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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 Rλ, measuring the relative sensitivity to sources of different orientation and calculated as the ratio of the smallest and largest singular values of the dipole gain matrix for MEG (top) and EEG (bottom). a Spatial maps of Rλ for the left hemisphere are shown in a lateral and medial view of an inflated representation of the cerebral cortex. The curvature of the cortex is indicated by darker (sulci) and lighter (gyri) regions through the semi-transparent color-coded map of Rλ. Note the different color scales for MEG and EEG. The location of the MEG sensors and EEG electrodes with respect to the cortical surface and the scalp are shown on the left. b Histograms of the Rλ values across all locations on the cortex, showing the number of vertices for MEG and EEG (nMEG, nEEG) with a given value of Rλ
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Fig1: Distribution of Rλ, measuring the relative sensitivity to sources of different orientation and calculated as the ratio of the smallest and largest singular values of the dipole gain matrix for MEG (top) and EEG (bottom). a Spatial maps of Rλ for the left hemisphere are shown in a lateral and medial view of an inflated representation of the cerebral cortex. The curvature of the cortex is indicated by darker (sulci) and lighter (gyri) regions through the semi-transparent color-coded map of Rλ. Note the different color scales for MEG and EEG. The location of the MEG sensors and EEG electrodes with respect to the cortical surface and the scalp are shown on the left. b Histograms of the Rλ values across all locations on the cortex, showing the number of vertices for MEG and EEG (nMEG, nEEG) with a given value of Rλ

Mentions: Distributions of the ratio Rλ across the cerebral cortex are shown in Fig.1. For MEG, the median value of Rλ was 0.06 and 95% of the values were below 0.23. The prominence of small values for Rλ for MEG suggests that for most locations on the cortex there was a source orientation with which little if any MEG signals was generated. In contrast, for EEG the median value of Rλ was 0.63 and 95% of the values were above 0.42.Fig. 1


Sensitivity of MEG and EEG to source orientation.

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

Distribution of Rλ, measuring the relative sensitivity to sources of different orientation and calculated as the ratio of the smallest and largest singular values of the dipole gain matrix for MEG (top) and EEG (bottom). a Spatial maps of Rλ for the left hemisphere are shown in a lateral and medial view of an inflated representation of the cerebral cortex. The curvature of the cortex is indicated by darker (sulci) and lighter (gyri) regions through the semi-transparent color-coded map of Rλ. Note the different color scales for MEG and EEG. The location of the MEG sensors and EEG electrodes with respect to the cortical surface and the scalp are shown on the left. b Histograms of the Rλ values across all locations on the cortex, showing the number of vertices for MEG and EEG (nMEG, nEEG) with a given value of Rλ
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2914866&req=5

Fig1: Distribution of Rλ, measuring the relative sensitivity to sources of different orientation and calculated as the ratio of the smallest and largest singular values of the dipole gain matrix for MEG (top) and EEG (bottom). a Spatial maps of Rλ for the left hemisphere are shown in a lateral and medial view of an inflated representation of the cerebral cortex. The curvature of the cortex is indicated by darker (sulci) and lighter (gyri) regions through the semi-transparent color-coded map of Rλ. Note the different color scales for MEG and EEG. The location of the MEG sensors and EEG electrodes with respect to the cortical surface and the scalp are shown on the left. b Histograms of the Rλ values across all locations on the cortex, showing the number of vertices for MEG and EEG (nMEG, nEEG) with a given value of Rλ
Mentions: Distributions of the ratio Rλ across the cerebral cortex are shown in Fig.1. For MEG, the median value of Rλ was 0.06 and 95% of the values were below 0.23. The prominence of small values for Rλ for MEG suggests that for most locations on the cortex there was a source orientation with which little if any MEG signals was generated. In contrast, for EEG the median value of Rλ was 0.63 and 95% of the values were above 0.42.Fig. 1

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