An SPM model can have controls for global factors with ANCOVA, grand mean scaling, and global normalization that is via ANCOVA or proportional.
Global scaling
In a nutshell: never use
Do not use. It aims to correct for drift over time. This is a remnant from the PET days.
Global normalization
In a nutshell: use for VBM OR add TIV as covariate, never for fMRI, usually never for quantitative MRI
This option controls for global values by either scaling the images or including the global mean as a covariate.
...
ANCOVA regressors in design
In a nutshell: never for fMRI, maybe for repeated measures VBM and quantitative MRI
In some designs (multiple groups, 2 sample or ANOVA), you can specify "ANCOVA-by-factor" regressors. The factor would usually be the group, and the covariate the global value (mean over the mask).
"These options allow different subjects to have different relationships between local and global measurements." If not repeated measures, it will do the group x global effects.
References
Dartmouth wiki: useful compilation about global scaling.
See also VBM manual in DARTEL.
Cambrige group page.
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