The specializations that channel current through the receptor cells and the electrical linearity of the cells allow the experimenter considerable control over the presynaptic potential and has made possible the determination of the input-output relationship. The presynaptic POtential cannot be well controlled at phasic receptors, which do however demonstrate the possibility of electrical excitability and regenerative responses in receptor cells. A series capacity is an elegant way to achieve accommodation, but no other receptors are likely to have it. The only other cells where a membrane is known to act as a series capacity are electrocytes of several electric organs. The mechanisms of central analysis in electrosensory systems are largely unknown and may not turn out to be easier to analyze than other

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systems of more widespread occurrence. Nonetheless, the ease of experimental preparation and of stimulating known numbers of individual receptors makes the system worthy of further exploration. The macroscopic and cytological specialization of the underlying brain structures ( Section V, C ) also makes them highly intriguing subjects for physiological investigation. One can conclude that the electrosensory systems have provided information of considerable general as well as comparative physiological interest. It can be anticipated that further study will continue to be rewarding. ACKNOWLEDGMENTS

I am indebted to Dr. R. B. Szamier for many of the morphological figures. Supported in part by grants from the National Institutes of Health ( 5 PO1 NB 07512 and HD-04248) and the National Science Foundation (GB-6880).

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Lissmann, H. W., and Machin, K. E. (1963). Electric receptors in a non-electric fish (Chrias). Nature 199, 88-89. Lissmann, H. W., and Mullinger, A. M. (1968). Organization of ampullary electric receptors in Gymnotidae (Pisces). Proc. Roy. SOC. B169, 345-378. Machin, K. E. (1962). Electric receptors. Symp. SOC. Exptl. Biol. 16, 227-2.44. Machin, K. E., and Lissmann, H. W. (1960). The mode of operation of the electric receptors in Gymnarchus niloticus. J. Exptl. Biol. 37, 801-811. Mandriota, F. J., Thompson, R. L., and Bennett, M. V. L. (1965). Classical conditioning of electric organ discharge rate in Mormyrids. Science 150, 1740-1742. Mandriota, F. J., Thompson, R. L., and Bennett, M. V. L. (1968). Avoidance conditioning of the rate of electric organ discharge in Mormyrid fish. Animal Behav. 16, 448-455. Mauro, A., Conti, F., Dodge, F., and Schor, R. (1970). Subthreshold behavior and phenomenological impedence of the squid giant axon. J. Gen. Physiol. 55, 497523. Meader, R. G. (1934). The optic system of the teleost, Holocentrus. I. The primary optic pathways and the corpus geniculatum complex. J. Comp. Neurol. 60, 361-

407. Mohres, F. P. (1957). Elektrische Entladung im Dienste der Revierabgrenzung bei Fischen. Naturwissenschuften 44, 431432. Moller, P. ( 1970). “Communication” in weakly electric fish, Gnathonemus niger, (Mormyridae). I. Variation of electric organ discharge ( E O D ) frequency elicited by controlled electric stimuli. Animal Behao. 18, 768-786. Mullinger, A. M. (1964). The fine structure of ampullary electric receptors in Amiurus. PTOC.Roy. SOC. B160, 345-359. Mullinger, A. M. (1969). The organization of ampullary sense organs in the electric fish Gymnarchus niloticzrs. Tissue Cell 1, 31-52. Murray, R. W. (1962). The response of the ampullae of Lorenzini of elasmobranchs to electrical stimulation. 1. Exptl. Biol. 39, 119-128. Murray, R. W. (1965). Electroreceptor mechanisms: The relation of impulse frequency to stimulus strength and responses to pulsed stimuli in the ampullae of Lorenzini of elasmobranchs. J . Physiol. (London) 180, 592-606. Murray, R. W. (1967). The function of the ampullae of Lorenzini of elasmobranchs. In “Lateral Line Detectors” (P. Cahn, ed.), pp. 277-293. Indiana Univ. Press, Bloomington, Indiana. Nicholson, C., Llinhs, R., and Precht, W. (1969). Neural elements of the cerebellum in elasmobranch fishes: Structural and functional characteristics. In “Neurobiology of Cerebellar Evolution and Development” (R. Llinis, ed.), pp. 215-242. Am. Med. Assoc., Chicago, Illinois. Nieuwenhuys, R. (1967). Comparative anatomy of the cerebellum. Progr. Brain Res. 25, 1-93. Nieuwenhuys, R., and Nicholson, C. (1969a). A survey of the general morphology, the fiber connections and the possible functional significance of the gigantocerebellum of mormyrid fishes. In “Neurobiology of Cerebellar Evolution and Development” ( R . Llinhs, ed.), pp. 107-134. Am. Med. Assoc., Chicago, Illinois. Nieuwenhuys, R., and Nicholson, C. (196913). Aspects of the histology of the cerebellum of mormyrid fishes. In “Neurobiology of Cerebellar Evolution and Development” (R. LlinL, ed.), pp. 135-169. Am. Med. Assoc., Chicago, Illinois, Obara, S., and Bennett, M. V. L. (1968). Receptor and generator potentials of ampullae of Lorenzini in the skate, Raia. Biol. Bull. 135, 430-431.

11. ELECTRORECEPTION

573

Pickens, P. E., and McFarland, W. N. (1964). Electric discharge and associated behaviour in the stargazer. Animal Behav. 12, 362-367. Rall, W., Shepherd, G. M., Reese, T. S., and Brightman, M. W. (1966). Dendrodendritic synaptic pathway for inhibition in the olfactory bulb. Exptl. Neurol. 14, 44-56. Roth, A. ( 1967). Propriktks fonctionnelles et morphologique des diffkrents organes de la ligne laterale de Mormyrides. J. Physiol. (Paris) 59, 486. Roth, A. (1968). Electroreception in the catfish, Amiurus nebulosus. Z. Vergleich. Physiol. 61, 196-202. Roth, A. ( 1969). Elektrische Sinnesorgane beim Zwergwels Ictalurus nebulosus ( Amiurus nebulosus) Z. Vergleich. Physiol. 65, 368-388. Roth, A., and Szabo, T. (1969). The effect of sensory nerve transection on the sensory cells and on the receptor potential of the tuberous (Knollen) organ in mormyrid fish (Gnathonemus sp.). Z. Vergleich. Physiol. 62, 395-410. Schnitzlein, H. N., and Faucette, J. R. (1969). General morphology of the fish cerebellum. In “Neurobiology of Cerebellar Evolution of Development” ( R. Llinb, ed. ), pp. 77-105. Am. Med. Assoc., Chicago, Illinois. Spoendlin, H. (1966). Some morphofunctional and pathological aspects of the vestibular sensory epithelia. In “2nd Symposium on the Role of Vestibular Organs in Space Exploration,” pp. 99-115. NASA, Washington, D. C. Steinbach, A. B., and Bennett, M. V. L. (1971). Effects of divalent ions and drugs on synaptic transmission in phasic electroreceptors in a mormyrid fish. J. Gen. Physiol. ( I n press.) Stendell, W. ( 1914). Morphologische Studieren an Morniyriden. Verhandl. Deut. Zool. Ges. 24, 254-261. Suga, N. (1967a). Coding in tuberous and ampullary organs of a gymnotid electric fish. I. Comp. Neurol. 131, 437452. Suga, N. (196713). Electrosensitivity of canal and free neuromast organs in a gymnotid electric fish. J. Comp. Neurol. 131, 453-458. Szabo, T. (1962). The activity of cutaneous sensory organs in Gymnarchus niloticus. Life Sci. 7, 285-286. Szabo, T. (1965). Sense organs of the lateral line system in some electric fish of the Gymnotidae, Mormyridae, and Gymnarchidae. J. Morphol. 117, 229-250. Szabo, T. ( 1967). Activity in peripheral and central neurons involved in electroreception. In “Lateral Line Detectors” (P. Cahn, ed.), pp. 295-312. Indiana Univ. Press, Bloomington, Indiana. Szabo, T. (1970). Uber eine bisher unbekannte funktion der sog. ampullaren organe bei Gnathonemus petersii. Z. Verleich. Physiol. 66, 164-175. Szabo, T., and Fessard, A. (1965). Le fonctionnement des Blectrortkepteurs Ctudik chez les Mormyres. J. Physiol. (Paris) 57, 343-360. Szabo, T., and Hagiwara, S. (1967). A latency change mechanism involved in sensory coding of electric fish (mormyrids) Physiol. Behav. 2, 331-335. Szabo, T., and Wersall, J. ( 1970). Ultrastructure of an electroreceptor (niormyromast) in the mormyrid fish, Gnathonemuy petersii. J. Ultrastruct. Res. 30, 47-90. Szamier, R. B., and Wachtel, A. W. (1969). Special cutaneous receptor organs of fish. 111. The ampullary organs of Eigenmannia. J. Morphol. 128, 261-290. Szamier, R. B., and Wachtel, A. W. (1970). Special cutaneous receptor organs of fish. VI. The tuberous and ampullary organs of Hypopomus. J. Ultrastruct. Res. 30, 450-471.

574

M. V. L. BENNETT

Usherwood, P. N. R., and Machili, P. (1968). Pharmacological properties of excitatory neuromuscular synapses in the locust. J. Exptl. Biol. 49, 341-361. Wachtel, A. W., and Szamier, R. B. (1966). Special cutaneous receptor organs of fish: The tuberous organs of Eigenrnannia. J. Morphol. 119, 51-80. Wachtel, A. W., and Szamier, R. B. (1969). Special cutaneous receptor organs of fish. IV. Ampullary organs of the non-electric catfish, Kryptopterus. J. Morphol. 128, 291-308. Waltman, B. (1966). Electrical properties and fine structure of the ampullary canals of Lorenzini. Acta Physiol. Scand. Suppl. 264, 1-60. Waltman, B. ( 1968). Electrical excitability of the ampullae of Lorenzini in the ray. Acta Physiol. Scund. 74, 29A-30A. Watanabe, A,, and Takeda, K. (1963). The change of discharge frequency by A.C. stimulus in a weak electric fish. J. Exptl. Bwl. 40, 57-66. Wersall, J., Flock, A., and Lindquist, P. G. (1965). Structural basis for directional sensitivity in cochlear and vestibular sensory receptors. Cold Spring Harbor S y m p . Quant. Biol. 30, 115-132. Westby, G. W. M., and Box, H. 0. ( 1970). Prediction of dominence in social groups of the electric fish, Gyrn!iotus curupo. Psychon. Sci. 21, 181-183. Zipser, B. ( 1971 ). Tetrodotoxin resistant electiically excitable responses of receptor cells. Biophys. SOC. Abstr. 15th Ann. Meeting, 44a.

