Ganglion cells of the cat retina that are neither alpha nor beta cells are often lumped for convenience into a single anatomical group—the gamma cells (Boycott & Wässle, 1974; Stone, 1983; Wässle & Boycott, 1991). Defined in this way, gamma cells are the morphological counterpart to the physiological W-cell class, which includes all ganglion cells that are neither Y (alpha) nor X (beta) cells. We have estimated the retinal distribution of gamma cells by using retrograde transport to label ganglion cells innervating the superior colliculus and by assuming that these included virtually all gamma cells and no beta cells. We excluded labeled alpha cells on the basis of soma size. Our data suggest that gamma cells represent just under half of the ganglion cells in most of the nasal retina, but only about a third of those in the area centralis and temporal retina. Gamma cells do not appear to be more highly concentrated in the nasal visual streak than are other ganglion cells. In the temporal retina, gamma cells with crossed projections to the brain are apparently at least twice as common as those with uncrossed projections.

Cat-301 is a monoclonal antibody which recognizes a cell surface associated antigen of selected neurons in the central nervous system (CNS). In the visual system, cat-301 selectively labels Y-like cells in several visual structures, including portions of the lateral geniculate nucleus complex and visual cortex. The cat superior colliculus (SC) also receives Y input and contains cells driven by Y input which are selectively distributed in the deep superficial gray and deeper laminae. If cat-301 is selective to the Y-cell system in SC, labeled cells should be restricted to those laminae. To test this hypothesis, we have examined quantitatively the laminar distribution, percentage, size, and morphology of cells in SC labeled by the cat-301 antibody. Cat-301 labeled a variety of cells in the cat SC. Labeled cells were found within the deep portion of the superficial gray layer (6.6%), optic layer (27.6%), intermediate gray layer (26.9%), and the deep gray and white layers (38.5%). By contrast, only 2 of 667 labeled cells (0.3%) were found within that part of the upper superficial gray layer innervated exclusively by W input and thought to contain only W-driven cells. When considered as a percentage of the total cell population, cat-301 labeled cells represented less than 3% of cells in the superficial gray layer and approximately 15% in the deeper layers. Neurons labeled by cat-301 were all of medium to large size (mean average diameter = 33.3μm; range = 15–84μm) and included vertical fusiform and stellate cells in the upper layers and the very large neurons found in the intermediate gray and deeper layers. These results provide further evidence that the cat-301 antibody selectively recognizes the Y channel of the cat visual system.

Sensory representations in the brain exhibit topographic variations in magnification. These variations have been thought to reflect regional differences in the density of innervation at the sensory receptor surface. In the primate visual cortex, for example, local magnification factors have been reported to be proportional to the corresponding densities of retinal ganglion cells. We sought to learn whether this principle also operates in a second major retinofugal pathway—the projection to the superior colliculus. In cats, we first used retrograde transport to determine the retinal distributions of the ganglion cells that project to the colliculus. Then, we compared the numbers of colliculopetal ganglion cells in selected retinal sectors to the areas of the corresponding collicular representations. Collicular areal magnification was not simply proportional to the density of afferent ganglion cells, being instead at least 5-fold greater than expected in the representation of the central visual field. These data imply that incoming retinal afferents are more widely spaced in the central regions of the tectal map than in the map's periphery. Such variations in afferent density appear to play as large a role as the distribution of ganglion cells in determining the metric of the collicular map.

The spatio-temporal receptive-field structure of 54 phasic W cells in cat retinas has been examined using the reverse-correlation method of Jones and Palmer (1987). Within this sample, 12 cells had on-center, 16 off-center, and 26 on-off receptive fields. Three of the on-center and seven of the on-off cells were directionally selective. Forty percent of the cells in this sample had local receptive fields consisting of two or more distinct subregions. However, no correlation was observed between the number of subregions in the local receptive field and other response properties such as center sign or direction selectivity. In all cases, individual subregions, including those in on-off cells, appear to be produced by a half-wave rectification of the input signal. For 76% of the cells, these local receptive fields were contained within large suppressive fields which could be seen to extend for at least 10 deg in all directions with no apparent spatial structure. The mechanism producing the suppressive field also appears to involve a rectification of the input signal, and has a relatively high spatial resolution. Furthermore, the suppressive field itself is only responsive to moving or flickering stimuli; large, stationary gratings have no effect on the output of the local receptive-field mechanism. Thus, the overall receptive-field organization of these cells is particularly well suited for detecting local motion. The remaining 24% of cells in the sample lacked suppressive fields, and consequently responded well to large moving stimuli, but these cells were otherwise similar in their receptive-field properties to cells with suppressive fields. The significance of these properties is discussed in the context of the projections of phasic W cells to the superior colliculus and accessory optic system.