[Eeglablist] weight matrix export

[Arnaud Delorme]

Dear Paul,
> I am just starting to use EEGLab 5.03 and have a question out
> EEG.icaweights that might indicate a bug, but probably just indicates
> my misunderstanding of the data structure. 
>  
> My platform is Windows XP SP2 running Matlab 6.1.0.450.
>  
> After running pop_runica using the interactive interface, I exported
> weights to a text file.  The weights exported do not agree with
> EEG.icaweights.
>  
> I note, however, that if those exported weights are reloaded into 
> matlab as,
> say, goo, then:
>  
> EEG.icawinv * goo
> gives the identity matrix
>  
> however:
>  
> EEGicawinv * EEG.icaweights
> does NOT give the identity matrix
>  
> is EEG.icaweights NOT supposed to be the weight (unmixing) matrix?
This is because the exported "weight" matrix is equal to 
EEG.icaweights*EEG.icasphere. The sphere matrix is a matrix for 
decorelating the data prior to running ICA. For more information see the 
part on whitening the data at
http://www.sccn.ucsd.edu/~arno/indexica.html

If you type EEG.icawinv*EEG.icaweights*EEG.icasphere

you will indeed get the identity matrix.

Hope this helps,

Arno

ps: I have forwarded this message to the EEGLAB list as I thought it 
might be of interest to other users.
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