A 58-year-old man from Delhi presents with a 2-year history of progressive tremor at rest in his right hand, which disappears with intentional movement. He also reports difficulty initiating gait and a stooped posture. On examination, he has a pill-rolling tremor, cogwheel rigidity in both upper limbs, and bradykinesia. His wife notes he has become emotionally flat and speaks in a monotone. MRI brain is normal. Which of the following basal ganglia circuits is primarily affected in this patient's condition?