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

Makoto Miyakoshi mmiyakoshi at ucsd.edu
Mon May 12 13:31:38 PDT 2014


Dear Peter,

Try trimOutlier plugin to visualize your whole channel whole datapoints in
figure.
http://sccn.ucsd.edu/wiki/Plugin_list_process
If you have an outlier, trimOutlier plot should show it to you.

Makoto


2014-05-12 12:34 GMT-07:00 Iman M.Rezazadeh <irezazadeh at ucdavis.edu>:

> Hi Pete,
> In general ripples in Fourier transforms mean there are some sudden changes
> in the data for example the Fourier transform square wave is a Sinc wave (
> like a Guassian with a lot of ripples- Gibb's phenomena). Anyhow, could you
> try to filter your data with a low pass filter to see if you still see the
> sawtooth stuff?
> Best
> Iman
>
> -----Original Message-----
> From: eeglablist-bounces at sccn.ucsd.edu
> [mailto:eeglablist-bounces at sccn.ucsd.edu] On Behalf Of Bachman, Peter
> Sent: Monday, May 12, 2014 12:04 PM
> To: Matt Craddock; eeglablist at sccn.ucsd.edu
> Subject: Re: [Eeglablist] triangular / sawtooth / zig-zag pattern in
> spectral data
>
> Thanks, Matt!  That would also explain why my results look fairly normal,
> after epoching (and demeaning), and other offline processing.  I appreciate
> the suggestion.
>
> Thanks,
> Pete
> ________________________________________
> From: Matt Craddock [matt.craddock at uni-leipzig.de]
> Sent: Monday, May 12, 2014 11:31 AM
> To: Bachman, Peter; eeglablist at sccn.ucsd.edu
> Subject: Re: [Eeglablist] triangular / sawtooth / zig-zag pattern in
> spectral data
>
> On 12/05/2014 18:53, Bachman, Peter wrote:
> > 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
> >
> >
>
> Hi Pete,
>
> This is typical with Biosemi. Here's an old post from Bradley Voytek about
> this very issue:
>
> http://sccn.ucsd.edu/pipermail/eeglablist/2008/002229.html
>
> It's probably some sort of rounding error somewhere down the line. He
> suggests removing the mean from each channel during import. I tried that
> and
> it didn't solve the issue; I think sometimes demeaning based on the full
> continuous data might not work if there's an abrupt shift in offset
> somewhere in the data - the abrupt shift means the data either side of the
> shift isn't centred on zero as it should be after subtraction of the mean.
> Anyway, if you're planning on epoching the data, baseline correction will
> also solve the problem, in my experience.
>
> Cheers,
> Matt
>
> --
> Dr. Matt Craddock
> Research Fellow
> Institute of Psychological Sciences
> University of Leeds
>
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-- 
Makoto Miyakoshi
Swartz Center for Computational Neuroscience
Institute for Neural Computation, University of California San Diego
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