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ANCOVA regressors in design
In a nutshell: use for VBM, never for fMRI, usually never maybe for 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).
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Dartmouth wiki: useful compilation about global scaling.
See also VBM manual in DARTEL.