Modulation of somatosensory evoked magnetic fields by sensory and motor interferences.

Modulatory influences of skilled exploratory finger movements on somatosensory evoked magnetic fields evoked by median nerve stimulation were investigated in six healthy subjects using a whole head magnetometer (MEG) system. The exploratory finger movements caused major changes in the somatosensory evoked fields. The most prominent effect was a reversal of the dipolar magnetic field around 30 ms after median nerve stimulation. Similar but less pronounced effects were exerted by repetitive finger movements and tactile stimulation of the hand. A dipole analysis and super-imposition of resulting sources on individual MRI scans showed that all somatosensory evoked fields up to 60 ms after stimulation, and the modulation of these responses were located in the primary somatosensory cortex (SI).