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

[Iman M.Rezazadeh]

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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