Monocular Segment Research Articles

An anomalous uncrossed retinal input to lamina A of the cat's dorsal lateral geniculate nucleus

A small anomalous group of uncrossed retinogeniculate axons has been found terminating in one or two small pericellular terminal nests in the lateral, monocular segment of lamina A of the cat's dorsal lateral geniculate nucleus. Such anomalous islands, demonstrable by fibre degeneration and by autoradiographic methods, have been found in 6 out of 13 normally pigmented cats, and in 2 out of 7 Siamese cats. Thus, in apparently normal cats, the highly ordered geniculate map of the visual field can be interrupted by small islands that do not fit into the overall order.

Three groups of cortico-geniculate neurons and their distribution in binocular and monocular segments of cat striate cortex.

Among 409 neurons recorded from binocular and monocular segments of the cat striate cortex, 91 were identified as cells (C-G cells) projecting to the dorsal lateral geniculate nucleus (LGN) on the basis of antidromic activation from LGN and of histological localization of cortical layer VI. The axonal conduction velocity of these C-G cells was calculated from differences in latency between antidromic responses to electrical stimulation of LGN and the optic radiation. According to this velocity, 70 C-G cells from the binocular segment could be classified as fast (13--32 m/sec), intermediate (3.2--11 m/sec) and slow (0.3--1.6 m/sec) cells. The fast cells (47% of the total) were spontaneously active and had receptive fields of complex type. Histologically they were located mostly in the upper half of layer VI. The intermediate cells (31%) were mostly simple. The slow cells (21%) were completely silent, not driven by visual stimuli, and located mostly in the lower VI. From the monocular segment of the cortex, the intermediate cells could not be recorded, while the other two groups of cells were sampled with the same frequency as from the binocular segment. These findings suggest an existence of three functionally distinct groups of C-G cells and a possible participation of the intermediate cells in binocular vision.

Development of the dorsal lateral geniculate nucleus in normal and visually deprived Siamese cats.

Neuronal cell bodies in the lateral geniculate nucleus of normal and of monocularly-deprived Siamese cats have been measured. Seventeen normally reared Siamese cats, ranging in age between 20 and 120 days, were used to determine rates of normal geniculate cell growth. A second group of five adult Siamese cats reared from bith with the lids of one eye closed were used to study the effects of monocular visual deprivation upon geniculate cell size. For each of the normal and visually deprived Siamese cats, the cross-sectional areas of 600 lateral geniculate cells were measured from camera lucida drawings of Nissl preparations. During normal development the geniculate cells rapidly increase in size during the first postnatal month of life and reach their adult size sometime between days 28 and 56. While this course of geniculate cell growth is similar to that seen in normally pigmented cats, the pattern of change seen after monocular deprivation is quite different in Siamese cats from that found in normally pigmented cats. In Siamese cats the regions of the nucleus receiving a contralateral projection from the deprived eye appear to be shielded from the effects of binocular competition. Cells throughout lamina A and in the abnormal, contralaterally innervated segment of lamina A1 show only about a 10% reduction in cell size. There are no noticeable differences between the parts of lamina A in the binocular and monocular segments of the nucleus. Cells in the ipsilaterally innervated segment of lamina A1, in contrast, show deprivation-induced changes that average 27.1%. Two mechanisms are proposed to explain why some geniculate cells in Siamese cats appear to be shielded from binocular competition: one depends on possible interactions between geniculo-cortical cells lying in adjacent parts of the same geniculate lamina, and the other depends on an anatomical segregation of the cell type ("Y-cells") most heavily affected by the binocular competition. Each proposed mechanism is related to earlier observations on monocularly deprived, normally pigmented cats.

Development of the dorsal lateral geniculate nucleus in normal and visually deprived cats.

