The effect of disparity change on binocular visual evoked potential parameters elicited by convergent dynamic random-dot stereogram stimuli in humans.

Binocular visual evoked potentials (VEP) were recorded from the left and right occipital cortices of right-handed subjects in response to six different levels of convergent disparity using the stimulus of dynamic random-dot stereogram (DRDS). The VEP obtained consisted of a negative (N1) and a positive peak (P1) within intervals of 200-400 ms and 400-600 ms, respectively. The latency of the N1 wave decreased with disparity and the amplitude of the same wave displayed a concave-down curve. The tuning-curve of N1 showed that the cortical focus, which produced this wave, was sensitive to mid-disparities. The fact that the tuning-curve of the right hemisphere was sharper than that of the left implied that the right hemisphere was more disparity-selective. In contrast to N1, the change of the latency and the amplitude of the P1 with disparity consisted of concave-up curves. The inverse correlation between the amplitudes of N1 and P1 with disparity made me think that the activation of the neuron population, which gave rise to N1 wave, changed the synchronization level of the cortical focus responsible for the P1 wave. If we keep in mind that the perception of depth is the result of the disparity detection, then we may conclude that N1 is related to disparity detection and P1 is the electrophysiological correlate of a depth percept.