Difference in somatosensory evoked fields elicited by mechanical and electrical stimulations: Elucidation of the human homunculus by a noninvasive method

We recently recorded somatosensory evoked fields (SEFs) elicited by compressing the glabrous skin of the finger and decompressing it by using a photosensor trigger. In that study, the equivalent current dipoles (ECDs) for these evoked fields appeared to be physiologically similar to the ECDs of P30m in median nerve stimulation. We sought to determine the relations of evoked fields elicited by mechanically stimulating the glabrous skin of the great toe and those of electrically produced P40m. We studied SEFs elicited by mechanical and electrical stimulations from the median and tibial nerves. The orientations of dipoles from the mechanical stimulations were from anterior-to-posterior, similar to the orientations of dipoles for P30m. The direction of the dipole around the peak of N20m from median nerve electrical stimulation was opposite to these directions. The orientations of dipoles around the peak of P40m by tibial nerve stimulation were transverse, whereas those by the compression and decompression stimulation of the toe were directed from anterior-to-posterior. The concordance of the orientations in ECDs for evoked fields elicited by mechanical and electrical stimulations suggests that the ECDs of P40m are physiologically similar to those of P30m but not to those of N20m. The discrepancy in orientations in ECDs for evoked field elicited by these stimulations in the lower extremity suggests that electrical and compression stimulations elicit evoked fields responding to fast surface rubbing stimuli and/or stimuli to the muscle and joint.