[Eeglablist] Clustering / ICA sign ambiguity

[Ralf Schmälzle]

Hi EEGLAB-users,
currently I am testing the clustering-program and ran into a problem
regarding the ICA-sign-ambiguity. I have learned (and found many examples)
that the ICA algorithm has a sign ambiguity (because of the random starting
parameters), that is the signs  of the IC-timecourse and
scalp-projection-map are arbitrary in that only the product (=EEG.data) has
fixed sign. This can be reached either by (ScalpMap*ICtimecourse) or by
((-1)*ScalpMap)*((-1)*ICtimecourse) since the (-1) cancels out. 
It seems to me that this may cause a problem when it comes to clustering
since there - according to my understanding of the clustering process - a
sign ambiguity e.g. in the ComponentERP or CompScalpMap would cause
corresponding Components to lie on opposite ends of the to-be-clustered
dimension. At least when doing the grandaverage of a componentERP this
information is essential, isn't it? 
Is that correct and how could I deal with this? 
In principle it should be allowed to flip the results (Map and TimeCourse)
but how do I know in advance (before clustering) whether one special
component must be flipped or not.

Many thanks, Ralf.
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Ralf Schmaelzle
Rheingasse 6
78462 Konstanz
Germany
Tel. +49 (0) 7531 367501
Ralf_Schmaelzle at web.de
 
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Ralf Schmaelzle
Allgemeine Psychologie
Universität Konstanz
Fach D 36
D-78457 Konstanz
Germany
 
Raum: C 528
Tel: +49 (0) 7531 88 2449
Fax: +49 (0) 7531 88 2971
Ralf.Schmaelzle at uni-konstanz.de
www.uni-konstanz.de/psychologie/ag-schupp
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