Interactions between chromatic adaptation and contrast adaptation in color appearance

Color appearance depends on adaptation processes that adjust sensitivity both to the average color in the stimulus (through light or chromatic adaptation) and to the variations in color (through contrast adaptation). We explored how these different forms of adaptation interact, by examining how the state of chromatic adaptation depends on the time-varying color contrasts in the stimulus, and conversely, how adaptation to the mean determines the stimulus contrasts underlying contrast adaptation. Light adaptation levels remain very similar whether observers adapt to a static chromaticity or to large temporal modulations in cone excitation that vary at rates of 0.5 Hz or higher. This suggests that up to the sites of light adaptation, the response to moderate contrasts is effectively linear and that the adaptation effectively averages over several seconds of the stimulus. For slower flicker rates color is differentially biased by the last half-cycle of the flicker, and perceived contrast may be altered by response polarization. This polarization selectively saturates responses to moderate (but not low) contrasts along the color direction complementary to the mean color bias, implying that the response changes occur within multiple mechanisms tuned to different chromatic axes. Chromatic adaptation often adjusts only partially to the mean color of the stimulus, and thus leaves a residual bias in the color appearance of the field. Contrast adaptation reduces perceived contrast relative to this residual color, and not relative to the stimulus that appears achromatic. Similarly, contrast discrimination thresholds appear lower around the residual color than around the achromatic point. Thus under biased states of chromatic adaptation alternative measures of 'zero contrast' can be dissociated, suggesting that they do not depend on a common null point within the channels encoding chromatic contrast.