Binocular beat VEPs: losses of cortical binocularity in monkeys reared with abnormal visual experience.

Binocular beat VEPs were recorded from anesthetized macaque monkeys with diverse visual rearing histories, including surgically induced esotropia, optical prism dissociation, optical anisometropia, monocular form deprivation (MD), and normal rearing. Dichoptic visual stimulation was produced by temporally modulating the luminances of uniform fields presented to each eye. Five pairs of temporal frequencies were used, all of which had interocular differences of 2 Hz. While normally reared animals exhibited robust binocular beat responses strongly tuned to temporal frequency, the responses from monkeys with abnormal rearing experiences showed losses in beat signal-to-noise ratios that correlated with the age of onset or duration of the abnormal visual experience. Surgical esotropia induced early in life (2 months of age) produced a virtually complete loss of the binocular beat response; the cortical losses were less severe as the age of surgery rose to 10 months. Monkeys reared with either anisometropia or optical dissociation also manifested substantial reductions in evoked beat nonlinearity. MD monkeys sutured relatively late in development (8 and 25 months) showed mild reductions. The correspondence of these results to earlier psychophysical data obtained from these animals, and the similarity of these results to previous findings with binocularly normal and abnormal humans, supports the use of the binocular beat as an objective, noninvasive index of binocular neural integrity.