Tovuu. Adviser College O F Optometry ACKNOWLEDGMENTS

RESPONSE CHARACTERISTICS OF SINGLE NEURONS IN THE VISUAL CORTEX OF THE VIRGINIA OPOSSUM Presented in Partial Fulfillment of the Requirements of the Degree Doctor of Philosophy in the Graduate School o f The Ohio State U niversity By Jerry Lee Christensen, B.Sc., M.Sc. The Ohio State University 1969 Approved by tovUU. Adviser College o f Optometry ACKNOWLEDGMENTS I wish to thank my Adviser, Dr. Richard M. H ill, whose patient guidance made this study possible. This study was also supported in part by U.S.P.H.S. Grants NBO6983 and NB05416 and by an N.I.H. predoctoral fellowship. I also want to thank Dr. Jess B. Eskridge whose inspiring teaching made me want to study the subject of physiological optics further and eventually go on to make this my career. I also must acknowledge the tolerant understanding of my wife, Betsy, without whose help this would have been impossible. 11 VITA June 24, 1940 ......................................... Born - Columbus, Ohio 1964 ............................................... .............. B.Sc. Optometry, The Ohio State U n ive rs ity, Columbus, Ohio 1965*1968 ....................................................N.I.H. Predoctoral fellow, The Ohio State University, Columbus, Ohio 1966 .............................. .................. ............ M.Sc., The Ohio State University, Columbus, Ohio 1968-1969 .................................................... Teaching Associate, College of Optometry, The Ohio State U n iv e rs ity , Columbus, Ohio PUBLICATIONS "Receptive Field Properties of Single Cells in the Dorsal Lateral Geniculate Nucleus of the Guinea Pig". British J. of Physiological Optics. In press. __ "A Review of the Anatomy and Neurophysiology o f the Opossum (Didelphis virginiana) Visual System". Am. J. Optom and Arch. Am. Acad. Optom., In press. FIELDS OF STUDY Major Field: Physiological Optics Studies in Visual Neurophysiology, Professor Richard M. H ill. i l l TABLE OF CONTENTS Page Acknowledgments ....................................................... ............ .. ............ II V ita .......................................... l i t List of Tables ................ v List of Figures ................................... vi Introduction ............................ ............................................................... 1 Chapter I . H is to ric a l Review A. The visual system of the opossum ............................ 3 B. Response o f c o rtic a l units ............... 13 I I . Methods and Procedures ..................................................... A. O ptical stimulus system ................................................ 2k B. Recording sys te m............................................................ 25 C. The experimental animal and its preparation .. 27 0. The experimental procedure........................................ 29 E. Histological methods...................................................... 32 III. Results ........................................ 33 IV. Discussion ............ 43 Bibliography .......... 53 iv TABLES — - Page Table 1 Receptive Field Diameters and Eccentricities .... 58 Table 2 Ocular Dominance....................................................................... 59 v FIGURES Page Figure 1 Photograph of the stimulus system ........................... 61 Figure 2 Schematic of the stimulus system ............................. 63 Figure 3 Photograph of the recording apparatus ............ 65 Figure 4 Schematic of the recording apparatus..................... 67 Figure 5 Photomicrograph o f an electrode track ............... 69 Figure 6 Photograph of nerve impulse recordings ............... 71 Figure 7 Plot of an "on" receptive field ........................ 73 Figure 8 Plot of an "off" receptive field ............................ 75 Figure 9 Plot of an "on-off" receptive field ...................... 77 Figure 10 Graph of spikes versus area ...................................... 79 Figure 11 Number o f nerve impulses produced in various regions of a receptive field ............................ 81 Figure 12 Graph of average receptive field diameters .... 83 Figure.13 Plot of a "binocular" receptive field................. 85 Figure 14 Number o f nerve impulses produced in various regions of a "binocular" receptive field ........... 87 Figure 15 Histogram o f ocular dominance classes ................. 