Sensory Stimulation for Essential Tremor and Parkinson's Disease

Inclusion Criteria for Healthy Participants: * Age from 18 to 80 years * No history of a brain and/or skull lesion * Normal hearing and (corrected) vision * Able to understand and give informed consent * No neurological disorders, no tremor * Absence of pathology that could cause abnormal movements of extremities (e.g., epilepsy, stroke, marked arthritis) * Able to understand and speak English Inclusion Criteria for Patients: * Age from 18 to 80 years * No prior history of skull lesions or craniotomy * Normal hearing and (corrected) vision * Able to understand and give informed consent * Diagnosis of ET (Tremor Research investigation Group criteria) or diagnosis of PD (UK PD Society Brain bank diagnostic criteria) by a physician * Tremor in at least an upper limb with pure flexion-extension wrist tremor with posture (ET) and rest (PD). * Tremor at least moderate-severe by clinician judgment and tremor scales (Fahn Tolosa Marin Tremor Rating Scale (TETRAS), Movement Disorder Society-Unified Parkinson's Disease Rating Scale (MDS-UPDRS)) * Absence of pathology that could cause abnormal movements of extremities (e.g., epilepsy, stroke, marked arthritis, moderate to severe dyskinesias in PD) * Stable medication doses for at least 30 days prior to study enrollment * Able to understand and speak English Exclusion Criteria for Healthy Participants: * Cardiac pacemaker or pacemaker wires; neurostimulators; implanted pumps * Metal in the body (rods, plates, screws, shrapnel, dentures, IUD) or metallic particles in the eye * Surgical clips in the head or previous neurosurgery * Any magnetic particles in the body * Cochlear implants * Prosthetic heart valves * Epilepsy or any other type of seizure history * Any neurological diagnoses or medications influencing brain function * History of significant head trauma (i.e., extended loss of consciousness, neurological sequelae) * Known structural brain lesion * Significant other disease (heart disease, malignant tumors, mental disorders) * Significant claustrophobia; Ménière's disease * Pregnancy (ruled out by urine ß-HCG if answers to screening questions suggest that pregnancy is possible), breast feeding * Non prescribed drug use * History of current substance abuse (exception: current nicotine use is allowed) * Recreational marijuana * Tremor, parkinsonism; neurological diseases; medical (cardiological, renal, hepatic, oncological) or psychiatric disease that would interfere with study procedures for asES, HD-EMG, TMS, or EEG * Dementia; severe depression; or prior neurosurgical procedures * Failure to perform the behavioral tasks or neuropsychological evaluation tests * Prisoners Exclusion Criteria for Patients: * Cardiac pacemaker or pacemaker wires; neurostimulators; implanted pumps * Metal in the body (rods, plates, screws, shrapnel, dentures, IUD) or metallic particles in the eye * Surgical clips or shunts in the head * Any magnetic particles in the body * Cochlear implants * Prosthetic heart valves * Epilepsy or any other type of seizure history * Significant claustrophobia; Ménière's disease * Pregnancy, breast feeding * Medications increasing risk for seizures * History of current substance abuse (exception: current nicotine use is allowed) * Failure to perform tasks (e.g., follow instructions to stay still in the scanner) or fill in safety screening forms * Prisoners * Atypical or secondary parkinsonism * Co-existence of other neurological diseases * Mixed or complex tremors * Inability or unwillingness to discontinue medications for tremor on the day of study assessments * Medical (cardiological, renal, hepatic, oncological) or psychiatric disease that would interfere with study procedures for asES, HD-EMG, TMS, or EEG; dementia; severe depression; prior neurosurgical procedures