Speaker 1: Today's episode is about color. What is the very specific
Speaker 1: reason that hunters wear orange? Why do birds and bees
Speaker 1: go to different flowers? Why do most mammals look like
Speaker 1: they've evolved at least in part for moving around at night?
Speaker 1: And what does that have to do with hairless humans
Speaker 1: getting angry? And what does any of this have to
Speaker 1: do with road signs or camouflage or mantis shrimp or
Speaker 1: the sun or the dress that broke the Internet or
Speaker 1: women who can see more colors than you can. Welcome
Speaker 1: to Inner Cosmos with me David Eagleman. I'm a neuroscientist
Speaker 1: and an author at Stanford, and in these episodes we
Speaker 1: look at the world inside us and around us to
Speaker 1: understand why and how our lives look the way they do.
Speaker 1: Today's episode and next week's as well, is about the
Speaker 1: absolutely amazing and often underappreciated topic of color. And if
Speaker 1: I do my job right, this is going to allow
Speaker 1: you to see the world around you with.
Speaker 2: Totally fresh eyes.
Speaker 1: As a neuroscientist, I've always been obsessed with color, what
Speaker 1: purposes it serves, how different people see it differently, why
Speaker 1: some people or some animals see more or fewer colors,
Speaker 1: or why kings and queens always love to wear purple,
Speaker 1: Or why some birds evolve to be red, or why
Speaker 1: blue animals are so rare, or what having to scavenge
Speaker 1: at night two hundred and fifty million years ago means
Speaker 1: for my dog's color vision. Today, in my laboratory, I've
Speaker 1: studied color illusions, and next week I'll show you how
Speaker 1: to see impossible colors that you've never seen before.
Speaker 2: So let's get started.
Speaker 1: Of all the qualities of our experience, color is one
Speaker 1: of the most intimate and vivid. It's tied to our
Speaker 1: emotions and our memories and our reactions. We remember the
Speaker 1: red of a childhood wagon, the green of summer leaves,
Speaker 1: the yellow of a favorite sweater. Color feels fundamental, like
Speaker 1: something that exists out there in the world, waiting for
Speaker 1: us to notice it. But of course it doesn't exist
Speaker 1: in the outside world. The weird thing is that color
Speaker 1: is not a property of light itself. Photons the particles
Speaker 1: of light. They don't carry color. They carry energy defined
Speaker 1: by their wavelength. The sensation of orange or cobalt blue
Speaker 1: or chartruse that only happens inside your head. Color is
Speaker 1: a construct that your brain invents based on electrical signals
Speaker 1: from your eyes. The world reflects and emits different wavelengths
Speaker 1: of light, but it's your brain that assigns those wavelengths
Speaker 1: different experiences, and we give these different names and feelings
Speaker 1: and meanings. So we live in a colorless world until
Speaker 1: a brain comes along to paint it. This episode and
Speaker 1: the next is a journey into that strange truth. We're
Speaker 1: going to explore what color really is and the strange
Speaker 1: and idiosyncratic ways that we and other animals perceive it.
Speaker 1: But we're not going to stop just with the biology.
Speaker 1: Color also shows up in language and culture, so next
Speaker 1: week tackles several surprising aspects about that, and along the
Speaker 1: way we'll meet animals with very different visual worlds. Color
Speaker 1: turns out to be a relationship between physics and perception,
Speaker 1: between the wavelengths of light and the circuits of the brain.
Speaker 1: It's part physics, part neuro So today let's look at
Speaker 1: what we see, what we don't see, and how our
Speaker 1: brains fill in the gaps with stories painted in light. Okay,
Speaker 1: so starting at the beginning, light is electromagnetic radiation, which
Speaker 1: is a traveling wave of electric and magnetic fields. Now
Speaker 1: here's the thing. These waves can span a huge range
Speaker 1: of wavelengths.
Speaker 2: You've got radio waves that.
