The investigators determine the half maximum effective concentration (EC50) of the Cet value of propofol in target-controlled infusion (TCI) when co-administered with dexmedetomidine for the maintenance of general anesthesia during microelectrode recording in the deep brain surgery in patients with Parkinson's disease.

General anesthesia is sometimes required for DBS (deep brain stimulation) surgery in those with excessive fear, anxiety, reduced cooperation, or severe movements due to ' off drug' state. Dexmedetomidine and propofol are both used individually in the sedation for DBS surgery, and were reported to affect the MER (microelectrode recording) to some extent, especially in the higher zone of the dose range. In this study, the investigators plan to use the combination of dexmedetomidine and propofol for the maintenance of general anesthesia in patients with Parkinson's disease. In this study, dexmedetomidine is given with a loading dose of 0.5 mcg/Kg, and continuous infusion with the dose of 0.4 mcg/Kg/hr. Propofol is given within 0.8~1.8 mcg/Kg during the MER recording (based on institutional guideline). MER recording will be observed and correlated wtih the dose of propofol. The Dixon "up-and-down" sequential allocation method was used to determine the EC50 of the value of Cet of propofol. The study aims to determine the dose that is adequate for maintaining general anesthesia with endotracheal intubation and least interference with the MER recording.

Inclusion Criteria:

1. Aged 20-85
2. Clinical diagnosis of Parkinson's disease
3. Scheduled for deep brain stimulation surgery under general anesthesia

Exclusion Criteria:

1. Allergic history to dexmedetomidine and propofol
2. Clinical diagnosis of congestive heart failure
3. Clinical diagnosis of liver cirrhosis
4. Clinical diagnosis of third degree A-V block

Adult patients, aged 20-85, with clinical diagnosis of Parkinson's disease, who are undergoing deep brain stimulation for which gnenral anesthesia with endotracheal intubation is required.