Experimental analysis of distortion of magnetoencephalography signals by the skull

Objectives: Magnetoencephalography (MEG) signals are, on theoretical ground s, thought to be relatively undistorted by the skull in contrast to electro encephalographic (EEG) signals, This assumption was experimentally tested i n an animal preparation with a brain similar to the human brain in many res pects, Methods: Possible skull effects on MEG were evaluated directly using an in vivo porcine preparation, by measuring the somatic evoked magnetic field (S EF) above the skull with and without the skull under, otherwise, the same c ondition. Results: The SEF was virtually undistorted by the skull with no obvious vis ible change in its waveform and amplitude under these two conditions. Howev er, there was some small, but significant attenuation when the skull was re moved, the distortion being greater for deeper sources. Conclusion: Our results are consistent with a theoretical expectation that the skull should be virtually 'transparent' to the magnetic fields for shal low sources, but less so for fields generated by deeper sources. (C) 1999 E lsevier Science Ireland Ltd. All rights reserved.