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Abstract:
Task-induced deactivation (TID) refers to regional
suppression of blood flow during an active task relative to a
"resting" baseline. We tested the hypothesis that TID results from
reallocation of processing resources by parametrically manipulating
task difficulty along three dimensions: target discriminability,
stimulus presentation rate, and short-term memory load. Subjects
performed an auditory target detection task during functional
magnetic resonance imaging, responding to a single target tone or,
in the memory load conditions, to target sequences. Seven task
conditions (a canonical version and two additional levels for each
of the three dimensions) were alternated with "rest" in a block
design. Correlation analysis identified brain regions in which TID
occurred. ANOVAs identified four regions (left and right angular
gyri, left precuneus/posterior cingulate, and left superior frontal
gyrus/sulcus) in which TID magnitude varied across task conditions.
Follow-up tests indicated that for each of the three dimensions,
TID magnitude increased with task difficulty. Increasing short-term
memory load was particularly effective in increasing TID. These
results suggest that TID represents reallocation of processing
resources from areas in which TID occurs to areas involved in task
performance. Short-term memory load and stimulus rate also predict
suppression of spontaneous thought, and brain areas showing TID
have also been linked with semantic processing, supporting claims
that TID results from interruption of spontaneous semantic
processes that occur during "rest" (Binder et al., J. Cogn.
Neurosci., 1999).
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