The early negative potential evoked by stimulation of the tibial nerve in man.

The scalp response to stimulation of the tibial nerve at the level of the medial malleolus was systematically analysed. It was recorded 2 cm posterior to the vertex and at the sites corresponding to cortical representation of the hand. The existence of an early negative wave with a peak latency of 37.2 2.29 ms and amplitude of -0.69 0.40 micro V was established (being half the amplitude of the first positive wave (P40) over the vertex). This wave was named N37 in respect of the peak latency and polarity. N37 was the first event recorded after stimulation of the tibial nerve at this level as the onset latency was 32.2 1.75 ms and that of P40 over the vertex 33.8 2.28 ms. It was recorded with the highest amplitude over the hand primary somatosensory area after stimulation of the opposite foot. N37 evoked by stimulation of the tibial nerve at the ankle and N20 evoked by stimulation of the arm nerve are both the primary negativities of the evoked potential. However, N37 is not recorded with maximum amplitude over the leg primary somatosensory area and it is rounded and longer lasting than N20. In spite of these differences the two initial negative electrical phenomena are not necessarily generated by different functional structures. The possible generators of N37 are discussed.