Evaluation of short-term cue recollection following entorhinal cortical lesions in rats

Abstract

To evaluate the performance of rats with entorhinal cortical (EC) lesions in a task which requires the short-term recollection of a cue, we designed a T maze with a start arm appended to the stem of the T which could be adjusted to require either a right or left turn into the stem. This forced turn was then used as the cue to guide performance at the choice point of the T. Two groups were trained with different contingencies. In the first (same turn), rats were trained to turn right at the choice if their forced “cue” turn had been to the right, and left if the cue turn had been left. In the second, the opposite contingencies applied (opposite turn). Normal animals found the former of these tasks considerably more difficult than the latter, requiring an average of 30 days (at 10 trials/day) to learn the “same turn” contingency and 15 days to learn the “opposite turn.” In addition, the version of the task which was easier for normal animals (opposite turn) was essentially unaffected by bilateral EC lesions, while the “same turn” contingency was profoundly disrupted. In addition to exhibiting no ability to perform according to the contingencies of the “same turn” task, the animals with bilateral EC lesions exhibited a high degree of perseveration of position preference, choosing one goal arm exclusively regardless of the cue turn. These results are considered in the light of the “cognitive map” and the “working memory” hypotheses which have been advanced to account for the behavioral deficits resulting from damage to the hippocampus and its circuitry.