[Freesurfer] Demeaning TIV

Hi all,

I have a question with regards to using the TIV as a covariate in comparing the subcortical volumes between the two cohorts. In particular I'm wondering:

1-Should you demean the TIV at all or not?

2-1-If you are using both age and TIV as your covariates, will it make sense to demean the age but not demean the TIV?

2-If you do demean each or both, should you demean these for each of the cohorts separately or use the mean of all of your subjects?

I should add that the mean age of both of my cohort are pretty much the same and I'm just interested in seeing if there is a difference in the subcortical volumes between the two cohorts.

Thanks for your advices in advance, Nas