Silvia,

> How can I implement FWHM in my analysis? More specifically, in the second-level analysis, I should use the acf obtained
> from the mean group (3dttest++) or that one at individual level (from 3dDeconvolve)?

There is no definite answer for this. It seems that people have been empirically estimating ACFs using the individual-level or population-level data.

> Is it correct to use ClustSim in a small volume (e.g. VTA, which has 40 voxels)? I would think that a peak-level analysis
> (as SVC in SPM) would be more appropriate here.

The correctness of SVC is likely in the eye of the beholder. TBH, I personally consider the SVC approach controversial, and would rather not use it. Multiplicity is a complicated issue, not just in neuroimaging. Even though some common methods have been proposed/adopted in the field, controversies remain. As discussed in the paper I mentioned earlier, if the analysis is performed at the whole brain level, I would present as much information as possible at the whole brain level. You can highlight and discuss the regions you would like to focus on, but it would be beneficial to the field and promote reproducibility without limiting the full results to a smaller amount of data.

Gang