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

Hamza Fawzi Altakroury (Alumni) hamzaf at sabanciuniv.edu
Wed Jun 25 13:41:21 CEST 2014


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 = [117.1920  215.8287  118.9998]

And using the fieldtrip function, I got:

NAS = [14.3973   49.3926   49.6322]


What is the difference between the two?


Thank you

-- 
Hamza Fawzi Altakroury
Graduate student - MA
Faculty of Engineering and Natural Sciences
Sabanc─▒ University
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