# Reference electrode in lead field

Joseph Dien jdien07 at MAC.COM
Wed Sep 30 04:10:50 CEST 2009

```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
> 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)
>
> 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
>  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/

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