AUTHOR INDEX Numbers in italics refer to the pages on which the complete references are listed. Banner, A., 181, 186, 192, 199, 232, 236 Abrahamson, E. W., 15, 27 Bannister, L. H., 84, 85, 86, 114 Adler, J., 95, 114 Barber, S . B., 151, 192 Adrian, E. D., 34, 35, 53, 94, 98, 114, Bardach, J. E., 87, 88, 89, 102, 103, 163, 192 106, 113, 114, 115, 120, 122, 124, Adrian, R. H., 359, 459, 484 13.0, 133 Agassiz, J. L., 136, 192 Barets, A., 300, 306, 344, 503, 524, 539, Akerman, M., 385, 487 568 Albe-Fessard, D., 351, 357, 373, 385, Barnett, R., 450, 453, 486 449, 458, 474, 476, 484, 485 Baron, J., 4, 27 Albers, R. W., 352, 459, 485 Barron, S. E., 343, 345 Albers, V. M., 138, 155, 192 Bartelmez, G. W., 266, 274, 325, 344 Alderdice, D. F., 109, 114 Bartels, E., 389, 459, 488 Alexander, R. M., 152, 192 Bauer, R., 496, 568 Alford, R. S., 137, 143, 144, 197 Baylor, E. R., 2, 6, 7, 27 Ali, M. A., 10, 11, 27 Beatty, D. D., 18, 21, 22, 27, 30 Aljure, E., 461, 463, 465, 486 Beccari, N., 274, 300, 344 Allen, W. R., 383, 487 Beidler, L. M., 87, 118 Allison, A. C., 90, 91, 98, 114 Belbenoit, P., 496, 568 Altamirano, M., 349, 383, 386, 388, 448, Benamy, D. A., 391, 415, 458, 459, 489 485, 486 Benjamins, C. E., 229, 236 Amatniek, E., 373, 489 Bennett, M. V. L., 257, 262, 340, 342, Andersen, R., 166, 194 344, 348, 349, 351, 354, 357, 359, Andrews, C. W., 124, 132 361, 362, 363, 364, 366, 371, 372, Arden, G. B., 43, 53 373, 375, 376, 377, 378, 379, 380, Ariens-Kappers, C. U., 553, 568 390, 391, 392, 393, 395, 396, 397, Aronov, M. I., 187, 200 398, 399, 401, 402, 406, 408, 412, Aronson, L. R., 89, 98, 114, 267, 340, 413, 414, 415, 416, 417, 421, 424, 344 427, 428, 429, 433, 436, 437, 441, Asada, Y., 463, 490 443, 444, 445, 446, 447, 448, 449, Atema, J., 106, 120 450, 452, 453, 454, 455, 456, 457, Auerbach, A. A., 340, 342, 344, 482, 485, 514, 568 458, 459, 461, 462, 463, 464, 465, Autrum, H., 170, 192 466, 467, 468, 469, 470, 471, 472, 474, 476, 477, 478, 482, 485, 486 B 488, 489, 490, 494, 495, 499, 503, 504, 509, 511, 512, 513, 514, 515, Backus, R. H., 137, 138, 143, 177, 189, 519, 529, 530, 531, 532, 533, 534, 190, 192, 198, 200 535, 536, 537, 538, 539, 540, 541, Bailey, S. E. R., 129, 132 542, 544, 545, 546, 548, 549, 550, Baker, A. L., 459, 489 575

A

576 554, 556, 557, 558, 559, 564, 566, 568, 569, 570, 572, 573 Beranek, R., 539, 569 Berkelbach van der Sprenkel, H., 500, 553, 569 Berkowitz, E. C., 342, 344 Bernhard, C. G., 38, 53 Bernstein, J. J., 26, 29, 60, 76 Best, A. C. G., 11, 27 Bigelow, H. B., 162, 192, 229, 236, 374, 486 Bjorklund, R. G., 122, 133 Black-Cleworth, P., 354, 391, 395, 478, 486, 496, 558, 569 Blaxter, J. H . S., 10, 27 Bloom, F. E., 450, 453, 486 Bodenheimer, T. S., 317, 319, 346 Bodian, D., 274, 275, 300, 320, 344 Bogatyrev, P. B., 8, 27 Bondesen, P., 155, 192 Boudreau, J. C., 98, 114 Box, H. O., 496, 558, 574 Boycott, B. B., 503, 567, 570 Brahy, B. D., 138, 143, 188, 193, 201 Brauer, A., 5, 11, 12, 13, 14, 27 Brawn, V . M., 158, 187, 192 Breder, C. M., 106, 114 Breder, C. M., Jr., 188, 192 Brett, J. R., 1, 7, 9, 10, 27, 109, 114 Bridge, T. W., 136, 147, 163, 192 Bridges, C. D. B., 4, 18, 21, 22, 24, 27, 28, 36, 39, 54 Brightman, M. W., 463, 486, 567, 573 Brindley, G. S., 35, 43, 53 Brock, L. G., 378, 486 Bronghton, W. B., 155, 192 Brown, K. T., 39, 43, 50, 53 Brown, P. K., 25, 31 Brown, P. S., 25, 31 Brunner, G., 60, 76 Buerkle, U., 176, 192 Bull, H. O., 122, 133, 162, 167, 193, 228, 236 Bullock, T. H., 354, 385, 395, 402, 409, 417, 432, 478, 486, 496, 514, 550, 551, 557, 560, 569 Burkenroad, M. D., 137, 142, 143, 144, 145, 186, 193 Burkhardt, D. A., 36, 53 Burne, R. H., 82, 83, 114

AUTHOR INDEX

Buser, P., 474, 484 Busnel, R.-G., 155, 189, 193 Byzov, A. L., 40, 49, 53, 56

C Cahn, P. H., 135, 142, 168, 173, 193, 262 Caldwell, D. K., 144, 187, 193 Caldwell, M. C., 144, 187, 193 Carregal, E. J. A., 87, 119 Case, J., 87, 89, 103, 114 Cass, A., 358, 486 Chagas, C., 385, 474, 485 Chagnon, E. C., 171, 180, 197, 227, 229, 230, 234, 237 Chandler, W. K., 359, 459, 484 Chapman, C. J., 158, 176, 187, 196, 199 Charlton, T., 303, 344 Chichibu, S., 514, 550, 551, 569 Churchill, J. A., 257, 262 Clark, E., 69, 77 Clarke, W. D., 13, 28, 190, 192 Coates, C. W., 349, 383, 386, 388, 485, 486 Cohen, L. B., 449, 458, 487 Cohen, M. J., 129, 133, 150, 158, 175, 188, 193 Cole, K. S., 391, 458, 487 Cone, R. A,, 43, 53, 55 Conti, F., 536, 572 Cordier, R., 87, 114, 494, 569 Couceiro, A., 385, 487, 554, 569 Couteaux, R., 430, 487 Craddock, J. E., 190, 192 Crescitelli, F., 19, 21, 28 Creutzberg, F., 113, 114, 115 Crickmer, R., 113, 114 Cronly-Dillon, J. R., 26, 30, 60, 63, 68, 76, 77 Crosby, E. C., 98, 117, 553, 568 Cumniings, W. C., 138, 143, 188, 193, 201 Curtis, H. J., 391, 487 Cushing, D. H., 189, 193

D Dalilgren, U., 362, 364, 456, 487 Dann, R., 138, 190, 197 Darlington, P. J., Jr., 20, 28

577

AUTHOR INDEX

Dartnall, H. J. A,, 15, 18, 19, 21, 23, 24, 25, 28 Davies, D. H., 177, 193 Davis, H., 565, 570 Davis, L. I., 155, 192 Daw, N. W., 43, 54 de Burlet, H. M., 163, 193, 211, 225, 234, 236 Deelder, C. L., 113, 115 del Castillo, J., 402, 487 Delco, E. A., Jr., 187, 193 Denker, A., 229, 236 Denton, E. J., 3, 11, 12, 13, 19, 21, 25, 28 De Oliveira Castro, G., 417, 487, 499, 563, 570 Derbin, C., 524, 533, 570 Desgranges, J. C., 87, 115 Detweiler, S. R., 34, 54 de Vries, H., 169, 170, 196 Diamond, J., 276, 278, 285, 290, 291, 297, 301, 302, 306, 312, 316, 319, 343, 344, 346, 481, 482, 487 Diesselhorst, G., 170, 174, 180, 193, 228, 229, 230, 236 Dietrich, G., 183, 193 Dijkgraaf, S., 122, 123, 122, 129, 133, 136, 146, 163, 165, 167, 168, 169, 170, 174, 178, 179, 181, 187, 193, 194, 203, 228, 229, 230, 231, 234, 236, 239, 242, 262, 495, 496, 499, 500, 501, 555, 557, 558, 559, 563, 570 Disler, N. N., 168, 194 Dixon, R. H., 166, 181, 198, 338, 345 Djahanparwar, B., 98, 118 Dobrin, M. B., 137, 143, 194 Dodge, F. A., 449, 487, 536, 572 Doving, K. B., 84, 90, 97, 98, 99, 115 Dohlman, G., 215, 236 Dorai Raj, B. S., 143, 170, 194 Doran, R., 257, 262 Dotterweich, H., 500, 570 DBtu, Y., 147, 149, 194 Dowling, J. E., 33, 47, 51, 54, 56, 503, 567, 570 Drujan, B., 47, 54 Dudok van Heel, W. H., 179, 194 Dufosst., M., 136, 137, 146, 149, 194

E Eccles, J. C., 519, 570 Eccles, R. M., 378, 486 Edstrom, A., 343, 344 Eigenmann, C. H., 383, 487 Eimer, T., 218, 236 Eisenberg, F. A., 449, 487 Eisenberg, J. F., 144, 199 Eisenberg, R. S., 361, 487 Ellis, M. M., 381, 383, 384, 456, 487 Emling, J. W., 137, 143, 144, 197 Enger, P. S., 165, 166, 167, 171, 172, 173, 180, 194, 227, 231, 232, 235, 236, 550, 551, 557, 570, 571 Entine, G., 26, 29 Evans, H. M., 162, 194 Ewart, J. C., 375, 456, 487 Eyzaguime, C., 565, 570

F Fange, R., 150,194 Fagerlund, U. H. M., 109, 113, 114, 115, 116 Farkas, B., 163, 164, 174, 194, 195, 228, 236, 237 Farquhar, M. G., 463, 487, 504, 570 Fatehchand, R., 47, 54, 55, 56 Faucette, J. R., 553, 573 Fawcett, D. W., 151, 195 Fessard, A., 441, 491, 551, 554, 557, 569, 573 Ffowcs-Williams, J. E., 142, 195 Fidone, S., 565, 570 Finkelstein, A., 358, 486 Fish, M. P., 137, 143, 144, 145, 146, 149, 150, 154, 156, 157, 158, 186, 187, 195 Fletcher, H., 173, 180, 195 Flock, A., 165, 195, 221, 222, 237, 249, 250, 253, 254, 257, 262, 503, 566, 567, 571, 574 Fox, H., 343, 345 Frankenhaeuser, B., 389, 449, 458, 487 Franz, V., 34, 54, 494, 570 Freygang, W. H., Jr., 391, 491 Frings, H., 184, 195 Frings, M., 184, 195 Frishkopf, L. S., 249, 250, 262, 557, 571

578

AUTHOR INDEX

Fritsch, G., 487 Froese, H., 163, 167, 195 Froloff, J. P., lG2, 195, 228, 229, 237 Fujimoto, K., 26, 29 Fujiya, M., 102, 103, 114, 115 Furakawa, T., 166, 195 Furshpan, E. J., 271, 273, 275, 276, 288, 315, 316, 317, 322, 324, 325, 327, 333, 337, 344, 345, 481, 482, 487 Furukawa, T., 258, 262, 271, 273, 275, 276, 315, 320, 322, 324, 325, 327, 333, 334, 337, 341, 342, 345, 481, 482, 487, 565, 570

Grosse, J. P., 442, 448, 488 Gruber, S. H., 4, 28, 177, 178, 181, 186, 199, 204, 232, 238 Grundfest, H., 349, 351, 357, 3591, 362, 363, 364, 366, 367, 371, 372, 373, 374, 377, 383, 386, 388, 389, 390, 391, 392, 393, 395, 396, 415, 427, 428, 429, 430, 437, 441, 443, 444, 445, 446, 447, 448, 449, 450, 452, 453, 454, 456, 458, 459, 477, 482, 485, 486, 488, 489, 490, 514, 519, 539, 549, 557, 558, 564, 569, 570

G

Haddle, G. P., 154, 160, 201 Haddon, A. C., 136, 147, 163, 192 Haedrich, R. L., 190, 192 Haempel, O., 229, 237 Hardig, J., 101, 116 Hafen, G., 229, 237 Hager, H. J., 63, 7 6 Hagiwara, S., 129, 133, 273, 346, 482, 488, 543, 550, 551, 552, 557, 560, 571, 573 Hahn, W. E., 107, 110, 118 Haller, H., 300, 345 Hama, K., 257, 262 Hamasaki, D. H., 4, 28 Hamasaki, D. I., 4, 28, 36, 39, 54 Hanaoka, T., 26, 29 Hanyu, I., 3, 29 Hara, T. J., 96, 97, 98, 99, 106, 110, 111, 112, 113, 115, 116, 120 Hardenberg, J. D. F., 137, 196 Harden Jones, R. R., 63, 76 Harder, W., 432, 488, 497, 498, 528, 541, 543, 560, 571 Harris, A. J., 73, 76 Harris, G. G., 141, 153, 169, 170, 183, 196, 242, 249, 250, 258, 261, 262, 566, 567, 571 Hartig, G. M., 174, 204 Hartline, H. K., 35, 40, 41, 54 Hashimoto, H., 39, 44, 50, 51, 54 Hashimoto, T., 189, 190, 196 Hashimoto, Y., 47, 56 Hasler, A. D., 80, 92, 105, 110, 116, 119, 146, 152, 187, 200 Hawkins, A. D., 158, 176, 187, 196, 199 Hawkins, J. E., 180, 196

