It seems the stuff of nightmares. To lie in a hospital bed, unable to speak or move - not even an eyelid. To understand what people say to you, and not be able to respond. Even to hear your fate being discussed, without being able to have your say.
But it can be all too real. It happened to me. And last week research showed that it may also be reality for many patients diagnosed as in vegetative states, sometimes condemned as brain dead and beyond recovery.
The research, published in Science, showed that a 23-year-old woman, so diagnosed after a road accident, responded in the same way as healthy people when asked to imagine walking round her house or playing tennis.
Dr Narender Ramnani, of Royal Holloway University, said it suggested that many similar victims "might be quite capable of decision-making and have a rich and complex internal life".
And one - Kate Bainbridge, now recovering - recalled: "I felt trapped inside my body. I had loads of questions like 'Where am I?', 'Why am I here?', 'What has happened?'. But I could not ask anyone."
I felt the same when I was in a coma after a simple operation went wrong almost 16 years ago. I did not suffer nearly as much as her, or many others. I was out for just a month, received no neurological damage, and was never thought to be in a persistent vegetative state - though my chances of recovery were rated at 1 per cent early on, and went on falling. At one stage my wife, Judy, was told there was no hope.
For much of the time I knew what was going on. I could not see, but I could feel and hear. I knew what was in the news - and, when I awoke, could repeat anecdotes that had been recounted to my apparently deaf ears.
I remember being told what the doctors thought was wrong and consciously setting out to fight it. I once overheard a discussion of how seriously ill I was.
Yet it was more puzzling than scary. This, I think, was partly because the nurses took the trouble to tell me what was happening. They would say that they were going to give me an injection, or take my blood pressure, before I felt the needle going in or the tourniquet tightening.
I could also feel my dear wife's hand in mine, our fingers entwined. I could hear her telling me that the children were all right and that their schools and my office were being supportive. I could not work out what she was doing in the strange world I now inhabited, but her presence was enormously reassuring.
I even consciously planned, in my "unconsciousness", how to make contact with her. Once, when she was giving me a blanket bath, I latched on to the idea that I could say "Thank you" when she finished. But just as she was about to do so, she handed over to a nurse, apologising that she had to get the kids from school!
Twice I knew I was on the cusp of death - a remarkably matter-of-fact, unscary experience. The second time I felt I had a choice, and having decided to live, never - against all logic - doubted I would fully recover.
Visiting some other people in comas, I have felt that they were also alert inside. I could well be wrong, but something in the way they twitched - slightly but visibly - when I spoke to them made me feel my words were going in.
I would never suggest that this applies to all patients in comas, let alone those diagnosed as being in vegetative states.
But ever since I recovered, I have had the uneasy feeling that some people were being denied food and water and allowed to die, knowing what was happening, but unable to communicate. This week's research strengthens my fears. If it leads to doctors becoming more cautious about condemning patients, I, for one, will be relieved.