Although much is known about the cell size changes that take place in the cat dorsal lateral geniculate nucleus as a result of visual deprivation, very little is known about the time course of either of these changes or the changes that occur during normal development. In addition, all previous studies of lateral geniculate nucleus cell size have been confined to the dorsal laminae A and A1 since the more ventral "C" laminae are impossible to identify in normal Nissl stained material. However, it is possible to extend the cell measurement data to laminae C, C1, and C2 by using autoradiographic techniques. Cross-sectional area measurements of dorsal lateral geniculate nucleus cells were made in 47 normally reared kittens and 45 monocularly deprived kittens. All of the normal kittens and 39 of the 45 deprived kittens were studied during the first 70 days of postnatal life. Six deprived cats used to study the deprivation induced changes in cell size in the "C" laminae were allowed to survive for longer periods. In normal kittens, lateral geniculate nucleus cells grow rapidly during the first four weeks of life. Cells in the deprived layers also grow rapidly during this time, however, at the end of the first month their growth stops and a slow shrinkage takes place over the next several weeks. In the "C" laminae of deprived cats significant changes in cell size are confined to layer C. Although many of the deprived cats show greater deprivation induced changes in cell size in the binocular segment of the lateral geniculate nucleus than in the monocular segment, other cats show approximately equal changes in cell size in the two regions. In addition, some cats exhibit little, if any, deprivation induced change in lamina A cell size but do show quite severe cell shrinkage in lamina A1.

A quantitative study of the occurrence and distribution of cytoplasmic laminated bodies in the lateral geniculate nucleus of the normal adult cat.

The proportion of neurons containing cytoplasmic laminated bodies (CLB) in the dorsal lateral geniculate nucleus (dLGN) of normal cats was determined by using a new method which permitted examination of large areas of this nucleus. Near the medial border of the dLGN (projection of the area centralis) in the A-laminae 30--60% of the neurons contain CLBs. Within the same region in lamina B only 10--30% of the nerve cells contain CLBs. In the monocular segment (MS) of lamina A 27--48% of the nerve cells contain CLBs. In the medial interlaminar nucleus (MIN) 25% of the neurons contain CLBs; these cells were distributed throughout the medio-lateral extent of the MIN. CLBs were also observed in neurons of the perigeniculate nucleus (PN) and in nerve cells embedded in the fiber sheet separating the dLGN from the pulvinar complex. The distribution of cells containing CLBs is extremely variable in all portions of the dLGN studied. The data presented in this paper indicate that CLB-containing cells in the dLGN may not be X-cells, as has been suggested by LeVay and Ferster ('77) because: (1) the distribution of neurons containing CLBs in lamina A does not match the distribution of X-cells recorded in this lamina; (2) some nerve cells in the MIN contain CLBs but very few X-cells have been found in this region.

Visual deficits and recovery following monocular lid closure in a prosimian primate

The behavioral consequences of long-term monocular lid closure in 6 deprived and 2 normal prosimian primates (Galago crassicaudatus) were examined using a visual field perimetry technique and a variety of visuomotor tests. Deprived galagos had their right lid sutured either within 8 (n = 4) or 38 (n = 2) days postpartum and were reared under these conditions for at least 8 months. Normal galagos responded briskly on all visuomotor tests including tracking, response to visual threat, ability to descend from a platform, and obstacle avoidance. Perimetry tests of these animals showed that the visual field with both eyes open was 180 degrees while each eye tested individually had a 135 degrees field. Deprived galagos tested with their experienced eye exhibited normal visuomotor behavior and normal visual fields. Following reverse suture all deprived animals behaved initially as if blind on all tests. Over the next 4-8 weeks these animals showed some limited recovery on the visuomotor tasks. Tests of perimetry indicated that by the end of four weeks of testing, vision was limited to the deprived monocular segment where it remained stable to the end of the testing period. With both eyes open, deprived galagos exhibited a normal 180 degrees visual field. Curiously, however, when stimuli were introduced simultaneously in corresponding points in both monocular segments of the field, the majority of orienting responses of deprived subjects favored the right deprived eye. Under similar testing conditions one normal subject favored its right eye and the other its left eye. The reversibility of monocular deprivation was explored further either by extending the period of forced use of the deprived eye in one animal or by removing the non-deprived eye in another animal. No further improvement in visual field perimetry was seen following a longer period of forced use of the deprived eye. In contrast, a substantial increase in the visual field could be demonstrated one month following removal of the originally experienced eye: the deprived eye field expanded to include the entire 90 degrees ipsilateral hemifield. The contralateral field of the deprived eye, however, continued to remain blind even after one year. These behavioral results suggest that although the original binocular competitive disadvantage suffered by the deprived eye during development is relatively permanent, it can partially be ameliorated by late removal of the non-deprived eye.

