Prior binocular dissociation reduces monocular form deprivation amblyopia in monkeys

The profound visual deficits associated with early monocular form deprivation (MD) are caused largely by competitive binocular interactions in the visual cortex. We tested the hypothesis that disrupting normal cortical binocularity prior to the onset of MD would reduce the degree of form deprivation amblyopia compared to that produced in animals that had normal binocular vision prior to MD. An optical strabismus was maintained in two rhesus monkeys between 30 and 90 d old, a rearing strategy that substantially reduces excitatory cortical binocular interactions. Subsequently, the lids of one eye were sutured closed for 9 mo. Comparison data were obtained from a series of form-deprived monkeys reared in a normal manner prior to the onset of MD. Psychophysical procedures were employed to measure the effects of the rearing procedures on the spatial contrast sensitivity functions for each eye. By itself, MD resulted in a severe amblyopia. The spatial resolution of the deprived eyes of monkeys lid sutured by 5 mo of age was about 6 octaves lower than normal values (Snellen acuities of about 20/2000). In contrast, equivalent periods of MD had a much smaller effect on the spatial vision of the prism-reared monkeys. In both cases, the deprived-eye cut-off spatial frequencies were within 1 octave of the nondeprived-eye values and were comparable to cut-offs for binocularly form-deprived monkeys. The results demonstrate that prior binocular image dissociation protects young monkeys from the effects of MD, probably by reducing the potential for antagonistic, competitive binocular interactions in the visual cortex.