Somatosensory evoked magnetic fields following passive finger movement

The somatosensory evoked magnetic field (SEF) following passive finger movement and electrical stimulation of finger was studied in 10 normal subjects. Four main components were identified in SEFs recorded at the hemisphere contralateral to the moved finger: 1M(P), 2M(P), 3M(P) and 4M(P). The 1M(P) was clearly identified only in three subjects and was smaller than other components. The equivalent current dipoles (ECDs) of 1M(P) were located around the finger area of the primary sensorimotor cortex and oriented either posteriorly or anteriorly. We speculate that it was generated in areas 3a or 2 of the primary sensory cortex. The 2M(P) and 3M(P) were usually combined as one large deflection with two peaks. Because the ECDs of 2M(P) and 3M(P) were located around the finger area of the sensorimotor cortex and both oriented posteriorly, they were considered to be generated in area 4 and/or 3b, and their activities have temporal overlapping. The 4M(P) has large inter-individual difference in terms of amplitude and latency. The ECD of 4M(P) was also located around the finger area of the primary sensorimotor cortex, and oriented anteriorly. The 4M(PI), the main component recorded from the hemisphere ipsilateral to the moved finger, was located in the upper bank of the sylvian fissure, probably the second sensory cortex (SII). Five components, 1M(E), 2M(E), 3M(E), 4M(E) and 4M(EI), corresponding to 1M(P), 2M(P), 3M(P), 4M(P) and 4M(PI), were identified following electrical stimulation of the same finger. However, SEFs following passive movement were clearly different from SEFs following electrical stimulation, in terms of waveforms and source locations, probably due to differences of ascending fibers and receptive fields.