Effects of early monocular lid suture upon neurons in the cat's medial interlaminar nucleus

Single unit extracellular recordings, cell size measurements, and cell packing density measurements were made in the medial interlaminar nucleus (MIN) of nine adult cats that had been monocularly deprived by lid suture prior to natural eye opening. The electrophysiological properties of neurons in the nondeprived regions of MIN (areas receiving input from the nondeprived eye) remained unaffected by monocular lid suture. The latencies to optic chiasm stimulation and receptive field properties, including receptive field center size, were essentially the same as those found for MIN neurons of normal adult cats. In contrast, cells in the deprived regions were severely affected by monocular deprivation. We encountered in the deprived regions of MIN only about one half as many active neurons per mm of electrode penetration as we did in the nondeprived regions. Of the physiologically active cells remaining, about one half had abnormal receptive field and/or response properties. This resulted in a sampling density of 5.1 normal Y-cells per mm of penetration in nondeprived regions of MIN compared to 1.0 normal Y-cell per mm in deprived regions of MIN. Histological effects of deprivation were also seen. Deprived regions of MIN were distinguished from nondeprived regions in four cats by autoradiography following intravitreal injection of tritiated proline into the deprived or nondeprived eye (2 cats each). The man cell size of deprived regions of MIN was 34% smaller than that of nondeprived regions. We did not find a difference in cell packing density between these two regions. It appears that the effects of monocular lid suture upon MIN are in most respects similar to the effects of monocular lid suture previously reported for the A laminae. Since MIN is composed solely of Y-cells, these data support the idea that the Y-pathways are more severely affected by visual deprivation than are the X-pathways. Further, since MIN projects largely outside the striate cortex, these data give the first clear demonstration of a primary effect of early lid suture upon extrastriate visual pathways.     

Studies of the cat's medial interlaminar nucleus: A subdivision of the dorsal lateral geniculate nucleus

The medial interlaminar nucleus (MIN) of the cat lies medial to the laminated region of the dorsal lateral geniculate (lamLGN). This latter region includes the A and C laminae. As does lamLGN, MIN receives direct retinal input and projects to various visual cortical areas. We examined the MIN of 15 normal adult cats with electrophysiological and anatomical techniques. Autoradiographs processed from cats that had one eye injected with tritiated fucose and proline indicate that MIN is composed of at least two laminae, one for each eye. The area which receives input from the ipsilateral eye is a small central region surrounded dorsally, medially, and ventrally by a larger crescent shaped region that receives input from the contralateral eye. This pattern was also evident from electrophysiological recording experiments. Extracellular recordings from 102 single-units in MIN indicate that these cells have properties essentially identical to lamLGN Y-cells. That is, they had short latencies to orthodromic stimulation of the optic chiasm and antidromic stimulation of the visual cortices, responded in a phasic manner to the presentation of a standing contrast within the receptive field center, responded to rapidly moving visual stimuli, and showed non-linear spatial summation properties typical of lamLGN Y-cells. We discovered two difference between MIN cells and lamLGN Y-cells. First the mean receptive field center size of MIN cells is considerably larger than that of lamLGN Y-cells, and second, MIN cells do not have the non-dominant eye inhibitory receptive fields found for many lamLGN Y-cells. Cell size measurements indicate that while the mean cell size in MIN is approximately 30% greater than in the A laminae of lamLGN, the distribution of MIN cell sizes extends over the full range of cell sizes in the A laminae. Since the A laminae are comprised mostly of X- and Y-cells, this suggests that, although Y-cells on average are larger than X-cells, considerable overlap exists in their size distribution. No differences between the ipsilateral and contra lateral terminal zones were found on any measure. Since MIN cells share most or all the fundamental features of lamLGN Y-cells, we suggest that these cell groups should be considered subpopulations of a more general group of geniculate Y-cells. Accordingly, we refer to these two subpopulations as lamLGN Y-cells and MIN Y-cells.     

