Beamforming - filter dimensions

Michael Wibral wibral at BIC.UNI-FRANKFURT.DE
Thu Jul 2 12:09:36 CEST 2009


Dear Christine, dear Frederic,

I think Christine's initial guess was right: 
The option " cfg.projectmom='yes' " seems to not only project the dipole moment timecourse on the direction of maximal power as it says in the beamformer-lcmv function but also compute an effective 1-D filter coefficient for this purpose and export that if you additionally specify cfg.keepfilters='yes'.

To explain things in a little more detail:
beamformer_lcmv.m could in principle compute 3D (or 2D for MEG) filters, compute the three timecourses, then compute the vector direction with the maximal power and export that power value but still keep all filter coefficients. But what it does (and what I think is more straight forward) is to do this projection by having a effective filter coefficient for that direction.

On the other hand, the option cfg.dim = [Nx Ny Nz] is used to give the results of beamforming some regular dimensions when using reverse warped grids for single subjects like Ingrid does. I actually do not know whether the current versions of fieldtrip still need it. It was there to let sourcegrandaverage and sourcestatistics and the source plotting functions know how to handle results over subjects  that had different, non-evenly spaced grids in true space but matching grids when transformed to MNI space (see Ingrid's tutorial/wiki page on this at the fieldtrip website . It simply gives the rectangular dimensions of the common grid in MNI space.

Michael



> -----Urspr√ľngliche Nachricht-----
> Von: "Frederic Roux" <fredericroux at HOTMAIL.DE>
> Gesendet: 01.07.09 21:13:25
> An: FIELDTRIP at NIC.SURFNET.NL
> Betreff: Re: [FIELDTRIP] Beamforming - filter dimensions

Hi Christine,
> 
> I am wondering if this problem could result from the fact that the 
> grids and headmodels were computed in a different way for each 
> analysis.
> 
> In your code it says:
> 
> %for config 1
> fullnameGrid = strcat(mripath, DesignLCMVSources{i,2});
> cfg = load(fullnameGrid);
> cfg.vol = cfg.hdm;
> 
> %for config 2
> cfgLCMV.grid=ForwardModel.grid;
> cfgLCMV.vol=ForwardModel.hdm;
> 
> Cheers,
> frederic
> 
> 
> Kein Werbe-Blabla - hier klicken!



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