Today's episode is about color. What is the very specific reason that hunters wear orange? Why do birds and bees go to different flowers? Why do most mammals look like they've evolved at least in part for moving around at night? And what does that have to do with hairless humans getting angry? And what does any of this have to do with road signs or camouflage or mantis shrimp or the sun or the dress that broke the Internet or women who can see more colors than you can. Welcome
to Inner Cosmos with me David Eagleman. I'm a neuroscientist and an author at Stanford, and in these episodes we look at the world inside us and around us to understand why and how our lives look the way they do. Today's episode and next week's as well, is about the absolutely amazing and often underappreciated topic of color. And if I do my job right, this is going to allow you to see the world around you with.
Totally fresh eyes.
As a neuroscientist, I've always been obsessed with color, what purposes it serves, how different people see it differently, why some people or some animals see more or fewer colors, or why kings and queens always love to wear purple, Or why some birds evolve to be red, or why blue animals are so rare, or what having to scavenge at night two hundred and fifty million years ago means
for my dog's color vision. Today, in my laboratory, I've studied color illusions, and next week I'll show you how to see impossible colors that you've never seen before.
So let's get started.
Of all the qualities of our experience, color is one of the most intimate and vivid. It's tied to our emotions and our memories and our reactions. We remember the red of a childhood wagon, the green of summer leaves, the yellow of a favorite sweater. Color feels fundamental, like something that exists out there in the world, waiting for us to notice it. But of course it doesn't exist in the outside world. The weird thing is that color is not a property of light itself. Photons the particles
of light. They don't carry color. They carry energy defined by their wavelength. The sensation of orange or cobalt blue or chartruse that only happens inside your head. Color is a construct that your brain invents based on electrical signals from your eyes. The world reflects and emits different wavelengths of light, but it's your brain that assigns those wavelengths different experiences, and we give these different names and feelings and meanings. So we live in a colorless world until
a brain comes along to paint it. This episode and the next is a journey into that strange truth. We're going to explore what color really is and the strange and idiosyncratic ways that we and other animals perceive it. But we're not going to stop just with the biology. Color also shows up in language and culture, so next week tackles several surprising aspects about that, and along the
way we'll meet animals with very different visual worlds. Color turns out to be a relationship between physics and perception, between the wavelengths of light and the circuits of the brain. It's part physics, part neuro So today let's look at what we see, what we don't see, and how our brains fill in the gaps with stories painted in light. Okay, so starting at the beginning, light is electromagnetic radiation, which is a traveling wave of electric and magnetic fields. Now
here's the thing. These waves can span a huge range of wavelengths.
You've got radio waves that.
Are kilometers long, to gamma rays that are subatomic, and things even outside of that. But of the entire spectrum, our eyes are sensitive to just a very narrow band, really narrow, less than a ten.
Trillionth of the spectrum.
The human eye is able to pick up on wavelengths roughly four hundred to seven hundred nanometers, and this narrow little band is what we label visible light. This little band includes all the colors of the rainbow, and it's the only part of the spectrum that we can directly see. Now, what that means is that all the rest of the spectrum, microwaves and radio programs and X rays and cosmic rays and infrared and ultraviolet, that's all passing right.
Through your body.
And this is totally invisible to you because you don't have the receptors to pick up on these It's all electromagnetic radiation, it's all light. But you can't see these other things. So we're just going to concentrate on that little bit that you can see, which we call visible light, which spans ROYGBIV red, orange, yellow, green, blue, indigo, violet. Now here's the important thing which I mentioned before. Electromagnetic radiation has no intrinsic color. A photon doesn't know that
it's red or blue. It just has a certain wavelength. When that photon hits the back of your eye, your retina, it interacts with particular cells depending on its wavelength, and then your brain starts the journey of constructing the experience of color, that perceptual quality, that private experience synthesized by your neural networks. It's your internal model's response to the frequency of the incoming light. So when we look at
a red apple, what's happening. The surface of the apple absorbs most wavelengths of light, but it reflects those around seven hundred nanometers, the long waves that we associate.
With the color red.
