[FieldTrip] Incorrect BEM leadfields

Klink-3, N.E.C. van N.vanKlink-2 at umcutrecht.nl
Thu May 4 17:05:18 CEST 2017

Dear all,

I am trying SAM beamformer on EEG data, but the results I get are strange. I found out that my BEM model is probably the cause of this, because the leadfields obtained by the model look scattered, see figure. (Black dot is one dipole location, around the head are 60 EEG electrodes with leadfield value for that dipole.) This is the optimum leadfield at optimum orientation as calculated in beamformer_sam. The three original orientations look scattered as well.

When I make a similar figure with concentric spheres, the leadfield looks like a dipolar field, as expected. When I make the figure for the simultaneously measured MEG, with the same BEM, the leadfield also looks good. (Probably because MEG doesn't care so much about the headmodel.)

I have created my BEM with OpenMEEG (in Windows), BEMcp gives similar results. The meshes look fine to me, and changing the number of vertices to [3000 3000 3000] does not help. Does anyone have an idea what could be the problem here?

Thanks in advance,
Kind regards,

Some code:
%make headmodel
cfg                = [];
cfg.output         = {'brain' 'skull' 'scalp'};
seg = ft_volumesegment(cfg, MRI);

%create mesh
cfg        = [];
cfg.method = 'projectmesh';
cfg.numvertices     = [1000 1500 3000];
cfg.tissue          = {'scalp','skull','brain'};
mesh = ft_prepare_mesh(cfg,segi);

% compute the subject's headmodel/volume conductor model
cfg                = [];
cfg.method         = 'openmeeg';
cfg.tissue          = {'brain' 'skull' 'scalp'};
vol                = ft_prepare_headmodel(cfg, mesh);
vol = ft_convert_units(vol,'cm');

%create grid
cfg                 = [];
cfg.grid.pos        = load([ptfile filesep pts]);  % predetermined grid positions in mm
cfg.grid.pos        = cfg.grid.pos./10; %make unit = cm
cfg.headmodel       = vol;
cfg.moveinward      = 0.01; %check if all grid points are inside the brain
grid = ft_prepare_sourcemodel(cfg);

% Create leadfields
hdr                  = ft_read_header('data.fif');
cfg                  = [];
cfg.elec             = hdr.elec;  % electrode positions
cfg.headmodel        = vol;
cfg.grid             = grid;
cfg.channel          = {'EEG', '-EEG043', '-EEG036'};
cfg.normalize        = 'column';
lf                   = ft_prepare_leadfield(cfg);

% beamformer
cfg.method = 'sam';
cfg.senstype = 'EEG';
cfg.grid = lf;
cfg.headmodel = vol;
cfg.sam.lambda = '5%';
cfg.sam.meansphereorigin = 1; %unused but necessary to not crash the script
cfg.sam.fixedori = 'robert';
source = ft_sourceanalysis(cfg, timelock);


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