Temporal summation characteristics of the human eye have been studied by various authors by measuring the increment threshold when two test stimuli are presented successively. Among other things, some have observed an inhibition between the effects of two flashes for a certain inter-flash interval and others have not. Here a similar experiment is carried out with a red test stimulus superposed upon a red adapting field. Inhibition is observed at an interval of 52 or 70 msec, depending on the adapting level. Such an inhibition is also found when two stimuli are both negative. The introduction of a negative test stimulus into the double-flash, increment-threshold technique is a new aspect of the present work. Some new phenomena are observed, particularly, that a positive and a negative flash summate with each other at the interval where double positive or double negative flashes yield inhibition. The luminance ratio of the two stimuli (positive or negative) was freely adjusted and new information concerning the linearity of the summation was obtained. Based on these findings, hypothetical response-potential functions have been derived, which are assumed to be responses in the visual system at some peripheral level.
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