The goal of this research study is to optimize the parameter of Low-Intensity Focused Ultrasound Stimulation (LIFUS) that is most effective in changing cortical excitability and motor learning skills in patients who suffered a stroke. The researchers hope to answer two questions.

• with increasing power, would LIFUS be more effective?
• with the same power, what LIFUS timing is the best

For Aim 1, the study will compare 8, 4 W/cm^2 to a zero(sham) stimulation to see if higher power is the better.

For Aim 2, the study will compare 500 vs 1000 vs 2000 Hz "timing" to see which one is better.

Inclusion Criteria:

• age >=21 years old of any gender or race
• First-ever ischemic or hemorrhagic stroke (neuroimaging verified) at least 6 months from the stroke onset of symptoms
• Unilateral arm weakness measured by the Fugl-Meyer Upper-Extremity Scale ≤ 62/64
• Inducible rest motor threshold and testing motor threshold recorded from the affected abductor pollicis brevis muscle

Exclusion Criteria:

• Bilateral strokes (infarcts and/or hematoma)
• Other co-existent neuromuscular disorders affecting upper extremity motor impairment
• History of medically uncontrolled depression or other neuropsychiatric disorders despite medications either before or after a stroke that may affect the subject's ability to participate in the study
• History of confirmed dementia or taking dementia drugs
• Uncontrolled hypertension despite medical treatment(s) at the time of enrollment, defined as SBP≥185 mmHg or DBP≥110 mmHg (patient can be treated, reassessed and enrolled later)
• Presence of any MRI/TMS/LIFUS risk factors
• Concurrent enrollment in another interventional stroke recovery study
• Concerns that the subject cannot comply with study procedures and visits
• Pregnant