[FieldTrip] Coordinate System (Align electrode positions) Brainstrom VS fieldtrip

"Jörn M. Horschig" jm.horschig at donders.ru.nl
Wed Jun 25 16:10:11 CEST 2014

Dear Hamzi,

this might be related to different coordinate systems, see

FieldTrip is using whatever coordinate system the data was spceified in, 
and it seems that brainstorm is using some fixed coordinate system and 
applied a transformation beforehands. you could e.g. notice that in 
Brainstorm the coordinates for the first and third dimension have about 
the same value, where in FieldTrip this happens for the second and third 
value. So I would assume the difference is due to a difference in 
coordinate systems.


On 6/25/2014 1:41 PM, Hamza Fawzi Altakroury (Alumni) wrote:
> Hello,
> I was checking the one of the Fieldtrip's pages to perform an have 
> change the coordinate system of my electrodes position
> The page:
> http://fieldtrip.fcdonders.nl/example/align_eeg_electrode_positions_to_bem_headmodel
> For doing an alignment, the fiducials positions should be given, I did 
> not have them first, so I use the brainstorm toolbox to get them from 
> the patient MRI.
> In brainstrom I found this equation for transforming from voxel to mm. 
> (given the voxel size)
> Position_mm = Position_voxel .* Voxel_size.
> While in fieldtrip I found another statement which gives (given the 
> transformation matrix) a different answer:
> warp_apply(vox2head, vox_Nas, 'homogenous');
> For example using brainstorm equation I got:
> NAS = [117.1920215.8287118.9998]
> And using the fieldtrip function, I got:
> NAS = [14.397349.392649.6322]
> What is the difference between the two?
> Thank you
> -- 
> Hamza Fawzi Altakroury
> Graduate student - MA
> Faculty of Engineering and Natural Sciences
> Sabancı University
> _______________________________________________
> fieldtrip mailing list
> fieldtrip at donders.ru.nl
> http://mailman.science.ru.nl/mailman/listinfo/fieldtrip

Jörn M. Horschig
PhD Student
Donders Institute for Brain, Cognition and Behaviour
Centre for Cognitive Neuroimaging
Radboud University Nijmegen
Neuronal Oscillations Group
FieldTrip Development Team

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