Walsh & Hoyt: Color Constancy and Achromatopsia

A defect in color constancy should result in color percepts that vary with changes in illumination. Patients with achromatopsia have a more severe deficit in that they lack any color percept at all. Testing such patients for constancy of something they do not perceive is paradoxical; however, this can be done on dyschromatopsic patients, who have some residual hue sensitivity. One such patient was reported to have impaired color constancy. Three patients, two of whom had transient symptoms of achromatopsia, and all with subtle color-matching problems at the time of testing, had abnormal shifts of color matches when lighting was changed. Despite possessing some local color constancy based upon cone contrasts, one patient had impaired constancy for complex scenes, indicating abnormal global constancy mechanisms. Another shifted his color naming to black with increasing background luminance; this indicated that his naming depended heavily on relative luminance, representing a failure of ""lightness"" (rather than hue) constancy. All of these reports included patients with some dyschromatopsia and bilateral lingual and fusiform gyral lesions, except for one unilateral case. However, a report of a large series of 27 patients found 5 subjects with impaired color constancy on matching tasks, all asymptomatic, who did not have dyschromatopsia, and who had right or left lateral parietotemporal lesions. The authors argued that this was evidence of a larger network of cortical regions beyond the lingual/fusiform region that participated in generating color constancy.