Hello

From the forum, it seems essential to demean a covariate if one wants to test for correlations between say FA or some behavioral or fMRI measure. My question is when should one demean the covariate and are there instances when the covariate can be used as is? For example, if the behavioral RT are expressed as Z scores then is a Demean on the Z score necessary. Could someone briefly explain the rationale behind demean?

Many thanks as always

Cibu