The timing of visual evoked potential activity in human area V4.

Studies of human occipital visual cortex have demonstrated functional specializations for colour and for motion, with a pivotal area for colour processing (area V4) being located in the fusiform gyrus. To study the timing of arrival of signals in area V4 we have recorded multi-channel visual evoked potentials (VEPS) to colour and grey 'Mondrian' stimuli, spatio-temporal dipole source analysis being computed on two independent group averages of five and six subjects respectively. Three active brain regions were identified, which we interpret to correspond to areas V1, V2 and V4; they showed sequential but overlapping activity in time with no difference in magnitude between colour and grey stimulated VEPs. Source analysis of the difference potentials, colour minus grey, isolated source activity resulting from colour stimulation and located it in the region of area V4. Activity in area V4 started at 100 ms and peaked at 135 ms after the onset of the visual stimulus.