afni.nimh.nih.gov

> COV (mean framewise displacement) is (I believe) a within-subject variable.

So, COV varies across the combinations among the three factors W1-3 for each subject?

> Is it possible to specify corSymm instead of corAR1?

corSymm() is used to specify the variance-covariance structure in the residuals. I would not worry too much about the residuals. Instead, it might make sense to try

-ranEff “~1+COV”

Gang