[Eeglablist] Single-trial ERSPs

[Arnaud Delorme]

Dear Simon,

it is better to work with the newtimef function to start with (which is used to compute STUDY ERSP measures).
The data saved in the .dattimef file is the same as the "tfdata" output of newtimef.
To start with, set the baseline to NaN, then the ERSP image should be the same (if you use the defaults) as

figure; imagesc(10*log10(mean(abs(tfdata).^2,3)));

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).

Sincerely,

Arno

On 20 Sep 2012, at 16:46, Simon Ruch wrote:

> Dear all
> 
> 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.
> My basic question is: how can I compute single-trial ERSPs from the *.dattimef-data?
> I used the following approach:
> 1)    Compute the absolute power-values from the complex *.dattimef-data:
> ABSPOWER = abs(tf-data).^2
> 2)    Compute the mean baseline power for each frequency and each trial in db:
> BASELINE(freq, trial) = 10*log10(mean(ABSPOWER(freq, time < 0, trial)))
> 3)    Compute baseline corrected power values (in dB) for each time-frequency point for each trial:
> CORRECTEDPOWER(freq,:,trial)=  10*log10(ABSPOWER(freq,:,trial)) -  BASELINE(freq,trial)
> 
> 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.
> To do so, I averaged my baseline-corrected power values across trials for each subject:
> MEANPOWER = mean(CORRECTEDPOWER,3);
> 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).
> Is my approach to compute single-trial ERSPs wrong?
> Is my approach to average these single-tiral ERSPs wrong?
> I assume that eeglab performs some adjustments in the power-baselines (across trials, or even across subjects?) before removing the baseline.
> 
> 
> -- 
> 
> 
> ________________________________________________
> 
> Simon Ruch, Lic. phil.
> University of Bern
> Department of Psychology
> Division of Experimental Psychology and Neuropsychology
> Office D101
> Muesmattstrasse 45
> CH-3012 Bern
> Switzerland
> Phone +41 (0)31 631 37 28
> Fax +41 (0)31 631 82 12
> mailto:simon.ruch at psy.unibe.ch
> http://www.apn.psy.unibe.ch/content/team/henke
> 
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