I am running a command line group analysis, and I want to include a covariate that is zero for all subjects in a group, and diverse for all subjects in the second group (medication intake, which is null for all subjects in the control group).

This generates a lack of range of that continuous variable within a class (control group), and thus mri_glmfit ends with errors.

Is it possible to perform such analysis? Could demeaning procedure work here?

If not, how could I "regress out" the effect of such covariate in between-groups thickness/area differences?

Many thanks in advance,

Pedro.