The topography of scalp potentials evoked by pattern pulse stimuli

Pattern pulse stimuli evoke activity in two distinct regions of the human visual cortex in temporal sequence. The earlier responding region (implicit time 93 msec) is localized to a relatively small region near Oz. Its topography is very sensitive to the position of the evoking stimulus in the visual field; it is dominated by a representation of the visual field near the vertical meridian. The later responding cortical region (implicit time 131 msec) is larger than the earlier one. Its medial border touches the lateral border of the earlier responding cortical region and it extends several cm laterally. Its topography is much less sensitive to the position of the evoking stimulus in the visual field. Stimuli presented to the ipsilateral field evoke either weak or no measurable activity in the earlier responding cortical region but often evoke measurable activity near the lateral boundary of the later responding cortical region. These results suggest that the earlier responding cortical region is striate cortex and that the later responding cortical region is composed of visual areas V2, V3, V3A, and V4.