<div dir="ltr"><div>Hi Pete,<br><br>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!<br><br></div>James<br><div><div><div class="gmail_extra">
<br><br><div class="gmail_quote">On Mon, May 12, 2014 at 4:32 PM, <span dir="ltr"><<a href="mailto:eeglablist-request@sccn.ucsd.edu" target="_blank">eeglablist-request@sccn.ucsd.edu</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<br>Today's Topics:<br>
<br>
1. Re: triangular / sawtooth / zig-zag pattern in spectral data<br>
(Bachman, Peter)<br>
2. Re: Cross Channel Coherence analysis (Makoto Miyakoshi)<br>
3. Re: triangular / sawtooth / zig-zag pattern in spectral data<br>
(Iman M.Rezazadeh)<br>
4. Re: triangular / sawtooth / zig-zag pattern in spectral data<br>
(Makoto Miyakoshi)<br>
<br><br>---------- Forwarded message ----------<br>From: "Bachman, Peter" <<a href="mailto:bachman@psych.ucla.edu">bachman@psych.ucla.edu</a>><br>To: Matt Craddock <<a href="mailto:matt.craddock@uni-leipzig.de">matt.craddock@uni-leipzig.de</a>>, "<a href="mailto:eeglablist@sccn.ucsd.edu">eeglablist@sccn.ucsd.edu</a>" <<a href="mailto:eeglablist@sccn.ucsd.edu">eeglablist@sccn.ucsd.edu</a>><br>
Cc: <br>Date: Mon, 12 May 2014 19:03:47 +0000<br>Subject: Re: [Eeglablist] triangular / sawtooth / zig-zag pattern in spectral data<br>Thanks, Matt! That would also explain why my results look fairly normal, after epoching (and demeaning), and other offline processing. I appreciate the suggestion.<br>
<br>
Thanks,<br>
Pete<br>
________________________________________<br>
From: Matt Craddock [<a href="mailto:matt.craddock@uni-leipzig.de">matt.craddock@uni-leipzig.de</a>]<br>
Sent: Monday, May 12, 2014 11:31 AM<br>
To: Bachman, Peter; <a href="mailto:eeglablist@sccn.ucsd.edu">eeglablist@sccn.ucsd.edu</a><br>
Subject: Re: [Eeglablist] triangular / sawtooth / zig-zag pattern in spectral data<br>
<br>
On 12/05/2014 18:53, Bachman, Peter wrote:<br>
> Hi everyone,<br>
><br>
><br>
> When I plot spectral data from continuous EEG, the resulting<br>
> Power-by-Frequency graph looks very strange. Specifically, starting at<br>
> about 5 Hz and continuing on to higher frequencies, the plots for each<br>
> channel have a repeating triangular, or sawtooth-like, or zig-zag shape.<br>
><br>
><br>
> Here is a Dropbox link to an image of representative data:<br>
> <a href="https://www.dropbox.com/s/lddxgzlti9m20vj/FFT_triangle_pattern.JPG" target="_blank">https://www.dropbox.com/s/lddxgzlti9m20vj/FFT_triangle_pattern.JPG</a><br>
><br>
><br>
> It looks like some kind of wacky filter has been applied, but the only<br>
> filter in the processing pipeline is a very conventional 0.16-100 Hz<br>
> bandpass.<br>
><br>
><br>
> The problem isn't a feature of the raw data itself, because opening the<br>
> same continuous data in different EEG analysis programs and applying a<br>
> FFT produces very normal looking spectra. This happens across different<br>
> versions of EEGLAB, Matlab, and Windows.<br>
><br>
><br>
> The continuous data were recorded using a Biosemi system and saved in<br>
> .bdf format.<br>
><br>
><br>
> Has anyone run into this before? I’d be very grateful for any suggestions.<br>
><br>
><br>
> Thanks!<br>
><br>
> Pete<br>
><br>
><br>
<br>
Hi Pete,<br>
<br>
This is typical with Biosemi. Here's an old post from Bradley Voytek<br>
about this very issue:<br>
<br>
<a href="http://sccn.ucsd.edu/pipermail/eeglablist/2008/002229.html" target="_blank">http://sccn.ucsd.edu/pipermail/eeglablist/2008/002229.html</a><br>
