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Abstract:
Task-switching paradigms, in which subjects must switch back
and forth between different tasks, provide an operationalization of
executive control. Using event-related fMRI, we attempted to
identify changes in neural activity that can be attributed to
task-switching. We used a paradigm similar to that of Rogers and
Monsell (1995), in which stimuli were letter-digit pairs, and the
two tasks were consonant-vowel and odd-even judgement. Stimuli
appeared in successive trials in one of four locations on the
screen, in a predictable clockwise pattern. The task for a given
trial was cued by the location of the stimulus, so that trials
alternated between task switches and task repetitions. Stimuli were
presented at an SOA of 8 seconds. This long interval served two
purposes: to reduce any observed shift cost to the "residual shift
cost" (Rogers & Monsell, 1995); and to facilitate analysis of
event-related activity in an unrandomized design. Within this
paradigm, we assessed changes in neural activity that predicted the
differences between shift and repeat trials. The results suggest
that neural activity predictive of shift trials is not limited to a
monolithic executive mechanism, but is instead distributed among
several sites, including the regions recruited by the tasks
themselves. These results suggest a neural basis for the residual
shift costs observed at long ITIs in task-switching
paradigms.
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