The summation index for double flashes was psychophysically obtained as a function of interflash interval t for five different adapting levels. Clear inhibition was found at about t = 50 and at t = 70 msec for the two brightest adapting fields. Inhibition was also observed at very large intervals, including t = 300 msec. To explain the curve, a temporal response with a positive component and a negative component (type-B response) was hypothesized for the human visual system, whose physiological correlates are discussed. For relatively low adapting levels, including dark adaptation, the summation-index curves showed a somewhat different shape. They did not show strong inhibition, but rather weak and long-continued inhibition, ranging from about t = 100 through 300 msec. Therefore, the temporal response for those conditions may be composed of a small but long-continued negative component (type-A response). The transfer from type-A response to type-B response is rather abrupt; it occurs at the adapting level where the increment threshold begins to increase from the value that corresponds to the absolute threshold.
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