[Eeglablist] Clustering / ICA sign ambiguity

[Julie Onton]

The ERP clustering should be set to take the absolute value of the PCA weights during clustering,
which would account for sign ambiguity. This should be made clear in the documentation if it is
not already (please let us know if it is not).

The scalp maps I am less sure about, but I believe they are clustered by the square of the scalp
maps. Again, please let us know if this is not explained in the STUDY documentation. It should be
made clear.

Best,  Julie

-- 
Julie Onton, PhD
Swartz Center for Computational Neuroscience
Institute for Neural Computation
University of California, San Diego
(858) 458-1927 ext 17

>
>
> Hi Ralf,
>   I've dealt with ICA sign ambiguity a couple ways for clustering:
>
> 1. Only use clustering features that are independent of component sign.
> For example, localize each component with Dipfit and use the dipole
> location and orientation (I don't believe EEGLAB's clustering software can
> let you use dipole orientation as a cluster feature yet, but the info is
> there).  Component spectra is independent of sign.  And you can take the
> absolute value of ERPs.
>
> 2. Set the sign of each IC so that its maximal absolute topography weight
> is positive.  In my experience, most of the reliable, non-artifact ICs have a
> rather focal distribution with a clear maximum.  If that's the case for
> you, this strategy could also work.
>    cheers,
>      -David
>
> dgroppe at cogsci.ucsd.edu
>
>
>
> On Fri, 16 Feb 2007, Ralf Schmälzle wrote:
>
>> 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.
>> ______________________________________________________
>>
>> Ralf Schmaelzle
>> Rheingasse 6
>> 78462 Konstanz
>> Germany
>> Tel. +49 (0) 7531 367501
>> Ralf_Schmaelzle at web.de
>>
>>  o      o     o    o     o    <o     <o>    o>    o
>> .|.    \|.   \|/   //    X     \      |    <|    <|>
>>  /\     >\   /<    >\   /<     >\    /<     >\   /<
>>
>> 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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>
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