The Perirhinal Cortex and Implicit Memory

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Many theories of human long term memory assume that explicit memory for prior events relies on the medial temporal lobes (MTLs) and its subregions. In contrast, implicit memory, measured in tasks where experience with a prior item facilitates the subsequent identification or production of that item, is thought to rely on cortical regions outside of the MTL. However, it has recently been hypothesized that the perirhinal cortex, a region within the MTL, holds high level conceptual and perceptual representations irrespective of awareness or intention. In one recent study, we provided evidence that the perirhinal cortex is sensitive to certain forms of implicit memory. We demonstrated that amnesic stroke and temporal lobectomy patients exhibited impaired conceptual implicit memory, and in a separate experiment showed that perirhinal activations at encoding predicted subsequent conceptual implicit memory. The experiments proposed here seek to extend these previous findings. In the first experiment, a novel paradigm that measures both conceptual implicit memory and explicit familiarity-based recognition will be used to assess the role of the perirhinal cortex in these two tasks. Participants will be scanned during recognition and free association memory tasks that measure familiarity and conceptual implicit memory, respectively. In the second experiment we will test the hypothesis that sufficiently complex perceptual discriminations will recruit the perirhinal cortex during both the encoding and retrieval phases of a perceptual implicit memory task. Both experiments will utilize high-resolution fMRI and region of interest alignment derived from manual MRI brain tracing. Moreover, for the first experiment we will develop a unique fMRI sequence designed to counteract movement artifacts intrinsic to overt speech. Understanding the conditions under which the perirhinal cortex processes and represents conceptual and perceptual information is critical to understanding the role of the MTLs in long term memory and beyond. The insights from these studies can be applied to diagnose, understand, and treat memory deficits and mental disorders that adversely affect memory.