On and off responses to a conditioning stimulus fluctuating between a primary adapting level of 2.7 ft-L and a secondary level of 0.3 ft-L are investigated using the increment-threshold technique. Single negative conditioning flashes (momentary diminution from 2.7 to 0.3 ft-L) are shown to yield discrete off and then on responses, even for flashes as short as 3 msec. Positive conditioning flashes, presented after a 17-msec diminution of the primary adapting level and followed by 57 msec at the secondary level, are also capable of generating discrete on and off responses, but only if longer than about 40 msec. Very short positive flashes produce little or no effect and are treated by the visual system as if they were part of a continuous dark interval. Flicker is investigated by presenting series of N negative conditioning flashes, with N varying from 2 to 5; increment thresholds obtained under these conditions are compared with those for a stimulus continuously fluctuating between the two levels at 29 cps. The results indicate that the visual system treats the flicker train to a considerable extent as it would a single negative flash, with a ripple superposed that is associated with all individual flashes in the train except the first positive flash, which is always ignored. It is tentatively suggested—pending further experimentation—that the responses to the individual flashes in a flicker train are neither on nor off responses; rather they may be responses to the integrated energy contained within each flash.
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