I have long thought there to be a connection between PD and schizophrenia. Consider the following:
From http://www.sciencedaily.com/releases...1105180809.htm <My comments bracketed>

"ScienceDaily (Nov. 6, 2008) — Research from the University of Pittsburgh could expand the options for controlling schizophrenia by identifying a brain region that responds to more than one type of antipsychotic drug.

...Bita Moghaddam, a professor in the Department of Neuroscience in Pitt's School of Arts and Sciences and the paper's lead author, found that schizophrenia-like activity in the orbitofrontal cortex—a brain region responsible for cognitive activity such as decision making—could be triggered by the two different neurotransmitters linked to schizophrenia: dopamine and glutamate....<"decision making", in my experience of PD, is what seems to be going on when I am Off and approaching a doorway or trying to turn. The data stream becomes too large and I am overwhelmed and I freeze.>

"Schizophrenia appears to be caused by very diverse and sometimes rare genetic mutations. Diverse mutations can end up causing the same disease if they disrupt the function of a common group of neurons or networks of neurons.... <Different paths to the same door explains a lot about PD's oddities>

Studies within the last few years—including work by Moghaddam at Yale University—have shown that under-functioning glutamate receptors known as NMDA receptors can produce schizophrenia-like symptoms. Moghaddam and Homayoun found that stunting the NMDA receptors resulted in schizophrenia-like effects in the orbitofrontal cortex. The team also used a dose of amphetamine to simulate dopamine-related schizophrenia symptoms in the orbitofrontal cortex; schizophrenia is often linked to an excess of dopamine in the brain....
<So, do "over-functioning" receptors, as might result from glutamate stimulation, produce PD-like actions?>

Moghaddam and Homayoun then tested the currently prescribed medication—a treatment developed more than 50 years ago that targets neural receptors of dopamine—and new experimental drugs that work on the glutamate system. They found that both medications normalized brain activity." <Presumably we are talking about levadopa here.>