Effects of visual deprivation upon the geniculocortical W-cell pathway in the cat: Area 19 and its afferent input

We studied the receptive field properties of 206 single units in area 19 of normal cats and 228 single units in area 19 of cats deprived of vision for 9-14 months by monocular lid suture. The ocular dominance of a sample of cells in area 17 of normal cats was studied for comparison. In some of these monocularly deprived animals, we also studied the sizes of relay cells in the parvocellular C laminae of the dorsal lateral geniculate nucleus labeled by electrophoretic injections of horseradish peroxidase into area 19. In area 19 of normal cats, the large majority of cells, regardless of their laminar location and the retinal eccentricity of their receptive fields, were binocular. Most responded equally well to the two eyes. In area 17, (see also Leventhal and Hirsch, ′78, ′80) but not in area 19, the cells which had the narrowest receptive fields tended to be activated unequally by the two eyes. In area 19 of monocularly deprived cats, virtually all cells (97%), re-gardless of their laminar location and receptive field eccentricity, responded only to stimulation of the normal eye. Thus, the effects of monocular dep-rivation upon area 19 are apparently more severe than those reported for area 17. In area 17 significant numbers of neurons in layer 4 can be activated by the deprived eye (Shatz and Stryker, ′78). Within the limits of our tech-nique, measurements of relay cells in the parvocellular C laminae labeled by injections into area 19 of deprived cats indicated that cell size in the deprived C laminae was unaffected by the deprivation. In contrast, cells in the deprived A laminae of these cats were severely shrunken. These findings suggest that the types of relay found in the parvocellular C laminae (referred to collectively as W-cells) are not affected by visual deprivation as severely as are the X- and Y-cells in the A laminae. Since laminar location and receptive field width are related to binocularity in area 17 but not in area 19 and the sizes of relay cells in the parvocellular C laminae (see also Hickey, ′80) are not seriously affected by monocular deprivation, it is suggested that binocular interactions in area 19aremninly determined by connections among cortical cells.     

The effect of monocular deprivation on cells in the C-laminae of the cat lateral geniculate nucleus

Five cats monocularly deprived by lid suture between 2 and 3 weeks of age were used experimentally between the ages of 16 and 52 weeks. An intraocular injection of horseradish peroxidase was used to reveal the individual C laminae (C, C1, C2) in the dorsal lateral geniculate nucleus. Soma size measurements of cells in the C laminae showed significant differences between cells in the deprived layers C and C1 and the corresponding nondeprived layers, but not between cells in deprived and nondeprived layer C2.     

Morphology and physiology of single neurons in the medial interlaminar nucleus of the cat's lateral geniculate nucleus

Physiological studies have shown that the cat's retinogeniculocortical system is comprised of at least three parallel and independent pathways, the W-, X-, and Y-cell pathways. The morphological correlates of the constituent W-, X-, and Y-cells have been determined both in the retina and in the A and C laminae of the lateral geniculate nucleus. The aim of this study was to extend these structure/function relationships to neurons in laminae 1 and 2 of the medial interlaminar nucleus (MIN), which is a division of the cat's dorsal lateral geniculate nucleus. We used intracellular injection of horseradish peroxidase (HRP) into individual, physiologically identified MIN neurons. Since this procedure may yield an unrepresentative sample of MIN neurons, two controls were performed. First Nissl staining showed that the soma sizes of intracellularly labeled cells were representative of those of all MIN cells. Second, retrograde labeling following HRP injections into the optic radiations or specific visual cortical areas showed that the intracellularly labeled MIN cells were representative of MIN relay neurons. Many of the retrogradely labeled cells were so well filled that their entire dendritic arbors were revealed. Of 70 MIN neurons recorded physiologically, 22 were injected with HRP and successfully recovered. We also completely labeled the somata and dendrites of 114 MIN neurons from HRP injections into the optic radiations and retrogradely labeled 165 MIN neurons by injection of HRP into visual cortical areas. Our sample of intracellularly injected neurons, which were all Y-cells, were morphologically representative of all MIN relay cells. We thus conclude that laminae 1 and 2 of the MIN contain a nearly homogeneous population of Y-cells with properties essentially identical to those of Y-cells in the A and C laminae of the lateral geniculate nucleus. When viewed in the coronal plane, MIN projection neurons typically exhibited oval or elongated somata. In the medial and ventral parts of the MIN, these somata were smaller and more flattened. MIN soma sizes extended over the full range of those seen in the A laminae. Dendritic arbors of most MIN relay neurons radiated in a fairly spherical fashion. In the medial and ventral parts of the MIN, however, dendrites were oriented in a more bipolar fashion, but intermediate forms between spherical and bipolar arbors were also common. Dendrites of MIN projection neurons were typically smooth; most primary dendrites were straight, but secondary dendrites were more variable in structure.(ABSTRACT TRUNCATED AT 400 WORDS)     

