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Abstract:
We investigated updating, the process of rearranging the
ordinal tags associated with representations of items held in
working memory, as a prefrontal cortically (PFC) mediated process
that may make important contributions to such complex working
memory tasks as n-back and alphabetization. Subjects viewed serial
displays of letter stimuli, presented in groups of 1, 2, or 3
letters, and maintained in memory the 4 most recently presented
items. A single-letter probe, appearing after 4, 8, or 12 letters,
evaluated memory for the 4 critical items. Performance was
insensitive to manipulation of encoding demands (e.g., encoding
3-item groups vs. encoding 1-item groups), but was sensitive to
manipulations of forgetting. Performance declined when the
integrity of letter groups was violated (e.g., if 3 letters were
encoded together, but only 2 were subsequently discarded together
from the active memory set). This suggests that rich episodic
information about context at encoding, although irrelevant to task
demands, was encoded automatically as a feature of stimulus
representations in working memory. In a distinct manipulation,
performance also declined when the probe matched a letter that had
appeared in the trial prior to the final four letters - a proactive
interference (PI) effect. Subsequent experiments investigated the
behavioral and anatomical independence of the forgetting and PI
effects, hypothesized to index updating and inhibition,
respectively.
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