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