<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">Dear Simon,<div><br></div><div>it is better to work with the newtimef function to start with (which is used to compute STUDY ERSP measures).</div><div>The data saved in the .dattimef file is the same as the "tfdata" output of newtimef.</div><div>To start with, set the baseline to NaN, then the ERSP image should be the same (if you use the defaults) as</div><div><br></div><div>figure; imagesc(10*log10(mean(abs(tfdata).^2,3)));</div><div><br></div><div>Then examine the newtimef code to see how the baseline is computed (by default it is a divisive baseline although you can also use an additive baseline).</div><div><br></div><div>Sincerely,</div><div><br></div><div>Arno</div><div><br><div><div>On 20 Sep 2012, at 16:46, Simon Ruch wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite">
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<font size="-1"><font face="Arial">Dear all<br>
<br>
I am interested in single-trial ERSP-data. Since I have found
some interesting activities in specific time-frequency-bins on
the study level (I used std_precomp to compute ERSPs), I would
now like to look at single-trial ERSPs.<br>
My basic question is: how can I compute single-trial ERSPs from
the *.dattimef-data?<br>
I used the following approach:<br>
1) Compute the absolute power-values from the complex
*.dattimef-data:<br>
ABSPOWER = abs(tf-data).^2<br>
2) Compute the mean baseline power for each frequency and
each trial in db:<br>
BASELINE(freq, trial) = 10*log10(mean(ABSPOWER(freq, time <
0, trial)))<br>
3) Compute baseline corrected power values (in dB) for each
time-frequency point for each trial:<br>
CORRECTEDPOWER(freq,:,trial)= 10*log10(ABSPOWER(freq,:,trial))
- BASELINE(freq,trial)<br>
<br>
In order to validate my newly computed single-trial ERSPs, I
wanted to compare the averaged ERSPs provided by std_precomp
(*.datersp-files) with my self-made ERSPs.<br>
To do so, I averaged my baseline-corrected power values across
trials for each subject:<br>
MEANPOWER = mean(CORRECTEDPOWER,3);<br>
The ERSP-data that I produced with this approach for each
subject and each electrode differed strikingly from the ERSPS
computed with std_precomp (provided in the *.datersp-files).<br>
Is my approach to compute single-trial ERSPs wrong?<br>
Is my approach to average these single-tiral ERSPs wrong?<br>
I assume that eeglab performs some adjustments in the
power-baselines (across trials, or even across subjects?) before
removing the baseline.<br>
<br>
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<br>
________________________________________________<br>
<br>
<b>Simon Ruch, Lic. phil.</b><br>
University of Bern<br>
Department of Psychology<br>
Division of Experimental Psychology and Neuropsychology<br>
Office D101<br>
Muesmattstrasse 45<br>
CH-3012 Bern<br>
Switzerland<br>
Phone +41 (0)31 631 37 28<br>
Fax +41 (0)31 631 82 12<br>
<a href="mailto:simon.ruch@psy.unibe.ch">mailto:simon.ruch@psy.unibe.ch</a><br>
<a href="http://www.apn.psy.unibe.ch/content/team/henke">http://www.apn.psy.unibe.ch/content/team/henke</a><p><br>
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