<div>Eelke,</div><div><br></div><div>I know this is a slow reply, but I was waiting to see if someone would comment on Joe's questions. Anyway, thank you for your explanation. It makes sense that the subtraction would control for 1/f within each frequency. However, I am curious about the points that Joe brought up and I hope someone can still comment on them.</div>
<div><br></div><div>Joe,</div><div><br></div>Your first point seems quite important, especially when averaging across a range of frequencies is a common thing to do in the literature. Does anyone know if it's correct that higher frequencies will get washed out by lower ones when averaging within a frequency band?<div>
<br></div><div>To add to Joe's questions, could normalizing power ever be a bad thing to do? It seems like it would be reasonable for FieldTrip to at least have the option so that one could use cfg.keeptrials='no' with ft_freqanalysis and would not need to have cfg.keeptrials='yes' for followed by the ft_freqbaseline steps that Stephan mentioned.<div>
<br></div><div>Not sure about your second point regarding spectral density, but I would also like to know more.</div>
<div><br></div><div>Thanks, everyone, for your knowledge in these matters.</div><div>Matt</div><div><div><br></div><div>--<br>Univ. of Colorado at Boulder<br>Dept. of Psychology and Neuroscience<br><a href="mailto:matthew.mollison@colorado.edu" target="_blank">matthew.mollison@colorado.edu</a><br>
<a href="http://psych.colorado.edu/~mollison/" target="_blank">http://psych.colorado.edu/~mollison/</a></div><div><br><div class="gmail_quote">On Tue, Apr 24, 2012 at 12:41 AM, Joseph Dien <span dir="ltr"><<a href="mailto:jdien07@mac.com" target="_blank">jdien07@mac.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
I'm new to spectral analysis so take anything I say with a grain of salt:<br>
<br>
1) If one intends on taking the average of a band (like 8-12Hz for alpha), seems like maybe helpful to correct for 1/f so the lower bands don't dominate?<br>
<br>
2) Another issue is spectral density (correcting for frequency bin width for discrete Fourier). As far as I can tell, FieldTrip isn't doing this. Seems like it should be standard. Or at least it should say in the documentation whether it is being done. Am I wrong?<br>
<br>
Cheers!<br>
<br>
Joe<br>
<div><div><br>
<br>
<br>
On Apr 23, 2012, at 5:54 AM, Eelke Spaak wrote:<br>
<br>
> Hi Matt,<br>
><br>
> When you are comparing power across conditions, it is not really<br>
> necessary to apply an explicit correction for the dominant 1/f<br>
> component of the raw spectrum. Since this 1/f component is present in<br>
> both conditions, when you subtract power in one condition from power<br>
> in another condition (or compute the ratio, or log-ratio, or relative<br>
> change, or whatever), the 1/f will cancel out and you will only be<br>
> left with whatever is due to your experimental manipulation. This is<br>
> true because the contrast is done per frequency. (Note that comparing<br>
> activity versus baseline is just a special case of looking at a<br>
> contrast between conditions, so the same argument holds there.)<br>
><br>
> The only time when an explicit correction for 1/f is useful, is when<br>
> you want to look at raw power. The most dominant oscillatory features<br>
> (visual alpha, visual contrast induced gamma...) will usually be<br>
> evident in raw spectra without such a correction, by the way.<br>
> Correcting for 1/f can be done in many ways, the most simple one is<br>
> simply taking the logarithm of power, something like:<br>
><br>
> freqCorrected = freqUncorrected;<br>
> freqCorrected.powspctrm = log10(freqCorrected.powspctrm);<br>
><br>
> Or you could take the first derivative in the time domain (equivalent<br>