Those photons hit your eye and they activate this cascade of cells, and your brain interprets that signal as red. But it's not the apple that's red, it's your brain painting that surface with meaning. So to get everything set up for this episode, let's start at the beginning. The retina, which is the lawn of cells at the back of your eye. This has two types of cells that are sensitive to light. These are called photoreceptors. These two types
of cells are rods and cones. Now you have tons of rods, like one hundred and twenty million of them, and they're extremely sensitive to light, but they don't give you any color information. They're what allows us to see in dim light, but all the air information is just black and white. It's just telling you how much light is there at that spot, whatever the wavelength. But the other type of light sensitive cell is a cone, and cones.
Are at the center of our story today.
Because these are the microscopic cells that lead to our experience of color. Now, first, there are many fewer cones than rods. There's only about six million of them, and they're all concentrated at the center of your vision, not in the periphery.
And cones come in three flavors.
You've got those that are most responsive to red, which is a long wavelength.
Then you've got those that are most.
Responsive to green, which is a medium wavelength, and those that are most responsive to the light that we perceive as blue. These are short wavelength, so what you are picking up on is essentially red, green, blue, And your perception of color doesn't come from any one cone, but from the pattern of activity.
Across all three types.
For example, if both your red and green cones are activated, you will perceive yellow. Now what I just described with the cones, that's only the very first layer straight away. Even before the signals get to your visual cortex, other cells begin to compare and contrast the signals.
So colors get analyzed in opposing pairs.
Red versus green, blue versus yellow, black versus white. And this is why you can't see reddish green or bluish yellow. Those combinations cancel out in our visual system. Okay, Then the signals get to the visual cortex and an enormous amount of further processing takes place. I'm going to skip all the details here. I'll post a chap from my textbook on the show notes. But the point I want to make is that you might wonder, wait, why is
there so much computation involved here? Why don't you just look at the wavelength hitting the retina and have your perception that way. Well, it turns out that wouldn't be nearly enough because the lighting conditions totally change what's bouncing off an object and hitting your eye. But if you're going to assign colors to things and that's going to carry some sort of information, you need to somehow account
for the lighting changes. In other words, one of the most important tricks that the brain pulls off is color constancy. This is your brain's ability to perceive and objects color as stable even when the lighting changes.
So let's say I'm wearing a white T.
Shirt that looks white to you out in the sunlight, and it also looks white when I'm in the yellowish light of an indoor lamp, or when I'm standing at a campfire, or when I'm in the.
Bright lights of a store.
But technically the wavelengths bouncing off my white T shirt are very different in those conditions. Your brain accounts for the context by looking at all the other colors in the scene and subtracting that to keep your perception stable. This is, by the way, in the laboratory why we can make so many color illusions. If you manipulate the surrounding light and shade, you can make two identical patches
appear wildly different in color. If you're interested more on this, I'm linking a paper I wrote in Nature Reviews Neuroscience on visual illusions.
But here's the.
Point I want to get to how weird it is that we can study all the pieces and parts of the brain and the physiology, but that doesn't really tell us anything about why you experience the color a particular way. Why don't our brains just register something about wavelength like, oh, that's four hundred and fifty animeters, instead of experiencing purpleness or a yellowness or a greenness. So consider this thought experiment called Mary's Room, which was proposed by the philosopher
Frank Jackson in nineteen eighty two. He was essentially asking how our private, subjective experiences like color can be reduced to physical information. Here's the setup he proposed. Mary is a brilliant scientist who knows everything there is to know about this science of color.
She understands the wavelengths.
