Brain Research, 159 (1978) 69-83 © Elsevier/North-HollandBiomedicalPress

69

RECEPTIVE FIELD DEVELOPMENT IN THE DORSAL LATERAL GENICULATE NUCLEUS IN RABBITS SUBJECTED TO MONOCULAR EYELID SUTURE

H. DALE BAUMBACHand KAO LIANG CHOW Department of Neurology, Stanford University School of Medicine, Stanford, Calif. 94305 (U.S.A.)

(Accepted April 6th, 1978)

SUMMARY Receptive field properties of cells in the dorsal lateral geniculate nucleus (LGNd) were examined in 3 groups of rabbits, each subjected to monocular visual deprivation by lid suture at differing age periods. Monocular deprivation occurring from 6-8 to 20--25 days of age affected the normal development of LGNd receptive fields. A significant proportion of the cells in the deprived LGNd were either unresponsive to visual stimulation or had vague, indefinite receptive fields. Significantly fewer cells with uniform fields were found in the deprived LGNd than in the control. Percentages of concentric, motion and directional cells did not differ between the deprived and control LGNd. The diameters of receptive fields for concentric cells with sustained response properties, however, were smaller in the deprived than in the control LGNd. When deprivation was continued to 87-121 days of age, the percentage of uniform, indefinite and non-responsive cells found in the deprived LGNd approached more normal levels. Percentages of concentric, motion and directional cells were also normal. Monocular deprivation commencing at 21-22 days of age also had disruptive effects on LGNd receptive field organization, as reflected in the lower percentage of uniform and increased percentages of indefinite and non-responsive cells. These deficits, however, were not as severe as those seen in the animals deprived at an early age. A fourth group of adult rabbits subjected to monocular lid suture showed no such detrimental deficits in receptive field organization. These results demonstrated that visual deprivation affects the predominantly monocular LGNd of the rabbit, and that a critical period exists for such effects.

INTRODUCTION Receptive field properties of single cells in the rabbit visual system are not fully developed at birth but, rather, achieve adult characteristics in a temporal sequence

70 during the early postnatal period. Much of this development occurs after the eyes open. The receptive field characteristics of cells in the dorsal lateral geniculate nucleus (LGNd) reach the adult status at about 19-20 days of age 37, those in the superior colliculus at 23-25 days12,49, and those in the visual cortex at 25 days34. Experience with patterned light appears to be a critical factor governing the normal developmental sequence in both the superior colliculus and visual cortex. Monocular patterned deprivation by lid suture prior to the time of eye opening leads to a disruption in the development of cells in the rabbit superior colliculus in that the number of cells with oriented-directional receptive fields is drastically reduced lz. In the visual cortex, monocular deprivation leads to a general delay in the age at which classes of cells with oriented receptive field types reach their normal proportions. Rabbits subjected to monocular lid suture during the first two weeks following normal eye opening exhibit cortical receptive fields similar to those seen in normal immature rabbits just prior to the time of eye opening22. With prolonged deprivation ~ the proportion of cells in each receptive field type gradually changes until after 3-5 months of continuous deprivation such proportions eventually reach normal adult levels. While visual deprivation during the period that neuronal development is taking place within the superior colliculus and visual cortex significantly alters the manner in which such development occurs, the precise mechanisms underlying this disruption are not understood. In addition to its retinal input, the superior colliculus receives a major source of afferents from the visual cortex 18. The primary afferents to the visual cortex, in turn, emanate from the LGNd20,39,40. One alternative is that the disruption of at least visuocortical development consequent to monocular deprivation may reflect a primary disruption in functional development of cells in the LGNd. The present study was designed to examine this possibility, and represents a continuation of a series of studies designed to systematically examine neuronal development in the LGNd of monocularly deprived rabbits. The effects of monocular deprivation on receptive field organization in the LGNd were studied. The rabbit visual system has the advantage of being an almost entirely crossed system. Over 9 0 ~ of the retinal efferent fibers cross at the optic chiasm19,z0. There is thus only a small area of binocular overlap in the LGNd with the major component exclusively monocular. By recording cells in the monocular region, the effects of visual deprivation per se can be studied without the complicating factors of binocular competition. Such binocular competition has been invoked as the major factor in causing the well-known effects of monocular deprivation on binocularity of cortical cells and on shrinkage of LGNd cells in cat and monkey21,z3-2s,as,54. Four separate experiments were conducted. The first examined the effects on LGNd receptive field development of monocular lid suture from about 8 to 20-25 days of age, immediately following the time of normal eye opening, which usually occurs about the 9th or 10th postnatal day; the second examined the effects of monocular deprivation from about 8 to 87-121 days of age; the third was designed to examine the effects of monocular lid suture commencing at 21-22 days of age and continuing for 9-16 consecutive days; and the fourth examined monocular deprivation effects in adult rabbits.

