Functional and neuronal binocularity in kittens raised with rapidly alternating monocular occlusion.

1. In order to determine the degree of synchrony of binocular activation required for the development of binocularity we reared 11 kittens with rapidly alternating monocular occlusion. Alternating occlusion was achieved with microprocessor-controlled electrooptic solid-state shutters, which were fitted to individually moulded goggles. The intervals of alternating occlusion were varied from 50 to 1,000 ms. Two normally reared kittens and three kittens that were reared with the shutters operating synchronously with open/close intervals of 50/50 ms, 200/200 ms, and 400/100 ms, respectively, were used as controls. Toward the end of the critical period we examined the kittens' ability for binocular depth discrimination and tested binocular luminance summation of the pupillary light reflex. Single-cell recordings were made from the visual cortex in order to determine the percentages of binocularly excitable neurons. 2. There was a good correlation between the degree of asynchrony of binocular experience, the impairment of depth discrimination, and the percentage of binocular neurons. Kittens reared with alternation rates of 200, 330, and 400 ms, respectively, had developed normal binocularity and were indistinguishable from the controls. Alternation rates of 500 ms or longer prevented the development of normal depth discrimination and luminance summation and resulted in reduced cortical binocularity. 3. A linear relationship between depth discrimination, binocular luminance summation, and percentages of binocular neurons was found. 4. Our findings indicate that an asynchrony of binocular activation of several hundred milliseconds is compatible with the development of normal binocularity in the kitten visual system.