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Abstract:
Previous results from quantitative electroencephalography
indicate that directed attention to either visual hemifield
enhances stimulus-driven activity at occipitoparietal scalp
locations over the contralateral hemisphere. The present study
applied fMRI during a visual directed-attention task in order to
localise these hemispheric modulations more precisely. In each of
ten sixty-second trials, red (target) and green (background)
coloured squares were flashed at 9Hz in the upper quadrant of each
hemifield while subjects fixated on a central cross. At any given
time, subjects attended to only one hemifield. A target in the
attended hemifield cued not only an overt, behavioural response (a
finger movement) but also a covert shift of attention into the
contralateral hemifield. Echo-planar images (TR=3s, effective
TE=40ms) collected during performance of the task were correlated
with a waveform representing the direction of the subject's
attention at each point in time, as determined from behavioural
data. In all subjects, activity in the lateral occipitotemporal
gyrus correlated with attention to the contralateral hemifield.
Furthermore, in a majority of subjects, activity in lateral
intraparietal sulcus correlated inversely with attention to the
contralateral hemifield. These results confirm earlier fMRI work
implicating inferior occipitotemporal cortex as the source of
attention-related occipitoparietal EEG modulation. In addition, the
inverse correlation in parietal cortex is compatible with the
notion of the lateral intraparietal region as an active suppressor
of irrelevant stimuli.
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