The late, acoustically evoked, averaged magnetic field from the right hemisphere of the human brain is composed of two signals. One is dominant, appears generated by an equivalent current dipole within or near the primary auditory cortex and shows a frequency dependent location and/or orientation (tonotopical organization). The other, denoted the 'residual' signal, resembles the electric T-complex and is possibly generated more diffusely in the auditory and adjacent cortical areas.

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.

The late averaged magnetic field evoked by contra- and ipsilateral auditory stimulation is recorded by means of a SQUID magnetometer from both hemispheres in four normally hearing, right-handed male adults. The stimuli consist of 1 kHz, 500 ms tone pulses with intensities from 5 to 85 dB HL and averaging is based on 60 sweeps. Stimulating the right ear the averaged magnetic field from the left hemisphere is approx. twice as great as that from the right hemisphere, whereas stimulating the left ear no difference in magnitude is found. The amplitude input-output functions are steeply rising near threshold and more shallow at high intensities. The responses from contralateral stimulation are approx. 9 ms earlier than those from ipsilateral stimulation with no interhemispheric difference.

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By means of a magnetic sensor, SQUID (Superconducting Quantum-Interference Device) the late acoustically evoked magnetic field was recorded from the right and left side of the skull in 5 humans in response to ipsi- and contralateral 1 kHz tone bursts at 80 dB SPL. The '100 ms' component of the magnetic field has opposite polarity on the two sides of the head and when crossing the primary auditory cortex at the Sylvian fissure in a posterior--anterior track, polarity inversion of this component takes place within a highly localized region. The evoked magnetic field is widely distributed across the scalp and seems to be produced by an equivalent magnetic dipole located in or near the primary auditory cortex. In the present experiment neither right--left hemisphere nor ipsi--contralateral differences could be demonstrated.