Fig. 5.

Primate retinal ganglion cells that receive cone selective connections and have the potential for playing a role in color vision. Upper and lower panels show how the addition of a third cone type changes the chromatic inputs to different ganglion cells in the primate. The cell bodies of ganglion cells are drawn as diamonds. Bipolar cells have circular cell bodies. Horizontal cells bodies are hexagonal. For the trichromat (lower panel), four ON/OFF pairs of midget ganglion cells are drawn. Different combinations of cone connectivity distinguish the four ganglion cell pairs. Two ON/OFF pairs of ganglion cells, one pair with an L cone center and one with an M cone center, receive input from cones that make contacts with H2 horizontal cells, which contact nearby S cones. Two other ganglion cell pairs (an L center and an M center) receive input from cones that do not have the potential for significant S cone input from the surround. Assuming that a small subset of ganglion cells receive S cone input from the surround, the M cones with S in the surround give rise to an OFF center ganglion cell with (S+L)-M opponency and an ON center ganglion cell with M-(S+L) providing the potential retinal basis for a red and green, respectively, hue pathway. L cones with S in the surround give rise to an OFF center ganglion cell with (S+M)-L opponency and an ON center ganglion cell with L-(S+M) providing the potential retinal basis for a blue and yellow, respectively, hue pathway. ON midget ganglion cells with no S cone input to the surround have L-M opponency when the center cone is L and M-L opponency when the center cone is M. M cones provide the center of one spectrally opponent ganglion cell but the surround of neighboring ganglion cells. If neighboring L-M and M-L are indiscriminately combined in the cortex, chromatic opponency for diffuse spots of light will cancel. This would make L-M and M-L ganglion cells the substrate for edge detectors that would also signal chromatic borders. Thus, the four pairs of midget ganglion cells could provide the retinal basis to serve red, green, blue and yellow hue perception and luminance/chromatic edge detection. One S cone bipolar cell is illustrated. It connects specifically to an S cone. It provides the S-ON input to the small bistratified ganglion cell, which is drawn showing dendritic arbors in both the ON and OFF sublamina (labeled OFF-CENTER and ON-CENTER). The single S cone bipolar cell also provides an S-OFF input via an inhibitory interneuron to the melanopsin ganglion cell (drawn in yellow with an “X” shaped dendritic arbor). Both the melanopsin ganglion cell and the small bistratified cells have large receptive fields so the ON component of the melanopsin and the OFF component of the small bistratified cell have M+L cone inputs, giving them (L+M)-S and S-(L+M) spectral opponency, respectively. A comparison of the “DICHROMAT” top panel with the “TRICHROMAT” bottom panel shows how the spectral opponent properties of each of the 10 ganglion cells illustrated change when the retina is transformed from having two cone types to having three. Those midget cells that are capable of only transmitting luminance information in the dichromat become L vs. M opponent in the trichromat. Putative midget ganglion cells with S vs. L inputs that could serve blue color vision are transformed into two pairs to serve blue-yellow and red-green color vision in the trichromat. An attempt was made to preserve some of the anatomical details of the retina in the cartoon. Cones and bipolar cells are shown with ribbon synapses. ON bipolars make connections to the ribbon and terminate in the ON lamina. OFF bipolar cells are shown making more lateral connections representing flat contacts and they terminate in the OFF sublamina. The inset illustrates that horizontal cells make reciprocal synapses.