Transient memory impairment in monkeys with bilateral lesions of the entorhinal cortex

Performance on five behavioral tasks was assessed post-operatively in Macaca fascicularis monkeys prepared with bilateral lesions of the entorhinal cortex (E group). Three of the tasks were also readministered 9–14 months after surgery. Initial learning of the delayed nonmatching-to-sample (DNMS) task was impaired in the E animals relative to unoperated control monkeys. On the delay portion of DNMS, the performance of E animals was nearly at control levels at short delays (up to 60 sec) but was impaired at 10 min and 40 min retention intervals. On the retest of DNMS, the E animals performed normally at all retention intervals. The E animals were unimpaired on the four other memory tasks. Neuroanatomical studies revealed a significant transverse expansion of the terminal field of the perirhinal cortical projection in the CA1 region of the hippocampus. Compared to unlesioned, anatomical control monkeys, the transverse length of the perirhinal terminal field in CA1 increased approximately 70% in the E monkeys. Although this was a striking morphological alteration, it is not known whether the sprouting of this projection influenced the behavioral recovery. The results of these studies suggest that the entorhinal cortex may normally participate in the learning and performance of tasks that are dependent on the medial temporal lobe memory system. However, recovery of normal DNMS performance demonstrates that the entorhinal cortex is not, by itself, essential for learning and performance of such tasks.