Gage, P. W., 361, 449, 487 Gainer, H., 151, 152, 195 Galler, S. R., 189, 195 Gardner-Medwin, A. R., 43, 53 Gasser, H. S., 90, 96, 115 Gautron, J., 460, 488 Gaze, R. M., 63, 64, 76 Gemne, G., 84, 90, 98, 99, 115 Geoffroy St. Hilaire, I., 138, 195 GBrard, P., 494, 550, 570 GBry, J., 383, 487 Gilbert, P. W., 9, 12, 29, 105, 115 GimBnez, M., 412, 461, 463, 4 6 5 467, 469, 470, 471, 474, 486, 533, 569 Glaser, D., 93, 115 Gleisner, L., 213, 222, 239 Comer, P., 242, 259, 262 GO,, H., 106, 115 Gorbman, A., 96, 97, 98, 99, 106, 107, 110, 111, 112, 115, 116, 118, 120 Goronowitsch, W., 26~3,345 Grangaud, R., 19, 28 Granit, R., 35, 36, 37, 38, 39, 40, 54 Grass&,P.-P., 164, 167, 195 Gray, E. G., 302, 303, 304, 305, 306, 344, 345 Gray, G.-A., 187, 188, 195 Green, W. C., 138, 190, 197 Greene, C . W., 136, 137, 149, 150, 158, 195 Greenwood, P. H., 383, 437, 487, 488 Griffin, D. R., 166, 170, 189, 196, 495, 570 Grimm, R. J., 105, 115 Groen, J. J., 216, 237

H

579

AUTHOR INDEX

Hays, E. E., 138, 139, 203 Hazlett, B. A., 146, 147, 151, 158, 187, 188, 196, 204 Heinecke, P., 187, 199 Held, R., 72, 76 Hemmings, C. C., 107, 112, 116, 176, 199 Hemmings, G., 73, 76 Hensel, H., 131, 132, 133 Hering, E., 52, 54 Herrick, C. J., 87, 116, 164, 196, 554, 571 Herrniknd, W. F., 138, 188, 193 Hersey, J. B., 138, 200 Herter, K., 60, 76 Hester, F. J., 189, 196 Hibbard, E., 343, 345 Hidaka, I., 100, 101, 116, 117 Higman, H. B., 389, 459, 488 Hille, B., 358, 362, 449, 454, 458, 487, 488 Hirata, Y., 87, 88, 116 Hoagland, H., 100, 116, 123, 125, 132, 133 Hodgkin, A. L., 349, 352, 358, 359, 458, 459, 484, 488 Hodgson, E. S., 105, 115 Hoglund, L. B., 101, 116 Hoff, I., 166, 198 Holl, A., 83, 84, 102, 103, 114, 116 Holmgren, F., 54 Hopkins, A. E., 84, 85, 86, 116 Horton, J. W., 166, 196 Hoyle, G., 482, 491 Hubel, D. H., 61, 76 Huber, G. C., 98, 117, 553, 568 Hudson, R. C. L., 343, 345 Hiittel, R., 108, 116 Humbach, I., 94, 116 Huxley, A. F., 389, 449, 458, 487, 488 Hyvarinen, J., 99, 115

I Idler, D. R., 105, 109, 114, 116, 117 Ikeda, H., 43, 53 Ingle, D., 62, 63, G4, 65, 67, 68, 69, 70, 72, 73, 74, 76 Inouye, K., 8, 29 Ishii, Y., 166, 195, 258, 262, 320, 334, 342, 345, 565, 570

Ishikawa, T., 33, 49, 57 Ishiyama, R., 375, 488 Israel, M., 372, 460, 488 Iversen, R. T. B., 175, 176, 196 Iwai, T., 87, 116, 168, 196

J Jacobs, D. W., 171, 173, 179, 180, J96 Jacobson, M., 63, 64,75, 76 Jagodowski, K. P., 84, 85, 86, 116 Jakubowski, M., 243, 262 Jasinski, A., 98, 116 Jielof, R., 169, 170, 196 John, K. R., 11, 29 Johnels, A. G., 427, 456, 488, 563, 571 Johnson, M. W., 137, 196 Jonas, R. E. E., 105, 109, 116, 117 Jones, F. R. H., 117, 166, 196 Jones, M. P., 10,27 Jurand, A., 343, 345

K Kahmann, H., 4, 29 Kaiserman-Abramof, I. R., 553, 571 Kalmijn, A. J., 495, 496, 501, 551, 558, 559, 570 Kandel, E. R., 98, 117 Kaneko, A., 26, 31, 39, 40, 44, 45, 46, 50, 51, 52, 54, 55, 56 Kappers, C. U. A., 98, 117 Karlin, A., 459, 488 Karnovsky, M. J., 463, 490 Katsuki, Y., 103, 117 Katz, B., 257, 262, 288, 345, 402, 477, 487, 488, 510, 512, 513, 514, 539, 566, 571 Keenleyside, M. H. A., 107, 117 Kellaway, P., 348, 349, 488 Kellogg, W. N., 158, 196 Kelsey, A. S., Jr., 137, 156, 195 Kendall, J. I., 103, 117 Kennedy, D., 3, 29 Keynes, R. D., 349, 383, 389, 427, 428, 429, 449, 456, 458, 477, 487, 489 Kilarski, W., 152, 197 Kinzer, J., 161, 197 Rlancher, J. E., 152, 195 Klausewitz, W., 145, 197

AUTHOR INDEX

Kleerekoper, H., 80, 86, 107, 108, 112, 117, 166, 169, 171, 180, 181, 197, 227, 229, 230, 231, 234, 237 Kluver, H., 65, 76 Knudsen, V. O., 137, 143, 144, 197 Kobayaslii, H., 37, 54 Koczy, F. F., 138, 201 Kolster, R., 300, 345 Konislii, J., 100, 101, 102, 117 Koynno, H., 565, 570 Kramer, E., 187, 199 Krausse, A., 229, 237 Kreidl, A., 162, 197 Krespi, V., 358, 486 Kriebel, M. E., 463, 482, 489 Krinner, M., 93, 117 Kritzler, H., 167, 170, 177, 178, 197, 232, 237 Kronengo!d, M., 138, 190, 197, 201 Kruger, L., 8, 31 Kubo, I., 218, 237 Kuchnow, K. P., 9, 12, 29 Kuffler, S. W., 40, 41, 54 Kuiper, J. W., 169, 170, 197 Kume, S., 459, 490 Kuroki, T., 181, 197 Kusano, K., 151, 195

1 Lafite-Dupont, J., 162, 197 Lander, M. R., 18, 21, 28 Lanyon, W. E., 155, 197 Larimer, J. L., 417, 489, 557, 560, 571 Laufer, M., 47, 54, 55, 56 Leghissa, S., 300, 345 Leitner, L. M., 565, 570 Lele, P. P., 123, 129, 133 Lesbats, B., 460, 488 Leuzinger, W., 459, 489 Licklider, J. C. R., 179, 197 Liebnian, P. A,, 26, 29 Lindeman, V. F., 3, 29 Lindquist, P. G., 503, 574 Lissmann, H. W., 349, 374, 391, 408, 432, 437, 489, 494, 495, 496, 498, 503, 524, 545, 556, 558, 559, 561, 563, 566, 572, 573 Locliner, 1. P. A., 177, 193 Loewenstein, J. M., 138, 190, 197 Losey, G. S., Jr., 109, 119

Lowenstein, O., 130, 133, 162, 165, 197, 208, 210, 211, 212, 213, 215, 216, 217, 218, 219, 220, 221, 222, 223, 224, 225, 226, 233, 234, 235, 237, 238 Lowrey, A., 405, 489 Loye, D. F., 137, 197 Ludwig, C., 94, 98, 114 Luft, J. H., 2.44, 262, 385, 489 Lundquist, P.-G., 213, 222, 239 Lyon, E. T., 218, 238 Lythgoe, J. N., 15, 18, 21, 23, 24, 25, 28, 29

M MacBain (Spires), J. Y., 138, 201 McBride, J. R., 105, 109, 113, 115, 116, 117 McCleary, R. A., 26, 29, 60, 69, 76 McDonald, H. E., 162, 197, 229, 238 MacDonald, J. A,, 417, 489, 557, 560, 571 McFarland, W. N., 23, 29, 364, 490, 496, 572 Machili, P., 539, 573 hlachin, K. E., 495, 496, 558, 559, 563, 566, 571, 572 hhckenzie, K. V., 138, 197 MacKinnon, D., 109, 114 Mackintosh, J., 74, 77 Mackintosh, N. J., 61, 62, 73, 74, 77 McNally, W. J., 219, 221, 226, 238 MacNaughton, I. P. J., 219, 221, 226, 238 MacNichol, E. F., 26, 29, 41, 42, 43, 44, 47, 48, 49, 53, 54, 55, 56, 57 Mahajan, C . L., 144, 198 Maier, H. N., 229, 238 Maksimova, E. M., 52, 55 Malar, T., 181, 197 Maliukina, G. A., 137, 170, 174, 198 Mandriota, F. J., 441, 478, 489, 496, 536, 558, 559, 566, 572 Maniwa, Y., 189, 190, 196 Mann, H., 102, 117 Manning, F. B., 163, 164, 198, 218, 229, 238 Zlansueti, R. J., 143, 202 Marage, M., 162, 198 Marcstrom, A., 92, 118

581

AUTHOR INDEX

Marks, W. B., 26, 29, 44, 51, 52, 55, 60,

77 Marler, P., 184, 198 Marhall, J. A., 137, 143, 147, 151, 152, 158, 160, 187, 188, 198, 204 Marshall, N. B., 5, 13, 29, 138, 142, 148, 149, 156, 166, 198 Martins-Ferreira, H., 383, 389, 449, 476, 485, 489 Massonet, R., 19, 28 Mathews, R. D., 109, 119 Mathewson, R. F., 105, 115, 151, 195, 364, 367, 373, 374, 388, 430, 450, 453, 454, 489 Matsuda, H., 101, 117 Matthews, R., 34, 35, 53 Matthews, W. A., 62, 77 Mattocks, J. E., 207, 240 Mauro, A., 349, 373, 489, 536, 572 Mauthner, L., 266, 345 Maxwell, S. S., 215, 218, 238 Mayoh, H., 109, 116 Mayser, P., 266, 345 Mead, G. W., 190, 192 Meader, R. G., 554, 572 Meder, E., 137, 198 Meesters, A., 63, 77 Melzack, R., 129, 133, 339, 346 Meyer, E., 152, 166, 198 Meyer, S. L., 13, 30 Midttun, L., 166, 198 Miesner, H.-J., 92, 118 Miledi, R., 257, 262, 345, 477, 488, 510, 512, 513, 514, 539, 566, 571 Miles, S. G., 113, 118 Milkman, R. D., 3, 29 Miller, P. L., 539, 569 Milne, D. C., 258, 262 Mitarai, G., 47, 49, 54, 55, 56 Mittelstaedt, H., 129, 133 Moatti, J.-P., 19, 28 Mohres, F. P., 496, 558, 572 Mogensen, J. A., 108, 117 Moller, P., 354, 441, 469, 478, 489, 551, 557, 558, 559, 560, 572 Moncrieff, R. W., 87, 118 Moorhouse, V. H. K., 162, 198, 228, 238 Moreau, A., 136, 150, 198 Mori, Y., 102, 117 Morita, H., 543, 551, 552, 5f30, 571

Morlock, N. L., 391, 415, 458, 459, 489 Morton, R. A,, 15, 29 Motais, R., 3, 29 Motokawa, K., 38, 39, 41, 47, 55 Moulton, D. G., 87, 95, 118 Moulton, J. M., 135, 136, 138, 143, 144, 145, 150, 153, 156, 157, 158, 160, 162, 164, 166, 181, 187, 188, 189, 190, 198, 338, 343, 345 Mowbray, W. M., 137, 151, 156, 187, 192, 195 Miiller, J., 136, 147, 148, 198, 199 Mullinger, A. M., 494, 496, 498, 503, 512, 515, 524, 540, 551, 557, 558, 559, 560, 572 Munk, O., 5, 11, 13, 14, 29, 30 Munson, W. A., 173, 180, 195 Muntz, W. R. A., 26, 30, 60, 77 Munz, F. W., 15, 18, 21, 22, 23, 24, 25, 28, 29, 30 Murakami, M., 26, 31, 39, 40, 43, 45, 46, 51, 52, 53, 55, 56 Murray, R. W., 124, 130, 131, 132, 133, 247, 262, 494, 495, 500, 517, 544, 547, 559, 561, 572 Myers, G. S., 383, 488 Myrberg, A. A., Jr., 186, 187, 199

