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Sensitivity of MEG and EEG to source orientation.

Ahlfors SP, Han J, Belliveau JW, Hämäläinen MS - Brain Topogr (2010)

Bottom Line: Similar to the simpler case of a spherical head model, in which MEG cannot see radial sources at all, for most cortical locations there was a source orientation to which MEG was insensitive.The median value for the ratio of the signal magnitude for the source orientation of the lowest and the highest sensitivity was 0.06 for MEG and 0.63 for EEG.The difference in the sensitivity to the source orientation is expected to contribute to systematic differences in the signal-to-noise ratio between MEG and EEG.

View Article: PubMed Central - PubMed

Affiliation: MGH/HST Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital/Harvard Medical School, 149 13th Street, Rm 2301, Charlestown, MA 02129, USA. seppo@nmr.mgh.harvard.edu

ABSTRACT
An important difference between magnetoencephalography (MEG) and electroencephalography (EEG) is that MEG is insensitive to radially oriented sources. We quantified computationally the dependency of MEG and EEG on the source orientation using a forward model with realistic tissue boundaries. Similar to the simpler case of a spherical head model, in which MEG cannot see radial sources at all, for most cortical locations there was a source orientation to which MEG was insensitive. The median value for the ratio of the signal magnitude for the source orientation of the lowest and the highest sensitivity was 0.06 for MEG and 0.63 for EEG. The difference in the sensitivity to the source orientation is expected to contribute to systematic differences in the signal-to-noise ratio between MEG and EEG.

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Distribution of the sensitivity to three orthogonal source components at each location on the cortex, indicated by the singular values (λk,1 = λk,max, λk,2, λk,3 = λk,min) of the dipole gain matrix Ak for MEG (a) and EEG (b). Note that for MEG, the color scales in the spatial map and the vertical scales in the histogram are different for the singular value for the orientation of the lowest sensitivity (λk,3) from those for λk,1 and λk,2. The units for λk,i are pT/nAm for MEG and μV/nAm for EEG
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Fig2: Distribution of the sensitivity to three orthogonal source components at each location on the cortex, indicated by the singular values (λk,1 = λk,max, λk,2, λk,3 = λk,min) of the dipole gain matrix Ak for MEG (a) and EEG (b). Note that for MEG, the color scales in the spatial map and the vertical scales in the histogram are different for the singular value for the orientation of the lowest sensitivity (λk,3) from those for λk,1 and λk,2. The units for λk,i are pT/nAm for MEG and μV/nAm for EEG

Mentions: The sensitivity of MEG and EEG for three orthogonal source orientations at each location on the cortex is shown in Fig. 2. The sensitivity was quantified by the three singular values of the dipole gain matrix Ak. The singular value corresponding to the highest sensitivity (λk,max) showed a strong depth dependency for MEG. The spatial distribution of the smallest singular value (λk,min) for MEG was similar to that of the ratio Rλ shown in Fig. 1, except that the large values of Rλ for MEG occurred in the subcortical regions near the center of the head where λk,max was very small.Fig. 2


Sensitivity of MEG and EEG to source orientation.

Ahlfors SP, Han J, Belliveau JW, Hämäläinen MS - Brain Topogr (2010)

Distribution of the sensitivity to three orthogonal source components at each location on the cortex, indicated by the singular values (λk,1 = λk,max, λk,2, λk,3 = λk,min) of the dipole gain matrix Ak for MEG (a) and EEG (b). Note that for MEG, the color scales in the spatial map and the vertical scales in the histogram are different for the singular value for the orientation of the lowest sensitivity (λk,3) from those for λk,1 and λk,2. The units for λk,i are pT/nAm for MEG and μV/nAm for EEG
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2914866&req=5

Fig2: Distribution of the sensitivity to three orthogonal source components at each location on the cortex, indicated by the singular values (λk,1 = λk,max, λk,2, λk,3 = λk,min) of the dipole gain matrix Ak for MEG (a) and EEG (b). Note that for MEG, the color scales in the spatial map and the vertical scales in the histogram are different for the singular value for the orientation of the lowest sensitivity (λk,3) from those for λk,1 and λk,2. The units for λk,i are pT/nAm for MEG and μV/nAm for EEG
Mentions: The sensitivity of MEG and EEG for three orthogonal source orientations at each location on the cortex is shown in Fig. 2. The sensitivity was quantified by the three singular values of the dipole gain matrix Ak. The singular value corresponding to the highest sensitivity (λk,max) showed a strong depth dependency for MEG. The spatial distribution of the smallest singular value (λk,min) for MEG was similar to that of the ratio Rλ shown in Fig. 1, except that the large values of Rλ for MEG occurred in the subcortical regions near the center of the head where λk,max was very small.Fig. 2

Bottom Line: Similar to the simpler case of a spherical head model, in which MEG cannot see radial sources at all, for most cortical locations there was a source orientation to which MEG was insensitive.The median value for the ratio of the signal magnitude for the source orientation of the lowest and the highest sensitivity was 0.06 for MEG and 0.63 for EEG.The difference in the sensitivity to the source orientation is expected to contribute to systematic differences in the signal-to-noise ratio between MEG and EEG.

View Article: PubMed Central - PubMed

Affiliation: MGH/HST Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital/Harvard Medical School, 149 13th Street, Rm 2301, Charlestown, MA 02129, USA. seppo@nmr.mgh.harvard.edu

ABSTRACT
An important difference between magnetoencephalography (MEG) and electroencephalography (EEG) is that MEG is insensitive to radially oriented sources. We quantified computationally the dependency of MEG and EEG on the source orientation using a forward model with realistic tissue boundaries. Similar to the simpler case of a spherical head model, in which MEG cannot see radial sources at all, for most cortical locations there was a source orientation to which MEG was insensitive. The median value for the ratio of the signal magnitude for the source orientation of the lowest and the highest sensitivity was 0.06 for MEG and 0.63 for EEG. The difference in the sensitivity to the source orientation is expected to contribute to systematic differences in the signal-to-noise ratio between MEG and EEG.

Show MeSH