[FieldTrip] BEMCP

Vladimir Litvak litvak.vladimir at gmail.com
Sat Mar 12 17:14:40 CET 2016

Dear Debora,

The critical thing with BEMCP is that your sources are not too close to the
inner skull boundary. When they are closer than the safe distance the
solution you get is completely wrong. When they are not too close, the
solution is OK and comparable at least to concentric spheres. The safe
distance is is more or less the average edge length in your  skull mesh. In
SPM meshes this is 6 mm. In ft_prepare_sourcemodel there is an option
cfg.moveinward that corrects the mesh to ensure that it is not too close.
It would be interesting if you use this option and report whether this
resolves your problem.

This issue should occur in any BEM but depending on the implementation the
critical distance might be shorter.



On Sat, Mar 12, 2016 at 3:30 PM, Debora Desideri <deb.desideri at gmail.com>

> Dear John,
> thanks a lot for your promt reply and your suggestion. I guess that I was
> sticking to the BEM head model just because I read the tutorial for EEG BEM
> head model first, but going quickly through the studies you have mentioned
> and considering the tests you have performed, I may also consider to switch
> to the FEM head model or to simply use the concentric spheres (which
> appeared to work similar to BEMCP in your comparisons). I am relatively new
> to source localization and had no crash/errors in my pipeline, and this
> combination makes always difficult to find possible pitfalls.
> As soon as I have them ready, I will post the results obtained with the
> FEM head model as well, maybe they will be useful for some other users :)
> Thanks again for your support!
> All the best,
> Debora
> On Sat, Mar 12, 2016 at 3:42 PM, RICHARDS, JOHN <RICHARDS at mailbox.sc.edu>
> wrote:
>> Deb:
>> I have had problems with BEMCP.
>>  1—one of the practical / technical problems is that if the compartments
>> overlap, or even come close, then the procedure would not work—the
>> ft_sourceanlaysis crashed.
>>  2—There are a couple of studies comparing BEMCP and dipoli, among other
>> methods.  The BEMCP method does not fare well, the dipoli fares better,
>> E. g., among three or four studies I have seen.
>> Gramfort, A., et al. (2010). "OpenMEEG: opensource software for
>> quasistatic bioelectromagnetics." Biomed Eng Online *9*: 45.
>> Gramfort, A., et al. (2011). "Forward field computation with OpenMEEG." Comput
>> Intell Neurosci *2011*: 923703.
>> 3—I have directly compared BEMCP, a 4-shell concentric spheres, dipoli,
>> and the Simbio FEM model, by correlating results from each model (e.g.,
>> correlations of all the dipole CDR estimates; or correlations across
>> anatomical ROIs).  The BEMCP and concentric spheres work similarly, and the
>> dipoli and Simbio-FEM work similarly.   This was with infants using
>> participant-computed realistic models for the compartments and the FEM.
>> I am not claiming from this and superiority of the dipoli (or FEM) models
>> over the BEMCP, but I suggest looking at the research literature formally
>> comparing these methods before going much further with BEMCP.
>> John
>> ***********************************************
>> John E. Richards Carolina Distinguished Professor
>> Department of Psychology
>> University of South Carolina
>> Columbia, SC  29208
>> Dept Phone: 803 777 2079
>> Fax: 803 777 9558
>> Email: richards-john at sc.edu
>> HTTP: jerlab.psych.sc.edu
>> ***********************************************
> --
> Debora Desideri, PhD Student
> BNP Lab - Neurology Department
> University Hospital Tübingen
> Eberhard Karls University Tübingen
> Hoppe-Seyler Str. 3
> D-72076 Tübingen
> Tel: +49 7071/29 80478
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