Dual functions of the perirhinal cortex in both perception and memory for objects

Abstract

Animals including humans guide their actions (e.g., where to go) based on a critical stimulus (e.g., object) in an environment, and can do so even when the stimulus is no longer present owing to the memory of the stimulus. The capability of processing, and recognizing a stimulus for informing prospective behaviors is fundamental to an organisms survival. Since Scoville and Milners characterization of patient H.M., who demonstrated severe recognition memory deficits following damage to the medial temporal lobe, the perirhinal cortex (PER) has been extensively studied as one of the key neural substrates for object recognition memory. Two dominant hypotheses have been prevalent in neuroscience regarding the role of the PER. The first hypothesis is that the PER is important for object recognition memory, as has long been acknowledged by human patients, and animal models. The second hypothesis is relatively a recent one and it suggests that the PER involvement in object recognition occurs not only during object memory but also during perceiving object especially when ambiguous objects that share many visual features need to be discriminated. The functional role of the PER has built primarily on behavioral findings using animal models in which behavioral deficits were found in object memory paradigms following focalized damages to the PER. While more direct evidence may be obtained by recording neural signals, only a handful of physiological studies have been conducted so far to measure neural correlates for objects in the PER. Also, in those studies, recordings were made from animals whose physical body movements were severely restrained, or in tasks that were difficult to correlate neural firings to an object due to multiple confounding factors. The present thesis is focused on providing physiological evidence that may help fill the gap that remained in the field for decades. In the thesis, I will present a novel object memory paradigm that was designed to address key issues related to object information processing in the rodent PER. Building on the previous anatomical, behavioral findings on the PER, I sought to address whether (i) the rodent PER neurons would show differential firing patterns for object identities, and (ii) the PER neuronal firings would be modulated by the perceptual features of an object that contains significant feature ambiguity.