Aging affects contrast response functions and adaptation of middle temporal visual area neurons in rhesus monkeys.

In the present study we studied the effects of aging on the coding of contrast in area V1 (primary visual cortex) and MT (middle temporal visual area) of the macaque monkey using single-neuron in vivo electrophysiology. Our results show that both MT and V1 neurons in old monkeys are less sensitive to contrast than those in young monkeys. Generally, contrast sensitivity is affected by aging more severely in MT cells than in V1 cells. Specifically, MT cells were affected more severely than motion direction selective V1 cells. Particularly, we found that MT neurons in old monkeys exhibited enhanced maximum visual responses, higher levels of spontaneous activity and decreased signal-to-noise ratios. In addition, we also found age-related changes in neuronal adaptation to visual motion in MT. Compared with young animals, the contrast gain of MT neurons in old monkeys is less affected, but the response gain by adaptation of MT neurons is more affected. Our results suggest that there may be an anomalous visual processing in both the magnocellular and parvocellular pathways. The neural changes described here are consistent with an age-related degeneration of intracortical inhibition and could underlie some deficits in visual function during normal aging.

Functional degradation of extrastriate visual cortex in senescent rhesus monkeys.

The receptive field properties of striate cortical (V1) cells degrade in senescent macaque monkeys. We have now carried out extracellular single unit studies of the receptive field properties of cells in extrastriate visual cortex (area V2) in very old rhesus (Macaca mulatta) monkeys. This study provides evidence that both the orientation and direction selectivities of V2 cells in old monkeys degrade significantly. Decreased selectivity is accompanied by increased visually driven and spontaneous responses. As a result, V2 cells in old animals exhibit markedly decreased signal-to-noise ratios. A significant degradation of neural function in extrastriate cortex may underlie the declines in higher order visual function that accompany normal aging.

Abnormal retinotopic organization of the dorsal lateral geniculate nucleus of the tyrosinase-negative albino cat.

Visual defects associated with hypopigmentation have been studied extensively in Siamese and albino cats. Previous research on tyrosinase-negative albino cats has shown that (1) approximately 95% of all nasal and temporal retinal fibers cross at the optic chiasm, and (2) ocular dominance columns normally found in cortex are replaced with hemiretinal domains. In this study, we compared the retinotopic organization of the dorsal lateral geniculate nucleus (LGNd) and visual cortex in albino cats. Extracellular recordings were conducted in the LGNd, area 17, and area 18 of six albino cats. Receptive fields (RFs) were plotted for all sites. We find that, as in albino visual cortex, the albino LGNd contains (1) normal cells with RFs in the visual hemifield contralateral to the recording site (RFc), (2) abnormal cells with RFs in the ipsilateral hemifield (RFi), (3) abnormal cells with dual, mirror-symmetric RFs, one in each hemifield (RFd), and (4) abnormal cells with broad RFs that span the vertical meridian (RFb). Our data indicate that lamina A and lamina A1 consist predominantly of normal RFc and abnormal RFi cells, respectively. The C laminae contain a mixture of RFc, RFi, RFd, and RFb cells. The interlaminar zones contained RFd cells, RFb cells, or both. Thus, the albino LGNd is arranged into hemiretinal and not ocular dominance laminae. Finally, the percentage of normal cells is significantly larger in area 17 (84%) and area 18 (70%) than in the LGNd (46%), suggesting a suppression of abnormal activity in albino cat cortex, which could underlie the existing competence of visual function in albinos.

Degradation of stimulus selectivity of visual cortical cells in senescent rhesus monkeys.

Human visual function declines with age. Much of this decline is probably mediated by changes in the central visual pathways. We compared the stimulus selectivity of cells in primary visual cortex (striate cortex or V1) in young adult and very old macaque monkeys using single-neuron in vivo electrophysiology. Our results provide evidence for a significant degradation of orientation and direction selectivity in senescent animals. The decreased selectivity of cells in old animals was accompanied by increased responsiveness to all orientations and directions as well as an increase in spontaneous activity. The decreased selectivities and increased excitability of cells in old animals are consistent with an age-related degeneration of intracortical inhibition. The neural changes described here could underlie declines in visual function during senescence.

