I'm pretty new to EEG analysis. I measured EEG on six scalp positions over night. When I plot the power of the frequencies, I get the following:
What I want to explain (and didn't manage to) is the origin of the 38 Hz peak in my case.
The most important steps to arrive at my graph were (using Python-MNE):
- band-pass filter of data (1 to 45 Hz)
- resampling from 512 Hz to 200 Hz
Then for every of the six channels:
- cut recording into consecutive segments of 2 sec duration
exclude segments if:
- maximum allowed voltage step of 50 µV is exceeded
- activity is lower than 0.5 µV
- maximum and minimum amplitude exceed +/- 200 µV
- maximum absolute difference of values in the segment > 200 µV
further:
- retain only segments that are ok for all electrodes
- randomly choose 15 such good segments (all electrodes) from every participant
- calculate FFT for every 2 sec time segment for every electrode and participant
- calculate mean of all these FFTs
- plot
My question is: is someone familiar with such an artifact at 38 Hz? Where could it come from? These are the things I already checked:
- it is pronounced in every participant
- it is pronounced in every electrode
- eye movements are unlikely since this peak is there after filtering out horizontal eye movements via ICA
- electrical devises are unlikely since they should peak at 50 Hz
- it is pronounced when segments are 6 sec instead of 2 sec
- I also used a notch filter at 50 Hz instead of the bandpass filter, with the following result (here the individual 15 segments are shown, but the 38 Hz peak is still there):
Does someone have an idea where this feature could come from?