[Eeglablist] ICs with identical topographies

[John J.B. Allen]

Max

I have observed that when the data are not full rank.   You can test the
rank of your data by reshaping your epoched data to a 2D matrix, and running
the rank command, like this:

rank(reshape(EEG.data,EEG.nbchan,EEG.trials*EEG.pnts))

When I did this, your rank is 63, but you have 69 channels, indicating that
some channels are linearly dependent on others.  I think this is the source
of your problem, and if you remove those channels before running ICA, you
should no longer see this issue.

Best

John




On Tue, Aug 23, 2011 at 07:24, Maximilien Chaumon <
maximilien.chaumon at gmail.com> wrote:

> Hi eeglabbers,
>
> I sometimes get ICs with extremely similar topographies and time courses,
> like on this slide<http://oszilla.hgs.hu-berlin.de/public/Similar_ICs.PNG>
> .
> I know that ICA returns independent components.
> Does that not mean that they should not look the same?
> I know the components are independent in a statistical sense, which is not
> the same as uncorrelated, but still. I'm a bit surprised. What do these two
> components mean if they cancel one another? well, do they?
>
> Sorry if my question is naive, but what is happening?
>
> The data is here <http://oszilla.hgs.hu-berlin.de/public/Similar_ICs.mat>.
>
> Best,
> Max
>
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