Evidence that Circuits for Spatial and Color Vision Segregate at the First Retinal Synapse

Different psychophysical “channels” mediate our perception of fine spatial patterns and color. The spatial channel compares the photon catch between adjacent cones, regardless of type, and thus supports perception of patterns as fine as the cone mosaic (Williams 1986). Color channels compare the photon catch between different cone types (Hurvich and Jameson 1957; Krauskopf et al. 1982). The “blue/yellow” channel compares the catch in cones sensitive to short (S) wavelengths with the catch in neighboring cones sensitive to middle (M) and long (L) wavelengths (Pugh and Mollon 1979). The “red/green” channel compares the photon catch in L and S cones with that in neighboring M cones (Thornton and Pugh 1983; Calkins et al. 1992). Each comparison is equivalent to a subtraction: for blue/yellow, S−(M+L); for red/green, L−M+(S). The color channels are rather coarse, with an acuity about one-third that of the spatial channel (Anderson et al. 1991; Sekiguchi et al. 1993).