71 METHODS Successive experiments were performed on 27 Dutch Belted rabbits. They ranged in age from 20 to 130 days of age at the time of the recording session. For Experiment A, one eye of 10 pups was sutured closed under halothane anesthesia at 6-8 days of age (just prior to the time of normal eye opening) and remained closed until 20-25 days of age, at which time they were prepared for recording. For the animals in Experiment B, one eye of 8 pups was sutured at 6-8 days of age but kept closed until 87-121 days of age. In both of these groups, the non-sutured eye was allowed to open at the normal time. In both Experiments C (6 rabbits) and D (3 rabbits), both eyes were left to open in the normal manner and the animals allowed normal visual experience. The C group were then subjected to monocular lid suture at 21-22 days of age and the D group at 82-116 days of age. In each case the sutured eye was kept closed for 9-16 days, after which the animals were prepared for recording. Since over 90 ~ of the optic fibers in the rabbit are crossed, most of the cells in the LGNd are activated exclusively by the eye contralateral to each respective LGNd. Each animal, therefore, served as its own control in Experiments A and C. The effects of patterned light deprivation were examined in the LGNd contralateral to the sutured eye and compared against response properties of cells in the control LGNd contralateral to the normally opened eye. The normative data of Rapisardi et al. 37 and Stewart et al. 5° served as a reference point for further comparison and as controls for Experiments B and D. Those few cells in each LGNd which were found to be driven by the ipsilateral eye were not included. The surgical and electrophysiological procedures involved in preparation of the animals for single unit recording were as described previously~7. Animals were anesthetized with halothane (Fluothane, Ayerst) anesthesia throughout all surgical procedures. During the recording session they were paralyzed with gallamine triethiodide (Flaxedil, Davis-Geck) and artificially respired. All wound margins were infiltrated with a long-acting local anesthetic (Anucaine, Calvin Chemicals) prior to the cessation of halothane anesthesia. Body temperature was maintained by the use of a water heating pad, and a steady heart rate was taken as an indication that the animal was not experiencing discomfort. To effect an additional degree of anesthesia throughout the recording session, approximately 50 9/00of the animals were administered ketamine HC1 (Ketalar, Parke-Davis) immediately after the withdrawal of halothane. In these animals an initial dose of 44 mg/kg was given i.m., followed every 30 min by supplemental doses of 22 mg/kg. The presence of ketamine did not affect receptive field properties, and the data will not be considered separately. Procedures for recording unit activity and determining receptive field properties were identical to that described previously87, except that tungsten microelectrodes of 2-3 M ~ (tested at 1000 Hz) were used. Histological verification of the electrode placement was made by placing two small electrolytic lesions separated by 1 mm along the electrode track at the end of recording. Animals were sacrificed, perfused, the brain removed, sectioned at 52/~m, and stained with cresyl violet. The entire extent of the electrode penetration was then reconstructed. Only those units clearly located within the boundaries of the LGNd were included in the analysis.

