THE SKY HAS NEVER LOOKED SO BLUE
I was recently blessed with an eye exam. I’d never been to an optometrist and thought my vision was fine, but I’ve spent about ten years sitting in the dark staring at little screens so I was happy to go and get things checked out.
At the optometrist office there were posters and pamphlets charting the structure of the eye and explaining how our vision works. It seemed to me that act of seeing couldn’t possibly be more plain and direct, like a simple transmission, a kind of package delivery, of light information from the world into your brain. And this is essentially what I learned in school. In order to see, the lesson went, light from the world around you passed through your cornea (the transparent front part of the eye that you could touch with your finger – which I do not recommend). This refracted light then passes through the lens (that little flexible curvature just inside the front of your eye) which further bends the light, focusing an inverted image of the world in front of your eyes onto the membrane at the back of the eye, called the retina. (You must have learned this at some point as well. Somehow it strikes me as particularly bizarre now, that the whole field of information your retina receives is actually upside-down. Just think about this fact next time you’re standing in front of a mirror parting your hair. If you don’t fall straight up onto the ceiling, backward, surely you’re not thinking about it hard enough.) But, as it turns out, that’s just where our crazy vision begins: twice-refracted and topsy-turvey.
The optometrist’s assistant, the lovely Angela, performed several tests and took photos of my retinas. Looking at the images on a screen, she told me I had perfect retinas, that they were in better shape than the model retinas on her poster up on the wall. That was good news. What was really interesting and surprising was what I learned later about retinas. As it turns out, your retina is actually part of your brain, but in your eyeballs. Isolated there from the rest of your encephalon, your retinas act as a transducer, an energy converter. To do so, each retina is full of specialized neurons called cones and rods (you probably know this, but there’s also a third type called photosensitive retinal ganglion cells. They’re interesting too, but we won’t worry about them here.) These neurons convert the light signals delivered through your cornea and lens into electrical signals that they then send down the optic nerve into the visual system of your brain. While this does seem pretty straightforward, the retina is actually a very strange place.
To start with, the architecture of the retina is fundamentally broken. The optic nerve (which shows up as a white spot in the middle of a retinal photograph) is smack in the middle of, and busts straight through, your retina – creating a spot where no light-detecting neurons reside. This means you’re stuck with two holes, two blind spots, right at the center of your vision, and right where you feel your sight is best. We don’t tend to notice this flaw because our visual system effectively lies to us, replacing the missing information with an inference (based on the visual information coming from the neurons surrounding the missing section) and interjects that misinformation in place of a blank spot.
(Interestingly, the octopus avoids these failings and workarounds altogether by simply having their optic nerve interfaced with the under side of the retina, and not obscuring their vision by breaking the surface of the retina. In this way the eye is really the perfect case against and not for Intelligent Design and human superiority. Is there any doubt?)
Not only is the light input that strikes the retina broken by the placement of the optic nerve but the signalling of that information is funky too. Unlike other sensory systems in the body that increase their activity when stimulated (which makes sense), these neurons actually do the opposite and decrease their firing rate, their release of neurotransmitters, when stimulated. So, counter to what you’d expect, when light strikes a rod or cone the system actually goes silent. There’s something beautiful about that, but it’s strange too, no?
Stranger still, you don’t just have two eyes, each with their own retina, receiving and converting light information. Nope, it’s so much messier than that. The right and left sides of each retina transmit different information, along different pathways, to different parts of the brain. Yup. So, light striking the right side of the retina of your right eye sends information to the right side of your brain. In that same right eye, light striking the left side of the retina results in information travelling over to the opposite side of the brain, to left side. And this process is mirrored in the other eye.
As if all that wasn’t strange enough, your retina and it’s neurons only signal the presence of light, they don’t actually directly signal colour. To experience colour your cones come in three different types, each responding to light of different wavelengths. But it’s nothing like a simple binary system of on/off switches keyed to discrete sections of the light spectrum: red, blue, green. Nope. All three cone types actually overlap a great deal in their sensitivity to the light spectrum. Cones detecting the lowest frequencies detect light in the blue and green range of the spectrum, a pretty broad range actually. Cones detecting middle frequencies can register almost the entire spectrum, everything but the very extremes, the deepest blues at one end and highest frequency reds at the other. Our third set of cones detect those highest reds the medium wavelength cones cannot and much of the rest of the spectrum too, right down into the blues. Therefore, given this overlap, in order to determine what colour is present no one cone can be relied upon. Instead, the visual system takes a kind of poll of all the cones in a region. Comparing the light spectrum and intensity absorbed by each, and computing how many neurons are responding to the light, a kind of vague consensus is reached. Wild, hey?
