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Source activity correlation effects on LCMV beamformers in a realistic measurement environment.

Belardinelli P, Ortiz E, Braun C - Comput Math Methods Med (2012)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: MEG Center, University of Tübingen, Otfried Mueller Street 47, Tübingen, Germany.

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.

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Setting of the three dipoles within the coverless phantom. Squares have a 5 mm side. In the left upper corner, the complete setup is shown.
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Related In: Results  -  Collection


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fig1: Setting of the three dipoles within the coverless phantom. Squares have a 5 mm side. In the left upper corner, the complete setup is shown.

Mentions: Two different dipole configurations were considered: one with two parallel dipoles (#1 and #2) placed on the same coronal plane in the two different hemispheres (Figure 1). The dipoles had different distances from the surface (2 and 3 cm, resp.). The distance between the dipoles was 3 centimeters. In the second configuration the two dipoles (#1 and #3) were placed in the left hemisphere at a slightly height (2 and 0.5 cm from the surface, resp.), with dipole #3 more external with respect to #1. Their distance in this case was 1.5 centimeters.


Source activity correlation effects on LCMV beamformers in a realistic measurement environment.

Belardinelli P, Ortiz E, Braun C - Comput Math Methods Med (2012)

Setting of the three dipoles within the coverless phantom. Squares have a 5 mm side. In the left upper corner, the complete setup is shown.
© Copyright Policy - open-access
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3351244&req=5

fig1: Setting of the three dipoles within the coverless phantom. Squares have a 5 mm side. In the left upper corner, the complete setup is shown.
Mentions: Two different dipole configurations were considered: one with two parallel dipoles (#1 and #2) placed on the same coronal plane in the two different hemispheres (Figure 1). The dipoles had different distances from the surface (2 and 3 cm, resp.). The distance between the dipoles was 3 centimeters. In the second configuration the two dipoles (#1 and #3) were placed in the left hemisphere at a slightly height (2 and 0.5 cm from the surface, resp.), with dipole #3 more external with respect to #1. Their distance in this case was 1.5 centimeters.

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: MEG Center, University of Tübingen, Otfried Mueller Street 47, Tübingen, Germany.

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.

Show MeSH