Bifurcation of P9 far-field potentials induced by changes in the shoulder position

Somatosensory evoked potentials to stimulation of the left median nerve were recorded from normal adults with reference to the right knee in the usual shoulder position and in an elevated shoulder position. A single peak of the P9 potentials in the former position bifurcated into two peaks in the latter position without changing the onset latency. This waveform change can be accounted for by changes in the resistance of the volume conductor around the nerve trunk.