Neural limitations of visual excitability: II. Retrochiasmal interaction

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Abstract
Binocular visual excitability changes were obtained from two trained observers by presenting a "conditioning" flash of light to one eye and a brief "test" flash to a homologous retinal position in the other. Interval between flashes ranged from –200 msec. (test flash preceding conditioning) to +600 msec. (test following onset of conditioning flash). Results showed that threshold rose when test preceded conditioning flash, reaching a maximum increment of 0.5 log ml at –25 msec. separation. When test was superimposed upon conditioning flash in time, threshold dropped sharply to reach a relatively constant but elevated value for the duration of the conditioning flash. No excitability changes were obtained, however, if the test and conditioning flashes were displaced so as to stimulate adjacent rather than "corresponding" retinal points. These findings indicate that central (retrochiasmal) factors can influence visual sensitivity, the magnitude of this influence varying with the spatial and the temporal relations between successive light flashes. This conclusion is compatible with recent electrophysiological data on excitability changes in the cerebral visual system.