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Inclusion Criteria:

1. Diagnosis of idiopathic PD with self-reported balance impairment or postural instability.
2. Age 40 to 85 years.
3. Have at least moderate motor impairment (defined by minimum UPDRS Part III (motor) score of 20 while ON medication state).
4. On stable Parkinson's disease medications for at least 30 days prior to enrollment, based on self-report. Dose changes of up to 20% in dopaminergic medications during this period are considered stable.

Exclusion Criteria:

1. Early or advanced stages of disease progression as defined by the modified Hoehn and Yahr stages scores of 2 or less and greater than 4.
2. Active epilepsy or conditions with significantly increased seizure risk including severe traumatic brain injury, congenital birth defects leading to seizures, brain tumors, metabolic disorders associated with seizures, intracranial or subarachnoid hemorrhage, and nonlacunar strokes.
3. Cognitive or communication impairment preventing comprehension of instructions or valid consent (e.g., dementia, severe cognitive impairment, aphasia, intellectual disability, or language barrier).
4. Severe musculoskeletal conditions (e.g., advanced arthritis, joint deformities) that substantially interfere with independent movement or balance.
5. Clinically unstable medical conditions (e.g., uncontrolled diabetes, decompensated cardiac disease, heart failure, decompensated pulmonary disease, COPD).
6. Recent (≤2 months) initiation or planned participation in a physician-prescribed physical therapy program targeting balance or postural instability during the study period. Participation is allowed if the program ended more than 2 months before enrollment, or if the participant has been in a stable program for at least 2 months and plans to continue without major changes during the study.
7. Contraindications to noninvasive brain stimulation or transcranial ultrasound according to the latest published safety tDCS and TUS guidelines.
8. Active, uncontrolled psychiatric illness that may interfere with safe participation or study compliance, including schizophrenia, bipolar disorder, uncontrolled psychosis, or suicidality.
9. Current or recent alcohol or substance use disorder within the past 6 months associated with functional, social, occupational, or legal consequences, or requiring acute treatment.
10. History of neurosurgical interventions including deep brain stimulation, brain ablation, or documented malignant intracranial mass lesions.
11. Current use of medications that may increase seizure risk, as follows: trihexyphenidyl, benztropine, or methylphenidate. Participants taking amantadine at a stable therapeutic dose with normal renal function or bupropion at a stable therapeutic dose of ≤300 mg/day will be eligible following review by the study principal investigator prior to enrollment.

• Cambridge, Massachusetts, United States