Thanks Gang!

> The model seems fine in general. Is 'Category' a within-subject variable?

Yes. In reality it has 4 levels.

> Do the groups differ substantially in "Corr"?

I compared the scores between groups for each level in the within-subject variable "Category". When doing that I find no significant difference between the groups, except for one level (0.04).
When comparing the scores between groups across all within-subject levels, I find a significant difference between the groups.

How do you think is best to proceed? Shall I include -qVarsCenters '1.0532', with 1.05532 as the overall average across "Group" and "Category", or shal I run it without qVarsCenters?

Also, would it be justified to run independent 3dmvms with covariate on each within-subject level separately instead of having them all in the same model? Our main interest is to see group differences in the relationship between BOLD & covariate at each level, and not necessarily to compare the levels.

Thanks so muich for your help.

All the best,
Irene