Neural correlates of boundary perception.

The responses of neurons in areas V1 (17) and V2 (18) of anesthetized and paralyzed rhesus monkeys and cats were recorded while presenting a set of computer-generated visual stimuli that varied in pattern, texture, luminance, and contrast. We find that a class of extrastriate cortical cells in cats and monkeys can signal the presence of boundaries regardless of the cue or cues that define the boundaries. These cue-invariant (CI) cells were rare in area V1 but easily found in V2. CI cortical cells responded more strongly to more salient boundaries regardless of the cue defining the boundaries. Many CI cortical cells responded to illusory contours and exhibited the same degree of orientation and direction selectivity when tested with boundaries defined by different cues. These cells have significant computational power inherent in their receptive fields since they were able to generalize across stimuli and integrate multiple cues simultaneously in order to signal boundaries. Cells in higher order cortical areas such as MT (Albright, 1992), MST (Geesaman & Anderson, 1996), and IT (Sary et al., 1993) have previously been reported to respond in a cue invariant fashion. The present results suggest that the ability to respond to boundaries in a cue-invariant manner originates at relatively early stages of cortical processing.

Signal timing across the macaque visual system.

The onset latencies of single-unit responses evoked by flashing visual stimuli were measured in the parvocellular (P) and magnocellular (M) layers of the dorsal lateral geniculate nucleus (LGNd) and in cortical visual areas V1, V2, V3, V4, middle temporal area (MT), medial superior temporal area (MST), and in the frontal eye field (FEF) in individual anesthetized monkeys. Identical procedures were carried out to assess latencies in each area, often in the same monkey, thereby permitting direct comparisons of timing across areas. This study presents the visual flash-evoked latencies for cells in areas where such data are common (V1 and V2), and are therefore a good standard, and also in areas where such data are sparse (LGNd M and P layers, MT, V4) or entirely lacking (V3, MST, and FEF in anesthetized preparation). Visual-evoked onset latencies were, on average, 17 ms shorter in the LGNd M layers than in the LGNd P layers. Visual responses occurred in V1 before any other cortical area. The next wave of activation occurred concurrently in areas V3, MT, MST, and FEF. Visual response latencies in areas V2 and V4 were progressively later and more broadly distributed. These differences in the time course of activation across the dorsal and ventral streams provide important temporal constraints on theories of visual processing.

Evidence that retinal ganglion cell density affects foveal development.

The monkey's foveola normally contains significant numbers of retinal ganglion cells. The somata of foveola cells are larger than those of other cells in the central retina. Their dendritic fields are up to 50 times larger in area than those of nearby cells in the foveal slope. Experimentally induced reductions in the number of ganglion cells in central retina results in alterations in the size and distribution of cells within the foveola. In these animals the foveola is abnormally small and contains an abnormally large number of cells having smaller than normal cell bodies and dendritic fields. These studies indicate that the formation of the foveola as well as the development of the morphology of cells within the foveola and foveal slope depend during development on high densities of retinal ganglion cells within the central retina.

Direction biases of X and Y type retinal ganglion cells in the cat.

1. It has been reported that in the cat only a specialized group of retinal ganglion cells constituting approximately 1% of the overall population are direction sensitive. Two major groups of retinal ganglion cells, the X and Y cells, have been reported not to be sensitive to the direction of stimulus motion. 2. We recorded action potentials of retinal ganglion cells intraocularly. We studied quantitatively the visual responses elicited by drifting sinusoidal gratings of various spatial frequencies, bars, and spots. 3. The results confirm previous reports that most cat retinal ganglion cells exhibit orientation biases when tested with gratings of relatively high spatial frequency. 4. Additionally, we find that 22% of X and 34% of Y type retinal ganglion cells exhibit direction biases. Overall, Y cells displayed significantly stronger direction biases than did X cells. 5. In general, direction biases are clearest when the test gratings are of relatively low spatial frequency. 6. The direction biases of X and Y cells subserving the central 15 degrees of retina were weaker than those of cells subserving more peripheral regions. 7. The direction-biased responses of cat ganglion cells were similar to those of X and Y type relay cells in the cat dorsal lateral geniculate nucleus (LGNd). Thus we suggest that the direction biases of LGNd cells are a reflection of their retinal inputs.

