Scalp-recorded short latency cortical and subcortical somatosensory evoked potentials to peroneal nerve stimulation

Short latency SEPs to peroneal nerve stimulation were recorded in 26 normal adults. These potentials consisted of 3 positive potentials (P1, P2, P3) peaking at about 17, 21 and 27 msec, followed by a negative potential (N1) peaking at about 34 msec. P1 and P2 were small and inconsistently recorded. P1 was widespread in its scalp distribution in non-cephalic reference leads, and was poorly defined or absent in scalp bipolar and ear reference leads. P2 was widespread in its scalp distribution in both non-cephalic and ear reference leads. It was poorly defined in scalp bipolar leads. P3 and N1 were consistently recorded from the scalp in both reference and bipolar leads. They were much greater in amplitude than the earlier potentials and had steep potential gradients. They were most prominent around the vertex. They showed significant differences in their peak latencies at different scalp recording locations. The short latency, positive polarity and widespread scalp distribution of P1 and P2 suggest they are far-field events arising in subcortical structures. The longer latency and steep potential gradient of P3 and N1 suggest that they are near-field events arising in cerebral cortical structures. Both P3 and N1 were stable potentials and, therefore, may prove to be clinically useful.