72 There were no reliable stereotaxic maps or practical head holders for use in young rabbits. We have made our own maps and measurements for locating the L G N d but with only partial success. This was reflected by the fact that useful data were obtained from 27 out of 32 animals studied, and that about 20 Yo of the recorded cells could not be located to LGNd. RESULTS A total of 331 cells were sampled in the deprived and 130 cells in the control LGNd for all groups combined. A distinction was made between action potentials arising from cell bodies and those arising from fibers according to standard criteria a,29. Only those units displaying clear cell body action potentials were included. Receptive field properties for both the deprived and control LGNd were consistently identical to those previously described for adult rabbit LGNd 5°. These are concentric, uniform, motion-sensitive and directionally selective. In addition, cells with indefinite response properties and non-responsive cells were found as described previously in young rabbits aT. Units classified as indefinite were responsive to either whole eye illumination or to light in local areas of the visual field. Their responses were urlreliable, inconsistent, and lacked any clearly definable field. Other cells did not respond to any type of visual stimulation employed and were thus classified as non-

responsive. The visual deprivation paradigm employed in the present study had no demonstrable effect on the qualitative nature of receptive fields found in the LGNd. All receptive field types were those typically encountered in normally developing rabbits. Visual deprivation, however, significantly altered the frequency with which certain of the several receptive field types are found. The data are summarized in Table I, which also includes data from normal adult and normal 19-20day-old animals taken from two earlier studiesaT, 50. As can be seen, the degree of disruption is dependent upon the age at which deprivation begins and the total duration of deprivation.

Effects of deprivation during early development Rapisardi et al. 37 have demonstrated that L G N d receptive field properties achieve adult characteristics in normal rabbits by 19-20 days of age (Table I, F). The purpose of Experiment A was to examine whether LGNd cells would acquire normal receptive field characteristics in the absence of patterned visual input during this early period. A total of 100 cells were sampled from the L G N d contralateral to the sutured eye and a total of 103 cells in the control L G N d contralateral to the normally opened eye in 10 animals. The percentage of cells in each receptive field class are presented in Table I (A). The proportional distribution of receptive fields in the control LGNd did not differ statistically (;t~ test) from either that of the 19-20-day-old normative data in the Rapisardi et al. study a7 (Table I, F) or that of the normative adult data of Stewart et al. 5° (Table I, E). In contrast, the proportional distribution of receptive fields in the deprived LGNd was significantly different from that obtained in the control LGNd of the same animals (P < 0.001, ;t2 test). There was a higher percentage of indefinite and

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74 non-responsive cells in the deprived L G N d than in the normal LGNd. These comprised 24 ° o of the cells sampled in the deprived L G N d , significantly greater than the 2°0 found in the control L G N d (P < 0.001)*. Fewer cells with uniform fields were also found in the deprived L G N d : 27 0o in the control L G N d against only 9 o/ O/ i n the deprived LGNd. This difference was statistically significant (P < 0.001). Percentages of cells in the other receptive field categories (e.g. concentric, motion and directional) were comparable between the deprived and control L G N d showing no statistically significant differences. The indefinite and non-responsive cells found in the deprived L G N d do not appear to be restricted to any particular area in the LGNd. Reconstruction of microelectrode penetrations in the deprived L G N d from 4 different animals are presented in Fig. 1. As can be seen, the indefinite and non-responsive cells were scattered throughout the I G N d .

Effects of late deprivation These two experiments were conducted in order to determine whether the disruption of receptive field organization following visual deprivation during the first 20-25 days of age was restricted to an early developmental period similar to critical periods found in other species4,11,30,56, or, alternatively, whether the disruption was a general consequence of decreased visual stimulation during any developmental stage. A total of 78 units were sampled in the deprived and a total of 27 units in the control L G N d of 6 rabbits in Group C (Table I, C). A higher percentage of non-responsive and indefinite cells were found in the deprived L G N d than is typical of normal rabbits. Together, the non-responsive and indefinite cells constituted 17 ° o of the cells sampled in the deprived LGNd, significantly more than found in the control L G N d (P < 0.05) or the normal adult L G N d (P < 0.001). With the exception of uniform fields, all other receptive field types in the deprived L G N d were found in approximately normal proportions. While fewer cells with uniform fields were again found in the deprived LGNd, this decrease did not reach statistical significance when compared to either the control L G N d or the normal adult LGNd. In Group D, 3 animals were allowed normal visual experience until 82-116 days of age, at which time one eye was sutured. Visual deprivation was then continued for 12-16 days. A total of 51 cells were sampled in the deprived L G N d only (Table I, D). All cells displayed clearly classifiable receptive field properties. No indefinite or nonresponsive cells were encountered. Z2 test revealed no significant deviation in the proportional distribution of receptive field types from that expected in a normal adult L G N d (Table I, E).

