Posts tagged ‘chimerical’

Is colour the problem or the solution? Last year we heard about a way of correcting colour blindness with glasses. It only works for certain kinds of colour blindness, but the fact that it works at all is astonishing. Human colour vision relies on three different kinds of receptor cone cells in the retina; each picks up a different wavelength and the brain extrapolates from those data to fill in the spectrum. (Actually, it’s far more complex than that, with the background and light conditions taken into account so that the brain delivers a consistent colour reading for the same object even though in different conditions the light reflected from it may be of completely different wavelengths. But let’s leave that aside for now and stick with the simplistic view.) The thing is, receptor cells actually respond to a range of wavelengths; in some people two kinds of receptors have ranges that overlap so much the brain can’t discriminate. What the glasses do is cut out most of the overlapping wavelengths; suddenly the data from the different receptor cells are very different, and the brain can do a full-colour job at last.

Now a somewhat similar approach has been used to produce glasses that turn normal vision into super colour vision. These new lenses exploit the fact that we have two eyes; by cutting out different parts of the range of wavelengths detected by same kind of receptor in the right and left eyes, they give the effect of four kinds of receptor rather than three. In principle the same approach could double up all three kinds of receptor, giving us the effective equivalent of six kinds of receptor, though this has not been tried yet.

This tetrachromacy or four-colour system is not unprecedented. Some animals, notably pigeons, naturally have four or even more kinds of receptor. And a significant percentage of women, benefiting from the second copy of the relevant genes that you get when you have two ‘X’ chromosomes, have four kinds of receptor, though it doesn’t always lead to enhanced colour vision because in most cases the range of the fourth receptor overlaps the range of another one too largely to be useful.

There is no doubt that all three kinds of tetrachromat – pigeons, women with lucky genes, and people with special glasses – can discriminate between more colours than the rest of us. Because our trichromat eyes have only three sources of data, they have to treat mixtures of wavelengths as though they were the same as pure wavelengths with values equivalent to the average of the mixtures – though they’re not. Tetrachromats can do a bit better at this (and I conjecture that colour video and camera images, which use only the three colours needed to fool normal eyes, must sometimes look a bit strange to tetrachromats).

Do tetrachromats see the same spectrum as we do, but in better detail, or do they actually see different colours? There’s never been a way to tell for sure. Tetrachromats can’t tell us what colours they see any more than we can tell each other whether my red is the same as yours, or instead is the same as what you experience for green.The curious fact that the ends of the spectrum join up into a complete colour wheel might support the idea that the spectrum is in some sense an objective reality, based on mathematical harmonic relationships analogous to those of sound waves; in effect we see a single octave of colour with the wavelength at one end double (or half) that at the other. I’ve sort of speculated in the past that if our eyes could see a much wider range of wavelengths we would see lower and higher octaves of colour; not wholly new colours like Terry Pratchett’s octarine, but higher and lower reds, greens and blues. I speculated further that ‘lower’ and ‘higher’ might actually be experienced as ‘cooler’ and ‘hotter’. That is of course the wildest guesswork, but the thesis that everyone – tetrachromats included – sees the same spectrum but in lesser or greater detail seems to be confirmed by the experimenters if I’m reading it right.

Of course, colour vision is not just a matter of what happens in the retina; there is also a neural colour space mapped out in the brain (which interestingly is a little more extensive than the colour space of the real world, leading to the hidden existence of ‘chimerical’ colours).  Do pigeons, human tetrachromats, and human trichromats all map colours to similar neural spaces? I haven’t been able to find out, but I’m guessing the answer is yes. If it weren’t so, there would be potential issues over neural plasticity. If your brain receives no signals from one eye during your early life, it re-purposes the relevant bits of neural real estate and you cannot get your vision back later even if the eye starts sending the right kind of signal. We might expect that people who were colour blind from birth would be affected in a similar way, yet in fact use of the new glasses seems to bring an intact colour system straight into operation for the first time. So it might be that a standard spectral colour space is hard-wired into the genes of all of us (even pigeons), or again it might be that the spectrum is a mathematical reality which any visual system must represent, albeit with varying fidelity.