The neural processes, in other words, the physics and the biology. But Mary has lived her entire life in a black and white room, and she has never actually seen color. She reads about red in the sense that it's a seven hundred nanimeter wavelength. She understands how it stimulates particular cones in the retina, how it's processed in the visual cortex, but she's never experienced red. Then one day she steps
outside and sees a ripe tomato for the first time. Now, Jackson's question is does Mary learn something new when she sees red for the first time. If she does, then there's something about the experience of color, some qualitative first person knowledge that isn't captured just by the objective physical facts. And that thought experiment illustrates the difficulty in explaining subjective
experience just in terms of physical mechanisms. And the weirdest part is that the way the brain constructs this subjective experience is not necessarily the same for you and me. This is why the dress broke the Internet. You remember that viral photo that you might have seen as black and blue, or you might have seen as white and gold. It comes down to the assumptions that eat brain makes about the lighting in the photo. Your brain sees this little picture of the dress in a shop, and it
makes dozens of assumptions totally unconsciously. What is the light source in the photograph? Is the dress being lit mostly by fluorescent lights or by sunlight? Is the dress facing a window or is the window behind it? What time of day is it, what season is it? The wild part is that you just open your eyes and there
it is. There's the color of the dress. But under the hood, your brain is doing an enormous amount of computation, asking questions and making assumptions you never have any awareness of. And you believe the colors that your brain tells you. But illusions like the dress tell us that your head may be making those assumptions differently than the head sitting next to you, and therefore the color you're seeing isn't something true, it's just your brain's result of the computations.
For more on the dress, listen to episode thirty one. Okay, there's a lot more to say about color perception and how different people see things differently, and we're going to return to that next week, but for now, I want to see how this gets even weirder when we compare against other species. Because while you think you're just seeing what's out there, what you're actually experiencing is one evolutionary
solution among many. For example, we humans have three types of color photoreceptors, these cones, but most other mammals have only two types. Now, why do mammals tend to be limited in this way? One idea about this is what's called the nocturnal bottleneck hypothesis.
The idea is that the.
Common ancestor of mammals and reptiles and birds had more photoreceptor types, but we he lost them.
Why.
It's because two hundred and fifty million years ago, during the Mesozoic era, the ancestors of mammals had to become mostly nocturnal to avoid getting stomped and eaten by dinosaurs. The large predatory dinosaurs dominated the daytime, and so early mammals adapted to nocturnal living to avoid them. So these many tens of millions of years of nocturnal activity led
to several adaptations that reflect nighttime living. So, for example, mammals developed excellent senses of hearing and smell, which you need for navigating and foraging in the dark, and they evolved eyes optimized for low light conditions, like large pupils and lots of really good rod cells which allow them to see better in dim environments. So, in other words, when we look at mammalian eyes today, they seem to have been shaped during this prolonged period of nighttime activity.
And here's the key for today day since detailed color vision is less useful at night, mammals lost their capacity for trichromatic vision, in other words, seeing three primary colors. Instead, they got a deeper reliance on senses better suited for nocturnal activity, like better hearing and smell. So this is presumably why modern mammals retain nocturnal traits even after the extinction of dinosaurs, which allowed them to diversify into daytime niches.
In other words, evolutionary pressures.
Can shape an entire class of animals, leaving a long lasting imprint on their physiology and behavior even after the environmental conditions change.
So just as an.
Example, my dog and your dog, they are di chromatic. They only have two types of cones, and their vision resembles red green color blindness in humans. They see a world of muted blues and yellows, and they can't distinguish reds from greens. This is the same with cats, with horses, with rodents, and over ninety percent of mammals. Okay, so the nocturnal bottleneck hypothesis suggests why a lot of mammals have not so great color vision, but some mammals, like humans,
have evolved better color vision. We have become trichromatic again. Now why would it make sense to regain the ability to distinguish red and green. Well, one common argument is that if you can distinguish red and green, then you can see a ripe fruit against a tree canopy at a distance, and that's really useful.
But maybe it's something even deeper than that.
So let me tell you one of my favorite hypotheses about why we humans re evolved red green color vision. This comes from my colleague Mark Changizi, and it flips the usual story.
On its head.
It suggests something deeply social. He argues that we didn't evolve red green vision to forage better.
We evolved to read each other better. So here's the idea.
