Dear Eric,
I don't know if this is the reason for what you're seeing, but whether
you specify proportional scaling or not, SPM carries out an additional
Grand-Mean scaling procedure within sessions. Basically, per session,
each voxel's intensity values are scaled according to the mean global (
i.e. volume) intensity value. I didn't understand the motivation for
this step until it was explained to me that this is due to the potential
scaling effects (assumed to be stationary) of the scanner's amplifiers
across sessions. The relevant part of the code is from line 511 in
spm_fmri_spm_ui.m onwards.
Perhaps this could explain your observations?
Best,
Dave McG.
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