All of this is skating around the classic philosophical issues. Does Mary, who never saw colours, know something new when she has seen red? Well, we can say with confidence that the redness will be registered and mapped properly; she will not have lost the ability to see colour through being brought up in a monochrome world. More importantly, the scientifically tractable aspects of colour vision have moved another step closer to the subjective experience. We have some objective reasons for supposing that Mary’s colour experience will be arranged along the same spectral structure as ours, though not necessarily graduated with the same fineness.

None of this will banish the Hard Problem, or dispel our particular sense that colours especially are subjective optional extras. For a long time some have thought of colour as a ‘secondary’ property, in the observer, not the world; not like such properties as mass or volume, which are more ‘real’. The newly-understood complexity of colour vision leads to new arguments that it is in fact artificial, a useful artefact in the brain, in some sense not really there in objective reality.  My feeling though is that if we can all experience tetrachromacy, the gap between the objective and the subjective will not be perceived as being so unbridgeable as it has been to date.


Picture: Paul Churchland. There is a lot of interesting stuff over at the The Third Annual Online Consciousness Conference; I particularly enjoyed Paul Churchland’s paper Consciousness and the Introspection of Apparent Qualitative Simples (pdf), which is actually a fairly general attack on the proponents of qualia, the irreducibly subjective bits of experience.  Churchland is of course among the most prominent, long-standing and robust of the sceptics; and it seems to me his scepticism is particularly pure in the sense that he asks us to sign up to very little beyond faith in science and distrust of anything said to be beyond its reach. He says here that in the past his arguments have been based on three main lines of attack: the conditions actually required for a reduction of qualia; the actual successes of science in explaining sensory experience, and the history of science and the lessons to be drawn from it. Some of those arguments are unavoidably technical to some degree; this time he’s going for a more accessible approach and, as it were, coming for the qualophiles on their own ground.

The attack has two main thrusts. The first is against Nagel, who in his celebrated paper What is it like to be a bat? claimed that it was pointless to ask for an objective account of matters that were quintessentially subjective. Well, to begin with, says Churchland, it’s not the case that we’re dealing with two distinct realms here: objective and subjective overlap quite a bit. Your subjective inner feelings give you objective information about where your body is, how it’s moving, how full your stomach is, and so on. You can even get information about the exhausted state of certain neurons in your visual cortex by seeing the floaty after-image of something you’ve been staring at.  Now that in itself doesn’t refute the qualophiles’ claim, because they go on to say that nevertheless, the subjective sensations themselves are unknowable by others. But that’s just nonsense. Is the fact that someone else feels hungry unknowable to me? Hardly: I know lots of things about other people’s feelings: my everyday life involves frequent consideration of such matters. I may not know these things the way the people themselves know them, but the idea that there’s some secret garden of other people’s subjectivity which I can never enter is patently untrue.

I think Churchland’s aim is perhaps slightly off there: qualophiles would concede that we can have third-person knowledge of these matters: but in our own experience, they would say, we can see there’s something over and above the objective element, and we can’t know that bit of other people’s feelings: for all we’ll ever know, the subjective feelings that go along with feeling hungry for them might be quite different from the ones we have.

But Churchland has not overlooked this and addresses it by moving on to the bat thought-experiment itself. Nagel claims we can’t know how it feels to be a bat, he says, but this is because we don’t have a bat’s history. Nagel is suggesting that if we have all the theoretical information about bat sensitivity we should know what being a bat is like: but these are distinct forms of knowledge, and there’s no reason why the possession of one should convey the other. What we lack is not access to a particular domain of knowledge, but the ability to have been a bat. The same unjustified claim that theoretical knowledge should constitute subjective knowledge is at the root of Jackson’s celebrated argument about Mary the colour scientist, says Churchland: in fact we can see this in the way Jackson equivocates between two senses of the word ‘know’: knowing a body of scientific fact, and ‘knowing how’ to tell red from green.

The second line of attack is directed against Chalmers, and it’s here that the simples of the title come in. Chalmers, says Churchland, claims that a reductive explanation of qualia is impossible because subjective sensations are ultimately simples – unanalysable things which offer no foothold to an inter-theoretical reduction.  The idea here is that in other cases we reduce away the idea of, say, temperature by analysing its properties in terms of a different theoretical realm, that of the motion of molecules. But we can’t do that for subjective qualities. Our actual experiences may consist of complex combinations, but when we boil it down enough we come to basic elements like red. What can we say about red that we might be able to explain in terms of say neurons? What properties does red have?  Well, redness, sort of. What can we say about it? It’s red.

