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Four experiments examined the effects of similarity between the sample and an interpolated stimulus in a modified delayed-matching-to-sample (DMTS) paradigm. The basis trial sequence was as follows: (a) a sample was presented, and the response to it was either rewarded or nonrewarded, (b) an interpolated stimulus was presented, and the response to it was either rewarded or nonrewarded, and (c) after a delay interval a choice test was given between the initial sample and a new stimulus, with the sample being correct if it had been initially rewarded and incorrect if it had been initially nonrewarded. When the sample and the interpolated stimulus were associated with opposite outcomes (sample rewarded, interpolated stimulus not rewarded or sample not rewarded, interpolated stimulus rewarded), retroactive interference was observed to increase as the similarity of the two stimuli increased. When the two stimuli were either both rewarded or both nonrewarded, retroactive facilitation was observed so long as the sample and interpolated stimuli were similar along either the relevant or an irrelevant dimension. Finally, test probes involving the interpolated stimulus and a new stimulus revealed asymmetrical interactions such that the interpolated stimulus influenced performance on the initial stimulus more than the initial stimulus altered performance on the interpolated stimulus. Taken as a whole, the results are inconsistent with the idea that alternative stimuli are encoded independently of each other. At best, independence may hold only for cases in which the stimuli are not similar to each other.

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