Reference electrode in lead field

[Robert Oostenveld]

Hi Mark

On 28 Sep 2009, at 21:29, Mark Drakesmith wrote:
> 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.

At the moment there is no other reference choise for EEG source
modelling than the average over all electrodes. However, more
elaborate bipolar referencing schemes are since recently needed for
out own research and for that we'll implement a very flexible
referencing scheme. You can expect that to be included in the
fieldtrip release in a month or so. However, the average reference
will remain the defaulty, as it has been shown in simulations to give
the most robust results (although it makes little difference). I don't
immediately know the reference for that, but you should be able to
find it in pubmed.

See also the reply from Burkhard and yesterdays message from Joseph.

> 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.

R is not a single physical value, it is just a shord-hand notation for
one of the terms in the equation. See the reference to implemented
methods on the fieldtrip wiki for publication details.

best
Robert

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