Apathy is defined as a lack of feeling, emotion, interest, or concern. Apathy also involves reduction or loss of motivation and goal-directed behavior. Clinically significant apathy, where meaningful activities are given up and quality of life is diminished, is common in people with Parkinson's disease (PD).

Many individuals with Parkinson's disease experiencing fluctuations in the severity of their movement problems and medication "off" time. "Off" time refers to periods of the day between doses of PD medication when your motor symptoms (e.g., tremor, stiffness, slowness, walking problems, etc.) are worse and interfere with your ability to complete tasks of daily living. The investigational drug, Istradefylline, is an FDA-approved medication to treat motor fluctuations and "off" time in PD.

The purpose of this study is to investigate whether people with Parkinson's disease (PD) who are treated with istradefylline (ISD) show improvements in motivation and apathy over a 12-week period. Specifically, we wish to see whether people with PD who are treated with ISD engage in more physical and recreational activities, such as hobbies and other interests.

We will ask 80 patients with Parkinson's disease, representing the full spectrum of motor and cognitive symptoms, to participate. Participation will include measurement of eye movements using two methods: the new computer-based saccade battery and the best available video-based eye-tracking equipment. The evaluation will be repeated about 30 days later. Data will be analyzed to determine whether the computer-based tasks are reliable and able to provide the same quality of information as the gold standard in eye-tracking. A comparison sample of 80 healthy older adults will also complete the behavioral saccade tests in order to establish normative data that will enable application in clinical settings.