Hi,
The zstats are always computed voxelwise so masking will never change
those. Thresholding (in general) takes the specified p-value and
calcualtes what the zstat value should be to give you that p-value -
and that correspondence from p to z will change depending on things
like the p-value, the number of voxels being considered, the spatial
smoothness, etc. So the zstat threhsold being applied will in
general be changing, but the original zstats themselves will not.
Cheers.
On 29 May 2009, at 19:11, Nick Wymbs wrote:
> 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
>
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Stephen M. Smith, Professor of Biomedical Engineering
Associate Director, Oxford University FMRIB Centre
FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
+44 (0) 1865 222726 (fax 222717)
[log in to unmask] http://www.fmrib.ox.ac.uk/~steve
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