I am a postdoctoral fellow at Vanderbilt University. I use positron emission tomography (PET) and magnetic resonance imaging (MRI) to study the neural basis of cognitive and affective processes underlying reward-driven behavior. My research focuses on the neurotransmitter dopamine and neural networks associated with the dopamine system.
1) Effects of Bromocriptine on Dopamine-Dependent Processes - Dysfunction in the dopamine system is associated with a wide range of disorders, including Parkinson’s disease, schizophrenia, ADHD, and drug addiction. Treatment of these disorders often involves medication targeting the dopamine system (e.g. levodopa for Parkinson’s disease and Adderall for ADHD). Complicating this treatment approach is evidence showing that effects of dopaminergic drugs vary across behavioral domains and across individuals, suggesting that individual differences in baseline dopamine affect their responsivity to dopaminergic drugs. We administer bromocriptine, a dopamine agonist, to healthy subjects who have undergone PET-[18F]fallypride imaging, which indexes basal dopamine receptor levels, to examine effects of bromocriptine on several dopamine-dependent cognitive and reward processes and to study the role of basal dopamine receptor levels on bromocriptine-induced changes. These results will lead to more personalized and targeted treatments of dopaminergic disorders. Additionally, eye blink rate has been used by some researchers as an inexpensive alternative to PET in measuring dopamine function. We will record eye blink rates and correlate them with measured basal dopamine receptor levels to examine the strength of eye blink rates as a proxy for dopamine function.
2) Dopamine Function in Reward-Driven Decision Making - Two views of dopamine function in reward-driven behavior have emerged. Evidence from the reinforcement learning literature shows that dopamine encodes a prediction error, or the difference between expected and received reward, and updates the reward values of stimuli, affecting which action among available actions will be taken. By contrast, in the motivation literature, dopamine is thought to invigorate behavior by reducing response cost barriers or inducing ‘wanting’. Although these two perspectives have significant implications for the understanding of dopamine and its role in reward-driven behavior, no study has examined these hypotheses concurrently to understand how the brain differentiates response choice from response vigor and how the dopamine system mediates these two functions. Niv and colleagues proposed that dopamine could affect both reward-driven decision-making and the vigor with which the decision is executed: positive reward prediction error are modulated by stimulus-induced dopamine release while response vigor is associated with more basal or statewise features of dopamine function. To date, only isolated parts of this model have been examined. We measure basal dopamine receptor levels and induced dopamine release with PET-[18F]fallypride to examine their relationships to the performance and neural correlates of tasks assessing reward prediction error and response vigor. We also examine how personality traits associated with addiction relate to the above measures and develop a multifaceted model of dopamine function in reward.
3) Dopaminergic Modulation of Subjective Valuation - Nearly all decisions require the weighing of expected benefits with any associated “costs”, such as varying amounts of temporal delay until outcomes are realized, the exertion of physical effort required to achieve various outcomes, or uncertainty about the outcome of a choice. Despite their ubiquity in everyday life, scientists have only recently started to examine how these features may be differentially integrated for decision making across adulthood and into old age. Emerging theories suggest that changes in cognition, emotion, and motivation across adulthood influence age differences in decision making. However, there is currently little empirical evidence linking specific changes in psychological processes and neural system function with age differences in choice. Our integrative approach, which includes measurement of psychological individual differences, decision making behavior and functional and structural brain imaging (fMRI, sMRI, PET), will be applied to the study of individual and age differences in preferences for time, effort, and uncertainty across a range of decision making domains (financial, social, health). Using PET-[18F]fallypride in a large life-span sample of healthy adults between the ages of 22 and 80, the project will provide the first whole-brain examination of the specific role of dopamine receptors and systems-level functional activity in supporting the core motivational valuation processes underlying decision making.