<br>
It's probably some sort of rounding error somewhere down the line. He<br>
suggests removing the mean from each channel during import. I tried that<br>
and it didn't solve the issue; I think sometimes demeaning based on the<br>
full continuous data might not work if there's an abrupt shift in offset<br>
somewhere in the data - the abrupt shift means the data either side of<br>
the shift isn't centred on zero as it should be after subtraction of the<br>
mean. Anyway, if you're planning on epoching the data, baseline<br>
correction will also solve the problem, in my experience.<br>
<br>
Cheers,<br>
Matt<br>
<br>
--<br>
Dr. Matt Craddock<br>
Research Fellow<br>
Institute of Psychological Sciences<br>
University of Leeds<br>
<br>
<br>
<br><br>---------- Forwarded message ----------<br>From: Makoto Miyakoshi <<a href="mailto:mmiyakoshi@ucsd.edu">mmiyakoshi@ucsd.edu</a>><br>To: 张雪 <<a href="mailto:zhangxue@psych.ac.cn">zhangxue@psych.ac.cn</a>><br>
Cc: EEGLAB List <<a href="mailto:eeglablist@sccn.ucsd.edu">eeglablist@sccn.ucsd.edu</a>><br>Date: Mon, 12 May 2014 12:40:32 -0700<br>Subject: Re: [Eeglablist] Cross Channel Coherence analysis<br><div dir="ltr">Dear Xue,<div>
<br></div><div>> I want to know whether could I just treat different channels as different trials in the newtimef. </div><div><br></div><div>Yes. EEGLAB also has crosstimef() function which you want to check out (there is GUI menu for it too).</div>
<div><br></div><div>> And another question is whether the ITC values of different conditions could be averaged all subtracted directly.<br></div><div><br></div><div>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).</div>
<div><br></div><div>Makoto</div><div class="gmail_extra"><br><br><div class="gmail_quote">2014-05-09 8:10 GMT-07:00 张雪 <span dir="ltr"><<a href="mailto:zhangxue@psych.ac.cn" target="_blank">zhangxue@psych.ac.cn</a>></span>:<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex"><br><br>Dear all<span></span><div> 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?</div>
<div>Thanks</div><div>Zhang Xue</div><div>Psychological institute,</div><div>Chinese Academy of Science</div><div><br></div><br><br><br><br>_______________________________________________<br>
Eeglablist page: <a href="http://sccn.ucsd.edu/eeglab/eeglabmail.html" target="_blank">http://sccn.ucsd.edu/eeglab/eeglabmail.html</a><br>
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<div><br></div>
-- <br><div dir="ltr">Makoto Miyakoshi<br>Swartz Center for Computational Neuroscience<br>Institute for Neural Computation, University of California San Diego<br></div>
</div></div>
<br><br>---------- Forwarded message ----------<br>From: Iman M.Rezazadeh <<a href="mailto:irezazadeh@ucdavis.edu">irezazadeh@ucdavis.edu</a>><br>To: "'Bachman, Peter'" <<a href="mailto:bachman@psych.ucla.edu">bachman@psych.ucla.edu</a>>, 'Matt Craddock' <<a href="mailto:matt.craddock@uni-leipzig.de">matt.craddock@uni-leipzig.de</a>>, <<a href="mailto:eeglablist@sccn.ucsd.edu">eeglablist@sccn.ucsd.edu</a>><br>
Cc: <br>Date: Mon, 12 May 2014 12:34:35 -0700<br>Subject: Re: [Eeglablist] triangular / sawtooth / zig-zag pattern in spectral data<br>Hi Pete,<br>
In general ripples in Fourier transforms mean there are some sudden changes<br>
in the data for example the Fourier transform square wave is a Sinc wave (<br>
like a Guassian with a lot of ripples- Gibb's phenomena). Anyhow, could you<br>
