Residual binocular interactions in the striate cortex of monkeys reared with abnormal binocular vision

We investigated the nature of residual binocular interactions in the striate cortex (V1) of monkey models for the two most common causes of visual dysfunction in young children, specifically anisometropia and strabismus. Infant rhesus monkeys were raised wearing either anisometropic spectacle lenses that optically defocused one eye or ophthalmic prisms that optically produced diplopia and binocular confusion. Earlier psychophysical investigations had demonstrated that all subjects exhibited permanent binocular vision deficits and, in some cases, amblyopia. When the monkeys were adults, the responses of individual V1 neurons were studied with the use of microelectrode recording techniques while the animals were anesthetized and paralyzed. The manner in which the signals from the two eyes were combined in individual cells was investigated by dichoptically stimulating both eyes simultaneously with drifting sine wave gratings. In both lens- and prism-reared monkeys, fewer neurons had balanced ocular dominances and greater numbers of neurons were excited by only one eye. However, many neurons that appeared to be monocular exhibited clear binocular interactions during dichoptic stimulation. For the surviving binocular neurons, the maximum binocular response amplitudes were lower than normal; fewer neurons, particularly complex cells, were sensitive to relative interocular spatial phase disparities; and the remaining disparity-sensitive neurons exhibited lower degrees of binocular interaction. In prism-reared monkeys, an unusually high proportion of complex cells exhibited binocular suppression during dichoptic stimulation. Binocular contrast summation experiments showed that for both cooperative and antagonistic binocular interactions, contrast signals from the two eyes were combined by individual neurons in a normal linear fashion in both lens- and prism-reared monkeys. The observed binocular deficits appear to reflect a reduction in functional inputs from one eye and/or spatial imprecision in the monocular receptive fields rather than an aberrant form of binocular interaction. In the prism-reared monkeys, the predominance of suppression suggests that inhibitory connections were, however, less susceptible to diplopia and confusion than excitatory connections. Overall, there were many parallels between V1 physiology in our monkey models and the residual vision of humans with anisometropia or strabismus.