Fig. 6.

Generation of color contrast and constancy by the evolution of double-opponent circuitry. (A) Contrast. The evolved responses of an evolved yellow (LM)-ON network to patterns in which the input to the central target remains the same but the input to the surround varies from S cone domination (bluish; Left) to LM cone domination (yellowish; Right) are shown. As a result of double opponency, the network’s output varies when the same spectra activate the target. These responses accord with psychophysical observations, evident in the appearance of the central targets here, showing that the perception is biased toward yellow by blue surrounds (Left) and toward blue by yellowish surrounds (Right); the mixed surround (Center) has little effect. (B) Constancy. In this case, the input values activating the central target vary, whereas the surrounds remain the same, as in A (also Fig. 1). Thus, the network responses tend to remain the same despite variation in the target input.