Binocularity in the human visual evoked potential: facilitation, summation and suppression.

The electrophysiology of normal binocular function was investigated by studying the binocular interactions from monocular and binocular visual evoked potentials (VEPs) recorded under a range of stimulus conditions. The amplitude and phase of the steady-state VEPs, which were obtained with sinusoidal gratings temporally modulated in counterphase, were measured with synchronous narrow-band filtering techniques. Binocular interactions were investigated as functions of spatial frequency, temporal frequency and contrast. Detailed sampling and testing within each stimulus domain revealed an unprecedented degree of VEP specificity in the extent of binocular interaction, which varied from zero summation to pronounced facilitation. Binocular facilitation was explored in terms of its relation to the neural mechanisms subserving binocular function. VEP correlates of rivalry, fusion and stereopsis were obtained. The facilitatory binocular interactions, revealed by careful spatial and temporal stimulus manipulations, were akin more to the neurophysiological responses of single neurons than to previously reported evoked potential work. The relationship between the specificity and narrow tuning of the binocular interactions recorded with VEP techniques and those recorded with single unit techniques is discussed.