Devdutta, Nick, Bruce, Mike

I'm trying to understand these measures in terms of how they would map to the brain physically.

Would it be correct that the size of the triangles touching a vertex would be related to the amount curvature near a particular vertex and therefore the amount of unfolding that takes place? Thus surface area would seem to relate to both brain size and the foldiness of the brain, and brain size could relate to a variety of factors including the cortical thickness but also the amount of underlying white matter.  So overall surface area seems to be a complex metric that could reflect changes in underlying white matter, cortical thickness and foldiness. Is this correct?

I guess one would have to correct for overall brain size (using TIV or similar measure as a covariate) when examining surface area because of the effect of amount of gray and/or white matter.

Volume then would be the amount of cortex underlying a vertex and the triangles connected to it (estimated as an average of the pial and white matter surface areas).

Darren


 
I think the area probably should be corrected to preserve the native area of the subject, but as you say we haven't gotten to the correction yet. On Tue, 17 Feb 2009, Nick Schmansky wrote:

   Devdutta,

   The 'area' of a vertex on a surface in freesurfer is just the average of
   the area of the triangles which touch that vertex .

   The volume data associated with a vertex created for a subject is not
   buggy in how it is calculated now.  it is the area of the mid-point
   between the white and pial surfaces times the thickness at the vertex.
   The question that has been bandied about internally is whether this
   volume calculation should be corrected when it is mapped to the
   fsaverage surface during a group analysis (qdec does this mapping of the
   volume calcs of each subject to the fsaverage surface, when you select
   'volume' as one of the measures).  At some point we will visit this
   issue (and determine if it is an issue) but other stuff seems to take
   priority.  Maybe someone on the list can chime in.

   Nick

   On Tue, 2009-02-17 at 11:28 -0600, Devdutta W wrote:

       Hi,
       In Qdec, what exactly does the area option measure?  As I understand,
       qdec does its calculations vertex by vertex, which makes sense in case
       of cortical thickness.  But if area refers to surface area how does
       that change from vertex to vertex?  Or is it calculating the area by
       ROI?  Is my understanding of this correct?

       Also, as regards to the volume option, I was told that it is not
       working.  But when I ran it it seems to work fine.  Should I take that
       to mean that the calculation has bugs and that the results it displays
       are incorrect?  Or has it been fixed?

       I greatly appreciate any help in this matter.

       Thank you,
       Devdutta
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