The projection from the lateral geniculate nucleus onto the visual cortex in the cat. A quantitative study with horseradish-peroxidase

Horseradish-peroxidase (HRP) was injected (9-18 μg in 0.03-0.06 μl) into cortical areas 17, 18 or 19 of 11 adult cats. After survival times of 17 hours to 7 days, the thalamus was examined for retrogradely HRP labelled nerve cells in serial transverse sections. From these sections, the percentage of labelled cells occurring in each subdivision of the dorsal lateral geniculate nucleus (LGNd) was calculated for each animal. One case each for injections in areas 17, 18 and 19 was then chosen for nerve cell size measurements in each LGNd subdivision. The peri-karyal area of each labelled cell (N=689), and of representative samples of unlabelled cells (N=1137), was measured by planimetry. Size distribution histograms, mean values, standard deviations, and statistical significance levels were obtained by computer. It was found that area 17 receives a projection almost exclusively from laminae A and Al, and that the projecting cells belong to all cell size classes. Area 18 receives a projection mainly from laminae C and Al, and from the medial interlaminar nucleus (MIN). The projecting cells belong mainly to the large cell size classes. Area 19 receives a projection largely from MIN, and also from the C-laminae and extrageniculate cell groups. The projecting cells belong to all cell size classes, with some emphasis on the large cells of lamina C. A significant projection was found to exist from the parvocellular laminae of LGNd onto area 19 and, to a lesser degree, area 18. In conclusion, as one goes from area 17 to 18 and to 19 the projection source shifts from the A-laminae through the C-laminae on to MIN and extrageniculate cell groups. The cells which project to area 18 are on the whole larger, than those which project to areas 17 and 19. A significant proportion of the contralateral visual input to area 18 is relayed via lamina G. These results provide a quantitative confirmation and extension of previous anatomical findings, and are in close relationship with physiological results regarding parallel channel processing in the visual system.     

The effects of monocular deprivation on the visual latency of geniculate X- and Y-cells in the cat

Monocular lid suture deprivation during early visual development of the cat alters the temporal flow of retinal information as it passes through the dorsal lateral geniculate nucleus. The latencies of Y-cells located in the deprived layers and of both X- and Y-cells in the non-deprived layers are shorter than the latencies of their counterparts in normally reared cats. The visual response onset latencies of X-cells located in the deprived geniculate layers lag those of X-cells in the non-deprived layers.     

Effects of monocular deprivation on geniculocortical synapses in the cat

In monocularly deprived (MD) cats, many cells in the lateral geniculate nucleus (LGN) but few cells in the visual cortex respond to input from the deprived eye, suggesting that the connections to visual cortex from the deprived geniculate laminae may have been disrupted. I have examined these connections in MD cats by using electron microscopic autoradiography of visual cortex after injections of tritiated lysine into single laminae of LGN. After injections into either deprived or experienced laminae, there was label over terminals that contained mitochondria and round synaptic vesicles and that made asymmetric contacts with dendritic profiles. However, the terminals of deprived afferents differed from those of experienced afferents: They were 25% smaller, contained 33% fewer mitochondria, were more likely to make synapses that were presynaptically convex (and thus, perhaps, immature), and synapsed onto smaller spines. These morphological changes were greater for afferents to upper layer IV than for afferents to lower layer IV. The geniculocortical synapses from deprived laminae were also reduced in number. To correct for variations in injection size and for a probable reduction in protein synthesis by cells in the deprived laminae, I computed the ratio of labeled synaptic terminals to labeled myelinated axons. Injections into the deprived laminae labeled 43% fewer synaptic terminals per labeled myelinated axon than did injections into the experienced lamina. The finding that the synaptic terminals of deprived afferents are both abnormal morphologically and fewer in number can help to explain the reduced effectiveness of the deprived eye in driving cortical cells.     

