ICA based artifact correction and phase-locking

Fri Feb 23 14:36:43 CET 2007

Dear Christian,

> Why do you remove these epochs? Do the signals saturate or is there
> excessive electrode(-cap) or subject movement in these?
>
Yes, that's correct. I remove epochs before any further analysis, if I
find saturated channels or excessive subject movements.

>
>> This means, my trials are a mixture of clearly contaminated as well
>> as clearly uncontaminated trials. Because my main goal is to identify
>> the eye and muscle artifacts, would it improve the detection of
>> components reflecting artifacts if I do the ICA decomposition only on
>> pre-identified artifactual trials? (My idea is that then the
>> components reflecting artifacts would catch less 'true' brain
>> activity. But perhaps I have a missconception here ...)
>
> The accuracy of the ICA decomposition relies fundamentally on the
> accuracy of the mixing matrix estimate A from which you form the
> de-mixing matrix (or vice versa). So if only some columns of A (e.g.
> artifact source sensor projections) are accurately estimated while the
> sensor projection estimates of brain activity are inaccurate
> (individually or as a subspace), then the de-mixing the data using W =
> inv(A) will not be fully successful in removing artifact activity from
> components reflecting the brain activity (subspace). In order to
> accurately estimate the sensor projections of ALL sources, the data
> needs to comprise example activity from ALL sources, i.e. BOTH
> artifact activity and brain activity.
>

Thanks for this clarification.

> So since you need to estimate your brain activity (subspace) as
> accurately as your artifact sources, you should use as much of the
> data, i.e. contaminated and clean segments, as possible.
>
> How many EEG channels do you have, by the way? I need to know in order
> to continue with the 'slightly longer answer'
>

I recorded my data from 60 Ag/Ag-Cl electrodes embedded in an elastic
cap (with BrainAmp amplifiers) positioned according to the extended
10/20 system. Additionally, I record EOG with two electrodes placed at
the outher canthi and one electrode placed below the left eye. All
channels are referenced to the right mastoid during recording, while the
left mastoid electrode is recorded as an additional active channel
(off-line re-refrenced to the mean of both mastoids).

Best regards,
Markus

> Regards,
> Christian
>
>
>
>>
>> Thank you very much for your help.
>>
>> Best regards,
>> Markus
>>
>> Christian Hesse schrieb:
>>> 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 <mailto:c.hesse at fcdonders.ru.nl>
>>> Web: www.fcdonders.ru.nl <http://www.fcdonders.ru.nl>
>>> ----------------------------------------------------------------------
>>>
>>>
>>>
>>>
>>
>>
>> --
>> **************************************************************
>> 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 <mailto:c.hesse at fcdonders.ru.nl>
> Web: www.fcdonders.ru.nl <http://www.fcdonders.ru.nl>
> ----------------------------------------------------------------------
>
>
>
>

--
**************************************************************
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
**************************************************************