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“Error Bounds for EEG and MEG Dipole Source Localization”
by John C. Mosher, Michael E. Spencer, Richard M. Leahy, and Paul S. Lewis
July 1992
General formulas for computing the lower bound in both localization and moment error for electroencephalographic (EEG) or magnetoencephalographic (MEG) current dipole models with arbitrary sensor array geometry are presented. Specific EEG and MEG formulas are presented for the case of multiple dipoles in a four spherical shell head model. Localization error bounds are presented for both EEG and MEG for several different sensor configurations. Graphical error contours are presented for 127 sensors covering the upper hemisphere, for both 37 sensors and 127 sensors covering a smaller region, and for the standard Ten-Twenty EEG sensor arrangement. One and two dipole cases are examined for all possible dipole orientations and locations within a head quadrant. The results show a strong dependence on absolute dipole location and orientation. Fusion of EEG and MEG measurements into a combined model is shown to reduce the lower bound. A Monte-Carlo simulation is performed to check the tightness of the bounds for a selected case. The simple head model, the white and relatively low power noise, and the few relatively strong dipoles were all selected in this study as optimistic conditions in order to establish possibly fundamental resolution limits for any localization effort. Results under these favorable assumptions show comparable resolutions between EEG and MEG, but accuracy for a single dipole in either case appears limited to several millimeters for a single time slice. The lower bounds markedly increase with just two dipoles. Observations are given to support the need for full spatio-temporal modeling to improve these lower bounds. All simulation results presented are easily scaled to other instances of noise power and dipole intensity.