[Eeglablist] help with power spectral density

Jerry Zhu jerryzhu at siu.edu
Fri Feb 6 14:01:20 PST 2015

Hi Makoto,
Thanks for your script!  I have been using it for our data analysis. But I
came to realized that the calculation might be a bit different.  Here is
how:

the output from pop_spectopo or spectopo is in unit of 10*log10(absolute
power) for each freq bin (say bins b1, b2, b3...  and their corresponding
absolute powers are x1, x2, x3...)

when averaging them directly as in mean(spectra()) that is essentially:
(10*log10(x1) + 10*log10(x2) + 10*log10(x3) + ... )/n =
10*log10(x1*x2*x3...)/n  (because the rule of log, plus becomes
multiplication)

Ideally, this would be 10*log10( (x1+x2+x3+...)/n )

-----------------Real example----------------
The difference of the calculations is smaller with my real data (directly
from spectopo function):
m = [
-11.0966824712145    -12.2067403734393    -13.1537220992757
-13.7874266498803    -14.1237495677973    -14.0469685506013
-13.6313946102344    -13.7130254954176    -14.5080218939225
-15.2868369198729    -15.3919990544936    -15.3197288848941]
the mean of above numbers directly is -13.8555, convert to absolute power
is 10^(-13.8555/10) = 0.0412

if we convert each number to absolute power first and then calculate mean
would be mean(10.^(m/10)) =  0.0429

Hope this makes sense to you.  whew! ...

Can you comment on that?  Thanks!

Jerry

--
Jian Zhu, M.A., Ph.D candidate
Department of Psychology
Southern Illinois University Carbondale
Web: http://zhupsy.com

We have two halves in the brain: left and right. Nothing is right in the
left. Nothing is left in the right.

On Sun, Nov 16, 2014 at 3:54 PM, Makoto Miyakoshi <mmiyakoshi at ucsd.edu>
wrote:

> Dear Omar,
>
> Sorry for slow response.
> You have to use the spectopo() function in the command line to obtain the
> raw values. There were related posts in the past in which I have been
> repeatedly showing the following. Hope it helps.
>
> %%%%%%%%%%%%%%%%%%%
> % for your epoched data, channel 2
> [spectra,freqs] = spectopo(EEG.data(2,:,:), 0, EEG.srate);
>
> % delta=1-4, theta=4-8, alpha=8-13, beta=13-30, gamma=30-80
> deltaIdx = find(freqs>1 & freqs<4);
> thetaIdx = find(freqs>4 & freqs<8);
> alphaIdx = find(freqs>8 & freqs<13);
> betaIdx  = find(freqs>13 & freqs<30);
> gammaIdx = find(freqs>30 & freqs<80);
>
> % compute absolute power
> deltaPower = 10^(mean(spectra(deltaIdx))/10);
> thetaPower = 10^(mean(spectra(thetaIdx))/10);
> alphaPower = 10^(mean(spectra(alphaIdx))/10);
> betaPower  = 10^(mean(spectra(betaIdx))/10);
> gammaPower = 10^(mean(spectra(gammaIdx))/10);
> %%%%%%%%%%%%%%%%%
>
> If you don't want to code, maybe you want to try the following toolbox.
> http://www.poil.dk/s/#.VDRGNZTWXNU
>
> Makoto
>
> On Mon, Nov 3, 2014 at 12:24 PM, Omar Ortega <tourette95 at gmail.com> wrote:
>
>> Hi there!
>>
>> I have a question about computing psd,
>>
>> I loaded an neuroscan .ee file and I wan to compute the power spectral
>> density, I saw in webpage that is possible to do that by using the plot
>> function, the problemm is:
>>
>> I clicked on plot function, but how can i get that into numerical data?
>>
>> I need to export the psd in text file in order to work whit it, so first
>> i hope using plot function from eeglab was enough to get the psd.
>>
>>
>>
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>
>
>
> --
> Makoto Miyakoshi
> Swartz Center for Computational Neuroscience
> Institute for Neural Computation, University of California San Diego
>
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