Stationary peaks from a moving source in far-field recording.

In 20 radial nerves, referential recording of antidromic sensory potentials from the digits consisted of two stationary far-field peaks, PI-NI and PII-NII. When compared with a bipolar recording of a moving source, the onset of PI and PII was coincident with the sensory potential approaching the wrist and the base of the digit, respectively. Of the two stationary positive peaks, PI was identical in latency irrespective of the digits tested. In contrast, the latency of PII was smaller for the first digit than the others, reflecting different distances from the stimulus point to the base of the respective digit. The amplitude of PII was proportional to that of the propagating sensory nerve action potential recorded at the volume conductor junction. A bipolar recording registers a near-field potential over the sensory fibers along the length of the nerve. In contrast, a referential recording represents a mixture of the near- and far-field potentials with the latter frequently producing major alterations of the classical triphasic wave. The standing far-field peaks show a temporal relationship to the traveling volleys approaching the main borders of the volume conductor. At the moment the sensory impulse reaches the boundary, current density changes suddenly in the two adjacent conducting media, giving rise to an apparent standing potential. The same mechanism probably plays an important role for the generation of some of the short latency peaks in the scalp recorded SEPs.