Binocular inhibition: psychophysical and electrophysiological evidence.

Binocular summation, defined as an increase in the binocular response compared with the monocular, occurs when the sensitivities of the two eyes are equal. We investigated the psychophysical and electrophysiological binocular response to a difference in monocular retinal illuminance. Different levels of unequal monocular sensitivities were induced by means of neutral density filters placed in front of one eye. Both studies produced similar results. In the absence of filters, maximum binocular summation was produced. With increasing difference in monocular illuminance, the binocular response decreased steadily until it reached a level below the monocular. The clinical implications of binocular inhibition, a perceptual phenomenon similar to Fechner's Paradox, are discussed.