[FieldTrip] question about virtual electrode MNI coordinates
"Jörn M. Horschig"
jm.horschig at donders.ru.nl
Wed Aug 7 14:52:32 CEST 2013
thanks very much for pointing to this, it is indeed an error in the
appendix of the tutorial. cfg.grid.pos should be based on the
subject-specific MRI or sourcemodel. So, correctly it should say that
you need to load sourcemodel.mat (i.e. subject specific grid) and then
use sourcemodel.pos(maxpowindx, :)
The rationale in principle is that the warped grid has the same size and
is indexed the same as the original grid; that is why you can use the
index variable obtained from the MNI-warped source reconstructed data.
The virtual channel reconstruction should however still be done in
whatever coordinate system the subject's anatomical data is in. I will
update the appendix accordingly.
On 8/6/2013 8:24 PM, Charidimos Tzagarakis wrote:
> 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 = ;
> 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.
> Charidimos [Haris] Tzagarakis MD, PhD, MRCPsych
> University of Minnesota Dept of Neuroscience and Brain Sciences Center
> fieldtrip mailing list
> fieldtrip at donders.ru.nl
Jörn M. Horschig
Donders Institute for Brain, Cognition and Behaviour
Centre for Cognitive Neuroimaging
Radboud University Nijmegen
Neuronal Oscillations Group
FieldTrip Development Team
P.O. Box 9101
NL-6500 HB Nijmegen
E-Mail: jm.horschig at donders.ru.nl
Trigon, room 2.30
NL-6525 EN Nijmegen
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the fieldtrip