[Eeglablist] A second ICA after removing components?

[Tarik S Bel-Bahar]

yes, ica does not care whether data is epoched or continuous.
However, of course, the cleanliness and nature of the data given is crucial.


On Tue, Aug 16, 2011 at 9:10 AM, Maximilien Chaumon <
maximilien.chaumon at gmail.com> wrote:

> Thanks Ronald,
> I understand your point with the dimensions.
> I thought ICA does not care about time, though? So concatenated, epoched or
> not, the results should be the same, right?
> Max
>
>
> 2011/8/16 Ronald Phlypo <Ronald.Phlypo at ugent.be>
>
>>  Dear Max,
>>
>> the problem lies in the maximally allowed number of components in both
>> decompositions. The first decomposition may allow for as many sources as
>> sensors (15 in your case). However, once 3 artefacts have been removed, your
>> data dimension reduces to 12, which allows to estimate a maximum of 12
>> sources only. To circumvent this problem, I suppose what the wiki means is
>> to do local decompositions first (trial by trial or epoch by epoch) and then
>> concatenating the trials/epochs again after their correction and before the
>> second "joint" decomposition. Since in this case the artefact removal is
>> nonlinear, it does not reduce the dimension of your concatenated data.
>>
>> Hope this helps,
>>
>> Ronald
>>
>> PS: you might also want to have a look at
>> http://www.hindawi.com/journals/cin/2007/075079/cta/ where short time and
>> long term windows are used jointly for artefact removal. The text refers to
>> literature on the mean duration of electrophysiological processes to
>> motivate this decision.
>>
>> Le 15/08/2011 18:09, Maximilien Chaumon a écrit :
>>
>> Hello all,
>>
>> I'm currently cleaning data before working with components.
>>
>>    - I cut my dataset into epochs
>>    - reject epochs where signal is bad
>>    - run an ICA
>>    - find blink and muscle components, reject them
>>    - run an ICA again, and look at my components.
>>
>> ... as I understood was suggested at the bottom of this page
>> http://sccn.ucsd.edu/wiki/Chapter_01:_Rejecting_Artifacts
>>
>> Then the ICs look very nice but come in pairs of extremely similar
>> topographies with different time courses, as shown on this picture<http://oszilla.hgs.hu-berlin.de/public/2ICAs.png>
>> .
>> I am wondering what happened here. I can imagine that rejecting components
>> before running the second ICA is what went wrong... But why did I read that
>> on the wiki?
>>
>> Thanks a lot for any advice.
>> Max
>>
>>
>>
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>
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