BINOCULAR INTERACTION IN NORMAL VISION STUDIED BY PATTERN-REVERSAL VISUAL-EVOKED POTENTIALS (PR-VEPS)

Monocular and binocular visual evoked potentials (VEPs) in response to different check sizes (15-21-38-84 minutes of are) were studied in 14 subjects with normal visual acuity and stereopsis. The binocular VEP amplitude is slightly higher than the VEP amplitude on stimulation of the ''better eye'' and significantly higher than the VEP amplitude on stimulation of the ''worse eye''; this effect is observed using small checks and almost exclusively involves N75-P100. Both the N75 and P100 peaks occur earlier after binocular than monocular stimulation. The s hortening of the N75 mean latency is significantly greater than that o f the P100 mean latency when larger check sizes are used. The mean lat ency of the N145 potential is not significantly different in monocular and binocular stimulus conditions. The slight summation effect and la tency shortening in the binocular VEPs are not consistent with the hyp othesis that it is the sum of separate monocular signals originating f rom the visual cortex that gives rise to the response. The early compo nents of both monocular and binocular VEPs are thought to be of post-s ynaptic origin (outside layer 4c of area 17), where the imputs become mixed so that most cells receive information from both eyes. The ampli tude enhancement of binocular VEPs, which mainly occurs when using sma ll checks, may be related to the increase in the total amount of corti cal activity representing the macular region; this may account for bin ocular superiority in fine spatial resolution. The latency shortening in binocular conditions can be explained by considering that the criti cal determinant of the latency is the fundamental spatial frequency of the pattern. When coarse patterns are used, their effectiveness in pa rafoveal stimulation may affect the VEPs, with a significant contribut ion coming from the more peripheral retina. The enlargement of the vis ual field when the eyes see simultaneously may therefore further reduc e the latency of the response when using the larger checks suitable fo r eccentric stimulation.