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

Makoto Miyakoshi mmiyakoshi at ucsd.edu
Fri May 15 18:14:42 PDT 2015 

Dear Ahmed,

0 in the vertical scale means your sine wave has amplitude of 1. If it is
10, the scale should point to 20. -40 means 1/100, -60 means 1/1000, etc...
somehow your signal seems very small. Usually eye blinks etc are around 20.

Makoto

On Sat, May 9, 2015 at 9:29 AM, Ahmed Almurshedi <ahmed19875000 at yahoo.com>
wrote:

> Dear All,
>
> Thanks for the useful discussion.
>
> In the same regard, I have tried to run spectopo on single dataset but,
> I'm blurred with the result. Why the power is negative? what does it mean?
>
> The spectopo is  spectopo topography.png
> <https://www.dropbox.com/s/q4v0nappwhvk1p5/spectopo%20topography.png?dl=0>
>
>
> [image: image]
> <https://www.dropbox.com/s/q4v0nappwhvk1p5/spectopo%20topography.png?dl=0>
>
>
>
>
>
> spectopo topography.png
> <https://www.dropbox.com/s/q4v0nappwhvk1p5/spectopo%20topography.png?dl=0>
> Shared with Dropbox
> View on www.dropbox.com
> <https://www.dropbox.com/s/q4v0nappwhvk1p5/spectopo%20topography.png?dl=0>
> Preview by Yahoo
>
>
>
> AHMED
>
>
>
>
>
> ------------------------------
>  *From:* Makoto Miyakoshi <mmiyakoshi at ucsd.edu>
> *To:* #SHAN XIN# <SHAN0055 at e.ntu.edu.sg>
> *Cc:* "eeglablist at sccn.ucsd.edu" <eeglablist at sccn.ucsd.edu>
> *Sent:* Friday, May 8, 2015 2:40 AM
>
> *Subject:* Re: [Eeglablist] How can I get the power of each frequency
> band?
>
> Dear Shan,
>
> Correct me if I'm wrong--I guess you are concerned with the number of
> sampling points in frequency. For example, if there are 4, 5, 6, 7 Hz in
> theta, and 8, 9, 10, 11, 12, 13 Hz in alpha, the former has 4 points while
> the latter has 6 points, therefore they should be normalized by the number
> of the frequency sampling points (i.e. to use the mean value within each
> frequency range 'bin'). If that's the case, yes I agree with you.
>
> Makoto
>
>
>
> On Tue, May 5, 2015 at 11:23 PM, #SHAN XIN# <SHAN0055 at e.ntu.edu.sg> wrote:
>
>  Dear Makoto,
>
>  Thank you for your explanation!!
>
>  For the "length of each discrete frequency", I mean the length of
> each frequency bin.  Let's say the density is "5uV^2/Hz" for one
> particular alphaIdx (one frequency bin), and assume the length of this
> frequency bin is ​0.5 Hz.  If we can directly sum all values, then actually
> you mean 5uV^2 is already the power in this frequency bin.  Or the power is
> actually 2.5uV^2, such that the density is 2.5uV^2/0.5Hz=5uV^2/Hz ? If it
> is the second case, then before summation we should first multiply the
> density by the length, right?  I don't know which one is correct...
>
>  Best wishes,
> SHAN Xin
>
>  ------------------------------
> *From:* Makoto Miyakoshi <mmiyakoshi at ucsd.edu>
> *Sent:* Tuesday, 5 May 2015 11:04 PM
>
> *To:* #SHAN XIN#
> *Cc:* eeglablist at sccn.ucsd.edu
> *Subject:* Re: [Eeglablist] How can I get the power of each frequency
> band?
>
>  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
>
>
>
> _______________________________________________
> Eeglablist page: http://sccn.ucsd.edu/eeglab/eeglabmail.html
> To unsubscribe, send an empty email to
> eeglablist-unsubscribe at sccn.ucsd.edu
> For digest mode, send an email with the subject "set digest mime" to
> eeglablist-request at sccn.ucsd.edu
>
>
>
>
>  --
>  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
>
>
>
>
> --
> Makoto Miyakoshi
> Swartz Center for Computational Neuroscience
> Institute for Neural Computation, University of California San Diego
>
> _______________________________________________
> Eeglablist page: http://sccn.ucsd.edu/eeglab/eeglabmail.html
> To unsubscribe, send an empty email to
> eeglablist-unsubscribe at sccn.ucsd.edu
> For digest mode, send an email with the subject "set digest mime" to
> eeglablist-request at sccn.ucsd.edu
>
>
>


-- 
Makoto Miyakoshi
Swartz Center for Computational Neuroscience
Institute for Neural Computation, University of California San Diego
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://sccn.ucsd.edu/pipermail/eeglablist/attachments/20150515/048a33be/attachment.html>

More information about the eeglablist mailing list