try to filter your data with a low pass filter to see if you still see the<br>
sawtooth stuff?<br>
Best<br>
Iman<br>
<br>
-----Original Message-----<br>
From: <a href="mailto:eeglablist-bounces@sccn.ucsd.edu">eeglablist-bounces@sccn.ucsd.edu</a><br>
[mailto:<a href="mailto:eeglablist-bounces@sccn.ucsd.edu">eeglablist-bounces@sccn.ucsd.edu</a>] On Behalf Of Bachman, Peter<br>
Sent: Monday, May 12, 2014 12:04 PM<br>
To: Matt Craddock; <a href="mailto:eeglablist@sccn.ucsd.edu">eeglablist@sccn.ucsd.edu</a><br>
Subject: Re: [Eeglablist] triangular / sawtooth / zig-zag pattern in<br>
spectral data<br>
<br>
Thanks, Matt! That would also explain why my results look fairly normal,<br>
after epoching (and demeaning), and other offline processing. I appreciate<br>
the suggestion.<br>
<br>
Thanks,<br>
Pete<br>
________________________________________<br>
From: Matt Craddock [<a href="mailto:matt.craddock@uni-leipzig.de">matt.craddock@uni-leipzig.de</a>]<br>
Sent: Monday, May 12, 2014 11:31 AM<br>
To: Bachman, Peter; <a href="mailto:eeglablist@sccn.ucsd.edu">eeglablist@sccn.ucsd.edu</a><br>
Subject: Re: [Eeglablist] triangular / sawtooth / zig-zag pattern in<br>
spectral data<br>
<br>
On 12/05/2014 18:53, Bachman, Peter wrote:<br>
> Hi everyone,<br>
><br>
><br>
> When I plot spectral data from continuous EEG, the resulting<br>
> Power-by-Frequency graph looks very strange. Specifically, starting<br>
> at about 5 Hz and continuing on to higher frequencies, the plots for<br>
> each channel have a repeating triangular, or sawtooth-like, or zig-zag<br>
shape.<br>
><br>
><br>
> Here is a Dropbox link to an image of representative data:<br>
> <a href="https://www.dropbox.com/s/lddxgzlti9m20vj/FFT_triangle_pattern.JPG" target="_blank">https://www.dropbox.com/s/lddxgzlti9m20vj/FFT_triangle_pattern.JPG</a><br>
><br>
><br>
> It looks like some kind of wacky filter has been applied, but the only<br>
> filter in the processing pipeline is a very conventional 0.16-100 Hz<br>
> bandpass.<br>
><br>
><br>
> The problem isn't a feature of the raw data itself, because opening<br>
> the same continuous data in different EEG analysis programs and<br>
> applying a FFT produces very normal looking spectra. This happens<br>
> across different versions of EEGLAB, Matlab, and Windows.<br>
><br>
><br>
> The continuous data were recorded using a Biosemi system and saved in<br>
> .bdf format.<br>
><br>
><br>
> Has anyone run into this before? I'd be very grateful for any<br>
suggestions.<br>
><br>
><br>
> Thanks!<br>
><br>
> Pete<br>
><br>
><br>
<br>
Hi Pete,<br>
<br>
This is typical with Biosemi. Here's an old post from Bradley Voytek about<br>
this very issue:<br>
<br>
<a href="http://sccn.ucsd.edu/pipermail/eeglablist/2008/002229.html" target="_blank">http://sccn.ucsd.edu/pipermail/eeglablist/2008/002229.html</a><br>
<br>
It's probably some sort of rounding error somewhere down the line. He<br>
suggests removing the mean from each channel during import. I tried that and<br>
it didn't solve the issue; I think sometimes demeaning based on the full<br>
continuous data might not work if there's an abrupt shift in offset<br>
somewhere in the data - the abrupt shift means the data either side of the<br>
shift isn't centred on zero as it should be after subtraction of the mean.<br>
Anyway, if you're planning on epoching the data, baseline correction will<br>
also solve the problem, in my experience.<br>
<br>
Cheers,<br>
Matt<br>
<br>
--<br>
Dr. Matt Craddock<br>
Research Fellow<br>