Abnormal ipsilateral visual field representation in areas 17 and 18 of hypopigmented cats.

We compared the central projections of retinal ganglion cells in temporal retina and the cortical representation of visual fields in areas 17 and 18 in cats with various hypopigmentation phenotypes (albino, heterozygous albino, Siamese, and heterozygous Siamese). In all cats studied, we found that the extent of abnormal ipsilateral visual field representation varied widely, and more of the ipsilateral visual field was represented in area 18 than in area 17. The greatest degree of ipsilateral visual field representation was found in albino cats, followed by Siamese, heterozygous albino and heterozygote Siamese cats, respectively. Additionally, in the different groups there was wide variation in the numbers of contralaterally projecting alpha and beta ganglion cells in temporal retina. In all cases, however, contralaterally projecting alpha cells were found to extend further into temporal retina than beta cells. We found that in each cat studied, the maximum extent of the abnormal ipsilateral visual field representation in areas 18 and 17 corresponded to the location of the 50% decussation line (i.e., the point where 50% of the ganglion cells in temporal retina project to the contralateral hemisphere) for alpha and beta cells, respectively, for that cat. Our results suggest that the extent of the abnormal visual field representations in visual cortex of hypopigmented cats reflects the extent of contralaterally projecting retinal ganglion cells in temporal retina.

Concomitant sensitivity to orientation, direction, and color of cells in layers 2, 3, and 4 of monkey striate cortex.

The receptive field properties of cells in layers 2, 3, and 4 of area 17 (V1) of the monkey were studied quantitatively using colored and broad-band gratings, bars, and spots. Many cells in all regions studied responded selectively to stimulus orientation, direction, and color. Nearly all cells (95%) in layers 2 and 3 exhibited statistically significant orientation preferences (biases), most exhibited at least some color sensitivity, and many were direction sensitive. The degree of selectivity of cells in layers 2 and 3 varied continuously among cells; we did not find discrete regions containing cells sensitive to orientation and direction but not color, and vice versa. There was no relationship between the degree of orientation sensitivity of the cells studied and their degree of color sensitivity. There was also no obvious relationship between the receptive field properties studied and the cells' location relative to cytochrome oxidase-rich regions. Our findings are difficult to reconcile with the hypothesis that there is a strict segregation of cells sensitive to orientation, direction, and color in layers 2 and 3. In fact, the present results suggest the opposite since most cells in these layers are selective for a number of stimulus attributes.

Visual deprivation does not affect the orientation and direction sensitivity of relay cells in the lateral geniculate nucleus of the cat.

Visual deprivation in early life profoundly affects the characteristic sensitivity of visual cortical cells to stimulus orientation and direction. Recently, relay cells in the lateral geniculate nucleus (LGNd) have been shown to exhibit significant degrees of orientation and direction sensitivity. The effects of visual deprivation upon these properties of subcortical cells are unknown. In this study cats were reared from birth to 6-12 months of age in total darkness; the orientation and direction sensitivities of area 17 (striate cortex) and LGNd cells were compared. All cells were studied using identical quantitative techniques and statistical tests designed to analyze distributions of angles. The results confirm previous work and indicate that the orientation and direction sensitivities of cells in area 17 are profoundly reduced by dark rearing. In marked contrast, these properties of LGNd relay cells are unaffected. The result is that, unlike in the normal cat, in dark-reared cats the orientation and direction sensitivities of cells in the LGNd and visual cortex do not differ. It is concluded that (1) the orientation and direction sensitivities of cortical cells contribute little, if at all, to the sensitivities of LGNd cells since LGNd cells exhibit normal sensitivities even though the cortical cells projecting to them exhibit greatly reduced sensitivities and (2) during normal development intracortical mechanisms appear to expand upon and/or modify the weak orientation and direction sensitivities of their inputs. These intracortical mechanisms depend upon normal visual experience since in dark-reared cats, but not normal ones, the orientation and direction sensitivities of cells in the LGNd and visual cortex do not differ quantitatively or qualitatively.

