Recent evidence suggests the perirhinal cortex (PRc), a medial temporal lobe structure involved in memory, may also be involved in visual perception. We conducted an event-related fMRI experiment to compare the role of PRc in perceptual discrimination and recognition memory decisions. When matched for task difficulty, no significant differences in PRc activation for memory and perception conditions were found. Instead, a conjunction analysis revealed a region in right PRc whose activity was related to accuracy of both recognition memory and perceptual discriminations. These findings show that the functional role of PRc is not limited to long-term declarative memory.

Hebb Award Abstract
The prevailing view of the medial temporal lobe (MTL) holds that its structures are dedicated to long-term declarative memory. Recent evidence challenges this position, suggesting that perirhinal cortex (PRc), a MTL structure, may also play a role in online processing of visual objects. It has been proposed that this processing facilitates discriminations of stimuli when comprised of highly overlapping visual features. Relevant neuropsychological findings in humans have been inconclusive however, likely because studies have relied on patients with large, variable MTL lesions, resulting in inconsistent findings across studies. Also, perceptual and mnemonic tasks have not always been matched for difficulty. Here, we conducted an event-related fMRI experiment in 18 healthy participants to directly examine the role of PRc in perceptual discriminations and in recognition memory. We maximized the degree of visual feature overlap in our stimuli by digitally morphing together photographs of human faces, a stimulus category which naturally has a high degree of feature overlap. Prior to scanning, subjects were familiarized with a subset of faces for the recognition memory task. During scanning, all trials employed displays with three faces presented simultaneously. Perception trials contained three novel faces. Memory trials contained a previously studied face as well as two novel faces. Perception trials required identification of an ‘oddball’, i.e. the face least similar to the other two images in the display. The memory task required the forced-choice recognition of the previously studied face. Task difficulty was manipulated independently in the two conditions. When contrasted against a baseline task, a region in PRc showed increased activation not only for the memory task, but also the perception task despite its lacking apparent long-term declarative memory demands. A further analysis restricted to trials matched in reaction time and accuracy was completed in order to obviate the effects of task difficulty. This analysis revealed that the addition of long-term declarative memory demands does not result in increased PRc activation when compared to perception trials that were behaviourally matched. The effects of accuracy were also analyzed, revealing greater activation in the PRc for correct than incorrect trials in both memory and perception tasks. A conjunction analysis revealed that these two accuracy effects activated a common region in right PRc whose activity was related to accuracy of both recognition memory and perceptual discrimination tasks. These findings show that the functional role of PRc is not limited to long-term declarative memory.