Source localization scale correction for Beamformer analysis

•Scale correction should be applied for valid current estimates of source activity.•A diagonal matrix does not reflect the true nature of the noise level.•A new method for accurate noise power estimation is suggested.•Empty room recording (ERR) should be added to each experimental recording session.•ERR serves as scale correction and beamformer performance evaluation.

BackgroundMagnetoencephalography measurements are often processed by using imaging algorithms such as beamforming. The estimated source magnitude tends to suffer from unbalanced scaling across different brain locations. Hence, when examining current estimates for source activity it is vital to rescale the estimated source magnitude, in order to obtain a uniformly scaled image.New methodWe present a generalized scale correction method (Nempty) that uses empty room MEG measurements to evaluate the noise level.ResultsThe location bias and spatial resolution of the estimated signal indicated that some scaling correction needs to be applied. Of all the scale correction methods that were tested, the best correction was achieved when using Nempty.Comparison with existing methodsWe show that a diagonal matrix does not reflect the true nature of the noise covariance matrix. Hence, diagonal matrix based methods are sub-optimal.ConclusionWe recommend adding empty room MEG measurements to each experimental recording session, for purposes of both scale correction and beamformer performance verification.