[Eeglablist] How can I get the power of each frequency band?

#SHAN XIN# SHAN0055 at e.ntu.edu.sg
Mon Apr 20 03:59:40 PDT 2015

Hi Makoto,

Oh I didn't realize the scale on the figure might already in dB...is it?  I know dB is defined as log ratio, but uV^2/Hz does not seem to be a ratio since the units (volt square and hz) do not match...  Also, do you therefore mean that the definition of absolute power is uV^2/Hz?

I've gone through the related topics in archives and still don't understand these... I also search on Google but still cannot find a clear explanation. Why divided by  Hz?

Thank you so much!!

Shan Xin

#SHAN XIN# <SHAN0055 at e.ntu.edu.sg<mailto:SHAN0055 at e.ntu.edu.sg>> ? Apr 20, 2015 17:29 ???

Dear Makoto,

I have a doubt for the power unit. I read the "help spectopo" in Matlab, for the variable "spectra":

         spectra  = (nchans,nfreqs) power spectra (mean power over epochs), in dB
         freqs    = frequencies of spectra (Hz)

If the unit of variable "spectra" is in dB unit already, then when computing the absolute power, do we still need to do the transformation as you suggested (i.e. 10.^(spectra(deltaIdx)/10))?

I know when plotting the figure, the spectrum is plotted in log scale. But if the unit of spectra is in dB, then there's no need to transform, right?

I don't know if this is correct... Could you look more into it? Thank you so much!!

Shan Xin

From: Makoto Miyakoshi <mmiyakoshi at ucsd.edu<mailto:mmiyakoshi at ucsd.edu>>
Sent: Thursday, 16 April 2015 11:11 AM
Cc: eeglablist at sccn.ucsd.edu<mailto:eeglablist at sccn.ucsd.edu>
Subject: Re: [Eeglablist] How can I get the power of each frequency band?

Dear Shan,

> I'm not very proficient in Matlab programming...

This could be a concern but you can try!

% 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 = mean(10.^(spectra(deltaIdx)/10));
thetaPower = mean(10.^(spectra(thetaIdx)/10));
alphaPower = mean(10.^(spectra(alphaIdx)/10));
betaPower  = mean(10.^(spectra(betaIdx)/10));
gammaPower = mean(10.^(spectra(gammaIdx)/10));


On Tue, Apr 14, 2015 at 6:23 AM, #SHAN XIN# <SHAN0055 at e.ntu.edu.sg<mailto:SHAN0055 at e.ntu.edu.sg>> wrote:

Hi EEGLab team,

Does EEGlab has plugin or script so that I can get the power of individual band (e.g. power of alpha band or beta band) from your computation, instead of just a continuous line in the power spectrum figure?  I'm not very proficient in Matlab programming...thank you so much!!

Best wishes,

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Makoto Miyakoshi
Swartz Center for Computational Neuroscience
Institute for Neural Computation, University of California San Diego
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