[Mne_analysis] Power peak at 38 Hz?

Nico Adelhöfer nico.adelhoefer at st.ovgu.de
Sun Sep 4 11:31:06 EDT 2016
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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:

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>:

> 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>
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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