Characteristics of the background fields in multichannel-recorded magnetic field responses

We have studied the background fields in the auditory evoked magnetic field responses recorded with a 37-channel SQUID magnetometer. The background fields were found to have a main contribution from the spontaneous fields, which originate in the neural activities of the brain. The spontaneous fields had strong spatial correlation across the recording sites even after averaging over 100 epochs. The spatial distribution of the spontaneous fields consisted of 3 main components of single extremum pattern, dipolar pattern, and a dipolar pattern with some distortion. Computer simulations of the localization of a single dipole source of the evoked field response showed that the spontaneous background field could bring about large location errors in an unpredictable manner, as compared with the location errors caused by a spatially random Gaussian noise field.