Speaker 1: Are kilometers long, to gamma rays that are subatomic, and
Speaker 1: things even outside of that. But of the entire spectrum,
Speaker 1: our eyes are sensitive to just a very narrow band,
Speaker 1: really narrow, less than a ten.
Speaker 2: Trillionth of the spectrum.
Speaker 1: The human eye is able to pick up on wavelengths
Speaker 1: roughly four hundred to seven hundred nanometers, and this narrow
Speaker 1: little band is what we label visible light. This little
Speaker 1: band includes all the colors of the rainbow, and it's
Speaker 1: the only part of the spectrum that we can directly see. Now,
Speaker 1: what that means is that all the rest of the spectrum,
Speaker 1: microwaves and radio programs and X rays and cosmic rays
Speaker 1: and infrared and ultraviolet, that's all passing right.
Speaker 2: Through your body.
Speaker 1: And this is totally invisible to you because you don't
Speaker 1: have the receptors to pick up on these It's all
Speaker 1: electromagnetic radiation, it's all light. But you can't see these
Speaker 1: other things. So we're just going to concentrate on that
Speaker 1: little bit that you can see, which we call visible light,
Speaker 1: which spans ROYGBIV red, orange, yellow, green, blue, indigo, violet.
Speaker 1: Now here's the important thing which I mentioned before. Electromagnetic
Speaker 1: radiation has no intrinsic color. A photon doesn't know that
Speaker 1: it's red or blue. It just has a certain wavelength.
Speaker 1: When that photon hits the back of your eye, your retina,
Speaker 1: it interacts with particular cells depending on its wavelength, and
Speaker 1: then your brain starts the journey of constructing the experience
Speaker 1: of color, that perceptual quality, that private experience synthesized by
Speaker 1: your neural networks. It's your internal model's response to the
Speaker 1: frequency of the incoming light. So when we look at
Speaker 1: a red apple, what's happening. The surface of the apple
Speaker 1: absorbs most wavelengths of light, but it reflects those around
Speaker 1: seven hundred nanometers, the long waves that we associate.
Speaker 2: With the color red.
Speaker 1: Those photons hit your eye and they activate this cascade
Speaker 1: of cells, and your brain interprets that signal as red.
Speaker 1: But it's not the apple that's red, it's your brain
Speaker 1: painting that surface with meaning. So to get everything set
Speaker 1: up for this episode, let's start at the beginning. The retina,
Speaker 1: which is the lawn of cells at the back of
Speaker 1: your eye. This has two types of cells that are
Speaker 1: sensitive to light. These are called photoreceptors. These two types
Speaker 1: of cells are rods and cones. Now you have tons
Speaker 1: of rods, like one hundred and twenty million of them,
Speaker 1: and they're extremely sensitive to light, but they don't give
Speaker 1: you any color information. They're what allows us to see
Speaker 1: in dim light, but all the air information is just
Speaker 1: black and white. It's just telling you how much light
Speaker 1: is there at that spot, whatever the wavelength. But the
Speaker 1: other type of light sensitive cell is a cone, and cones.
Speaker 2: Are at the center of our story today.
Speaker 1: Because these are the microscopic cells that lead to our
Speaker 1: experience of color. Now, first, there are many fewer cones
Speaker 1: than rods. There's only about six million of them, and
Speaker 1: they're all concentrated at the center of your vision, not
Speaker 1: in the periphery.
Speaker 2: And cones come in three flavors.
Speaker 1: You've got those that are most responsive to red, which
Speaker 1: is a long wavelength.
Speaker 2: Then you've got those that are most.
Speaker 1: Responsive to green, which is a medium wavelength, and those
Speaker 1: that are most responsive to the light that we perceive
Speaker 1: as blue. These are short wavelength, so what you are
Speaker 1: picking up on is essentially red, green, blue, And your
Speaker 1: perception of color doesn't come from any one cone, but
Speaker 1: from the pattern of activity.
Speaker 2: Across all three types.
Speaker 1: For example, if both your red and green cones are activated,
Speaker 1: you will perceive yellow. Now what I just described with
Speaker 1: the cones, that's only the very first layer straight away.
