Dear All,
I've just run a VBM analysis twice, once using absolute thresholding (0.05),
and once using proportional thresholding (0.8).
Contrary to what I expected, I get slightly more significant findings using
absolute thresholding than with proportional. However when I display the
mask.img files, the absolute mask seems to cover all of the brain, while the
proportional mask seems much more constrained to grey matter. Similarly,
accoridng to the results, the search volume for the absolute mask is 1128180
voxels (843 resels), but is 605617 voxels (528 resels) for the proportional
thresholding.
Given the search volume is much larger for the absolute threshold results, I
would have expected lower significance here because of the greater number of
tests, compared to the proportional thresholding. Does anyone have any
suggestions why the opposite might be happening?
Many thanks
Xavier
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