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 deprivatin 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.     

Rearing cats with eyelid suture has both early and late effects on cells in the lateral geniculate nucleus

We have assessed the effects of duration of infant-onset deprivation, and therefore the age of the subject at the time of data collection, on the physiology and morphology of cells in the lateral geniculate nucleus (LGN) of cats. Twenty-two kittens underwent lid suture. Electrophysiological experiments were performed in 12 of these subjects when they were between 5 and 16 months of age. The remaining 10 cats were studied between 17 and 29 months. In 16 of these same subjects we also measured LGN soma sizes to permit a direct within-subject comparison of the morphological and physiological effects of lid suture. Physiological data from cats recorded before 17 months of age showed a reduction in the encounter rate of Y-cells in deprived LGN laminae. In contrast, none of the cats which were 17 months or older at the time of recording showed a reduction in the encounter rate of deprived Y cells, giving the appearance of a more normal X/Y-cell ratio. Preliminary observations suggest that these late changes in the physiological effects of deprivation are not due to a recovery of Y-cells, but are more likely due to the superimposition of a reduction in the encounter rate for X-cells known to be typical of a variety of adult-onset deprivations. Finally, the physiological and morphological differences between non-deprived and deprived LGN laminae are correlated for the individual subjects.     

Spatial and temporal sensitivity of lateral geniculate cells in dark-reared cats

Electrophysiological recordings were made from single-units in the dorsal lateral geniculate nucleus of normally-reared and dark-reared cats. In agreement with previous studies, significantly fewer Y-cells were encountered in dark-reared cats than in normally-reared cats. In addition, a large number of cells having abnormal receptive field and/or response properties were observed in the dark-reared animals. Included in this latter group were a number of cells that had short latencies to electrical stimulation of the optic chiasm, which is indicative of Y-cells, but did not display the nonlinear response component usually observed in Y-cells. Measures of contrast sensitivity across a wide range of spatial and tempora modulation frequencies were made with counterphased grating stimuli. No differences were observed for these measures between X-cells in normally-reared and dark-reared cats. In addition no differences were observed between Y-cells in normally-reared cats and the few Y-cells in dark-reared cats.While previously reported data from monocularly deprived cats indicate that geniculate X-cells can be affected by an abnormal visual environment created through lid-suture, the present data indicate that the X-system can develop normal electrophysiological properties in the absence of light. This observation suggest that the development of the X-system is not necessarily dependent on the influence of normal visual expereince. In contrast, it appears that the cat must experience a normal visual environment in order for all of its Y-system to develop properly.     

Physiological and morphological changes in cells of the lateral geniculate nucleus in monocularly-deprived and reverse-sutured cats

In the present study the size of large samples of cells and the relative frequency of Y-cells were measured in the lateral geniculate nucleus of monocularly-deprived and reverse-sutured cats. Due to deprivation large cells shrink more than smaller cells. This shrinkage is irreversible even over two years of reverse suture although over the same time the animals showed a remarkable pattern discrimination ability, which, in these animals, depends on the integrity of the geniculo-cortical system. Physiologically, a significant increase in the probability of recording Y-cells in the early-deprived laminae of the LGNd was found after a reverse suture. The relative frequency of Y-cells in the early-deprived layers and in the late-deprived layers was almost equal and not different from normal, although after the early deprivation by itself only 20% Y-cells were found in the deprived layers. The latter results confirm our previous observation (Sherman et al., '72). The increase in the number of recorded Y-cells following a reverse suture after monocular deprivation is accompanied by only small changes in the distribution of cell sizes in the LGNd. The number of very large cells in the early-deprived layers was increased but the mean cell size remained unaltered after long forced usage of the deprived eye.     

