Hi Arjen,<br><br>my point is that I am interested not in the relative difference between signals but in the actual difference in microvolt. I know that this is not the standard thing to do but that is what I need .<br><br>
After playing aroud a bit i come to the following conlcusion <br><br>Lets say I have two signals (pure sine 20 Hz; sampling freq 512) one with an amplitude of 100uV and one with an aplitude of 200uV. <br>Computing a power spectrum using matlab fft function I can eventually read out that the difference between my two signals is 100uV. <br>
<br>In ft_freqanalysis_mtmfft the signal is multiplied by a taper<br>line 340 dum=fft([data.trial{perlop}(sgnindx(sgnlop),:).*tap(taplop,:) pad],[],2);<br><br>The output can hence not be transformed back right away to the real microvolt values, but happens to be scaled (in my case ) by a factor of ~0.036 (which is the mean value of the taper)<br>
<br>At least that is what I think happens. <br><br>Regards<br>Martin<br><br><br><br><br><br><div class="gmail_quote">2010/8/13 <a href="mailto:a.stolk@fcdonders.ru.nl" target="_blank">a.stolk@fcdonders.ru.nl</a> <span dir="ltr"><<a href="mailto:a.stolk@fcdonders.ru.nl" target="_blank">a.stolk@fcdonders.ru.nl</a>></span><br>
<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
If you use the same mtmfft settings for both conditions, in theory it should be fine. Make sure that you compute the fourier transformation on equal window lenghts.<br>
<br>
If you happen to have variable triallengths per condition, my suggestion would be to cut the trials in (e.g.) 1 second pieces. This way your 'frequency resolution' is the same for condition A and B.<br>
<br>
If your experimental setup allows you, you could choose to normalize your data (e.g. a baselinecorrection) first before making the A and B comparison. This way you correct for (non-)cognitive powerchanges over time (case it's not an ABABABAB etc. design). Such as conductivity reducal of the EEG paste over time, subject habituation effects and whatelse you have.<br>
<br>
Best,<br>
<div>Arjen<br>
<br>
<br>
<br>
<br>
----- Original Message -----<br>
From: "Martin Bleichner" <<a href="mailto:bleichner.martin@GMAIL.COM" target="_blank">bleichner.martin@GMAIL.COM</a>><br>
To: <a href="mailto:FIELDTRIP@NIC.SURFNET.NL" target="_blank">FIELDTRIP@NIC.SURFNET.NL</a><br>
</div><div>Sent: Thursday, August 12, 2010 10:48:14 PM GMT +01:00 Amsterdam / Berlin / Bern / Rome / Stockholm / Vienna<br>
Subject: Re: [FIELDTRIP] Units powspctrm<br>
<br>
Yes, typically, but is it the case for the powspctrm you get from the mtmfft<br>
freqanalysis?<br>
I am asking for the following reason:<br>
<br>
Lets say, I have two experimental conditions A and B and I compute the<br>
powerspectra for both of them.<br>
When the units actually are uV^2/Hz i could tell for each frequency by how<br>
much uV A and B differ.<br>
<br>
What confuses me is that in<br>
</div>ft_freqanalysis_mtmfft<<a href="http://fieldtrip.fcdonders.nl/reference/ft_freqanalysis_mtmfft" target="_blank">http://fieldtrip.fcdonders.nl/reference/ft_freqanalysis_mtmfft</a>>the<br>
<div><div></div><div>signal is multiplied with a taper (tap) and thereby scaled.<br>
The resulting powspctrm hence does not reflect the the true changes in<br>
microvolt.<br>
Do I see that correctly, or am I missing something here? Is the<br>
powerspectrum corrected for that somewhere else?<br>
<br>
Regards,<br>
Martin<br>
<br>
<br>
<br>
2010/8/12 <a href="mailto:a.stolk@fcdonders.ru.nl" target="_blank">a.stolk@fcdonders.ru.nl</a> <<a href="mailto:a.stolk@fcdonders.ru.nl" target="_blank">a.stolk@fcdonders.ru.nl</a>><br>
<br>
> Hello Martin,<br>
