[Freesurfer] Problem with covariates in design for surface based analysis

Dear Freesurfur users,

I have a question about the design for a surface based analysis! I want to compare controls and patients. I have 5 covariates. One is a continous variable. The others are categorial variables (4 with 2 levels and 1 with 5 levels). I want to ajust my result with all of thoses variables. I am interested in the difference in thickness between patients and controls. Is it possible to construct such design? Can I put some of my variables in the "nuisance factors" ? I don't really understand the difference between "nuisance factor" and covariabes. I really hope somebody can help me. Thanks for all! All the best, Charlotte