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Abstract:
The processing of task-relevant stimuli presented at
infrequent, random intervals has been postulated to involve many
different brain regions. However, recent functional magnetic
resonance imaging (fMRI) studies using the oddball paradigm have
identified only a few regions that respond to such stimuli
(McCarthy et al., J. Neurophys., 1997; Menon et al., Neuroreport,
1997). The present study used rapidly presented, interleaved
stimuli and whole-brain fMRI, which provides greater sensitivity
and coverage than that offered by previous techniques (Clark et
al., J. Neurophys., 1998). Responses were identified in 6 subjects
using 3 stimulus types presented in pseudo-random order, including
frequent non-target stimuli (the letter "T", p=0.8), rare
non-target stimuli ("C", p=0.1), and rare target stimuli ("X",
p=0.1) to which subjects made a speeded button press response.
Stimuli were presented for 200 msec, with ISIs varied randomly
between 550 and 2000 msec. T2*-weighted gradient echo, echo planar
images were acquired for the whole brain and cerebellum in 20, 6 mm
thick oblique slices (TR=2.125 sec). A multiple regression analysis
was used to identify responses evoked by the 3 stimulus types.
Target stimuli evoked increased fMRI signal in a wide range of
brain regions, including prefrontal, cingulate, parietal,
occipital, and temporal cortices, and in the thalamus and
cerebellum. This topography agrees well with regions previously
identified for similar tasks using evoked potentials and brain
lesion analyses.
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