Graham,
On Oct 26, 2006, at 6:35 PM, Graham Wideman wrote:
Donna:
> All the PALS_B12 surfaces are mid-cortical thickness surfaces.
Hmmm, this will send us on a closer inspection of the surfaces in
conjunction with volumes.
I think we've been somewhat disoriented at looking at an average
surface (ie: surfaces averaged across subjects) and how it does or
doesn't manage to intersect blobs of activation, and indeed the
degree to which it does or doesn't follow actual anatomy. Ie:
during these initial experiences with Caret, the B12 average
surface gives the impression of being "small". Perhaps this is
just what you get when surfaces are averaged, and the detailed
anatomy individual differences gets averaged out...
That's indeed what happens. The PALS-B12 average fiducial surface
area is 530 cm-sq only about 55% of the vs 962 cm-sq for the average
surface area for the 12 contributing individuals. This is to be
expected as a consequence of individual variability.
The average fiducial surface is particularly compressed in regions of
high variability. The overall distortion/compression pattern is
quantified in a metric file that comes with the atlas dataset. This
allows surface areas computed for one or another region of interest
on the average fiducial surface to be multiplied by an appropriate
factor, so that the adjusted area represents the value for a
'typical' individual fiducial surface.
so an interesting lesson on how anatomy varies.
Anyhow, this discussion has been very helpful.
Good! The issue of whether the average fiducial surface intersects
various blobs of activation is an important one. Our empirical
observation is that generally it does, but there are assuredly some
important exceptions. This is precisly why we went to the trouble of
implementing multi-fiducial mapping. It allows you see the outcome
of mapping to a dozen individuals, then taking the average.
All this is detailed in http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?
db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=16172003
Van Essen DC. A Population-Average, Landmark- and Surface-based
(PALS) atlas of human cerebral cortex.
Neuroimage. 2005 Nov 15;28(3):635-62
David
Graham
