Dear Matthew,
when you have to mean-center and when you don't always confuses me. In the context of my analysis, you suggested that I should demean the two covariates so that the intercept contrast will answer the question: "Is the activation different from 0 when the effect of the covariate is on average?" while in the present form the analysis answers the question: "Is the activation different from 0 when the effect of the covariate is at 0?". Am I right?
If so, why is the difference between the two question important, if the covariates have a meaningful 0 value?
Thanks for your clarification,
Antonio
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