N Nachmansohn, D., 383, 386, 485 Naito, K., 19, 22, 30 Naka, K. I., 52, 55 Nakajima, S., 389, 415, 448, 449, 458, 490 Nakajima, Y., 412, 461, 463, 465, 467, 469, 470, 471, 472, 474, 486, 512, 531, 533, 569 Nakamura, Y., 389, 415, 448, 449, 458, 490 Nanba, R., 98, 118 Negishi, K., 47, 54 Nelson, D. R., 167, 177, 178, 180, 181, 186, 199, 204, 232, 236, 238 Nelson, E. M., 167, 199 Nelson, K., 146, 187, 199 Neurath, H., 92, 118 Nicholson, C., 553, 554, 572 Nicol, J. A. C., 1, 6, 9, 11, 12, 13, 19, 27, 28, 30

582

AUTHOR INDEX

Nieuwenhuys, R., 300, 345, 553, 554, 572 Nishi, K., 565, 570 Nishimura, M., 190, 196 Niwa, H., 4, 26, 31, 49, 56, 101, 117 Noell, W. K., 39, 55 Nosaki, H., 46, 50, 56 Noto, S., 8, 29 Nursall, J. R., 154, 199

0 Ohara, S., 519, 572 O’Connell, C. P., 4, 30 Ogawa, T., 38, 39, 41, 47, 55 Ohtsu, K., 22, 30 Oikawa, T., 38, 39, 47, 55 Olmsted, J. M. D., 104, 118 Orcutt, B., 459, 490 Orlov, 0. Yu., 52, 55 Oshorn, C. M., 9, 32 Osborne, M. P., 208, 210, 211, 212, 213, 221, 223, 224, 238 Oshima, K., 96, 97, 98, 106, 107, 110, 112, 118 Ostroy, S. E., 15, 27 Otsuka, N., 343, 346 Ottoson, D., 95, 96, 118

P Packard, A., 151, 199 Pak, W. L., 43, 55 Palade, G. E., 463, 487, 504, 570 Palay, S. L., 87, 89, 119, 553, 571 Palmer, E., 123, 133 Pappas, G. D., 392, 401, 402, 406, 412, 416, 433, 444, 450, 453, 454, 456, 459, 461, 463, 465, 467, 469, 470, 471, 472, 474, 482, 489, 490, 512, 531, 533, 569 Parker, G. H., 80, 104, 118, 162, 163, 177, 199, 218, 219, 227, 229, 238 Parkhurst, R. M., 109, 119 Parrish, B. B., 176, 199 Parvulescu, A., 142, 155, 173, 199 Pautler, E. L., 26, 31, 45, 46, 51, 52, 56 Payton, B. W., 463, 490 Pearcy, W. G., 13, 30 Pearson, A . A,, 163, 164, 199 Pellegrin, J., 437, 490 Pfeiffer, W., 83, 108, 109, 119, 144, 199

Piatt, J., 343, 346 Pickens, P. E., 364, 490, 496, 572 Piper, H., 227, 238 Pipping, M., 82, 119 Pitt, G. A. J., 15, 29 Plack, P. A., 19, 30 Poggendorf, D., 170, 171, 174, 192, 199, 227, 231, 238 Poll, M., 437, 439, 490 Polleski, T. R., 389, 459, 488 Popper, A. N., 171, 199 Post, R. L., 459, 490 Prince, J. H., 1, 4, 30 Protasov, V. R., 135, 137, 138, 161, 187, 198, 200 Proudfoot, D. A., 137, 197 Pumphrey, R. J., 5, 6, 7, 31, 166, 182, 200

R Rall, W., 567, 573 Ramsay, D. A,, 110, 119 Rashcheperin, V. K., 161, 200 Rauther, M., 149, 200 Reed, J. R., 108, 119 Reese, T. S., 86, 119, 463, 486, 567, 573 Reickel, A,, 137, 200 Reinhardt, F., 163, 181, 200, 234, 238 Remmler, W., 490 Retzius, G., 208, 210, 239 Retzlaff, E., 279, 280, 320, 346 Revel, J. P., 151, 195, 463, 490 Rhodin, J., 244, 262 Richard, J. D., 177, 178, 181, 186, 190, 199, 200, 204 Richardson, E. G., 155, 200 Riddell, L. A., 36, 37, 54 Roberts, T. D. M., 165, 197, 233, 234, 235, 237 Robertson, J. D., 317, 319, 346 Rochon-Duvigneaud, A., 1, 4, 31 Rode, P., 163, 200 Rodgers, W. L., 339, 346 Roggenkamp, P. A., 166, 169, 171, 197, 227, 231, 237 Ronianenko, E. V., 161, 200 Romanes, G. H., 218, 239 Roper, S., 343, 346 Rosen, D. E., 383, 488 Rosenberg, H., 490

583

AUTHOR INDEX

Rosenblatt, R. H., 109, 119 Roth, A., 494, 495, 500, 514, 515, 516, 530, 563, 564, 573 Rovainen, C. M., 342, 346 Ruhin, M. A., 123, 124, 133 Rushton, W. A. H., 52, 55 Russell, I. j., 257, 261, 262

S Safriel-jorne, O., 74, 77 Saito, N., 474, 482, 488, 490 Salmon, M., 143, 145, 152, 187, 200 Sand, A,, 130, 131, 132, 133, 163, 200, 215, 216, 217, 237, 238, 258, 262 Sasaki, Y., 39, 55 Sato, Y., 47, 56 Saxena, A., 63, 65, 66, 77 Scharf, B., 180, 200 Scharrer, E., 87, 89, 119 Schevill, W. E., 138, 200 Schief, A., 560, 571 Schneider, H., 138, 144, 145, 146, 147, 149, 151, 152, 155, 157, 158, 166, 174, 180, 187, ZOO, 228, 230, 239 Schneider, J. E., 72, 77 Schneirla, T. C., 184, 191, 200, 201 Schnitzlein, H. N., 90, 91, 119, 553, 573 Schoen, L., 219, 226, 239 Schone, H., 226, 239 Schor, R., 536, 572 Schriever, H., 163, 201 Schroeder, W. C., 374, 486 Schrodinger, E., 52, 55 Schukneckt, H. F., 257, 262 Schulte, A., 65, 66, 69, 70, 77 Schutz, F., 108, 119 Schwanzara, S. A., 15, 16, 17, 18, 20, 21, 23, 24, 30, 31 Schwartz, E., 242, 263 Schwartz, I. R., 392, 401, 402, 406, 416, 433, 444, 450, 453, 454, 456, 459, 490 Schwasqman, H. O., 8, 31, 408, 489 Sebeok, T. A., 184, 201 Segall, M., 11, 29 Seliger, H. H., 9, 31 Sen, A. K., 459, 490 Shapiio, S. M., 70, 77 Shaw, E., 6, 7, 27, 168, 191, 193, 201 Shaw, T. I., 21, 28

Sheldon, R. E., 90, 98, 104, 119 Shepherd, G. M., 95, 118, 567, 573 Sheridan, M. N., 372, 450, 453, 490 Sherrington, C., 471, 490 Shihuya, T., 94, 95, 119 Shimada, H., 112, 118 Shishkova, E. V., 153, 201 Shores, D. L., 190, 192 Siler, W., 173, 193 Silvester, C. F., 362, 364, 487 Simpson, G. G., 21, 31 Sims, R. T., 337, 346 Sinclair, D. C., 129, 133 Sjostrand, F. S., 33, 55 Sjostrand, J., 343, 344 Skinner, W. A., 109, 119 Skoglund, C. R., 38, 53, 151, 201 Skudrzyk, E. j., 154, 160, 201 Smith, E. D., 177, 193 Smith, H. M., 136, 201 Smith, I. C., 271, 346 Smith, M., 113, 115 Smith, S. W., 87, 89, 119 SGrensen, W., 136, 137, 144, 148, 201 Sorgente, N., 143, 145, 152, 187, 200 Spath, M., 125, 126, 127, 128, 133 Sperry, R. W., 69, 77 Spires, J. Y., 138, 143, 193 Spoor, A., 169, 170, 196 Spoendlin, H., 533, 565, 573 Sprengling, G., 108, 116 Stage, D. E., 317, 319, 346 Stampehl, H., 137, 201 Stefanelli, A., 267, 340, 343, 346 Stein, R. B., 479, 490 Steinhach, A. B., 417, 421, 424, 474, 486, 490, 531, 532, 539, 554, 566, 569, 573 Steinherg, J. C., 138, 201 Steinhausen, W., 215, 239 Stell, W. K., 33, 47, 56 Stendell, W., 573 Sten\io, E., 208, 239 Stetter, H., 162, 163, 164, 170, 179, 201, 203, 219, 226, 228, 229, 230, 231, 239 Stevens, D. M., 107, 119 Stevens, S. S., 180, 196 Stipetii., E., 163, 174, 179, 201, 228, 230, 239

584

AUTHOR INDEX

Stone, H., 87, 119 Stout, J. F., 187, 201, 204 Strieck, F., 91, 93, 119 Strother, W. F., 174, 204 Suckling, E. E., 169, 170, 202, 432, 491 Suckling, J. A,, 169, 170, 202 Suga, N., 499, 503, 550, 551, 560, 561, 563, 573 Sullivan, C. M., 123, 124, 133 Sutherland, N. S., 61, 62, 65, 66, 67, 68, 74, 76, 77 Svaetichin, G., 35, 47, 48, 49, 54, 55, 56 Swanson, R. T., 23, 30 Szabo, T., 392, 430, 432, 441, 442, 443, 447, 448, 456, 472, 473, 474, 487, 488, 490, 491, 494, 498, 503, 514, 520, 524, 528, 530, 533, 539, 540, 543, 550, 551, 553, 557, 560, 561, 568, 570, 571, 573 Szaniier, R. B., 494, 498, 500, 503, 505, 506, 507, 508, 515, 522, 524, 549, 573, 574

T Tagliani, G., 300, 346 Takeda, K., 354, 409, 491, 557, 560, 574 Takeuchi, A., 448, 491 Takeuchi, N., 359, 365, 448, 491 Tamura, T., 4, 26, 31, 49, 56 Tarrant, R. M., Jr., 92, 119 Tasaki, I., 273, 346, 391, 491 Tasaki, K., 38, 39, 55 Tateda, H., 102, 120 Tavolga, W. N., 106, 120, 135, 136, 138, 143, 144, 145, 146, 147, 148, 149, 150, 151, 152, 153, 155, 156, 157, 158, 159, 160, 161, 164, 165, 167, 171, 173, 174, 175, 179, 180, 184, 185, 186, 187, 188, 190, 196, 197, 202, 204, 231, 239 Taylor, J. R., 565, 570 Taylor, M., 143, 202 Teal, J. M., 190, 192 Teichniann, H., 80, 82, 83, 92, 120 Tesch, F. W., 113, 115, 120 Tester, A. L., 103, 117 Thompson, R. L., 441, 478, 489, 546, 547, 558, 563, 566, 572 Thornhill, R. A., 208, 210, 211, 223, 224, 225, 238

Thurm, U., 263 Tiegs, 0. W., 300, 302, 346 Tobin, T., 459, 490 Todd, J. H., 106, 113, 114, 120 Tomaschek, H., 163, 202 Tomita, T., 26, 31, 35, 39, 44, 45, 46, 47, 50, 51, 52, 56 Tomlinson, N., 105, 109, 113, 115, 116, 117 Tosaka, T., 47, 56 Tower, R. W., 136, 146, 149, 150, 203 Toyoda, J., 46, 50, 56 Toyota, M., 101, 117 Tracy, S. C., 167, 203 Trevarthen, C. B., 63, 72, 73, 77 Trifonov, Yu. A., 49, 53, 56 Trinkaus, J. P., 436, 486, 504, 569 Trudel, P. J., 93, 120 Trujillo-Cen6z, O., 84, 87, 120 Tschiegg, C. E., 138, 139, 203 Tsvietkov, V. I., 161, 200 Tucker, D., 95, 118