* Unless otherwise specified, statistics were computed by the test for differences between proportions (two-tailed test) and corrected for discreteness (Edwards, A. L., Experimental Design in Psychological Research, Holt, Rinehart and Winston, New York, 1968).

75

Prolonged deprivation The above data clearly indicate a developmental disruption of L G N d receptive field properties in rabbits deprived of patterned vision during a specific period in early postnatal development. Data obtained in the visual cortex of visually deprived rabbits~t,z2 indicate that the principal effect of early deprivation is to impose a delay in neuronal development with respect to receptive field organization. This experiment was designed to test this hypothesis in the L G N d and to examine the possibility that L G N d cells would ultimately achieve more adult characteristics in the continued absence of patterned visual input. In this group, 8 rabbit pups were subjected to continuous monocular lid suture from the time of normal eye opening to 87-121 postnatal days. A total of 102 cells were recorded in the deprived L G N d of these animals. No attempt was made to obtain a comprehensive sample in the control LGNd, since the receptive field properties and proportional distribution in adult rabbit L G N d have been well documentedaT, 50 (Table I, E and F), and can be referred to for ready comparison. An adequate number of units, however, were sampled in the control L G N d as a means of verifying the general physiological condition of the animals following recordings in the deprived LGNd. Units thus sampled typically showed normal response properties. The data for the deprived L G N d is presented in Table I, Group B. There continues to be a high percentage of non-responsive and indefinite cells in the deprived LGNd. These two cell types combined represent 1 6 ~ of the cells sampled, significantly higher than that found in the normal adult L G N d (Table I, E, P < 0.001) but less than those in the 20-25-day-old deprived L G N d (Table I, A). The indefinite and

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Fig. 2. Percentage of cells recorded in the LGNd contralateral to the sutured eye of each monocularly deprived group. Percentages shown are for cells which were either not responsive to visual stimulation (NO-RESP.) or which were responsive but had indefinite receptive fields (INDEF.), and for those which were classified as having either uniform (UNIF) or concentric (CON) receptive fields. Group identification is the same as in Table I. Note the decrease in the percentage of indefinite and nonresponsive and the increase in uniform cells for Group B as compared to Group A.

76 TABLE I Percentage of cells in each receptive field category recorded in the LG Nd of monocularly deprived rabbits

'Deprived' denotes cells in the LGNd contralateral to the sutured eye, and 'control' cells in the LGNd contralateral to the normally opened eye. Group A: rabbits monocularly sutured at 6-8 days of age and recorded at 20-25 days of age (early deprivation); group B: rabbits sutured at 6-8 days of age and recorded at 87-121 days of age (prolonged deprivation); Group C: rabbits sutured at 21-22 days of age and recorded at 31-37 days of age (late deprivation); Group D: rabbits sutured at 82-116 days of age and recorded at 98-130 days (adult deprivation); Groups E and F are normative data taken from Stewart et al. 5° and Rapisardi et al.SL

