[Eeglablist] A second ICA after removing components?

[Maximilien Chaumon]

Thanks Ronald,
I understand your point with the dimensions.
I thought ICA does not care about time, though? So concatenated, epoched or
not, the results should be the same, right?
Max

2011/8/16 Ronald Phlypo <Ronald.Phlypo at ugent.be>

>  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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