Magnetoencephalography signals are influenced by skull defects

•We present the first in vivo experimental evidence of the substantial influence of skull defects on MEG.•The MEG signal amplitude deviates more if the source is central under the skull defect, whereas the EEG signal amplitude deviates more if the source is under the edge of the defect.•Dense spatial sampling reveals high spatial frequencies in MEG and EEG signals due to skull defects that are not detectable with current human helmet-type MEG devices and standard EEG setups.

ObjectiveMagnetoencephalography (MEG) signals had previously been hypothesized to have negligible sensitivity to skull defects. The objective is to experimentally investigate the influence of conducting skull defects on MEG and EEG signals.MethodsA miniaturized electric dipole was implanted in vivo into rabbit brains. Simultaneous recording using 64-channel EEG and 16-channel MEG was conducted, first above the intact skull and then above a skull defect. Skull defects were filled with agar gels, which had been formulated to have tissue-like homogeneous conductivities. The dipole was moved beneath the skull defects, and measurements were taken at regularly spaced points.ResultsThe EEG signal amplitude increased 2–10 times, whereas the MEG signal amplitude reduced by as much as 20%. The EEG signal amplitude deviated more when the source was under the edge of the defect, whereas the MEG signal amplitude deviated more when the source was central under the defect. The change in MEG field-map topography (relative difference measure, RDM∗ = 0.15) was geometrically related to the skull defect edge.ConclusionsMEG and EEG signals can be substantially affected by skull defects.SignificanceMEG source modeling requires realistic volume conductor head models that incorporate skull defects.