No. of rabbits

A 10

B 8

C 6

D 3

E 25

F 4

Deprived

Control Deprived

Deprived Control

DeprivedNormal adult

No. of cells

100

103

102

78

27

51

191

Normal 19-20day 36

Non-responsive Indefinite Concentric Uniform Motion Directional

15 9 53 9 11 3

1 1 52 27 15 4

9 7 55 17 10 2

16 1 51 14 10 8

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non-responsive cells of this group again showed no clustering in any particular region within the LGNd. As in the 20-25-day deprived group, there is also a slight decrease in the percentage of uniform cells. However, in contrast to the 20-25-day group, this decrease does not reach statistical significance when compared to the percentage found in the normal adult LGNd. This developmental trend toward normal adult status is depicted in Fig. 2. The major effect of deprivation during early development is reflected in an increase in the percentage of indefinite and non-responsive cells with a concomitant decrease of the percentage of cells with uniform fields. This effect is most severe in the 20-25-day group (Fig. 2, Group A), but becomes much less when deprivation is prolonged (Fig. 2, Group B). Deprivation commencing at 21-22 days (Fig. 2, Group C) has intermediate effects, and that in the adult (Fig. 2, Group D) no effects. Therefore, a sensitive period for visual deprivation to affect rabbit L G N d development appears to terminate between 35 and 80 days. Additional studies are needed to determine the precise time of this critical period. Percentages of cells with concentric receptive fields were not changed by deprivation in any of the groups. Fewer concentric cells were found in the L G N d of all groups in the present study than were previously reported for the normal adult or the normal 19-20-day-old animal (Table I). These differences, however, are problaby not attributable to the present experimental conditions, for they appeared in both the control and deprived data, and they are not statistically significant. It is unlikely that our results from the deprived eye are due to any significant extent by changes in refractive error consequent to lid suture. While the effects of lid

77 suture on the refractive properties of the rabbit eye are not known, several studies have indicated that lid suture in young animals does produce a myopic condition of the eye44,5a,5~. In each case, however, myopia and the associated refractive errors were evident only after long periods of lid suture. In the monkey, 19 days oF lid fusion commencing at l 1 days of age resulted in only a minimal refractive error, and the severity of myopia and refractory error increased as the duration of lid suture was extended5a. In our animals, deficits in receptive field organization were most severe in young animals subjected to lid suture for 14-19 days, while longer periods of lid suture were associated with a return toward normality. It is to be noted that retinoscopy could not be performed in Rapisardi et al.'s a7 study on the normal development of rabbit LGNd due to the cloudy nature of the optic media in young rabbits. To avoid complications in comparing our data with this normative data, we chose to replicate their procedures. DISCUSSION During the postnatal period, cells in the rabbit LGNd with clearly definable receptive fields were initially encountered on the 7th or 8th dayaT. While all adult types of receptive fields were present at this age, the majority of cells were either not responsive or showed indefinite response characteristics. At the time their eyes opened (9-11 days), roughly half of the cells isolated continued to be either non-responsive or indefinite. The proportion of these two cell types subsequently decreased while the proportion of cells with clear receptive fields increased until 19-20 days of age, when all cells isolated displayed clear receptive field properties. The data from the present study demonstrate that visual experience is an important element regulating the temporal sequence of such receptive field development. The classes of concentric, motion and directional receptive fields continued to develop in normal proportions in the absence of patterned visual stimulation. The development of cells with uniform fields, however, was significantly affected by monocular deprivation. In 20-25-day-old rabbits deprived of patterned input during the first 3 weeks of life, the LGNd contained a high percentage of indefinite and nonresponsive cells, with relatively few cells displaying uniform fields. This percentage of non-responsive and uniform cells is virtually identical to that found in non-deprived 9-11-day-old rabbits at about the time of eye opening. When deprivation was continued to 87-121 days, the percentage of uniform, non-responsive and indefinite cells became more like that found in normal 14-18-day-old rabbits. In visually deprived rabbits, therefore, neuronal development underlying LGNd receptive field properties does continue, but at a slower rate than in animals allowed visual experience. These data are in agreement with that of the deprived rabbit visual cortex, where visual deprivation also results in a delay in development of a particular class of cells. When tested at 20-25 days of age22, the visual cortex of monocularly deprived rabbits contained a significantly greater number of non-responsive and indefinite cells. In addition, there was a concomitant absence of cells displaying oriented receptive fields.