Stimulus dependence of orientation and direction sensitivity of cat LGNd relay cells without cortical inputs: a comparison with area 17 cells.

The cortical contribution to the orientation and direction sensitivity of LGNd relay cells was investigated by recording the responses of relay cells to drifting sinusoidal gratings of varying spatial frequencies, moving bars, and moving spots in cats in which the visual cortex (areas 17, 18, 19, and LS) was ablated. For comparison, the spatial-frequency dependence of orientation and direction tuning of striate cortical cells was investigated employing the same quantitative techniques used to test LGNd cells. There are no significant differences in the orientation and direction tuning to relay cells in the LGNd of normal and decorticate cats. The orientation and direction sensitivities of cortical cells are dependent on stimulus parameters in a fashion qualitatively similar to that of LGNd cells. The differences in the spatial-frequency bandwidths of LGNd cells and cortical cells may explain many of their differences in orientation and direction tuning. Although factors beyond narrowness of spatial-frequency tuning must exist to account for the much stronger orientation and direction preferences of cells in area 17 when compared to LGNd cells, the evidence suggests that the orientation and direction biases present in the afferents to the visual cortex may contribute to the orientation and direction selectivities found in cortical cells.

Direction-sensitive X and Y cells within the A laminae of the cat's LGNd.

Drifting sinusoidal gratings, moving bars, and moving spots were employed to study the direction sensitivity of 425 neurons in the A laminae of the cat's LGNd. Thirty-two percent of X- and Y-type LGNd relay cells exhibit significant direction sensitivity when tested with drifting sinusoidal gratings. X and Y cells exhibit the same degree of direction sensitivity. Moving spots and bars elicit direction specific responses from LGNd cells that are consistent with those elicited when drifting sinusoidal gratings are employed. For cells that are both orientation and direction sensitive, the preferred direction tends to be orthogonal to the preferred orientation. In general, direction sensitivity is strongest at relatively low spatial frequencies, well below the spatial-frequency cutoff for the cell. The presence of significant numbers of direction-sensitive LGNd cells raises the possibility that subcortical direction specificity is important for the generation of this property in the visual cortex.

Postnatal development of different classes of cat retinal ganglion cells.

Previous investigators have documented the postnatal development of alpha and beta type ganglion cells in cat retinae (Ramoa et al. [1987] Science 237:522-525; Ramoa et al. [1988] J. Neurosci. 8:4239-4261; Dann et al. [1987] Neurosci. Lett. 80:21-26; Dann et al. [1988] J. Neurosci. 8(5):1485-1499). The development of the remaining cells (about 50%), which constitute a heterogeneous group and are referred to here collectively as gamma cells (Boycott and Wässle, '74), has not been studied in detail. The purpose of this study was to compare the postnatal development of alpha, beta, and gamma cells in kitten and adult retinae using horseradish peroxidase histochemistry and the fluorescent dye DiI. In the kitten, alpha, beta, and gamma cells are recognizable. We find, as have others, that kitten alpha and beta cell bodies and dendritic fields are significantly smaller than in the adult. However, kitten gamma cells are nearly adult sized. In fact, at birth the cell bodies of beta cells throughout the retina are significantly smaller than those of gamma cells. During the first 12 weeks of life, alpha and beta cell bodies increase in size from 90% to 680% depending upon eccentricity. Gamma cells hardly increase in size at all. Also, the normal adult center-to-peripheral cell size gradient for alpha and beta cells is not seen in the neonate. Gamma cells show no such gradient in the neonate or adult. Our results suggest that the morphological development of alpha and beta cells occurs later than that of gamma cells and may explain some of the differences in the effects of visual deprivation and surgical manipulation upon the parallel Y-, X-, and W-cell pathways.

Retinal ganglion cells within the foveola of New World (Saimiri sciureus) and Old World (Macaca fascicularis) monkeys.

