Units powspctrm

[a.stolk@fcdonders.ru.nl]

If you use the same mtmfft settings for both conditions, in theory it should be fine. Make sure that you compute the fourier transformation on equal window lenghts.

If you happen to have variable triallengths per condition, my suggestion would be to cut the trials in (e.g.) 1 second pieces. This way your 'frequency resolution' is the same for condition A and B.

If your experimental setup allows you, you could choose to normalize your data (e.g. a baselinecorrection) first before making the A and B comparison. This way you correct for (non-)cognitive powerchanges over time (case it's not an ABABABAB etc. design). Such as conductivity reducal of the EEG paste over time, subject habituation effects and whatelse you have.

Best,
Arjen




----- Original Message -----
From: "Martin Bleichner" <bleichner.martin at GMAIL.COM>
To: FIELDTRIP at NIC.SURFNET.NL
Sent: Thursday, August 12, 2010 10:48:14 PM GMT +01:00 Amsterdam / Berlin / Bern / Rome / Stockholm / Vienna
Subject: Re: [FIELDTRIP] Units powspctrm

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.pad ='maxperlen'
> 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
>
> ----------------------------------
> 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
> and EEG analysis. See also
> http://listserv.surfnet.nl/archives/fieldtrip.html and
> http://www.ru.nl/neuroimaging/fieldtrip.
>
> ----------------------------------
> 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
> and EEG analysis. See also
> http://listserv.surfnet.nl/archives/fieldtrip.html and
> http://www.ru.nl/neuroimaging/fieldtrip.
>

----------------------------------
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 and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/neuroimaging/fieldtrip.

----------------------------------
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 and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/neuroimaging/fieldtrip.