Somatotopic organization of the human somatosensory cortex revealed by neuromagnetic measurements.

The primary projection areas in the human somatosensory cortex activated by electrical stimulation of the digits of the hand and the ankle were localized by measuring the magnetic field outside the head contralateral to the side of stimulation. Most of the spatial variation in the amplitude of the field component normal to the scalp could be accounted for by representing each source as a single current dipole in a spherical conducting medium with solely concentric variations in electrical conductivity, although the fit of this model to the data showed some statistically significant deviations. Based on the best-fitting parameter values of the model, we found that the projection areas of the thumb, the index finger, the little finger and the ankle were located at successively more medial positions along the primary somatosensory cortex, at an average depth of 2.2 cm from the scalp surface.