EEG source localization and imaging using multiple signal classification approaches

Equivalent current dipoles are a powerful tool for modeling focal sources. The dipole is often sufficient to adequately represent sources of measured scalp potentials, even when the area of activation exceeds 1 cm(2) of corte x. Traditional least-squares fitting techniques involve minimization of an error function with respect to the location and orientation of the dipoles. The existence of multiple local minima in this error function can result i n gross errors in the computed source locations. The problem is further com pounded by the requirement that the model order, i.e. the number of dipoles , be determined before error minimization can be performed. An incorrect mo del order can produce additional errors in the estimated source parameters. Both of these problems can be avoided using alternative search strategies based on the MUSIC (multiple signal classification) algorithm. Here the aut hors review the MUSIC approach and demonstrate its application to the local ization of multiple current dipoles from EEG data. The authors also show th at the number of detectable sources can be determined in a recursive manner from the data. Also, in contrast to least-squares, the method can find dip olar sources in the presence of additional non-dipolar sources. Finally, ex tensions of the MUSIC approach to allow the modeling of distributed sources are discussed.