[FieldTrip] questions about forward calculations

[Micheli, C.]

Dear Margit,
the bug with the sign of the FT call to OpenMEEG leadfield is now fixed.
Please try again to generate one and let me know if it is consistent to the 3 concentric spherical solution, as you tried before.

I am looking into the units issue in these days.
Cristiano

----- "C. Micheli" <c.micheli at fcdonders.ru.nl> schreef:

> Van: "C. Micheli" <c.micheli at fcdonders.ru.nl>
> Aan: "Email discussion list for the FieldTrip project" <fieldtrip at donders.ru.nl>
> Verzonden: Woensdag 26 oktober 2011 11:24:46
> Onderwerp: Re: [FieldTrip] questions about forward calculations
>
> Hi Margit
> I could replicate the behavior that you described in point 1) the last
> email and at the moment we are checking that all required options
> within FieldFrip and OpenMEEG are set correctly.
> For your points 2) and 3) I could not replicate the problem.
> Everything should work fine if the units of the sensors and the volume
> conductor are consistent.
> However, we are working to make units management more handy for the
> users, so that ambiguities like this can be solved. Please, keep
> posted on the mailing list to track the changes in the near future.
> 
> Could you maybe paste your source code here in the mailing list?
> 
> Cheers,
> Cristiano
> 
> 
> 
> 
> ----- "Alexandre Gramfort" <alexandre.gramfort at inria.fr> schreef:
> 
> > Van: "Alexandre Gramfort" <alexandre.gramfort at inria.fr>
> > Aan: "Email discussion list for the FieldTrip project"
> <fieldtrip at donders.ru.nl>, "c micheli"
> > <c.micheli at fcdonders.ru.nl>, "Margit Schoenherr"
> <Margit.Schoenherr at uk-erlangen.de>
> > Verzonden: Dinsdag 25 oktober 2011 15:41:10
> > Onderwerp: Re: [FieldTrip] questions about forward calculations
> >
> > 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/
> > 
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