Scalp response topography to dynamic random dot stereograms.

Responses were recorded to dynamic random dot stereograms from 32 sites (acquired in separate 8-channel montages). The disparate stimulus comprised a central, 3 degrees square of 30 arc min crossed disparity, superimposed on a 10.7 degrees square background field. Stimulus onset occurred 100 msec into a 500 msec recording epoch, with a 100 msec stimulus duration. Wave forms for 8 subjects were normalised across the 100 msec pre-stimulus period. Average amplitudes were calculated for the post-onset intervals: 120-140, 140-180, 180-220, 220-260 and 260-300 msec. Contour lines of evoked response amplitude were plotted from these data. These plots are compared with theoretical plots generated from a '3 concentric sphere' model of the head. The results lend support to the proposal that preliminary stereoscopic processing takes place in the primary visual cortex, followed by secondary processing in more anterior cortical regions.