U Uchida, M., 102, 117 Uchihashi, K., 90, 91, 120 Uchizono, K., 304, 346 Ueda, K., 110, 111, 116, 120 Uhlemann, H., 432, 488, 560, 571 Ulrich, H., 218, 239 Usherwood, P. N. R., 539, 573

V Vallecalle, E., 47, 54, 55, 56 van Bergeijk, W. A., 141, 164, 169, 173, 181, 182, 183, 196, 203, 261, 262, 263 Vanderwalker, J. G., 189, 203 van Heusen, A. P., 162, 199, 229, Vendrik, A. J. H., 216, 237 Verheijen, F. J., 109, 120, 179, 194, 236 Verrier, M.-L., 4, 27 Versteegh, C., 211, 225, 236 Vigonrenx, P., 138, 203 Villegas, G. M., 33, 56 Villegas, J., 47, 54, 55, 56 Villegas, R., 33, 56

170, 242, 238 230,

585

AUTHOR INDEX

Vilter, V., 5, 13, 31 Vincent, F., 158, 162, 203 Vinnikov, J. A., 86, 95, 120 von Baumgarten, R., 92, 98, 118 von BAkAsy, G., 180, 203 von Boutteville, K. F., 170, 171, 203, 229, 231, 239 von Buddenbrock, W., 89, 120 von Frisch, K., 89, 107, 120, 162, 163, 164, 165, 171, 181, 203, 208, 219, 226, 227, 228, 229, 234, 239, 320, 321, 346 von Holst, E., 129, 133, 219, 226, 239 von Ihering, R., 136, 203 von Kries, J., 52, 56 Vu-TBn-Tu&,383, 487

W Wachtel, A. W., 364, 367, 374, 375, 388, 430, 450, 453, 454, 456, 489, 491, 494, 498, 500, 503, 505, 506, 507, 508, 515, 522, 524, 549, 573, 574 Wagner, H. G., 26, 29, 41, 42, 43, 53, 55. 56. 57 Wald, G., 15, 19, 25, 31 Walker, M. A., 19, 28 Walker, T. J., 168, 203 Wall, P. D., 129, 133 Walls, G. L., 1, 2, 4, 5, 7, 8, 9, 11, 13, 31 Walters, V., 149, 203 Waltman, B., 494, 504, 517, 547, 549, 574 Warner, L. H., 162, 204 Warren, F. J., 19, 25, 28 Warwick, R. T . T., 90, 114 Watanabe, A., 273, 346, 354, 409, 482, 488, 491, 557, 560, 574 Watanabe, K., 47, 56 Watkins, W. A., 155, 158, 190, 204 Waxman, S. G., 463, 482, 489 Weale, R. A., 13, 31 Weber, E. H., 162, 163, 204 Weddell, G., 123, 129, 133 Weiler, I. J., 60, 77 Weiss, B. A., 171, 172, 173, 174, 204 Weitzman, S. H., 383, 488 Welsh, J. H., 9, 32 Wenz, G. M., 140, 141, 155, 204

Werblin, F. S., 33, 51, 54, 56 Werner, C. F., 219, 239 Werner, H., 77 Wersa11, J., 165, 195, 212, 213, 221, 222, 238, 239, 253, 262, 503, 52.4, 573, 574 Westby, G. W. M., 496, 558, 574 Westerfield, F., 180, 204, 228, 230, 240 Westerman, R. A,, 61, 77, 85, 86, 90, 98, 120 Weston, D. E., 189, 204 Whitear, M., 88, 120 Whittaker, V. P., 372, 491 Wiesel, T. N., 39, 50, 53, 61, 76 Williams, B., 87, 119 Willmer, E. N., 21, 32 Willows, A. 0. D., 482, 491 Wilson, D. M., 342, 346 Wilson, H. V., 207, 240 Wilson, J . A. F., 85, 86, 90, 98, 120 Wilt, F. H., 19, 22, 30, 32 Wing, A. S., 190, 192 Winn, H. E., 143, 145, 146, 147, 150, 151, 152, 156, 158, 160, 175, 187, 188, 193, 195, 196, 2.00, 204 Wisby, W. J., 92, 116, 177, 178, 181, 204 Witkovsky, P., 36, 52, 53, 57 Wittenberg, B. A., 3, 32 Wittenberg, J. B., 3, 32, 150, 194 Wodinsky, J., 153, 167, 171, 173, 174, 175, 193, 202, 204, 231, 239 Wohlfahrt, T. A., 163, 167, 179, 204, 205, 209, 226, 230, 234, 240 Wolbarsht, M. L., 26, 29, 41, 42, 43, 53, 55, 56, 57 Wolfe, J., 68, 76 Wolff, D. L., 174, 176, 177, 189, 205 Wood, L., 167, 170, 177, 178, 197, 232, 237 Wrede, W. L., 106, 120 Wiirzel, M., 351, 357, 371, 372, 373, 449, 452, 454, 486, 549, 569 Wyllie, J. H., 19, 28 Y

Yager, D., 26, 32, 61, 77 Yamada, E., 33, 49, 57 Yamashita, E., 39, 41, 55 Yanagisawa, K., 103, 117

AUTHOR INDEX

586 Yasargil, G. M., 276, 278, 285, 290, 291,

Yokota, S., 100, 116 Young, J. Z., 9, 32 Young, T., 51, 57

Z

Zipser, B., 530, 536, 566, 574 Zotterman, I., 165, 205 Zotterman, Y., 100, 117, 129, 133

SYSTEMATIC INDEX Note: Names listed are those used by the authors of the various chapters. No attempt has been made to provide the current nomenclature where taxonomic changes have occurred. 366, 373, 389, 451, 462, 468, 471, A 480,481, 483, 553, 554, 562 Abramis, 98 Astyanax mexicanus, 171 Acerina cernua, 177, 229 Atherinops asinis, 109 Acipenser fdvescens, 88 Adontosternarchus, 383, 423-425, 456, Auks, 339 461 B Aequidens pulcher, 62 Bagre marinus, 144, 148, 151, 159, 160 Agnatha, 208 Balistes, 144, 145 Alburnus lucidus ( A . alburnus), 229 Barracuda, see Sphyraena Alosa, 6 Ameiurus, 9, 87, 104, 106, 234, 515, 516, Bathygobius, 106 Bathylychnops, 13, 14 553 Bathystoma rimator, 107 A . nzelas, 102 Bathytrocetes, 5 A. nebulosus, 122, 229, 231 Batrachoididae, 158, 188 Amia, 3 Amphibia, 75, 96, 106, 154, 266, 267 Belonesox, 21 Beta splendens, 228 Amphioxus, 123 Bittern, 340 Amphiprion, 144 Blennius, 161, 167, 187 Anabantidae, 228, 230, 231 B. pholis, 122 Anabas scandens ( A . testudineus), 228 Blenny, see Blennius Anableps, 8, 14 Blue acara, see Aequidens pulcher Anacanthini, 98 Botia hymenophysa, 145 Anchovies, 4, 160 Brotulidae, 148, 149 Anglefish, 3 Bullhead, see Ictalurus Anguilla, 9, 82-84, 90, 91, 174, 180 Yellow bullhead, see Ictalurus natalis A. anguilla, 81, 83, 92, 109, 113, 228, Burbot, see Lota 230 A . iaponica, 94 C A . rostrata, 113 Caranx, 156, 160, 161 A. vulgaris, 113 Carassius, 11, 90, 98 Anguillidae, 174, 228 C. auratus, 92, 96, 110, 122, 171, 172, Anoptichthys iordani, 93, 94, 108 179, 218, 229 Apeltes quadracus, 144 C . auratus L., 276, 283, 304, 305, 322, Aplodinotus, 146, 187 323, 338 Apteronotus, 383, 448, 455, 456 C. carassius, 85, 90 Argyropelecus, 13 Carcharinus, 4 Astronotus ocellatus, 60, 108 C. leucas, 177, 232 Astroscopus, 349, 351, 362, 363, 365, Carp, 26, 35, 36, 38, 39, 4446, 49-51, 587

SYSTEMATIC INDEX

53, 66, 69, 70, 90, 91, 94, 95, 100, 101, 103, 181, 553, see also Carassius European, see Carassius carassius Japanese, 100 Swedish, 100 Carpiodes, 553, 554 Catfish, 91, 94, 95, 98, 100, 102, 136, 146, 147, 150-152, 158, 231, 494, 495, 497, 549, 562, 563, see also Ameiurus, Ictalurus, Clarias, Leptops, Parasilurus Gafftopsail, see Bagre marinus Japanese, 273 Marine, 10, 157, 186, 188, see also Galeichthys felis, Plotosus anguillariS Centrarchid, 22 Centronotus gunnellus, 122 Centropomidae, 48, 49 Cetacea, 138, 143, 495 Channa, 94 C . argus, 95, 100 Characidae, 20, 146, 187 Characinidae, 229, 231 Cichlasoma seuerum, 108 Cichlidae, 187 Clarias, 558, 559 Clown fish, see Amphiprion Clupea, 10, 166 C. harengus, 167, 231 Clupeidae, 167, 231, 234 Clupeoididae, 2 Cnesterodon, 84 Cobitidae, 229 Cobitis, 88 Codfish, 158, see also Gadus Atlantic cod, see Gadus morhua Colisa lalia, 228 Congiopodus, 151 Cormorants, 339 Coruina, 174, 179, 187 C . nigra, 22s. 230 Corydoras, 87 Cottidae, 228 Cottus, 180 C . bulbalis, 122 C. gobbio, 228 C. pnuo, 228 C . scorpius, 122, 165, 166, 228, 235 Crab, 104, 105

Crenilabrus C. griseus, 228 C . melops, 122, 228 Croacher, 152, 158, 160, 187 Croaching gourami, see Trichopsis uittatus Crustacea, 132 Ctenobrycon, 17, 18 Cyclopterus lumpus, 122 Cyclostoma, 2, 208, 223 Cymatogaster aggregatus, 228 C ynoscion C. nobilis, 158, 174 C . regalis, 218 Cyprinodont, 84 Cyprinus, 83, 86, 88, 94 Cyprodontidae, 228 C ypselurus heterurus, 2 Cyrpinidae, 11, 20-23, 51, 52, 84, 87, 208, 229, 231 D Dactylopteridae, 150 Dactylopterus, 149, 150 Dasyatis, 4, 553 Diplodus sargus, 112 Dogfish, see Acanthias, Mustelus, Scylliorhinus, Squalus, Scyllium smooth, see Mustelus canis spiny, see Squalus acanthias spotted, see Scyliorhinus caniculus spur, see Squalus acanthias Drumfish, 152, 158, 160, 187 fresh-water drumfish, see Aplodinotus

E Eel, 3, 34, 81, 95, 146 American, see Alosa rostrata Electric, see Electrophorus electricus European, see Alosa alosa Eigenmannia, 361, 382, 383, 409411, 415-417, 433, 450, 452, 479, 506508, 526, 533 Elasmobranchi, 2-4, 6, 8, 9, 12, 13, 21, 34, 39, 87, 89, 130, 143, 162, 165, 177, 181, 207, 208, 211, 215, 218, 219, 221-223, 225, 232, 234-236, 374, 451, 457, 494, 497, 500, 517, 550, 553, 555, 562, 563

SYSTEMATIC INDEX

Electrophoridae, 350, 382 Electrophorus electricus, 229, 349, 350, 360, 380-382, 388, 391, 392, 448, 451, 453, 457459, 471, 474476, 478, 480, 481, 483, 499, 515, 546, 550, 554, 555, 561, 562 Elephant nose fish, see Mormyridae Emboitocidae, 228 Entosphenus, 2, 94 Epinephelus striatus, 146, 147 E. guttatus, 151-153 Esocidae, 228 Esox, 82-84, 86, 90 E. estor, 96 E. Eucius, 215, 217 E. niger, 108

F Fitzroyia, 84 Flatfish, see Solea, Microstomus, Pleuronectes platessa, Platessa flesus Flying fish, see Cypselurus heterurus Flying gurnard, see Dactylopterus volitans Fundulus, 4 F. heteroclitus, 104, 228 G Gadid, 21 Gadidae, 174, 228 Gadus G. calkmias, 122, 187, 228 G. merlangus, 122 G. morhua, 176, 177 G. vivens, 122 Gaidropsarus, 174 Galeichthys, 159 G. felis, 112, 140, 144, 148, 188 Gasteropelecus, 342, 343, 482, 514, 545 Gasterosteus, 82-84 G. aculeatus, 93, 144 Gekko gekko, 46 Genomyrus, 437 G. donnyi, 437,439 Giguntura, 13 Ginglymostoma, 4 G. cirratum, 103, 105 Glandulocauda inequalis, 146 Gnathonemus, 228, 230, 437, 441443, 453, 456, 465, 477, 505, 521, 523,

