Participants with Parkinson disease will complete between 50 and 100 postural perturbations (via support surface translations), ON and OFF their dopamine replacement therapy (i.e. levodopa). Adaptation of responses to these perturbations will be tracked. Participants will also undergo transcranial magnetic stimulation to capture cortical excitability of the brain (in particular the motor cortex). Cortical excitability will be correlated to adaptation of stepping (i.e. postural motor learning) ON and OFF levodopa. Investigators will also capture postural motor learning and cortical excitability in age-matched healthy adults.