Retinal projections in the Tasmanian devil, Sarcophilus harrisii.

Retinal projections were mapped in Tasmanian devils which had one eye injected with 3H-proline. The retinal fibers terminate in seven regions in the brain. These are (1) dorsal lateral geniculate nucleus (LGNd), (2) ventral lateral geniculate nucleus, (3) lateral posterior nucleus, (4) pretectum, (5) superior colliculus, (6) hypothalamus and (7) accessory optic system. The pattern of retinal input to six of these regions is similar to that seen in other marsupials. The pattern of retinal projections to the LGNd, while basically similar to that observed in other polyprotodont marsupials, is much simpler than that seen in the related native cat, Dasyurus viverrinus. The LGNd of Sarcophilus presents the simplest cytoarchitectural organisation of any marsupial examined so far. Each LGNd receives overlapping projections from both eyes. Suggestions of an intermittent lamination are seen in the LGNd contralateral to an eye injection of 3H-proline. On the ipsilateral side there are two patches of label, a large lateral patch and a smaller medial patch, both of which occupy areas receiving contralateral input. The monocular segment, occupying the ventral 40% of the nucleus, is more extensive than has been reported in any other polyprotodont marsupial.

Effects of visual cortex lesions upon the visual fields of monocularly deprived cats.

AbstractThe visual fields of 16 cats raised with monocular eyelid suture were measured by means of a visual orienting test. We separately measured the fields of nondeprived and deprived eyes. Each cat was tested preoperatively, and 13 of the cats were tested following lesions of the visual cortex, superior colliculus, and/or optic chiasm.Preoperatively with the nondeprived eye, every cat had a normal monocular field extending roughly from 90μ ipsilateral to 45μ contralateral to the eye being tested. Fields for the deprived eye seemed to depend upon the nature of the deprivation. Fourteen of the cats had complete lid fusions, and 13 of these had virtually identical deprived eye fields which essentially included only the monocular segment (i.e., roughly 45v to 90μ ipsilateral). Only these 13 cats were tested postoperatively. The fourteenth cat with complete lid closure may have had a visual field for the deprived eye that included the entire ipsilateral hemifield, but its responses were extremely unreliable. Two of the cats had incomplete lid fusions which exposed the cornea and thus permitted some pattern vision during development. Their visual fields for the deprived eye included the entire hemifield. We conclude that rearing a cat with complete monocular lid occlusion produces for the deprived eye a field which is effectively limited to the monocular segment.Following postoperative testing, histological verification of neural lesions was obtained for every cat except one. An optic chiasm transection in one cat rendered its deprived eye totally blind on these tests, presumably because crossing nasal fibers which represent the monocular segment were cut. The chiasm transection also reduced the nondeprived eye's field to 0μ to 45μ contralateral. Cortical ablations in the other 12 cats were contralateral to the deprived eye or bilateral, and they ranged in size from lesions of areas 17 and 18 to total occipitotemporal ablations. (Cats with the latter ablations also had tectal lesions to counteract hemianopia due to large cortical lesions.) Each of these 12 cats showed a dramatic postoperative increase of the deprived eye's visual field to include most or all of the ipsilateral hemifield. The smallest lesion (involving areas 17 and 18 contralateral to the deprived eye) produced such an expansion of the deprived eye's field. Collicular ablations in another cat suggest that these expanded fields following cortical lesions depend upon retinotectal pathways. Postoperative fields for the nondeprived eyes were more variable. Generally, smaller lesions caused little change in these fields from preoperative measurements; larger lesions tended to reduce the fields to include only the ipsilateral hemifield. Two cats with bilateral occipitotemporal cortical ablations and transections of the commissure of the superior colliculus exhibited no obvious behavioral differences between use of the nondeprived and deprived eyes, and the monocular fields included the ipsilateral hemifield for each eye.One interpretation of these results is based upon prior suggestions that retinotectal pathways develop fairly normally in monocularly deprived cats, while geniculocortical pathways do not. The animals' preoperatively tested visual behavior and collicular reponse properties tend to reflect the status of cortical pathways, but following cortical lesions, the orienting functions of retinotectal pathways are more fully expressed. Since these retinotectal pathways are dominated by nasal retina, the entire nasal retina of the deprived eye after appropriate cortical lesions is functional for visual orienting.

