Serial pattern retention in male and female rats

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Serial pattern learning is a model paradigm for studying parallel-processing in complex learning in rats. The current experiment extends the paradigm to the study of sequential memory by examining forgetting curves for the component element types that make up a serial pattern. Adult male and female rats were trained in a serial multiple choice (SMC) task in which rats learned a serial pattern of nose-poke responses in a circular array of 8 receptacles mounted on the walls of an octagonal operant chamber. The pattern was 123–234–345–456–567–678–781–818, where digits represent the clockwise positions of successive correct receptacles. Previous work has shown that chunk-boundary elements (the first element of each 3-element chunk), within-chunk elements (the second and third elements in all but the last chunk), and the violation element (the last element of the pattern) are learned via different cognitive mechanisms. After each rat was trained to an 85% correct performance criterion on the violation element, we then assessed serial pattern retention at 24-h, 2-week, and 4-week retention intervals. For chunk-boundary and within-chunk elements, forgetting was observed only at the 4-week retention interval. Sex differences were observed; females performed better than males on within-chunk elements at 24-h and 4-week retention intervals. For the violation element, significant forgetting was observed earlier at the 2-week retention interval as well as at the 4-week retention interval. Thus, pattern elements that were learned slower were forgotten faster. The experiment provides a proof of concept for evaluating forgetting curves separately for the multiple memory systems rats appear to employ concurrently in this paradigm, a method that may prove useful in characterizing the impact of relevant neurobiological manipulations on forgetting in multiple sequential memory systems.

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