Neural response to stimulus repetition in the rat perirhinal and postrhinal cortex

Abstract

It has been suggested that the perirhinal and postrhinal cortex plays key roles in recognition memory. A possible neural mechanism for the recognition memory is phenomenon called repetition suppression, which refers to decremental change in the response of neuron when a stimulus is repeated. Monkey studies have found a proportion of perirhinal cortex neurons that changed with changes in stimulus familiarity. However, there were few comparable studies in rodent literature. Furthermore, there has been a conflict between existing rat physiological studies. For this reason, recordings of single neuronal activity were made from the perirhinal and postrhinal cortex of rats while performing object cued response selection (OCRS) task. The neural responsiveness to familiar and unfamiliar stimuli were compared in each regions. The postrhinal cortex neurons show a large proportion of (79%) decremental response change across multiple repetition of familiar stimulus and greater decremental response change when unfamiliar stimulus was repeated between the first trial and subsequent trial, but no such response changes were found in the perirhinal cortex. When the learning state of animals were accounted for neural response changes, both perirhinal and postrhinal cortex show a greater decremental response to the unfamiliar stimulus during learned state than during acquisition phase. This result suggests that both perirhinal and postrhinal cortex process relative familiarity of unfamiliar stimulus, but not of highly familiar stimulus, only when the rat successfully acquired recognition memory. This finding is the first to show decremental neural response changes in the perirhinal and postrhinal cortex that were modulated by object-response associative recognition memory in rodents.