Below is a lightly edited transcript of the btrmt. lectures podcast. For the article that inspired it, see Purple doesn’t exist.

Welcome to the Betterment Lectures. My name is Dr Dorian Minors, and if there’s one thing I’ve learnt as a brain scientist, it’s that there’s no instruction manual for this device in our head. But there are patterns. Patterns of thought, patterns of feeling and patterns of action, because that’s what brains do: they create the patterns that gracefully handle the predictable shapes of everyday life. So let me teach you about them. One pattern, one podcast, and you see if it works for you.

Now, I want to do something a little more fun and speculative today. As I’ve done more and more of these lectures, I’ve deferred more and more of the process to AI, and my last lecture was uncomfortable to listen to, because you can hear me fighting with the speech notes that the AI derived from the article the lecture is based on. Very uncomfortable to listen to. So I’m going to go back to what I was doing at the beginning of this, which is just speaking off the article itself. And for that, I thought I’d choose an easy one.

¶Reason is lazier than we like to think

There’s one idea that’s really come to colour an enormous proportion of what I teach as an associate professor of behavioural science here at the Royal Military Academy Sandhurst. It’s the idea that reason—deliberative reasoning—isn’t the rational, logical, evidence-driven process we like to think it is, but might actually be more of a lazy process of rationalising our intuitions to ourselves.

I’m not going to talk about that in detail today, but I’ll link to an excellent book on the subject, The Enigma of Reason by Mercier and Sperber, as well as an article or two of mine where I do talk about it.

As it’s coloured more and more of my teaching, I’ve started to think about how it might be a reflection of some of the tendencies of the brain that used to puzzle me when I was working as a brain scientist at Cambridge. And I want to illustrate that today with a pretty curious phenomenon, which is how the brain processes the colour purple.

I think this might end up being a bit of a controversial take, but at least it’ll be fun. So let’s talk about how the colour purple doesn’t really exist.

¶What we learn about colour in primary school

In primary school, we learn that colour is a property of what light is reflected back at us. The light comes from the sun, it hits an object, the object absorbs some proportion of the light and reflects back some of it, and that’s the colour we see. So a leaf absorbs everything in the light spectrum that isn’t green and reflects back the greenness—from the chlorophyll, I think. I don’t actually know how that works, but I’m a brain scientist, not a leaf scientist, so you’ll forgive me. And as a consequence, we see a leaf as green, because that’s what’s left of the light spectrum after the leaf has absorbed whatever it absorbs.

So now, moving away from my ignorance about the interaction between leaves and light, and more towards my domain of strength, the brain.

What’s interesting about studying the brain is that you learn colour actually isn’t that. It’s something more to do with how the brain processes that information. Colour, for humans, comes about because we have cells in the eye that are sensitive to different wavelengths of light.

¶Three cells, and the whole spectrum

Light comes in a range of wavelengths. At one end you have the very short wavelengths—the kind of radiation that comes from a nuclear bomb—and at the other end the very long ones, the sort picked up by radio antennas. In the middle of those two things is the visible spectrum, which we call light.

The eyes of humans have three photoreceptors, each tuned to a different part of that range. What differs between them is the rate at which each one captures photons from the light arriving.

So we’ve got two things going on here. Light comes and hits our leaf, and the leaf absorbs some of those photons but reflects back the ones in the green part of the range. And the reason we see it as green is that it comes into the eye and there’s a photoreceptor perfectly placed to capture light at that wavelength. We have a green photoreceptor, essentially—the M-cone.

We have two other kinds. You have S-cones, which are sensitive to shorter wavelengths—faster-oscillating waves—and we see those as blue. And then we have L-cones, which are sensitive to longer wavelengths, and we see those as red.

What’s interesting is that obviously we can see more than just three colours. We don’t just see blue, green and red. We see yellow. And we see purple, and we see pink.

The reason we see those things is that, kind of like mixing colours in paint, the brain works out that if you’re getting a little bit of the M-cone, the green photoreceptor, and a little bit of the L-cone, the red photoreceptor, then probably you’re seeing something reflecting light in between the two. And so we see that as orange—a kind of mix between green and red.

