Tuesday, February 26, 2008

dlPFC promotes LTM formation via WM organization

Blumenfeld, R.S. & Ranganath, C. (January 18, 2006). Dorsolateral prefrontal cortex promotes long-term memory formation through its role in working memory organization. The Journal of Neuroscience, 26, 3, 916-925.

This paper speculates that the different regions of the prefrontal cortex contribute differentially to working memory (WM). Specifically, they argue that the dorsolateral prefrontal cortex (dlPFC) is recruited during tasks requiring organization of items active in WM, while the ventrolateral prefrontal areas (vlPFC) are involved in WM maintenance, or simply holding items in short-term memory.

The study begins behaviorally by showing that overall memory is increased during tasks which require organization by re-ordering (in comparison to tasks which are merely rehearsals). Single items encoded on re-ordering trials were significantly more likely to be judged as "remembered" than from rehearsal trials. And organization of items during re-ordering trials resulted in much higher recollection together, suggesting the strengthening of associative links between items in these organization trials. Then the paper shifts to the fMRI data to attempt to implicate the dlPFC specifically. Their evidence: the dlPFC showed increased activation during the delay (encoding) period of re-ordering trials relative to rehearse trials.

However, one very possible alternative explanation is the effect of nonspecific factors correlated with task difficulty. Reorder and rehearse trials differ in a number of ways, difficulty being the most significant. Perhaps the increased attention required in the re-ordering trials led to changes in behavioral results and a general increase in activation in the dlPFC (and elsewhere). This raises concerns that the activity in the dlPFC could be attributed to processes other than organization.

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