Cell growth in the monocular segment of the lateral geniculate nucleus following the opening or closing of one eye.

Both eyes were closed in 12 kittens soon after birth, and one eye was reopened at 23 days. From 14 to 26 days later, the cells in the monocular segment of the LGN connected to the open eye became significantly larger than the corresponding cells on the other side of the brain that were connected to the eye that was still closed. Up to 26 days after eye opening, the response of the monocular segment was similar to that of the binocular segment, so the latter may not be dependent on mutual inhibition or competition between sets of cells connected to each eye. From 30 to 60 days after eye opening, however, there were no differences between the monocular segments, but marked differences between the binocular A laminae where mutual inhibition and competition can occur. In seven more kittens both eyes were untouched until 23 days, when one eye was closed. Between 3 and 31 days later the deprived cells in the monocular segment of the LGN were not more than 8% smaller than those on the other side of the brain, and the measurements were not significantly different in any one cat. Within the binocular part of lamina A, cell growth is retarded by eye closure in two phases. Neither the early phase from 4--6 days nor the later phase from 21 days onwards occurred in the monocular segment, so both may be dependent on mutual inhibition of competition between the sets of cells connected to each eye. Two possible explanations of this difference between the effects of eye opening and eye closing are discussed.

Changes in the visual system of monocularly sutured or enucleated cats demonstrable with cytochrome oxidase histochemistry

Endogenous cytochrome oxidase activity within the mitochondria of neurons and neuropil was demonstrated histochemically under normal and experimental conditions. Since enzymatic changes were noted with chronic neuronal inactivity in the auditory system (Wong-Riley et al), the present study sought to examine functionally induced enzymatic changes in the visual system of kittens. Eight kittens were used experimentally: 5 had monocular lid suture for varying periods of time; one had binocular lid suture followed by monocular suture followed by binocular opening; two had monocular enucleation. All initial procedures were performed before eye opening. Materials from other normal kittens and cats were also used as controls. At the end of the experiments, the animals were perfused with aldehyde solutions and frozen sections of the brains were incubated for cytochrome oxidase activity (a detailed protocol was outlined). The results indicated that the deprivation caused by monocular suture produced a decrease in the cytochrome oxidase staining of the binocular segment of the deprived geniculate laminae. Enucleation yielded a greater decrease in the cytochrome oxidase activity in the affected geniculate laminae. However, the staining in the 'normal' lamina extended across the interlaminar border to include a row of surviving large cells in the 'denervated' lamina. The staining of the monocular segment appeared not to be affected by lid suture, but was decreased by enucleation. At the cortical level, lamina IV in area 17 of normal cats was stained darkly as a continuous band. Following lid suture, this pattern was replaced in part by alternating columns of light and dark staining, suggestive of ocular dominance columns. Thus, a decrease in neuronal activity due to reduced visual stimulation or destruction of the primary afferent nerves led to a significant decrease in the level of oxidative enzyme activity one to several synapses away.