So you don’t have a cell in the eye that tells you what things are reflecting yellow light. You have the brain processing a green cell and a red cell and saying, oh, that’s probably yellow.

And that’s the entire visible light spectrum to us. These three cells, these three photoreceptors. The entire colour spectrum is your brain working out what colours it can see based on the activity of three photoreceptors.

¶Where violet stops

What’s really interesting is that the way this whole thing is set up means sometimes the brain has to invent colours. Purple is an example of this.

The shortest wavelengths of light we can see come through as this sort of violet colour. And this is a really good way of illustrating how the brain processes colour, because it’s only got these three photoreceptors and it’s got to try to express to us what information it has available. The information it has is the blue photoreceptor responding weakly to something. So it wants to show us something, but it knows the green photoreceptor isn’t firing at all, so it can’t be anything longer-wavelength than blue. So it shows us this sort of sub-blue colour, this violet.

Now, as the waves get longer, what we see becomes bluer and bluer, because the brain’s trying to tell us the blue cell is getting more and more excited. Then eventually the green cell starts to kick in, and the brain wants to tell us that, so we start to see cyan. It changes from blue to something else. And as the waves get longer still, we see this colour change to become more and more green, because the brain is telling us the blue cell is firing less and less but the green cell is firing more and more.

And the process continues. The waves get longer, and now you’ve got the red cell kicking in, getting more and more excited. So the brain says: the green cell is starting to fire less and less, but the red cell is getting more and more excited. So you’re going to see yellow through orange and into red, which eventually becomes this sort of muted colour as the red cell fires more and more weakly to the increasing length of the light waves.

And then it stops being something we can perceive at all. On that side it goes into infrared. And on the other side of the spectrum, past violet, where the wavelengths are too short for our photoreceptors to pick up, we get ultraviolet.

And that’s the interesting point, because we shouldn’t see any more colours than violet. There’s no wavelength of light that corresponds to more purple than violet. We don’t get more colours when we go past violet—we get into ultraviolet, which is invisible to us. We don’t have a photoreceptor that responds to light at that wavelength.

¶The colour the brain invents

So the question is: where do these other kinds of purple come from?

The answer is that it’s a sort of invention of our brain, for something that’s confusing to this triadic system of photoreceptors. What we’re seeing when we see other kinds of purple is an invention of the mind.

Things that reflect both short- and long-wavelength light are usually also reflecting the middle, which our green photoreceptor picks up. But some things reflect just the short and the long without reflecting that middle, green wavelength. And the brain has to solve a problem here. It knows there’s something between blue and red. It has a solution to that problem already—it has green, the colour it shows you when the light is between blue and red. But in this case the visual system isn’t getting any of those green signals, because whatever is reflecting the light to us isn’t reflecting green. It’s only reflecting the fast and the slow.

So the brain has to do something. It can’t show you green, because that would be wrong—it’s not getting any signals from the green photoreceptors. So what it does is make up a brand new colour for us to see. It makes up purple.

Purple is the brain telling us that this is something that isn’t, but should be, green. It’s what we call a non-spectral colour. A colour that doesn’t exist on the light spectrum.

¶Half the colour wheel is a fiction

I really love this illustration, and not just because it’s kind of fun that the colour purple doesn’t really exist. I love it because it illustrates that the world we see reflects our purpose in being in the world.

It’s something I write about a lot here. We don’t care so much about the objective reality of the world, some objective model of it. What we care about is what matters to us. And at some point, things that reflect both very short and very long wavelengths of light mattered to us. So our brain had to invent a way of representing that.

And so a colour wheel is a beautiful illustration of that fact. Half of it reflects an objective fact—something about the wavelength of the light we’re seeing. And half of that colour wheel is an invention. Not reflecting some objective fact about the world, but reflecting a human need. Something the brain needs to make up in order to represent it to us.

