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, 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.
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ANCOVA regressors in design
In a nutshell: use for VBM, never for fMRI, usually never for 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).
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