Speaker 1: Even before the signals get to your visual cortex, other
Speaker 1: cells begin to compare and contrast the signals.
Speaker 2: So colors get analyzed in opposing pairs.
Speaker 1: Red versus green, blue versus yellow, black versus white. And
Speaker 1: this is why you can't see reddish green or bluish yellow.
Speaker 1: Those combinations cancel out in our visual system. Okay, Then
Speaker 1: the signals get to the visual cortex and an enormous
Speaker 1: amount of further processing takes place. I'm going to skip
Speaker 1: all the details here. I'll post a chap from my
Speaker 1: textbook on the show notes. But the point I want
Speaker 1: to make is that you might wonder, wait, why is
Speaker 1: there so much computation involved here? Why don't you just
Speaker 1: look at the wavelength hitting the retina and have your
Speaker 1: perception that way. Well, it turns out that wouldn't be
Speaker 1: nearly enough because the lighting conditions totally change what's bouncing
Speaker 1: off an object and hitting your eye. But if you're
Speaker 1: going to assign colors to things and that's going to
Speaker 1: carry some sort of information, you need to somehow account
Speaker 1: for the lighting changes. In other words, one of the
Speaker 1: most important tricks that the brain pulls off is color constancy.
Speaker 1: This is your brain's ability to perceive and objects color
Speaker 1: as stable even when the lighting changes.
Speaker 2: So let's say I'm wearing a white T.
Speaker 1: Shirt that looks white to you out in the sunlight,
Speaker 1: and it also looks white when I'm in the yellowish
Speaker 1: light of an indoor lamp, or when I'm standing at
Speaker 1: a campfire, or when I'm in the.
Speaker 2: Bright lights of a store.
Speaker 1: But technically the wavelengths bouncing off my white T shirt
Speaker 1: are very different in those conditions. Your brain accounts for
Speaker 1: the context by looking at all the other colors in
Speaker 1: the scene and subtracting that to keep your perception stable.
Speaker 1: This is, by the way, in the laboratory why we
Speaker 1: can make so many color illusions. If you manipulate the
Speaker 1: surrounding light and shade, you can make two identical patches
Speaker 1: appear wildly different in color. If you're interested more on this,
Speaker 1: I'm linking a paper I wrote in Nature Reviews Neuroscience
Speaker 1: on visual illusions.
Speaker 2: But here's the.
Speaker 1: Point I want to get to how weird it is
Speaker 1: that we can study all the pieces and parts of
Speaker 1: the brain and the physiology, but that doesn't really tell
Speaker 1: us anything about why you experience the color a particular way.
Speaker 1: Why don't our brains just register something about wavelength like, oh,
Speaker 1: that's four hundred and fifty animeters, instead of experiencing purpleness
Speaker 1: or a yellowness or a greenness. So consider this thought
Speaker 1: experiment called Mary's Room, which was proposed by the philosopher
Speaker 1: Frank Jackson in nineteen eighty two. He was essentially asking
Speaker 1: how our private, subjective experiences like color can be reduced
Speaker 1: to physical information. Here's the setup he proposed. Mary is
Speaker 1: a brilliant scientist who knows everything there is to know
Speaker 1: about this science of color.
Speaker 2: She understands the wavelengths.
Speaker 1: The neural processes, in other words, the physics and the biology.
Speaker 1: But Mary has lived her entire life in a black
Speaker 1: and white room, and she has never actually seen color.
Speaker 1: She reads about red in the sense that it's a
Speaker 1: seven hundred nanimeter wavelength. She understands how it stimulates particular
Speaker 1: cones in the retina, how it's processed in the visual cortex,
Speaker 1: but she's never experienced red. Then one day she steps
Speaker 1: outside and sees a ripe tomato for the first time. Now,
Speaker 1: Jackson's question is does Mary learn something new when she
Speaker 1: sees red for the first time. If she does, then
Speaker 1: there's something about the experience of color, some qualitative first
Speaker 1: person knowledge that isn't captured just by the objective physical facts.
