This study looks at how the brain's *dopamine* signals work in healthy people and people with Parkinson's Disease (PD). Dopamine is a chemical in the brain that helps with movement and feeling happy. The study will check if there are differences in *optimistic* (expecting good things) and *pessimistic* (expecting bad things) signals in people with and without PD. It uses MRI (Magnetic Resonance Imaging, a type of scan) to see how the brain reacts when people play a game to win money. People with PD will be tested on and off their medication to see how it affects them.

• The study involves 1-2 visits where participants play games in an MRI scanner.
• People need to be 35 or older and meet certain health criteria to join.
• Participants will not take their regular medication before one visit.