Effects of local variations in skull and scalp thickness on EEG's and MEG's.

Many studies have been performed on the effects of various features of head geometry on electroencephalograms (EEG's) and magnetoencephalograms (MEG's) and on the accuracy with which electrical sources in the brain can be localized using these measurements. However, to date no studies have been performed of the effects of local variations in skull and scalp thickness. This paper presents a computer modeling study of the effects of such local variations. The results obtained in this study indicate that local variations in skull and scalp thickness have effects on EEG's and MEG's which range from a simple intuitive effect to complex effects which depend on such factors as source depth and orientation, the geometry of the variation in skull and scalp thickness, etc. These results also indicate that local variations in skull and scalp thickness cause EEG localization errors which are generally much less than 1 cm and MEG localization errors which are even smaller. These results also indicate that multichannel and single-channel MEG measurements will produce localization errors of approximately the same amplitude when there is a bump on the external surface of the head but that multichannel measurements will produce significantly smaller localization errors than single-channel measurements when a depression is present in that surface.