Computational and Mathematical Methods in Medicine
Volume 2012 (2012), Article ID 190513, 8 pages
http://dx.doi.org/10.1155/2012/190513
Research Article

Source Activity Correlation Effects on LCMV Beamformers in a Realistic Measurement Environment

Paolo Belardinelli,1 Erick Ortiz,1 and Christoph Braun1,2,3

1MEG Center, University of Tübingen, Otfried Mueller Street 47, 72076 Tübingen, Germany
2CIMeC, Center of Mind/Brain Sciences, University of Trento, Via Delle Regole 101, 38123 Mattarello, Italy
3DiSCoF, Department of Cognitive and Educational Sciences, University of Trento, Corso Bettini no. 31, 38068 Rovereto, Italy

Received 17 November 2011; Revised 1 February 2012; Accepted 9 February 2012

Academic Editor: Luca Faes

Copyright © 2012 Paolo Belardinelli et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

In EEG and MEG studies on brain functional connectivity and source interactions can be performed at sensor or source level. Beamformers are well-established source-localization tools for MEG/EEG signals, being employed in source connectivity studies both in time and frequency domain. However, it has been demonstrated that beamformers suffer from a localization bias due to correlation between source time courses. This phenomenon has been ascertained by means of theoretical proofs and simulations. Nonetheless, the impact of correlated sources on localization outputs with real data has been disputed for a long time. In this paper, by means of a phantom, we address the correlation issue in a realistic MEG environment. Localization performances in the presence of simultaneously active sources are studied as a function of correlation degree and distance between sources. A linear constrained minimum variance (LCMV) beamformer is applied to the oscillating signals generated by the current dipoles within the phantom. Results show that high correlation affects mostly dipoles placed at small distances (1, 5 centimeters). In this case the sources merge. If the dipoles lie 3 centimeters apart, the beamformer localization detects attenuated power amplitudes and blurred sources as the correlation level raises.