Hi,
Regarding covariates, the OASIS example indeed includes covariates: for instance, when you make inference on sex, age is a covariate. The philosophy is that everything is user-specified, and you need to check that the design matrix represents what you would like to have.
Regarding global values, I have to say, I don’t understand what they represent, as this is not explained in the SPM12 documentation. Is this used as a normalizaton, or simply a covariate ?
Best,
Bertrand