Selective hippocampal lesions in rats do not affect retrieval processes promoted by prior cuing with the conditioned stimulus or the context

Abstract

Following partial training in an avoidance brightness discrimination task in a Y-maze, the retention performance can be enhanced by a pretest exposure to various training features (prior cuing). The effectiveness of these cues may vary as a function of the length of the retention interval. These results demonstrate that prior cuing promotes the retrieval of the memory trace and that the memory trace is submitted to a long-lasting time-dependent reorganization. The present experiments were designed to examine the involvement of the hippocampal formation in retrieval processes promoted by prior cuing. Rats with bilateral ibotenic acid hippocampal lesions and sham-operated rats were postoperatively trained in the avoidance brightness discrimination task. Retrieval processes promoted by prior cuing were not affected by hippocampal lesions: After a 1-day training-to-test interval (TTI), the rats in both conditions demonstrated a dramatic improvement of retention performance following a pretest exposure to the conditioned stimulus (CS; Experiment 1). After a 21-day TTI, exposure to the CS was no longer effective, and a pretest exposure to the experimental context enhanced performance (Experiment 2). In contrast, when the lesioned rats were tested in a radial arm maze task, they showed severely impaired performance, demonstrating the effectiveness of the hippocampal lesions on behavior. These results demonstrated that damage to the hippocampus does not disrupt the promotion of retrieval processes by prior cuing. In particular, the lesions did not prevent the rats from being able to relate the CS and the experimental context to the initial training episode. The fact that the hippocampal lesions also did not disrupt time-dependent changes in the effectiveness of certain retrieval cues suggests that this structure is not involved in the long-term maturation processes evidenced by prior cuing.