I want to identify brain areas that are sensitive to the scores my subjects have on the Root variable (which is a continuous, between-subject variable). The logic is that these scores represent some cognitive capacity that might also be reflected on their brain activation during the task that they performed inside the scanner. The two other variables (Sem and Phon) are there as controls. So assuming I find a brain area whose activation during the task is modulated by the Root score, I want to show that this modulation is significantly higher than the modulation of Sem and Phon. Hence, the contrast I was thinking of. Does this make sense? All three variables are standartized, if it matters.