[FieldTrip] Leadfield magnitude with openmeeg method

[Schoffelen, J.M. (Jan Mathijs)]

Hi Yiyao,

Is there a specific reason why you expect the potential to go up with increasing conductivity values?

I think there’s some confusion with respect to your expections versus reality. Also, it’s not specific to openmeeg as a method, if you’d use an analytic (concentric) sphere model you would probably observe the same thing.

With respect to units I’d recommend to stick to S.I. units throughout. FieldTrip has quite some checks and balances in order to try to match the units of different objects that are needed for the computations (e.g. meshes, sensor positions etc.), but I am not sure whether there’s an explicit specification of a conductivity unit somewhere. Therefore, I suspect that the unit thereof is assumed to be S/(the internally used metric units, which can be mm/m/cm/km/lightyear). Perhaps it’s safest to explicitly input everything in meters (this can be achieved typically with the ft_convert_units function).

Good luck,

Jan-Mathijs


On 25 Apr 2023, at 09:50, Ye Lin via fieldtrip <fieldtrip at science.ru.nl<mailto:fieldtrip at science.ru.nl>> wrote:

Dear Colleagues,

I attempted to solve volumen conduction problem for another organ rather than brain using openmeeg method. The geometry was expressed in mm. As for conductivity, the leadfield value (magnitude order 10^-5) was about 1000 times higher than expected when I introduced the tissue conductivity in siemen/metre. Then I thought that maybe I should express the conductivity in siemens/mm, and multiplied by 0.001 the tissue conductivity, curiosly the leadfield value increased 1000 times (magnitude order 10^-2). Analogly, when I multiplied by 1000 the tissue conductivity, the leadfield decreased 1000 times (magnitude order 10^-8). This phenomenon is very strange because I expected to have higher surface potential for higher tissue conductivity. The last experiment I did is that I set the tissue resistivity in ohmio.meter in tissue conductivity field, the leadfield value was in expected range.
The surface potential distribution I obtained was physically interpretable, the surface potential amplitude is the unique issue that generating confusion.
How should I set the tissue conductivity of ft_prepare_headmodel when using openmeeg method?


Best regards
**********************************
Yiyao Ye Lin
Universitat Politècnica de València,
Departamento de Ingeniería Electrónica,
Ed, 7F, 4 planta,
46022, València,
València, España


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