Reference electrode in lead field

[Mark Drakesmith]

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

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