Postcritical‐period reversal of effects of monocular deprivation on dorsal lateral geniculate cell size in the cat

AbstractIf cats are raised with monocular lid suture, cells in dorsal lateral geniculate (DLG) laminae which receive inputs from the deprived eye are smaller than normal when the animals reach maturity. Previous studies have shown that after a critical period of development, these morphological changes cannot be reversed by closing the initially experienced eye and opening the initially deprived eye (reverse suture). The present experiment investigated whether removal of the experienced eye, a procedure which rapidly reverses some of the physiological effects of monocular deprivation on striate cortex neurons (Kratz et al., '76), also would produce a reversal of the effects of monocular deprivation on DLG cell size. In addition, if a morphological reversal in DLG cell size could be observed, we were interested in knowing its time course relative to the physiological reversal in striate cortex.Nine control kittens were raised with monocular lid suture until they were four to eight months old (group MD). Comparison with a group of six normally reared kittens (group N) showed the usual effects of monocular deprivation on DLG cell size (cross‐sectional area) which have been reported previously; i.e., a marked reduction in cell size in the deprived binocular segments of laminae A and A1, and a smaller reduction in cell size in the deprived monocular segment of lamina A. Ten additional kittens were raised with monocular lid suture until they were four to five months old, at which time the experienced eye was enucleated. Five of these kittens (group MD‐DE‐Immed) were allowed to survive 30 hours after the enucleation. There was no significant difference in DLG cell size between these animals and the MD control animals. The remaining five kittens (group MD‐DE‐3 mo) were allowed to survive three months after enucleation of the experienced eye, and the deprived eye remained closed during this time. In these kittens, cells in the deprived DLG laminae were significantly larger than cells in the deprived laminae of kittens in both the MD‐DE‐Immed and the MD control groups. In addition, they were not significantly different from normal. The increased cell size in the MD‐DE‐3 mo cats was present in the deprived binocular segments of both laminae A and A1, and to a lesser extent in the deprived monocular segment of lamina A. Cell sizes also were measured in the deafferented (initially experienced) laminae of cats in the MD‐DE‐Immed and MD‐DE‐3 mo groups. They showed a slight decrease in size relative to DLG cells in the experienced laminae of MD control cats.These results indicate that deprived DLG cells can resume their growth after the previously defined critical period. Comparisons with previous studies indicate that it is necessary to completely remove the inputs from the experienced eye for the new growth to occur. However, it is not necessary to provide the deprived eye with visual experience. In addition, the increase in deprived DLG cell size occurs sometime after the increase in ability of the deprived eye to drive striate cortex cells (Kratz et al., '76). Thus, the functional recovery of cortical cells precedes the morphological recovery of DLG cells.

Lateral Geniculate Nucleus in Dark-Reared Cats: Loss of Y Cells Without Changes in Cell Size

In cats reared in the dark from birth until 4 months of age, the dorsal lateral geniculate nucleus contained few normal Y cells in either the binocular or monocular segments. Although most of the neurons appeared to be normal X cells unaffected by light deprivation, many cells with abnormal receptive field and response charcteristics were encountered. These effects were permanent, since 1 to 2 years of normal visual experience following initial light deprivation did not lead to any functional recovery. The sizes of cell bodies in cats reared in the dark were similar to those of normal animals, an indication that changes in geniculate cell physiology need not be related to changes in cell size.

Variability of laminar patterns in the human lateral geniculate nucleus.

The structure of the human lateral geniculate nucleus has been studied on serial Nissl stained sections from 57 human brains. Most of the brains were from neurologically normal individuals and were obtained during routine autopsy procedures. The laminar arrangement within the human nucleus is surprisingly variable. It is always possible to recognize a small segment with two layers (the monocular segment), one with four layers and one with six layers. Often an 8-layered segment can also be seen. The posterior half of the nucleus, within which central vision is represented, is made up mainly of six layers, and is the least variable part of the nucleus. Here the layers lie roughly parallel to each other. In the anterior half of the nucleus the laminar arrangement is more variable, and the layers often form complex and irregular interdigitations with each other. The 8-layered segment varies greatly in size and may be absent. It generally lies at the edge of the 4-layered segment not, as might have been expected, within the borders of the 6-layered segment. In many parts of the nucleus nerve cells are organized in short rows that run perpendicular to the layers; also, individual nerve cells are elongated in this direction. This neuronal orientation follows the lines of projection that are defined by the borders of a zone of retrograde degeneration, and also corresponds to the orientation of a cellular discontinuity that probably is the geniculate representation of the blind spot. Thus, we conclude that the neuronal orientation indicates the lines of projection within the nucleus.

