Stimulating Specific Brain Areas (VOP/VIM) With Electricity to Improve Movement and Muscle Control

Inclusion Criteria: * Essential tremor patients who are scheduled to undergo deep brain stimulation surgery at UPMC Presbyterian with asymmetric essential tremor symptoms will be included, with the tested limb corresponding to the least affected arm to better approximate normal function. These patients are implanted bilaterally regardless of symptom laterality, as the natural history of essential tremor indicates eventual bilateral symptoms. * Parkinson's Disease patients who are scheduled to undergo deep brain stimulation surgery at UPMC Presbyterian with a history of Parkinson's Disease will also be recruited for intra-operative stimulation of their STN-targeted implants, to assess if VIM/VOP has high specificity for improving motor output. Confirmation that subjects' terms of insurance coverage for their standard of care procedure will not be altered by study enrollment. Exclusion Criteria: * Patients will be excluded from the study if there is any seizure history, to avoid the increased epileptogenic risk of intraoperative stimulation. * Patients with significant bilateral tremor that precludes completion of motor tasks.