Patients with schizophrenia have significant problems in thinking, referred to as cognitive dysfunction, which make it difficult for them to function well, and have no effective treatment at this time. A number of existing medications currently used in other medical or psychiatric conditions have been considered, however, none of them has yet shown unequivocal signs of effectiveness for these problems in schizophrenia. One important reason for this, that has not been properly tested, is how antipsychotic treatment may affect the impact of these newer medications for cognition. In the present study, the investigators will evaluate how two different antipsychotic medications may have a different impact on the brain processes that give rise to important cognitive functions that are impaired in schizophrenia. They will conduct functional MRI in schizophrenia patients while they perform a cognitive task, both before and after 8 weeks of treatment with either aripiprazole or risperidone. Then the investigators will evaluate the effects of a new medication called modafinil, and how it may improve brain and cognitive function, depending on which antipsychotic medication the patients are taking. The results of this study may inform researchers, doctors and patients about how to understand the effects of these drugs on the brain and cognition, and which treatments might be best to improve cognition in schizophrenia, in order to improve these patients' ability to function in their lives.