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 bac kground fields were found to have a main contribution from the spontan eous fields, which originate in the neural activities of the brain. Th e spontaneous fields had strong spatial correlation across the recordi ng sites even after averaging over 100 epochs. The spatial distributio n of the spontaneous fields consisted of 3 main components of single e xtremum pattern, dipolar pattern, and a dipolar pattern with some dist ortion. Computer simulations of the localization of a single dipole so urce of the evoked field response showed that the spontaneous backgrou nd field could bring about large location errors in an unpredictable m anner, as compared with the location errors caused by a spatially rand om Gaussian noise field.