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Abstract:
Abstract: When subjects are required to switch between two
conflicting tasks, performance is worse on Switch trials than on
Repeat trials: response times (RTs) are longer, and more errors are
made. This decrement in performance is generally taken to represent
an index of the extent to which control processes are active on
Switch trials. Furthermore, because performance improves somewhat
when more preparation time is allowed, these control processes are
thought to be involved in `setting up' the cognitive system for the
new task. Therefore, control processes should be more active on
Switch trials than Repeat trials; and the same processes should be
engaged when subjects are asked to prepare for a Switch -- even if
they are not subsequently required to perform the new task. We
investigated this by assessing performance (RT and errors) as well
as regional cerebral blood flow (rCBF) in the four conditions
resulting from the factorial crossing of Switching (Switch vs.
Repeat) and Preparation (prepare-only vs. prepare and perform). We
found that RTs were longer, and there was an increase in rCBF in
dorso-lateral prefrontal cortex (DLPFC), on Switch trials relative
to Repeat trials. However, the DLPFC activation was only evident
when subjects were actually presented with a stimulus -- suggesting
that this activation represents overcoming competition in the
system and not activity related to `setting up' the system. A
discussion follows.
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