I would like to constrain my 2nd-level MRI analysis using an explicit
mask to restrict the search volume. In SPM8, there are at least three
ways to do this:
A. When estimating the model, specify an explicit mask;
B. After running whole-brain analysis and results, do a small-volume
correction and specify the mask image;
C. When running results, during masking options specify an image to
inclusively mask by.
And of course, there is (D), the unmasked whole-brain analysis on the
same images.
When running these 4 analyses, approaches A-C give different results,
although I would expect them to be the same (because it's the same
search volume, and the same voxelwise uncorrected threshold I'm
using).
One difference is the smoothness (as reported at the bottom of the
results table):
B==C==D, A is different.
The volume (i.e. just below the FWHM in the results table) also
differs, with A==B and C==D.
The cluster-level corrections (both FWE and FDR) also differ:
A, B, C, D all differ (D I would of course expect to be different, but
I also expected A==B==C)
The FDR p value for the peak voxel changes as well:
A, B, C, D all differ.
(Needless to say, the t/Z stats for the peaks are identical across all
of these approaches.)
Any insight into why these 3 methods of constraining the search volume
give differing results would be appreciated...as well as which method
might be most sensible.
Thanks in advance,
Jonathan
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