Color contrast processing in human striate cortex

Abstract

Color constancy refers to the unchanging nature of the perceived color of an object despite considerable variation in the wavelength composition of the light illuminating it. The color contrasts between objects and their backgrounds play a crucial role in color constancy. We tested a patient whose right striate cortex had been removed and demonstrated that he made no use of color contrast in judging color appearance but instead made judgments based simply on wavelength comparison. This was shown by presenting pairs of colored stimuli against a background color that gradually changed across space. When presented with such displays, both normal observers and those with cerebral achromatopsia (cortical color blindness) judge the color appearance of such stimuli on the basis of the chromatic contrast the stimuli make against their background rather than on the physical wavelengths of the light emitted from them. However, our patient made no such use of color contrast but, instead, made color discriminations simply on the basis of wavelength composition. This is consistent with recent findings from monkey electrophysiology that identify cells in early cortical visual areas that signal local contrast and so contribute to the likely mechanism for achieving color constancy.

Fig. 1.

Stimuli. (a and b) Pairs of discs presented against a graduated background. Most observers judge the pair in b as being more similar to one another than those in a. (c and d) Exactly the same pairs of discs as those in a and b, respectively, but against uniform backgrounds. This shows that the pair in a are physically identical, although they appear different because they differ in contrast relative to their immediate backgrounds. (e and f) The same illusion in the color domain. The upper and lower discs are spectrally identical in e, although they appear quite different from one another when seen against the graduated background. In f, the color of the lower disc has been adjusted to make its contrast against its background, and hence its appearance, more similar to the upper disc. Note that print color reproduction may not represent the stimuli accurately.

Fig. 2.

Results. Percentage of trials on which the physically matching pair of stimuli was selected as more similar to one another than the contrast matching pair. In D.B.'s right hemianopic visual field, he judges a pair of discs such as those seen in Fig. 1e as being similar, although, in both his normal and damaged fields, he is subject to the luminance illusion corresponding to that which makes us see the discs presented in Fig. 1e as different. Error bars are 95% binomial confidence intervals.