589 529, 531, 537, 538, 542, 551, 554, 560 G. compressirostris, 441, 447 G. leopoldianus, 439 G. moorii, 447 G. numenius, 439 G. petersii, 439, 497, 498 Gnathostoma, 8, 208-211, 223-225 Gobies, 161, 187 Gobiidae, 106, 174, 228 Gobio, 82, 83 Gobius, 167, 174, 179 G. flavescens, 122 G. iozo, 219 G. niger, 228, 230 G. paganellus, 228 Goldfish, 11, 41, 42, 44, 60-70, 73-75, 90, 92, 97, 105, 267, 269, 273-275, 482, 545, see also Carassius Gourami, see Trichogaster Grouper, 150, 152, 174, 187 black, see Mycteroperca bonaci Nassau, see Epinephelus striatus Grunt, see Haemulon blue-striped, see H. sciurus white, see H. album Guitarfish, 553 Gymnarchidae, 350 Gymnarchus, 350, 355, 361, 393, 399, 415, 416, 419, 421, 432, 434, 435, 446, 450453, 456, 473, 494, 497, 512, 540, 556559, 562 G. niloticus, 350 Gymnocymbus ternetzii, 219, 226 Gymnorhamphichthys, 402, 408, 417, 478 G. hypostomus, 382 Gymnotidae, 229, 349, 351, 354, 355, 360, 361, 380-382, 384, 416, 432, 441, 451, 453, 456458, 467471, 474, 476, 494, 497-499, 503, 503, 506, 507, 509, 511, 512, 515, 516, 522, 524-526, 533, 534, 535, 541, 545, 547, 550, 553, 555-560, 562, 563 Gymnotus, 354, 390-397, 402, 417, 421, 451, 453455, 470, 471, 476, 479, 502, 509, 511, 533-535, 541, 545, 551, 555-558, 560 G. carapo, 351, 382

590

SYSTEMATIC INDEX

L

H Haddock, see Melanogrammus aeglefinus

Labridae, 228

Haemulon, 143, 145, 175, 186 H . album, 142 H . plumieri, 142 H . sciurus, 171 Hake, see Urophycis Hatchetfish, see Gasteropelecus Hemigrammus caudovittatus, 93, 229 Hepsitia stripes, 107 Herring, 160, see also Clupea Holocentrus, 158-160, 175, 553, 554 H . ascensionis, 151, 167, 171 H . rufus, 147, 151 Huso huso, 161 Hyborhynchus notatus, 92, 105 Hydrolagus afinis, 12 Hyperopisus, 437, 442 H . bebe, 439 Hyperprosopon, 168 Hyphessobrycon flammeus, 229 Hypopomus, 382, 383, 395402, 406, 409, 417, 428, 451, 453, 478, 479, 522, 525, 550, 560 H . artedi. 396

Lagodon rhomboides, 144 Lamna, 4 Lampetra, 86, 208 L . fluviatilis, 211 Lamprey, 3-5, 95, 224, see also Petromyzon Lebistes, 164, 174 L. reticulatus, 228 Lepomis, 22 L. gibbosus, 112, 122 Leptops, 554 Leuciscus L. dobula, 229 L. rutilus, 92, 101, 125-128, 223 Liparis montagui, 122 Lophius, 9 Lota, 21 L . lota, 82-84, 90, 97, 98, 242, 243, 249-251, 259, 260 L. vulgaris, 221, 222 Lucioperca sandra, ,177 Lungfish, see Protopterus sp. Lutjanidae, 48

I

M

Ictalurus, 9, 83, 90, 103, 109, 110, 166, 515 1. natalis, 102, 106, 107, 113, 122 1. nebdosus, 112, 113, 171, 174 ldus melanotus (Leucisctis i d u s ) , 229 Indian loach, see Botia hymenophysa Ipnops, 13 Isichthys, 437 I . henryi, 437, 439

Macro podus M . cupanus, 228 M . opercularis, 228, 230 Macrouridae, 149 Malapteruridae, 351 Makzpterurus, 441, 453, 454, 456 M. caballus, 439 M . rume, 441, 444446 Marcusenius M . isodori, 229, 230 M . plagiostoma, 439 Margate fish, see Haemulon album Melanogrammus aeglefinus, 158, 187 Menidia, 6 Mexican blind cave fish, see Anoptichthys jordani hlexican blind characin, see Astyanax mexicanus Af icrogadus tomcod, 103 Micropterus punctrilatus, 108 Microstomiis, 11 Midshipman, see Porichthys notatus

J Jack, see Caranx

K Kiaeraspis, 208 Killifish, see Fundulus sp. Knifefish, see Electrophortis electricus Kokanee, see Oncoihynchus nerka Kryptoperus bicirrhus, 500, 513, 515, 516, 559

SYSTEMATIC INDEX

Minnow, see Phoxinus bluntnose, see Hyborhynchus notatus freshwater, 187 Misgurnus, 94 Monomitpus, 148 Moray, see Muraenidae Mormyridae, 149, 350, 355, 360, 432, 436, 437, 439, 441, 446, 451, 453, 456, 465, 468, 472, 473, 475, 477479, 482, 494, 497, 498, 503, 505, 512, 515, 516, 521, 523, 524, 528, 529, 531, 532, 536-538, 540, 542544, 547, 548, 550, 551, 553-555, 557, 559, 560, 562, 563 Mormyrops, 437, 441, 442, 446, 473, 554 M . deliciosus, 439 Mugil, 2, 48, 174 Mugilidae, 48, 174 Mullet, see Mugil Mullidae, 174 Mullus, 174 Muraenidae, 82 Mustelus, 4, 12 M . cannis, 104, 218 M . mustelus, 112 Mycteroperca bonaci, 160 Mylinae, 108 Myliobatis, 4, 11 Myoinyrus macrodon, 439 Myxine, 208, 210, 211, 223, 225, 226 Myxinoididae, 210 Myxocephalus, 84, 145, 151

N Narcine, 369, 370, 373, 457, 495 Necturis maculosus, 46, 244, 247-250 Negaprion, 4, 12, 105 N. breuirostris, 178, 180, 181, 186, 232, 235 Nemacheilus barbatula, 229 Nerophis lumbriciformes, 122 Nolemigonus, 21 Notopterus ofer, 382 Nurse shark, see Ginglystoma cirratum

0 Oedemognathus exodon, 383 Omosidis, 13 Oncorhynchus, 10, 22, 109-111 0. kisutch, 92, 107, 109, 110

591 0. nerka, 22, 92, 105, 109 0. tshawytscha, 107, 109-111 Onos mustela, 122 Opisthoproctus, 13 Opsanus, 149, 150, 187 0. beta, 188 0. tau, 151, 157-160, 188 Orthosternarchus t a m n d u a , 383 Ostariophysi, 98, 108, 140, 152, 162, 164, 166, 170, 174, 179, 180, 227, 229232, 234, 235, 320, 321 Ostracodermi, 208

P Paralichthys, 8 Paramyomyrus, 437 P. aequipinnis, 437 Parasilurus, 88, 94 P. asotus, 102 Parrotfish, 144 Perca, 174 P. fluviatilis, 177, 229 Perch, see Perca common, see Perca fluviatilis Pike, see Lucioperca sandra stone, see Acerina cernua Percidae, 11, 174, 177, 228 Periophthalamus koelreuteri, 228 Petrocephalus, 437, 441, 442 P. sauvagei, 439 Petromyzon, 21 P. marinus, 108 Petromyzontidae, 210 Phoxinus, 63, 81-86, 88, 92, 93, 101, 106, 108, 164, 219, 234, 364 P. laevis, 107, 122, 123, 179, 209, 226 P. phoxinus, 81, 91, 93, 94, 214, 229, 230 Pigfish, see Congiopodus Pike, see Esox Pimephales notatus, 229 Pinfish, see Lagodon rhomboides Pipefish, see Syngnathus louisianae Platessa flesus, 226 Platichthys flesus, 122 Platyrhinoides triseriata, 554 Platyroctegen, 5 Platytrocetes, 5 Pleuronectes, 11 P. platessa, 122, 226

592

SYSTEMATIC INDEX

Plotosus, 550 P. anguillaris, 101, 102 Poeciliidae, 21, 174 Pollachius pollachius, 176 P. virens, 176 Pollack, see Pollachius Pomadasyidae, 142, 145, 174, 186 Porichthys, 137, 149, 150 P. notatus, 150, 157, 175, 188 Porotergus, 383 Priacanthidae, 143 Prionotus, 8, 87, 89, 149, 150 P. carolinus, 103 P. scitulus, 151 Protopterus, 271 Pseudopleuronectes, 8 P. americanus, 218 Puffer, 3, 82, 144 Pyrrhulina rachoviana, 229

R Rabdolichops longicaudatus, 382 Raja, 8, 130, 131, 225, 348, 350, 362, 472, 495, 500, 517, 547, 557-559 R. clavata, 216, 219, 220, 225, 233, 234 R. eghntaria, 375, 376 R. erinacea, 375-379 Rana, 22 R. esculenta, 106 R. palustris, 226 R. sylvatica, 226 R. temporaria, 106 Ray, 4, 81, 132, 143, 212, see abo M yoblatis, Dasyatis electric, see Torpedo marmorata thornback, see Platyrhinoides triseriata Rhamphichthys, 409 R. rostrattis, 382 Roach, see Leuciscus rutilus, Rutilus rutilus

5 Salamander, see Triturus, Salamandra, Necturus maculosus Salamandra, 106 Salmo, 11 S . fario, 553, 554 S. gairdneri, 110, 122 S . irrideus, 92, 109 S . salar, 101

Salmon, 10, 113, 189, 207 atlantic, see S. salar chinook, see Oncorhynchus tshwytscha coho, see 0. kiszctch pacific, see Oncorhynchus sockeye, see 0. nerka South American, see Hemigrammus caudovittatus Salmoniidae, 23, 95 Salvelinus, 11, 23, 123, 124 Sargus, 174, 179 S . annularis, 229, 230 Scardinus, 21, 22 S. erythrophthalamus, 107 Scaridae, 144 Scatophagus, 16 Schreckstoff, 185 Sciaenidae, 136, 137, 146, 151, 152, 158, 187, 229 Scombroid, 2 Scorpaenidae, 147 Sculpin, see Myoxocephalus, Cottus Scup, see Stenotomus chrysops Scyliorhinus, 8, 11 S. canicula, 178 S . stellaris, 112 Scyllium, 131, 132, 215 Sea bass, see Cynosion nobilis Sea horse, see Hippocampus Sea robin, 150, 151, 188, see also Prionotus, Trigla slender, see Prionotus scitulus Searsia, 5 Sebasticus, 147, 149 Selachians, 104 Semotilus, 181 S . atromaculatus, 122, 234 Serranidae, 48, 146, 174 Serrasalminae, 108 Shark, 21, 41, 81, 143, 178, 181, 186, 553, 555, 559, see also Ginglymostoma, Negaprion, Scyliorhinus, Sphyrna, Squatina, Mustelus bull, see Carcharhinus leucas lemon, see Negaprion brevirostris squaloid, see H ydrolagus affinis, Raja richardsonii Siluridae, 229, 231, 425 Siluroidea, 147, 163 Silversides, see Menidia Snapper, 174

