Hippocampal lesions enhance configural learning by reducing proactive interference

Rats were trained on an operant conditional discrimination in which an ambiguous stimulus (X) indicated both the occasions on which responding in the presence of a second cue (A) would be reinforced and the occasions on which responding in the presence of a third cue (B) would not be reinforced (X --> A+, A-, X --> B-, B+). Both rats with neurotoxic lesions of the hippocampus and control rats learned this discrimination more rapidly when the training trials were widely spaced (mean intertrial interval of 8 min) than when they were massed (mean intertrial interval of 1 min). With spaced practice, lesioned and control rats learned this discrimination equally well. But when the training trials were massed, lesioned rats learned more rapidly than controls. At the end of training, performance of all rats was tested at three different intertrial intervals, 0.5 min, 1 min, and 8 min. The control rats trained with 8-min intertrial intervals showed deficits in discrimination performance when the test intertrial interval was 0.5 min or 1 min. An analysis of sequential effects indicated that a major source of this performance deficit was the control rats' failure to withhold responding on nonreinforced trials when those trials were immediately preceded by reinforced trials within 0.5 min or 1 min. In contrast, performance of lesioned rats was not affected by either the test intertrial interval or by the nature of preceding trials. Thus, with short intertrial intervals, hippocampal lesions may have improved learning or performance on a given trial by reducing proactive interference from the previous trial.