Time course of the subjective contrast enhancement for a second stimulus in successively paired above-threshold transient forms: perceptual retouch instead of forward masking.

Subjects estimated apparent contrast of an invariant test stimulus (the letter H), exposed either after an inducing stimulus (from the set of 12 alternatives) with SOAs varying from 20 to 290 msec, or alone (a control condition). With inducing stimuli, at certain SOAs the subjective contrast of the TS was enhanced relative to single-stimulus control. This enhancement appeared as a curvilinear function of SOA with the most pronounced enhancement at SOA = 110 msec. The effect was present also if the Ss had to recognize the inducing, backward-masked stimulus (the recognition function being U-shaped). The enhancement effect increased with increase in the intensity-level of the inducer. The separate functions for the conditions of inducing letters of high and low figural similarity with TS were coinciding, thus pointing to the nonspecific nature of the obtained effect within the given spatial constraints (1 deg of the visual angle). A small forward masking effect present with SOAs less than or equal to 40 msec was rapidly replaced by the enhancement with increase in SOA. The results are discussed in terms of the visual masking theories and the hypothetical perceptual retouch mechanism.