Temporal resolution during rapid light adaptation was studied using a <i>suprathreshold</i> two-pulse discrimination paradigm with stimulus conditions modeled after the classic increment threshold experiments of B. H. Crawford. The two-pulse stimulus (1° diam, two 20-ms pulses separated by 40 ms) was presented before, during, and after a 6° diam, 500 ms background conditioning field, and discriminability was measured as <i>d</i>’ using signal detection methods. Discriminability is depressed at background field onset and offset, but increases during the exposure time of the background. Immediately following background offset, two-pulse discriminability is dramatically enhanced: two pulses are discriminated from one pulse with a <i>d</i>’ of 4 or more. When the two-pulse stimulus follows background offset by 80 to 240 ms, a novel <i>temporal illusion</i> occurs: a single pulse consistently appears as a double pulse. Several hypotheses for understanding these results are discussed.
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