On Fri, Feb 11, 2011 at 11:56 PM, Carsten Stahlhut <[log in to unmask]> wrote:
> Dear Vladimir,
> I have approx three questions (or issues) that I hope you can help me out
> Do you know what the units are for the lead field matrix and the source
> estimates in SPM8?
These are two different questions.
> If you apply one of the EEG imaging methods (as I understood it) then the
> source estimates should be current density i.e. the unit should be something
> like A/m^2 or uA/mm^2 depending on the scaling of the EEG data and lead
> field matrix.
> And then the units of the lead field should be something like m^2 * Ohm ( =
> m^2 / S ).
In theory the forward computation done by the 'forward' toolbox should
be unit-blind so the units of the lead field should depend on units of
the head model (mm in SPM). In practice this might not be the case. We
recently found a difference in scaling between 'single shell' and
other MEG models. This will be fixed in the next update. There might
be something similar also for EEG. SPM is not sensitive to it but it's
still something worth looking into.
> Does this also holds for SPM8 or do there exist any type of scaling of the
> lead field matrix such as a normalization of the columns or something like
Yes, there are several scaling and normalization steps so the units of
the output images are not related to original physical units but are
normalize to the total power across sources and conditions.
> The second question is related to the lead field matrix as well. If I
> compare two different head models a BEM and 3-spheres model then they seem
> to be on two very different scales - is that really the case? Or am I
> missing an important step? Of course it is two very different head model
> assumptions so they should be quite different but I'm a bit surprised that
> there exists a scaling factor of 100 between the largest element in the BEM
> gain matrix and the 3-spheres gain matrix.
> What I just did was to load the mat-files with the gain-matrices and used
> the imagesc-function to show the amplitudes of the two lead field matrices.
> The 28th Jul 2010 you sent an answer to a post regarding head models and you
> here mentioned that FEM head models from SimBio might be an option in the
> very nice MEEGTool. I was wondering what the status is on integrating the
> SimBio FEM implementation with SPM, as I'm very interested to try out the
> FEM head models. I guess there is no reason that I will try to write the
> code my self if the option is almost done and will be part of the MEEGTool.
I am aware of some efforts to integrate SimBio support in the forward
toolbox. Even if this is done I don't expect it to become one of the
standard options in SPM because SimBio runs only on a particular Unix
platform (I'm not even sure it's LINUX) and I suspect has very lengthy
computing time. But if you compute a gain matrix for the canonical
mesh in SimBio you can just replace the mat file generated by SPM with
your file and it will be used by spm_eeg_invert. If there is
inconsistency in units, it shouldn't be a problem.
There is also a possibility to use another BEM implementation OpenMEEG
(http://www-sop.inria.fr/athena/software/OpenMEEG/). That's something
that works and I tested it last year, but found it impractical for
most users again because of difficulties with installation and long
computation time (about 24 h for normal mesh). The differences in the
lead fields with our BEM were only for a small number of vertices
close to the surface. But I can give you the glue code for this if you