Selective losses in binocular vision in anisometropic amblyopes

Human anisometropic amblyopes typically exhibit reduced contrast sensitivity in the amblyopic eye, especially at higher spatial frequencies. We determined whether this spatial frequency selective loss in contrast sensitivity is accompanied by selective losses in binocular function. Binocular summation (the improvement in one eye's detection performance produced by a subthreshold pattern presented to the fellow eye) was measured at several spatial frequencies. Normal observers exhibited equivalent binouclar summation at all spatial frequencies, whereas all anisometropic amblyopes exhibited normal summation at low spatial frequencies but none at high spatial frequencies. Stereoacuity (minimum resolvable disparity) was also measured as a function of spatial frequency. For normal observers, stereoacuity was best at the highest spatial frequency; for anisometropes stereoacuity was normal at low spatial frequencies, subnormal at intermediate spatial frequencies, and unmeasurable at higher spatial frequencies. Anisometropia may represent a form of selective binocular deprivation that affects neural mechanisms underlying binocular summation and stereopsis.