[Eeglablist] negative power values in spectrum

[Yuan-fang Chou]

In the default,the value of power spectra is transformed to
10*log10(power),I think that's why you got negative values.

2012/5/22 Christian Scharinger <c.scharinger at gmx.net>

> Dear eeglab users,
>
> I'm still struggling with the question why eeglab calculates spectra
> with negative power values for my data in a study design.
> Reading the eeglab documentation and surfing through the eeglablist
> archives I didn't find a solution how one could get spectra with
> absolute power values in eeglab study design.
>
> When I use fieldtrip instead of eeglab for calculating spectra I get the
> data I'd like to have (that is positive absolute power values). But I
> really like the opportunities eeglab offers in study design, so I'd like
> to stay within this framework if possible.
>
> Maybe someone has any suggestions for my problem (see code below)?
>
> I'm sorry to bother the list again with the same question, but as I got
> always very helpful comments from the list for other questions I had in
> the past (many thanks!), I'd be glad about any helpful comments this time.
>
> Best
> Christian
>
>
>
> 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...
>
> 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
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-- 
Yuan-Fang Chao
School of Psychology
SouthWest University
Beibei,Chongqing,China
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