‘Imbalance model’ of adolescent brain development [64]. NAC = nucleus accumbens, PFC=prefrontal cortex. Dotted line refers to activity levels in adults. During adulthood, both NAC and PFC have reached functional maturity and their activity levels are in balance. Similarly, during pre-adolescence, both regions are immature and activity levels are lower than in adults, but still balanced in relationship to each other. In contrast, there is an imbalance that arises in adolescents because maturation of PFC lags behind that of NAC, and NAC activity is increased.