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 <i>t</i> = 50 and at <i>t</i> = 70 msec for the two brightest adapting fields. Inhibition was also observed at very large intervals, including <i>t</i> = 300 msec. To explain the curve, a temporal response with a positive component and a negative component (type-<i>B</i> 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 <i>t</i> = 100 through 300 msec. Therefore, the temporal response for those conditions may be composed of a small but long-continued negative component (type-<i>A</i> response). The transfer from type-<i>A</i> response to type-<i>B</i> 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.
TOSHIO UETSUKI and MITSUO IKEDA, "Study of Temporal Visual Response by the Summation Index," J. Opt. Soc. Am. 60, 377-381 (1970)