A source model is proposed to simulate spatial distributions of abnormal MEG and EEG activities, such as the delta activity associated with brain tumors. The brain tumor itself is electrically silent, but the spherical shell around the tumor may cause abnormal neural activities. The sources of these neural activities are represented by combinations of multiple dipoles. The magnetic fields and electrical potentials arising from these dipoles are calculated over the surface of the head. The results show that, in a special case where the dipoles are oriented in the same direction or are oriented radially, the spatial MEG and EEG patterns are analogous to those generated by a single dipole or a pair of dipoles. The electrical conductivity of brain lesions varies with such pathological circumstances as edemas and calcification. The effect of inhomogeneities of brain lesions on spatial MEG and EEG patterns is also studied. It is found that only the magnitude of the magnetic fields and potentials are influenced by the inhomogeneities, while the spatial patterns are not influenced.
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