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Imaging frontal and medial temporal lobe interaction during memory retrieval and disentangling the effects of the default network


Exploring how the medial temporal lobe interacts with different areas of the brain during memory tasks is an important component in understanding the dynamics of memory retrieval. To achieve this goal, functional magnetic resonance imaging (fMRI) was used to determine how the frontal and medial temporal lobes interact during memory retrieval and subsequent use of recollected information and how memory retrieval may integrate with or affect default network activity. The first study used blocked and event-related fMRI to examine hippocampal activity during long term memory recollection and post- retrieval processing of paired items. Subjects were asked to make living/nonliving judgments about items visually presented (classify) or about items retrieved from memory (recollect-classify). In addition, active (odd/even digit classification) and passive (fixation) baselines were used to differentiate task-related activity from default network activations. During the r̀ecollect-classify' task, activity in the anterior hippocampus was selectively reduced relative to 'classify' and baseline tasks (active and passive), and hippocampal activity was inversely correlated with bilateral DLPFC activity. The finding was that frontal and hippocampal activity are dissociated during memory retrieval and post-retrieval processing. The second study examined the effects of retrieval instruction on brain regions implicated in episodic memory retrieval. The finding was that the default network showed a greater decrease in activation for poorly-remembered than strongly -remembered responses regardless of whether subjects were instructed to suppress the cued material or to perform non -elaborative or elaborative retrieval. The hippocampus showed an increase in activation only with successful memory recall. Further examination revealed that the retrieval network, commonly identified by differences in activation during successful retrieval and baseline, seems to be modulated somewhat by task instruction, but not recall success. The hippocampus showed differential activity based on top-down modulation elicited by retrieval instructions while the default network did not. The third study examined recollection and familiarity with attention to reaction-time to explore its contribution to regional activations. The finding was that the hippocampus is functionally dissociated from other regions of the retrieval-network during recollection. Portions of the retrieval-network are generally influenced by reaction- time and show suppressed signal when subjects are task- engaged in either recollection or familiarity; suppression is greater for longer trials. The hippocampus, however, shows a positive response only for recollection trials, where activation is greater for longer recollection trials, but not longer familiarity trials. It is concluded from these experiments that the hippocampus is dissociated from the rest of the default network during tasks of memory retrieval. While many studies have examined how the hippocampus, pre-frontal cortex, retrieval network, and default network are related to memory retrieval, these studies take into account additional components of the memory retrieval tasks like reaction time and false alarm rate in order to disentangle memory retrieval itself from related components. Prior studies have shown functional connectivity of the hippocampus and the default network, but these studies, taken together, suggest that hippocampus is affected by task instruction as well as task success, and is dissociated from the default network during tasks involving memory retrieval. The summation of these studies exposes how the modulation of activity in non-memory related networks in the brain may affect activation attributed to memory retrieval, and why it is so important to take these confounds into account in memory recall studies

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