Institute of Psychological Sciences<br>
University of Leeds<br>
<br>
_______________________________________________<br>
Eeglablist page: <a href="http://sccn.ucsd.edu/eeglab/eeglabmail.html" target="_blank">http://sccn.ucsd.edu/eeglab/eeglabmail.html</a><br>
To unsubscribe, send an empty email to <a href="mailto:eeglablist-unsubscribe@sccn.ucsd.edu">eeglablist-unsubscribe@sccn.ucsd.edu</a><br>
For digest mode, send an email with the subject "set digest mime" to<br>
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<br>
<br>
<br><br>---------- Forwarded message ----------<br>From: Makoto Miyakoshi <<a href="mailto:mmiyakoshi@ucsd.edu">mmiyakoshi@ucsd.edu</a>><br>To: Iman M.Rezazadeh <<a href="mailto:irezazadeh@ucdavis.edu">irezazadeh@ucdavis.edu</a>><br>
Cc: EEGLAB List <<a href="mailto:eeglablist@sccn.ucsd.edu">eeglablist@sccn.ucsd.edu</a>>, "Bachman, Peter" <<a href="mailto:bachman@psych.ucla.edu">bachman@psych.ucla.edu</a>><br>Date: Mon, 12 May 2014 13:31:38 -0700<br>
Subject: Re: [Eeglablist] triangular / sawtooth / zig-zag pattern in spectral data<br><div dir="ltr">Dear Peter,<div><br></div><div>Try trimOutlier plugin to visualize your whole channel whole datapoints in figure.</div><div>
<a href="http://sccn.ucsd.edu/wiki/Plugin_list_process" target="_blank">http://sccn.ucsd.edu/wiki/Plugin_list_process</a><br>
</div><div>If you have an outlier, trimOutlier plot should show it to you.</div><div><br></div><div>Makoto</div></div><div class="gmail_extra"><br><br><div class="gmail_quote">2014-05-12 12:34 GMT-07:00 Iman M.Rezazadeh <span dir="ltr"><<a href="mailto:irezazadeh@ucdavis.edu" target="_blank">irezazadeh@ucdavis.edu</a>></span>:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hi Pete,<br>
In general ripples in Fourier transforms mean there are some sudden changes<br>
in the data for example the Fourier transform square wave is a Sinc wave (<br>
like a Guassian with a lot of ripples- Gibb's phenomena). Anyhow, could you<br>
try to filter your data with a low pass filter to see if you still see the<br>
sawtooth stuff?<br>
Best<br>
<span><font color="#888888">Iman<br>
</font></span><div><div><br>
-----Original Message-----<br>
From: <a href="mailto:eeglablist-bounces@sccn.ucsd.edu" target="_blank">eeglablist-bounces@sccn.ucsd.edu</a><br>
[mailto:<a href="mailto:eeglablist-bounces@sccn.ucsd.edu" target="_blank">eeglablist-bounces@sccn.ucsd.edu</a>] On Behalf Of Bachman, Peter<br>
Sent: Monday, May 12, 2014 12:04 PM<br>
To: Matt Craddock; <a href="mailto:eeglablist@sccn.ucsd.edu" target="_blank">eeglablist@sccn.ucsd.edu</a><br>
Subject: Re: [Eeglablist] triangular / sawtooth / zig-zag pattern in<br>
spectral data<br>
<br>
Thanks, Matt! That would also explain why my results look fairly normal,<br>
after epoching (and demeaning), and other offline processing. I appreciate<br>
the suggestion.<br>
<br>
Thanks,<br>
Pete<br>
________________________________________<br>
From: Matt Craddock [<a href="mailto:matt.craddock@uni-leipzig.de" target="_blank">matt.craddock@uni-leipzig.de</a>]<br>
Sent: Monday, May 12, 2014 11:31 AM<br>
To: Bachman, Peter; <a href="mailto:eeglablist@sccn.ucsd.edu" target="_blank">eeglablist@sccn.ucsd.edu</a><br>
Subject: Re: [Eeglablist] triangular / sawtooth / zig-zag pattern in<br>
spectral data<br>
<br>
On 12/05/2014 18:53, Bachman, Peter wrote:<br>
> Hi everyone,<br>
><br>
><br>
> When I plot spectral data from continuous EEG, the resulting<br>
