Subjects judged as “same” or “different” pairs of visual stimuli, each either an X or an O, whose onsets were separated by intervals of 50, 100, 150 or 200 ms. When the interstimulus interval (ISI) was held constant within a block of 40 trials (Experiment I), decision time was independent of ISI. But when ISI was varied unpredictably from trial to trial (Experiments II and III), decision times increased the shorter the ISI. This effect was more marked for “same” than for “different” decisions. These results fail to support a single-channel interpretation, but suggest that variations in decision time depend on the subject's expectancy as to when the second stimulus will follow the first.
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