# Units powspctrm

Martin Bleichner bleichner.martin at GMAIL.COM
Thu Aug 12 22:48:14 CEST 2010

```Yes, typically, but is it the case for the powspctrm you get from the mtmfft
freqanalysis?
I am asking for the following reason:

Lets say, I have two experimental conditions  A and B and I compute the
powerspectra for both of them.
When the units actually are uV^2/Hz i could tell for each frequency by how
much uV A and B differ.

What confuses me is that in
ft_freqanalysis_mtmfft<http://fieldtrip.fcdonders.nl/reference/ft_freqanalysis_mtmfft>the
signal is multiplied with a taper (tap) and thereby scaled.
The resulting powspctrm hence does  not reflect the the true changes in
microvolt.
Do I see that correctly, or am I missing something here? Is the
powerspectrum corrected for that somewhere else?

Regards,
Martin

2010/8/12 a.stolk at fcdonders.ru.nl <a.stolk at fcdonders.ru.nl>

> Hello Martin,
>
> A powerspectrum typically has the frequencies on one axis and the power of
> the corresponding frequencies on the other. The power represents how strong
> a certain frequency occurs in a signal, given by the square of the voltage
> per frequency.
>
> In your case: square of the microvolts per frequency: uV^2/ Hz
>
> Best regards,
> Arjen
>
> ----- Original Message -----
> From: "Martin Bleichner" <bleichner.martin at GMAIL.COM>
> To: FIELDTRIP at NIC.SURFNET.NL
> Sent: Thursday, August 12, 2010 5:52:19 PM GMT +01:00 Amsterdam / Berlin /
> Bern / Rome / Stockholm / Vienna
> Subject: [FIELDTRIP] Units powspctrm
>
> Hello together,
>
> What units does the powspctrm have?
> Starting with an EEG signal in microvolts and computing the powerspectrum
> using freqanalysis what are the units of powspctrm?
> I tried to figure it out on my own but I am stuck somehow.
>
> Here some example code
>
> cfg=[];
> cfg.method='mtmfft';
> cfg.output='pow';
> cfg.foilim=[1 130]
> cfg.taper='hanning'
> cfg.channel=1
> cfg.keeptrials='no'
>
> R=ft_freqanalysis(cfg,data);
>
> I am interested in the units of R.powspctrm
>
> Thanks
> Martin
>
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> ----------------------------------
> The aim of this list is to facilitate the discussion between users of the
> FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG