Additional Study Information: Why? To evaluate how brain networks are affected when frequency-specific, non-invasive, brain stimulation is applied to people with ataxia or essential tremor as compared to healthy control subjects. Who? Healthy adults over the age of 40, who would like to participate as controls. What? Participation involves one clinic visit for 2-3 Hours, involving EEG measurements from the scalp (noninvasive), wearable sensors on the hand, feet, and back to measure postural movement. This will be followed by non-invasive brain stimulation during resting-state and during movement tasks. Cost to me? There is no cost involved for participating in the study. You will receive $75 after completion of the study. Location: 710 W 168th St, New York, NY 10033