[Eeglablist] ICA and signal phase content

Thu Sep 18 12:54:19 PDT 2014

Hi, 

The EEG reconstruction after removing bad components/sources MAY change the phase value of the signal at any electrode. Each source and its back projection into the channel space at any time point have individual phase value; however, the EEG phase value is a “summation” of all the phase values from each individual sources ( in other words, the EEG phase value at any given channel is the summation of phase values of each source/component reconstructed signal at that given channel). So, if some sources are removed because of artifact removing procedure then the summation of the phase values of the  remainder sources may not be matched to the initial un-cleaned EEG. 

Also, each source has a specific IC map ( spatial filter) so the effect of that source is distributed/propagated throughout all channels. The magnitude of the effect at any channel is proportional to the IC map value at that channel ( in EEGLAB: if the IC Map is dark red/blue, its effect is high; the greenish color means the effect is low) . So, by removing a source which may correspond to eye blink , the effect of that source/component  to other channels (other than ‘Fz’, for example)  will be also eliminated. As a result the component rejection procedure  MAY change the phase value/content at any given channel. 

Best

Iman 

============================================

Iman M.Rezazadeh, Ph.D. , M.Sc., B.Sc.

UCLA David Geffen School of Medicine

Semel Institute for Neuroscience and Human Behavior

760 Westwood Plaza, Ste 47-448

Los Angeles, CA  90095

 <http://www.linkedin.com/pub/iman-m-rezazadeh/10/859/840/> http://www.linkedin.com/pub/iman-m-rezazadeh/10/859/840/

From: eeglablist-bounces at sccn.ucsd.edu [mailto:eeglablist-bounces at sccn.ucsd.edu] On Behalf Of otte georges
Sent: Thursday, September 18, 2014 9:26 AM
To: mmiyakoshi at ucsd.edu
Cc: eeglablist at sccn.ucsd.edu
Subject: Re: [Eeglablist] ICA and signal phase content

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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