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

Somatosensory representations occupy parietal postcentral gyral (S1) andlateral sulcal-opercular cortex (S2). To address the issue of possiblemultiple activation foci in these regions and possible differences due tostimulating skin directly or through an imposed tool, we studied changes incerebral blood flow with positron emission tomography during passivetactile stimulation of one or two fingertips. Restrained fingers wererubbed with embossed gratings using a rotating drum stimulator in 11subjects. For different scans, gratings touched the skin directly foroptimal stimulation of cutaneous receptors (called skin mode stimulation)or indirectly through an imposed guitar plectrum snugly fitted to the samefingers (called tool mode stimulation). The latter was expected tostimulate deep receptors better. Subjects estimated roughness after eachscan. Direct skin contact activated statistically validated foci in bothhemispheres. On the contralateral side these foci occurred in the anteriorand posterior limbs of the postcentral gyrus and on the ipsilateral sideonly in the posterior limb. Tool mode stimulation activated onecontralateral focus that was in the posterior limb of the postcentralgyrus. These results suggest at least two maps for distal fingertips in S1with the anterior and posterior foci corresponding, respectively, toactivations in area 3b and the junction between areas 1 and 2. Incontralateral S2, skin mode stimulation activated a peak that was anteriorand medial to a focus associated with tool mode stimulation. The magnitudeof PET counts contralateral to stimulation was greater in the anterior S1and the S2 regions during initial scans but reversed to more activation inthe posterior S1 during later scans. These short- term practice effectssuggest changes in neural activity with stimulus novelty.