Speaker 1: And that thought experiment illustrates the difficulty in explaining subjective
Speaker 1: experience just in terms of physical mechanisms. And the weirdest
Speaker 1: part is that the way the brain constructs this subjective
Speaker 1: experience is not necessarily the same for you and me.
Speaker 1: This is why the dress broke the Internet. You remember
Speaker 1: that viral photo that you might have seen as black
Speaker 1: and blue, or you might have seen as white and gold.
Speaker 1: It comes down to the assumptions that eat brain makes
Speaker 1: about the lighting in the photo. Your brain sees this
Speaker 1: little picture of the dress in a shop, and it
Speaker 1: makes dozens of assumptions totally unconsciously. What is the light
Speaker 1: source in the photograph? Is the dress being lit mostly
Speaker 1: by fluorescent lights or by sunlight? Is the dress facing
Speaker 1: a window or is the window behind it? What time
Speaker 1: of day is it, what season is it? The wild
Speaker 1: part is that you just open your eyes and there
Speaker 1: it is. There's the color of the dress. But under
Speaker 1: the hood, your brain is doing an enormous amount of computation,
Speaker 1: asking questions and making assumptions you never have any awareness of.
Speaker 1: And you believe the colors that your brain tells you.
Speaker 1: But illusions like the dress tell us that your head
Speaker 1: may be making those assumptions differently than the head sitting
Speaker 1: next to you, and therefore the color you're seeing isn't
Speaker 1: something true, it's just your brain's result of the computations.
Speaker 1: For more on the dress, listen to episode thirty one. Okay,
Speaker 1: there's a lot more to say about color perception and
Speaker 1: how different people see things differently, and we're going to
Speaker 1: return to that next week, but for now, I want
Speaker 1: to see how this gets even weirder when we compare
Speaker 1: against other species. Because while you think you're just seeing
Speaker 1: what's out there, what you're actually experiencing is one evolutionary
Speaker 1: solution among many. For example, we humans have three types
Speaker 1: of color photoreceptors, these cones, but most other mammals have
Speaker 1: only two types. Now, why do mammals tend to be
Speaker 1: limited in this way? One idea about this is what's
Speaker 1: called the nocturnal bottleneck hypothesis.
Speaker 2: The idea is that the.
Speaker 1: Common ancestor of mammals and reptiles and birds had more
Speaker 1: photoreceptor types, but we he lost them.
Speaker 2: Why.
Speaker 1: It's because two hundred and fifty million years ago, during
Speaker 1: the Mesozoic era, the ancestors of mammals had to become
Speaker 1: mostly nocturnal to avoid getting stomped and eaten by dinosaurs.
Speaker 1: The large predatory dinosaurs dominated the daytime, and so early
Speaker 1: mammals adapted to nocturnal living to avoid them. So these
Speaker 1: many tens of millions of years of nocturnal activity led
Speaker 1: to several adaptations that reflect nighttime living. So, for example,
Speaker 1: mammals developed excellent senses of hearing and smell, which you
Speaker 1: need for navigating and foraging in the dark, and they
Speaker 1: evolved eyes optimized for low light conditions, like large pupils
Speaker 1: and lots of really good rod cells which allow them
Speaker 1: to see better in dim environments. So, in other words,
Speaker 1: when we look at mammalian eyes today, they seem to
Speaker 1: have been shaped during this prolonged period of nighttime activity.
Speaker 1: And here's the key for today day since detailed color
Speaker 1: vision is less useful at night, mammals lost their capacity
Speaker 1: for trichromatic vision, in other words, seeing three primary colors. Instead,
Speaker 1: they got a deeper reliance on senses better suited for
Speaker 1: nocturnal activity, like better hearing and smell. So this is
Speaker 1: presumably why modern mammals retain nocturnal traits even after the
Speaker 1: extinction of dinosaurs, which allowed them to diversify into daytime niches.
