Depth-related visually evoked potentials by dynamic random-dot stereograms in humans: negative correlation between the peaks elicited by convergent and divergent disparities.

Abstract

Binocular visually evoked potentials (VEP) were recorded from the left and right occipital cortices of right-handed subjects in response to convergent and divergent stimuli, each having six disparity levels, using dynamic random-dot stereograms (DRDS). The VEP recorded consisted of a negative peak (N300) and a positive peak (P500) within intervals of 200-400 ms and 400-600 ms, respectively, with respect to the stimulus onset. For convergent disparities, the relationships between the amplitude of N300 and the degree of disparity showed convexity towards the disparity axis, whereas the same relationships displayed concavity for P500. For divergent disparities, on the other hand, the amplitude and the degree of disparity relationships showed concavity towards the disparity axis for N300, in contrast to those obtained by convergent disparities. Although the disparity profile of P500 displayed concavity in the left hemisphere, its right hemisphere counterpart turned out to be bi-modal in behaviour, indicating a relative loss of disparity sensitivity in the mid-disparity range. The significant negative correlation between the N300 and P500 behaviour in response to both stimulus modalities suggests that the activity of the N300 centre is effective in changing the synchronization level of the cell population comprising the P500 centre. The significant negative correlations between the profiles of the N300 wave in response to convergent DRDS and its divergent DRDS counterpart in both hemispheres, and between the profiles of the convergent and divergent P500 waves in the left hemisphere imply that the N300 and the P500 foci are sensitive to both the magnitude and the direction of the disparity. The latency differences between the two response modalities revealed that the N300 wave in response to convergent DRDS always led the N300 wave elicited by divergent DRDS, indicating that convergent disparities are processed faster than the divergent disparities.