I would say, maybe the other way around - gait is very much a cognitive task (think of all the compute power w/ reinforcement learning to get a robot to stand up or balance a stick). but it is probably a self-reinforcing cycle.

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).

My father had Parkinson's and in my experience the lack of agency he had when trying to do daily tasks even other than walking was a real mind-killer. He was dependent on my mom (and later caretakers and skilled nursing facility) to do anything like feeding, toileting etc. Obviously when he was later bedridden mostly due to the inability to move, it probably drove him even more crazy.

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.

My father was disqualified from getting DBS due to concerns around amplifying his dementia. Had I known then what I know now, I would have gladly signed a waiver, because DP has completely destroyed his body and mind.

Disagreed, exercise of all kind is known to help with neuroplasticity, complex movements such as dancing and compound lifts even more so. I can easily see how being unable to do any of those things could at least contribute to mental decline.

As I understand the medical science, it all comes down to stimulating the brain and there is no harder stimulation than physical feedback. Lack of mobility means lack of the biggest source of brain stimulation, which leads to accelerated degeneration.

I think we're talking about 2 different things.

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.