Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Brain and Cognitive Sciences, September 2011.Cataloged from PDF version of thesis. "September 2011."Includes bibliographical references (p. 125-156).Parkinson's disease (PD) is an age-related degenerative disease of the brain, characterized by motor, cognitive, and psychiatric symptoms. Neurologists and neuroscientists now understand that several symptoms of the disease, including hallucinations and impulse control behaviors, stem from the dopaminergic medications used to control the motor aspects of PD. Not all patients experience these nonmotor symptoms and tools that can predict a priori which patients are likely to have an adverse response to medication do not exist. This thesis begins to fill this gap by elucidating the mechanisms underlying the adverse effects of dopaminergic medications. Converging evidence from animals and humans shows that individual differences in particular genes that affect the dopamine system may alter the response of PD patients to dopaminergic medication. We examined the hypothesis that patients taking dopamine replacement therapy who carry candidate alleles that increase dopamine signaling experience a dopamine overdose, causing unwanted psychiatric symptoms.by Paymon Ashourian.Ph.D.