A role of monkey perirhinal cortex in relational memory and learning

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To clarify roles of perirhinal cortex in memory and learning, especially in relational or configural ones, effects of perirhinal cortex lesion on delayed matching-to-sample tasks (DMS) with small (3 stimuli) and large (300 stimuli) sets of stimuli (DMS-S and DMS-L, respectively) and transverse patterning task (TPT) were examined using 6 macaque monkeys. Preoperatively, monkeys were trained on DMS-L and DMS-S. Postoperatively, DMS-S was first tested. Thereafter, DMS-L was given to the monkeys. Last, TPT and its control task, concurrent discrimination task (CDT), were trained. As a result, no impairment was seen in relearning of DMS-S or DMS-L or in their delay tests with 30, 60 and 120 sec. Perirhinal cortex lesion, however, produced a severe impairment in transverse patterning task without impairment in CDT. Our monkeys with perirhinal cortex lesion were not impaired in this task, despite that It is well-established that perirhinal cortex is critically involved in DMS-L. It might be possible that they resolved DMS-L by using their learned DMS-S strategy which remained intact.The present results are sharply contrasted with our previous study demonstrating that monkey brain ischemia, which caused cell loss in the hippocampus, produced impairment in DMS-S, but not in DMS-L, TPT or CDT. It has been controversial whether TPT should be regarded as a configural task or as a relational one. However, perirhinal cortex lesion did not impair in DMS-S which might be regarded as a relational task, but in TPT. Thus, the present study suggests that perirhinal cortex might be involved in configural association, but not in relational association.