Selective color constancy deficits after circumscribed unilateral brain lesions

The color of an object, when part of a complex scene, is determined not onl y by its spectral reflectance but also by the colors of all other objects i n the scene (von Helmholtz, 1886; Ives, 1912; Land, 1959). By taking global color information into account, the visual system is able to maintain cons tancy of the color appearance of the object, despite large variations in th e light incident on the retina arising from changes in the spectral content of the illuminating light (Hurlbert, 1998; Maloney, 1999). The neural basi s of this color constancy is, however, poorly understood. Although there se ems to be a prominent role for retinal, cone-specific adaptation mechanisms (von Kries, 1902; Poppel, 1986; Foster and Nascimento, 1994), the contribu tion of cortical mechanisms to color constancy is still unclear (Land et al ., 1983; D'Zmura and Lennie, 1986). We examined the color perception of 27 patients with defined unilateral lesions mainly located in the parieto-temp oro-occipital and fronto-parieto-temporal cortex. With a battery of clinica l and specially designed color vision tests we tried to detect and differen tiate between possible deficits in central color processing. Our results sh ow that color constancy can be selectively impaired after circumscribed uni lateral lesions in parieto-temporal cortex of the left or right hemisphere. Five of 27 patients exhibited significant deficits in a color constancy ta sk, but all of the 5 performed well in color discrimination or higher-level visual tasks, such as the association of colors with familiar objects. The se results indicate that the computations underlying color constancy are me diated by specialized cortical circuitry, which is independent of the neura l substrate for color discrimination and for assigning colors to objects.