Study on Neuromagnetic Imaging and Modeling of MEG Activities

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During the past decade, substantial advances in the understanding of the functional organization of the human brain have been made through the technique of MEG topographic mapping. Most of these investigations were concerned with the estimation and localization of sources which were modeled as single current dipoles positioned in a semi infinite volume conductor with homogeneous conductivities. However, the sources in the brain are complex, and the head as a volume conductor consisits of different materials with different electrical conductivities. The influence of these inhomogeneities on the MEG topography is studied by a computer simulation, modeling the sources as single or multiple dipoles located in inhomogeneous volume conductors. The computer simulation suggests some important aspects in estimation of source localization. (1) Flux reversal phenomena can be observed in MEG to pographic patterns in special cases where conductivities of the inhomogeneous regions change with pathol … More ogical conditions. The flux reversal can be caused by the change of flow patterns of currents which flow on the surface of boundary between different inhomogeneous regions in the head. (2) Radially oriented dipoles produced magnetic fields outside the sphere due to the effects of inhomogeneities. Normal components of magnetic fields perpendicular to the surface of the sphere generated by the effects of inhomogeneities were determined by volume current. The sources of MEG activities in human subject during sleep were also studied, The following results were obtained ; (3) Sources of K-complexes can be modeled by two current dipoles positioned in the deeper parts of the head. (4) Delta activities in sleep stages 3 and 4 have many sources with different frequencies. Some of these sources can be detected by MEG measurements, but are difficult to detect with EEG measurements. The MEG technique has a potential advantage to provide important information concerning brain function which cannot be obtained using the EEG measurements. Less