Hello FSLers-

I have a question regarding the pre-threshold masking option in FEAT. I'm working on a higher-level group analysis and would like to isolate voxels in a set of contrasts that fall within a task > baseline mask. When I apply the mask, I get only those voxels that fall within the mask. However, when I run the same contrast without the mask, those voxels that show up in both the unmasked and masked versions have the same intensity values. These were run without correcting for multiple comparisons.

I was under the impression that my stats values would be higher for those voxels that are present in the masked analysis. What am I missing here? Is the masking option only important for multiple comparisons correction? For further clarification, here is the logfile WITH the mask:

echo 73969 > thresh_zstat1.vol
zstat1: DLH=0.0197981 VOLUME=73969 RESELS=242.621

/usr/local/fsl/bin/ptoz 0.005
2.575829

And here is WITHOUT the mask:

echo 260833 > thresh_zstat1.vol
zstat1: DLH=0.0197981 VOLUME=260833 RESELS=242.621

/usr/local/fsl/bin/ptoz 0.005
2.575829

Clearly, the search volumes change but the RESELS remains the same. Is this correct, or is something amiss with my mask?

Thanks for your help,
Nick