78 When deprivation was continued to 46-55 days of age, however, a substantial reduction appeared in the number of indefinite and non-responsive cells with a concomitant increase of cells with oriented receptive fields (unpublished data). When deprivation continued to 90-300 days, cortical cell populations achieved normal status ~. Thus, in both the visual cortex and LGNd, early monocular deprivation apparently leads to a delay in the development of specific classes of receptive fields. In the cortex it is the oriented fields and in the LGNd the uniform fields that are principally affected. The fact that receptive field development in the rabbit LGNd precedes that of the visual cortex, together with the results of the present study, leads to the parsimonious conclusion that the delay in receptive field organization in the visual cortex of visually deprived rabbits may be in large part due to the delay in the LGNd. It is also possible that delays in LGNd development may reflect retinal dysfunctions in monocularly deprived rabbits. Recent anatomical evidence obtained in the rabbit LGNd 33, however, has revealed that synaptogenesis in the LGNd continues until about 18-20 days of age. This hypothesis, however, does not account for the fact that, although receptive field properties in the deprived LGNd approached normality with prolonged deprivation, some deficits were still apparent, as evidenced by the few non-responsive and indefinite cells found. Percentages of receptive field types found in the striate cortex of rabbits monocularly deprived for prolonged periods, in contrast, are more completely normal. This lack of direct correspondence between LGNd and cortical development after prolonged monocular deprivation awaits further experimentation. Although the data on the normative development of both LGNd 37 and visual cortexa4 lead one to assume that LGNd development, preceding that of the visual cortex, takes place independently of cortical development, other processes may also be involved. As one possibility, Sherman et al. 48, in accounting for the loss of Y-cells in the monocularly deprived cat LGNd, have suggested that the functional development of LGNd Y-cells may depend on normal corticofugal input to the LGNd from the visual cortex. Deficits in LGNd receptive field organization consequent to visual deprivation may thus reflect disturbances in geniculate-cortex interactions. Complex corticofugai projections from the cortex to the LGNd have been described in both the cat ~1 and rabbitl~, 19, and functional interactions between the visual cortex and LGNd with respect to receptive field organization have been described in the cat~0,42. Physiologically, neuronal hyperactivity generated by topical application of penicillin to area 17 leads to concomitant complex activity in the LGNd of both cats41,4a and rabbitsL Such cortical hyperactivity occurring in neonatal rabbits also has a disruptive effect on LGNd receptive field development similar in many respects to that seen following visual deprivation2, 7. That the effects of monocular deprivation in rabbits are not limited to a delay in initial development is indicated by the data from animals who had lid suturing delayed until 21-22 days of age. The principal effect of deprivation at this age is reflected in the increased number of non-responsive and decreased number of uniform cells found. Only one indefinite cell was found. The deprivation effects at this age are much less pronounced than deprivation in younger animals. This reflects the fact that depriva-

