Nucleus Basalis Deep Brain Stimulation for Thinking & Memory Problems in Parkinson's.

Inclusion Criteria: * All patients will meet Queen Square brain bank criteria for the diagnosis of PD and will have motor fluctuations (off periods and/or L-dopa induced dyskinesias) in response to medications that are known to improve with GPi DBS, and will be appropriate candidates for GPi DBS aside from the coexistence of cognitive impairment. Patients will be aged between 35 and 80 years. Patients will be able to give informed consent. Patients will meet criteria for PD dementia. Patients will have a MMSE score between 26 and lower cutoff of 21 to restrict the sample to those with mild dementia and cognitive impairment. This will be equivalent to an age and education adjusted scaled score of greater than 5 and lower than 9 (mildly impaired range) on the Mattis Dementia Rating Scale-2. Patients will have only minimal atrophy on pre-operative brain MRI scans. Patients will be living at home and will have a carer living with them e.g. their spouse Able to comply with trial protocol and willing to attend clinic necessary visits Exclusion Criteria: * Diagnosis or suspicion of other cause for parkinsonism or dementia. Known abnormality on CT or MRI brain imaging considered likely to compromise compliance with trial protocol. Prior intra-cerebral surgical intervention for Parkinson's disease including Deep Brain stimulation, lesional surgery, growth factor administration, gene therapy or cell transplant.