Magnetic auditory responses from the human brain. A preliminary report.

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.