Jerry,
This mail may help.
http://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind0010&L=spm&P=R32477
*******
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 spatially 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.
******
Best,
Dave.
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David J McGonigle
Biomagnetic Imaging Lab,
Box 0628, Dept. of Radiology
UCSF, 513 Parnassus Ave
CA 94143 USA
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