Cardinal axes are not independent in color discrimination

Abstract

We measured chromatic discrimination under conditions where the target fields could be distinguished only by the ratio of excitation of the long- (L) and middle-wavelength (M) cones. The excitation level of the short-wavelength (S) cones was varied in the experiments, although for any given measurement the S-cone excitation was common to the two target fields and could not be directly used for discrimination. Adaptation was maintained by a steady neutral background metameric to Illuminant D65. Thresholds varied substantially and systematically with the S-cone level of the target probes, but in a complex way: when the ratio of L:M cone excitation was low, an increase in S-cone excitation reduced the thresholds, but when the L:M ratio was higher, an increase in S-cone excitation raised the thresholds. To account for the pattern of results, we postulate a neural channel that draws synergistic inputs from L and S cones and an opposed input from M cones. The proposed channel has a compressive response function and is most sensitive at the point set by the steady background.