Physiology, morphology and spatial densities of identified ganglion cell types in primate retina

The use of in vitro preparations of primate retina provides new perspectives on the mosaic organization and physiological properties of three ganglion cell types that project to the lateral geniculate nucleus: the parasol, midget and small bistratified cells. Dendritic field sizes and coverage for the three types suggest that their relative densities vary with eccentricity. Of the total ganglion cells in the human fovea, midget cells constitute about 90%, parasol cells about 5%, and small bistratified cells about 1%. In the periphery, midget cells make up about 40-45%, parasol cells about 20% and small bistratified cells about 10% of the total. Thus from peripheral to central retina the number of midget ganglion cells progressively increases relative to the parasol and small bistratified types. Physiological properties of these cells have recently been studied in macaque (Macaca nemestrina) retina by combining intracellular recording and dye injection. As expected, parasol cells, projecting to geniculate magnocellular layers, give phasic, non-opponent light responses. Midget cells, which project to geniculate parvocellular layers, show opponent responses sensitive to only mid and long wavelengths; no evidence of short-wavelength-sensitive cone (S-cone) input to any midget ganglion cell has been found. However, the small bistratified cells, which also project to the parvocellular geniculate layers, give a strong blue-ON response to stimuli designed to modulate S-cones. Thus, S-cone and medium- or long-wavelength-sensitive cone opponent signals arise from morphologically distinct ganglion cell types that project in parallel to the lateral geniculate nucleus.