Human skin is packed with tiny blood vessels just beneath the surface, and when your emotions shift, when you're embarrassed or you're angry, or you're afraid or you're aroused, blood flow changes. Your skin flushes red or it pails. It happens in fractions of a second, and we pick up on this in other faces without even realizing it. So Tchannghisi suggests that our color vision system is tuned to detect these subtle changes in oxygenation of the hemoglobe and
under the skin. The spacing of our red and green cone sensitivities is almost perfect for distinguishing these tiny shifts in skin tone. In other words, we're not just seeing red and green on apples and trees. We're seeing it in faces, in ears, in emotional states, and those social signals is why CHANGESI suggests we evolved the ability in
the first place to track the emotion of the other. Now, this potentially helps explain something I mentioned that most mammals don't have trichromatic vision, but about two thirds of primates do, and these animals, including us, tend to have less fur on their faces. Less fur means more exposed skin. More skin means more visible blood flow, and more reason to evolve a visual system that can pick up on the language of color as it plays out in living flesh.
So maybe red green vision isn't about navigating the jungle, about navigating each other. By the way, you might wonder does this apply to people with more pigment in their skin, Because in people with darker skin, melanin absorbs more light and that can make those blood float changes harder to see. But the signals are still there, They're just more subtle. Even with darker skin. Changes in coloration can be seen in eras where the skin is thinner, like the lips and the.
Eyelids and the palms and the cheeks.
So we see the world differently than most of our mammalian cousins, and understanding this sort of thing can clarify a lot of the world around you. For example, why do hunters wear bright orange vests? You might correctly assume it so they can spot one another, right, But why don't they wear chartreuse or bright yellow or any other equally detectable color. Well, as we just saw, the ability to detect reddish colors was lost during the Mesozoic, and the great apes, including humans, regained it.
Deer did not, so deer.
Can only see two ranges of color, blue and yellow green. Deer have great smell and hearing, and they've got great night vision, but they're not sensitive to red or orange light. And that's why hunters wear blaze orange, which is easily spotted by the other humans but not by the deer. This color that helps hunters spot each other at a distance makes them nearly invisible to their prey. In this case, our evolutionary divergence from other species our trichromatic vision, and
their dichromatic becomes a tactical advantage. We can design ourselves to be invisible to them. And by the way, this is why you see nature red birds. It's the same idea. The red birds can be there against the green leaves and they don't have to worry about getting spotted because their predators are red green colorblind. And it turns out its getically easier to express red than green, So expressing red feathers is a perfectly good way to go. Now,
let's come back to the deer hunters. Note that they never wear blue jeans. Why not because deer are much more sensitive to blue even than we are to them. Blue jeans shine like a warning beacon, so the hunters wear camouflage or earthstone pants to blend in. Okay, So back to humans with their three types of photoreceptors. It
turns out that's most humans. A small fraction of humans have some forms of color blindness where they're missing one type of color photoreceptor, or two types, or all three, and you can guess what their experience is. They can distinguish fewer and fewer colors. What you might not know is that a small fraction of the human female population. They have a fourth type of cone. They are called tetrachromatic instead of the typical chromatic vision.
With three types, these women can.
Distinguish something like one hundred million shades of color, so for them, a sunset contains hues that the rest of us have no capacity to see and no words for. By the way, it's only women because the mutation in the photoreceptor is on one of the X chromosomes and not on the other. And it's not just the rare
human female who's tetrachromatic. A whole lot of birds and reptiles and insects are tetrachromats, and in animals, the fourth cone type often extends into the ultraviolet, so bees, for example, can see ultraviolet patterns on flowers that are completely invisible to us. These patterns help them locate the nectar. They're like little runways on the flower pedal that don't look like anything to us. And then there's the mantis shrimp.
This little crustacean has sixteen types of photoreceptors. These photoreceptors can detect ultraviolet and polarized light, and presumably colors we can't imagine, but are useful in their niche. Of course, it's impossible for us to know exactly what the experience of the shrimp is because it depends what its brain is doing with that data. But what all these examples show is that color vision is about usefulness. Millions of years of evolution tends to equip species with the ability
to perceive what enhances survival and reproduction. Our color vision is not what a mantis shrimps is, but it was good enough to spot ripe fruit, and detect emotional signals and flushed skin and navigate a multicolored environment. Okay, now I want to talk about the birds and the bees.
For a moment.
