Retention of concurrent conditional discriminations in rats with ibotenate lesions of hippocampus

Abstract

We studied the effects of removing the hippocampus on the retention of appetitive Pavlovian conditional discrimination. In Experiment 1, rats were trained preoperatively on two concurrent conditional discrimination problems in which a stimulus (i.e., a facilitator) signaled the reinforcement of another stimulus (i.e., a target). The results indicated that performance was poorer when a “trace” interval separated the facilitator and target than when the facilitator overlapped with the target. Data obtained from transfer and probe trials yielded results compatible with the idea that the problems were solved using “if-then” rules rather than by either direct associations between facilitators and reinforcement or unique configurai associations. In Experiment 2, half of the rats had the hippocampus removed using multiple injections of small amounts of ibotenic acid, and after recovery all rats were tested for retention of the preoperatively learned conditional discriminations. In contrast with previous reports, the results in this study showed that lesions of the hippocampus did not disrupt conditional performance. Thus, rather than supporting the hypothesis that the hippocampus plays an important role in the control of conditional behaviors, our results indicate that the structure is not necessary for the performance of complex Pavlovian conditional discriminations. © 1989, Psychonomic Society, Inc.. All rights reserved.