Auditory evoked magnetic fields of subcortical origin

Magnetoencephalography (MEG) is generally dealing with cortical sources, since the sensitivity of MEG declines rapidly with increasing source depth. Two factors are responsible for this loss of sensitivity. First, the amplitude of the measured MEG is inversely proportional to the squared distance between sensors and source, and this distance naturally increases with source depth. Second, the MEG amplitude rapidly decreases when the source approaches the center of the volume conductor [9]. The first factor applies to the electroencephalogram (EEG) as well. It explains, for example, why evoked potentials arising from brainstem nuclei have, in general, a considerably lower amplitude then evoked potentials of cortical origin. The second factor, however, represents a peculiarity of MEG. Its nature is especially clear in the case of a spherical head model, where the MEG amplitude is proportional to the distance between source and center of sphere [5] so that sources located in the center itself are “silent”. This factor is the actual reason for the enormous difficulties to measure deep sources with MEG. Nevertheless, already in the eighties several attempts were made to record auditory evoked activity from sources as deep as the brainstem. The most successful study was performed by Erné et al. [1, 2]. In one subject, stimulated by brief 1000 Hz tone pulses, they found a clear magnetic wave coinciding with a wave in the brainstem auditory evoked potential (BAEP). The latency of this wave was 9.8 ms, and its amplitude was about 6 fT. At two other recording locations a magnetic counterpart of BAEP wave V was described as well, but that wave hardly exceeded the noise level so that its nature was not completely clear. In a second subject, stimulated by clicks instead of tone pulses, no BAEF could be detected at any of the locations measured. In what follows, recent BAEF measurements are presented which clearly show a magnetic correlate of the click-evoked BAEP wave V. In one subject, clicks were presented also at longer interstimulus intervals (ISIs), to look for slower waves of subcortical origin. 2 Methods