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.