The effect of artifact rejection by signal-space projection on source localization accuracy in MEG measurements

The consequences of artifact suppression by means of signal-space projectio n on dipole localization accuracy for magnetoencephalography measurements a re studied. Approximate analytical formulas, equivalent to the Cramer-Rao b ound, are presented and verified by Monte Carlo simulations which relate th e increase of localization error for individual coordinates to the similari ty of the artifact field and respective (contravariant) quadrupole fields o btained by differentiating the dipole field with respect to its origin. The expressions simplify significantly for dipoles placed below the center of the measuring system giving rise to highly symmetric field patterns, Formul as are presented both for single- and for multiple-artifact rejection. As i llustrative examples artifact fields are constructed which a) lead to highl y decreasing signal-to-noise ratio and goodness of-fit (GOF), while the loc alization error is unaffected for all coordinates and b) lead to an increas e of localization error while the SNR and the GOF stays constant. Finally, the rich structure of localization error increase is demonstrated for a cla ss of artifact fields originating from artifact current dipoles.