Hello list,
I am doing a combined analysis of T1 (grey and white matter) and DTI (FA and
mean diffusivity=MD. n=2 x 20) data. For DTI I used the non-parametric
toolbox (version 2, SPM2) with 1000 permutations, no variance smoothing, and
FDR on the voxellevel to control for multiple testing. The model is a group
comparison with a covariate of no interest.
Questions:
1. Does the spm_filtered.img represent the T-map thresholded acc. to the
specified 'q' in the FDR routine?
2. FA-Analysis: I have attached the permutation distribution and logplot-
matlab figures.
At FDR-q of 0.005 (p=0.002) a huge cluster appears, at q = 0.001 no voxel
'survives'. So the window for is very narrow - at higher thresholds as e. g.
q = 0.01 matlab (version 7) crashes (segmentation fault) - does this indicate
a bug, or does it mean that all voxels are (would be) significant, and can
the crash be prevented?
3. The lowest uncorr. p-value is 0.001. To dissect p-values below, should just
more permutations be used? Sorry, if this is trivial.
4. In addition, the sensitivity of FA and MD to detect group differences seems
to be different - does it make sense to try to present data at a common FDR
threshold or should the two modalities be treated separately with resp. to
multiple test correction?
5. For 34 degrees of freedom is variance smoothing recommended for DTI data?
Thanks a lot in advance for any help,
Philipp Saemann
Max-Planck-Institute of Psychiatry
NMR Group
80804 Munich
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