MEP onset latencies from the hand muscle using two different stimulus methods will be compared from each study participant. Conventional MEP data will be collected prior to skin incision. Following the DBS insertion hand muscle evoked responses will be generated via the DBS electrode. A hand held DBS programmer will be connected to the proximal end of the implanted DBS for delivery of electrical stimuli. Stimulus intensity will be adjusted until a hand muscle evoked potential is registered from the same recording electrodes used for the conventional MEP acquisition. A STN-implanted electrode can produce hand muscle responses due to it's close proximity to the motor fibres of the internal capsule. When the conventional and DBS-induced hand muscle responses have been acquired the study data objectives have been met. Data will be analyzed off-line.

Purpose: Hand muscle MEP generated via the DBS electrode will provide an onset latency standard from which we can compare conventional MEP from the same hand muscles. It is critical for validation of conventional MEP to know where in the brain the motor pathway is being stimulated. This will avoid false negative MEP test results.

Objectives: We will measure the onset latencies from hand muscle MEP generated via the DBS electrode as well as using conventional MEP methods. The DBS onset latency data will provide a known anatomical standard to compare with conventional hand MEP.

Inclusion Criteria:

• Male or female patients scheduled for DBS surgery under general anesthesia; age 35-75 years; no history of stroke or peripheral nerve disease

Exclusion Criteria:

• peripheral nerve disease (e.g. type I diabetes); demyelinating disease (e.g. multiple sclerosis); previous stroke; allergy to propofol based anesthesia

The patient population will be those diagnosed with medically refractory Parkinson's Disease or Essential tremor and scheduled for DBS surgery.

onset latencies derived from conventional and DBS-induced MEP will be available upon study completion