A challenge to studying automaticity is that central nervous system circuitry for locomotion cannot be directly assessed in humans so shifts between automaticity and executive control have been inferred with a dual-task paradigm. If gait slows or deteriorates during a concurrent cognitive task, gait is deemed less automatic and using more cortical control. Recently, a more direct measure of automaticity has been studied with functional near infrared spectroscopy (fNIRS). fNIRS is used to measure changes in cortical oxygenated hemoglobin (HbO2) levels using light-tissue interaction properties of light within the near infrared range, similar to the way the HbO2 is detected with magnetic resonance imaging. Increased oxygenated hemoglobin are related to increased blood flow, which, in turn, reflects increased cortical activity.

A recent study shows higher than normal activation of the prefrontal cortex during usual walking in patients with PD, an indicator of increased cortical control and reduced automaticity. An enhanced understanding of the neurochemistry underlying gait automaticity in PD will have implications for pharmacologic management of gait impairment in PD. If reduced gait automaticity is due to depressed central dopamine, then more aggressive dopaminergic strategies may be in order. On the other hand, if reduced gait automaticity is due to depressed central cholinergic tone (as the investigators hypothesize), then cholinergic strategies will be more effective for optimizing gait in PD. Few studies have explored the positive effects of cholinergic augmentation on gait and falls but none have examined its direct effects on gait automaticity.

Aim I. Examine the effect of donepezil or levodopa on walking automaticity, as measured by prefrontal cortex activity while walking. This study will employ a double-blind, two-way crossover design in which 20 participants with idiopathic PD will be tested either after 14 days of placebo and their regular dose of levodopa or after 14 days of donepezil (5 mg/day oral) and their regular dose of levodopa. Levodopa doses are not determined or controlled by the investigators, as the recruited participants are already on levodopa specific to their own needs. The investigators will measure automaticity by measuring the activity of the prefrontal cortex while walking in single- and dual-task conditions with a wireless fNIRS system. The investigators predict that donepezil plus levodopa will decrease the prefrontal cortex activity, indicating more automaticity, during walking compared to placebo and levodopa.

Aim II. Relate walking automaticity with spatio-temporal measures of walking. In addition to walking automaticity measured with fNIRS while walking, subjects will be equipped with 3 inertial sensors to measure spatio-temporal features of gait, such as stride time, stride length and their variability. The investigators hypothesize that variability of gait will be related to level of prefrontal cortex activity. The investigators predict that variability of gait will not change with levodopa, reflecting prefrontal control of walking. In contrast, the investigators predict that donepezil will improve variability of gait, both with and without a concurrent, cognitive task.