Auditory magnetic fields from the human cerebral cortex: location and strength of an equivalent current dipole

Auditory evoked cortical magnetic fields are recorded from human subjects by means of a SQUID gradiometer. The spatial and temporal distributions of the averaged evoked fields normal to the surface of the skull are measured from both hemispheres in response to contra- and ipsilateral 1 kHz stimulation. The evoked magnetic response can be separated into a dominant and a 'residual' signal and the former is analysed with a particular source model consisting of a single equivalent current dipole in each hemisphere. We find that the equivalent current dipoles are located near the superior surface of the temporal lobes approximately 20 mm below the surface of the skull. The dipoles are oriented in the superior-inferior direction. In the left hemisphere the dipole is located approximately 14 mm posterior to that in the right hemisphere, but otherwise no hemisphere/ear difference in dipole location or orientation is found. The strength of the dipole in the left hemisphere is found to be twice as great as that in the right hemisphere when stimulating the right ear, whereas no difference is found when stimulating the left ear. The strength of the dipole is greater in response to contralateral than ipsilateral stimulation. By means of a statistical experiment and using estimates of the variance of the recorded evoked fields we show that the model suggested is adequate to describe the experimental data and that the overall confidence of the extracted dipole parameters can be estimated.