Somatotopy of human hand somatosensory cortex revealed by dipole source analysis of early somatosensory evoked potentials and 3D-NMR tomography.

Somatosensory evoked potentials (SEPs) to median nerve and finger stimulation were analyzed by means of spatio-temporal dipole modelling combined with 3D-NMR tomography in 8 normal subjects. The early SEPs were modelled by 3 equivalent dipoles located in the region of the brain-stem (B) and in the region of the contralateral somatosensory cortex (T and R). Dipole B explained peaks P14 and N18 at the scalp. Dipole T was tangentially oriented and explained the N20-P20, dipole R was radially oriented and modelled the P22. The tangential dipole sources T were located within a distance of 6 mm on the average and all were less than 9 mm from the posterior bank of the central sulcus. In 6 subjects the tangential sources related to finger stimulation arranged along the central sulcus according to the known somatotopy. The radial sources did not show a consistent somatotopic alignment across subjects. We conclude that the combination of dipole source analysis and 3D-NMR tomography is a useful tool for functional localization within the human hand somatosensory cortex.