Two-pulse discrimination and rapid light adaptation: complex effects on temporal resolution and a new visual temporal illusion.

Temporal resolution during rapid light adaptation was studied using a suprathreshold two-pulse discrimination paradigm with stimulus conditions modeled after the classic increment threshold experiments of B. H. Crawford. The two-pulse stimulus (1 degree diam, two 20-ms pulses separated by 40 ms) was presented before, during, and after a 6 degree diam, 500 ms background conditioning field, and discriminability was measured as d' 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 d' of 4 or more. When the two-pulse stimulus follows background offset by 80 to 250 ms, a novel temporal illusion occurs: a single pulse consistently appears as a double pulse. Several hypotheses for understanding these results are discussed.