[Eeglablist] A second ICA after removing components?

[Ronald Phlypo]

Dear Max,

the problem lies in the maximally allowed number of components in both 
decompositions. The first decomposition may allow for as many sources as 
sensors (15 in your case). However, once 3 artefacts have been removed, 
your data dimension reduces to 12, which allows to estimate a maximum of 
12 sources only. To circumvent this problem, I suppose what the wiki 
means is to do local decompositions first (trial by trial or epoch by 
epoch) and then concatenating the trials/epochs again after their 
correction and before the second "joint" decomposition. Since in this 
case the artefact removal is nonlinear, it does not reduce the dimension 
of your concatenated data.

Hope this helps,

Ronald

PS: you might also want to have a look at 
http://www.hindawi.com/journals/cin/2007/075079/cta/ where short time 
and long term windows are used jointly for artefact removal. The text 
refers to literature on the mean duration of electrophysiological 
processes to motivate this decision.

Le 15/08/2011 18:09, Maximilien Chaumon a écrit :
> Hello all,
>
> I'm currently cleaning data before working with components.
>
>   * I cut my dataset into epochs
>   * reject epochs where signal is bad
>   * run an ICA
>   * find blink and muscle components, reject them
>   * run an ICA again, and look at my components.
>
> ... as I understood was suggested at the bottom of this page 
> http://sccn.ucsd.edu/wiki/Chapter_01:_Rejecting_Artifacts
>
> Then the ICs look very nice but come in pairs of extremely similar 
> topographies with different time courses, as shown on this picture 
> <http://oszilla.hgs.hu-berlin.de/public/2ICAs.png>.
> I am wondering what happened here. I can imagine that rejecting 
> components before running the second ICA is what went wrong... But why 
> did I read that on the wiki?
>
> Thanks a lot for any advice.
> Max
>
>
>
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