Measurements of auditory and somatically evoked magnetic fields and localization of their dipole sources

We have measured neuromagnetic fields evoked by auditory and somatic stimulations using a SQUID. Responses of the auditory evoked field (AEF) have two peaks at about 100 and 180 ms after the onset of tone burst stimulus. Responses of the somatically evoked field (SEF) have two peaks at about 80 and 160 ms after the stimulus of current pulse. Magnetic field data measured at various positions of the lateral side of the head have been used to calculate parameters of an equivalent current dipole source based on a least-squares fit. The determined source locations indicate the auditory cortex for the AEF and the primary somatosensory cortex for the SEF, in agreement with anatomical structures obtained from NMR images. A simulation study has been done to estimate errors in the calculated dipole parameters due to the field noise. For typical measurement conditions of a signal-to-noise ratio of approximately 5 and measurement points of approximately 40, location errors of the order of 5 mm are estimated.