This study involves Parkinson's disease (PD). Symptoms include slow movement, tremor, and muscle rigidity. Current medications for the treatment of PD do not improve gait and balance difficulties in individuals with PD. Donepezil (study drug) has been found to reduce falls in individuals with PD. The mechanism in which this reduction of falls occurs is unclear. The investigators study will look at what aspects of gait and balance are improved by the study drug. The study drug is not approved to treat PD in the United States or other countries because we do not know enough about it.