QUANTIFYING COLOR CONSTANCY - EVIDENCE FOR NONLINEAR PROCESSING OF CONE-SPECIFIC CONTRAST

Color constancy was studied by the method of comparing color samples u nder two different illuminants using a CRT color monitor. In addition to the classical approach in which one of the illuminants is a (standa rd) white, we performed experiments in which the range of differential illumination was extended by using pairs of lights that were both col ored. The stimulus pattern consisted of an array of thirty-five color samples (including five neutral samples) on a white background. A tric hromatic illuminant-object interaction was simulated analogous to that resulting from illumination by three monochromatic lights. The test s amples, as seen under ''test'' and ''match'' illumination, were succes sively presented to the left and right eye (haploscopic matching). The data show systematic deviations from predictions on the basis of cone -specific normalization procedures like those incorporated in the Reti nex algorithm and the von Kries transformation. The results can be des cribed by a nonlinear response transformation that depends on two fact ors, receptor-specific sample/background contrast and the extent to wh ich the illuminant stimulates the receptor system in question. The lat ter factor explains the deviations. These are mainly caused by the sho rt-wave-sensitive system, as a consequence of the fact that this syste m can be more selectively stimulated than the other, spectrally less s eparated, cone systems.