Distribution of scalp somatosensory potentials evoked by stimulation of the tibial nerve in man.

The scalp distribution of the response to stimulation of the tibial nerve at the medial malleolus was systematically analysed. The somatosensory evoked potential (SEP) was recorded with electrodes placed in a transversal line over the ipsilateral and contralateral postcentral gyri and in a sagittal line over the longitudinal brain fissure. The SEPs recorded over the ipsilateral hemisphere and along the sagittal line were similar to the F response (the response over the foot primary somatosensory region). Over the contralateral hemisphere the waveform of the responses changed obviously from point F to the point C (contralateral hand primary somatosensory region). The C response started with N37, P40 had a longer latency, N50 was not present and the subsequent waves were also considerably different. Mathematical simulation of the responses recorded from the electrodes between points F and C has shown that they represent an electrical algebraic summation of the activity over points F and C. Although the F and C responses may be 2 potentials arising from the opposite sides of a single dipole generator which is located in the medial fissure, it is more probable that the somatosensory evoked potential on tibial nerve stimulation reflects the activity of 2 separate generators.