[Eeglablist] Fwd: Applying ICA weight matrix on another dataset

[Makoto Miyakoshi]

Dear Maarten,

I asked it to Nima offline at SCCN B177E. He said categorically no because
ICA weight matrix is very dependent of the sphering matrix.

By the way you can download a function getMIR() which is included in the
amica toolbox downloadable from the following URL link:
http://sccn.ucsd.edu/wiki/Amica_Download
This function would eventually tells you which W works better in terms of
mutual information reduction (this unmixing matrix W should be a product of
sphering matrix x ica weight matrix). Let us know if you find something
interesting.

Makoto

2013/2/22 Maarten De Schuymer <maartendeschuymer at gmail.com>

> Dear list,
>
> I am still trying to figure out which is the correct sphering matrix when
> applying ICA weights to another version of the same dataset.
> Is there an expert who can weight in on this issue?
>
> Thanks a lot,
> Maarten De Schuymer
>
>
> ---------- Forwarded message ----------
> From: Maarten De Schuymer <maartendeschuymer at gmail.com>
> Date: 2013/2/14
> Subject: Applying ICA weight matrix on another dataset
> To: eeglablist at sccn.ucsd.edu
>
>
> Dear list,
>
>
> I have a question concerning the role of the sphering matrix (which
> decorrelates the channels) in the rather common scenario where I compute an
> ICA on one version of a dataset, but then apply the ICA results to another
> version of the same data (e.g. epoched vs. continuous, filtered vs.
> unfiltered).
>
> To remove artifacts in my study, I compute the ICA on high-pass filtered
> (e.g. 1 Hz) data, because this results in much better ICA decompositions.
> However, I would like to apply the results of this ICA to my original,
> unfiltered version of the same dataset, because would like to keep slow
> potentials (< 1 Hz) in the data. After running ICA on the filtered data, I
> save both EEG.icaweights and EEG.icasphere.
>
>
> When I now apply the ICA weight matrix to the original data it is unclear
> to me which sphering matrix needs to be used.
>
> Should I (A) also import the ICA sphering matrix from the filtered data or
> (B) recompute the sphering matrix (cmd: sphere(EEG.data)) for the original
> unfiltered data and consequently use that one. Both possibilities result in
> different outcomes since sphering matrices are different for both versions
> of the datasets. Which of these possibilities are recommended and more
> importantly, why exactly?
>
> A related question concerns the exporting-importing of the weight matrix
> in the GUI of EEGLAB. When exporting weights, a single exported file
> contains the combined weight*sphering matrix. However, when importing, two
> different files need to be imported, i.e. both weight matrix and sphere
> matrix separately. This does not seem practical. Or is there a rationale
> behind this distinction between import and export?
>
>
> Thanks for any input on this,
>
>
> Best
> Maarten De Schuymer
>
>
>
>
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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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