[Eeglablist] ICA and signal phase content

[otte georges]

Dear Makoto

 

Thank You very much for the quick reply. I carefully checked the material You indicated in the link to the 2013 workshop (quite extensive but also  very interesting material) but could not find much specfic on the phase content (imaginary part) of the reconstructed EEG signals. In your mail You wrote to me  “of course it will change phase”. Is that “of course” because of a regression process in reconstructing the time points from the remaining components (after the artefact components have been removed) or is there another reason ? I would like to understand the mathematics and can only come up with this explanation but maybe I see it wrongly. 

 

I read the article of Roberto Montefusco who concluded ( on both reaml EEG and simulated signals) that the phase changes after reconstructing the signal s are non stationary in time , location and frequency. For instanced he found major changes (after removing blink components ) at Fz, around 6 hz. But they were also present at other sensor locations and frequencies.

Phase was determined by using the Hilbert transform.

 

All help and explanations would be very much appreciated.

 

-Sincerely

 

Georges Otte

 

De : Makoto Miyakoshi [mailto:mmiyakoshi at ucsd.edu] 
Envoyé : dinsdag 16 september 2014 4:09
À : otte georges
Cc : EEGLAB List
Objet : Re: [Eeglablist] ICA and signal phase content

 

Dear Georges,

 

If you remove IC and reconstruct channel EEG by backprojecting the remaining ICs, of course it changes channel EEG phase!

 

Here is our EEGLAB workshop 2013 materials. You can find several powerpoint/pdf files that explains how ICA works on data cleaning.

http://sccn.ucsd.edu/wiki/EEGLAB_2013_UCSD

 

Makoto

 

On Sat, Sep 13, 2014 at 7:50 AM, otte georges <georges.otte at pandora.be <mailto:georges.otte at pandora.be> > wrote:

Dear ICA experts

 

There is some discussion going on about the evidence that reconstructing an multichannel EEG after ICA decomposition and removing one or two artefact components could result in  scrambling the phase content (and coherence) of the reconstructed signals. Fi starting from a 19 ch EEG, removing two components with fi electrode pops and sweat artefacts , reconstructing the 19 ch EEG from 17 components (taking out the artefact containing ones)  without those artefacts. 

 

Others say that this is not the case and that ICA respects the phase. However many authors (Zeman, Thatcher..) have published very convincing data to the contrary.

 

Any suggestions how to evaluate this ? Is it known from the algorithm what the reconstruction  will do to the phase content (imaginary part of the signal) ?

 

All help is very welcome

 

Sincerely

 

Georges

 

 


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-- 

Makoto Miyakoshi
Swartz Center for Computational Neuroscience
Institute for Neural Computation, University of California San Diego

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