COLOR CONSTANCY IN VARIEGATED SCENES - ROLE OF LOW-LEVEL MECHANISMS IN DISCOUNTING ILLUMINATION CHANGES

For a visual system to possess color constancy across varying illumina tion, chromatic signals from a scene must remain constant at some neur al stage. We found that photoreceptor and opponent-color signals from a large sample of natural and man-made objects under one kind of natur al daylight were almost perfectly correlated with the signals from tho se objects under every other spectrally different phase of daylight. C onsequently, in scenes consisting of many objects, the effect of illum ination changes on specific color mechanisms can be simulated by shift ing all chromaticities by an additive or multiplicative constant along a theoretical axis. When the effect of the illuminant change was rest ricted to specific color mechanisms, thresholds for detecting a change in the colors in a scene were significantly elevated in the presence of spatial variations along the same chromatic axis as the simulated c hromaticity shift. In a variegated scene, correlations between spatial ly local chromatic signals across illuminants, and the desensitization caused by eye movements across spatial variations, help the visual sy stem to attenuate the perceptual effects that are due to changes in il lumination. (C) 1997 Optical Society of America.