[Eeglablist] A second ICA after removing components?

[Maximilien Chaumon]

Thank you all for your answers.
I updated the final section of the tutorial to reflect this discussion.
http://sccn.ucsd.edu/wiki/Chapter_01:_Rejecting_Artifacts
What I understood is that, if two ICAs are to be performed, ICs should NOT
be rejected as a whole after the first ICA, but only after the second one.
The time courses of the first ICs can be used to reject data epochs, prior
to running the second ICA.

Thanks,
Max

2011/8/16 Makoto Miyakoshi <mmiyakoshi at ucsd.edu>

> Dear Max,
>
> Thank you for your question. Artifact rejection with ICA is often
> (mis)understood in that way. I (mis)understood exactly the same way
> when I first learn how to use ICA, so it's pretty understandable for
> me.
>
> If you want to obtain better ICA solution, what you should actually do
> is to click 'Tools - Reject data using ICA - Reject data (all
> methods)' and not 'Reject components by map'. You may have run epoch
> rejection before running ICA. Similarly, you can run the same epoch
> rejection after ICA on IC activities. If you want to know why and how
> epoch rejection on IC activity is effective compared to raw EEG data,
> see
>
> Arnaud Delorme, Terrence Sejnowski, Scott Makeig. (2007). Enhanced
> detection of artifacts in EEG data using higher-order statistics and
> independent component analysis. NeuroImage 34, 1443-1449.
>
> Makoto
>
>
>
> 2011/8/15 Maximilien Chaumon <maximilien.chaumon at gmail.com>:
> > 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.
> > 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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>
>
> --
> Makoto Miyakoshi
> JSPS Postdoctral Fellow for Research Abroad
> Swartz Center for Computational Neuroscience
> Institute for Neural Computation, University of California San Diego
>
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