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.

 

11 Comments

  1. 1. Lloyd Rice says:

    Very interesting, Peter. I had heard about the glasses, but did not pursue it at the time.

    It is my guess that the new NCCs that arise are essentially random numbers, created on the spot at the point (in the parietal ??) where the color information is decoded. We, of course, learn from others, the language constructs that correspond, “red”, “pink”, etc. To me, the sensation is not much different from what happens with respect to depth perception when you open and close the second eye. Of course, in that case, you also have, and almost always had, physical clues that correlate with the perceptual change, so it seems natural to assume that the physical knowledge immediately “jumps into” the neural correlate for depth. But I would argue that is not the case. That the depth correlate arises at a different point in the brain from the correlate for the physical relationship and that the two get immediately linked. So no such second correlate is available for color and, I believe, we just plug in some arbitrary values and give them the names we are told apply.

  2. 2. Lloyd Rice says:

    And so, you might counter, what’s that have to do with the “hard problem”? Why should a random number be perceived as red? “Well”, I would respond, “but that’s exactly what the world model does”. It takes numbers from everywhere, for depth, color, shape, texture, and everything else, and puts it all together into a coherent model of what seems to be out there.

  3. 3. Paul Torek says:

    Cool trick with the glasses; another good find to advance phil. mind. Since you brought it up, I’ve never found the primary-secondary quality distinction to be useful. All attempts wind up either hopelessly vague, or with (at least) one disjunct empty of cases. It all stems from a silly attempt to establish some kind of metaphysical penis envy among properties. All logical combinations of properties are genuine properties; some might not be very *useful* properties, but that’s another story. And anyway, it’s not likely that evolution has made your eyes sensitive to properties which aren’t useful.

  4. 4. Tom Clark says:

    Peter: “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.”

    This seems to me what usefully distinguishes secondary properties, e.g., colors, tastes, smells, from primary, namely that we can plausibly wonder whether they appear or feel different ways to different observers since they are functions of private qualitative phenomenal experience. Whereas as for scalar primary properties such as length, mass, etc. there is no possibility of things appearing differently to different observers since these properties don’t depend on qualities for their specification, only publicly observable quantities.

  5. 5. Lloyd Rice says:

    On an irrelevant side note: A tetrachromat sees about the same thing we see, anyhow. After all, we do not see three colors, we see a spectrum. The same spectrum will be reconstructed from four “primaries” as from three. Why the “same spectrum”? For the same reason that blue fades into red. As the frequency increases, the cones begin to pick up the sub-harmonic, half the frequency. And what was blue becomes red.

  6. 6. Jochen says:

    Color vision is complicated, and I don’t know anything about it beyond the simplest details, but it seems to me that there’s three basic possibilities for what could happen in tetrachromacy. Basically, with three types of cone cells, every color can be described by a set of three numbers, corresponding to the level of stimulation each receives. Thus, for a trichromat, color can be ordered in 3D-space.

    In principle, the addition of a fourth kind of cone cell thus will elevate this to a 4D-space, which could conceivably indeed lead to new phenomenology—new colors—, provided the way the brain’s wired up is such as to be capable of taking advantage of the additional information. (Anecdotally, I know people who claim to be able to imagine four-dimensional solids; personally, I can’t even imagine what that might be like.)

    However, the values of stimulation of the three cones aren’t independent—there are overlaps in sensitivity to different wavelengths. So there’s really just a part of the theoretical 3D-color space that we can map (a cone, in fact; I’m not sure if I have this right, but if all cone responses were independent, we ought to be able to map the full first octant of 3D-space—negative responses being impossible).

    With these glasses, you would essentially have two different 3D-solids for each eye. Now, how your brain combines them, I have no clue—both are subspaces of the same theoretical color space, so one might just superimpose them; or, one might conceive of the different coordinate in each as being along different, orthogonal directions, thus integrating them into a 4D-space.

    I’m not sure the above makes sense; but I would definitely like to try these glasses…

  7. 7. Richard Kennaway says:

    Tetrachromats may not be able to give us their experience, but what they can do is tell us whether they can distinguish the colours of two objects. “Tell” is to be understood broadly, as including tasks of sorting presented colours, or detecting patterns as in the Ishihara colour plates. Another way of mapping the objective shape of their colour space is to elicit judgements about whether this pair of similar colours are as different from each other as this other pair. Is the difference they see between spectral yellow and the subjectively closest red-green mixture a large one or a small one?

    Has anyone tried doing this? Formulating the test materials might be difficult, given that computer screens and printing processes are all designed for standard trichromats.

  8. 8. Jorge says:

    “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.”

    I’m not convinced by this.

    A person who sees only black and white definitely lacks the ‘qualia’ associated with red, green and blue. A person who is a dichromat is also probably “missing” qualia without being aware of it.

    In my view, the way to resolve this issue once and for all is to engineer a human being (via CRISPR or whatever) with a photopigment that responds to UV. Ideally, this will be an inducible transgene, so that the person experiences tytpical vision first and then reports what UV is like after a period of neuroplasticity to accommodate the new input.

    I volunteer my firstborn, if anyone wants to crowdsource me.

  9. 9. Lloyd Rice says:

    Considering Jochen’s suggestions, I would argue for two reasons that we would not see, in any sense, a 4-D color space. First, as I suggested before, we do not sense a 3-D space, but a smooth range, actually, a 1-D space. Second is that the values of the “so-called” primaries can be shifted around. RGB is just one such choice, each of which seem to give similar percepts. I have no physiological knowledge of this, but I would guess that the cone firings are more or less algebraically added, giving rise to a single color firing rate.

  10. 10. Jochen says:

    Lloyd:

    First, as I suggested before, we do not sense a 3-D space, but a smooth range, actually, a 1-D space.

    This is only true for spectral colors, i.e. those of the rainbow, corresponding to light of a single frequency—but there are many other colors corresponding to combinations of multiple frequencies of light (there’s no brown in a rainbow, for example). See wiki:

    Weighting a total light power spectrum by the individual spectral sensitivities of the three types of cone cells gives three effective stimulus values; these three values make up a tristimulus specification of the objective color of the light spectrum. The three parameters, denoted S, M, and L, can be indicated using a 3-dimensional space, called LMS color space, which is one of many color spaces which have been devised to help quantify human color vision.

    Second is that the values of the “so-called” primaries can be shifted around.

    This is true, but merely means that every point in a 3D-space has multiple (actually, infinitely many) decompositions into basis vectors.

  11. 11. David Duffy says:

    If you’re bored, you can look at some binocular colour mixing here, using the power of crosseyedness as opposed to fancy glasses:

    http://www.bicovi.com/bcv/demos.html

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