593

SYSTEMATIC INDEX

Solea, 11 Sparidae, 144, 174, 229, 231 Sphenodon, 5 Sphyraena, 160 Sphyrna, 4, 105 Spinachia oulgaris, 122 Squalina, 4 Squalus, 12, 73 Squirrelfish, 140, 150, 152, 153, 187, 188, see also Holocentrus Stargazer, see Astroscopus Steatogenys, 383, 396, 402, 403, 405, 406, 413, 417, 451, 453, 454, 549, 551 S . elegam, 382, 404, 407-409 Stenesthes, 84, 86 Sternachidae, 349, 351, 361, 381-383, 416, 424, 433, 450452, 455, 479 Sternarchella, 383 Sternarchogiton, 383 Sternarchorhamphus, 383, 421, 423 S. oxyrhynchus, 383 Sternarchus, 354, 383, 416, 419423, 498 S . albifrons, 417 Sternopygidae, 351, 382, 451 Sternopygus, 354, 361, 382, 383, 409, 411-417, 419, 421, 433, 450, 452, 453, 479, 533 Stickleback, see Gasterosteus aculeatus Stingray, see Dasyatis Stomatorhinus, 437 S . corneti, 439 Sturgeon, see Acipenser fulvescens beluga, see Huso huso Surfperch, see Hyperprosopon Syngnathus louisianae, 145

Tilapia, 74 Tinca tinca, 94, 229, 276, 283, 303 Toadfish, 151-153, 187, see also Opsanrrs Tomcod, see Microgadus tomcod Torpedinidae, 350, 362, 369-371, 374, 451, 453, 456, 474, 483, 496, 562 Torpedo, 348, 350, 362, 371-374, 388, 449, 454, 460, 474, 480, 481, 548, 553, 561 T . marmorata, 161, 350, 371, 372 T . nobiliana, 371 Trichogaster, 89 T . leeri, 228 T . trichopterus, 228 Trichopsis uittatus, 137, 143 Triggerfish, 143, 145, 146, 187, see also Balistes, Rhinecanthus rectagulus Trigla, 136, 149, 150 Triglidae, 88, 150 Trout, 6, 63, 66, see also Saloelinuj, Salnio brook, 108 brown, 108 rainbow, 109, see Salmo irrideus, S . gairdneri Tuna, 105 yellowfin, sce Thunuzrs nlbacares Turtle, 49

U Umbra, 180, 230 U . limi, 228 U . pygmaea, 228 Unzbridus, 4 Uranoscopidae, 351, 361 Uranoscopus, 9 Urophycis, 87, 89

T Teleosti, 2-7, 9, 11, 13, 25, 34, 87-89, 97, 124-130, 132, 142, 143, 146, 152, 170, 227, 232, 243, 266, 267, 301, 451,457, 465, 468, 553 Tench, see Tinca tinca Tetraodontidae, 82 Tetraodontiformes, 144 Tetrapoda, 152 Therapon, 137, 147, 151, 157 Three-spined stickleback, see Gasterosteus Thunnus albacares, 175, 176 Tigerfish, see Therapon

W W'iiiteria, 13 X Xenoinystus, 382 Xenopus, 247, 258, 261 X. laeuis, 337

Z Zeus, 149 Zoarces uiuiparus, 122

SUBJECT INDEX A Accessory organs, 406, 408, 424, 425, 451, 457, 477 caudal filament, 381, 432 chin appendage, 437, 439 chin organ, 424, 425, 451, 456, 461 dorsal filament, 383, 416 postopercular organ, 406, 409 rostra1 organ, 402, 404, 405, 425, 457, 476, 550 submental organ, 405-409, 413 Acetylcholine, 460 Acetylcholinesterase, 459 Acousticolateralis system, 169, 21 1, 262, 500, 503, 567 Acoustics, see also Sounds, Auditory nerve, Auditory sac communication in fish, 183-189, 227 underwater, 135, 137-142, 155, 189191 Adenosine triphosphatase, 459 Alarm substances, 107-109 Ainacrine cells, 34, 47 Ampullae of Lorenzini, 131, 132, 164, 210, 211, 216, 219, 374, 494, 500, 501, 503, 517-520, 543, 544, 547, 550, 561 Aphakic space, 4, 5 Aqueous humor, 3 ATPase, see Adenosine triphosphatase Auditory nerve, 163, 164, 166, 170, 210, 213, 215, 225, 231, 266, 267, 269, 271, 273, 277, 278, 280, 315-320, 322, 324-329, 334, 338, 341 Auditory sac, 207, 500

B Barbels, 87, 88, 100, 103, 113, 114, 437 of catfish, 100, 102, 103 Basilar membrane, 180, 230, see also Ear Behavior, 101, 105, 123, 124, 137, 184, 186, 191, 480, 496, 555-561 aggressive, 185, 558 594

alarm, 142, 187, 188, 266, 278, 329, 332-337, 339-343 approach, 181 avoidance, 167, 171, 177, 338440, 441, 557, 558 courtship, 106, 143, 558 escape response, 123, 124, 181, 266, 335, 337, 339, 482, 556 feeding, 105, 107, 143, 185, 188 fighting, 106, 107 light avoidance, 124 parental, 79 reproductive, 102 schooling, 106, 107, 137, 143, 185, 188, 189, 191 searching, 104, 105, 122 social, 108 sonic, 157, 186-188, 191 spawning, 185, 187, 188 reproductive, 102 visual, 59-75 Brain, 95, 97, 98, 110, 112, 215, 231, 236, 266, 267, 269, 278-280, 285, 286, 311, 315, 321, 329, 331, 337, 338, 340, 341, 460, 553, 568 forebrain, 91, 105 morphology of, 89-91, 554, 555 Branchial muscles, 456, see also Gills Branchial nerves, 100 Bulbospinal relay system, 474, 475

C Canal organs, 242, 244-246, 248, 249, 251, 252, 258, 261, 499, see also Mechanoreceptor, Lateral line organs, Neuromast organ, Epidermal organs goblet cells, 246, 247 mantle cells, 246 mulberry cells, 248 supporting cells, 245, 249 Canalis transversus, 320, 321 Carr-Price reaction, 18

SUBJECT INDEX

595

Caudal filament, 381, 432 Caudal peduncle, 441, 497 Central nervous system, 92, 97-100, 105,

124, 127, 129, 131, 164, 182, 214, 217, 242, 329, 330, 343, 461, 471, 478, 481, 555, 560, 561, 567 Cerebellum, 110, 267, 269, 270, 554, 562 Cerebral cortex, 305 Chemical senses, 104 orientation by, 109-1 14 reproductive behavior and, 105, 106 Chemoreception, 79-114 biological aspects of, 104-114 chemical perception of foods, 104, 105 discrimination of body odors and schooling, 106, 107 chemical sense organs, anatomy of, 80-91 chemoreceptive functions, behavioral studies of, 91-94 alarm substances, 107-109 orientation by chemical senses, 109-

114 repellents, 109 Chemoreceptor responses, electrophysiological studies of, 94-100 Chemosensory system, 80 chemorensory organ, 109 Chin organ, 424, 425, 451, 456, 461 chin appendage, 437,439 Cholinesterase, 373, 430, 483 Cochlea, 180, 208, 227, 565, 566 Command system, 461, 467, 468, 471,

472, 474-476, 479-482 Corpus cerebelli, 553, 555 Corpus striatum, 305 Crista, 208-211, 217, 221 Crus commune, 208, 211 Cupula, 242, 244, 247-249, 251, 252,

258, 261, 499 Cutaneous structures, see also Skin nerves, 124, 128, 130 sense, 162, 167 sensory system, 163, 234

E Ear, 136, 140, 164,218,227 ampullae, 131, 132, 164 basilar membrane, 180, 230 canalis transversus, 321 cochlea, 180, 208, 227, 565, 566 crista, 208-210, 217, 221 crus commune, 211 ductus endolymphaticus, 207, 208 endolymph, 209, 211, 215, 218, 320 fenestra sacculi, 165 inner ear, 162-166, 170, 173, 174, 181-183, 208, 234, 252, 253, 258, 261 lagena, 163, 165, 166, 208, 209, 214, 223, 226, 2 3 4 2 3 6 macula, 165, 180, 210, 219, 221-226, 235 middle ear, 162, 182, 544 otolith, 164, 180, 208, 214, 223, 226, 234-236 sacculus, 163-165, 175, 208-210, 218. 219, 221, 223, 225, 226, 235, 274, 320, 321 sagitta, 164, 209 semicircular canals, 130, 164, 208, 210, 214-217, 219, 221 utriculus, 165, 208, 209, 218, 219, 221-223, 226, 321 Weberian ossicles, 145, 162-166, 174, 180, 182, 227, 231, 232, 234, 320, 321 Electric organs, 347484, 495, 496, 556, 560-563, 567 adaptations and convergent evolution of, 450-455 embryonic origin and development of,

455-457 evolution of, 561-564 of freshwater fish, 380-448 of marine fish, 362-380 membrane properties of, 4 5 7 4 6 2 neural control of, 4 6 0 4 8 3 Electrocytes, 352, 353, 355-460, 473,

475-481, 483

D Denticles, 142, 143, 145, 156 Dorsal filament, 383, 416 Ductus endolymphaticus, 207, 208

synchronization of activity of, 4 7 5 4 7 8 as experimental material, 457-460 Electrolocation, 495, 496, 543, 552, 556 Electromotor system, 477, 483

596

SUBJECT INDEX

Electroreceptor, 123, 257, 456, 484, 547, 552, 553, 558, 561-563, 566, 567 ampullae of Lorenzini, 131, 132, 164, 210, 211, 216, 219, 374, 494, 500, 501, 503, 517-520, 543, 544, 547, 550, 561 ampullary, 494, 496, 498, 500 in gymnotids and mormyrids tonic, 503515 characteristics in catfish of tonic, 515516 distribution of, 496-503 phasic, 494, 496, 497, 502, 520-541, 543, 548, 551, 559-561, 565, 567 pit-organs, 103, 549 tonic, 494, 496, 497, 500, 502-520, 524, 545, 548, 550, 556, 559, 561, 567 tuberous, 494, 496, 520, 540 Electrosensory system, 354, 385, 408, 424, 450, 452, 478, 483, 495-497, 552-558, 561-564, 567, 568 evolution of, 561-564 Eminentia granulosa, 555 Epidermal organs, 243, 244, 247, 248, see also Canal organs, Lateral line organs, Mechanoreceptors, Neuromast organs Eye, 1, 3, 4, 6-9, 11-14, 35, 36, 39, 69, 70, 73, 74, 82, 83, 104, 214, 215, 217, 219, 226, 253, 273, 278, 329, 341, 382, 437, 544, 553, 556, 563 choroid, 3, 4, 6, 12, 14 choroid gland, 3 ciliary body, 6 cornea, 2, 3, 5, 6, 8, 13, 14, 35-38 dilator, 9 irideal sphincter, 9 iris, 3-6, 8, 13, 14 lens, 3-8, 13, 14 oculomotor, 2 protractor, 3 structure of, 1-5 tubular, 8, 13, 14

F Facial lobes, 91 Facial nerve, 80, 87, 100-102, 164, 371, 500 Falciform process, 3

Fenestra sacculi, 165 Fins, 87, 89, 103, 124, 141, 153, 156, 186, 214, 215, 219, 278, 306, 343, 381, 382, 416, 437, 441, 497, 499, 515, 516 anal, 124, 381, 382, 385, 437, 556 caudal, 267 dorsal, 123, 382, 425, 432, 437, 556 fin spines, 137, 142, 144, 437 pectoral, 87, 145, 342, 343, 382, 437, 482 pelvic, 132 ventral, 437 First cranial nerve, see Olfactory nerve Foliaceous cell, 86

G Gas gland, 3, see also Swim bladder Genital papilla, 381 Gills, 82, 87, 276, 331, 335, 406, 409, see also Branchial muscles arches, 80, 87 rakers, 87, 100 slits, 161, 278 ventilation, 125 Glossopharyngeal nerve, 80, 87, 100, 164, 371 Gustatory organ, 87-89 receptors, 100-104 sense, 91-94, 103, 113 system, 105, 553 H Hair cell, 182, 211, 213, 214, 222-225, 241, 242, 247-253, 255-258, 261, 320, 566 kinocilia, 168, 169, 212, 213, 221, 222, 224, 245, 248, 253, 254 sensory excitation in, 248-255 sterocilia, 212, 225, 245, 248, 253-255 Hearing, 142, 163, 227-236, see also Acoustics bongo drum theory of, 180 capacities, 170-181 evolution of, 182 place theory, 180, 190 pitch discrimination, 229-234 volley theory, 180 Heart, 461, 482, 559 Hemoglobin, 15

597

SUBJECT INDEX

Herring’s theory, 52 Hodgkin-Huxley equation, 479-536 Hunter’s organ, 385, 457, 474, see also Canal organs, Epidermal organs, Lateral line organs, Mechanoreceptors Hypoglossal nerve, 152 Hypophysis, 98 Hypothalamus, 90, 105