Speaker 2: In other words, evolutionary pressures.
Speaker 1: Can shape an entire class of animals, leaving a long
Speaker 1: lasting imprint on their physiology and behavior even after the
Speaker 1: environmental conditions change.
Speaker 2: So just as an.
Speaker 1: Example, my dog and your dog, they are di chromatic.
Speaker 1: They only have two types of cones, and their vision
Speaker 1: resembles red green color blindness in humans. They see a
Speaker 1: world of muted blues and yellows, and they can't distinguish
Speaker 1: reds from greens. This is the same with cats, with horses,
Speaker 1: with rodents, and over ninety percent of mammals. Okay, so
Speaker 1: the nocturnal bottleneck hypothesis suggests why a lot of mammals
Speaker 1: have not so great color vision, but some mammals, like humans,
Speaker 1: have evolved better color vision. We have become trichromatic again.
Speaker 1: Now why would it make sense to regain the ability
Speaker 1: to distinguish red and green. Well, one common argument is
Speaker 1: that if you can distinguish red and green, then you
Speaker 1: can see a ripe fruit against a tree canopy at
Speaker 1: a distance, and that's really useful.
Speaker 2: But maybe it's something even deeper than that.
Speaker 1: So let me tell you one of my favorite hypotheses
Speaker 1: about why we humans re evolved red green color vision.
Speaker 1: This comes from my colleague Mark Changizi, and it flips
Speaker 1: the usual story.
Speaker 2: On its head.
Speaker 1: It suggests something deeply social. He argues that we didn't
Speaker 1: evolve red green vision to forage better.
Speaker 2: We evolved to read each other better. So here's the idea.
Speaker 1: Human skin is packed with tiny blood vessels just beneath
Speaker 1: the surface, and when your emotions shift, when you're embarrassed
Speaker 1: or you're angry, or you're afraid or you're aroused, blood
Speaker 1: flow changes. Your skin flushes red or it pails. It
Speaker 1: happens in fractions of a second, and we pick up
Speaker 1: on this in other faces without even realizing it. So
Speaker 1: Tchannghisi suggests that our color vision system is tuned to
Speaker 1: detect these subtle changes in oxygenation of the hemoglobe and
Speaker 1: under the skin. The spacing of our red and green
Speaker 1: cone sensitivities is almost perfect for distinguishing these tiny shifts
Speaker 1: in skin tone. In other words, we're not just seeing
Speaker 1: red and green on apples and trees. We're seeing it
Speaker 1: in faces, in ears, in emotional states, and those social
Speaker 1: signals is why CHANGESI suggests we evolved the ability in
Speaker 1: the first place to track the emotion of the other. Now,
Speaker 1: this potentially helps explain something I mentioned that most mammals
Speaker 1: don't have trichromatic vision, but about two thirds of primates do,
Speaker 1: and these animals, including us, tend to have less fur
Speaker 1: on their faces. Less fur means more exposed skin. More
Speaker 1: skin means more visible blood flow, and more reason to
Speaker 1: evolve a visual system that can pick up on the
Speaker 1: language of color as it plays out in living flesh.
Speaker 1: So maybe red green vision isn't about navigating the jungle,
Speaker 1: about navigating each other. By the way, you might wonder
Speaker 1: does this apply to people with more pigment in their skin,
Speaker 1: Because in people with darker skin, melanin absorbs more light
Speaker 1: and that can make those blood float changes harder to see.
Speaker 1: But the signals are still there, They're just more subtle.
Speaker 1: Even with darker skin. Changes in coloration can be seen
Speaker 1: in eras where the skin is thinner, like the lips
Speaker 1: and the.
Speaker 2: Eyelids and the palms and the cheeks.
