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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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Time-courses for three different levels of correlation. Dashed and solid curves represent the dipole activities on the two source locations. Due to the addition of Gaussian noise, the signals diverge slightly from sine waves. Their phase synchrony grows as correlation increases.
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fig2: Time-courses for three different levels of correlation. Dashed and solid curves represent the dipole activities on the two source locations. Due to the addition of Gaussian noise, the signals diverge slightly from sine waves. Their phase synchrony grows as correlation increases.

Mentions: For each time sample, an instantaneous frequency was drawn from a Gaussian distribution centered on a 10 Hz frequency: N(10 Hz, 3 Hz). The final time course consisted of the sine of the cumulative sum of such instantaneous frequencies, with a random initial phase. The dipole time courses were controlled either for low (0.15 ± 0.05) medium (0.55 ± 0.05) or high (0.95 ± 0.05) correlation (Figure 2).


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

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

Time-courses for three different levels of correlation. Dashed and solid curves represent the dipole activities on the two source locations. Due to the addition of Gaussian noise, the signals diverge slightly from sine waves. Their phase synchrony grows as correlation increases.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

fig2: Time-courses for three different levels of correlation. Dashed and solid curves represent the dipole activities on the two source locations. Due to the addition of Gaussian noise, the signals diverge slightly from sine waves. Their phase synchrony grows as correlation increases.
Mentions: For each time sample, an instantaneous frequency was drawn from a Gaussian distribution centered on a 10 Hz frequency: N(10 Hz, 3 Hz). The final time course consisted of the sine of the cumulative sum of such instantaneous frequencies, with a random initial phase. The dipole time courses were controlled either for low (0.15 ± 0.05) medium (0.55 ± 0.05) or high (0.95 ± 0.05) correlation (Figure 2).

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