Stationary waves recorded at the shoulder after median nerve stimulation.

A new recording method with a reference electrode on the stimulated arm defined two discrete far-field potentials just before the propagated near-field nerve action potential was recorded at Erb's point. Both potentials were stationary waves with the same latency at all recording sites. The first potential had the same onset and peak latencies as the propagated wave at the axilla; it corresponded to the first component of the P9 far-field potential recorded with scalp to noncephalic reference montages. The latency of the second potential coincided with that of the propagated wave entering the neck and corresponded to the peak of the P9 potential. The occurrence of these potentials where there are significant changes in the morphology of the volume conductor suggests that the P9 far-field potential is due to a change in the conducting medium that surrounds the nerve.