The investigators hypothesize that the emotional and cognitive areas of the Subthalamic Nucleus (STN) have distinct electrophysiological properties and that specific stimulation of these areas can influence the mental and the emotional state. Parkinson's disease (PD; n=5) and obsessive-compulsive disorder (OCD; n=5) patients that are candidates for treatment with Subthalamic Nucleus (STN) Deep Brain Stimulation (DBS) will be recruited over a period of two years. The patients will be enrolled in a prospective, open label, clinical trial and will be followed for one year. No randomization or blinding will be used in this study.

Inclusion criteria of PD patients:

1. Patients with PD who meet accepted criteria for DBS surgery:

   i. advanced idiopathic PD ii. long-term levodopa use that leads to decreased efficacy and increased incidence of adverse effects iii. normal cognitive function or mild-moderate cognitive decline (Addenbrooke's Cognitive Examination (ACE) > 75 and the Frontal Assessment Battery (FAB) > 10.

2. Age 40-75 years

3. Male or female

4. Competent and willing to give written informed consent

Exclusion Criteria of PD patients:

1. A diagnosis of severe major depression disorder (MDD) with psychotic features
2. Significant suicidal risk [Hamilton Depression scale item 3 (suicide) >2]
3. Lifetime history of substance or alcohol dependence or of abuse in the preceding 12 months
4. Significant cognitive decline, as measured by Addenbrooke's Cognitive Examination (ACE) < 75 and the Frontal Assessment Battery (FAB) < 10.
5. Significant cognitive impairment that would affect a participant's ability to give informed consent or provide interview or self-report data reliably
6. Any clinically significant abnormality on preoperative MRI
7. Any DBS contraindication, infection, coagulopathy, significant cardiac risk factors, or other medical risk factors for surgery
8. Pregnant and/or woman of childbearing age not using effective forms of birth control

Inclusion criteria of OCD patients:

1. Obsessive-compulsive disorder (OCD), diagnosed by Structured Clinical Interview for Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) (SCID-IV)
2. Presence of disabling severity, as assessed by a Yale-Brown Obsessive Compulsive Scale (YBOCS) score of at least 25
3. Documented highly treatment refractory OCD; the documentation must demonstrate persistence of severe symptoms and impairment for 5 or more years despite both pharmacological and psychological treatments.
4. Either drug free or on a stable drug regimen for at least 6 weeks before study entry
5. General good overall health
6. Age 20-70 years
7. Male or female
8. Competent and willing to give written informed consent.

Exclusion Criteria of OCD patients:

1. A diagnosis of severe major depression disorder (MDD) with psychotic features
2. Significant suicidal risk [Hamilton Depression scale item 3 (suicide) >2].
3. Comorbidity with any primary Psychotic Disorder, Bipolar Disorder, Post Traumatic Stress Disorder (PTSD), Eating Disorder.
4. Lifetime history of substance or alcohol dependence or of abuse in the preceding 12 months
5. Significant cognitive decline, as measured by Addenbrooke's Cognitive Examination (ACE) < 75 and the Frontal Assessment Battery (FAB) < 10.
6. Significant cognitive impairment that would affect a participant's ability to give informed consent or provide interview or self-report data reliably
7. Any another current clinically significant neurological disorder or medical illness affecting brain function, other than a tic disorder
8. Any clinically significant abnormality on preoperative MRI
9. Any DBS contraindication, infection, coagulopathy, significant cardiac risk factors, or other medical risk factors for surgery
10. Pregnant and/or woman of childbearing age not using effective forms of birth control