Episodic Retrieval

Retrieval of episodic memory is mediated by regions that generally are functionally and anatomically distinct from those used in encoding. Most neuroima-ging studies of retrieval use a task design similar to that used in studies of encoding, in which participants must study a set of items and are subsequently tested for their memory of the items. The difference is that participants are scanned while they retrieve (recall or recognize) rather than while they encode the material. In recognition tasks, an item is shown, and it is the task of the participant to indicate whether that item was presented during the study phase. Hence, it is necessary only to access the source and not the item. In recall tests, it is necessary to generate the item as well. Neuroimaging studies of both recall and recognition typically show activity in the right prefrontal cortex, hippocampus, medial as well as inferior parietal cortex, anterior cingulate, and cerebellum. There are some important variations in this pattern, however, that are discussed next.

The hippocampus is typically considered to be involved in the consolidation of long-term memories, as discussed earlier. Although this function implies that the hippocampus should not be involved in retrieval, some studies have found it to be activated during retrieval tasks. To test whether the effort required for retrieval might influence activation of the hippocampus, one study varied the amount of effort required to search memory. In a "high-recall" condition, words were deeply encoded and, hence, less effortfully retrieved. When the recall phase was scanned, this manipulation revealed activity in the hippocampus bilaterally, supporting the view that the hippocampus is involved in effortless, conscious recall. In a "low-recall" condition, words were encoded more superficially and, hence, required more effortful retrieval. Scanning during this more effortful recall phase showed bilateral prefrontal but not hippocam-pal activation. The finding that the prefrontal cortex is involved in effortful retrieval is consistent with the view that one function of the prefrontal cortex is to implement retrieval strategies. The hippocampus, by contrast, may be involved in relatively more automatic retrieval.

Certain neuroimaging studies of episodic retrieval have found not only increased activity in the right prefrontal cortex but also decreased activity in other areas such as the left prefrontal cortex. On the basis of this effect, some have suggested that episodic retrieval is not just an active process of search and retrieval but involves the active inhibition of certain regions of the brain by other areas of the brain. By this model, the right prefrontal cortex could be actively inhibiting left frontal regions as well as inferior temporal regions, areas that sometimes show deactivations in retrieval tasks. In the case of the temporal regions, for example, this might indicate the suppression of language processes during episodic retrieval. This effect has been termed "ensemble inhibition'' and suggests that episodic retrieval may be carried out, in part, by inhibitory processes.

Retrieval processing involves an interplay between the right prefrontal cortex and the hippocampus in the implementation of search strategies and conscious, effortless retrieval, respectively. The involvement of other areas of the brain such as the precuneus, parietal cortex, anterior cingulate, and cerebellum has yet to be fully elucidated, so much further research is required on this problem.

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