There's a link to a story about a man who was presumed to be comatose for 23 years. When a state-of-the-art PET scanner was used, doctors found that the man's brain was functioning normally.
But, he was paralyzed and unable to speak.
He lived in a nightmare of inability to communicate with the rest of the world. Thankfully, he has been hooked up to a device by which he can "talk" to others.
My big objection to the euthanasia of Terry Shiavo was the fierce determination of her husband NOT to determine whether or not she had any consciousness left. In other words, he WANTED to think of her as "non-human", so he could put her down like a dog.
Scratch that. Dogs are better treated than Terry was. She was slowly starved and dehydrated over a period of weeks.