Development of the dorsal lateral geniculate nucleus in the cat.

The development of the lateral geniculate nucleus has been studied systematically in Nissl preparations from a series of cats that ranged in age from newborn to adult. In addition, preliminary observations are reported at two stages of fetal development. It was found that laminae develop in the lateral geniculate nucleus near the time of birth and continue to differentiate during the first postnatal week. During development the major axis of the lateral geniculate rotates approximately 180 degrees in the sagittal plane. The rotation begins prenatally and is not completed until after the twentieth postnatal week. The volume of the lateral geniculate was computed at different ages and it was determined that during the first postnatal month the nucleus attains two-thirds of its adult size. However, the rate of growth declines markedly thereafter, and final volume, like final position, is not achieved until late in development. The cross-sectional areas of lateral geniculate neurons were measured at four locations in the nucleus in each animal. The locations represented the following parts of the visual field: the paracentral and inferior peripheral fields in the binocular segment of lamina A; the monocular segment of lamina A; and the paracentral field in lamina A1. Neurons in each of these locations grow at approximately the same rate and are essentially fully grown by 56 days. Cell size histograms show that more large cells are found in lamina A1 and more small cells in the monocular segment than elsewhere in the dorsal laminae. Unlike the retina, there appears not to be a gradient of development in the lateral geniculate nucleus from center to periphery, at least in terms of cell body size at the ages studied. On the contrary, that part of the lateral geniculate nucleus which represents the paracentral visual field is the last segment in the dorsal laminae to achieve a mature cell size distribution. Finally, a discrete class of small spindle-shaped neurons was observed in the lateral geniculate nucleus ventral and caudal to the C laminae during the first two postnatal weeks. These cells possess a leading and trailing cytoplasmic process and are distinctly different from cells in the main laminae. It is suggested that these spindle-shaped cells may be neurons that are still in the process of migration or differentiation in the postnatal animal.

Differential effects of monocular deprivation seen in different layers of the lateral geniculate nucleus

Recent investigations have suggested that the morphological effects of monocular deprivation can be explained by a developmental competitive interaction between the pathways from the two eyes. This study presents evidence in the tree shrew for binocular competition and for an unequal effect of such competition on the different layers of the lateral geniculate nucleus. The effects of monocular deprivation were evaluated by comparing cell size changes in the binocular and monocular segments of the lateral geniculate nucleus in three tree shrews raised with one eye sutured. In two of these animals the open eye was injected with 3H proline in order to identify accurately geniculate layers innervated by the non-deprived eye. Cell sizes in three normal animals and one monocularly enucleated animal were measured for comparison. The results show the following main effects: First, that monocular deprivation significantly changes cell size in the binocular but not the monocular segment of the geniculate nucleus. Comparisons with cell size in normal animals indicates that non-deprived cells may grow in response to deprivation. Second, that cell size in geniculate lamina 3 is not affected by monocular deprivation, suggesting that cells in this layer are morphologically or functionally secluded from competitive interactions affecting the other layers. Finally, that monocular enucleation in the adult tree shrew affects all parts of the geniculate nucleus including layer 3 and the monocular segment, demonstrating that these parts of the geniculate nucleus are responsive to lack of retinal innervation.

Receptive-field properties of neurons in binocular and monocular segments of striate cortex in cats raised with binocular lid suture.

