ELECTROPHYSIOLOGICAL STUDIES OF COLOR PROCESSING IN HUMAN VISUAL-CORTEX

Electrophysiological recordings from human visual cortex were carried out with electrodes chronically implanted in 13 patients for localizat ion of an epileptogenic focus. Visual evoked potentials (VEPs) elicite d by red or blue checkerboard stimuli were recorded using an adaptatio n stimulus-test stimulus design in which color was the most salient fe ature. A ''significant color effect,'' defined as a statistically sign ificant effect of the adaptation stimulus on test stimulus VEPs evoked by the same or a different color, was determined for various cortical regions: medial lingual gyrus, 20%; lateral lingual gyrus, 38%; poste rior fusiform gyrus, 50%; anterior fusiform gyrus, 0%; inferior tempor al gyrus, 5%; occipital pole, 30%; lateral surface of non-visual corte x, 6%; inferior parietal and temporal cortex, 5%. The time course of t he significant color effects suggests that wave length-selective neuro nal activity occurs initially at the first stage of cortical processin g in the medial lingual gyrus' followed by progressively later activat ion of the lateral lingual gyrus, the posterior fusiform gyrus, and th e inferior temporal gyrus. In two patients, stimulation of the lateral lingual and fusiform gyri elicited color sensations in the contralate ral half-field, whereas stimulation of the medial lingual and cuneate gyri evoked retinotopically appropriate quadrantic ''shimmering'' devo id of color. These results suggest that a region of inferior occipital cortex, primarily the posterior portion of the fusiform gyrus, is inv olved in color perception and may be homologous with area V4 in monkey s. There is also a region of dorsolateral surface cortex which exhibit s a fairly high percentage of significant color effects and when stimu lated may evoke sensations of color. This region may be the same as th e dorsolateral region thought to be involved in selective attention to color.