FAR-FIELD POTENTIALS

Far-field potentials are produced by neural generators located at a distance from the recording electrodes. These potentials were initially characterized incorrectly as being of positive polarity, widespread distribution, and constant latency; however, recent advances have clearly demonstrated that far-field potentials may be either positive or negative depending upon the location of the electrodes with respect to the orientation of the dipole generator. Additionally, peak latencies in the far-field can vary with alterations in body position and the spatial distribution of far-field potentials, while widespread, is not uniform. Recent studies of far-field potentials suggest how such waveforms are produced when the symmetry of an action potential, as recorded by distant electrodes, is broken by such factors as differing conductivities of volume conductor compartments, direction of action potential propagation, size differentials in adjoining body segments, or the termination of action potential propagation in excitable tissue. Human, animal, and computer experiments support the preceding generalizations. These new explanations are directly applicable to such far-field potentials as the short latency somatosensory-evoked potential. Furthermore, since far-field potentials can also occur in muscle tissue, one should expect that these generalizations will hold with respect to electromyographic potentials.