Are you sure about your image extents? That looks like it was probably the extent you collected at, but not the extent of the normalized data (because 64x64x32 is less than 208,000 voxels, so your actual data doesn't match that).
You need to use the values for the data as analyzed. So your first command is fine because it reads voxel size and number of voxels from the mask. Your second command would need to be updated to use the actual x,y,z extents of your normalized data and then could be used, though it will be more conservative than your masked version because it's using the whole cubic volume. Your third command would only be correct if you had analyzed the data in native space.
-Mike
--
Mike Angstadt
Research Computer Specialist / PANLab Lab Manager
Department of Psychiatry / University of Michigan
(734) 936-8229
> -----Original Message-----
> From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]]
> On Behalf Of Mike
> Sent: Thursday, January 14, 2016 1:27 PM
> To: [log in to unmask]
> Subject: [SPM] 3dClustSim further question
>
> Hi,
>
> I have a 3dClustSim question and hope some experts can clarify it for me. In
> my fMRI study, the BOLD acquisition matrix is 64x64x32, with voxel sizes of
> 3.5 x 3.5 x 3.5. Previously I used the command below to calculate extent
> threshold for my fMRI group analysis in SPM8:
>
> 3dClustSim -mask mask.hdr -fwhmxyz 11.2 12.0 11.1 -iter 10000 -pthr 0.001 -
> athr 0.05 (let's call it command A)
>
> where "mask.hdr" is the one produced during fMRI group analysis in SPM8. It
> contains about 208000 voxels. Since in SPM all data have been normalized to
> standard space (i.e., 2x2x2), the volume of the mask is 208000 x (2x2x2) =
> 1664000 mm3. After executing command A, for an alpha level of 0.05 at
> uncorrected p=0.001, the necessary minimal cluster size is 167 voxels.
> However, if I use command B as listed below (the voxel size in standard
> space), the necessary minimal cluster size is 141 voxels.
>
> 3dClustSim -nxyz 64 64 32 -dxyz 2 2 2 -fwhmxyz 11.2 12.0 11.1 -iter 10000 -pthr
> 0.001 -athr 0.05 (this is command B)
>
> If I use command C as listed below (the voxel size in original space), the
> necessary minimal cluster size is just 45 voxels!
>
> 3dClustSim -nxyz 64 64 32 -dxyz 3.5 3.5 3.5 -fwhmxyz 11.2 12.0 11.1 -iter 10000
> -pthr 0.001 -athr 0.05 (this is command C)
>
> My question is, which command should I use to threshold my group results?
>
> Thanks a lot. Mike
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