Reference electrode in lead field

knyazev knyazev at PHYSIOL.RU
Thu Oct 1 05:22:48 CEST 2009


I wonder how many electrodes should be considered enough to provide a
 representative sampling of the head surface? I remember reading somewhere
that using an average reference from too few channels distorts results
severely. May someone provide an appropriate citation on the topic? Thanks
in advance.

Gennady Knyazev

----- Original Message -----
From: "Joseph Dien" <jdien07 at MAC.COM>
To: <FIELDTRIP at NIC.SURFNET.NL>
Sent: Wednesday, September 30, 2009 9:10 AM
Subject: Re: [FIELDTRIP] Reference electrode in lead field


> The reasoning behind the average reference is that it is a physics
> principle that the sum of the voltages over an enclosed surface must
> equal zero.  To the extent that the electrode locations provide a
> representative even sampling of the head surface (an important caveat)
> the sum of the voltages therefore provides an estimate of the true  zero
> voltage.  The reason for using the average reference rather than  a single
> reference site is that using a reference site arbitrarily  defines that
> point as being zero voltage (which is to say, inactive),  which is not
> biophysically reasonable as there are no inactive sites  on the head (due
> to volume conduction).  Also, to clarify, an average  reference does not
> result in a reference-free solution since, as you  say, a voltage
> measurement is by definition a relative measure  (although ideally it
> should be relatively independent of the electrode  montage, given enough
> electrodes).  It's just that the comparison  "site", which is the zero
> equipotential line (as estimated by the  average reference computation),
> is a more biophysically reasonable one  (given enough recording sites)
> than arbitrarily picking a single fixed  electrode site as the reference.
>
> For an extended discussion of these issues, see:
>
> Dien, J. (1998). Issues in the application of the average reference:
> Review, critiques, and recommendations. Behavior Research Methods,
> Instruments, and Computers, 30(1), 34-43.
>
> Cheers!
>
> Joe
>
>
> On Sep 28, 2009, at 3:29 PM, Mark Drakesmith wrote:
>
>> Hi all
>>
>> I am experimenting with source reconstruction and was wondering how  a
>> reference electrode is defined in the lead field. Looking through  the
>> scripts it looks like the average reference is used, but this is  a
>> physical impossibility, as there must be a physical reference to  which
>> differences in electrical potential can be measured. The lead  field will
>> be differ depending on the location of the reference  electrode.
>>
>> Firstly, is there a way to specify a reference electrode when
>> constructing an EEG lead field in fieldtri p and not jsut use the
>> average reference.
>>
>> Secondly, looking through  the code for  'inf_medium_leadfield' (called
>> from prepare_leadfield ->  compute_leadfield -> eeg_leadfieldb), the
>> equations used for  calculating the lead field look a little strange:
>>
>> radius = position (vox) - position(elec)
>> R (resistivity?) = 4 x pi x conductivity x sum(radius^2)^(1.5)
>> lead field(vox,elec)=radius / R.
>>
>> Where the the exponential to 1.5 come from? Is there a reference to
>> somewhere where this method is used. I'm confused as to sure how  this
>> calculation works.
>>
>> Many thanks
>>
>> Mark
>>
>> --
>>
>> Mark Drakesmith
>> PhD Student
>>
>> Neuroscience and Aphasia Research Unit (NARU)
>> University of Manchester
>>
>> ----------------------------------
>> 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.
>
>
> --------------------------------------------------------------------------------
>
> Joseph Dien,
> Senior Research Scientist
> Center for Advanced Study of Language
> University of Maryland
> 7005 52nd Avenue
> College Park, MD 20742-0025
>
> E-mail: jdien07 at mac.com
> Phone: 301-226-8848
> Fax: 301-226-8811
> http://homepage.mac.com/jdien07/
>
> ----------------------------------
> 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.

----------------------------------
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.



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