[Mne_analysis] 50Hz artefact for time frequency analysis

[Virginie van Wassenhove]

Hi Elisabeth,

I tend to agree with Tommi. We did not experience this sort of noise with
combined MEEG. When you look at the power spectra of your EEG signals, do
you actually see the HPI peak responses? Such high amplitude in the signals
likely correspond to lower frequency - e.g. muscle artifacts? Or possible
arterial artifact (were these specific EEG electrodes located at the point
of contact between the skull and the dewar?)

If the spectral artifact is steady over time, baseline correction will work
and so will contrasting conditions against each other - if you want to
avoid notch artifacts in higher frequency bands.

Best,
Virginie

On Wed, Oct 8, 2014 at 2:25 AM, <raij at nmr.mgh.harvard.edu> wrote:

>
> 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
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > _______________________________________________
> > Mne_analysis mailing list
> > Mne_analysis at nmr.mgh.harvard.edu
> > https://mail.nmr.mgh.harvard.edu/mailman/listinfo/mne_analysis
>
>
>
> ---
> Tommi Raij, MD, PhD
> MGH/MIT/HMS Athinoula A. Martinos Center for Biomedical Imaging
> Bldg 149, 13th St
> Charlestown, MA 02129
> U.S.A.
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-- 
Virginie van Wassenhove
+33(0)1.69.08.1667
Virginie.van.Wassenhove at gmail.com
https://sites.google.com/site/virginievanwassenhove/
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