My best guess is that the "grey matter threshold" was set to zero, which
resulted in lots of low intensity voxels being included in the analysis,
which causes problems. Either use explicit masking, or a grey matter
threshold that will exclude such low intensity regions.
Best regards,
-John
-----Original Message-----
From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]]
On Behalf Of Lynne Gauthier
Sent: Tuesday, January 02, 2007 6:08 AM
To: [log in to unmask]
Subject: [SPM] voxels per resel
Dear fellow SPMers,
I recently ran statistics using SPM with 12 mm smoothed VBM images.
When I
looked at the results, the SPM.mat's .FWHM is a vector of numbers less
than
1 (I think they should be around 12)? Also, there are .1 voxels per
resel
(or ~10 resels/voxel). I was under the impression that there should be
more voxels than resels in the image.... so I'm confused by these
numbers.
As a result of SPM thinking my data is very "rough", my expected cluster
size is less than 1 voxel, making any significant voxel show up as
significant in the cluster-size test. Does anyone have an idea of where
I
might have gone wrong? Is my understanding of the numbers I should have
obtained correct? Thanks in advance for any advice.
Lynne
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