The role of various sources of proactive and retroactive interference in Delayed-Matching-to-Sample (DMTS) performance of five capuchin monkeys (Cebus apella) was examined in a series of four experiments. The DMTS trials were presented in a semi-automated Wisconsin General Test Apparatus (WGTA) with junk objects serving as stimuli. The first experiment assessed the effects of delay interval (10 or 40 seconds), sample reward (present or absent), and nature of delay interval stimulation (dark or illuminated) on DMTS performance when sample and choice stimuli are drawn from a large pool of stimuli. Performance was better when the sample response was rewarded than when it was not. Correct responding was initially more frequent after dark delays than after illuminated delays; however, with continued training the effects of delay interval illumination disappeared. There was no significant difference in performance with a 10 second and a 40 second delay interval. The second experiment consisted of two replications of Experiment I: one with the sample and choice stimuli drawn from a large pool (480) of objects and the other with stimuli drawn from a small pool (2) of objects. When stimuli were drawn from the small pool of objects, performance was markedly poorer than when stimuli were drawn from the large pool. A significant delay interval effect was observed in the small pool task, but as in Experiment I no such effect was observed when stimuli were drawn from a large pool. Performance was poorer on the small pool condition when the delay interval was illuminated than when the houselights were extinguished. However, unlike the effect observed in Experiment I, illuminated delay performance decrements persisted after continued training. No illumination effect was observed for the large pool replication of Experiment II. In both the large and small pool replications, performance was again inferior after nonrewarded sample responses than after rewarded sample responses. Experiment III was designed to test the hypothesis that animals learn to use delay interval stimuli as cues to remember sample stimuli. After extended training on the small stimulus pool condition with delay intervals illuminated, a number of probe trials were presented. On probe trials, the stimuli previously displayed to the subjects during the delay interval (a yellow and white opaque screen) and the stimuli previously displayed during the intertrial interval (a black opaque screen) were reversed. Contrary to the predictions of the retention-cue hypothesis, DMTS performance was no lower on probe trials than on control trials. The final experiment was designed to test the hypothesis that emotionally significant events occurring after the sample presentation period can disrupt DMTS performance by displacing the sample stimulus trace from a limited capacity memory store. In order to insure that the effects of the independent variable not be obscured by floor effect, an intermediate sized (52) pool of objects was used in this experiment. After a pretraining phase, a number of probe trials were presented with a novel sample incentive. As predicted, performance was significantly poorer when the type of incentive was unexpected than when it was expected. These results were interpreted as evidence that animals are more sensitive to the effects of retroactive interference when proactive interference is high (small stimulus pool) than when it is low (large stimulus pool). It was also suggested that emotional reactions can serve as powerful sources of retroactive interference in DMTS performance. These effects occur regardless of whether the emotional response is negative (sample nonreward) or positive (novel sample reward).
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