Color contrast gain in anomalous trichromats

Abstract:

Anomalous trichromats have three cone types, but with diminished L vs M sensitivity due to shifted spectral sensitivities. We examined whether post-receptoral processing might amplify and thus compensate for these receptoral sensitivity losses. Compensation is predicted if – like color normals – the visual system of anomals adapts to match visual coding for their visual environment. Results from previous studies investigating this have been mixed, but recent evidence points to compensation in some tasks. The current study used threshold detection and two measures of suprathreshold perception (contrast matching and reaction times for discriminating color differences) to compare contrast coding in normal and anomalous observers. LM contrast thresholds were substantially higher in anomals. They also required more LM contrast to match a suprathreshold reference stimulus. However, the contrast losses were greater for thresholds than the suprathreshold matches. For each task, we modeled the cone-opponent signal expected from differences in the peak separation of the L and M cones. For anomals, the separation predicted by suprathreshold contrast matching was nearly twice that predicted by detection thresholds, suggesting neural compensation. Reaction times were measured at several contrast levels along the four cardinal directions of cone-opponent space. Anomals had slower average reaction times, consistent with a weaker contrast response, but had nearly identical reaction times at the highest contrast level, indicating a stronger contrast dependence. However, unlike the contrast matching, deficits were also observed for stimuli that varied in S cone contrast. This suggests that under the stimulus conditions used, the reaction times of anomalous trichromats cannot be accounted for by simple gain adjustments for their sensitivity losses.