Adaptation to Variegated Scenes and Colour Constancy

For a visual system to possess colour constancy across varying illumination, chromatic signals from a scene must remain constant at some neural stage. We found that photoreceptor and opponent-colour signals from a large sample of natural and man-made objects under one kind of natural daylight were almost perfectly correlated with the signals from those objects under every other spectrally different phase of daylight. Therefore, in scenes consisting of many objects, the effect of illumination changes on specific colour mechanisms can be simulated by shifting all chromaticities by an additive or multiplicative constant along a theoretical axis. When the effect of the illuminant change was restricted to specific colour axes, thresholds for detecting a change in the perceived colours in a scene were significantly elevated in the presence of spatial variations along the same axis. Probe-flash threshold curves revealed that adaptation to variegated scenes is qualitatively different from independent adaptation to the constituents or to the space-average, but is similar to adaptation to prolonged temporal modulation (Shapiro and Zaidi, 1992 Vision Research 32 2065 – 2076), which would be caused by small eye-movements across object boundaries. The data are consistent with a ‘response equalisation’ model, which modifies the response function of each mechanism to match the cumulative frequency distribution of its inputs (Zaidi and Shapiro, 1993 Biological Cybernetics 69 415 – 428). In a variegated scene, correlations between spatially local chromatic signals across illuminants, and adaptation caused by eye movements across spatial variations, help the visual system to attenuate the perceptual effects due to changes in illumination.