Binocular deficits associated with early alternating monocular defocus. I. Behavioral observations.

To study the binocular vision deficits associated with anisometropia, monkeys were reared with alternating monocular defocus, which allowed monocular mechanisms to develop normally while binocular mechanisms were selectively compromised. A defocusing contact lens of -1.5 D, -3 D, or -6 D was worn on alternate eyes on successive days (n = 3 per lens power) from 3 wk to 9 mo of age. The control subjects were two normally reared monkeys and two human observers. Functional binocular vision was assessed through behavioral measurements of stereoscopic depth discrimination thresholds as a function of spatial frequency. To characterize the extent of the deficits in disparity processing at a given spatial frequency, the contrast required to support stereopsis was determined for a range of disparities that exceeded the subjects' measured stereoacuity. The lens-reared monkeys showed spatial-frequency-selective deficits in stereopsis that depended on the magnitude of the simulated anisometropia experienced during the rearing period. For a given spatial frequency, the treated monkeys generally required higher than normal contrasts to support stereopsis even for large disparities. Moreover, a given increase in contrast produced smaller than normal improvements in stereo discrimination in our treated subjects, which suggests that in addition to deficits in contrast sensitivity, disparity-sensitive mechanisms exhibited low contrast gains. The spatial-frequency selective nature of the binocular deficits produced by the imposed anisometropia indicate that disparity processing mechanisms are normally spatial-frequency selective and that mechanisms tuned to different spatial frequencies can be differentially affected by abnormal binocular visual experience.