[Eeglablist] Average Reference

[Claudio Georgii]

Dear Makoto,

thanks for your reply! It's good to know that using 64 electrodes should be
no concern if you are using an online average reference system.

I now analyzed the data using the MPT (within EEGlab) and i know have 2
other concerns:

1) The electrode montage we use is not evenly distributed over the head
having more electrodes at posterior sites (PO9, PO10, Oz) than on frontal
sides. I now asked myself the question, whether this may lead to
topographic changes, and thus migration of sources (compared to 64
Electrodes evenly distributed over the head), if you are using an online
average reference system. I thought of this, because the fact that i have
lesser electrodes on frontal sides would mean that i am not that capable of
measuring the activity from all the underlying frontal sources. Thus, based
on the theoritical assumptions of the average reference the summed activity
of all electrodes would deviate from zero (more than it normally does),
resulting in a situation, where partial activity of frontal sources is
substracted from all electrodes. In theory this might lead sources migrate
to more posterior sides?

In fact, since quite some time we are wondering why the effects in our
experiments are emerging on posterior sides than reported in the literature.

2) The ICA (infomax, extended, runica function) i used creates to almost
identical ICs, which just differentiate in their respective polarity,
located at the center of the brain. In theory i'd say that this is "bad
luck" and the ICA is splitting one dipole into two different ICs. But the
problem is that almost every participant in our study has these two ICs and
based on the MPT they are later clustered into one source! Which is of
course not wrong, but it inflates the dimension of the source i got from
the MPT, because two dipoles (related two the two ICs) are used to cluster
the source instead of one.

Is there any way to prevent the ICA in splitting this one dipole into two
IC's?

Thanks for your help in advance!

Best,
Claudio

2015-03-19 20:10 GMT+01:00 Makoto Miyakoshi <mmiyakoshi at ucsd.edu>:

> Dear Claudio,
>
> > As far as i know by now, different reference electrodes do not affect
> the scalp distribution.
>
> Oh it does change the scalp distribution. For ICA it does not.
>
> > I asked me this question, because theoretically the summed activity of
> all electrodes equals zero only if you have a comprehensive electrode
> montage.
>
> That's a good point. And the head should be a perfect sphere (with no
> neck) too :-)
>
> > Thus, are 64 electrodes enough for using an online average reference?
>
> Practically no problem.
>
> Makoto
>
> On Fri, Mar 13, 2015 at 6:12 AM, Claudio Georgii <
> Claudio.Georgii at stud.sbg.ac.at> wrote:
>
>> Hi,
>> i am Claudio Georgii, a Ph.D. student at the University of Salzburg,
>> Austria. I am currently working with EEGlab to analyse my EEG data in the
>> source space using the measure projection toolbox (MPT).
>>
>> I try do evaluate know whether using an online average reference (64
>> electrode system) has an impact on the source distribution. As far as i
>> know by now, different reference electrodes do not affect the scalp
>> distribution. But have different electrode montages, while using an online
>> average reference, an impact on the scalp distribution?
>>
>> I asked me this question, because theoretically the summed activity of
>> all electrodes equals zero only if you have a comprehensive electrode
>> montage. Thus, are 64 electrodes enough for using an online average
>> reference?
>>
>> Best,
>> Claudio
>>
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>
>
>
> --
> Makoto Miyakoshi
> Swartz Center for Computational Neuroscience
> Institute for Neural Computation, University of California San Diego
>
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