[Eeglablist] triangular / sawtooth / zig-zag pattern in spectral data

[Bachman, Peter]

Hi everyone,

When I plot spectral data from continuous EEG, the resulting Power-by-Frequency graph looks very strange.  Specifically, starting at about 5 Hz and continuing on to higher frequencies, the plots for each channel have a repeating triangular, or sawtooth-like, or zig-zag shape.

Here is a Dropbox link to an image of representative data:  https://www.dropbox.com/s/lddxgzlti9m20vj/FFT_triangle_pattern.JPG

It looks like some kind of wacky filter has been applied, but the only filter in the processing pipeline is a very conventional 0.16-100 Hz bandpass.

The problem isn't a feature of the raw data itself, because opening the same continuous data in different EEG analysis programs and applying a FFT produces very normal looking spectra.  This happens across different versions of EEGLAB, Matlab, and Windows.

The continuous data were recorded using a Biosemi system and saved in .bdf format.

Has anyone run into this before?  I’d be very grateful for any suggestions.

Thanks!
Pete
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