# Power Units in Freqanalysis

A. Stolk A.Stolk at EWI.UTWENTE.NL
Wed Nov 12 16:26:51 CET 2008

```Dear Paco,

Power is normally given in microV^2/Hz. The amplitude (microV) of the EEG signal can indeed be around 70mV.

Nevertheless, your maximum value of 8*10^-3 (mV^2/Hz?) seems quite low. How have you acquisitioned your data? Have you used the get_data script in matlab? If so, my first impression would be that you have forgotten to undo the amplification. Divide the data by the amplification. Secondly, your data might be in voltages and not microvoltages. In matlab:

%undo amplification with factor <amp>. Display in microvolts (1e6).
data = (eegdata.*1e6)./<amp>;

If you do not use matlab for data acquisition you may forgot all that is written. ;) Regards,

Arjen

________________________________

Van: FieldTrip discussion list namens Paco Diaz
Verzonden: wo 11/12/2008 3:59
Aan: FIELDTRIP at NIC.SURFNET.NL
Onderwerp: [FIELDTRIP] Power Units in Freqanalysis

Hi all,

I have performed a time frequency analysis on the evoked activity for EEG
data whose units where (of course) in microVolts. It seems that it all work
fine, but I would appreciate some help to understand my results (my plot
actually). Below you can see the configuration structure that I have used.
Well, the question is as follows, which are the units for the z-axis (color
code)??

I think that they should be (microV)^2 but the values I'm obtainning are
very low when  compared with the literature, my maximum value is 8*10^-3 but
i have found in the literature values between 8 and 70 (microV)^2. Is there
a scale factor that I am missing or is it all simply wrong??

cfg=[];
cfg.method='tfr';
cfg.output='pow';
cfg.foi=[30:0.5:40];
cfg.width=7;
cfg.gwidth=5;

tfr_data=freqanalysis(cfg,data);

Thank you,
Paco.

----------------------------------
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/fcdonders/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/fcdonders/fieldtrip.

```

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