[Eeglablist] Computing a second ICA on a dataset

[Klados Manousos]

Dear Jan

If i understood well, your problem is that you permorf ICA decomposition
once you remove some components and then you reconstruct the signal and you
want to perform a second ICA on the already reconstructed signal. If is that
your problem forget it. It's totally pointless to perfome ICA on a
reconstructed signal after ICA decomposition and elimination of some
components. Because ICA decomposes the signal into statistical idependent
sources according to the ICA algorithm you use. After that the
reconstruction of the signal does'nt have all the real underlying cerebral
sources it mixes the aforementioned computed sources (where it supposed to
be very near to the original). So in the best second ICA will give you the
mixed sources...

Manousos

2008/11/7 Jan R. Wessel <Jan.Wessel at nf.mpg.de>

> Hey everybody,
>
> I'm analyzing an EEG dataset on an eye-movement task. In order to cope
> with the massive eye-movement artifacts, I computed a normal ICA on the
> dataset and got rid of all the oviously eye-movement related components.
> Now, I'm trying to compute a second ICA in order to clean up the data
> even more, because there is still considerable saccadic activity in most
> of the datasets.
>
> When I'm doing this, I'm running into a series of problems. Seemingly
> depending on the memory of the machine I am running it on, a) either the
> ICA takes days to compute for a single subject, b) or Matlab terminates
> without any prompt, leaving me with a "segmentation fault" in the unix
> terminal or c) if it runs until the end, it seems to give me (and here
> comes the really weird part) complex numbers (!) such as
> "0.009259085778863 + 0.013638045843933i" as ICA weights. When I display
> the component activations, I get some funny activation spots in the
> bottom left corner of the plot, while the rest of the plot is blank.
> This effect is regardless of which epochs I display.
>
> So obviously, the second ICA doesn't really seem to work out. Does
> anybody have any experience with consecutive ICA analyses or has even
> come across the same problem and can maybe help me out here?
>
> Thanks in advance,
> best,
> Jan
>
> --
> Jan R. Wessel, Dipl.-Psych.
> Cognitive Neurology Research Group
> Max Planck Institute for Neurological Research
> Gleueler Straße 50
> D-50931 Cologne
> Phone: 0221 - 4726 343
> Mail:  Wessel at nf.mpg.de
>
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-- 
Klados A. Manousos
Graduate Student, Research Assistant
Group of Applied Neurosciences
Lab of Medical Informatics, Medical School
Aristotle University of Thessaloniki
Thessaloniki, Greece
_________________________________________________
Tel: +30-2310-999332
Website :
http://lomiweb.med.auth.gr/gan/index_files/Page1269.htm
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