Cerebral magnetic fields evoked by peroneal nerve stimulation.

We measured evoked magnetic fields and evoked potentials to peroneal nerve stimulation in healthy humans. The results were consistent with activation of the primary sensorimotor foot area on the mesial surface of the hemisphere during several deflections between 40 and 200 msec. A cortical origin is suggested for the electrical P40. Distinct magnetic field patterns were observed over the primary and secondary somatosensory cortices (SI and SII). At SII, responses to ipsilateral stimulation were weaker and had longer latencies than those to contralateral stimulation. Simulations were used to clarify the differences between electrical and magnetic patterns when SI and SII were simultaneously active.