[Eeglablist] newtimef ersp output without any or with external baseline

Andero Uusberg Andero.Uusberg at ut.ee
Thu Feb 25 13:03:15 PST 2010


I have a few (maybe fairly naive) questions regarding running the 
newtimef function without using baseline from the dataset being analyzed.

Firstly, while running newtimef on a channel of a segmented dataset 
without either of the baselines (having both 'baseline' and 'powbase' 
set to NaN) I have trouble understanding the units of the output ersp 
matrix (I call the function from the command line and use freqrange 
2:20, padratio 2, cycles 1 6 and basenorm 'off'). The output ersp matrix 
is full of positive numbers in the range of 40 to 60. Although the 
figure legend states this is 10*log10 transform of power density in 
microvolts, this does not seem likely (the power densities would be 
around 100 000 microvolts). In addition, even with both baselines set to 
NaN the powbase output is created (consisting of numbers in the same 
range). All this probably has something to do with the normalization 
process and is completely meaningful, but could someone explain how 
exactly these values are generated and what is their unit?

Second and related question regards feeding externally created baseline 
into newtimef via the powbase key. I know one way would be to calculate 
the basline with newtimef in the first place and not to care about the 
units of the data :) However, I would also like to be able to use data 
calculated with FFT in Vision Anlayzer as the baseline. Do I understand 
correctly that the data should be in power denisty units (square of 
microvolt per hertz) and should I run the 10*log10 transform on the data 
before feeding in into the newtimef with powbase?

Finally, as I'm not sure my previous e-mail reached the list I repeat a 
related question concerning using std_precomp for calculating ERSPs for 
several datasets using a single external baseline. When referring to a 
channel-by-frequencies (as well as frequencies-by-channles) matrix of 
external power density values with the powbase key in erspparams field 
og std_precom, I receive the following error message: "std_ersp at 372: 
powbase should be of size (ncomps,nfreqs)". Can anyone estimate if this 
could be a bug or should I keep figuring out what I'm doing wrong?

I apologise if these issues have been dealt with before in the list in 
which case I am grateful for a kind reference to older posts.

With kind regards and general gratitude to eeglab :)
Andero Uusberg
University of Tartu

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