An FMRI Study of Complex Object and Scene Discrimination: The Contributions of Perirhinal Cortex, Hippocampus and Temporal Pole

Abstract

Previous research has investigated how the perirhinal cortex (PRC), hippocampus (HC) and temporal pole (TP) are involved in complex visual discrimination using a variety of stimuli. Results from these studies have shown that the PRC activates to a greater extent for object stimuli relative to scene stimuli and that familiar stimuli elicit greater activation than do novel stimuli. In contrast, the HC shows greater activation for scene stimuli relative to objects and has also been reported to show greater activation for familiar relative to novel stimuli. To our knowledge, however, no studies in humans have replicated the stimulus specificity findings reported for PRC and HC. Additionally, no studies have used a combination of perceptual difficulty and familiarity/novelty to investigate how varying these factors affects activation in PRC, HC and TP during visual discrimination tasks. Chapter 2 describes an fMRI study performed to investigate the PRC's and HC's involvement in object and scene visual discrimination. The results of this study showed that the PRC was activated similarly by scenes and objects and that the HC was activated similarly for objects and scenes. Chapter 3 describes an fMRI experiment that manipulated both familiarity and difficulty, measuring how this affected PRC, HC and TP activation. In PRC the results showed a significant interaction between novelty and level of difficulty, such that novel objects with high levels of overlapping features showed greater activation compared to all other conditions. In contrast, the HC only showed a main effect of difficulty, indicating that the stimuli with high, relative to low, levels of difficulty elicited greater activation regardless of familiarity. Cumulatively, the evidence above suggests that the involvement of the PRC and HC in visual discrimination is complex. We propose that PRC is engaged whenever visual discrimination is required for any stimuli with overlapping features not just objects, and is activated to a greater extent when stimuli are novel, while HC appears to respond to stimuli with overlapping features, regardless of familiarity.