As far as I know SPM provides two methods of removing global signal
effects: scaling with scan (volume) mean or session mean. I managed to
find the code responsible for calculating those means and applying
corrections, but I was wondering if those means were later on used as
nuisance regressors (and if yes where is it in the code). I also heard
heard that SPM was departing from removing global signal due to
negative activation one can get. Therefore it would be great if
someone could shed some light on this issues.