Visual field deficits in cats reared with unequal alternating monocular exposure.

Thirty-four kittens reared by allowing each eye patterned visual input, but on alternate days and for unequal periods of time (unequal alternating monocular exposure (AME)), were tested for their ability to orient to targets at different positions in visual space. In all unequal AME cats, the visual field of the more experienced eye (MEE) was normal, while that of the less experienced eye (LEE) was restricted. In contrast, in 14 cats reared with equal AME, the fields of both eyes were equal and of normal size. The field deficits observed in the unequal AME cats must therefore be due to the imbalance in stimulation and thus result from a competitive interaction between the afferents from the MEE and the LEE. The field deficits observed in the unequal AME cats differed from those observed in two monocularly deprived (MD) cats. Neither of the MD cats ever responded to targets presented in the region of normal binocular overlap when tested with the deprived eye (DE). The unequal AME cats all showed a nasal field loss in the LEE, but responded normally to targets throughout the temporal portion of the binocular visual field. When the imbalance in stimulation was large (8 to 1) or moderate (8 to 4), there was an abrupt drop in responsiveness (from 100% to zero) as the position of the target was changed from temporal to nasal. When the imbalance was slight (8 to 7), the drop in responsiveness was more gradual and occurred within the nasal field. Our results demonstrate that (1) the paradigm of unequal AME is a useful one for studying binocular competition quantitatively, (2) even the slightest imbalance in stimulation of the two eyes can affect the outcome of the competitive interaction, and (3) the pathways serving binocular vision are not uniformly affected by binocular competition: the ipsilateral pathway is more sensitive than the contralateral pathway.