When an increment threshold is measured as a function of the radiance of an adapting field, the so-called threshold-versus-radiance curve is obtained. Stiles has shown by the two-color threshold technique that this curve is not necessarily smooth but can have inflections whose nature depends upon the conditions. From this he reasoned that the mechanism responsible for the threshold changes from one to another with a change of adapting level, all conditions otherwise being kept constant. In the present paper it is shown that an analogous situation can occur with the threshold-versus-duration curve, which is sometime called a temporal summation curve. Instead of a change of adapting level, a change in the duration of the test stimulus is considered. An assumption is made that each mechanism has its own threshold-versus-duration curve, and that the over-all threshold-versus-duration curve is determined by their relative positions. As an example of the utility of this analysis, two sensitivity curves of the eye were obtained with test stimuli of two different durations, 100 and 12.5 msec. It is argued that they are different in shape because of a change in the contribution of the responsible mechanisms which can be interpreted in terms of the relative positioning of threshold-versus-duration curves.
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