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

Makoto Miyakoshi mmiyakoshi at ucsd.edu
Tue Apr 21 11:07:45 PDT 2015

```Dear Shan,

> Why divided by  Hz?

Here is (esoteric) discussion about the related topic.
http://sccn.ucsd.edu/pipermail/eeglablist/2014/008224.html

Makoto

On Mon, Apr 20, 2015 at 3:58 AM, #SHAN XIN# <SHAN0055 at e.ntu.edu.sg> wrote:

>  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> 於 Apr 20, 2015 17:29 寫道：
>
>   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> 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
>
>

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