Just my 2 cents: 

I guess you lookded at Colins origin, that was "off"? You can't make that decision based on Colin I would say. The average MNI template (as in your SPM template folder) is the "real" MNI brain, and used for normalization. When I remember well that brain is created from a bunch (100s at least) of Canadians in Montreal, first mapped (using indivisual anatomical landmarks) to Talaraich and then affine transformed to eachother. John, is that correct?

Colin is a very high resolution anatomical scan of one subject, it has too much detail to be referred to as the standard 'MNI' coordinate system.
Therefore, I wouldn't refer to his brain as being MNI, but only "roughly" MNI, although a great number of people present their data on Colin, poor guy ;-). More accurate would be to present functional data on the averaged brains (after warping to MNI) of the subjects that really took part in your experiment.

Hope this helps?

Bas

-------------------------------------------
Dr. S.F.W. Neggers
dept. of Psychonomics,Helmholtz Institute
Utrecht University
Heidelberglaan 2
3584 CS, Utrecht, room 17.09
the Netherlands
Tel: (+31) 30 253 4582 Fax: (+31) 30 534511
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-----Original Message-----
From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]]On
Behalf Of J Chris May
Sent: vrijdag 4 februari 2005 17:59
To: [log in to unmask]
Subject: [SPM] Error in MNI origin coordinates?


Greetings everyone,
I'm new to the list, so I hope this is a proper posting; I've looked thru
the archives and haven't found any explicit answers to my questions.

It appears to me that the origin on the MNI template (any of them, 152, 305,
Colin) is not on the Anterior Commissure; and it seems that others agree:
http://www.mrc-cbu.cam.ac.uk/Imaging/Common/mnispace.shtml
"...the coordinates are reported in terms of the system that Talairach
developed, with coordinate 0,0,0 being at the anterior commissure (AC), and
with the anterior / posterior commissural line (AC/PC line) defining the the
plane where z = 0. (In fact the AC is not exactly at 0,0,0 in the MNI brain,
but about 4mm below...)"

My first question is: Why is the origin off, or how was the origin determined?

My second question is: Do people make a correction for this when reporting
MNI coordinates? I would like to begin reporting MNI coordinates, but I'm a
little troubled by the fact that the default coordinate values reflect an
incorrect AC-relative anatomical location.

My last question is: Does the non-linear formula provided on the webpage
indicated above take into account the differences in origins between the 2
reference brains? Or better yet, does anyone have suggestions as to how to
handle coordinate reporting between the 2 reference brains?

Suppose that you wanted like to provide tables of both coordinate systems.
Perhaps one could talairach the MNI brain and extract those as talairach
coordinates. One could provide the MNI coordinates from SPM also but add a
footnote that the coordinate do not reflect an AC-origin.

Much thanks to all, cheers

-christopher
--
J. Christopher May
University of Pittsburgh
Dept. of Psychology