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Although we have suggestive evidence that storage-plus-processing activates the DLPFC, perhaps through the mechanism of divided attention, it is of interest to inquire into the status of the related hypothesis that pure storage tasks do not activate the DLPFC. The contrast we presented at the outset of this section between the item-recognition and 2-back tasks supports the related hypothesis in that the former task did not activate DLPFC. Also, neither of the other item-recognition experiments reported in this article (10, 13) showed significant activation in DLPFC.

In contrast, there are a couple of studies of verbal WM that on the face of it seem to require only pure storage, yet show activations of DLPFC. Activation of the left DLPFC was found in a recent fMRI experiment (30) in which subjects saw a series of letters and had to detect any X that had been preceded by an A. Although this paradigm seems to be a pure storage task, it is possible that subjects adopted the following strategy: search for an A, and if one is detected, check whether the next letter is an X. This strategy makes the paradigm a kind of dual task, requiring some attention switching whenever an A is detected, and the attention switching may have been the source of the DLPFC activation.§ In another relevant study (31), subjects were given five words or nonwords to remember for 60 sec, and then had to recall the items in any order they wanted. Subjects were scanned, by PET, only during the 60-sec interval they were remembering the items. Though this paradigm seems to be a pure storage task, there was activation in DLPFC. However, because the delay interval was very long for a WM study (far longer than the delay interval used in any other study mentioned in this paper), perhaps subjects engaged in some additional long-term processes, and the latter may have been the cause of the DLPFC activation.

The preceding accounts are speculative. We will need additional research to see how much credence they should be given. It may turn out that some pure storage tasks do activate the DLPFC, but that such activation increases disproportionately when a processing component is added to the storage component. This issue deserves further consideration because it will lead to a refinement of the neural circuitry of WM, a fundamental process in much of thought.

This work was supported by grants from the Department of Energy, McDonnel-Pew Program in Cognitive Neuroscience, National Institute of Aging, and Office of Naval Research.

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Note further that this task also may involve some inhibition because of the inclusion of “foils,” e.g., cases in which X occurred not preceded by A.

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