[Mne_analysis] noise covariance

[Alexandre Gramfort]

Hey Gus,

> So, I inherited several resting state datasets but empty room data were not collected for any of them. I was wondering about your experiences analyzing resting state in source space using an identity noise covariance matrix.

I have no such experience.

> Some questions that come to mind:
>
> - any suggestions for regularization levels?

I would take a noise cov with that machine from another experience
and make it diagonal. So the order of magnitudes should be ok.

> - if I'm interested in frequency domain, what about constructing my noise matrix with the filtered signals of a non-interesting band (i.e. band-stop on the frequency band of interest)?

I would clearly not do this. The amplitude/structure of noise depends
on frequencies.
If I can and make sense for the question, I tend to use a noise cov per band.

> - say I'm interested in the usual 4-5 frequency bands, maybe using the same noise covariance that's constructed using high frequency signals (> 100Hz, outside of the bands of interest)?

there is very little noise at these frequencies. I doubt it would be do the job.

> - how would the answers above change in the context of a MNE vs LCMV beamformer analysis?

good question.

HTH
Alex