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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.

cerebral cortex

Multiple foci in parietal and frontal cortex activated by rubbing embossed grating patterns across fingerpads: a positron emission tomography study in humans