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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.

Show MeSH
Plots of relative power normalized to the baseline obtained by means of DICS at four sites of interest: location of dipole #1 (deep blue), dipole #2 (light blue), and dipole #3 (yellow) and the median point between dipole #1 and #3 (red). The high relative values are a symptom of the good performance of the spatial filter.
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fig5: Plots of relative power normalized to the baseline obtained by means of DICS at four sites of interest: location of dipole #1 (deep blue), dipole #2 (light blue), and dipole #3 (yellow) and the median point between dipole #1 and #3 (red). The high relative values are a symptom of the good performance of the spatial filter.

Mentions: Here we focus on power levels at four locations of interest in order to discern the correlation effects in a realistic environment (Figure 5). The three dipole sites are considered (deep blue, light blue, and yellow). In addition, based on the previous results, we focus on the median point between dipole #1 and #3 (red bar). Due to the spatial proximity, the dipole couple #1 + #3 tends to merge when the correlation degree increases. This does not happen in the case of the couple #1 + #2. In this case the distance always prevents the detection of these dipoles as one merged source. Differently from the couple #1 + #3, the dipoles just attenuate each other's power in the localization process.


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

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

Plots of relative power normalized to the baseline obtained by means of DICS at four sites of interest: location of dipole #1 (deep blue), dipole #2 (light blue), and dipole #3 (yellow) and the median point between dipole #1 and #3 (red). The high relative values are a symptom of the good performance of the spatial filter.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig5: Plots of relative power normalized to the baseline obtained by means of DICS at four sites of interest: location of dipole #1 (deep blue), dipole #2 (light blue), and dipole #3 (yellow) and the median point between dipole #1 and #3 (red). The high relative values are a symptom of the good performance of the spatial filter.
Mentions: Here we focus on power levels at four locations of interest in order to discern the correlation effects in a realistic environment (Figure 5). The three dipole sites are considered (deep blue, light blue, and yellow). In addition, based on the previous results, we focus on the median point between dipole #1 and #3 (red bar). Due to the spatial proximity, the dipole couple #1 + #3 tends to merge when the correlation degree increases. This does not happen in the case of the couple #1 + #2. In this case the distance always prevents the detection of these dipoles as one merged source. Differently from the couple #1 + #3, the dipoles just attenuate each other's power in the localization process.

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