In macaque retina, rods are electrically coupled to red and green cones. In addition, about half of blue cones also show rod input. The magnitude of rod input is comparable in all three cone types. Our analysis suggests that rod-cone coupling will influence color perception. Indeed psychophysical studies demonstrate that rod excitation causes shifts in color percepts. Using our direct measurements of rod and cone light responses, we modeled the extent to which rod-cone coupling could account for the rod influence on color. For most dim stimuli, rod excitation is predicted to shift color percepts towards blue-green.

Under photopic conditions the receptive fields of blue cones display center-surround chromatic opponency. The excitatory center consists of input from a single blue cone. The inhibitory surround comes from neighboring of red and green cones. Surround sensitivity falls exponentially with a length constant of ~150 μm. The pharmacology and voltage sensitivity of the surround suggests that the inhibition is generated by horizontal cell feedback acting on L-type calcium channels in blue cones.