There were reports recently of a study which tested different methods for telling whether a paralysed patient retained some consciousness. In essence, PET scans seemed to be the best, better than fMRI or traditional, less technically advanced tests. PET scans could also pick out some patients who were not conscious now, but had a good chance of returning to consciousness later; though it has to be said a 74% success rate is not that comforting when it comes to questions of life and death.
In recent years doctors have attempted to diagnose a persistent vegetative state in unresponsive patients, a state i which a patient would remain alive indefinitely (with life support) but never resume consciousness; there seems to be room for doubt, though about whether this is really a distinct clinical syndrome or just a label for the doctor’s best guess.
All medical methods use proxies, of course, whether they are behavioural or physiological; none of them aspire to measure consciousness directly. In some ways it may be best that this is so, because we do want to know what the longer term prognosis is, and for that a method which measures, say, the remaining blood supply in critical areas of the brain may be more useful than one which simply tells you whether the patient is conscious now. Although physiological tests are invaluable where a patient is incapable of responding physically, the real clincher for consciousness is always behavioural; communicative behaviour is especially convincing. The Turing test, it turns out, works for humans as well as robots.
Could there ever be a method by which we measure consciousness directly? Well, if Tononi’s theory of Phi is correct, then the consciousness meter he has proposed would arguably do that. On his view consciousness is generated by integrated information, and we could test how integratedly the brain was performing by measuring the effect of pulses sent through it. Another candidate mught be possible if we are convinced by the EM theories of Johnjoe McFadden; since on his view consciousness is a kind of electromagnetic field, it ought to be possible to detect it directly, although given the small scales involved it might not be easy.
How do we know whether any of these tests is working? As I said, the gold standard is always behavioural: if someone can talk to you, then there’s no longer any reasonable doubt; so if our tests pick out just those people who are able to communicate, we take it that they are working correctly. There is a snag here, though: behavioural tests can only measure one kind of consciousness: roughly what Ned Block called access consciousness, the kind which has to do with making decisions and governing behaviour. But it is widely believed that there is another kind, phenomenal consciousness, actual experience. Some people consider this the more important of the two (others, it must be added, dismiss it as a fantasy). Phenomenal consciousness cannot be measured scientifically, because it has no causal effects; it certainly cannot be measured behaviourally, because as we know from the famous thought-experiment about philosophical ‘zombies’ who lack it, it has no effect on behaviour.
If someone lost their phenomenal consciousness and became such a zombie, would it matter? On one view their life would no longer be worth living (perhaps it would be a little like having an unconscious version of Cotard’s syndrome), but that would certainly not be their view, because they would express exactly the same view as they would if they still had full consciousness. They would be just as able to sue for their rights as a normal person, and if one asked whether there was still ‘someone in there’ there would be no real reason to doubt it. In the end, although the question is valid, it is a waste of time to worry about it because for all we know anyone could be a zombie anyway, whether they have suffered a period of coma or not.
We don’t need to go so far to have some doubts about tests that rely on communication, though. Is it conceivable that I could remain conscious but lose all my ability to communicate, perhaps even my ability to formulate explicitly articulated thoughts in my own mind? I can’t see anything absurd about that possibility: indeed it resembles the state I imagine some animals live their whole lives in. The ability to talk is very important, but surely it is not constitutive of my personal existence?
If that’s so then we do have a problem, in principle at least, because if all of our tests are ultimately validated against behavioural criteria, they might be systematically missing conscious states which ought not to be overlooked.