It's not brain death, but it's not quite like life as we know it, either: being in a vegetative or non-responsive state.
British-Canadian neuroscientist Adrian Owen has made great efforts to communicate with people in these states.
And to date, he's found roughly 20% who may still have minds intact, despite brain and/or body damage.
Owen tells the story of his research in the bookInto the Gray Zone: A Neuroscientist Explores the Border Between Life and Death. The author sits for a phone interview about his research.