[Eeglablist] Head model
r.oostenveld at fcdonders.ru.nl
Wed Sep 13 11:46:30 PDT 2006
No, it is not a precomputed leadfield. It is a matrix that describes
how the infinite medium potential at all 3000 vertices (i.e. the
potential if there were no conductivitu differences between brain-
skull-skin-air) should be corrected, to account for the conductivity
discontinuities at the three boundaries. This matrix is multiplied
with the infinite medium leadfield on all 3000 vertices to compute
the "bounded" potential distribution on all 3000 vertices.
Subsequently, the potential on the outer 1000 vertices is linearly
interpolated onto the ~128 electrodes (these two steps are done at
once for efficiency).
The formulation of the boundary element method allows for seperating
the volume conduction (geometry+conductivity) terms from the source
terms: the "mat" matrix contains only volume conduction aspects, and
depending on how you compute the leadfield (i.e. one dipole, or a
distributed patch) the infinite medium leadfield only contains the
The actual code in Fieldtrip is not so transparent, but that is
because of speed considerations. If you want to know how it is done,
you should look through the fieldtrip/private/prepare_vol_sens
(computes the linear interpolation from outer surface vertices
towards electrodes) and fieldtrip/private/compute_leadfield (computes
the infinite medium potential on all vertices and then the bounded
medium leadfield on all electrodes).
On 5 Sep 2006, at 10:24, Jose Rebola wrote:
> Dear All,
> Can anybody tell me what exactly is the meaning of the field mat of
> the structured vol in the BEM example for the head models in
> EEGLAB? It is a 3000x3000 double. Is it a pre-computed leadfield?
> And if so, how is it ordered?
> Thanks in advance,
> José Rebola
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