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<div style="direction: ltr;"><span style="font-size: 10pt;">Dear all, </span>
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<div style="color: rgb(0, 0, 0); font-family: Tahoma; font-size: 10pt;">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.</div>
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<div style="color: rgb(0, 0, 0); font-family: Tahoma; font-size: 10pt;">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.)</div>
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<div style="color: rgb(0, 0, 0); font-family: Tahoma; font-size: 10pt;">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?</div>
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<div style="color: rgb(0, 0, 0); font-family: Tahoma; font-size: 10pt;">Thanks in advance,</div>
<div style="color: rgb(0, 0, 0); font-family: Tahoma; font-size: 10pt;">Kind regards,</div>
<div style="color: rgb(0, 0, 0); font-family: Tahoma; font-size: 10pt;">Nicole</div>
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<div style="color: rgb(0, 0, 0); font-family: Tahoma; font-size: 10pt;">Some code:</div>
<div style="color: rgb(0, 0, 0); font-family: Tahoma; font-size: 10pt;">%make headmodel</div>
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<div>cfg = [];</div>
<div>cfg.output = {'brain' 'skull' 'scalp'};</div>
<div>seg = ft_volumesegment(cfg, MRI);</div>
</div>
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<div>%create mesh</div>
<div>cfg = [];</div>
<div>cfg.method = 'projectmesh';</div>
<div>cfg.numvertices = [1000 1500 3000];</div>
<div>cfg.tissue = {'scalp','skull','brain'};</div>
<div>mesh = ft_prepare_mesh(cfg,segi);</div>
</div>
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<div>% compute the subject's headmodel/volume conductor model</div>
<div>cfg = [];</div>
<div>cfg.method = 'openmeeg'; </div>
<div>cfg.tissue = {'brain' 'skull' 'scalp'};</div>
<div>vol = ft_prepare_headmodel(cfg, mesh);</div>
</div>
<div style="color: rgb(0, 0, 0); font-family: Tahoma; font-size: 10pt;"><span style="font-size:10pt">vol = ft_convert_units(vol,'cm');</span></div>
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<div><br>
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<div>%create grid</div>
<div>cfg = [];</div>
<div>cfg.grid.pos = load([ptfile filesep pts]); % predetermined grid positions in mm</div>
<div>cfg.grid.pos = cfg.grid.pos./10; %make unit = cm</div>
<div>cfg.headmodel = vol;</div>
<div>cfg.moveinward = 0.01; %check if all grid points are inside the brain</div>
<div>grid = ft_prepare_sourcemodel(cfg);</div>
</div>
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<div>% Create leadfields </div>
<div>hdr = ft_read_header('data.fif');</div>
<div>cfg = [];</div>
<div>cfg.elec = hdr.elec; % electrode positions</div>
<div>cfg.headmodel = vol; </div>
<div>cfg.grid = grid; </div>
<div>cfg.channel = {'EEG', '-EEG043', '-EEG036'}; </div>
<div>cfg.normalize = 'column'; </div>
<div>lf = ft_prepare_leadfield(cfg); </div>
<div><br>
</div>
<div>% beamformer</div>
</div>
<div>
<div>cfg=[];</div>
<div>cfg.method = 'sam';</div>
<div>cfg.senstype = 'EEG';</div>
<div>cfg.grid = lf; </div>
<div>cfg.headmodel = vol; </div>
<div>cfg.sam.lambda = '5%'; </div>
<div>cfg.sam.meansphereorigin = 1; %unused but necessary to not crash the script</div>
<div>cfg.sam.fixedori = 'robert'; </div>
<div>source = ft_sourceanalysis(cfg, timelock); </div>
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