Auditory Evoked Magnetic Fields

Abstract

Evoked potentials are remotely detectable traces of neural excitation processes. They are commonly used as scalar measures to monitor the functional state of a sensory system. The time domain waveform, however, if recorded in the far field of the generators, only very poorly reveals time-variant aspects (with respect to time elapsing since stimulus onset) of the underlying processes. Potentials from differently excited and differently located generators superimpose, and the anisotropic body tissue distorts considerably the distribution of the propagating electric field. The attribution of a voltage, measured over the scalp at a specified instant, to a specified process occurring in neural generators at a specified location is at least ambiguous, if not impossible. Even if the field distribution over the scalp is determined with multichannel recordings, the solution of the inverse problem still remains equivocal. Consequently, the only reliable measures of far-field recorded evoked potentials are peak latencies and, with less reliability, amplitudes — i.e., in a certain sense “static” measures — obtained at specified instants, while the remaining portion of the time domain waveform — i.e., the “dynamic” aspect (though I am aware of the fact that I use the term “dynamic” in a different sense) — is disregarded.