¶The brain’s job was never accuracy

What I’m hoping you’re taking away from this is that the brain’s job isn’t so much to reconstruct some objective fact about the world. It’s to convey information to you. To create some representation in your mind of the world that’s useful to you, that helps you navigate it.

It’s not really relevant whether purple exists or not. It’s not important to the brain that purple doesn’t follow neatly along with the rest of the visible light spectrum, because the brain isn’t interested in showing us something objective about the light spectrum. The brain is interested in representing to us whatever we needed to know about it. And sometimes we needed to know about things reflecting both very short and very long wavelengths, but not the middle. You can imagine something to do with foraging, for example.

There are other non-spectral colours. Pink is one of them. You’ll actually notice that if you look at an image of the light spectrum, pink doesn’t appear at all—because we only get pink when all three of our colour cells are active but the red photoreceptor is the most active. The brain gets sort of confused by this and shows you something that is kind of red, but also obviously not.

I tell you that because I think it’s kind of fun that the prettiest sunsets are basically a set of imaginary colours. Colours that are an artefact of the way our body has learnt to respond to the world, and that our brain has made up in response to that.

¶The split-brain patient and the snow shovel

And I guess that brings me back to the property of human reason.

There are a lot of examples in the empirical literature of people creating nonsense reasons to explain phenomena they can’t really articulate, because those phenomena sit outside our common experiences. Maybe the most famous examples come from split-brain patients.

Split-brain patients are, as it sounds like from the name, patients who’ve had their brains cut in half. Cut in half across what’s essentially connective tissue binding two functional halves of the brain, the left hemisphere and the right. This is a drastic step, but it’s done for patients who have such bad seizures that cutting the brain in half is preferable. Seizures are essentially an electrical fault that spreads from one part of the brain across the surface, and if you cut the brain in half it can only spread so far—it can’t spread to the other half.

What’s really interesting about these patients is what happens to their ability to describe what they’re seeing. A bit of context that’s necessary to understand this: a lot of the architecture responsible for helping you understand and produce speech sits on the left side of the brain. It’s one of the reasons you end up with these myths about creative and logical brains—a lot of speech production, or processing related to it, happens on the left side for most people. Not all people.

I’m going to simplify this experiment a bit for a podcast, but I’ll find a link to some of these findings for you to follow up on.

Let’s say you have one of these split-brain patients, understanding that each eye plugs in to a different side of the brain, just like each arm is controlled by a different side of the brain, and so on. In the famous experiment—the seminal one—researchers flashed two different images to the two eyes of the patient. In the eye connected to the language side of the brain, they showed a chicken claw. And to the other side of the brain, the one no longer connected to all that language infrastructure, they showed a snowy scene.

Then they asked the patient to use their hands to select associated images that related to the things they could see. So the language hand, remembering that this side of the brain saw a chicken claw, selected a chicken. And the non-language hand, remembering that this side of the brain saw a snowy scene, selected a snow shovel.

Then they asked the person to explain why they’d selected what they selected. And the person said: well, I selected the chicken because I saw a chicken claw. And obviously I selected the shovel so that I could clean the chicken poop out of the chicken shed.

They said nothing about the snowy scene.

Again, that’s a simplification of the experiment, but I’ll try to find a link for it. The point is that the faculty of reason obviously had no access to what the non-language part of the brain could see—and yet it had to come up with a reason for the behaviour of the person. They’d picked a shovel. Why could they have picked a shovel? Well, perhaps because you have to shovel out the chicken coop.

¶A storyteller all the way down

And I think that bears some non-trivial resemblance to the way the brain makes up colour. When it doesn’t understand why we’ve engaged in a certain kind of behaviour, it confabulates an explanation.

I guess what I’m wondering is: is the chicken shovel the same kind of thing as the sunset? An explanation made by the brain to represent stuff it doesn’t really understand. And in these cases, they’re pretty harmless. But if they’re happening in these cases, when are they happening and they’re harmful?

I think, going beyond that, the brain seems to be a storyteller down to the lowest level of perception. And I guess that’s something that troubles me a little bit.

And on that happy note, I guess I’ll leave it there.