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.