I believe that this question should be very easy to answer for many of you:

We recently submitted a manuscript about an fMRI study. We compared seed-based functional connectivity between two groups. There's one results derived from two sample ttest with the participants' mean headmovement as a covariate. One of the reviewers  asked for more detailed information about it, for which I couldn't find it in the SPM manual or internet.

I would appreciate it if I can get some mathematical explainations of the covariate inside the model or simply the right keywords to search for the answer.

Thanks & best regards,

Charles