MTL Cortex and Item Recognition
Cognitive theory has long advanced that one basis for recognition is item memory strength, which may support the perception of item familiarity. At present, the neural mechanisms supporting perceived differences in memory strength at retrieval remain poorly specified. To begin to understand how memory strength is signaled, we used fMRI and MEG to index the neural correlates of graded memory strength in the human brain, focusing on MTL cortex [Gonsalves et al., 2005]. The fMRI measures revealed decreased MTL cortical activation––observed along the anterior/posterior extent of the parahippocampal gyrus––that tracked parametric levels of perceived memory strength: Greater BOLD repetition suppression was associated with the perception of stronger item memory strength. These findings suggest that MTL cortex signals item memory strength via repetition suppression (or, conversely, signals stimulus novelty through increased activation).
Building on these fMRI findings, our MEG data revealed that strength-dependent activation reductions can onset within 150-300 ms in MTL cortex, consistent with behavioral data revealing that evidence regarding item memory strength is rapidly available and consistent with electrophysiological data in animals showing rapid repetition suppression in PRc. Motivated by these fMRI and MEG observations, we are currently using high-resolution fMRI to test directed hypotheses regarding MTL retrieval processes supporting item memory strength, including examining (a) their dependence on study-test perceptual similarity, (b) the importance of stimulus-directed attention at encoding and retrieval for their emergence, and (c) whether distinct item memory effects are observed across MTL subregions. Consistent with this latter hypothesis, we recently obtained evidence pointing to a dissociation between conceptual and perceptual item memory effects along the anterior-posterior axis of parahippocampal gyrus [O’Kane et al., 2005].
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