I am working on some .edf files.
I try to extract the frequency distributions from the EEG waves.
The thing is, which i can't be sure if it is a problem or not, the most dominant frequency appears to be 0.39 hz always.
I can remove/change it with bandpass or other filters, but i am not sure if i am missing some important point here.
I used to work on patch-clamp data and this is my first time working with EEG data, just checking the waters for a possible collaboration. I can't be sure if this 0.39 hz is a noise, or something that i do wrong, or is it just how Welch - Fourier works.
I just have the common knowledge that the common low frequency delta waves are 0.5-4 hz and that makes me think. But also, i see on various papers on the internet that the lower-low frequences seems to have a similar peak.
I am sharing single position EEG data from 4 different recordings, although all positions have the similar peak at 0.39 hz. Fs was 200 hz.
Matlab code:
win = hamming(1024);
nfft = 1024;
noverlap = nfft / 2;
[px, f] = pwelch(waveseq, win, noverlap, nfft, Fs);
plot(f, 10 * log10(px))...
If anyone wonders, I saw a paper:Article High Resolution Topography of Age-Related Changes in Non-Rap...
they applied 1-50 hz bandpass filter with their EEGs. Using the bandpass filter like that, my plot looks like theirs, although it felt like cheating and i still can't say where these ultra slow waves come from.
I also heard about things like DC offset, and i tried various methods to remove that (baseline correction etc.), but it didn't change the 0.39 hz dominance.
My data is polysomnography data, so mostly n-rem sleep. Like the article i cited above. With n-rem, I see similar shapes on other studies, but also very different ones too.
I also saw that delta waves can also be considered in range of 0.3-5 hz, or 0-4 hz even... So maybe it can be about interference and delta predominance in n-rem sleep both...
Thanks for the input.
Strictly speaking, I think the delta band is 0-4 Hz. However, it's generally a good practice to eliminate very low frequencies due to the aforementioned issues. Although I have no direct experience with polysomnography data, considering what you mentioned about the article, your EEG data might be as expected. To decide whether to eliminate frequencies below 0.5 Hz, it could be helpful to review other articles to see what they usually report as important frequencies.
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