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Abstract:
We examined a syntactic specialization of Broca's area by
using low-frequency transcranial magnetic stimulation (TMS).
Paired-pulse TMS (ISI, 2 ms) with a figure-of-eight coil centered
over the pars opercularis (PO) was applied every 6 s at an
intensity of 60 % of the active motor threshold for each subject.
The effect of TMS was assessed by measuring reaction times (RTs) in
reference to the RTs of separate trials with a control TMS, which
was applied at an intensity of 40 % over a region 40 mm apart from
PO. We compared two tasks involving visual stimuli of Japanese
sentences (a verb for 350 ms): a syntactic decision (Syn) task and
a semantic decision (Sem) task. There was no change in RTs during
both tasks, when TMS was applied at the verb onset. When TMS was
applied 150 ms after the onset, we observed a significant decrease
in RTs (facilitation), with this effect being observed in the Syn
task, but not in the Sem task. In contrast, we observed a
significant increase in RTs (inhibition) only in the Syn task, when
TMS was applied 400 ms after the onset. These results demonstrate
that Broca's area is selectively involved in syntactic processing
within this restricted time window.
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