I guess until I have written ten replies, I cannot post urls. Do a search ELECTROMAGNETIC RADIATION THERAPY, and you will find a full patent explaining their experiment.

The only research is on Alzheimer's so far. For Parkinson's, the area of the brain with issues is the substantia nigra and dopamine production. The substantia nigra is located in the brain stem, so I would guess you would place the led emitter over the back base of the skull. Not suggesting you try this, just illustrating theoretically where it should be applied.