Here's the thing. We usually think of a flower as just a flowersome color and nice scent, But every petal, every hue of the flower, every tiny detail is a carefully c afted advertisement. It's a billboard designed to say hey, I'm open for business to a very specific clientele. And the best part about this botanical dating game is how birds and bees have entirely different preferences when it comes
to the colors that they find attractive. So imagine you walk into a store and some of the products are completely invisible to you.
That's what happens in the floral world.
So let's start with the bees. When a bee flies into your garden, it's not seeing the same spectrum of colors.
That you are.
For the bee, red flowers are a complete no go. It's like a black screen. Bees see in shades of blue and purple and violet and yellow, So those are the flowers they go for. And as I mentioned, many flowers that look plain to us have hidden ultraviolet patterns that act like glowing landing strip for the bee, guiding it straight to the nectar. It's like a secret map that only they can read.
So if you're a.
Bee, that vibrant red rose might just be invisible. Now turn to our feathered friends like hummingbirds, which are big pollinators, and guess what their favorite color is. It's red and orange is a close second. So think about this. Hummingbird feeders are always red, right, that's no accident because birds, unlike bees, can see red perfectly and they're drawn to
it like a magnet. And you'll notice that these bird pollinated red flowers often have long tubular shapes, perfect for a hummingbird's beak to dip in so why this color divide. Over millions of years, flowers have evolved to attract the most effective pollinators for whatever their needs are. If a flower wants a bee to carry its pollen, it's going to evolve to be blue or yellow, and it'll have
those secret ultraviolet patterns. If it wants a bird, then it flaunts reds and oranges, knowing that the bees won't even notice.
This kind of.
Color specialization ensures that the pollen gets where it needs to go. It's the botanical equivalent of targeted advertising. So the next time you're outside, take a closer look at the flowers around you, and you can guess their intended audience by their color.
If you didn't.
Already know this, it gives you a new way to appreciate the intricate dance of life happening all around us. Okay, now, even though the human eye is limited to a narrow sliver of visible light, we've spent the recent century trees
building tools that let us see beyond that. So we have infrared cameras that reveal heat signatures of animals at night, or we have X rays that show us our bones, where we construct telescopes to pick up on microwave or ultraviolet frequencies, and that shows us a very different universe than the one we can see with our eyes. But now that we know that that huge spectrum of wavelengths is out there, why don't we see all the rest with our eyes. The answer to that question lies one
hundred and fifty million kilometers away. It's the Sun. Our star emits light, electromagnetic radiation across the whole spectrum.
But when this sunlight passes through.
Earth's atmosphere, shorter and longer wavelengths, light, gamma rays, and most ultraviolet that's all filtered out, and much of the infrared is absorbed as heat. So what punches through all the way to the surface where we're hanging out is a pretty narrow window of light wavelengths between about four hundred and seven hundred nanimeters. Of all the energy that gets released from the Sun, only this tiny sliver reaches us in abundance, And.
So that's what evolution seesed on.
Our ancestors' eyes evolved to pick up on what was available. This little band of light provided enough data to distinguish edges and shapes and motion and color plants reflect the particular wavelengths we see as green, right, fruit often reflects another set of wavelengths we see red or yellow. Blood looks red, fire looks orange, and presumably our ancestors who could detect these sorts of distinctions had a better shot
at survival. Color perception had nothing to do with esthetic pleasures, which we'll get into in the next episode, at least not originally it was about spotting. What mattered is that fruit ripe, is that animal bleeding, is that flash of color over there, a flower or a threat. Over tens of millions of years, animal brains became color detectives. It's not that we wanted to marvel at rainbows. It's that it was extraordinarily useful to decode a layer of information
bouncing off objects in the world. We extract meaning from the quality of the reflected light. I suspect that berry is full of sugar and calories because it's reflecting a wavelength that I see as red. That yellowing leaf tells me about decay. The flush in that guy's cheek reveals anger.
In this way, we.
