Colour identification and colour constancy are impaired in a patient with incomplete achromatopsia associated with prestriate cortical lesions.

We have examined visual functions, including colour vision, in a patient with bilateral cortical lesions involving mainly the fusiform and lingual gyri, areas known to be involved in the central processing of chromatic stimuli. The patient has near normal (6/9) acuity, and his responses to tests of binocular function and spatial vision are normal, as are his discrimination of changes in target speed and surface lightness. He does, however, exhibit minor losses in the upper visual field, mild prosopagnosia and topographical agnosia, all conditions commonly associated with cerebral achromatopsia. Colour matches and spectral response data establish that his cone photoreceptors have normal spectral characteristics and his spectral sensitivity measured against a white background reveals normal postreceptoral chromatic function. The patient's colour discrimination for differences in wavelength, hue or saturation is, however, impaired and his colour naming is significantly disturbed, particularly for blues and greens. We have determined the areas of the chromaticity chart that correspond to his naming categories for surface colours, and show that changes in illuminant cause him to alter the names of surface colours in a manner consistent with the changes in their chromaticities. Other subjects with normal or congenital red-green deficient colour vision make many fewer name changes under changes in illuminant. We conclude that the patient's colour constancy is impaired as a consequence of abnormal central processing of colour vision.