Clinical section
Demonstration of useful differences between magnetoencephalogram and electroencephalogramDémonstration de différences utiles entre magnétoencéphalogramme et electroencéphalogramme

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Abstract

For a dipole source, theory predicts 3 useful differences between the MEG and EEG spatial patterns over the head. These are seen when a comparison is made between theoretical MEG and EEG maps, due to the dipole in a spherical model of the head. If true, these differences would allow the MEG to better localize or differentiate neural sources in some ways than does the EEG. A first experimental test of the differences is made here. A comparison is made between MEG and EEG maps due to a neural source which appears to behave as a dipole (N20 of the somatic evoked response). The same 3 differences are seen, therefore the predicted differences are confirmed experimentally. The first 2 differences, due only to the tangential component of the dipole, are that the MEG pattern is rotated by 90° from the EEG pattern and is one-third tighter. The first allows the MEG to localize a tangential dipole better in a preferred direction, across the dipole (while the EEG does so along the dipole); the second allows the MEG to localize somewhat better in its preferred direction than the EEG does in its preferred direction. The third difference is due only to the radial component of the dipole; while the MEG receives no contribution from this component, the EEG pattern is asymmetrical and is heavily weighted by it. This allows the MEG to reveal tangential sources which are obscured by the radial sources in the EEG. For sources which cannot be approximated by a dipole, the MEG-EEG differences will depend on the particular case; however, the spherical model can now be used with more confidence to predict differences in these cases.

Résumé

Pour une source en dipôle, la théorie prévoit 3 différences utiles entre les patterns spatiaux MEG et EEG sur le crâne. Elles apparaissent lorsqu'une comparaison est effectuée entre cartes théoriques MEG et EEG, produites par un dipôle dans un modèle sphérique de la tête. Si ces différences sont réelles, le MEG pourrait d'une certaine façon mieux localiser ou distinguer les sources nerveuses que l'EEG. Une première épreuve expérimentale de ces différences est présentée ici. Une comparaison est effectuée entre les cartes MEG et EEG pour une source nerveuse qui semble se comporter comme un dipôle (l'onde N20 de la réponse évoquée somatique). Les 3 mêmes différences ont été observées, ce qui est une confirmation expérimentale. Les 2 premières différences, uniquement dues à la composante tangentielle du dipôle montrent que le pattern du MEG subit une rotation de 90° par rapport au pattern EEG ainsi qu'un resserrement d'un tiers. La première différence permet au MEG de mieux localiser un dipôle tangentiel dans une direction ‘préférentielle’ à travers le dipôle (tandis que l'EEG le fait le long du dipôle); la seconde permet au MEG une meilleure localisation dans sa direction préférentielle que l'EEG dans la sienne. La troisième différence est due uniquement à la composante radiale du dipôle; tandis que le pattern EEG est insensible à cette composante, le pattern EEG est assymétrique et massivement influencé par cette composante. Cette propriété permet au MEG de mettre en évidence des sources tangentielles masquées par des sources radiales dans l'EEG. En ce qui concerne les sources ne pouvant pas être assimilées à un dipôle, les différences MEG-EEG varient pour chaque cas; mais pour chacun d'eux, le modèle sphérique peut maintenant être utilisé avec plus de confiance pour prédire les différences.

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This work was supported by National Science Foundation Grant No. PCM-819973.

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