Reference electrode in lead field

Mark Drakesmith mark.drakesmith at POSTGRAD.MANCHESTER.AC.UK
Mon Sep 28 21:29:08 CEST 2009

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 Drakesmith
PhD Student

Neuroscience and Aphasia Research Unit (NARU)
University of Manchester

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