The projection of the lateral geniculate nucleus to area 18

The present study is concerned with the projection of the lateral geniculate nucleus onto cortical area 18. Horseradish peroxidase (HRP) was injected into area 18 of 15 cats. Drawings were made to determine the location of the injection site and the distribution of labeled neurons in the lateral geniculate nuclei of each cat. The local retinotopic maps constructed prior to the injections and the reconstructions of the lateral geniculate nucleus were used to determine the location and the extent of each of the HRP injections. In 15 of the 25 hemispheres studied, the ratio of the number of HRP-labeled neurons in lamina A relative to the number of labeled neurons in lamina A1 was calculated. This ratio varied from 1.06 to 0.28, indicating that at least some regions of area 18 are dominated by inputs from lamina A1. However, if the HRP-labeled neurons in lamina C are included in the counts for lamina A, then the ratio A + C/A1 has a mean of 1.11, suggesting that area 18 receives a balanced input, with inputs from the contralateral eye being relayed through laminae A and C, and inputs from the ipsilateral eye being relayed through lamina A1. When the distribution of HRP-labeled neurons in lamina A was plotted onto a dorsal view of the lateral geniculate nucleus, the labeled neurons formed an ellipse with the long axis of the ellipse oriented parallel to the isoelevation lines. The representation of azimuth is compressed in area 18 relative to the lateral geniculate nucleus. In six hemispheres the injections were restricted to a few layers of the area 18. Following small injections into layer IV of area 18, the HRP-labeled neurons occupied an extensive region of the lateral geniculate nucleus, indicating a considerable amount of convergence of the inputs to area 18. In hemispheres where the injections were restricted to layers I and II, labeled neurons were only seen in the medial interlaminar nucleus and the C laminae.     

Quantitative studies of cell size in the cat's dorsal lateral geniculate nucleus following visual deprivation

The effects of visual deprivation upon dorsal lateral geniculate (DLG) cell size were compared for seven kittens reared with monocular lid-suture (MD), seven with binocular lid-suture (BD), and six with one eye lid-sutured and the other eye enucleated soon after birth (MD-E). Six additional kittens were reared normally for comparison. For each kitten the cross-sectional areas of 300 cells were measured in one or both nuclei. Measurements were taken from the binocular segment of laminae A and A1 and th monocular segment of lamina A. In agreement with previous studies, cells in th binocular segment of the deprived laminae of MD cats were smaller (33-34%) than those in the non-deprived laminae. Comparisons with normal animals indicated that this difference was due to an increase (10-15%) in size of cells in the non-deprived laminae as well as a decrease (23-25%) in size of cells in the deprived laminae. Cells in the monocular segment also were affected by deprivation in MD cats, and this effect increased with the age (and duration of the deprivation) of the animal. However, it was always smaller than the decrease in sell size in the binocular portion of the DLG. In BD kittns, DLG cells were smaller (7-12%) than normal in all portions of the nucleus, including both the binocular and monocular segments. Direct comparisons between the deprived laminae of MD and BD kittens indicated that the decrease in cell size was greater for MD kittens in the binocular segment, but tended to be greater for BD kittens in the monocular segment. IN MD-E kittens, DLG cells in the deprived laminae were smaller (11-17%) than normal in all portions of the nucleus, including both the binocular and monocular segments. Thus, the effects of deprivation were similar to those in BD kittens, even though inputs from the deprived eye had been placed a competitive advantage in MD-E kittens. These results indicate that two factors may affect cell size in the DLG of visually deprived cats: deprivation per se and abnormal binocular competition. Finally, separate analyses for the ten largest and the ten smallest cells in each lamina of each cat were carried out in an attempt to determine if the changes in cell size were limited to the largest cells. In every case, differences observed for the total sample of cells were paralleled by differences from normal of both the largest cells present and the smallest cells present in the deprived laminae. Since at least two alternative interpretations can account for this finding, the question of whether the large cells are selectively affected by visual deprivation remains unanswered in the cat.     

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

The 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.     

