The relationship between current sources in a brain tissue and the magnetic field outside the brain will be studied directly on isolated brain preparations as well as on partially intact preparations in order to provide a firm basis for the interpretation of the extracranial magnetic field of the brain (MEG). The relationship of the MEG has been inferred on the basis of theoretical results that hold for model sources and volume conductors, but has not been directly determined, since the research has been limited to human MEGs. To directly study the relationship, a current source will be set up in the cerebellum of turtles and guinea pigs immersed in a Ringer solution and it will be characterized in terms of the distribution of """"""""current-sources"""""""" and """"""""current-sinks"""""""" in the active tissue by applying the current source-density (CSD) analysis. Then the magnetic field outside the brain tissue will be calculated by applying the law of Biot and Savart to each current element represented by a current source-sink pair. Finally the computed field will be compared with the actual field. The comparison should reveal whether the external field is indeed due solely to the component of the current in the active tissue tangential to the outer boundary of the volume conductor, without any contribution from the normal component of the current or from sources set up by the volume current, as commonly assumed today. Inversely, the location and size of single and multiple sources will be estimated from measured fields and compared with those determined with the CSD analysis in order to evaluate the accuracy of source localization based on point source and distributed source models that incorporate the assumptions about the relative contributions of tangential and normal current sources and sources set up by the volume current.
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