Speaker 1: So we see the world differently than most of our
Speaker 1: mammalian cousins, and understanding this sort of thing can clarify
Speaker 1: a lot of the world around you. For example, why
Speaker 1: do hunters wear bright orange vests? You might correctly assume
Speaker 1: it so they can spot one another, right, But why
Speaker 1: don't they wear chartreuse or bright yellow or any other
Speaker 1: equally detectable color. Well, as we just saw, the ability
Speaker 1: to detect reddish colors was lost during the Mesozoic, and
Speaker 1: the great apes, including humans, regained it.
Speaker 2: Deer did not, so deer.
Speaker 1: Can only see two ranges of color, blue and yellow green.
Speaker 1: Deer have great smell and hearing, and they've got great
Speaker 1: night vision, but they're not sensitive to red or orange light.
Speaker 1: And that's why hunters wear blaze orange, which is easily
Speaker 1: spotted by the other humans but not by the deer.
Speaker 1: This color that helps hunters spot each other at a
Speaker 1: distance makes them nearly invisible to their prey. In this case,
Speaker 1: our evolutionary divergence from other species our trichromatic vision, and
Speaker 1: their dichromatic becomes a tactical advantage. We can design ourselves
Speaker 1: to be invisible to them. And by the way, this
Speaker 1: is why you see nature red birds. It's the same idea.
Speaker 1: The red birds can be there against the green leaves
Speaker 1: and they don't have to worry about getting spotted because
Speaker 1: their predators are red green colorblind. And it turns out
Speaker 1: its getically easier to express red than green, So expressing
Speaker 1: red feathers is a perfectly good way to go. Now,
Speaker 1: let's come back to the deer hunters. Note that they
Speaker 1: never wear blue jeans. Why not because deer are much
Speaker 1: more sensitive to blue even than we are to them.
Speaker 1: Blue jeans shine like a warning beacon, so the hunters
Speaker 1: wear camouflage or earthstone pants to blend in. Okay, So
Speaker 1: back to humans with their three types of photoreceptors. It
Speaker 1: turns out that's most humans. A small fraction of humans
Speaker 1: have some forms of color blindness where they're missing one
Speaker 1: type of color photoreceptor, or two types, or all three,
Speaker 1: and you can guess what their experience is. They can
Speaker 1: distinguish fewer and fewer colors. What you might not know
Speaker 1: is that a small fraction of the human female population.
Speaker 1: They have a fourth type of cone. They are called
Speaker 1: tetrachromatic instead of the typical chromatic vision.
Speaker 2: With three types, these women can.
Speaker 1: Distinguish something like one hundred million shades of color, so
Speaker 1: for them, a sunset contains hues that the rest of
Speaker 1: us have no capacity to see and no words for.
Speaker 1: By the way, it's only women because the mutation in
Speaker 1: the photoreceptor is on one of the X chromosomes and
Speaker 1: not on the other. And it's not just the rare
Speaker 1: human female who's tetrachromatic. A whole lot of birds and
Speaker 1: reptiles and insects are tetrachromats, and in animals, the fourth
Speaker 1: cone type often extends into the ultraviolet, so bees, for example,
Speaker 1: can see ultraviolet patterns on flowers that are completely invisible
Speaker 1: to us. These patterns help them locate the nectar. They're
Speaker 1: like little runways on the flower pedal that don't look
Speaker 1: like anything to us. And then there's the mantis shrimp.
Speaker 1: This little crustacean has sixteen types of photoreceptors. These photoreceptors
Speaker 1: can detect ultraviolet and polarized light, and presumably colors we
Speaker 1: can't imagine, but are useful in their niche. Of course,
Speaker 1: it's impossible for us to know exactly what the experience
Speaker 1: of the shrimp is because it depends what its brain
Speaker 1: is doing with that data. But what all these examples
Speaker 1: show is that color vision is about usefulness. Millions of
Speaker 1: years of evolution tends to equip species with the ability
Speaker 1: to perceive what enhances survival and reproduction. Our color vision
Speaker 1: is not what a mantis shrimps is, but it was
Speaker 1: good enough to spot ripe fruit, and detect emotional signals
Speaker 1: and flushed skin and navigate a multicolored environment. Okay, now
Speaker 1: I want to talk about the birds and the bees.
