The Use of a Squid Third Order Spatial Gradiometer to Measure Magnetic Fields of the Brain^a

Abstract

It appears to be clear from the results that the third order gradiometer is able to detect small biomagnetic signals from the brain which are related to evoked potentials and spontaneous electrical activity. The instrument operates reasonably well within a noisy environment, however further development is necessary to balance the first gradient. We intend to pursue this direction with software systems. Some of the data presented suggest that components of MEG evoked activity may change independently of EEG. One interpretation which may derive from this is that the same current dipoles are probably not responsible for the entire configuration of evoked fields. This interpretation is consistent with EEG evidence which indicates that analogous components in the evoked potential may vary independently as a function of stimulus parameters and information processing. Perhaps a model of magnetic dipoles due to small current loops would be more compatible with the electrophysiological data.