Upper Extremity Weakness

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.