A perceptual-mnemonic role for the perirhinal cortex in age-associated cogntive decline

Abstract

Perirhinal cortical-dependent behavior and single-unit neuron activity in this brain region were compared between normal aged and young rats. Three different variants of the spontaneous object recognition task were used in these experiments, and the results confirmed previous reports that aged animals are impaired at stimulus recognition. The novel contribution of the present experiments was the identification that the behavioral deficit in the aged rats was due to the old animals treating novel objects as familiar, rather than to forgetting the previously experienced stimuli. This pattern of results in the old animals mirror data obtained from rats with perirhinal cortical lesions and promotes a hypothesis that this area of the brain serves a perceptual-mnemonic function. Additionally, multiple single-unit recordings were obtained from perirhinal cortical cells while young and aged rats traversed a track that contained several objects. Perirhinal neurons exhibited selective increases in their firing rates at object locations. We have called these areas of higher perirhinal cortical cell activity `object fields'. While both young and old rats expressed object fields, a lower proportion of perirhinal neurons showed this type of activity in the aged compared to the young rats. Although familiar and novel objects were placed on the track as part of a systematic design, there was no effect of novelty on the overall firing rates of perirhinal cortical neurons or the proportion of cells expressing object fields under these experimental conditions. These data suggest that the physiological correlate of stimulus recognition is not decrements in perirhinal cortical activity when a stimulus goes from novel to familiar. A final important observation made during these studies in young rats was that place fields in the middle hippocampal CA1 subregion are affected by placing objects in the track. Because this same manipulation increases perirhinal cortical activity, it could indicate that age-related changes in the perirhinal cortex might alter the function of other closely associated structures.