Attenuation of context-specific inhibition on reversal learning of a stimulus-response task in rats with neurotoxic hippocampal damage.

Rats with hippocampal or sham lesions were trained on a stimulus-response task developed for the 8-arm radial maze. After reaching a stringent learning criterion, different context manipulations were performed. In Experiment I, the different groups were transferred to an identical radial maze in a different room to determine the context specificity of the discrimination learning. Experiment I revealed that although rats with hippocampal lesions did not show a normal context detection effect, the expression of the discrimination was not context dependent for either the lesion or sham groups. In Experiment II, animals were trained to criterion on the discrimination task and then both groups were divided into sub-groups based on whether they would experience reversal training in the same or different context from original training. Experiment II indicated that animals with hippocampal lesions and shams reversed in a different context were significantly enhanced in reaching the learning criterion compared to either counterparts that were reversed in the same context. Reversal learning in rats with hippocampal lesions was faster than sham animals in the same context suggesting that the context-specific inhibition effect was hippocampal-based. After learning the reversal task, the groups of animals trained and reversed in different contexts were brought back into the original training context to test for competitive effects. Animals with hippocampal lesions that were reversed in the different context, did not show a competition between the most recently acquired discrimination and a context-specific association acquired during original training whereas sham animals in the same condition did. Taken together these results suggest that rats with hippocampal lesions do not acquire normal context-specific inhibition during discrimination learning.