Magneto-encephalogram artifacts caused by electro-encephalogram electrodes.

The electro-encephalogram (EEG) and magneto-encephalogram (MEG) are often measured simultaneously. By Faraday's law of induction, the changing magnetic field of the MEG can induce eddy currents in the EEG electrodes. These eddy currents produce their own magnetic field that adds to that produced by current sources in the brain, resulting in an artifact in the MEG recording. It is shown that, under typical conditions, this artifact is less than 1%, but, during measurements of high temporal frequency and high spatial resolution, the artifact can be as large as 3%.