The morphology and distribution of retinal ganglion cells within the foveola of New World (Saimiri sciureus) and Old World (Macaca fascicularis) monkeys labeled as a result of horseradish peroxidase injections into the dorsal lateral geniculate nucleus were studied. The results indicate that monkey's foveola normally contains significant numbers of retinal ganglion cells. Most of these project ipsilaterally. Cells within the foveola are larger than other cells in monkey central retina; their dendritic fields are up to 50 times larger in area than those of cells in the foveal slope. The dendritic fields of the ganglion cells within the foveola cover at least 70-100% of its area. Among ganglion cells within the foveola (as in most ganglion cells), there was a strong tendency for the axon and dendritic tree to arise from opposite poles of the soma. The axon-dendrite axes of ganglion cells within the foveola did not show a consistent pattern. In contrast, the axes of ganglion cells in the transition zone between the foveola and the foveal slope were directed tangentially to the circumference of the fovea. The dendritic coverage of the foveola by retinal ganglion cells suggests functional significance and provides a possible neural basis for 2-3 degrees of bilateral representation of the fovea within the central visual pathways. Alternatively, or in addition, these cells may be "remnants of foveation" and provide insight into the developmental processes that mediate the development of the fovea.

Selective depletion of beta cells affects the development of alpha cells in cat retina.

The results of previous studies suggest that class-specific interactions contribute to the development of the different classes of retinal ganglion cells. We tested this hypothesis by examining the morphologies and distributions of alpha (alpha) cells in regions of mature cat retina selectively depleted of beta (beta) cells as a result of visual cortex lesions at birth. We find that alpha cells in regions of central retina depleted of beta cells are abnormally large while alpha cells in regions of peripheral retina depleted of beta cells are abnormally small. The normal central-to-peripheral alpha cell soma-size gradient is absent in hemiretinae depleted of beta cells. The dendritic fields of alpha cells in the border of beta-cell-depleted hemiretina extend preferentially into the beta-cell-poor hemiretina. In spite of this, alpha cell bodies retain their normal retinal distribution and remain distributed in a nonrandom mosaic-like pattern. Thus, it appears that the development of alpha retinal ganglion cells is influenced by interactions both with other alpha cells (class-specific interactions) and with surrounding beta cells (nonclass-specific interactions).

Organized arrangement of orientation-sensitive relay cells in the cat's dorsal lateral geniculate nucleus.

We studied the physiological orientation biases of over 700 relay cells in the cat's dorsal lateral geniculate nucleus (LGNd). Relay cells were sampled at regular intervals along horizontally as well as vertically oriented electrode penetrations in a fashion analogous to that used previously in studies of visual cortex (Hubel and Wiesel, 1962). The strengths of the orientation biases and the distributions of the preferred orientations were determined for different classes of relay cells, relay cells in different layers of the LGNd, and relay cells subserving different parts of the visual field. We find that, at the population level, LGNd cells exhibit about the same degree of orientation bias as do the retinal ganglion cells providing their inputs (see also Soodak et al., 1987). Also, as in the retina (Levick and Thibos, 1982; Leventhal and Schall, 1983), most LGNd cells tend to prefer stimuli oriented radially, i.e., parallel to the line connecting their receptive fields to the area centralis projection. However, the radial bias in the LGNd is weaker than in the retina. Moreover, there is a relative overrepresentation of cells preferring tangentially oriented stimuli in the LGNd but not in the retina. As a result of the overrepresentation of cells preferring radial and tangential stimuli, the overall distribution of preferred orientations varies in regions of the LGNd subserving different parts of the visual field. Reconstructions of our electrode penetrations provide evidence that, unlike in the retina, cells having similar preferred orientations are clustered in the LGNd. This clustering is apparent for all cell types and in all parts of laminae A and A1. The tendency to cluster according to preferred orientation is evident for cells preferring radially, intermediately, and tangentially oriented stimuli and thus is not simply a reflection of the radial bias evident among retinal ganglion cells at the population level. It is already known that cells having inputs from different eyes, on-center, off-center, X-, Y-, W-type, and color-sensitive ganglion cells are distributed nonrandomly in the LGNd of cats and monkeys (for review, see Rodieck, 1979; Stone et al., 1979; Lennie, 1981; Stone, 1983). The finding that relay cells having similar preferred orientations are also distributed nonrandomly suggests that the initial sorting of virtually all properties segregated in visual cortex may begin in the LGNd.

