Monocular visual excitability was studied psychophysically by determining the threshold intensity for a small (40-min) brief (5-msec) “test” flash centered within a larger (3°) “conditioning” flash, both presented at 7° of arc in the temporal half-field. Interval between flashes was varied from −200 (test flash preceding onset of conditioning) to +1500 msec (test flash following), nine combinations of conditioning flash intensity and duration being used as parameters. For all combinations, threshold rose between −200 and −100 msec (before the start of the conditioning flash), reaching a maximum increment when the onsets of the two flashes were synchronized. When test was superimposed upon conditioning flash in time (positive intervals), threshold declined to approach an asymptote near the end of the latter flash. Following termination of the conditioning flash, threshold returned to resting level. The maximum rise in threshold, as well as the time taken to achieve an asymptote, varied with both intensity and duration of the conditioning flash. When a rough estimate of the photochemical contribution to these threshold changes was subtracted from the raw data, residual threshold increments were obtained and attributed to neural processes. On the basis of time course, it is suggested that these neural processes involve central (geniculocalcarine) as well as peripheral (neuro-retinal) factors.
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