The somatosensory evoked magnetic fields

Averaged magnetoencephalography (MEG) following somatosensory stimulation, somatosensory evoked magnetic field(s) (SEF), in humans are reviewed. The e quivalent current dipole(s) (ECD) of the primary and the following middle-l atency components of SEF following electrical stimulation within 80-100 ms are estimated in area 3b of the primary somatosensory cortex (SI), the post erior bank of the central sulcus, in the hemisphere contralateral to the st imulated site. Their sites are generally compatible with the homunculus which was reported by Penfield using direct cortical stimulation during surgery. SEF to passi ve finger movement is generated in area 3a or 2 of SI, unlike with electric al stimulation. Long-latency components with peaks of approximately 80-120 ms are recorded in the bilateral hemispheres and their ECD are estimated in the secondary somatosensory cortex (SII) in the bilateral hemispheres. We also summarized (1) the gating effects on SEF by interference tactile st imulation or movement applied to the stimulus site, (2) clinical applicatio ns of SEF in the fields of neurosurgery and neurology and (3) cortical plas ticity (reorganization) of the SI. SEF specific to painful stimulation is a lso recorded following painful stimulation by CO2 laser beam. Pain-specific components are recorded over 150 ms after the stimulus and their ECD are e stimated in the bilateral SII and the limbic system. We introduced a newly- developed multi (12)-channel gradiometer system with the smallest and highe st quality superconducting quantum interference device (micro-SQUID) availa ble to non-invasively detect the magnetic fields of a human peripheral nerv e. Clear nerve action fields (NAFs) were consistently recorded from all sub jects. (C) 2000 Elsevier Science Ltd. All rights reserved.