[Mne_analysis] 50Hz artefact for time frequency analysis

[Alexandre Gramfort]

hi Elisabeth,

> I have some data in which we did have some problem during the acquisition of
> MEG/EEG combined.
>
> Indeed some EEG electrodes picked up the frequency oscillation of the HPI
> coils. It should not be a problem as the coils frequency is above 300 HZ.
> However I just noticed  in my data that some EEG electrodes (4 out of 70 in
> average) have an harmonic around 50HZ.
>
> If I’m doing ERF/ERPs, my data will be filtering this artefact, but
> unfortunately I want to do time-frequency analysis on sensors & source
> level.
>
> On unfiltered data (1-100Hz) , those EEG electrodes are picked as artefact
> and rejected (amplitude of oscillation ~ 300-400uV). To pass the data I
> needed to mark those electrodes as bad in MNE.
>
> Now I thought about different solution to remove this and would like to have
> your advice on which one to do, or maybe you will have a better suggestion..
>
> a)      Use a notch filter, but then what about the gamma band? Will I still
> be able to analyse it or not?

why not?

> b)      Interpolate those specific electrodes, there are maximum 4 out of 70

I would say it depends on where are the 4 electrodes.

> c)       As power computation is baseline corrected it should not be a
> problem; the oscillations are also in the baseline and then will be
> automatically removed (same for PLV?)

yes indeed.

Best,
Alex