Perirhinal Cortex Lesions and Spontaneous Object Recognition Memory in Rats: Detecting Novelty But Not Distinguishing Novelty

Abstract Perirhinal cortex lesions in rats consistently impair object recognition memory, as shown by the failure to spend more time exploring a novel object in preference to a familiar object in a spontaneous object recognition task. Nevertheless, these same lesioned rats show normal levels of object exploration during the initial ‘Sample Phase’, when first familiarized to a novel object. This consistent finding is surprising as it indicates an intact ability to detect novelty. This apparent paradox was examined in greater detail using the ‘Bow-Tie maze’, which allows multiple, continuous trials within the same session. Experiments using this apparatus revealed that rats with perirhinal lesions show normal levels of object exploration when presented with just novel or just familiar objects to explore, i.e., when presented separately. Preference deficits reappear when novel and familiar objects are presented simultaneously. These results, which reveal novelty detection mechanisms outside perirhinal cortex, highlight how much is unknown about the mnemonic processes taxed by spontaneous object recognition tasks, despite their apparent simplicity. The results also reveal how the particular test procedure can lead to seemingly different findings.