He decided early on to never get this treatment and I feel sad to this day that he couldn't have lived a happier last few years.
movement/neuroplasticity/physical stimulation absolutely helps - hence PT, exercise classes, etc have being an important part of treatment
My point is, though - the onset of decline in gait ability typical of Parkinson's patients, is a sign of a decline of cognitive ability because the ability to perform gait and all the inherent brain commands associated with gait, is very much a cognitive function as much as it is motor.
One can argue that good DBS therapy by helping keep patients active and engaged does, at least indirectly, help cognition in that sense. BUT - it is consider not to directly treat Parkinson's dementia if a patient is unfortunate enough to come down with that aspect of disease. i.e. it's not like we can switch the device on or fiddle around w/ the programming and the patient's cognition directly improves from the brain stimulation itself.
That being said, from DBS clinical series - we know the technology helps w/ limb rigidity, bradykinesia and tremor. It helps initially with gait but sometimes the gait declines despite DBS and meds, say, roughly a decade after implant (even as the upper limb symptoms stay well treated).