Dear Meg,

2. When SPM prompts you "FWE", "FDR", and "none", choose "none" and try 0.001 (or 0.01, 0.005 etc). If you choose "FWE" p<0.05, then SPM uses an FWE-corrected threshold at the VOXEL-LEVEL, which is often creates spurious results on cluster p-values. The corrected cluster p-values are FWE corrected, even if you choose "none", or an uncorrected threshold. Cluster p-values are adaptive, meaning that they are adjusted for the threshold chosen by the user.

I'm not clear on exactly what is being said here.  Say i choose "none" and set my level to 0.001.  You say that the "cluster p-values are FWE corrected even if I choose "none" ?  Is that true?  Does this mean I don't need to do any further corrections?

You are correct. The three options SPM gives, “FWE”, “FDR”, and “none” (or uncorrected) only apply to voxel-level p-values. Although the threshold you choose is taken into account, cluster p-values are calculated differently from voxel-level p-values. So the “Corrected” p-values for the cluster-level in your SPM output are already corrected for FWE, if you are doing PET, SPECT, or fMRI analyses. In VBM, cluster p-values need an additional correction, as I noted in my email earlier.

Hope this helps.

-Satoru