[Visual evoked potentials and recognizing depths during changes in gaze direction]

The neurophysiological mechanism underlying a phenomenon of depth perception that can not be explained by only the horizontal binocular retinal disparity was analyzed by means of visual evoked cortical potentials (VECPs).

A pair of pictures showing fourteen vertical lines with the same magnitude of horizontal retinal disparity were presented to eight normal subjects stereoscopically. Subjects were required to view the pair while changing the visual line direction. VECPs were measured while changing both the visual line directions and the sizes of the horizontal retinal disparity.

The subjects perceived all lines as having the same magnitude of depth when they viewed the center position of the pictures stereoscopically (centro-version viewing). Shifting their visual line direction to the left end (laevo-version viewing), the depth magnitude of each line decreased gradually with increase of retinal eccentricity. Next, while maintaining the laevo-version the subjects were asked to make all lines of the same depth by changing the disparity magnitude of each line manually. Viewing this new version of the picture in the centro-version, the further each line was separated from the line located in the center, the greater was the magnitude of its depth. However, no significant differences in amplitudes of VECPs, corresponding to those pictures were found.

Our results suggest that differences of recognizable depth perception between the centro- and the laevo-version viewing occur in a higher order visual center than in the occipital and parietal zone when analyzing horizontal retinal disparity.