Model 2 permitted a comparison of the estimated marginal mean region-of-interest volume or thickness between nondependent control subjects and participants dependent on each substance. Significance was defined as in model 1. The large impact of alcohol dependence on the data (see the Results section) influenced the decision to examine whether dependence on any of the substances other than alcohol was related to differences in region-of-interest volume or thickness compared with nondependent control subjects. This was assessed with a secondary linear contrast within model 2 that grouped dependence on nicotine, cocaine, metham- phetamine, and cannabis (but not alcohol) in a comparison with nondependent control subjects.
