Coherence and dipole

jan-mathijs schoffelen jan.schoffelen at DONDERS.RU.NL
Mon Oct 25 12:25:32 CEST 2010


Dear Rodolphe,

The problem with coherence analysis on channel level (EEG) data that
widely separated electrodes can 'see' the same neuronal sources. Also,
they see the same noise (as 'injected' by the choice of reference
electrode, which can be a single channel, or the average across all
channels). So, every channel contains an unknown mix of interesting
brain signal, uninteresting brain signal, and uninteresting noise.
Depending on the spatial structure of these 3 components and the way
they are mixed per channel determines the observations.
Troublesome sources which may cause the effect you are observing, may
be quite distant 'dipoles' accounting for the cardiac activity picked
up by the EEG electrode array. Although in general I believe that EEG-
channel-level coherence analysis is very problematic due to
interpretational difficulties, you could see whether your reported
spatial patterns change when constructing a Laplacian derivation for
each of your electrodes (if the number of electrodes allows for this).

Good luck,

Jan-Mathijs


On Oct 23, 2010, at 12:12 AM, Rodolphe Nenert wrote:

> Dear Jan,
>
> yes i do analyze EEG data. The data was recorded using vertex  (CZ) as
> reference, but i modified to an average reference before doing any
> time-freq
> analysis.
>
> Rodolphe.
>
> On Fri, 22 Oct 2010 09:04:00 +0200, jan-mathijs schoffelen
> <jan.schoffelen at DONDERS.RU.NL> wrote:
>
>> Dear Rodolphe,
>>
>> I guess you are analyzing EEG data? What reference electrode(s) did
>> you use?
>>
>> Best,
>> JM
>>
>>
>>
>> On Oct 22, 2010, at 1:47 AM, Rodolphe wrote:
>>
>>> Dear Fieldtrip users,
>>>
>>> I recently run a coherence analysis on my data and i plotted a
>>> beautiful Topographic plot showing a strong coherence between one
>>> electrode in left parietal ( P5) and some right frontal electrodes.
>>> The thing is, when i try with the right parietal equivalent ( P6 )
>>> then i find a coherence with left frontal. Ans so on with a couple
>>> of other electrodes use as ref channels.
>>> As this symmetry looks awkward to me, is any of you have an idea of
>>> a posible artifact that can cause this, or a possible bias in the
>>> analysis? ( I tripled checked my code)
>>>
>>> Best,
>>>
>>> Rodolphe N., Ph.D.
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>>> between users of the FieldTrip toolbox, to share experiences
>>> and to discuss new ideas for MEG and EEG analysis.
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>>> and http://www.ru.nl/neuroimaging/fieldtrip.
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>>>
>>
>> Dr. J.M. (Jan-Mathijs) Schoffelen
>> Donders Institute for Brain, Cognition and Behaviour,
>> Centre for Cognitive Neuroimaging,
>> Radboud University Nijmegen, The Netherlands
>> J.Schoffelen at donders.ru.nl
>> Telephone: 0031-24-3614793
>>
>> ---------------------------------------------------------------------------
>> You are receiving this message because you are subscribed to
>> the  FieldTrip list. The aim of this list is to facilitate the
>> discussion
>> between  users of the FieldTrip  toolbox, to share experiences
>> and to discuss  new ideas for MEG and EEG analysis.
>> See also http://listserv.surfnet.nl/archives/fieldtrip.html
>> and http://www.ru.nl/neuroimaging/fieldtrip.
>> ---------------------------------------------------------------------------
>
> ---------------------------------------------------------------------------
> You are receiving this message because you are subscribed to
> the  FieldTrip list. The aim of this list is to facilitate the
> discussion
> between  users of the FieldTrip  toolbox, to share experiences
> and to discuss  new ideas for MEG and EEG analysis.
> See also http://listserv.surfnet.nl/archives/fieldtrip.html
> and http://www.ru.nl/neuroimaging/fieldtrip.
> ---------------------------------------------------------------------------
>

Dr. J.M. (Jan-Mathijs) Schoffelen
Donders Institute for Brain, Cognition and Behaviour,
Centre for Cognitive Neuroimaging,
Radboud University Nijmegen, The Netherlands
J.Schoffelen at donders.ru.nl
Telephone: 0031-24-3614793

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between  users of the FieldTrip  toolbox, to share experiences
and to discuss  new ideas for MEG and EEG analysis.
See also http://listserv.surfnet.nl/archives/fieldtrip.html
and http://www.ru.nl/neuroimaging/fieldtrip.
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