Hello,

There are 3 related methods for cluster correction, which might be causing confusion here.

Note that -Clustsim and -ETAC are different (but related) methods. Using both options would simply be for educational purposes, as only one method would be used for the cluster correction. Of course, the method that will be applied (including the uncorrected p-value threshold) should be chosen before viewing the results.

But those options do not require ACF parameters, and they do not require running 3dFWHMx. 3dttest++ with -Clustsim or with -ETAC will generate random clusters using permutation testing on the residuals of the t-test, so an estimate of the mixed model auto-correlation function (ACF) parameters is not needed.

Alternatively, if you were using afni_proc.py and estimated the blur (ACF params) in the residuals, those blurs could be applied using 3dClustSim to do the correction. But that is a different method from using 3dttest++ with -Clustsim or with -ETAC.

- rick