Dear Santiago,

Thank you for the lesson, my knowledge on the history of this matter was
clearly lacking. Particularly, I have overlooked the role of the
proportional grid system. In a way then, the Talairach atlas /is/ a
probabilistic one: "if your coordinate lands in cell X, we say it's
equally likely to be any one of the following structures: a,b,c...; take
your pick!". Keeping in mind this limitation, of a single mm coordinate
having an "accuracy" of ca. 3 cc, I'm ready to concede that the
Talairach atlas has it's place "even" today. I just want to bring up
mni2tal one last time, since it's what sparked my original post.

Quote from Matthew Brett on his webpage on the mni2tal.m-script
(http://www.mrc-cbu.cam.ac.uk/Imaging/Common/mnispace.shtml): "The
approach above is only intended to be preliminary." His 2-part fit of
the MNI to Talairach was based on a (visual) comparison between MNI
slices and atlas pages. I guess significantly more effort needs to be
put into a more accurate converter that conforms to the proportional
grid system. Automatic determination of the Talairach landmarks is
tricky, particularly for the PC, but perhaps someone out there has
already solved the problem or is now inspired to do so... Perhaps this
is what Cinly mentioned in one of his posts?

I couldn't find any estimates on how "much" mni2tal skews coordinates
(relative to the "true" coordinates); difficult problem, I would
imagine. In any case, the use of mni2tal induces some extra uncertainty
into the transformed coordinates, which will lead to some coordinates
"changing" cells. Since a Talairach coordinate has no label, but belongs
to a group of coordinates to which several labels are assigned,
belonging to cell A rather than B can have a significant effect. In
summary, mni2tal is not a shortcut to naming a coordinate using the
Talairach atlas or the daemon. It is a tool for obtaining a rough
estimate for a more detailed (manual?) examination.

Thank you once again, Santiago, for your input,

-Chris
--
Christopher Bailey <[log in to unmask]>
PET Centre and
Center for Functionally Integrative Neuroscience
Aarhus University Hospital, Denmark
http://www.cfin.au.dk/

On Fri, 2004-10-01 at 12:08, Santiago Reig wrote:
> Hi,
>
> I´ve followed with interest the discussion about the "standarness" of the
> Talairach and think that there are two important points about the work by
> Talairach (back in the ´60, don´t forget that) that have been forgotten.
>
> One is the choice of the AC and PC as the key reference points for
> localization. These points were chosen after tedious and painstaking work
> of dissecting long series of brains and realizing that these two points are
> among the most unvarying in location within the brain and more importantly,
> the distance between the two is invariant with regards to brain depth and
> length (the only measurements that they were able to get from
> pneumoencephalography).
>
> The other important (but left behind) issue about the Talairach Atlas is
> the grid system. I mean the tessellation of the brain into the 1,056 cells.
> The first major division yields 6 different regions on each hemisphere,
> three below and three above the AC-PC plane: anterior to AC, between AC and
> PC, and behind PC. A second division of those six hemispheric regions into
> grid cells originates the 1,056 cells. It´s precisely on those cells where
> the homology is grounded. You are most likely to find the same gyrus or the
> same brain nuclei in the same grid cell, despite the dimensions of that
> cell logically vary across subjects. Talairach stated that very clearly in
> his introduction: don´t pay attention to the mm coordinates but to the
> cells. If you follow the Talairach subdivision of the brain, your grid
> cells are homologous subsections across subjects. The beauty of it is that
> the brain size and shape (proportions) remain untouched, yet the grid cells
> locate the same structures labelled in the Atlas. Your resolution is then
> limited by the size of the cells (about 1.5 to 3 cc) which is fair enough
> for some structures and very accurate for major lobes.
>
> Part of the reputation and popularity of the Talairach Atlas comes from its
> use for neurosurgery (the original use of the Atlas in the '60). With the
> help of the Atlas neurosurgeons are able to locate very tiny nucleus in the
> brain, and believe it, it works!
>
> Just my 2 cents...
>
> Regards,
>
> Santiago
>
> ><((((º>`·.¸¸.·´¯`·.¸.·´¯`·...¸><((((º>`·.¸¸.·´¯`·.¸.·´¯`·...¸><((((º>
> `·.¸¸.·´¯`·.¸.·´¯`·...¸><((((º>`·.¸¸.·´¯`·.¸.·´¯`·...¸><((((º>`·.¸¸.·´
> Santiago Reig, PhD.
> Gregorio Maranon Hospital, Universidad Complutense de Madrid
> Experimental Surgery and Medicine Lab
> Dr. Esquerdo 46, 28007 Madrid, SPAIN
> Fax# 34 91 426 5108, Ph# 34 91 426 5067
> [log in to unmask] http://www.hggm.es/image