Somatosensory evoked magnetic fields following median nerve stimulation

Topography of somatosensory evoked magnetic fields (SEFs) following stimulation of the median nerve were investigated in normal subjects. N20m-P30m-N40m-P60m and their counterpart, P20m-N30m-P40m-N60m, were identified in the hemisphere contralateral to the stimulated median nerve. Their equivalent current dipoles (ECDs) were considered to be located in the hand area of area 3b in the primary sensory cortex (SI). Restricted deflections, P25m and N25m, were considered to be generated in area 1 in SI. Therefore, short-latency deflections less than 40 ms were considered to be hybrids of ECDs generated in areas 3b and 1. Middle-latency deflections, N90m-P90m, were considered to be generated in the second sensory cortex (SII), but they were greatly affected by the much stronger fields generated in SI. The N30m deflection, which was a magnetic reflection of the N30 potential of somatosensory evoked potentials (SEPs), were widely recorded in the frontal area. The generator site of N30 of SEPs is considered to be the supplementary motor area (SMA). However, ECDs of N30m were located in SI, and no significant ECD generated in the frontal area including SMA was detected. No significant deflections other than small N90m-P90m in SII were identified in the hemisphere ipsilateral to the stimulated nerve. No significant deflections whose ECDs were generated in the mid-parietal area were identified. In conclusion, short- and middle-latency SEFs are mainly generated in area 3b in SI contralateral to the stimulated median nerve, and responses generated in area 1 of SI and SII affect the SEFs to some degree.