[Mne_analysis] 50Hz artefact for time frequency analysis

[raij at nmr.mgh.harvard.edu]

Hi Elisabeth,

A note related to the origins of your 50-Hz artefact: The MEG HPI coils
should not induce any detectable currents in your EEG electrodes.
Moreover, 300 Hz (or, the 290-33 Hz range that is typically used for cHPI)
does not have sub-harmonics at 50 Hz. It is much more likely that you had
a bad EEG connection/electrode/wire at those 4 EEG electrodes, resulting
in that they picked up the 50 Hz in the electric mains.

Best regards,

Tommi


>
>
> Dear MNE users,
>
> 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?
>
> b)      Interpolate those specific electrodes, there are maximum 4 out of
> 70
>
> 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?)
>
>
>
> Thanks for your help
>
> Elisabeth
>
>
>
>
>
>
>
> __________________________________________________________
>
>
>
> Dr Elisabeth Fonteneau
>
> Neurolex Group
>
> Department of Psychology
>
> University of Cambridge
>
> Downing Street, Cambridge CB2 3EB, UK
>
> Phone: +44 1223 333 548
>
> Email:  <mailto:ef309 at cam.ac.uk> ef309 at cam.ac.uk
>
> Web:  <http://www.neurolex.psychol.cam.ac.uk/directory/ef309@cam.ac.uk>
> www.neurolex.psychol.cam.ac.uk/directory/ef309 at cam.ac.uk
>
>
>
>
>
>
>
>
>
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---
Tommi Raij, MD, PhD
MGH/MIT/HMS Athinoula A. Martinos Center for Biomedical Imaging
Bldg 149, 13th St
Charlestown, MA 02129
U.S.A.