Forward model conductivities

[Stanley Klein]

Dear Paul and Carsten,
The conductivity ratio of 1/20 that you mention is indeed on the low side of
many present estimates. It is way lower than the Rush  &  Driscoll  1/80
ratio that we used in our 30 year old "Ary Correction" paper that examined
the effect of skull and scalp thickness. My big worry however is
not simply to find the ratio (taking individual differences into account).
but
also to get estimates of the differences in conductivity across the skull.
I'm pretty confident that such inhomogeneities are present and that they
will
have significant effects on the BEM forward models. The inhomogeneities
will not only affect the effective depth of sources but also can laterally
shift
their locations to places on cortex with very different surface normals.

An important aspect of our EEG/MEG source localization work is to learn
how to calibrate each individual's conductivity partly based on differences
in EEG vs MEG localizations.

best,
Stan



On Mon, Oct 25, 2010 at 4:40 AM, Paul Czienskowski <paul_c at gmx.de> wrote:

> Dear all,
>
> Alexandre Gramford pointed out to me that there was some discussion ongoing
> on which conductivities to use for EEG-Forward-Modeling and I'd like to move
> this discussion more to public for it could be interesting for many of us.
> I was recently overflying some papers concerned with that very issues and I
> made some interesting and discoveries, especially in
>
> Conductivity of living intracranial tissues. by Latikka J, Kuurne T, Eskola
> H.
> The electrical conductivity of human cerebrospinal fluid at body
> temperature by Baumann et al.
>
> The first one measured the conductivities (or resistivities) of living
> brain tissue and came to values of 3.51 Ohms*m and 3.91 Ohms*m for grey
> respectively white matter and 0.80 Ohms*m for the CSF (which are about 0.28,
> 0.256 and 1.25 S/m). In contrary the second one found a value for 1.79 S/m
> for CSF at body temperature where it was about 1.4 S/m at room temperature.
> The Skull-To-Brain conductivity ratio was measured for example in
>
> Estimation of *in vivo* brain-to-skull conductivity ratio in humans by
> Yingchun Zhang, Wim van Drongelen, and Bin He
>
> where they found a ratio of about 1/18.7 which is way larger than the
> commonly assumed value.
>
> I would appreciate very much if many of you could contribute to this
> discussion by telling us which values you use for the conductivities and on
> which publications the are based.
>
> Best,
> Paul
>
>
> --
> Paul Czienskowski
> Max Planck institute for human development
> Lentzeallee 94
> 14195 Berlin
>
> Björnsonstr. 25
> 12163 Berlin
>
> Tel.: (+49)(0)30/221609359
> Handy: (+49)(0)1788378772
>
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