E. Williams and colleagues (2006) used linear regression to determine the relationship between levels of a brain

Question:

E. Williams and colleagues (2006) used linear regression to determine the relationship between levels of a brain chemical (5-HT, also known as serotonin) and the positive affect (or mood) of participants in their study. Identify the predictor variable and the criterion variable in their study based on the following description:
A positive [predictive] relationship was observed between 5-HT and positive affect, F(1,21) = 12.045, p = 0.002. 5-HT may reflect the subjective experience of ... mood states. (E. Williams et al., 2006, p. 173)