Stationary negative potentials near the source vs. positive far-field potentials at a distance

We studied the field distribution of referentially recorded negative potentials after stimulation of the median nerve at the wrist in 15 normal subjects. When recorded from multiple sites along the lateral aspect of the arm with the reference electrode at the knee, 3 negative peaks, N3, N6 and N9, appeared at fixed latencies. Of these N3 and N6 were highest in amplitude at the distal insertion of the brachioradialis and the distal end of the deltoid, respectively, and N9, at the acromion. With stimulation of the finger, the negative peaks shifted in latency by about 3 msec, indicating an anatomically fixed generator source for each component. When compared to far-field potentials, N9 was of the same latency as scalp recorded P9, that extended to the arm contralateral to the side of stimulation and to the upper half of the trunk. In contrast, N6 extended to the scalp with P6 spreading to the lower half of the body. When two subjects were connected by the arm, stationary negative or positive peaks were transmissible from the stimulated to the non-stimulated subject. When the stimulated arm of the first subject was in contact with the second subject, N3, N6 and N9 were recorded in the latter. Only P9 was registered when the unstimulated arm was in contact with the second subject. We conclude that N3, N6 and N9 are stationary negative potentials generated at certain points along the nerve pathway, probably representing a negative counterfield for positive far-field peaks, P3, P6 and P9. These stationary potentials can spread widely in a volume conductor and can even be detected in a non-stimulated subject making a close contact to the generator source.