3. (continued) Stronger 0.7 Hz & harmonics up to 10 Hz
This is no longer the case. The magnetic stirrers were removed from the room right above the MEG lab.
Unfortunately, this previously reported artefact is now much worse. The amplitude is much larger, causing harmonics to also rise above the noise floor. This also interacts with the power line noise producing side peaks around 60 Hz. This is what a standard noise recording now looks like:
I’m hoping that this can be cleaned with SSP as we did previously for a sensor issue. I will confirm later, but it would surely require at least 2 components since the phase of the signal varies across sensors, thus it is not a fixed topography. I will update this post once I figure this out. Note that this phase variation also causes the shape to deviate from a sine wave with the application of 3rd gradient noise suppression (aka CTF compensation in Brainstorm), while the raw signals are very sine-like.
Finally, please pay attention to the noise spectrum as this artefact can make it more difficult to see other issues. Recently, MRT27, which is disabled, regularly needs to be defluxed when it causes jumps in neighboring channels. This is what the spectrum looks like in such an example:
