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Abstract:
Abstract: Our goal was to examine the cortical processing of
simultaneously presented somatosensory and visual stimuli in
humans. Functional magnetic resonance imaging (fMRI) and
magnetoencephalography (MEG) were used to measure responses in
parietal and occipital cortex of 7 subjects. Somatosensory stimuli
were delivered via pneumatically driven diaphragms (25psi) to right
digits 2 and 3. The visual stimulus was a reversing checkerboard
(5.5x5.75) presented centrally. The three conditions consisted of
stimuli presented individually and simultaneously. fMRI was
performed using a GE 1.5T scanner, a gradient echo EPI sequence
(TR=2s, TE=60ms, flip angle=69, phases=60, slice thickness=5mm) and
a 20 second block design. MEG was performed using a 37-channel
biomagnetometer. For each condition 256 trials of 500ms were
sampled at 1kHz, averaged and filtered (1-40Hz). Active voxel count
and magnetic field strength were compared between conditions. Voxel
count was significantly reduced in visual cortex for both stimuli
presented together (mean SD, visual=307 164 vs.
somatosensory+visual=169 147 voxels, p<0.01), conversely,
magnetic field strength was significantly greater for both stimuli
(visual=1.76 0.39 vs. somatosensory+visual=2.04 0.44fT, p<0.01).
Activity in parietal cortex was not significantly different.
Increased activity in occipital cortex during simultaneously
presented stimuli is consistent with monkey electrophysiology. This
discrepancy between methods may be due to the direct (MEG,
electrophysiology) vs. indirect (fMRI) origin of the signal, or to
the unmonitored effects of attention.
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