Midget cell

Abstract: We quantified the spatial distribution of presumed ganglion cells and displaced amacrine cells in unstained whole mounts of six young normal human retinas whose photoreceptor distributions had previously been characterized. Cells with large somata compared to their nuclei were considered ganglion cells; cells with small somata relative to their nuclei were considered displaced amacrine cells. Within the central area, ganglion cell densities reach 32,000-38,000 cells/mm2 in a horizontally oriented elliptical ring 0.4-2.0 mm from the foveal center. In peripheral retina, densities in nasal retina exceed those at corresponding eccentricities in temporal retina by more than 300%; superior exceeds inferior by 60%. Displaced amacrine cells represented 3% of the total cells in central retina and nearly 80% in the far periphery. A twofold range in the total number of ganglion cells (0.7 to 1.5 million) was largely explained by a similar range in ganglion cell density in different eyes. Cone and ganglion cell number were not correlated, and the overall cone:ganglion cell ratio ranged from 2.9 to 7.5 in different eyes. Peripheral cones and ganglion cells have different topographies, thus suggesting meridianal differences in convergence onto individual ganglion cells. Low convergence of foveal cones onto individual ganglion cells is an important mechanism for preserving high resolution at later stages of neural processing. Our improved estimates for the density of central ganglion cells allowed us to ask whether there are enough ganglion cells for each cone at the foveal center to have a direct line to the brain. Our calculations indicate that 1) there are so many ganglion cells relative to cones that a ratio of only one ganglion cell per foveal cone would require fibers of Henle radiating toward rather than away from the foveal center; and 2) like the macaque, the human retina may have enough ganglion cells to transmit the information afforded by closely spaced foveal cones to both ON- and OFF-channels. Comparison of ganglion cell topography with the visual field representation in V1 reveals similarities consistent with the idea that cortical magnification is proportional to ganglion cell density throughout the visual field.

Abstract: 1 Three distinct morphological types of cat retinal ganglion cells have been identified and categorized as α, β and γ Alpha ganglion cells have dendritic field diameters from 180 to 1000 μm; β, about 25 to 300 μm; γ, 180 to 800 μm, possibly more 2 The dimensions of the α and β ganglion cell dendritic fields increase monotonically from the central area outwards to the periphery; those of the γ cells do not Seemingly a spectrum of sizes of the γ cells is found at most locations in the retina 3 All three morphological types of ganglion cells are found in the central area 4 Possible further anatomical types of ganglion cells are discussed Correlations are suggested between the morphological category α cells and the physiological class Y cells; between β cells and the X cells and between the γ cells and the W cells

Abstract: Colour vision in humans and Old World monkeys begins with the differential activation of three types of cone photoreceptor which are maximally sensitive to short (S), medium (M) and long (L) wavelengths. Signals from the three cone types are relayed to the retinal ganglion cells via cone-specific bipolar cell types. Colour-coding ganglion cells fall into two major physiological classes: the red-green opponent cells, which receive antagonistic input from M- and L-sensitive cones, and the blue-yellow opponent cells, which receive input from S-sensitive cones, opposed by combined M- and L-cone input. The neural mechanisms producing colour opponency are not understood. It has been assumed that both kinds of opponent signals are transmitted to the lateral geniculate nucleus by one type of ganglion cell, the midget cell. We now report that a distinct non-midget ganglion cell type, the small bistratified cell, corresponds to the physiological type that receives excitatory input from S cones, the 'blue-on' cell. Our results thus demonstrate an anatomically distinct pathway that conveys S-cone signals to the brain. The morphology of the blue-on cell also suggests a novel hypothesis for the retinal circuitry underlying the blue-yellow opponent response.

Abstract: Previously, we discovered that the broadband cells in the two magnocellular (large cell) layers of the monkey lateral geniculate nucleus (LGN) are much more sensitive to luminance contrast than are the color-sensitive cells in the four parvocellular (small cell) layers. We now report that this large difference in contrast sensitivity is due not to LGN circuitry but to differences in sensitivity of the retinal ganglion cells that provide excitatory synaptic input to the LGN neurons. This means that the parallel analysis of color and luminance in the visual scene begins in the retina, probably at a retinal site distal to the ganglion cells.