> to multiplying the spectrum with f, search for post by Robert on this<br>
> on the FT list). Or you could take the log of both the frequency- and<br>
> power axes, then fit a line, and subtract it, then transform back<br>
> (10^corrected data).<br>
><br>
> But, the main point is: in the vast majority of typical cognitive<br>
> experiments, correcting for 1/f is not needed.<br>
><br>
> Best,<br>
> Eelke<br>
><br>
> On 23 April 2012 05:54, Matt Mollison <<a href="mailto:matt.mollison@gmail.com" target="_blank">matt.mollison@gmail.com</a>> wrote:<br>
>> Hi FieldTrippers,<br>
>><br>
>> In almost all the papers I've read involving oscillatory power, some kind of<br>
>> transformation is done to the data due to the 1/f power spectrum effect<br>
>> (power decreases as frequency increases). I'm mostly looking at<br>
>> within-subjects experiments (every subject behaved in all conditions)<br>
>> comparing conditions across subjects, but it seems like normalizing the<br>
>> power spectrum should apply in any case (especially if any kind of<br>
>> parametric stats are done—right?).<br>
>><br>
>> Anyway, it's not apparent to me how to use FT functions like ft_freqanalysis<br>
>> to make these transformations (e.g., log10 normalization, dB normalization<br>
>> [EEGLab does this], vector length normalization, etc.; the only thing I see<br>
>> is in ft_sourcedescriptives, but I'm not doing source analyses), and it<br>
>> confuses me why this is the case. I can't find much discussion regarding the<br>
>> 1/f issue on the FT wiki or the mailing list. This seems like an important<br>
>> step that is missing from any frequency analysis workflow. Am I missing<br>
>> something (meaning I just don't see the option), am I misunderstanding<br>
>> something (meaning I'm incorrect in this assumption), or is this an issue<br>
>> that needs to be fixed?<br>
>><br>
>> Thanks,<br>
>> Matt<br>
>><br>
>> --<br>
>> Univ. of Colorado at Boulder<br>
>> Dept. of Psychology and Neuroscience<br>
>> <a href="mailto:matthew.mollison@colorado.edu" target="_blank">matthew.mollison@colorado.edu</a><br>
>> <a href="http://psych.colorado.edu/~mollison/" target="_blank">http://psych.colorado.edu/~mollison/</a><br>
>><br>
>> _______________________________________________<br>
>> fieldtrip mailing list<br>
>> <a href="mailto:fieldtrip@donders.ru.nl" target="_blank">fieldtrip@donders.ru.nl</a><br>
>> <a href="http://mailman.science.ru.nl/mailman/listinfo/fieldtrip" target="_blank">http://mailman.science.ru.nl/mailman/listinfo/fieldtrip</a><br>
><br>
> _______________________________________________<br>
> fieldtrip mailing list<br>
> <a href="mailto:fieldtrip@donders.ru.nl" target="_blank">fieldtrip@donders.ru.nl</a><br>
> <a href="http://mailman.science.ru.nl/mailman/listinfo/fieldtrip" target="_blank">http://mailman.science.ru.nl/mailman/listinfo/fieldtrip</a><br>
<br>
<br>
</div></div>--------------------------------------------------------------------------------<br>
<br>
Joseph Dien,<br>
Senior Research Scientist<br>
University of Maryland<br>
<br>
E-mail: <a href="mailto:jdien07@mac.com" target="_blank">jdien07@mac.com</a><br>
Phone: <a href="tel:301-226-8848" value="+13012268848" target="_blank">301-226-8848</a><br>
Fax: <a href="tel:301-226-8811" value="+13012268811" target="_blank">301-226-8811</a><br>
<a href="http://homepage.mac.com/jdien07/" target="_blank">http://homepage.mac.com/jdien07/</a><br>
<div><div><br>
<br>
<br>
<br>
<br>
<br>
<br>
<br>
<br>
<br>
_______________________________________________<br>
fieldtrip mailing list<br>
<a href="mailto:fieldtrip@donders.ru.nl" target="_blank">fieldtrip@donders.ru.nl</a><br>
<a href="http://mailman.science.ru.nl/mailman/listinfo/fieldtrip" target="_blank">http://mailman.science.ru.nl/mailman/listinfo/fieldtrip</a><br>
</div></div></blockquote></div> <br></div></div>
</div>