CIRCUITRY FOR COLOR CODING IN THE PRIMATE RETINA

Human color vision starts with the signals from three cone photorecept or types, maximally sensitive to long (L-cone), middle (M-cone), and s hort (S-cone) wavelengths. Within the retina these signals combine in an antagonistic way to form red-green and blue-yellow spectral opponen t pathways, In the classical model this antagonism is thought to arise from the convergence of cone type-specific excitatory and inhibitory inputs to retinal ganglion cells, The circuitry for spectral opponency is now being investigated using an in vitro preparation of the macaqu e monkey retina, Intracellular recording and staining has shown that b lue-ON/yellow-OFF opponent responses arise from a distinctive bistrati fied ganglion cell type. Surprisingly, this cone opponency appears to arise by dual excitatory cone bipolar cell inputs: an ON bipolar cell that contacts only S-cones and an OFF bipolar cell that contacts L- an d M-cones, Red-green spectral opponency has long been linked to the mi dget ganglion cells, but an underlying mechanism remains unclear. For example, receptive field mapping argues for segregation of L- and M-co ne signals to the midget cell center and surround, but horizontal cell interneurons, believed to generate the inhibitory surround, lack oppo nency and cannot contribute selective L- or M-cone input to the midget cell surround. The solution to this color puzzle no doubt lies in the great diversity of cell types in the primate retina that still await discovery and analysis.