[Eeglablist] Coordinate systems question

Robert Oostenveld roberto at smi.auc.dk
Fri Feb 18 06:36:52 PST 2005

Hi Laszlo

You can also have a look at 
http://www2.ru.nl/fcdonders/fieldtrip/faq.html#q5 where some coordinate 
systems are explained. In general it is possible to convert from one 
coordinate system to the other, but it may be tricky. Attached you will 
find a function that computes a homogenous coordinate transformation 
matrix, that can be used to convert a list of positions from one to the 
other coordinate system. Also have a look at 
http://bishopw.loni.ucla.edu/AIR5/homogenous.html to see how to use the 
homogenous coordinate transformation matrix.

For dipole modelling it is important to reaslie that all geometrical 
objects (i.e. electrode positions, headmodel and dipolemodel) have to 
be expressed in the same coordinate system (and in the same units of 

best regards,

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On 17 Feb 2005, at 17:13, Bal?zs L?szl? wrote:

> Hi,
> I am trying to write code to digest electrode positions data for a 
> Polhemus Patriot
> digitizer. I would like to match up results with some standard 
> coordinates, however I am
> puzzled with the various location files available. The problem is with 
> the origins/axes.
> Neuroscan sample files seem to have axes thru preauriculars and 
> nasion. These files
> were probably made with a digitzer, not having much attention paid to 
> precision. Other
> files, like the ones in the EEGLAB sample locations seem to have an 
> origin somewhat
> higher, close to the level of Fp1-Fp2-O1-O2. What are the origin and 
> axes here? If I
> understand correctly, the system should not influence the 
> localisation, until it is
> coregistered with te fiducial points (LE,RE, Nasion), since the head 
> model is getting
> transformed to the MRI head using the coordinates of the fiducials .
> Anyway, I would like to see a bit more clearly.
> Cheers,
> Laszlo
> -----------------------------------------------
> Laszlo Balazs, Ph.D.
> Institute for Psychology, Budapest
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Robert Oostenveld, PhD
F.C. Donders Centre for Cognitive Neuroimaging
Radboud University Nijmegen
phone: +31-24-3619695
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