Churchland begins by pointing out that our experiences may turn out to be more analysable than we realise. Our first taste of strawberry ice cream may seem like a simple, elemental thing, but later on we may learn to analyse it in terms of strawberry flavour, creaminess, sweetness, and so on. This in itself does not prove that there isn’t a final vocabulary of simples lurking at the bottom, of course. But, asks Churchland, how will I know when I’ve hit bottom?  Since every creature’s ability to discriminate is necessarily limited, it’s inevitable that at some point it’s going to seem as if I have gone as far as I could possibly go – but so what? Even temperature probably seemed like a simple unanalysable property once upon a time.

Moreover, aren’t these unanalysable properties going to be a bit difficult to handle? How do we ever relate them to each other or even talk about them? Of course, the fact that qualia have no causal properties makes this pretty difficult already. If they don’t have any causal effects, how can they explain anything? Qualophiles say they explain our conscious experience, but to do that they’d need to be registered or apprehended or whatever, and how can that happen if they never cause anything? As an explanation, this is ‘a train wreck’.

Churchland is quite right that all this is a horrible mess, and if Chalmers were offering it as a theory it would be fatally damaged. But we have to remember that Chalmers is really offering us a problem: and this is generally true of the qualophiles. Yes, they might say, all this stuff is impossible to make sense of; it is a train wreck, but you know, what can we do because there they are, those qualia, right in front of your nose. It’s pretty bad to put forward an unresolved mystery, but it would be worse to deny one that’s palpably there.

On the point about simples, Churchland has a point too: but there does seem to be something peculiarly ungraspable here. Qualia seem to be a particular case of the perpetual give-away argument; whatever happens in the discussion someone will always say ‘the trouble is, I can imagine all that being true, and yet I can still reasonably ask: is that person really having the same experience as me?’ So we might grant that in future Churchland will succeed in analysing experience is such a way that he’ll be able to tell from a brain scan what someone is experiencing, conclusions that they will confirm in great detail: we can give him all that and still feel we don’t know whether what we actually experience as red is what the subject experiences as blue.

Churchland thinks that part of the reason we continue to feel like this is that we don’t appreciate just how good some of the scientific explanations are already, let alone how good they may become. To dramatise this he refers back to his earlier paper on Chimerical colours (pdf).  It turns out that the ‘colour spindle’ which represents all possible colours is dealt with in the brain by a neuronal area which follows the Hurvich-Jameson model. The interesting thing about this is that the H-J model is larger than the spindle: so the model actually encodes many impossible colours, such as a yellow as dark as black. Presumably if we stimulated these regions with electrodes, we should experience these impossible colours.

But wait! There is a way to hit these regions without surgery, by selectively exhausting some neurons and then superimposing the after-image on a coloured area. See the paper for an explanation and also a whole series of practical examples where, with a bit of staring, you can experience colours not in nature.

These are well worth trying, although to be honest I’m not absolutely sure whether the very vivid results seem to me to fall outside the colour spindle: I think Churchland would say I’m allowing my brain to impose sceptical filtering – because some mental agent in the colour processing centre of my brain doesn’t believe in dark yellow, for example, he’s whispering in my ear that hey, it’s really only sort of brown, isn’t it?

For Churchland these experiments show that proper science can make predictions about our inner experience that are both remarkable and counter-intuitive, but which are triumphantly borne out by experience. I do find it impossible not to sympathise with what he says. But I can also imagine a qualophile pointing out that the colour spindle was supposed to be a logically complete analysis of colour in terms of three variables: so we might argue that these chimerical colours are evidence that analyses of sensory experience and the reductions that flow from them tend to fail and that the realm of colour qualia, quite contrary to the appraently succesful reduction embodied inthe colour spindle, is actually unconstrained and undefinable.  And why are these experiments so exciting, the qualophile might ask, if not because they seem to hold out the promise of new qualia?