Mechanism of binocular interaction in refraction errors: study using pattern-reversal visual evoked potentials

In this study we sought to determine whether a natural condition involving fine discrimination, for example moderately severe myopia, might yield inte resting information regarding the binocular interaction expressed by visual evoked potentials (VEPs). We studied ten normal subjects with a mild refra ction deficits. Transient VEPs were elicited by monocular and binocular sti mulation under conditions of natural and lens-corrected vision. The visual stimulus was a pattern-reversal checkerboard consisting of 15' and 40' chec ks. VEPs in response to binocular stimulation were compared with monocular VEPs. We plotted the monocular `better-VEP' and `worse-VEP' response, since significant differences between individual eye stimulations were present. We found no significant difference between the mean N75 and P100 latencies of the binocular VEP and the better monocular VEP, regardless of the check size used and of natural or corrected vision. Under all stimulus conditions , the mean amplitude of the N75-P100 of the binocular VEPs was also larger than the better monocular VEP response. The difference proved more signific ant when we stimulated our subjects with smaller squares and left vision un corrected. The mean P100-N145 amplitude obtained with binocular stimulation was larger than the better monocular VEP response only when using small ch ecks (15') and uncorrected vision. Overlapping latencies are consistent wit h an earlier hypothesis that monocular and binocular VEPs originate postsyn aptically from the binocular neurons in the primary visual cortex. The gain in amplitude achieved by binocular stimulation may depend upon the removal of `tonic interocular inhibition' and/or on a cortical modulatory mechanis m.