We examine the functional role of S-cone signals on reaction time (RT) variability in human color vision. Stimuli were selected along red–green and blue–yellow cardinal directions and at random directions in the isoluminant plane of the color space. Trial-to-trial RT variability was not statistically independent but correlated across experimental conditions and exhibited noise spectra with an exponent close to unity in most of the cases. Regarding contrast coding, noise for random chromatic stimuli at isoluminance was similar to that for achromatic stimuli, thus suggesting that S-cone signals reduce variability of higher order color mechanisms. If we regard spatial coding, the effect of S-cone density in the retina on RT variability was investigated. The magnitude of noise at 16 min of arc (S-cone free zone) was higher than at 90 min of arc in the blue–yellow channel, and it was similar for the red–green channel. The results suggest that S-cone signals are beneficial and they modulate noise spectra at postreceptoral stages. The implications related to random multiplicative processes as a possible source of noise and the optimal information processing in color vision are discussed.
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