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) a nd lateral sulcal-opercular cortex (S2). To address the issue of possi ble multiple activation foci in these regions and possible differences due to stimulating skin directly or through an imposed tool, we studi ed changes in cerebral blood flow with positron emission tomography du ring passive tactile stimulation of one or two fingertips. Restrained fingers were rubbed with embossed gratings using a rotating drum stimu lator in 11 subjects. For different scans, gratings touched the skin d irectly for optimal stimulation of cutaneous receptors (called skin mo de stimulation) or indirectly through an imposed guitar plectrum snugl y fitted to the same fingers (called tool mode stimulation). The latte r was expected to stimulate deep receptors better. Subjects estimated roughness after each scan. Direct skin contact activated statistically validated fool in both hemispheres. On the contralateral side these f oci occurred in the anterior and posterior limbs of the postcentral gy rus and an the ipsilateral side only in the posterior limb. Tool mode stimulation activated one contralateral focus that was in the posterio r limb of the postcentral gyrus. These results suggest at least two ma ps for distal fingertips in Sf with the anterior and posterior foci co rresponding, respectively, to activations in area 3b and the junction between areas 1 and 2. In contralateral S2, skin made stimulation acti vated a peak that was anterior and medial to a focus associated with t ool mode stimulation. The magnitude of PET counts contralateral to sti mulation was greater in the anterior S1 and the S2 regions during init ial scans but reversed to more activation in the posterior S1 during l ater scans. These short-term practice effects suggest changes in neura l activity with stimulus novelty.