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

Makoto Miyakoshi mmiyakoshi at ucsd.edu
Tue May 5 08:04:18 PDT 2015

```Dear Shan,

> (1) Since dB is the log ratio of two same quantities, so in the spectopo
plot 10log10(uV^2/Hz) , does EEGlab implicitly assume that the reference
level is 1uV^2 ?

I once tested it myself.
I generated a long sine wave with amplitude of 1 and 10 Hz and ran
spectopo. The peak at 10 Hz was at 0. When I change the amplitude to 10,
the peak level was at 20 (if I remember correctly).

> (2) The script line "deltaPower = mean(10.^(spectra(deltaIdx)/10)); "
gives uV^2/Hz, which is the mean value of densities.  If I hope to get the
total power within alpha band, what script should I write?  Intuitively it
seems something like: total power=sum(10.^(spectra(alphaIdx)/10)*(length
of each discrete frequency)).  Is this concept correct? If it is correct,
then what should be the value for "length of each discrete frequency" (say
for 128Hz sampling rate)? If not, then is there actually any method to get
the total power?

I have never wished to have the total power so I'm not completely sure, but

> total power=sum(10.^(spectra(alphaIdx)/10)*(length of each discrete
frequency))

What is the 'length of each discrete frequency' here? What 'length' is it
referring to? Isn't it simply sum(10.^(spectra(alphaIdx)/10)?

Makoto

On Thu, Apr 23, 2015 at 9:17 AM, #SHAN XIN# <SHAN0055 at e.ntu.edu.sg> wrote:

>   Dear Makoto,
>
>
>  Thank you so much for the link!!  I still have two questions:
>
>
>  (1) Since dB is the log ratio of two same quantities, so in the spectopo
> plot 10log10(uV^2/Hz) , does EEGlab implicitly assume that the reference
> level is 1uV^2 ?
>
>
>  (2) The script line "deltaPower = mean(10.^(spectra(deltaIdx)/10)); "
> gives uV^2/Hz, which is the mean value of densities.  If I hope to get
> the total power within alpha band, what script should I write?  Intuitively
> it seems something like: total power=sum(10.^(spectra(alphaIdx)/10)*(length
> of each discrete frequency)).  Is this concept correct? If it is correct,
> then what should be the value for "length of each discrete
> frequency" (say for 128Hz sampling rate)? If not, then is there actually
> any method to get the total power?
>
>
>  Best wishes,
>
> SHAN Xin
>
>
>  ------------------------------
> *From:* Makoto Miyakoshi <mmiyakoshi at ucsd.edu>
> *Sent:* Wednesday, 22 April 2015 2:07 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,
>
>  > 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
>

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