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CONCLUSIONS

At the outset, we noted that neuroimaging techniques are extremely powerful because they can provide information regarding particular brain regions involved in actually performing diverse types of cognitive tasks, including memory tasks. The findings discussed above illustrate this point. Tasks that promote long term memory encoding have tended to activate areas in the left prefrontal cortex as well as the anterior cingulate and the right-lateral cerebellum. These areas also are activated by word generation tasks and certain verbal working memory tasks, indicating the interdependency of the processing demands across these tasks that we often, possibly erroneously, think of as separate. Explicit memory retrieval has been associated with additional activation of several brain areas, including the anterior prefrontal cortex (often right>left), although the specific contribution that these areas make to retrieval remains largely unknown. Implicit retrieval manifesting priming has revealed reductions in brain areas activated for task performance, perhaps reflecting the facilitation of local processing regions as a consequence of item repetition.

It is also worthwhile to note that, although comparisons of the brain regions involved in specific tasks may in many cases be undertaken with the aim of identifying regions that are uniquely associated with a particular task, the nature of these comparisons will, unavoidably, also entail consideration of commonalties of activation. Regions may contribute to several kinds of cognitive tasks. Examining the correlations of brain activation with functional tasks may thus have an important side benefit of focusing attention on neural systems and their interactions across task types (90), such as was the case noted for left prefrontal involvement in long term memory encoding, elaborate word generation, and working memory.

Future attempts to reliably identify and understand the bases of both the differences and commonalities in activation patterns across tasks are thus likely to advance our understanding of the complex, and pervasive, function of memory in our lives. Such advances may arise directly, through providing new information on neural systems and correlates of memory, and indirectly: Efforts to interpret and integrate the functional neuroimaging findings may enhance our understanding of the tasks themselves and what it is, precisely, that we are doing—or not doing—when we deliberately set about to remember something (explicit memory) or use the products of past learning without ever becoming aware of an intention to remember (implicit memory) or engage in any of a multitude of other cognitive endeavors such as perception or attention for which memory is important.

We thank Bruce Rosen and Daniel Schacter for collaboration, support, and discussion and Nicholas Szumski for help with the preparation of this manuscript. Mieke Verfaellie, Michael Rotte, Anders Dale, Anthony Wagner, and John Gabrieli were also collaborators on several of the studies presented. Endel Tulving, Cheryl Grady, Anthony Wagner, Michael Posner, Steven Petersen, and an anonymous reviewer all provided valuable comments on earlier drafts of this manuscript. Support was provided by the McDonnell Center for Higher Brain Function, the Dana Foundation, the Human Frontiers Science Program, and National Institutes of Health grants: National Institute on Deafness and Other Communication Disorders DC03245 to R.L.B., National Institute of Mental Health AG-08377 to Steven Petersen, and National Institute on Aging AG08441 to Daniel Schacter in support of W.K.

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