Early extrastriate activity without primary visual cortex in humans

Damage to the primary visual cortex (V1) destroys the major source of anatomical input to extrastriate cortical areas (V2, V3, V4 and V5) and produces cortical blindness--an absence of any sensation of light and colour--in the visual field contralateral to the side of the lesion. Neuroimaging studies, nevertheless, have recently demonstrated dorsal and ventral extrastriate activation for stationary stimuli presented to the blind visual field in the absence of V1 activity in human subjects. To clarify the moment in time that visual information reaches extrastriate areas, by means of event-related potentials (ERPs) we tracked the temporal course of responses to complex visual stimuli (faces) presented in the blind field of a hemianopic patient. Stimulation of the normal visual field elicited a positive occipital deflection (P1) at 140 ms. A P1 response was also observed with stimulation of the blind field, although slightly delayed (20 ms) and reduced. Its topography and timing demonstrate that early neural activity for stationary stimuli takes place within extrastriate regions despite V1 denervation.