Who said anything about consent?
If the criteria that are important here are (1) finding a cure for diseases like Parkinsons, and (2) making sure that there is no suffering, then why in the world shoud it matter whether you -- or anyone else who might be comatose -- gives h is or her consent?
Why shouldn't the State, in the best interests of minimizing suffering caused by dieseases such as Parkinson's, simply allow the sacrifice of those sub-humans whose sub-humanity is defined by the fact that they are comatose? And isn't finding a cure for something as awful as Parkinson's much more important that giving you -- or anyone else who might be comatose -- your own dignity as a human being?
If consent/non-consent is reasonably available, it should be a requirement.