Effects of monocular deprivation on the cat's geniculate neurons projecting to both areas 17 and 18

When a kitten is reared with one eyelid sutured closed, there are profound changes in the developing visual system. In the lateral geniculate nucleus, the neurons in the laminae innervated by the deprived eye are smaller than normal, and some of these neurons may lose connections with the visual cortex. In the present study a variety of double label retrograde transport methods were used to define the effects of monocular deprivation on cortical projections of geniculate neurons. One marker was injected into area 17 and the other was injected into area 18. Neurons projecting to area 17 are on average 16.4% smaller than those in the nondeprived laminae. The neurons that normally would project to both areas 17 and 18 by an axon that branches are the most severely affected by monocular deprivation. These cells are nearly 40% smaller than their counterparts in the nondeprived laminae, and many of the neurons appear to lose their projection to one of the cortical areas. These neurons may be at a distinct disadvantage, since they must compete with neurons from the nondeprived laminae for a considerable amount of cortical territory in two different cortical areas. This competition may be so severe that some of the neurons are no longer capable of maintaining connections with both cortical areas.     

Abnormal Y/X ration in the area centralis of the siamese cat

The responses of X- and Y-type retinal ganglion cells were recorded in the optic tract of Siamese cats in order to investigate the encounter rate of Y-cells with respect to retinal eccentricity. The percentage of Y-cells in Siamese cats was highest in or near the area centralis and it decreased with eccentricity. This is in contrast to the proportion of Y-cells in normally pigmented cats, which was lowest in the area centralis and increased with eccentricity. Thus, the Y/X ratio in Siamese cats is higher in the area centralis, but significantly lower in the peripheral retina compared to those in normal controls. The lower percentage of Y-cells in the periphery parallels an additional finding that Y-cells exhibiting very high conduction velocities were missing from the Siamese cat optic tract. Finally, the receptive-field center (RFC) size of X-cells in the area centralis was larger in Siamese cats, and the correlation between the RFC size and conduction velocity was weak in these animals. The results are discussed in terms of behavioral deficits in Siamese cat vision.     

Relay cell classes in the lateral geniculate nucleus of the cat and the effects of visual deprivation.

This study presents evidence that the X- and Y-cells described physiologically in the A laminae of the cat's dorsal lateral geniculate nucleus (LGN) are two morphologically distinct cell types recognizable in Golgi preparations. It is shown firstly that the three cell types seen in Golgi preparations of the A laminae (large and medium-sized principal cells and small interneurons-types 1,2 and 3 in the classification of Guillery, '66) may be identified in 1-mum Epon sections of osmicated material. While cell-diameter histograms prepared from serial 1-mum sections show a unimodal distribution of cell sizes, three populations can be distinguished if attention is paid to the presence or absence of large cytoplasmic inclusions (laminar bodies). These three populations consist of large cells lacking laminar bodies (Class I), medium-sized cells possessing laminar bodies (Class II) and small cells lacking them (Class III). That these three classes correspond to the three morphological types has been shown by (i) size comparisons, and (ii) direct demonstration of laminar bodies in the Golgi-impregnated cell bodies of Guillery's type 2 cells. Histograms prepared in this way for samples taken at various positions in the LGN show that the numbers of class II cells decline from the representation of the area centralis to the monocular segment. This decline is compensated by a corresponding rise in the numbers of class I cells. This pattern of distribution is similar to the physiologically observed distribution of X- and Y-cells, indicating that X-cells are likely to be class II cells and Y-cells class I cells. The cortical projections of the various cell types have been examined by the horseradish peroxidase method. Class II cells project to area 17 only. Most class I cells also project to area 17 only, but a few very large class I cells project to area 18. From our results, it appears that very few if any cells in the A laminae have branching axons supplying both 17 and 18. The class III cells do not project to the visual cortex, a finding consistent with their identification as interneurons. Class I and II cells are also found in lamina C and in the MIN. In both these regions there is a predominance of very large class I cells, which project to area 18. Laminae Cl-C3 contain small cells lacking laminar bodies. These cells may project to both areas 17 and 18 with branching axons. They are likely to correspond to Guillery's type 4 cells (small relay cells confined to the C laminae) and to the physiologically described W-cells. Long-term monocular deprivation causes cell shrinkage which is much more severe for class I than for class II cells. There is in addition a decrease in the relative numbers of class I cells. This decrease is found in binocular deprivation also. These observations provide an anatomical basis for the reported loss of Y-cells from deprived laminae of the LGN. It is suggested that the effects of deprivation on Y-cells may be accounted for in terms of competition for synaptic space.     