><br>
> A powerspectrum typically has the frequencies on one axis and the power of<br>
> the corresponding frequencies on the other. The power represents how strong<br>
> a certain frequency occurs in a signal, given by the square of the voltage<br>
> per frequency.<br>
><br>
> In your case: square of the microvolts per frequency: uV^2/ Hz<br>
><br>
> Best regards,<br>
> Arjen<br>
><br>
> ----- Original Message -----<br>
> From: "Martin Bleichner" <<a href="mailto:bleichner.martin@GMAIL.COM" target="_blank">bleichner.martin@GMAIL.COM</a>><br>
> To: <a href="mailto:FIELDTRIP@NIC.SURFNET.NL" target="_blank">FIELDTRIP@NIC.SURFNET.NL</a><br>
> Sent: Thursday, August 12, 2010 5:52:19 PM GMT +01:00 Amsterdam / Berlin /<br>
> Bern / Rome / Stockholm / Vienna<br>
> Subject: [FIELDTRIP] Units powspctrm<br>
><br>
> Hello together,<br>
><br>
> What units does the powspctrm have?<br>
> Starting with an EEG signal in microvolts and computing the powerspectrum<br>
> using freqanalysis what are the units of powspctrm?<br>
> I tried to figure it out on my own but I am stuck somehow.<br>
><br>
> Here some example code<br>
><br>
> cfg=[];<br>
> cfg.method='mtmfft';<br>
> cfg.output='pow';<br>
> cfg.pad ='maxperlen'<br>
> cfg.foilim=[1 130]<br>
> cfg.taper='hanning'<br>
> cfg.channel=1<br>
> cfg.keeptrials='no'<br>
><br>
> R=ft_freqanalysis(cfg,data);<br>
><br>
> I am interested in the units of R.powspctrm<br>
><br>
> Thanks<br>
> Martin<br>
><br>
> ----------------------------------<br>
> The aim of this list is to facilitate the discussion between users of the<br>
> FieldTrip toolbox, to share experiences and to discuss new ideas for MEG<br>
> and EEG analysis. See also<br>
> <a href="http://listserv.surfnet.nl/archives/fieldtrip.html" target="_blank">http://listserv.surfnet.nl/archives/fieldtrip.html</a> and<br>
> <a href="http://www.ru.nl/neuroimaging/fieldtrip" target="_blank">http://www.ru.nl/neuroimaging/fieldtrip</a>.<br>
><br>
> ----------------------------------<br>
> The aim of this list is to facilitate the discussion between users of the<br>
> FieldTrip toolbox, to share experiences and to discuss new ideas for MEG<br>
> and EEG analysis. See also<br>
> <a href="http://listserv.surfnet.nl/archives/fieldtrip.html" target="_blank">http://listserv.surfnet.nl/archives/fieldtrip.html</a> and<br>
> <a href="http://www.ru.nl/neuroimaging/fieldtrip" target="_blank">http://www.ru.nl/neuroimaging/fieldtrip</a>.<br>
><br>
<br>
----------------------------------<br>
The aim of this list is to facilitate the discussion between users of the FieldTrip toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also <a href="http://listserv.surfnet.nl/archives/fieldtrip.html" target="_blank">http://listserv.surfnet.nl/archives/fieldtrip.html</a> and <a href="http://www.ru.nl/neuroimaging/fieldtrip" target="_blank">http://www.ru.nl/neuroimaging/fieldtrip</a>.<br>
<br>
----------------------------------<br>
The aim of this list is to facilitate the discussion between users of the FieldTrip toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also <a href="http://listserv.surfnet.nl/archives/fieldtrip.html" target="_blank">http://listserv.surfnet.nl/archives/fieldtrip.html</a> and <a href="http://www.ru.nl/neuroimaging/fieldtrip" target="_blank">http://www.ru.nl/neuroimaging/fieldtrip</a>.<br>
</div></div></blockquote></div><br>
<p>----------------------------------</p>
<p>The aim of this list is to facilitate the discussion between users of the FieldTrip toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis.</p>
<p> http://listserv.surfnet.nl/archives/fieldtrip.html</p>
<p> http://www.ru.nl/fcdonders/fieldtrip/</p>