Answer
Greengard's studies primarily focused on the basal ganglia, a brain region involved in motor control, learning, and reward. His research elucidated the molecular mechanisms underlying neurotransmission in this region, specifically the role of protein phosphorylation in regulating neuronal activity. Greengard's work laid the foundation for understanding the molecular basis of neurological disorders associated with the basal ganglia, such as Parkinson's disease.