79 tion in these older animals spanned the latter part of the developmental period, and indicates that, even after receptive field properties have developed to adult levels, some subsequent visual experience is needed to fully maintain and 'set' the underlying neuronal connections. These connections, however, do ultimately become impervious to alterations in afferent input. The monocularly deprived adult animals of the present study showed no abnormalities in receptive field properties of LGNd cells. Thus, as in the cat4,11,30,56, there is a critical period in rabbit LGNd development during which visual deprivation alters the course of receptive field development. In their study of monocularly deprived kittens, Wiesel and Hube155 reported that virtually all of the cells studied in the deprived LGNd had normal receptive field properties. Subsequent reports have generally agreed in finding little or no change in receptive field properties of LGNd cells following visual deprivation14, 2v,45, although subtle alterations have been described26,~v,32,4s,sL Postnatal neuronal plasticity in the mammalian geniculostriate system was thus considered as mainly restricted to the cortical level. Morphological analyses of the LGNd of monocularly deprived kittens, however, did reveal deficiencies, as indicated by a general reduction in the size of cells located within the deprived layers1,6,16,2a,2L More recent studies have attributed these decreases to the selective loss of the larger Y-cells in the binocular section of the LGNd in monocularly deprived kittens4a and tree shrew a6. Interpretation of these results, however, is still uncertain. The decrease in Y-cell populations seen in the deprived LGNd is principally restricted to the binocular segment. Cells in the monocular segment are typically reported not to be affected by monocular deprivation. This has been interpreted as reflecting some type of binocular competition between cells such that the cells in the deprived laminae are placed at some unspecified disadvantage with respect to the non-deprived cells. Whether the rabbit LGNd contains distinct populations of cells corresponding to the W, X, Y classification of the LGNd demonstrated in other speciesS,9,1a,2s,85,46, 47,58,59 is currently an open question. Our data demonstrate that the development of a particular class of receptive field types, the uniform fields, is disrupted by monocular deprivation during a sensitive period in early development. In addition, while concentric, motion and directional ceils were encountered with the same frequency in the deprived as in the control LGNd, analysis of receptive field diameters indicates some subtle effects of deprivation on concentric cells. The mean diameters of the receptive fields for all mapped cells in the 20-25-day age group are presented in Fig. 3. Analysis of variance on all cells combined in the 20-25-day age group revealed that field diameters for cells in the deprived LGNd were significantly smaller than those in the non-deprived LGNd (F = 8.40, df =1, 161, P < 0.01). Although fields in the deprived LGNd were smaller for each receptive field class than the controls, when subdivided according to receptive field type only the concentric cells in the deprived LGNd reached a statistically significant decrease when compared to controls (F = 4,52, df = 1,98, P < 0.05). Additional analyses revealed that this size decrease was principally due to the significant decrease (F ---- 6.49, df =- 1,25, P < 0.05), in the receptive field diameters of those concentric cells that showed sustained responses to light. Meaningful comparisons of receptive field diameters for the other experimental

80

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Fig. 3. Mean diameter in degrees of visual field of receptive fields of cells recorded in the LGNd of 20-25-day-old rabbits which had been monocularly deprived from the time of normal eye opening (Group A, Table I). NORMAL: cells in the LGNd contralateral to the non-deprived eye. DEPRIVED : cells in the LGNd contralateral to the deprived eye. TOTAL CELLS, all cells for which receptive field maps were obtained; CON, concentric; UNIF, uniform; MOT, motion; CON PHASIC, cells with concentric receptive fields which responded to visual stimulation with short bursts of firing; CON SUST., cells with concentric receptive fields which responded to visual stimulation with long trains of discharges which usually continued for the duration of the stimulus. * P < 0.05; ** P < 0.01.

groups could not be made due to the lack of adequate data in normal L G N d of rabbits at corresponding ages. The effects of monocular deprivation in cat L G N d (e.g. the loss of Y-cells and the decrease of cell sizes), being located mainly in the binocular segment, are thought to result from unbalanced binocular competition. The present data, together with an earlier report 5, show that deprivation effects also occur in the monocularly dominated L G N d of rabbit. This now appears to be also true in the monocular segment of the largely binocular system of the monkey and tree shrew. Von Noorden et al. 5z reported decreased cell sizes in the monocular segment of monkey LGNd, and Norton et al. a~ recorded some abnormal cell responses in the monocular part of tree shrew LGNd. Clearly, binocular competition is probably only one of several factors responsible for the effects on LGNd development consequent to monocular deprivation. ACKNOWLEDGEMENTS

This work was supported by USPHS Grants NS 00691 and NS 18512 and NASA Grant N G R 05-020-435 to K.L.C., and USPHS Grant NS 07012 and USPHS Postdoctoral Fellowship EY 05176 to H.D.B. We gratefully acknowledge the assistance of Dr. Barry Gordon and David Glanzman in recording some of the animals. We thank Robin Lawson and Jim Aase

81 for p r e p a r i n g the histological materials, P a m Vario for expert secretarial assistance, a n d J o h n J a c k s o n for t y p i n g the m a n u s c r i p t .

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