The visually evoked response. Binocular facilitation and failure when binocular vision is disturbed.

The visually evoked response (VER) caused by the horizontal sinusoidal movement of a display consisting of a checkerboard with 15-minute checks of relatively low contrast oscillating at 6.3 Hz is itself sinusoidal with a frequency of 12.6 Hz. When viewed binocularly, the VER is 25% to 30% greater in amplitude than the sum of the amplitudes for monocular viewing. This binocular faciltation may be a VER correlate of normal binocular single vision. It is lost in small-angle esotropes and in normals whose binocular function is disturbed by a vertical prism placed over one eye. Characteristic curves relating VER amplitude and phase angle to frequency to oscillation of the checker-board display suggest that two "systems" carry information to the visual cortex: a long and a short latency system. In amblyopia, the long latency system may be selectively impaired.