This study was structured in 2 phases, and was stopped after the first phase. The collected data will be used to improve the device and prepare future studies.

The purpose of this study is to provide proof-of-concept that directional stimulation and directional recording, in an intraoperative setting, is perceivable in a subject. The tests will be performed using a dedicated intraoperative lead connected to an external neuro-recorder/stimulator, during a deep brain stimulation implantation surgery.

Deep brain stimulation (DBS) technique involves implanting electrodes using a stereotactic method and connecting them to a pulse generator to deliver a current in localized areas in the brain.

The optimum location for DBS electrodes is determined by pre-operative brain imaging, often combined with an intraoperative microelectrode recording/stimulating (MER) sequence. Current MER practice involves inserting micro-recording/stimulating electrodes into guide tubes in the brain and lowering them towards the target, performing recordings at selected steps. The recordings differ based on the types of neuronal structures they are passing through.Based on the recordings, some promising locations are further discriminated by stimulation using macro-electrodes, and the target for final lead placement is chosen.

The spiderSTN lead (Aleva Neurotherapeutics SA, Switzerland) is capable of recording and stimulating from both its inner and outer surface, allowing the user to steer the direction of stimulation during the target location phase. This will allow the neurosurgeon greater flexibility in investigating the balance between therapeutic benefits and stimulation-induced side effects at a given location.

This feasibility study is designed to explore the effect of directional stimulation and the ability to differentiate between stimulation induced side effects at a given location, during the DBS surgery. It will be proposed to patients who suffer from Parkinson disease (PD) or essential tremor (ET) and for whom DBS is indicated according to standard criteria at the hospital. It will be restricted to a limited number of patients (10), sufficient to assess the recording and stimulating capacity of the spiderSTN device. The patient undergoes the normal DBS surgery until the position for the final lead has been chosen. Then the spiderSTN test phase takes place. Once test phase is completed, the conventional DBS surgery resumes.

Inclusion Criteria:

Patient foreseen to undergo DBS for Parkinson's disease or Essential Tremor according to routine criteria of the center.

For Parkinson Disease:

• Person is between 18 and 75 years of age
• Established diagnosis of idiopathic Parkinson's disease with progression of symptoms for a minimum of 2 years.
• Person has a history of at least 30% improvement on the UPDRS therapy in L-dopa (levodopa), except in the case of tremor dominant patient
• Weak control of symptoms by dopaminergic therapy (off phenomenon, fluctuations on / off, dyskinesia on)

For Essential Tremor:

• Person is between 18 and 80 years of age
• Established diagnosis of Essential Tremor for a minimum of 2 years
• Functional disability due to tremor was not adequately controlled by medication for at least 3 months prior to implant.

Person has given his/her written consent

Exclusion Criteria:

• Person suffering from an active major psychiatric disorder
• Mattis Dementia Rating Scale score <130 or otherwise not capable of discernment
• Presence of major co-morbidity or medical condition that may affect participation to the study
• Presence of an electrical or electromagnetic implant (e.g., cochlear implant, pacemaker)
• Person with a previous surgery for the treatment of Parkinson's disease or Essential Tremor
• Person with a previous brain ablation procedure
• Person who suffers from epilepsy
• Person who is pregnant: a pregnancy test will be performed in patients of childbearing age
• Person with coagulopathies
• Abuse of drugs or alcohol