Color contrast adaptation and compensation in color deficiencies

Anomalous trichromacy (AT) results from a reduced spectral separation between the L and M cone photopigments. This leads to smaller differential responses in the L and M cones and thus lower sensitivity to the colors signaled by the LvsM difference. Despite this, for stimuli above threshold, many color-anomalous observers report color experiences that resemble those of color-normal individuals, suggesting some form of perceptual compensation for their sensitivity losses. The nature and sites of this compensation remain uncertain, and may reflect many levels, from early sensory mechanisms to later post-perceptual processes. To assess potential sensory-level compensation, we compared the aftereffects of adaptation to chromatic contrast in 15 color-normal and 15 color-anomalous observers (10 deutan, 5 protan). Without compensation, the same adapting contrast should produce weaker adaptation effects in anomalous observers, because the same physical adaptor is a lower multiple of their threshold sensitivity. We quantified this prediction in color-normals by rescaling the LvsM contrasts to simulate the sensitivity losses. Although protan observers showed mixed results, the deutan observers exhibited adaptation effects that exceeded the predictions based on their threshold sensitivities, indicating partial compensation for the reduced LvsM signals. These findings are consistent with a post-receptoral sensory gain in contrast processing that compensates for the weaker LvsM cone signals available to anomalous observers, and could reflect a general normalization of contrast coding to match the color gamut of the observer's environment.