DEPTH-PERCEPTION AND EVOKED BRAIN ACTIVITY - THE INFLUENCE OF HORIZONTAL DISPARITY AND VISUAL-FIELD LOCATION

The perception of dynamic random-dot stereograms (RDS) depends on the physiological fusion of horizontally disparate binocular visual input. Thus, the use of RDS offers the possibility to study selectively cort ical processing of visual information in man. We investigated the infl uence of horizontal disparity on the scalp topography of RDS evoked br ain activity in 33 healthy subjects. Stereoscopic checkerboard pattern s were presented in the center or lateralized in the left or right vis ual field with horizontal disparities changing at temporal frequencies of six or eight depth reversals/s using different disparity values ra nging from 3.5 to 28 min of are. In 11 subjects evoked potential field s were recorded from 16 electrodes, and 21 subjects participated in 30 -channel recordings with electrodes located over the parietal and occi pital brain areas. Stimulation frequency-related brain activity was ob tained with all disparity values; however, with large or small dispari ties the potential field strength decreased significantly while larges t responses were obtained with intermediate disparities. Significant d ifferences were observed in RDS evoked brain activity when central and lateralized stimulus locations were compared. With lateral stimuli (e xtending from the fovea to 17.1-deg eccentricity) maximal amplitudes w ere obtained at larger disparities than with central stimuli. In addit ion there were pronounced differences between brain activity evoked wi th stimuli presented in the left or right visual field; however, there were very similar evoked potential signals recorded from electrodes l ocated over the left and right hemispheres. Our findings indicate that the processing of disparity information with lateralized stimuli is d ifferent from the processing in the center of the visual field. In add ition, lateralized stimulation yields a significant disparity tuning m ainly with stereoscopic targets occurring to the right from the fixati on point (but not with stimuli to the left) suggesting a functional di fference between the visual half-fields.