Topography of somatosensory evoked magnetic fields following posterior tibial nerve stimulation.

The topography of somatosensory evoked magnetic fields (SEFs) following stimulation of the right and left posterior tibial nerves was investigated in 5 normal subjects (10 nerves). The main deflections N37m-P45m-N60m-P75m and their counterparts P37m-N45m-P60m-N75m were identified in the hemisphere contralateral to the stimulated nerve. Their equivalent current dipoles (ECDs) were located in the foot area of the primary sensory cortex (SI), probably in area 3b. Restricted minor deflections, P40m-N40m and N50m-P50m, were considered to be generated in area 1 in SI. As the generator sources of P37m-N37m, P40m-N40m and N45m-P45m were temporarily changed and interfered with each other, the direction of ECDs appeared to be rotated with the passage of time. Small middle-latency deflections, N100m-P100m, were clearly identified in 2 subjects. ECDs of these deflections were found in the second sensory cortex (SII), in both hemispheres, although they were clearer in the hemisphere contralateral to the stimulated nerve. In conclusion, short- and middle-latency SEFs are mainly generated in area 3b in SI contralateral to the stimulated nerve, and responses generated in area 1 of SI and SII affect the SEFs to some degree, but interindividual differences are large compared with SEFs evoked by upper limb stimulation.