[FieldTrip] question about virtual electrode MNI coordinates

[Charidimos Tzagarakis]

Hi there,
I have a question regarding virtual electrodes: How can I double check that
the MNI coordinates that I enter are correctly interpreted?
More specifically:
I have followed the the extended beamforming tutorial in the "Appendix" and
have been able to transpose it to my data (I have 4D/BTi MEG files and
Dicom mri volumes).The part I am not certain about is the call to the LCMV
beamformer.
Here it is in my case ( I just want to get the voxel were the power is max
in the previous analysis):
cfg              = [];
cfg.method       = 'lcmv';
cfg.vol          = volfcm;
cfg.grid.pos     = source_diff.pos(maxpowindx, :);
cfg.grid.inside  = 1:size(cfg.grid.pos, 1);
cfg.grid.outside = [];
cfg.grad=senscm
source_idx       = ft_sourceanalysis(cfg, tlock);
My headmodel does not include MEG channel information so I need to also
define the cfg.grad parameter. What I don't understand is how cfg.grid.pos
(which is in MNI coordinates as per the previous part of the tutorial) can
be correctly applied to the coordinates of the headmodel and the channels
since these 2 are not in MNI coordinates (in my case they are in 4d/Bti
coordinates and rescaled to cm).In the previous part of the tutorial this
is (if I understand correctly) accomplished because the leadfield used when
calling ft_sourceanalysis was created using and MNI-warped template. I
don't see where the equivalent part is when estimating the virtual
electrode though.
Any advice you may have would be much appreciated.
Best,
Haris


Charidimos [Haris] Tzagarakis MD, PhD, MRCPsych
University of Minnesota Dept of Neuroscience and Brain Sciences Center
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