[Eeglablist] Resampling "artefacts" 512-500 Hz

[Ziereis, Annika Carina]

Dear all,

(I'd like to say beforehand that I am a total beginner in signal processing... so if you have an idea why this happened I'd be very happy if you'd consider it in your explanation)

For the EEG recording we use a sampling rate of 512 Hz which we want to down-sample to 500 Hz (we do simultaneous eye-tracking which records with 500 Hz and we want to do a co-registration).

During the pre-processing I looked at the power spectral density and found a power increase for the harmonics of 12 Hz (so peaks at 12, 24, 36 ... Hz) after resampling to 500 Hz. These peaks were not visible in the  power spectral density plot of the raw data.
It was especially pronounced for participants with a higher alpha power  and not so much for those with low alpha power.
I played with some resampling frequencies to check how it looks like for 499 Hz, 501 Hz 497 Hz... and 256 Hz (to compare to a integer ratio).
The "artefacts" only occur  for 499, 500 and 501 Hz-showing each the harmonics of the difference in Hz
e.g. for 499 Hz (which would be 13Hz less than the original sampling rate) I get the harmonic peaks for 13, 26 and so on.
It disappears for Frequencies that are not in the alpha range (e.g. 15 Hz, for resampling to 497 Hz).

Then I read that one could do low-pass filtering to avoid aliasing effects (which I have to admit- I don' t understand very well). So I tried to low-pass filtering of 40 Hz  (I just used 40 Hz because we did so in another study) and do the resampling again. This resulted in the same harmonics and super strange peaks in the filtered out frequencies.
I uploaded a pdf with pictures here (hopefully, it makes it more clear what I mean): https://github.com/aziereis/eegquestions/blob/master/downsampling.pdf


Has anybody encountered this problem before?
Is this artefact problematic?
Thank you very much for your help!!

Best
Annika



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Annika Carina Ziereis

Georg-Elias-Müller Institute for Psychology
- Affective Neuroscience and Psychophysiology -
University Göttingen
Goßlerstr. 14
37073 Göttingen

Phone +49 551 39 20623

Email: annika.ziereis at uni-goettingen.de