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Abstract:
Abstract: The present series of studies examined the effects
of inter-stimulus interval (ISI) and behavioral response task in
multiple versions of the three-stimulus oddball paradigm using
fMRI. Stimuli included frequent non-target stimuli (the letter "T",
82% of stimuli), rare distractor stimuli ("C", 9%), and rare target
stimuli ("X", 9%) to which subjects made a speeded button press
response (Exps. 1 and 2) or mentally counted the number of targets
(Exp. 3). ISIs were varied randomly between 550 and 2050 msec
(Exps. 1 and 3), or 800 and 1200 msec (Exp. 2). Gradient echo, echo
planar images were acquired in 20 contiguous 6 mm thick sections
from 5 subjects, and were analyzed using multiple regression.
Responses to target stimuli were observed in a wide range of brain
regions. The volume of this response was smaller frontally with
reduced ISI range, and increased in parietal and frontal cortex
with the counting task. Distractor stimuli evoked fMRI signal
change in inferior anterior cingulate, medial frontal, bilateral
inferior frontal, and right superior frontal gyri, and in
cerebellar, inferior parietal and left occipital-temporal cortex.
Significant variation in distractor evoked activity was observed
across stimulus repetitions. These event-related fMRI results shed
light on the sensitivity to task design in a type of paradigm that
has proven useful in many P300 ERP studies of healthy and
clinically-impaired populations.
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