89 yi INTRODUCTION The purpose of th is Investigation was to analyze the response characteristics of single cells In the visual cortex of the opossum, Didelphis vlrginlana by using microelectrodes to record the a c tiv ity o f c o rtic a l c e lls . Such methods have been employed on a large number of placentals, but never on a marsupial. The cell responses of this marsupial are those of a very primitive cortex in a modern representative of the marsupials which may possess the earliest expression of a mammal I an-type neocortex. The response characteristics of these marsupial cdTls-were compared and contrasted to those of the rabbit, the cat, and the monkey ( 1 ,2 ,3 ,4 ). The receptive fields of the cortical cells have been mapped and their main parameters quantified for comparison with other mammals. The features analyzed were the following: 1) geometry, (i.e . the response zones and diameters of the receptive fields and the temporal relation of the nerve Impulses to the onset and cessation of the light stimulus); 2) static and dynamic thresh olds; and 3) special consideration of those cells which are influ enced by both of the eyes. C ortical c e lls o f the rab b it possess certain specialized coding features such as habituation and directional selectivity. The opossum was studied for evidence of these response character- 2 I sties. Habituation, a diminution of response to a repeated stimulus, and directional selectivity, (a condition in which a c e ll responds maximally to a spot of lig h t moving across its receptive field in a certain direction and minimally to a spot moving the reverse direction) have been previously demon strated in cells in the superior colliculus of the opossum (5). it was of interest as well to determine If a functional cortical columnar arrangement as demonstrated by Mountcastle (6) and Hubei and Wiese1 (3,7) existed in this primitive neocortex. All cells were tested to determine If they could be stimu lated by either eye or both eyes. Giolli (8) reported that 80% of the optic nerve fibers of the opossum cross in the optic chiasm and so a certain degree o f b tnocu larity would be expected. Furthermore, H ill and Goodwin (5) found that about 20% of the cells in the superior colliculus of the opossum could be driven by both eyes. HISTORICAL REVIEW This review Is divided into two sections, the first being a coverage of anatomical and physiological studies of the visual system o f the V irg in ia opossum, and the second the various Inves tigations of the response properties of single cells in the visual cortices of the cat, monkey, and rabbit. The Visual System of the V irg in ia Opossum The transitional character of the opossum visual system, in corporating features both of the reptilian-like monotremes and the Eutherian3 mammals, has drawn interest since the last century (9). Paleontological evidence suggests that the opossum has not changed appreciably since the Eocene period (circa, sixty million years ago), this form represents, as w e ll, an example of a very early mammal ian neocortex. The earliest investigations on the common North American opossum (DIdeiphls virginiana) were anatomical. According to a. The Eutheria (i.e. the "placentals") are the highest infra order of the class Mammalia and contains, structurally and behaviorly, the most advanced mammalian orders, including the primates. The opossum belongs to another infraclass of the Mammalia, the M arsupialia. Among the distinguishing characteristics of this group are the presence of paired u te ri and vaginae, the b irth of th e ir young in an immature condition and the absence o f a corpus callosum. 3 h Walls (10) and Duke-Elder (11), the eye of the common North American opossum (Didelphls vlrglniana) seems well adapted for scotoplc vision. While there Is some debate whether the opossum is a noc turnal or crepuscular (i.e . a "twilight" animal) its optics are most characteristic of a nocturnal eye (e.g. the lens is very nearly spherical and 6 mm. thick in an eye measuring only 11 mm. in diam eter). In addition, there are several features of the opossum eye which are unique among the mammals. The tapetum lucidum, for example, is of the retinal type and is the only such tapetum found amongst mammals. Structurally it extends over just the superior half of the retina, and although It is clearly a modification of the pigment epithelium, its chemical basis is s till unknown. Further, there has been shown a direct correlation between the density of capillaries within this retina and the local thickness of the tapetum (10). The receptor cell distribution of this retina is another of its interesting features. According to Duke-Elder (11), the mosaic is most typically reptilian in type, resembling in many regards the Sauria.^ There are, in a ll, three types of visual cells present: rods, single cones, and double cones. Some of the single cones contain oil droplets, which, like the possession of double cones is b. The Sauria are the suborder of the class Reptilia which include the lizards and geckos. 5 distinctive only of the monotremes and marsupials among the mammals. The greatest density of rods is at the center of the tapetized area, their

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