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

James Jones-Rounds jj324 at cornell.edu
Mon May 12 15:53:59 PDT 2014


Hi Pete,

Yes, this happens to my BioSemi data also. For me, it goes away once I run
a high-pass 0.1 Hz filter, although I am not entirely sure why!

James


On Mon, May 12, 2014 at 4:32 PM, <eeglablist-request at sccn.ucsd.edu> wrote:

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> Today's Topics:
>
>    1. Re: triangular / sawtooth / zig-zag pattern in spectral data
>       (Bachman, Peter)
>    2. Re: Cross Channel Coherence analysis (Makoto Miyakoshi)
>    3. Re: triangular / sawtooth / zig-zag pattern in    spectral data
>       (Iman M.Rezazadeh)
>    4. Re: triangular / sawtooth / zig-zag pattern in    spectral data
>       (Makoto Miyakoshi)
>
>
> ---------- Forwarded message ----------
> From: "Bachman, Peter" <bachman at psych.ucla.edu>
> To: Matt Craddock <matt.craddock at uni-leipzig.de>, "
> eeglablist at sccn.ucsd.edu" <eeglablist at sccn.ucsd.edu>
> Cc:
> Date: Mon, 12 May 2014 19:03:47 +0000
> 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
>
>
>
>
> ---------- Forwarded message ----------
> From: Makoto Miyakoshi <mmiyakoshi at ucsd.edu>
> To: 张雪 <zhangxue at psych.ac.cn>
> Cc: EEGLAB List <eeglablist at sccn.ucsd.edu>
> Date: Mon, 12 May 2014 12:40:32 -0700
> Subject: Re: [Eeglablist] Cross Channel Coherence analysis
> Dear Xue,
>
> > I want to know whether could I just treat different channels as
> different trials in the newtimef.
>
> Yes. EEGLAB also has crosstimef() function which you want to check out
> (there is GUI menu for it too).
>
> > And another question is whether the ITC values of different conditions
> could be averaged all subtracted directly.
>
> A problem in comparing ITC measures across condition/subjects is that the
> signal to noise ratio depends on the number of trials. If you compare
> 30-trial average and 100-trial average, you'll see significant difference
> allover the plot because 'background' ITC SNR is very different and
> interestingly event-related phase modulation is left as nonsignificant! So
> I would suggest that for fair comparison you do something like subsampling
> to have the same number of trials across conditions for ITC (which people
> don't do very often though).
>
> Makoto
>
>
> 2014-05-09 8:10 GMT-07:00 张雪 <zhangxue at psych.ac.cn>:
>
>>
>>
>> Dear all
>>        I'm trying to do cross-channel coherence which is similar to the
>> inter-trial coherence. I want to know whether could I just treat different
>> channels as different trials in the newtimef. And another question is
>> whether the ITC values of different conditions could be averaged all
>> subtracted directly. Has there anybody ever done this through EEGlab before?
>> Thanks
>> Zhang Xue
>> Psychological institute,
>> Chinese Academy of Science
>>
>>
>>
>>
>>
>> _______________________________________________
>> Eeglablist page: http://sccn.ucsd.edu/eeglab/eeglabmail.html
>> To unsubscribe, send an empty email to
>> eeglablist-unsubscribe at sccn.ucsd.edu
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>
>
>
> --
> Makoto Miyakoshi
> Swartz Center for Computational Neuroscience
> Institute for Neural Computation, University of California San Diego
>
>
> ---------- Forwarded message ----------
> From: Iman M.Rezazadeh <irezazadeh at ucdavis.edu>
> To: "'Bachman, Peter'" <bachman at psych.ucla.edu>, 'Matt Craddock' <
> matt.craddock at uni-leipzig.de>, <eeglablist at sccn.ucsd.edu>
> Cc:
> Date: Mon, 12 May 2014 12:34:35 -0700
> Subject: Re: [Eeglablist] triangular / sawtooth / zig-zag pattern in
> spectral data
> 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
>
> _______________________________________________
> Eeglablist page: http://sccn.ucsd.edu/eeglab/eeglabmail.html
> To unsubscribe, send an empty email to
> eeglablist-unsubscribe at sccn.ucsd.edu
> For digest mode, send an email with the subject "set digest mime" to
> eeglablist-request at sccn.ucsd.edu
>
>
>
>
> ---------- Forwarded message ----------
> From: Makoto Miyakoshi <mmiyakoshi at ucsd.edu>
> To: Iman M.Rezazadeh <irezazadeh at ucdavis.edu>
> Cc: EEGLAB List <eeglablist at sccn.ucsd.edu>, "Bachman, Peter" <
> bachman at psych.ucla.edu>
> Date: Mon, 12 May 2014 13:31:38 -0700
> Subject: Re: [Eeglablist] triangular / sawtooth / zig-zag pattern in
> spectral data
> 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
>>
>> _______________________________________________
>> Eeglablist page: http://sccn.ucsd.edu/eeglab/eeglabmail.html
>> To unsubscribe, send an empty email to
>> eeglablist-unsubscribe at sccn.ucsd.edu
>> For digest mode, send an email with the subject "set digest mime" to
>> eeglablist-request at sccn.ucsd.edu
>>
>> _______________________________________________
>> Eeglablist page: http://sccn.ucsd.edu/eeglab/eeglabmail.html
>> To unsubscribe, send an empty email to
>> eeglablist-unsubscribe at sccn.ucsd.edu
>> For digest mode, send an email with the subject "set digest mime" to
>> eeglablist-request at sccn.ucsd.edu
>>
>
>
>
> --
> Makoto Miyakoshi
> Swartz Center for Computational Neuroscience
> Institute for Neural Computation, University of California San Diego
>
> _______________________________________________
> eeglablist mailing list eeglablist at sccn.ucsd.edu
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>



-- 
James Jones-Rounds
Laboratory Manager
Human Development EEG and Psychophysiology (HEP) Laboratory,
Department of Human Development,
--------------------------------------------
Cornell University | Ithaca, NY
607-255-9883
eeg at cornell.edu
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