Pain-related somatosensory evoked magnetic fields following lower limb stimulation.

Somatosensory evoked magnetic fields (SEFs) following painful electrical stimulation of the sural nerve were examined in 6 normal subjects. Equivalent current dipoles (ECDs) of the deflections shorter than 100 ms in latency were located in the foot area of the primary sensory cortex (SI) in the contralateral hemisphere following both weak and painful stimulations. Two main deflections, N150m-P150m and N250m-P250m, were independently identified only following painful stimulation. ECDs of the N150m-P150m were considered to be located in bilateral second sensory cortices (SII). ECDs of the N250m-P250m were identified in multiple areas including bilateral cingulate cortices and SII. These findings were consistent with the pain-related SEFs following upper limb stimulation. Therefore, we considered that bilateral SII and the cingulate cortices were activated by the painful stimulation and that pain-specific brain activities in those areas did not depend on the stimulation site.