Pain-related somatosensory evoked magnetic fields

Somatosensory evoked magnetic fields (SEFs) following painful electrical stimulation of the finger were investigated in 5 normal subjects. Equivalent current dipoles (ECDs) of deflections shorter than 100 msec in latency were located in the primary sensory cortex (SI) in the hemisphere contralateral to the stimulated finger following either non-painful or painful stimulation. Two main deflections, N100m-P100m and N250m-P250m, were independently identified following painful stimulation, although they were not found in SEFs following non-painful weak stimulation. ECDs of the N100m-P100m were considered to be located in the bilateral second sensory cortices (SII). ECDs of the N250m-P250m were identified in the bilateral cingulate cortices and SII, but the intersubject difference was large. Therefore, we considered that contralateral SI and bilateral SII were initially activated by painful noxious stimulation, and then multiple areas including bilateral SII and cingulate cortices were activated. In EEG recordings (evoked potentials), no potential corresponding to N100m-P100m was found, probably because it was difficult to record activation in SII by EEG recordings. The P250 potential which corresponded to the N250m-P250m was clearly identified, probably because activation of multiple areas generated large long-duration EEG potentials which were maximal around the vertex, unlike MEG recordings.