Df. Stegeman et al., NEAR-FIELDS AND FAR-FIELDS - SOURCE CHARACTERISTICS AND THE CONDUCTING MEDIUM IN NEUROPHYSIOLOGY, Journal of clinical neurophysiology, 14(5), 1997, pp. 429-442
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 pe
rformed 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. Fa
r-field potentials can be transiently generated when any propagating A
P constitutes a net ''real'' or ''virtual'' dipolar source. ''Real'' d
ipolar sources can occur if an AP encounters the termination of excita
ble tissue, an alteration in conduction velocity, curvature in excitab
le 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 or shape of
the extracellular medium or a transition in extracellular conductivity
.