Differential effects of experience on tuning properties of macaque MTL neurons in a passive viewing task.

The structures of the medial temporal lobe (MTL) have been shown to be causally involved in episodic and recognition memory. However, recent work in a number of species has demonstrated that impairments in recognition memory seen following lesions of the perirhinal cortex (PRh) can be accounted for by deficits in perceptual discrimination. These findings suggest that object representation, rather than explicit recognition memory signals, may be crucial to the mnemonic process. Given the large amount of visual information encountered by primates, there must be a reconsideration of the mechanisms by which the brain efficiently stores visually presented information. Previous neurophysiological recordings from MTL structures in primates have largely focused on tasks that implicitly define object familiarity (i.e., novel vs. familiar) or contain significant mnemonic demands (e.g., conditional associations between two stimuli), limiting their utility in understanding the mechanisms underlying visual object recognition and information storage. To clarify how different regions in the MTL may contribute to visual recognition, we recorded from three rhesus macaques performing a passive viewing task. The task design systematically varies the relative familiarity of different stimuli enabling an examination of how neural activity changes as a function of experience. The data collected during this passive viewing task revealed that neurons in the MTL are generally not sensitive to the relative familiarity of a stimulus. In addition, when the specificity (i.e., which images a neuron was selective for) of individual neurons was analyzed, there was a significant dissociation between different medial temporal regions, with only neurons in TF, but not CA3 or the PRh, altering their activity as stimuli became familiar. The implications of these findings are discussed in the context of how MTL structures process information during a passive viewing paradigm.