Recently, there has been intermittent interference in the EEG channels which can grow large and generate similar interference in MEG channels near the electrodes. This has happened both with EEG (for epilepsy scans) and without, with only ECG and EOG. In the latter case, the MEG interference appears around the top VEOG electrode.
Note that it can appear as broadband noise (left side) or more regular spikes (right side).
For now, to try and mitigate the issue, we recommend placing the ground electrode closer to the others, on the head, for example on the left mastoid bone behind the ear.
Another thing to try is to ask the participant to touch the grounding contact on the EEG box (brass circle), if/when the interference occurs, and reconnect the ground electrode and make sure it’s still well affixed on the participant. And of course, you can restart the electronics, though we’ve had mixed success doing that.
Please contact me if you see this issue, and let me know all the details so we can continue troubleshooting this with CTF. The leading theory is that it’s related to the EEG ground equalization system, which is designed to maintain the participant and EEG ground level matched to the MEG ground.
This was resolved a few weeks later by replacing the EEG power supply in the electronics rack. It should no longer be an issue, and would no longer be relevant with the imminent electronics upgrade.
Sadly, with the electronics not quite yet replaced, this issue has returned (Aug 2025). It shows up as waves of increased noise, often in only half the electrodes. It’s possibly mostly line noise (60 Hz and harmonics), but maybe it’s more complex, with broadband and even 30 Hz contributions. In some cases, the waves grow in amplitude and eventually may interfere with MEG sensors as well. Again, this can happen even with just EOG/ECG, which may go unnoticed, and in which case the artefact would be in the MEG channels around the top EOG electrode.
The recommendations are as before: EEG ground closer to the head, and ensuring low impedance on the ground, and sometimes restarting the electronics might help, and is recommended when it gets too strong.
Further investigation is needed regarding cleaning, but at least in some cases the usual notch filter for power lines is sufficient.