> Power-by-Frequency graph looks very strange. Specifically, starting<br>
> at about 5 Hz and continuing on to higher frequencies, the plots for<br>
> each channel have a repeating triangular, or sawtooth-like, or zig-zag<br>
shape.<br>
><br>
><br>
> Here is a Dropbox link to an image of representative data:<br>
> <a href="https://www.dropbox.com/s/lddxgzlti9m20vj/FFT_triangle_pattern.JPG" target="_blank">https://www.dropbox.com/s/lddxgzlti9m20vj/FFT_triangle_pattern.JPG</a><br>
><br>
><br>
> It looks like some kind of wacky filter has been applied, but the only<br>
> filter in the processing pipeline is a very conventional 0.16-100 Hz<br>
> bandpass.<br>
><br>
><br>
> The problem isn't a feature of the raw data itself, because opening<br>
> the same continuous data in different EEG analysis programs and<br>
> applying a FFT produces very normal looking spectra. This happens<br>
> across different versions of EEGLAB, Matlab, and Windows.<br>
><br>
><br>
> The continuous data were recorded using a Biosemi system and saved in<br>
> .bdf format.<br>
><br>
><br>
> Has anyone run into this before? I'd be very grateful for any<br>
suggestions.<br>
><br>
><br>
> Thanks!<br>
><br>
> Pete<br>
><br>
><br>
<br>
Hi Pete,<br>
<br>
This is typical with Biosemi. Here's an old post from Bradley Voytek about<br>
this very issue:<br>
<br>
<a href="http://sccn.ucsd.edu/pipermail/eeglablist/2008/002229.html" target="_blank">http://sccn.ucsd.edu/pipermail/eeglablist/2008/002229.html</a><br>
<br>
It's probably some sort of rounding error somewhere down the line. He<br>
suggests removing the mean from each channel during import. I tried that and<br>
it didn't solve the issue; I think sometimes demeaning based on the full<br>
continuous data might not work if there's an abrupt shift in offset<br>
somewhere in the data - the abrupt shift means the data either side of the<br>
shift isn't centred on zero as it should be after subtraction of the mean.<br>
Anyway, if you're planning on epoching the data, baseline correction will<br>
also solve the problem, in my experience.<br>
<br>
Cheers,<br>
Matt<br>
<br>
--<br>
Dr. Matt Craddock<br>
Research Fellow<br>
Institute of Psychological Sciences<br>
University of Leeds<br>
<br>
_______________________________________________<br>
Eeglablist page: <a href="http://sccn.ucsd.edu/eeglab/eeglabmail.html" target="_blank">http://sccn.ucsd.edu/eeglab/eeglabmail.html</a><br>
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<br>
_______________________________________________<br>
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</div></div></blockquote></div><br><br clear="all"><div><br></div>-- <br><div dir="ltr">Makoto Miyakoshi<br>Swartz Center for Computational Neuroscience<br>Institute for Neural Computation, University of California San Diego<br>
</div>
</div>
<br>_______________________________________________<br>
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Eeglablist page: <a href="http://www.sccn.ucsd.edu/eeglab/eeglabmail.html" target="_blank">http://www.sccn.ucsd.edu/eeglab/eeglabmail.html</a><br>
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Laboratory Manager<br>Human Development EEG and Psychophysiology (HEP) Laboratory,<div>Department of Human Development,<br>--------------------------------------------<br>Cornell University | Ithaca, NY<br></div><div>607-255-9883</div>
<div><a href="mailto:eeg@cornell.edu" target="_blank">eeg@cornell.edu</a></div></div>
</div></div></div></div>