Speaker 2: For a moment.
Speaker 1: Here's the thing. We usually think of a flower as
Speaker 1: just a flowersome color and nice scent, But every petal,
Speaker 1: every hue of the flower, every tiny detail is a
Speaker 1: carefully c afted advertisement. It's a billboard designed to say hey,
Speaker 1: I'm open for business to a very specific clientele. And
Speaker 1: the best part about this botanical dating game is how
Speaker 1: birds and bees have entirely different preferences when it comes
Speaker 1: to the colors that they find attractive. So imagine you
Speaker 1: walk into a store and some of the products are
Speaker 1: completely invisible to you.
Speaker 2: That's what happens in the floral world.
Speaker 1: So let's start with the bees. When a bee flies
Speaker 1: into your garden, it's not seeing the same spectrum of colors.
Speaker 2: That you are.
Speaker 1: For the bee, red flowers are a complete no go.
Speaker 1: It's like a black screen. Bees see in shades of
Speaker 1: blue and purple and violet and yellow, So those are
Speaker 1: the flowers they go for. And as I mentioned, many
Speaker 1: flowers that look plain to us have hidden ultraviolet patterns
Speaker 1: that act like glowing landing strip for the bee, guiding
Speaker 1: it straight to the nectar. It's like a secret map
Speaker 1: that only they can read.
Speaker 2: So if you're a.
Speaker 1: Bee, that vibrant red rose might just be invisible. Now
Speaker 1: turn to our feathered friends like hummingbirds, which are big pollinators,
Speaker 1: and guess what their favorite color is. It's red and
Speaker 1: orange is a close second. So think about this. Hummingbird
Speaker 1: feeders are always red, right, that's no accident because birds,
Speaker 1: unlike bees, can see red perfectly and they're drawn to
Speaker 1: it like a magnet. And you'll notice that these bird
Speaker 1: pollinated red flowers often have long tubular shapes, perfect for
Speaker 1: a hummingbird's beak to dip in so why this color divide.
Speaker 1: Over millions of years, flowers have evolved to attract the
Speaker 1: most effective pollinators for whatever their needs are. If a
Speaker 1: flower wants a bee to carry its pollen, it's going
Speaker 1: to evolve to be blue or yellow, and it'll have
Speaker 1: those secret ultraviolet patterns. If it wants a bird, then
Speaker 1: it flaunts reds and oranges, knowing that the bees won't
Speaker 1: even notice.
Speaker 2: This kind of.
Speaker 1: Color specialization ensures that the pollen gets where it needs
Speaker 1: to go. It's the botanical equivalent of targeted advertising. So
Speaker 1: the next time you're outside, take a closer look at
Speaker 1: the flowers around you, and you can guess their intended
Speaker 1: audience by their color.
Speaker 2: If you didn't.
Speaker 1: Already know this, it gives you a new way to
Speaker 1: appreciate the intricate dance of life happening all around us. Okay, now,
Speaker 1: even though the human eye is limited to a narrow
Speaker 1: sliver of visible light, we've spent the recent century trees
Speaker 1: building tools that let us see beyond that. So we
Speaker 1: have infrared cameras that reveal heat signatures of animals at night,
Speaker 1: or we have X rays that show us our bones,
Speaker 1: where we construct telescopes to pick up on microwave or
Speaker 1: ultraviolet frequencies, and that shows us a very different universe
Speaker 1: than the one we can see with our eyes. But
Speaker 1: now that we know that that huge spectrum of wavelengths
Speaker 1: is out there, why don't we see all the rest
Speaker 1: with our eyes. The answer to that question lies one
Speaker 1: hundred and fifty million kilometers away. It's the Sun. Our
Speaker 1: star emits light, electromagnetic radiation across the whole spectrum.
Speaker 2: But when this sunlight passes through.
