ICA based artifact correction and phase-locking

[Christian Hesse]

Hi Markus,

the short answer is: if (and only if, big if by the way) ICA  
correctly separates artifacts from brain activity, and you correctly  
identify the artifact components, removal of the artifact components  
from your data does not affect the time-frequency properties  
(including phase) of the other components (i.e. the rest of the data)  
if you project back to the sensor level. This is because ICA assumes  
a linear (instantaneous) mixing of statistically independent signals.

I would just try it out for the  time being - if you get funny  
results (or things too good to be true), please get back in touch as  
there is also a slightly longer answer.

Hope this helps,
Christian


On 23 Feb 2007, at 12:24, Markus Werkle-Bergner wrote:

> Dear all,
>
> in my studies, I'm investigating early preceptual binding (visual)
> across the lifespan (i.e., I have data form children, younger and  
> older
> adults) with EEG measures. My main interest concerns changes in
> gamma-power and measures of phase-synchronization in the gamma  
> frequency range(e.g., phase-locking index, n:m (theta:gamma) phase  
> synchronization).
>
> Currently  I use a 'semi-automatic' procedure for artifact  
> rejection, i.e., I use thresholding in the time-domain (min/max in  
> segment -/+ 100µV)to 'suggest' contaminated epochs. After that I  
> visually inspect the data again for eye-blink and muscle activity,  
> and completely reject the contaminated epochs.
>
> The problem with this procedure is that, especially in the older  
> adults group, for many subjects only too few trials remain in the  
> final sample.
>
> Therefore, I thought I could use ICA for artifact correction  
> (instead of complete rejection). After identification of the  
> components that reflect muscle activity (and also other artifacts),  
> I thought to recombine the remaining ICs and perform my analyses  
> (power, PLI, n:m synchronization) on the recombined (cleaned data).
>
> Now my question(s): Is there any experience whether removing  
> certain ICs
> may change the phase spectrum, i.e. may this approach induce some
> systematic bias? If there is a systematic bias, are different  
> frequency
> bands affected differentialy? Could anyone give me some references on
> these issues?
>
> Any comments are very much appreciated.
>
> Best regards,
> Markus
>
> -- 
> **************************************************************
> Markus Werkle-Bergner, Dipl. Psych.
> Predoctoral Research Fellow
>
> Center for Lifespan Psychology
> Max Planck Institute for Human Development
> Lentzeallee 94, Room 211, D-14195 Berlin, Germany.
> Phone: +49(0)30-82406-447       Fax: +49(0)30-8249939
> **************************************************************
>

----------------------------------------------------------------------
Christian Hesse, PhD, MIEEE

F.C. Donders Centre for Cognitive Neuroimaging
P.O. Box 9101
NL-6500 HB Nijmegen
The Netherlands

Tel.: +31 (0)24 36 68293
Fax: +31 (0)24 36 10989

Email: c.hesse at fcdonders.ru.nl
Web: www.fcdonders.ru.nl
----------------------------------------------------------------------




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