So, after my first set of eye tests, the optometrist came in and confirmed what her assistant thought of my retinas and then moved me over into another room. There she sat me down in front of another device. Watching her adjust all these lenses and knobs and dials in front of me, I couldn’t help but think about how these tools here are all keenly calibrated to measure one’s vision with great precision – and what a funny thing to want to do when perfection is at best a kind of fractured and broken mess of mixed-up and missing information. I asked the optometrist about lenses and refraction and retinas and vision. She reminded me that it was still more broken, quirky, complicated, vague, and interpretive. That, of course, we have binocular, stereoscopic vision. This means our brain receives four flipped, bifurcated, punctured, two-dimensional, electric misrepresentations of light information from the world around us, which it then translates into the seamless, dynamic, three-dimensional band of vision to which you are accustomed. (And this happens with your nose and your eyelashes right there, ever-present, constantly in the way, and something you must forever actively ignore.)
After looking into my eyes and asking a few questions, the optometrist suggested we could end the testing here, if I liked. She said everything looked great and that she was glad I came in because I would now have a solid benchmark from which to compare future examinations. That seemed just fine. My whole experience was as painless and easy as it was informative. Returning home, I just had to explore sight some more.
Despite all the above non-designed, comically-evolved convolution, how and what we see is still not as simple as I’ve outlined. Our vision is not a one-way flow (of information collection, translation, and interpretation.) Instead, we know that almost all of the light information first striking our cornea (estimated at about 90%) is lost by the time it passes through the many layers of the visual system and into the back of our brains. With this trickle of data, experiment shows, our brains construct a kind of assumption about what’s present in our visual field, based only partially on this visual data (or lack thereof.) In this way, it seems, our vision is a kind of interpretive projection based a great deal upon prior knowledge and experience, and then inflected through our beliefs, expectations, goals, and even our language.
For example, the colour blue is a recent cultural invention. As explained in my favourite Radiolab podcast, when you search through all our ancient texts, across all continents, you find a total absence of any mention of the colour blue. Ancient Greek texts, Icelandic and Chinese and Vedic texts, the Hebrew Bible: they are all missing the colour blue. Nobody ever wrote of the sky or the sea or their friends eyes or of anything as being blue. Crazy, right? This is because all cultures across the globe learn colours, learn to see them and then name them, progressively over time. Yes, all cultures everywhere learn red first and blue last – always. This is so because in order to perceive something we need to differentiate it out of the sea of noisy likeness in which it, and everything else, resides. Interestingly, we don’t tend to label colours, or so it seems, until we can produce them ourselves. (And it has been pointed out that red is the most common and easiest pigment to produce, while blue is the least common and hardest. So maybe this all makes sense.) It seems we build categories and labels in this way; and the labels themselves act as a kind of perceptual feedback, reinforcing the difference in our mind and so in the world. When you differentiate yellow from green, assign them different names and categories, the difference is heightened and becomes more important to you. You always saw yellow and green, only now you care about the subtle difference more and then notice both much more strongly than you would otherwise.
Amazingly, we can actually see all this happening in our children as they learn about colours and in cultures that have yet to label blue. Yes, there are communities who have no word for blue and whose members, as a result, do not notice it. You can give these folks a colour test, with panels of different greens and blues, and they simply will not discriminate between what seems a very obvious difference to you and me (people with a vocabulary full of greenness and blueness.) Instead, they will report that these swatches are indistinguishable. And they are. Blue and green are indistinguishable for cultures who have not yet pulled them conceptually apart and labelled them differently. As such, our experience of the world is only as rich as our language, and vice versa.
When I first heard this I thought it was simultaneously the most upsetting and wonderful idea ever. Like, how amazing and important are mere words? Words are a kind of ultimate psychedelic, no? We can see that whole new ecologies can unveil themselves through simple words and ideas, their invention and manipulation. In this light, Shakespeare was really the Timothy Leary of his day, was he not? And, well, publishing then seems as much a kind of pharmacological phenomena as anything. Still now, many years after first hearing the story of blue, I still sit up at night wondering how I can transform my perception and everyone else’s with new labels and language.
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