Relating cone signals to color appearance: Failure of monotonicity in yellow/blue

Observers performed red-green and yellow-blue hue cancellation tasks for a 0.8-deg circular test field on a dark surround, by manipulating the excitation level of one cone class while the other two classes were held constant. The results of the red-green judgments conformed to classical opponent color theory in that both L- and S-cone excitation levels were antagonistic to M-cone signals. The yellow-blue judgments revealed a nonmonotonic nonlinearity in which the S-cone signal could act either antagonistically or synergistically with M- and L-cone signals. These results demonstrate that fixed hue sensations should not be associated with a given class of cone, even at the level of opponent neural coding.