[Eeglablist] biosemi zig-zag pattern as a visualization issue? (eeglab13_4_4b vs eeglab14_0_0b)
widmann at uni-leipzig.de
Thu Mar 23 09:05:55 PDT 2017
> I was using EEGlab (eeglab13_4_4b) to load Biosemi data and plotting the power spectrum when I noted a repeating triangular, zig-zag shape, and I found the following discussion of the list,
> [Eeglablist] triangular / sawtooth / zig-zag pattern in spectral data
> [Eeglablist] EEGLAB, BioSemi, BioSig, and means
> The issue is that if I load my data with eeglab13_6_5b or eeglab14_0_0b, I don't find the zig-zag shape anymore. In fact the data obtained is exactly the same using any of these EEGlab versions (that's good news, it couldn't be otherwise!),
> This suggest that this issue is a visualization problem. Could it be?
Rather a change of analysis parameter defaults aiming to reduce spectral leakage. In particular window size now defaults to sampling rate and FFT size to window size avoiding zero padding unless necessary. There were several relevant changes between Nov 2015 to June 2016 (https://bitbucket.org/sccn_eeglab/eeglab/history-node/85265b6245b8/functions/sigprocfunc/spectopo.m?at=develop). Not sure which EEGLAB versions reflect the changes though.
> So, is not solved by applying a high-pass filter, maybe applying the filter change the visualization, or something like that?
I recommend DC correcting or applying a high-pass filter anyway for BioSemi or any other data with a strong DC offset as there will be still leakage for the lower frequencies.
Hope this helps,
> Could any of the developers comment on this, just for the sake of understanding,
> Maybe this a bit old, but I haven't find more recent discussion about this problem,
> Thanks in advance for any hint,
> José Luis ULLOA FULGERI, PhD
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