[Eeglablist] STUDY design: spectrum negative power values

[Arnaud Delorme]

Dear Christian,

yes, you are getting 10*log10(power) in these plots which is the usual way of visualizing EEG data.
You may recuperate the data values on the command line

[STUDY specdata freqs] = std_specplot(STUDY, ALLEEG, 'channels', { 'Cz' });

and convert the data back to absolute power values and visualize the data

for index = 1:length(specdata(:))
	specdata{index} = 10^(specdata{index}/10);
end;
std_plotcurve(freqs, specdata);

Note, if you want amplitude not power, use instead specdata{index} = 10^(specdata{index}/20); or specdata{index} = sqrt(10^(specdata{index}/10)); which is equivalent.
Best,

Arno

On May 7, 2012, at 12:56 AM, Christian Scharinger wrote:

> Dear eeglab users,
> 
> I use the following code for calculating the spectrum in a STUDY design:
> 
> [STUDY ALLEEG] = std_precomp(STUDY, ALLEEG, 'channels', 'design', [1], 
> 'erp', 'off', 'spec', 'on', 'ersp', 'off', 'specparams', { 'specmode', 
> 'psd', 'recompute', 'on', 'timerange', [1300 1800], 'freqfac', [4] }, 
> 'savetrials', 'off');
> 
> STUDY = pop_specparams(STUDY, 'groupstats','on', 'mcorrect','fdr', 
> 'threshold',0.05,'plotgroups','together','freqrange',[1 30], 
> 'rmsubjmean', 'off');
> 
> With this code, I'm getting negative power values when I use the 
> std_specplot function for plotting (or when I do the plotting via GUI).
> 
> So I guess I'm not getting absolute power values (what I would like to 
> have) but power values in relation to some kind of baseline.
> 
> Is it possible to get absolute power values for a spectrum in a STUDY 
> design?
> Where can I see/change what baseline eeglab uses for calculating power 
> values?
> 
> Many thanks in advance,
> 
> Christian
> 
> 
> 
> -- 
> Christian Scharinger, M.A.
> 
> Knowledge Media Research Center (KMRC)
> Schleichstraße 6 | 72076 Tuebingen
> 
> Phone: +49 (0)7071 979-360
> Internet: http://www.iwm-kmrc.de/c.scharinger
> E-Mail: c.scharinger at iwm-kmrc.de
> 
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