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991.
Somatosensory representations occupy parietal postcentral gyral (S1) and
lateral sulcal-opercular cortex (S2). To address the issue of possible
multiple activation foci in these regions and possible differences due to
stimulating skin directly or through an imposed tool, we studied changes in
cerebral blood flow with positron emission tomography during passive
tactile stimulation of one or two fingertips. Restrained fingers were
rubbed with embossed gratings using a rotating drum stimulator in 11
subjects. For different scans, gratings touched the skin directly for
optimal stimulation of cutaneous receptors (called skin mode stimulation)
or indirectly through an imposed guitar plectrum snugly fitted to the same
fingers (called tool mode stimulation). The latter was expected to
stimulate deep receptors better. Subjects estimated roughness after each
scan. Direct skin contact activated statistically validated foci in both
hemispheres. On the contralateral side these foci occurred in the anterior
and posterior limbs of the postcentral gyrus and on the ipsilateral side
only in the posterior limb. Tool mode stimulation activated one
contralateral focus that was in the posterior limb of the postcentral
gyrus. These results suggest at least two maps for distal fingertips in S1
with the anterior and posterior foci corresponding, respectively, to
activations in area 3b and the junction between areas 1 and 2. In
contralateral S2, skin mode stimulation activated a peak that was anterior
and medial to a focus associated with tool mode stimulation. The magnitude
of PET counts contralateral to stimulation was greater in the anterior S1
and the S2 regions during initial scans but reversed to more activation in
the posterior S1 during later scans. These short- term practice effects
suggest changes in neural activity with stimulus novelty.
相似文献
992.
Top-down modulation of early sensory cortex 总被引:11,自引:7,他引:4
Shulman GL; Corbetta M; Buckner RL; Raichle ME; Fiez JA; Miezin FM; Petersen SE 《Cerebral cortex (New York, N.Y. : 1991)》1997,7(3):193-206
Data from nine previous studies of human visual information processing
using positron emission tomography were reanalyzed to contrast blood flow
responses during passive viewing and active discriminations of the same
stimulus array. The analysis examined whether active visual processing (i)
increases blood flow in medial visual regions early in the visual hierarchy
and (ii) decreases blood flow in auditory and somatosensory cortex.
Significant modulation of medial visual regions was observed in six of nine
studies, indicating that top-down processes can affect early visual cortex.
Modulations showed several task dependencies, suggesting that in some cases
the underlying mechanism was selective (e.g. analysis-or feature-specific)
rather than non- selective. Replicable decreases at or near auditory
Brodmann area (BA) left 41/42 were observed in two of five studies, but in
different locations. Analyses that combined data across studies yielded
modest but significant decreases. Replicable decreases were not found in
primary somatosensory cortex but were observed in an insular region that
may be a somatosensory association area. Decreases were also noted in the
parietal operculum (perhaps SII) and BA 40. These results are inconsistent
with a model in which the precortical input to task- irrelevant sensory
cortical areas is broadly suppressed.
相似文献
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Pericardial fluid distribution: CT analysis 总被引:2,自引:0,他引:2
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