Experimental induction of an abnormal ipsilateral visual field representation in the geniculocortical pathway of normally pigmented cats

In the normally pigmented neonatal cat, many ganglion cells in temporal retina project to the contralateral dorsal lateral geniculate nucleus (LGNd) and medial interlaminar nucleus (MIN). Most of these cells are eliminated during postnatal development. If one optic tract is sectioned at birth, much of this exuberant projection from the contralateral temporal retina is stabilized (Leventhal et al., 1988b). To determine how the abnormal projection from the contralateral temporal retina is accommodated in the central visual pathways, neuronal activity was recorded in the visual thalamus and cortex of adult cats whose optic tracts were sectioned as neonates. The recordings showed that up to 20 degrees of the ipsilateral hemifield is represented in the LGNd and MIN. Recordings from areas 17 and 18 of the intact visual cortex showed that up to 20 degrees of the ipsilateral visual field is also represented and that the ipsilateral representation is organized as in a Boston Siamese cat (Hubel and Wiesel, 1971; Shatz, 1977; Cooper and Blasdel, 1980) or a heterozygous albino cat (Leventhal et al., 1985b). The extent of the ipsilateral visual field representation was greater in area 18 than in area 17; the extent of the ipsilateral hemifield representation in areas 17 and 18 varied with elevation, increasing with distance from the horizontal meridian. The receptive fields of cells in the LGNd and visual cortex subserving contralateral temporal retina were abnormally large. Otherwise, their receptive field properties seemed normal. In the same animals studied physiologically, HRP was injected into the ipsilateral hemifield representation in the LGNd and MIN of the intact hemisphere. The topographic distribution of the alpha and beta cells, respectively, labeled by these injections correlated with the elevation-related changes in the ipsilateral visual field representation in areas 18 and 17. Our results indicate that the retinotopic organization of the mature geniculocortical pathway reflects the abnormal pattern of central projections of ganglion cells in neonatally optic tract sectioned cats. Thus, if they do not die, retinal ganglion cells normally eliminated during development are capable of making seemingly normal, functional connections. The finding that an albino-like representation of the ipsilateral hemifield can be induced in the visual cortex of normally pigmented cats suggests that the well-documented defects in the geniculocortical pathways of albinos are secondary to the initial misrouting of ganglion cells at the optic chiasm (Kliot and Shatz, 1985) and not a result of albinism per se.     

Rearing with monocular lid suture induces abnormal NADPH-diaphorase staining in the lateral geniculate nucleus of cats

We investigated the changes in NADPH-diaphorase staining that occur in the lateral geniculate nucleus of cats following rearing with monocular lid suture. This staining allows visualization of the synthesizing enzyme of nitric oxide, a neuromodulator associated with plasticity. In the lateral geniculate nucleus of normally reared cats, NADPH-diaphorase exclusively labels the axons and terminals of an input from the parabrachial region of the brainstem; no geniculate cells in the A-laminae are labeled. Early monocular lid suture has no obvious effect on the NADPH-diaphorase staining of parabrachial axons. However, this lid suture results in the abnormal appearance of NADPH-diaphorase staining for geniculate somata. These cells are located primarily in the nondeprived laminae. Double-labeling experiments indicate that these cells with abnormal NADPH-diaphorase reactivity are Y relay cells: NADPH-diaphorase staining is found in cells retrogradely labeled from visual cortex; it is found in cells labeled with a monoclonal antibody for CAT-301, which selectively targets Y cells; it is not found in cells labeled with an anti-GABA antibody, which targets interneurons. Also, NADPH-diaphorase labeled cells are among the largest cells in the nondeprived laminae, again suggesting that they are Y relay cells. We cannot suggest a specific mechanism for this induction of NADPH-diaphorase labeling, but it does not seem to be due to abnormal binocular competition induced by the monocular lid suture. © 1994 Wiley-Liss, Inc.