[Eeglablist] Average Reference

Thu Mar 26 14:01:05 PDT 2015

Dear Makoto,

thank you very much for your explanation, i think this seems to be a
interesting topic and you helped me to understand that this finding is not
a artificial one.
I will now look deeper into my data structure, to fully identify possible
ICA subspaces!

Best,
Claudio

2015-03-26 18:45 GMT+01:00 Makoto Miyakoshi <mmiyakoshi at ucsd.edu>:

> Dear Claudio,
>
> First of all I'm not qualified to discuss it since my background is not so
> mathematical. That being said, let me tell you what I know.
>
> > But if there are not 64 (on a 64 electrode setup) unique brain-related
> sources in the data, the ICA tends to split one source into two,
>
> That's not the case. ICA by nature performs independent subspace
> decomposition. Our current interpretation of such IC subspace is that if
> there is a moving source ICA may decompose it into a subspace (imagine you
> take still picture of a moving car...)
>
> > (they are independent by means of the ICA, but occur on the same instant
> in the brain, thus having a entropy which deviates from zero).
>
> If you measure mutual information reduction (MIR) across the ICs, you'll
> see the subspace. I wonder, by the way, 'getMIR.m' is available outside
> SCCN... it's packaged in AMICA plugin.
>
> > And what would be the solution to the problem.
>
> I heard people explain it as if it is not a problem since it's a nature of
> ICA... but I'm still puzzled, yes!
>
> > I normally try to give the most cleaniest data into the ICA computation
> (excluding noisy channels, eye movement correction, raw data inspection
> etc.), so shouldn't i do that?
>
> Our clustering approach, either k-means or MeasureProjection, addresses
> the issue. However, if you need to choose one out of several ICs, you may
> want to use some criteria such as largest variance, or most similar
> spectrum/scalp map/ERP to the cluster centroid, etc... there is no
> established way to do it unfortunately. It is a weak point of ICA; it
> messes up later processes.
>
> Makoto
>
> On Mon, Mar 23, 2015 at 2:38 AM, Claudio Georgii <
> Claudio.Georgii at stud.sbg.ac.at> wrote:
>
>> Dear Makoto,
>>
>> Thank you for your help and advices. I never thought that the position of
>> cap would be source of the problem, but i think you are right, and it's a
>> simple solution to the problem as well!
>>
>> I watched the videos of the link you sent me. But i am not quite sure if
>> i get your line of argumentation right.
>>
>> Normally each artificial source is represented in one ICA component, thus
>> having cleaner results in having "only" brain-related ICA components. But
>> if there are not 64 (on a 64 electrode setup) unique brain-related sources
>> in the data, the ICA tends to split one source into two, which form an ICA
>> subspace and are residual dependent of each other (they are independent by
>> means of the ICA, but occur on the same instant in the brain, thus having a
>> entropy which deviates from zero).
>>
>> Did i got this right? And what would be the solution to the problem. I
>> normally try to give the most cleaniest data into the ICA computation
>> (excluding noisy channels, eye movement correction, raw data inspection
>> etc.), so shouldn't i do that? Or is there a midway between clean and dirty
>> data i should take.
>>
>> What would be your recommendations?
>>
>> Thanks in advance!
>>
>> Best,
>> Claudio
>>
>> 2015-03-21 0:56 GMT+01:00 Makoto Miyakoshi <mmiyakoshi at ucsd.edu>:
>>
>>> Dear Claudio,
>>>
>>> > 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.
>>>
>>> I think that is true.
>>>
>>> > In theory this might lead sources migrate to more posterior sides?
>>>
>>> No, that does not happen. If you see the effect of average referencing
>>> for each frame, it just adds constant values for all the channels.
>>> Therefore, it does not change the inter-channel relations. If a source
>>> moves, it should change the inter-channel relations.
>>>
>>> > 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.
>>>
>>> If you apply EEG cap wrongly it may happen. Make sure that your Fp1 and
>>> Fp2 are right above the eyebrows.
>>>
>>> > In theory i'd say that this is "bad luck" and the ICA is splitting one
>>> dipole into two different ICs.
>>>
>>> The more channels and the clearner data you have, the more subspace
>>> you'll see after ICA. For the nature of subspace, see the following video
>>> by Jason Palmer.
>>>
>>> Chapter 2 Part 2 (Don't miss the spectacular movie from 13:01)
>>> http://sccn.ucsd.edu/eeglab/Online_EEGLAB_Workshop/EEGLAB12_ica1.html
>>>
>>> Makoto
>>>
>>> On Fri, Mar 20, 2015 at 2:47 AM, Claudio Georgii <
>>> Claudio.Georgii at stud.sbg.ac.at> wrote:
>>>
>>>> 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
>>>>>>
>>>>>> _______________________________________________
>>>>>> Eeglablist page: http://sccn.ucsd.edu/eeglab/eeglabmail.html
>>>>>> To unsubscribe, send an empty email to
>>>>>> eeglablist-unsubscribe at sccn.ucsd.edu
>>>>>> For digest mode, send an email with the subject "set digest mime" to
>>>>>> eeglablist-request at sccn.ucsd.edu
>>>>>>
>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> Makoto Miyakoshi
>>>>> Swartz Center for Computational Neuroscience
>>>>> Institute for Neural Computation, University of California San Diego
>>>>>
>>>>
>>>>
>>>
>>>
>>> --
>>> Makoto Miyakoshi
>>> Swartz Center for Computational Neuroscience
>>> Institute for Neural Computation, University of California San Diego
>>>
>>
>>
>
>
> --
> Makoto Miyakoshi
> Swartz Center for Computational Neuroscience
> Institute for Neural Computation, University of California San Diego
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://sccn.ucsd.edu/pipermail/eeglablist/attachments/20150326/f4951e9a/attachment.html>