[FieldTrip] Leadfield magnitude with openmeeg method

[yiye at eln.upv.es]

Hi Jan-Mathijs,

 

I am using the attached file to compute leadfield. My source signa is of order of mV, the surface recorded signal should be of ten-hundred microvolts. Taking into account the computed leadfield and normal vector of source, for each electrode, I added the contribution of all the signal sources. The simulated surface data is of order of volt, whic is much bigger than surface recorded data. 

Can you tell me what’s wrong? Thank you very much for your help

Best regards

**********************************

Yiyao Ye Lin 

Universitat Politècnica de València,

Departamento de Ingeniería Electrónica, 

Ed, 7F, 4 planta,

46022, València,

València, España

 

 

De:  <fieldtrip-bounces at science.ru.nl> En nombre de Schoffelen, J.M. (Jan Mathijs) via fieldtrip
Enviado el: sábado, 6 de mayo de 2023 11:55
Para: FieldTrip discussion list <fieldtrip at science.ru.nl>
CC: Schoffelen, J.M. (Jan Mathijs) <janmathijs.schoffelen at donders.ru.nl>
Asunto: Re: [FieldTrip] Leadfield magnitude with openmeeg method

 

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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