[FieldTrip] inconsistent results from dipolefitting and beamforming

[Kathrin Müsch]

Hi Fieldtrip users,

I am trying to apply a beamformer to simulated data to check whether my headmodels are correct. While ft_dipolefit localizes the dipole in the original position, the beamformer results in a source at a totally different position (see figure 1). I kept everything as similar as possible to the example scripts (forward simulation & dipole fit, forward simulation & beamformer) but used individual gradiometer positions, grid and volume of a single subject (CTF space) and plotted the source in a template grid (SPM space). If I run the dipolesimulation with the individual gradiometer structure and the template headmodel vol and template grid, the dipole and the source result in the same position, although the grad and vol are not in the same coordinate system (figure 2). And importantly, shouldn't the dipole be in the right hemisphere anyway with the coordinates 5/2/3? I appended the code below.

Could the different coordinate systems lead to the inconsistent results of dipolefitting and beamforming? Will the dipolesimulation be in the same coordinate system as the volume or gradiometer structure I feed in (CTF space, in my case)? How could I use the dipolesimulation, dipolefitting and beamforming to properly check my headmodels?

I tried so many things now but I couldn't find the error. Any help is very appreciated.

Best,
Kathrin



These are the variables I used:

vol = headmodel of individual subject in CTF space (cm)
data.grad = gradiometer structure of individual subject in CTF space (cm)
mnigrid = grid of individual subject in CTF space (cm)
template_grid = grid of template brain in SPM space (cm)
mri = spm8 template brain in SMP space


%% compute forward simulated data + apply diplole fit
cfg     = [];
cfg.vol = vol;
cfg.grad = data.grad;
cfg.channel = 'MEG';
cfg.dip.pos = [5 2 3]; % pos of dipole in cm
cfg.dip.mom = [1 0 0]';
cfg.dip.frequency = 10;
cfg.ntrials = 20;
cfg.relnoise = 10;
raw = ft_dipolesimulation(cfg);
 
% compute the data covariance matrix, which will capture the activity of
% the simulated dipole
cfg = [];
cfg.covariance = 'yes';
avg = ft_timelockanalysis(cfg, raw);
 
% apply dipolefit
cfg = [];
cfg.vol = vol;
cfg.grad = data.grad;
cfg.dip.pos = [0 0 0];
cfg.gridsearch = 'no';
dip = ft_dipolefitting(cfg, avg);
 
cfg = [];
cfg.location = dip.dip.pos *10;
figure, ft_sourceplot(cfg,mri)

 
% do the beamformer source reconstuction on a 1 cm grid
cfg = [];
cfg.channel = 'MEG';
cfg.vol = vol;
cfg.grad = data.grad;
cfg.grid  = mnigrid;
cfg.method = 'lcmv';
cfg.projectnoise = 'yes'; 
source = ft_sourceanalysis(cfg, avg);

% compute the neural activity index, i.e. projected power divided by
% projected noise
cfg = [];
cfg.powmethod = 'none'; % keep the power as estimated from the data covariance, i.e. the induced power
source = ft_sourcedescriptives(cfg, source);
 
interp = ft_sourceinterpolate([], source, mri);
 
cfg = [];
cfg.method = 'ortho';
cfg.interactive = 'yes';
cfg.funparameter = 'nai';
cfg.funcolorlim = [1.5 4];  % the voxel in the center of the volume conductor messes up the autoscaling
figure, ft_sourceplot(cfg, interp);
 






_____________________________________
Kathrin Müsch

Dept. of Neurophysiology and Pathophysiology
University Medical Center Hamburg-Eppendorf
Martinistr. 52
20246 Hamburg
Germany
Phone: +49-40-7410-54680
Fax: +49-40-7410-57752
E-Mail: k.muesch at uke.uni-hamburg.de
_____________________________________


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