[Eeglablist] determine the origin of the coordinates of the electrode positions.

[Robert Oostenveld]

Dear Xian

The MNI or SPM coordinate system is related to the anterior and  
posterior commusura (AC and PC), i.e. two distinct points in the  
ventricular system *inside* the brain which can bee seen in a MRI.  
The standard way to describe electrode locations is by relating them  
to points that can be measured *outside* the brain, and it is common  
to use the preauricular points (or alternatively the earcanal) and  
the nasion, i.e. the bridge of the nose.

If you want to use the electrodes (which I presume you now have  
defined in mm with respect to the AC, i.e. MNI coordinates) in  
combination with the spherical BESA head model, you can use the  
inverse of the transformation matrix that Arno described in the  
previous mail:

besa2spm = [
     0.0101   -0.9400         0    0.5588
     1.1889    0.0080    0.0530  -18.0041
    -0.0005   -0.0000    1.1268    1.8045
          0         0         0    1.0000 ];

spm2besa = inv(besa2spm);

See Arno's previous mail which explains how to use this homogenous  
transformation matrix on each of your electrode positions (electrode  
positions can be transformed exactly in the same way as dipole  
positions).

More detailled information about various coordinate systems used in  
EEG, MEG and MRI analyses is pressent at http://www2.ru.nl/fcdonders/ 
fieldtrip/1041.html.

best regards,
Robert


On 10-feb-2006, at 21:02, xian zhang wrote:

> Dear EEG List:
> We acquired the xyz coordinates of the electrodes in T1 image space.
> I wonder how to translate those into standard electrode coordinate.
> In other words,  how to determine the origin and axes from the T1  
> image.
> Are the origin and axes fixed in MNI space? Or are they fixed for
> specific anatomic land marks?
> Xian
>
> _______________________________________________
> eeglablist mailing list eeglablist at sccn.ucsd.edu
> Eeglablist page: http://sccn.ucsd.edu/eeglab/eeglabmail.html
> To unsubscribe, send an empty email to eeglablist- 
> unsubscribe at sccn.ucsd.edu