[FieldTrip] problem with MEG leadfield

Hamid Mohseni hamid.mohseni at eng.ox.ac.uk
Mon Mar 11 12:55:30 CET 2013


Hi,

In MEG normally people use single layer volume conductor and It
is believed that three layer is not useful and can deteriorate the results
(this is in contrast to EEG). Therefore,  you should use a single shell or
a single sphere for MEG, which models only the brain (conductivity of the
scalp is assumed to be same as the air).

Thanks




On 10 March 2013 17:22, <s.rombetto at cib.na.cnr.it> wrote:

> Hi everybody
> I'm trying to generate a field for a simulated dipole using the command
> ft_dipolesimulation, for a MEG study.
>
> I do the following:
> -read the hdr
>
> -import the sensor structure with the following lines
> grad=hdr.grad;
> radius = sqrt(sum(grad.chanpos.^2,2));
> grad.chanpos = grad.chanpos./ [radius radius radius];
> grad.label=hdr.label(1:306)';
>
> -then I create the vol structure using the following code
> vol = [];
> vol.r = 12 * [0.88 0.92 1.00]; %   radii of spheres, the head radius is 12
> cm
> vol.c = [1 1/80 1];       % conductivity
> vol.o = [0 0 0];
>
> At this point I have the grad structure, and the vol structure.
> Then I give all the options for the dipole simulation with the code
>
> %% create a dipole simulation with a custom timecourse
> cfg      = [];
> cfg.vol  = vol;      % see above
> cfg.grad = grad;     % see above
> cfg.dip.pos = [0 0.5 0.3];
> cfg.dip.mom = [1 0 0]';
> cfg.dip.frequency = 10;     %Hz
> cfg.ntrials = 10;
> cfg.triallength = 20;        % seconds
> cfg.fsample = 250;          % Hz
>
> but when I try to esecute the line
>
> raw1 = ft_dipolesimulation(cfg);
>
> I find this error
>
> ??? Error using ==> ft_compute_leadfield at 301
> unsupported volume conductor model for MEG
>
> Error in ==> ft_dipolesimulation at 198
>   lf = ft_compute_leadfield(dippos{**trial}, sens, vol);
>
>
> I really cannot understand why  volume conductor model is not supported.
> Any suggestion? I'm pretty sure that the mistake is trivial, but I'm not
> able to see it.
>
> Thank you!
> Dott.ssa Sara Rombetto
>
>  Istituto di Cibernetica
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>
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-- 
Hamid R. Mohseni, PhD
Post-Doctoral Research Assistant
Institute of Biomedical Engineering
University of Oxford, OX3 7DQ, UK
Tel: +44 (0) 1865 2 83826
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