I Inner ear, 162-166, 170, 173, 174, 181

183, 208, 234, 252, 253, 258, 261 Interocular transfer, 69-71 Intragemmal plexus, 88 Iris, 3-6, 8, 13, 14

J Jamming avoidance response, 417, 432,

478,557, 559, 560 Jaws, 82, 223, 273, 278, 341, 382, 437

K Kinocilia, see Hair cell Klinokinesis, 112

L Labyrinth, 163, 164, 207-236, 321 structure of the, 207-214 Lagena, 163-166, 208, 210, 219-221,

223, 225, 226, 320, 321 Lateral line, 124, 132, 142, 162, 167,

176, 177, 183, 184, 207, 227, 235, 501, 558, see also Canal organs, Epidermal organs, Mechanoreceptors, Neuromast organs hearing and, 167-170 mechanoreceptors, 241-262, 562 nerves, 123, 124, 340, 497, 498, 500,

530, 554, 558 organs, 101, 123, 124, 131, 132, 170,

181, 182, 234, 241-248, 258, 259, 261, 263, 567 system, 163, 164, 169, 173, 177, 182, 257, 261, 340, 563 Lens, 3, 5-8, 13, 14 pad, 14 Lips, 383

Lissmann’s hypothesis, 562 Lithocyst, 218

M Macrosomatic, 80, 82 Microsomatic, 80, 82 Macula, 165, 180, 210, 219, 221-226,

235 Mauthner cell, 163, 166, 181, 266-344,

481, 482, 563 A, unit, 286-288, 290, 292-294, 296, 297, 301, 302, 304, 306, 308-312, 314, 335, 342 function of, 310-312 A? unit, 286, 287, 288, 290, 293, 294, 309, 313 system, 266, 267 activation of Mauthner neuron, 271277 anatomy of Mauthner neuron, 267-271 excitation of, 315-331 function of, 331-344 glia cells, 319, 325 group A, 286, 287, 289, 292, 302, 337 group B, 286, 287, 289, 292, 302, 337 Mauthner reflex, 278-284, 286, 287, 293, 295, 306, 308, 315, 330, 331, 338343 minimum discrimination time, 282284, 288, 294-297, 310-315, 338 Mechanoreceptor, 123, 125, 128, 182, 241-262, 558, 562, see also Canal organs, Epidermal organs, Neuromasts, Pacinian corpuscle Medulla oblongata, 91, 167, 231, 261,

266-269, 270, 273, 277, 281, 371, 411, 461, 465, 469, 471474, 476 Melanophore, 12, see also Pigment Memory, 92, 112 Mesencephalon, 91, 461 Midline nucleus, 472, 474, 481 Mitral cells, 90, 97 Mouth, 80, 87, 267, 382, 383, 406, 409, 437 Mucosal potentials, 94, 95 Muller cell, 34, 47

N Nares, 382, 383, 437 Nernst equation, 389

598

SUBJECT INDEX

Neuromast organ, 168, 182, 207, 247250, 259, 502, see also Canal organs, Lateral line organs, Mechanoreceptors Nictitating membrane, 2 Ninth cranial nerve, see Glossopharyngeal nerve Nose, 81, 83, 92, 96, 99, 104-106, 110, 111 Nucleus princeps trigemini, 274

0 Occipital nerve, 152 Oculoniotor muscles, 2 Oculomotor nerve, 9, 462, 553, 554 Ociilomotor neurons, 471, 472 Odors attractant, 112 predator, 108 repellent, 109 Olfaction, olfactory bulb, 80, 84, 89, 90, 96, 97, 99, 108111, 554, 567 center, 89 epithelial system, 84, 86 epithelium, 82-84, 86, 87, 93-95, 98, 99, 106, 107 knob, 83, 86 lobe, 90, 93, 106 nerve, 80, 84, 89, 90, 94-96, 108 activity, 95, 96 organ, 81-87, 104, 253 pits, 81, 82, 84 receptor, 82, 84-86 rosette, 81, 82 sac, 94, 96, 97, 105, 106 sense, 91-93, 104, 106-108 system, 81, 94-100, 105 tract, 80, 90, 91, 94, 97-99, 104 electrical activity and central regulatory system, 97-100 Operculum, 145, 273, 278, 280, 341, 406 Optic lobes, 110 Optic nerve, 13, 14, 33-35, 38 Optic sense, 104 Optic tectum, 39, 61, 68, 70, 72, 74, 553, 554 Orthokinesis, 112 Otolith organs, 164, 180, 208, 214, 223, 226, 234-236

function of, 218-226 structure of, 209-211 Otocyst, 218

P Pacemaker neurons, 461, 4 6 9 4 7 6 , 478, 479, 482 Pacinian corpuscle, 253, 255 Palatal organ, 87, 100 of carp, 100, 101 Palatine nerve, 87 Pars inferior, 164, 219, 234 Pars superior, 164, 219, 234 Pectoral girdle, 145 Perilyniphatic fluid, 167, 174, 320 Pharynx, 80, 87, 142, 143 Photomechanical movements, 9-12 Photopigments, see Visual pigments Photoreceptor, 31, 40, 44, 46, 47, 49-51, see also Retina, Vision early and late receptor potential, 43, 44 response of, 4 3 4 7 Pigments, 9, 12, 14, 26, 43, 44, 52, 53, 87, see also Retina, Vision black, 13 epithelium, 4, 9-12, 39 falciforni process, 3 melanophore, 12 opsin, 14, 15, 18, 23, 24 photophores, 13 porphyropsin, see Visual pigments retinal melanin, 9, 11 rhodopsin, see Visual pigments Pit organs, 103, 549 Placode, 207 Pneumatic duct, 146 Postopercuhr organ, 406, 409 Protractor muscle, 3 Pseudobranch, 3 Pupil, 3, 4, 8, 9, 11, 12

R Receptor potential, 248-250 origin of, 253-255 properties of, 251-253 Receptors auditory (acoustic), 565 bimodal, 128 cutaneous, 127

599

SUBJECT INDEX

electroreceptors, 123, 257, 347, 456, 484, 517, 552, 558, 561-563, 566, 567 exteroceptor, 565 mechanoreceptors, 128, 495, 562, 563, 565, 567 multimodal, 129 stretch, 132 thermoreceptors, 121-132 touch-temperature, 127 visual or optic, see Vision Repellents, 109 Rete mirabile, 3 Retina, 3, 5, 6, 8-14, 19, 26, 33-39, 41, 44, 48-50, 60, 70, 74, 75, 556, 557 accessory retina, 13, 14 electrophysiology of, 33-53 C response, 48-50 electroretinogram, 3 5 4 0 component analysis of, 37-39 L response, 48-50 localization of components of, 3 9 4 0 luminosity type, 48, 49 as mass response, 35-37 S cell, 49, 50 S response, 48-50 fovea, 4-6 ganglion cells, 3335, 39, 47, 50, 52, 53, 63, 75 receptive field, 4 1 4 3 response of single, 4 0 4 3 horizontal cells, 34, 47-50 inner nuclear layer, 34, 39, 40, 50, 51 responses in, 47-50 S cells, 49, 50 S potential, 40, 47-52 Retinomotor movements, 9, see also Photomechanical movements Retractor lentis muscle, 6 Retractor muscle, 3 Retroorbital muscle, 279 Rheotaxis, 112, 113, 142 Rostra1 accessory organ, see Accessory organs

5 Sacculus, 163-165, 175, 208-210, 218, 219, 221, 223, 225, 226, 235, 274, 320, 321 sagitta, 164, 209

Sach's organ, 385, 457, 474 Schooling, 106, 107, 137, 160, 161, 185, 188, 189, 191 Sclera, 2, 14 Segmental cutaneous system, 123-125 Semicircular canals, 130, 164, 208, 210, 219, 221 function of, 2 1 4 2 3 6 structure, 209 Sherrington concept, 471 Sinus impar, 320, 321 Skin, 243, 382, 501-505, 513, 515, 524, 529, 532, 540, 541, 544548, 552, 559, 563, 564, 567, see also Cutaneous structures Skull, 145, 147, 148, 166, 167, 177, 215, 222, 2-33, 234, 277 Snout, 82, 382, 383, 397, 409, 563 Sonic drumming muscle, 145 Sonic mechanisms, 142-154, 161, 482, 563 Sonic muscles, 146-151 Sonic organ, 164-170 Sonic species, 137-139, 158 Sounds, 154-162 detection mechanisms of, 136, 162182, see also Hearing hydrodynamic and swimming, 142, 153, 154, 186 production, 135-162 stridulatory, 142, 156, 157, 186 swim bladder, 157-160, 184 Spinal system, 315, 330, 331, 381, 412, 417, 424, 441, 465, 472474, 476, 479 anatomy of, 297-310 circuitry, 266, 281, 293-295, 297, 471, 476 cord, 124, 266, 271-273, 277-279, 281, 282, 284-286, 288-290, 292, 293, 295, 297, 298, 300, 303-305, 307, 311, 317, 327, 331, 33&338, 340343, 411, 412, 416, 417, 419, 425, 461, 468, 470, 471, 473, 475, 479 nerves, 80, 87, 89, 103, 152, 381, 385, 406 responses, 28C294 Spindle cells, 88 Statocyst, 218

SUBJECT INDEX

600 Steller ganglion cell, 34 Stretch receptors, 132 Stridulatory mechanisms, 142-145 Stridulatory sounds, 142, 156, 157, 186 Stridulatory teeth, 137, 142, 186 Submental filament, see Accessory organs Swim bladder, 3, 136, 142, 143, 162, 163,

177, 181-183, 186, 227, 231, 232, 234, 261, 312, 320, 329, 338, 342 elastic spring, 148, 150, 152 hearing and, 166, 167 red glands, 150 sound mechanisms, 145-153 sounds, 157-160, 184 Synaptic noise, 310, 311, 330

T Tail, 272, 273, 277, 278, 280, 281, 283,

286, 335, 376, 412, 455,

306, 321, 322, 329-331, 334, 337, 338, 340, 341, 354, 374, 382, 396, 397, 404, 409, 411, 415, 423, 425, 427, 432, 442, 456, 477, 482, 549, 556, 557 Tapetum lucidum, 3, 4, 9, 11-13 choroidal, 11, 12 retinal, 11 Taste buds, 87-89, 100, 102, 103 Taste receptors, 89, 100, 427 Teeth, 382, 383, 437, 439 Telencephalon, 97 Temperature receptors, 121-132 electrophysiology of, 125-132 in Elasmobranchs, 130-131 in Teleosts, 125-130 Tenth cranial nerve, see Vagus nerve Thermal sensitivity of fishes, 121-123 acclimation temperature, 124 Thyroxine, 22 Trigeniinal nerve, 86, 87 Trunk, 273, 277, 278, 281, 282, 285, 286, 322, 329, 331,332

U Utriculus, 165, 208, 209, 218-220, 222,

223, 226, 321 lapillus, 209

V Vagal lobes, 80, 87, 91, 105, 553, 554 Vagus nerve, 80, 89, 164, 371, 500 Vertebral column, 167, 276 Vestibular system, 275, 277, 320-322,

324, 329, 338, 339, 342 Vision, 8, 183, 503, 556, 563 accommodation, 5-8, 13 achromatic, 48 adaptation, light and dark, 8-12, 35-

39 binocular, 4, 5, 7, 13, 73 C cell, 50 C response, see chromaticity type chromaticity type, 48-50, 52 color, 25-26, 41, 43 retinal mechanisms of, 51-53 dichromatic, 48 emmetropic, 6-8 monocular, 70 myopic, 6, 7 opponent color theory, 61 retinoscopic, 7 Visual behavior configurational properties of shapes,

64, 67 experimental analysis of, 59-75 perceptual equivalence and change in spatial position, 68-72 relative discrimination weaknesses, 61-

64 selective attention, 72, 74 toward unified outlook on, 74-75 Visual pigments, 14-26, see also Pigments color, 25-26 methods of study of, 15-18 multiplicity of opsins, 23-25 photochemistry of, 14, 15 photopigments, 13 porphyropsin, 15, 18-23 retinene, 14-18 rhodopsin, 9, 15, 18-24 Vitreous humor, 3

W Weberian ossicles, 145, 162-166,

174, 180, 182, 227, 231, 232, 234, 320, 321