And other animals learned to read the mood and state of the world. Now I want to circle back to the deer hunter or the red bird to double click on this issue of camouflage and also the opposite of camouflage, because color can serve a couple of functions in nature in the sense that it can allow animals to blend in like camouflage, or to stand out like warning coloration. Both blending in and standing out rely on exploiting the visual systems of other species. So take a tiger in
the jungle. He's got these really bold orange and black stripes that might seem like a terrible disguise, But most of the tigers prey don't see red well, so the orange appears as a muted gray. The stripes help the tiger's body dissolve into the dappled shadows and reeds. Now flip to the opposite strategy, what's called warning coloration. So picture the bright yellow and black stripes on a wasp or the super bright red of a poisoned dart frog.
I'm putting some pictures in the show notes on Eagleman dot com slash podcast. The point is that these colors serve as honest ad advertisements. It says don't eat me, I'm dangerous, and other animals learn very quickly not to mess with those patterns.
So a young.
Bird who eats a monarch butterfly and becomes violently ill learns to avoid anything.
That looks like that.
In the future, we humans, of course, have co opted the same idea and our own signaling systems. We have road signs and hazard labels and life jackets and construction gear. We build these all with high contrast, high saturated colors, and this taps into our built in alert system designed to grab our attention and to hold it. Now, just note that not all warning colors are truthful. Nature has its con artists. Some animals mimic the colors of toxic
species without being toxic themselves. So think of the viceroy butterfly, which looks like the monarch butterfly. Predators who have tasted a monarch and regretted it are.
Likely to avoid both.
And this is deception through color, using color as a kind of costume. And then let's not forget about the theatricality of sexual selection.
Charles Darwin.
At first he found the peacock's tail to be a headache for his theory of natural selection, because it seems to hinder survival. But then Darwin realized that traits like the peacock's tail could provide an advantage in the competition for mates. In other words, it's for sex appeal. Now, this apparently scandalized many scientists when he first proposed it, but the evidence holds. In many species, the brightest, most colorful individuals are auditioning. They're working to signal health or
genetic quality or dominance. Just think about a male mandrill's brilliantly colored face and rump. Those are just for decoration. They're billboards of status. I'll also mention that in some species, color shifts with mood or with context. So you may have seen a cuttlefish who can change their skin pattern in seconds, and some fish develop intense coloration only during mating season. These color changes serve as social cues. They are visually broadcasting their intentions. So what we see is
that color is one of evolution's most versatile tools. That can hide, it can warn, it can deceive, it can seduce, and it can do all of this differently depending on who is watching and what their color capabilities are. So I want to zoom the camera out to see what we've covered today. Color isn't something out there in the world. It's something our brains create. Photons have no color. It's our brain that absorbs the raw wavelengths and transfer forms
them into experience. This transformation starts with light sensitive cells in your retinam goes all the way to your brain, and the color you perceive ends up being shaped by context, and your brain's best guess becomes your visual reality.
And we saw how.
Our color vision is a patchwork of evolutionary trade offs, shaped by ancient nocturnal life, and by the need to spot fruit, and perhaps by the need to read each other's emotions. We looked at how different animals live in very different color worlds, and how our narrow window of vision was sculpted by sunlight and survival. And we saw how evolutionary pressures shaped the palette of our world. Flowers evolved colors not for us, but for the eyes of pollinators.
The natural world is performing for eyes. Now that we have the basics down, next week we're going to move to deeper levels, include beyond biology into culture and language and art. Because once we understand how color is made in the brain, it opens up all sorts of new questions.
Why was purple the color of royalty.
How do we see colors that are recent ancestors never saw in their lives. Why are you unable to imagine a new color? Are there, in fact new colors that you could see? Could you lose.
Your color vision? And many other questions.
So to wrap today, the main lesson we see is that color is a construction. It's a useful fiction of the brain, a mental model painted on.
Top of physics.
But it's also one of the richest and most emotionally resonant parts of being alive. So just take a moment and look around at the light. What you're seeing is what your brain makes of that light. The colors aren't out there, they're in here. Your brain isn't just perceiving the world, it's actively constructing it. Go to eagleman dot com slash podcast for more information and to find further reading.
Join the weekly discussions on my substack and check out on Subscribe to Inner Cosmos on YouTube for videos of each episode and to leave comments until next time. I'm David Eagleman, and this is Inner Cosmos.