Biomagnetic Measurements Using Squids

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Abstract
Systematic studies of the magnetoencephalogram (MEG) in normal and pathological subjects (mainly with focal epilepsies) showed that the MEG may evidence significant brain activities even if they are not present in the electroencephalogram (EEG). They also showed that the MEG has a considerably higher spatial resolution than the EEG. A novel mapping technique was introduced to get such a representation of the data that would enable the investigator to draw his conclusions mainly from inspecting the plots. This technique is characterized by an isospectral amplitude (iso-SA) mapping of the scalp distribution of specified spectral components or frequency bands of the MEG power spectrum. With the above method we were able to localize an epileptiform focus using a noninvasive technique without applying an eliciting stimulus. Furthermore using SQUID measurements we were able to describe the behavior of the MEG when the brains of different subjects were subjected to low frequency sinusoidal binaural stimuli. Under these conditions it has been shown that the MEG tends to organize around discrete frequencies that depend on the interference pattern (beat) between the two inputs.