Thanks Gang!
I just want to make sure I am interpreting the results from 3dttest w/ covariates properly. The sub-bricks that reference means are the differences between groups with the covariate as a regressor (i.e. removed from the analysis so it's not contributing to the resulting differences)?

The sub-bricks looking at the difference between groups and the covariate (as help function says: difference of slopes wrt covariate IQ - or in my case, cognitive performance - Young-Aged_behavior) means that there are significant differences between the groups that is dependent on the behavioral score?

To visually show this would I have the Young-Aged_behavior sub-brick as the overlay and threshold with the corresponding z-score?

Thanks so much!