Interocular luminance differences and the binocular pattern-reversal visual-evoked response

Although the visual-evoked response (VER) is frequently used to assess binocularity, the contribution of the monocular components to the binocular VER is poorly understood. To more fully elucidate this relationship, we examined checkerboard (14 min arc checks) pattern-reversal (3.75 Hz) VERs evoked from observers with normal binocularity, with conditions in which interocular luminance differences (from 0.3 to 2.0 log units) were established. When compared with monocular VERs obtained with similar luminances, the binocular response was always less than the sum of the component monocular responses. In addition, the amplitude of the binocular signal was dependent on the amount of interocular luminance the difference. For interocular luminance differences of less than 0.6 log units the amplitude of the binocular response was consistently greater than either corresponding monocular VER. When the interocular luminance difference was 1.3 log units or greater the amplitude of the binocular response fell below the level of either corresponding monocular response. Furthermore, it does not appear that these results can be attributed to a passive spread of electrical potentials from monocular cortical cell populations. We therefore suggest that these results indicate the activity of a binocular neural process.