[FieldTrip] questions about forward calculations

[Alexandre Gramfort]

Hello Margit,

> I have some questions regarding forward computations.
>
> 1) How is the definition of the EEG leadfield - does the dipole point from red to blue or from blue to red? I obtain different results when I compute the leadfield of the same dipole either with a 3-concentric-spheres volume conductor or with BEM (openmeeg).

do you say that you get different polarity for sphere and OpenMEEG? if
it's the case my only
guess it that the triangles of the boundary meshes are not properly
oriented. Maybe Cristiano
who looked at this recently can comment.

does the toy example behave properly? In
external/openmeeg/openmeeg_eeg_leadfield_example.m

> 2) In which units are the leadfields given? For the 3-concentric-spheres volume conductor, the magnitude of the field is 70, for BEM 2e-5.

If the input meshes passed to OpenMEEG are in meters and dipole
currents in Am then the leadfield should give EEG potentials in V.

> 3) Also for MEG, the BEM leadfield is 1e-6 times smaller than the single sphere. For the single sphere model, the field strength is 1e-5. I think, this is Tesla and corresponds to a 1 Am dipole. But the BEM forward field of the same dipole has strength 1e-11. What is the unit here?

Make sure your BEM surfaces are expressed in meters and not
millimeters like it is often the
case in MRI coordinates, and tell me if you still see some inconsistent results.

Best,
Alex
-- 
Alexandre Gramfort, PhD
gramfort at nmr.mgh.harvard.edu
Dept. of Radiology MGH Martinos Center / Harvard Medical School
http://www-sop.inria.fr/members/Alexandre.Gramfort/