1. We studied the receptive fields of 171 striate cortical neurons from 17 cats raised with binocular lid suture. Of these, 102 fields were within 10 degrees of the area centralis and the remaining 69 were at least 38 degrees from the vertical meridian. 2. Based on their different response properties, cells were divided into three broad groups: the mappable cells (49%) had clearly defined receptive fields, the unmappable cells (31%) were activated by visual stimuli but had diffuse fields which could not be hand plotted, and the visually inexcitable cells (20%) could not be activated by visual stimuli. Very few (less than or equal to 12% of the total sample) normal simple or complex cells could be found. 3. Orientation selectivity was assessed in these cells. Only 12% displayed orientation selectivity within normal bounds, and these were all mappable cells. None of the unmappable cells had discernible orientation selectivity. 4. Ocular dominance was assessed for 62 of the centrally located receptive fields. Among mappable cells, there was an abnormally low proportion of binocular fields, while no such abnormality was seen for unmappable cells. 5. For 47 of the neurons, average response histograms were compiled for moving stimuli of various parameters in an effort to evoke the maximum discharge or peak response. This peak response was normal for mappable cells but reduced for unmappable cells. 6. We devised a technique for studying potential inhibitory receptive-field zones in these neurons, validated the method in normal striate cortex, and used it to test 20 mappable cells in the lid-sutured cats. None showed the pattern of strong inhibitory side bands seen in normal simple cells, although six showed weak or abnormal inhibitory zones. Interestingly, six of the seven visually inexcitable cells tested by this method had purely inhibitory receptive fields. 7. The effects of binocular suture were essentially identical for the binocular and monocular segments since the cell types and their response properties did not differ between these two areas of cortex. Furthermore, the cortical monocular segments of these cats seemed qualitatively different from the deprived cortical monocular segment after monocular suture. This extends an analogous difference for these cats reported for the monocular segments of the lateral geniculate nucleus. We thus conclude that monocularly and binocularly sutured cats develop by qualitatively different mechanisms. For the former, competition between central synapses related to each eye is a prominent feature of geniculocortical development, whereas, for the latter, such specific forms of geniculocortical development may not obtain.

Loss of Y-cells in the lateral geniculate nucleus of monocularly deprived tree shrews.

In tree shrews (Tupaia glis) reared with one eye closed, Y-cells were almost entirely absent in the binocular segment of the lateral geniculate laminae receiving input from the deprived eye. Y-cells were found in the monocular segment of these laminae, and in the binocular segment of the laminae with input from the normal eye. X-cells were present in both the deprived and normal laminae and appeared unaffected by the deprivation. A number of abnormal cells were also found, and these were located primarily in the binocular segment where Y-cells were absent.

The distribution of afferents representing the right and left eyes in the cat's visual cortex

The presumed anatomical basis for ocular dominance columns in the cat's visual cortex was demonstrated autoradiographically. Transneuronal transport of radioactive materials following an intraocular injection of tritiated proline and fucose was used to identify the set of thalamocortical afferents representing the injected eye. In normal cats, ipsilateral to the injected eye discrete patches of silver grains were visible in layer IV and the base of layer III of areas 17 and 18. The distribution of grains in the contralateral hemisphere of area 17 was almost continuous in layer IV, but there were clear indications of periodic fluctuations in concentration. This pattern of labeling presumably reveals the overall pattern of the physiologically defined ocular dominance columns. As expected, there was a strip of heavy labeling within the monocular segment of the contralateral hemisphere and a corresponding absence of label in this region ipsilateral to the injected eye. When large portions of the fourth layer in areas 17 and 18 of the ipsilateral hemisphere were reconstructed from serial parasagittal into a system of irregular bands roughly 0.5 mm wide separated by gaps of similar width containing less label. The bands ran mediolaterally and perpendicular to the anatomical 17–18 border. The combined size of a left and right eye band was about 1 mm, both in cortex representing central and peripheral visual fields. The bands almost doubled in width within area 18. In the contralateral hemisphere the bands were less distinct due to the widespread presence of label in layer IV. This arrangement suggested that geniculocortical afferents from the two eyes share a good deal of fourth layer territory. One cat with artificially produced squint and one short-term monocularly d deprived kitten were also studied. The primary effect of both forms of deprivation was to render the system of fourth layer bands more distinct, particularly within the hemisphere contralateral to the eye injection.

Differential effects of early monocular deprivation on binocular and monocular segments of cat striate cortex.

