Spatial contrast sensitivity deficits in monkeys produced by optically induced anisometropia.

An anisometropia was simulated in infant rhesus monkeys by securing a high-powered minus lens (-10 D) in front of one eye. The anisometropia rearing procedure was initiated at 30 days of age and was continued for durations of 30, 60, or 90 days. Behavioral measurements of spatial contrast sensitivity obtained when the animals were 9 months of age indicated that the monkeys treated for 30 days had equal or nearly equal contrast sensitivities and cut-off spatial frequencies in the two eyes. The 30-day monkeys also demonstrated normal binocular summation for threshold stimuli. In contrast, the monkeys treated for either 60 or 90 days showed a significant reduction in contrast sensitivity in the defocused eyes for spatial frequencies greater than 1.0 cycles/deg and failed to show an improvement in contrast sensitivity under binocular viewing conditions. The cut-off spatial frequencies obtained at moderate luminance levels for the defocused eyes of the 60- and 90-day monkeys were slightly more than 1.0 octave lower than the cut-offs for the nondeprived eyes and, like humans with anisometropic amblyopia, the deficits in the spatial resolving capacity of the defocused eyes were observed over a large range of background luminances. The results indicate that the lens-reared monkey is a promising model for anisometropic amblyopia in humans.