Chromatic and luminance signals in visual memory.

The efficiency of chromatic and luminance signals was studied in a set of tasks requiring the discrimination of two colors. Discrimination was measured around an adapting achromatic light and a number of other points in a three-dimensional color space. As a baseline, discrimination thresholds were measured under conditions permitting a side-by-side comparison of stimuli in space or time. For the spatiotemporal configurations used in these experiments, chromatic signals were more efficient than luminance signals in terms of the difference in cone excitation required at the discrimination threshold. When stimuli were separated in both space and time, so that memory was required for their comparison, the efficiency of luminance signals was attenuated further, while chromatic signals retained their efficiency. Further experiments showed that the addition of a memory requirement did not impair the accuracy of luminance discrimination when the two test colors could be placed in distinct perceptual categories with respect to the surround color. Our results indicate that chromatic signals are particularly efficient in simple color discrimination tasks requiring even the barest amount of memory, especially when the perceptual categorization scheme is not available for the comparison of stimuli.