1. Receptive-field properties were studied for 156 cells in 15 mqnocularly deprived cats. Particular emphasis was placed on comparisons between receptive fields located in the deprived monocular segment (i.e., far from the vertical meridian) and more centrally located fields. 2. Between 0” and 30’ of visual-field eccentricity from the vertical meridian, 36 of 37 cells were influenced, both for the excitatory and inhibitory components, exclusively by the nondeprived eye. Also, se.veral of these cells had abnormal receptive-field properties. At eccentricities greater than 30”, we found 40 cells which responded to stimuli presented to the deprived eye; of these, 4 were influenced binocularly. An additional 43 cells with fields between 30’ and 45’ eccentricity responded only to stimulation of the nondeprived eye, and 17 cells were studied in the nondeprived monocular segment. About one-third of the cells influenced by the deprived eye had abnormal fields. At least 18 other cells did not respond to any visual stimulus presented to either eye. 3. While the cortical monocular segment related to the nondeprived eye had normal percentages of cell types, the deprived monocular segment had a significant reduction in the ratio of normal complex to normal simple cells. Simple cells in the deprived monocular segment appeared to be normal in every respect we measured. 4. No gross anatomical changes were found which might account for this complex cell loss. That is, we found no differences in cell size or packing density between the deprived and nondeprived monocular segments, 5. The following conclusions were drawn from these results: a) in agreement with previous studies, these data suggest that binocular com-

Studies of the modifiability of the visual pathways in Midwestern Siamese cats.

AbstractIn Midwestern Siamese cats each cerebral hemisphere receives visual inputs from both temporal retinae, and the conflicting signals that arise from these two disparate afferent sources appear to prevent the establishment of some normal functional connections for the whole temporal retina. As a result, these cats show no behaviorally demonstrable visual orientation for the temporal retina, although they respond normally for the nasal retina. The visual capacities of Midwestern cats can be changed by modifying early visual experience, and some of the rules that govern the formation of central visual pathways can thus be defined.The apparent blindness of the temporal retina of a Midwestern cat can be modified by monocular rearing. When such a cat is raised with one eye sutured or removed, the other eye develops a field of view that is normal in terms of visual orientation. Cortical lesions show that the vision acquired by the temporal retina depends upon areas 17 and 18 of occipital cortex. The modification of vision in the open eye can be produced by deprivation starting between the 8th and 80th day of life; later stages remain to be tested. In contrast, modifications produced in the deprived eye show a different relation to the time of the suture: if the lid suture is done at eight to ten weeks, the deprived eye develops vision for only the nasal retina and thus demonstrates the visual connections that developed while both eyes were open: if the suture is done at about the age of eye opening, the sutured eye develops no demonstrable vision at all. This is in striking contrast to the effect seen in non‐Siamese cats in which the sutured eye shows vision for the monocular crescent (Sherman, 1973).Experiments using alternate deprivation (one eye closed on the tenth day and then opened when the other is closed or removed at the eighth to fourteenth week) show that if one eye has sole access to the geniculo‐cortical system during the first eight or more weeks of life, functional connections demonstrable by behavioral testing of visual orientation are established permanently for the temporal retina of this eye. Thereafter the other eye cannot also establish functional connections for its temporal retina, even when the “successful” eye is sutured or removed surgically. In contrast, opening a previously sutured eye at the eighth or tenth week will allow visual development for the pathways from the temporal retina of the opened eye, provided that both eyes had been kept closed initially. Thus, after the eighth week, vision for the temporal retina can still be established, but we have been unable to make both temporal retinae share the pathways available in one hemisphere.Some segments of the geniculo‐cortical system are essentially non‐functional in Midwestern cats and other segments are made non‐functional by monocular deprivation. However, since all parts show about the same degree of retrograde reaction after cortical lesions, it appears that all parts send geniculo‐cortical axons to the visual cortex. The monocular suture itself affects geniculate cell growth in the normal segments of lamina A1 more than in the rest of the nucleus. The large monocular segments of lamina A, which are characeristic of the Siamese nucleus, do not show the sparing effect that is seen for geniculate cell growth in normal cats (Guillery and Stelzner, 1970), all parts of lamina A showing approximately the same cell size after monocular rearing.