Magnetic auditory evoked fields: dipole orientation.

Magnetic auditory evoked fields (MAEFs) were recorded from the left hemisphere of 7 normal human subjects in response to 512 binaural click stimuli. The instrument was a figure-eight SQUID gradiometer that measured the transverse gradient of the field perpendicular to the scalp. Its response to a current dipole source inside the head was a cosine function of the angular orientation of the gradiometer about an axis perpendicular to the scalp. Measurements were made at various orientations from which were derived the orientation of the equivalent dipole source. Our findings suggest that when considering total magnetic energy, current dipoles in the vicinity of the transverse gyri of Heschl, whose primary orientation are approximately perpendicular to the sylvian fissure, appear to be the major contributors to the magnetic fields produced by click stimuli.