Extrinsic determinants of retinal ganglion cell development in primates.

As in all mammals studied to date, primate retina contains morphologically distinct classes of retinal ganglion cells (Polyak: The Retina. Chicago: University of Chicago Press, '41; Boycott and Dowling: Philos. Trans. R. Soc. Lond. [Biol.] 225:109-184, '69; Leventhal et al.: Science 213:1139-1142, '81; Perry et al.: Neuroscience 12:1101-1123, '84; Rodieck et al.: J. Comp. Neurol. 233:115-132, '85; Rodieck: In H.D. Steklis and J. Erwin (eds): Comparative Primate Biology, Volume 4: Neurosciences. New York: Alan R. Liss, Inc., pp. 203-278, '88). We have now studied the morphologies, central projections, and retinal distributions of the major morphological classes of ganglion cells in the normal adult monkey, the newborn monkey, and the adult monkey in which restricted regions of retina were depleted of ganglion cells at birth as a result of small lesions made around the perimeter of the optic disc. Both old-world (Macaca fascicularis) and new-world (Saimiri sciureus) monkeys were studied. Our results indicate that, at birth, the major morphological classes of monkey retinal ganglion cells are recognizable; cells in central regions are close to adult size whereas cells in peripheral regions are much smaller than in the adult. As in the adult (Stone et al.: J. Comp. Neurol. 150:333-348, '73), in newborn monkeys there is a very sharp division between ipsilaterally and contralaterally projecting retinal ganglion cells (nasotemporal division). Consistent with earlier work (Hendrickson and Kupfer: Invest. Ophthalmol. 15:746-756, '76) we find that the foveal pit in the neonate is immature and contains many more ganglion cells than in the adult. In the adult monkey in which the density of retinal ganglion cells in the central retina was reduced experimentally at birth, the fovea appeared immature, and an abnormally large number of retinal ganglion cells were distributed throughout the foveal pit. The cell bodies and dendritic fields of ganglion cells that developed within cell-poor regions of the central retina were nearly ten times larger than normal. In peripheral regions the effects were smaller. The dendrites of the abnormally toward the foveal pit. They did not extend preferentially into the cell-poor region as do the abnormally large cells on the borders of experimentally induced cell-poor regions of cat central retina (Leventhal et al.: J. Neurosci. 8:1485-1499, '88) or, as we found here, in paracentral regions of primate retina.(ABSTRACT TRUNCATED AT 400 WORDS)

Extrinsic determinants of retinal ganglion cell structure in the cat.

The degree to which a retinal ganglion cell's environment can affect its morphological development was studied by manipulating the distribution of ganglion cells in the developing cat retina. In the newborn kitten there is an exuberant ganglion cell projection from temporal retina to the contralateral lateral geniculate nucleus (LGNd) (Leventhal et al., 1988) and from nasal retina to the ipsilateral LGNd. Neonatal, unilateral optic tract section results in the survival of many of these ganglion cells (Leventhal et al., 1988). The morphology of ganglion cells which survive in regions of massively reduced ganglion cell density was studied. As reported previously (Linden and Perry, 1982; Perry and Linden, 1982; Ault et al., 1985; Eysel et al., 1985), we found that the dendritic fields of all types of ganglion cells on the border of an area depleted of ganglion cells extended into the depleted area. The cell bodies and dendritic fields of alpha and beta cells within depopulated areas, as well as on the borders of the depopulated areas, were larger than normal. The dendritic fields of these cells also exhibited abnormal branching patterns. For alpha and beta cell types the relative increase in size tended to be greatest where the relative change in density was the greatest. In fact, isolated beta cells within the cell-poor area centralis region resembled normal central alpha cells in the cell-rich region of the area centralis in the same retina. Interestingly, in the same regions of reduced density where alpha and beta cells were dramatically larger than normal, the cell body and dendritic field sizes of other cell types (epsilon, g1 and g2 were unchanged. These results indicate that neuronal interactions during development contribute to the morphological differentiation of retinal ganglion cells and that different mechanisms mediate the morphological development of different classes of cells in cat retina.

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.