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

#SHAN XIN# SHAN0055 at e.ntu.edu.sg
Mon Apr 20 02:29:27 PDT 2015

```Dear Makoto,

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

Outputs:
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>
Sent: Thursday, 16 April 2015 11:11 AM
To: #SHAN XIN#
Cc: 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));
%%%%%%%%%%%%%%%%%

Makoto

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,
SHAN Xin

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