Receptive fields of single optic nerve fibers in a mammal with an all-cone retina. 3. Opponent color units.

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Abstract
Some units (99 of 410 studied; 24%) in the ground squirrel optic nerve transmitted opponent color information. The activity of each fiber was controlled by 2 mutually antagonistic components (one excitatory, one inhibitory) which had different spectral sensitivities. Some units were excited by green and inhibited by blue light; others, the reverse. Units excited or inhibited by green gave the same type of response to yellow or red light; there was no evidence of red-green antagonism. Selective chromatic adaptation revealcd that the excitatory and the inhibitory responses were produced by 2 independent cone processes with peak sensitivities at about 525 nm and 460nm. There were 3 types of receptive fields: Class I (54%) - the spatial distributions of the green-and the blue-responsive components were identical; Class II (38%) -the spatial distribution of the blue-sensitive system included both the center and the surround of the receptive field, while the green-responsive component was confined to the field center; Class III (8%) - the green-sensitive component was present only in the field center; the blue-sensitive system, only in the surround.