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Near- and Far-Fields: Source Characteristics and the Conducting Medium in Neurophysiology

Stegeman, Dick F.; Dumitru, Daniel*; King, John C.*; Roeleveld, Karin

Journal of Clinical Neurophysiology: September 1997 - Volume 14 - Issue 5 - p 429-442
Original Contributions

It is possible to appreciate the production of far-field potentials by considering constant current dipolar source voltage distributions in bounded volumes, especially when they are stretched in one direction, e.g., a cylinder. An essentially nondeclining voltage is detected when the recording electrodes are on opposite sides of, and relatively far from, the dipolar source. This voltage maintains its (a) latency, (b) amplitude, (c) morphology, and (d) polarity even if recordings are performed a whole body length away. These four criteria define far-field potentials. A propagating action potential (AP) can be conceptualized as a linear quadrupole or the summation of two dipoles "back-to-back" (+ - - +). The far-field components of the summated dipoles cancel resulting in the anticipated triphasic waveform for APs with only near-field characteristics, not meeting the first three criteria above. Far-field potentials can be transiently generated when any propagating AP constitutes a net "real" or "virtual" dipolar source."Real" dipolar sources can occur if an AP encounters the termination of excitable tissue, an alteration in conduction velocity, curvature in excitable tissue resulting in a change in propagation direction, or an abrupt change in resistance of the excitable tissue. Virtual dipolar sources may be produced if an AP encounters a change in the size of shape of the extracellular medium or a transition in extracellular conductivity.

Department of Clinical Neurophysiology, Institute of Neurology, University Hospital Nijmegen, Nijmegen, The Netherlands; and *Department of Rehabilitation Medicine, The University of Texas Health Sciences Center at San Antonio, San Antonio, Texas, U.S.A.

Address correspondence and reprint requests to Dr. D. F. Stegeman at Department of Clinical Neurophysiology, Institute of Neurology, University Hospital Nijmegen, P.O. Box 9101, 6500 HB Nijmegen, The Netherlands.

© Lippincott-Raven Publishers