Participants in this study will complete four separate experiments, each with a different aim and testing panel [cognitive function tests with and without functional magnetic resonance imaging (fMRI)]. The first experiment will examine memory, attention, cognitive function, and impulsivity; the aim is to determine the relationship between impulsivity and cognitive function in cocaine dependent individuals receiving treatment. The second experiment will examine the relationship between impulsivity and the prefrontal cortical structure and function. Participants will complete an fMRI during the second experiment. The third experiment will consist of cognitive function tests and will examine the prefrontal cortex in relation to treatment response, based on four different treatments: 1) L-dopamine, 2) naltrexone, 3) modafinil, and 4) placebo. The fourth experiment will examine the effect of cocaine dependence treatment on prefrontal cortex, focusing on participants receiving modafinil.