Thx Erik for your quick reply,

So there is no way to bypass the skullstrip stage in the sense that I have to provide at least my mask instead of providing nothing.
Does all this imply that if I provide my skullstripped EPI as -d and the same EPI renamed EPI.brain+orig. I will gather nothing in the end?
If the algorithm simply subtracts, this means that all the voxels will be deactivated. correct?

thx
R.