[Mne_analysis] Power peak at 38 Hz?

[Ghuman, Avniel]

If I had to guess, this is likely to be something that is specific to your site.

Best wishes,
Avniel

________________________________
From: mne_analysis-bounces at nmr.mgh.harvard.edu [mne_analysis-bounces at nmr.mgh.harvard.edu] on behalf of Nico Adelhöfer [nico.adelhoefer at st.ovgu.de]
Sent: Sunday, September 4, 2016 11:31 AM
To: mne_analysis at nmr.mgh.harvard.edu
Subject: Re: [Mne_analysis] Power peak at 38 Hz?


Hello Eric,

thank you so much for pointing out the function plot_psd. I wasn't aware of this and it saves me a ton of work for testing...

My raw EDF files are too large so I only picked EEG channels, chose a segment of only some minutes duration and saved the file without filtering or resampling as a .fif file. Now plot_psd looks like this:

[cid:3Mi-NOIvUgtwf91WQgt-sg1 at webmail.uni-magdeburg.de]

So it seems it is indeed in the raw data. Also, each obvious line peak (50 Hz, 150 Hz) seems to have smaller peaks 12 Hz apart. So it seems to somehow originate from line-frequency electrical noise. Would you agree? If yes, shouldn't that be an ubiquitous issue in EEG experiments?

Thank you

Nico





Quoting Eric Larson <larson.eric.d at gmail.com<mailto:larson.eric.d at gmail.com>>:

> Can you try the same analysis without the resampling step? Maybe there is
> some aliasing of a line harmonic. How did you resample?
>
> You can also try raw.plot_psd to see if the original unprocessed data has
> it. That will tell you if something is in the original data, or if there is
> a processing problem.
>
> Eric
>
> On Sep 4, 2016 9:12 AM, "Nico Adelhöfer" <nico.adelhoefer at st.ovgu.de<mailto:nico.adelhoefer at st.ovgu.de>> wrote:
>
>> Hello,
>>
>> this question is not really related to MNE itself, but maybe someone has
>> seen something like this before (I am new to EEG analysis):
>>
>> I did a fast Fourier transform on 6 second long segments and calculated
>> mean power values (averaged over electrodes, participants and 15 segments
>> each). What perplexes me is the power peak at approx. 38 Hz:
>>
>>
>>
>> My steps for data preparation were as follows:
>>
>>    1. band-pass filter of data (1 to 45 Hz)
>>    2. resampling from 512 Hz to 200 Hz
>>    3. cut recording into consecutive segments of 6 sec duration
>>    4. exclude segment if maximum allowed voltage step of 50 µV is exceeded
>>    5. exclude segment if activity is lower than 0.5 µV
>>    6. exclude segment if maximum and minimum amplitude exceed +/- 200 µV
>>    7. exclude segment if maximum absolute difference of values in the
>>    segment > 200 µV
>>    8. retain only segments that are ok for all electrodes
>>    9. randomly choose 15 such good segments (all electrodes) from every
>>    participant
>>    10. calculate FFT for every 2 sec time segment for every electrode and
>>    participant
>>    11. calculate mean of all these FFTs
>>    12. plot
>>
>> Has someone seen a similar peak before?
>>
>> Thanks for any help and advice.
>>
>> Nico
>>
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