Hi Jorge,
I'll leave this for either Doug or Tom Nichols, who understand this much
better than me. I would guess that 5000 is not enough, but I could be
wrong.
cheers,
Bruce
On Mon, 30 Jun 2008, jorge luis wrote:
Thank you very much Bruce
I have another question:
I plan to use permutation testing in order to correct for multiple comparisons.
Is it enough to perform 5000 iterations for an initial voxel_wise threshold of
0.005?
Is there also any heuristic to select this arbitrary initial threshold and
theoretical results for the required number of iterations for the permutations?
I appreciate very much your help
Jorge
Phd Student
--- El dom, 29/6/08, Bruce Fischl <[EMAIL PROTECTED]> escribió:
De: Bruce Fischl <[EMAIL PROTECTED]>
Asunto: Re: [Freesurfer] About the area of the vertices in the surfaces
Para: "jorge luis" <[EMAIL PROTECTED]>
CC: freesurfer@nmr.mgh.harvard.edu
Fecha: domingo, 29 junio, 2008 5:22
Hi Jorge,
yes, the area of a vertex is 1/3 of the area of all
triangles that it is
a member of, and it should sum to the area of the surface.
The average
subject has a correction factor that accounts for the
"lost" area due to
averaging, so you'll need to apply this if you want it
to be in true mm
(in the average sense across your subjects). mris_info
~/local_subjects/fsaverage/surf/lh.white
reading group avg surface area 864 cm^2 from file
Reading in average area
/homes/4/fischl/local_subjects/fsaverage/surf/lh.white.avg.area.mgh
SURFACE INFO ========================================
type : MRIS_TRIANGULAR_SURFACE=MRIS_ICO_SURFACE
num vertices: 163842
num faces : 327680
num strips : 0
surface area: 70394.8
AvgVtxArea 0.429650
AvgVtxDist 0.757401
StdVtxDist 0.227951
group avg surface area: 86444.7
.
.
.
thus, you would need to multiply your areas by
86444.7/70394.8 to get mm in
the average cross-subject sense.
cheers,
Bruce
On Sun, 29 Jun
2008, jorge luis wrote:
Hello all
I have some questions:
I would like to know how freesurfer computes the
vertexÿÿs wise areas? eg. thouse in lh.area.
It should be expected that the sum of the areas of all
the vertices across a surface equals the total surface area?
To be much specific:
Can I compute the area of an 'activated'
cortical ROI in the average
subject by simply adding the area of all its vertices?
In adavance thank you
Jorge
Phd Student
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