Hello,
To summarize, you have two head shapes from different subjects: the head surface from the Colin27 template and the electrodes locations digitized on your subject’s head.
To match the two in an EEG study, you have two options to make them match:
- either you transform the electrodes to fit the template head surface (EEG-MRI registration > Project electrodes on scalp surface)
- or you transform the MRI and surfaces to fit the head shape defined by the electrodes positions (Digitized head points > Warp > Deform to fit)
It’s one or the other. If the case of #2, you should not edit the positions of the electrodes before, except for solid transformations (rotations+translations of the whole set of electrodes) to align roughly the subject’s head shape with the Colin27 brain.
I’m not sure what gives the best results in EEG. The approach #1 is probably easier. The warping of default anatomy (option #2) was developed more for MEG studies. In MEG, the positions of the sensors are fixed, so we cannot do something like “project sensors on the scalp surface”.
If you want to go for this option #2 anyway, you can get better results by adding more head points than just the electrodes. You can do this with the Brainstorm digitizer interface:
http://neuroimage.usc.edu/brainstorm/Tutorials/TutDigitize
In the database, the channel file must have exactly the same number of entries in the .Channel field as the recordings (number of rows of the .F field in the data files).
The fiducials are not supposed to be imported as “electrodes” in the channel file, they should be saved in the field .SCS in the channel file and they should not appear as points that you can edit using the menu “MRI registration > Edit”.
If they are saved as electrodes, you should delete them from the channel file immediately after importing it, either with the menu “MRI registration > Edit” or with the menu “Edit channel file”.
Please let me know if you have further questions.
Francois