The existing DBS setting in patients with DBS of the globus pallidus interna (GPi), which aims to treat motor symptoms by a high-frequency stimulation, will be temporarily reprogramed to stimulate the NbM at a low frequency using the distal electrodes positioned in the vicinity of the NBM.

This study aims to investigate the neuropsychological effect of low frequency stimulation of the nucleus basalis of Meynert (NBM) in advanced Parkinson's disease (PD) patients, who were previously treated with deep brain stimulation (DBS) of the globus pallidus interna (GPi).

The existing DBS setting in patients with GPi DBS, which aims to treat motor symptoms by a high-frequency stimulation, will be temporarily reprogramed to stimulate the NBM at a low frequency using the distal electrodes positioned in the vicinity of the NBM.This study consists of a three-day screening study of the NbM stimulation followed by a two-week, randomized, single-blind, crossover trial. Between the screening study and the crossover trial, one week of washout period will be applied to eliminate potential carryover effect from the previous stimulation.

This study aims to investigate the neuropsychological effect of low frequency stimulation of the nucleus basalis of Meynert (NBM) in advanced Parkinson's disease (PD) patients, who were previously treated with deep brain stimulation (DBS) of the globus pallidus interna (GPi).

Inclusion Criteria:

• Patients with GPi stimulation, with the NBM within the vicinity of the electric field of at least one DBS electrode.
• Patients should be able to give informed consent
• Patients should be on a stable medication regimen for at least 4 weeks

Exclusion Criteria:

• any unstable internal disease
• Subject with significant worsening of motor function during the pilot screening study, which is indicated by an increase of the UPDRS part III score more than 30%, will not continue to the crossover trial