Speaker 1: Earth's atmosphere, shorter and longer wavelengths, light, gamma rays, and
Speaker 1: most ultraviolet that's all filtered out, and much of the
Speaker 1: infrared is absorbed as heat. So what punches through all
Speaker 1: the way to the surface where we're hanging out is
Speaker 1: a pretty narrow window of light wavelengths between about four
Speaker 1: hundred and seven hundred nanimeters. Of all the energy that
Speaker 1: gets released from the Sun, only this tiny sliver reaches
Speaker 1: us in abundance, And.
Speaker 2: So that's what evolution seesed on.
Speaker 1: Our ancestors' eyes evolved to pick up on what was available.
Speaker 1: This little band of light provided enough data to distinguish
Speaker 1: edges and shapes and motion and color plants reflect the
Speaker 1: particular wavelengths we see as green, right, fruit often reflects
Speaker 1: another set of wavelengths we see red or yellow. Blood
Speaker 1: looks red, fire looks orange, and presumably our ancestors who
Speaker 1: could detect these sorts of distinctions had a better shot
Speaker 1: at survival. Color perception had nothing to do with esthetic pleasures,
Speaker 1: which we'll get into in the next episode, at least
Speaker 1: not originally it was about spotting. What mattered is that
Speaker 1: fruit ripe, is that animal bleeding, is that flash of
Speaker 1: color over there, a flower or a threat. Over tens
Speaker 1: of millions of years, animal brains became color detectives. It's
Speaker 1: not that we wanted to marvel at rainbows. It's that
Speaker 1: it was extraordinarily useful to decode a layer of information
Speaker 1: bouncing off objects in the world. We extract meaning from
Speaker 1: the quality of the reflected light. I suspect that berry
Speaker 1: is full of sugar and calories because it's reflecting a
Speaker 1: wavelength that I see as red. That yellowing leaf tells
Speaker 1: me about decay. The flush in that guy's cheek reveals anger.
Speaker 2: In this way, we.
Speaker 1: And other animals learned to read the mood and state
Speaker 1: of the world. Now I want to circle back to
Speaker 1: the deer hunter or the red bird to double click
Speaker 1: on this issue of camouflage and also the opposite of camouflage,
Speaker 1: because color can serve a couple of functions in nature
Speaker 1: in the sense that it can allow animals to blend
Speaker 1: in like camouflage, or to stand out like warning coloration.
Speaker 1: Both blending in and standing out rely on exploiting the
Speaker 1: visual systems of other species. So take a tiger in
Speaker 1: the jungle. He's got these really bold orange and black
Speaker 1: stripes that might seem like a terrible disguise, But most
Speaker 1: of the tigers prey don't see red well, so the
Speaker 1: orange appears as a muted gray. The stripes help the
Speaker 1: tiger's body dissolve into the dappled shadows and reeds. Now
Speaker 1: flip to the opposite strategy, what's called warning coloration. So
Speaker 1: picture the bright yellow and black stripes on a wasp
Speaker 1: or the super bright red of a poisoned dart frog.
Speaker 1: I'm putting some pictures in the show notes on Eagleman
Speaker 1: dot com slash podcast. The point is that these colors
Speaker 1: serve as honest ad advertisements. It says don't eat me,
Speaker 1: I'm dangerous, and other animals learn very quickly not to
Speaker 1: mess with those patterns.
Speaker 2: So a young.
Speaker 1: Bird who eats a monarch butterfly and becomes violently ill
Speaker 1: learns to avoid anything.
Speaker 2: That looks like that.
Speaker 1: In the future, we humans, of course, have co opted
Speaker 1: the same idea and our own signaling systems. We have
Speaker 1: road signs and hazard labels and life jackets and construction gear.
Speaker 1: We build these all with high contrast, high saturated colors,
Speaker 1: and this taps into our built in alert system designed
Speaker 1: to grab our attention and to hold it. Now, just
Speaker 1: note that not all warning colors are truthful. Nature has
Speaker 1: its con artists. Some animals mimic the colors of toxic
Speaker 1: species without being toxic themselves. So think of the viceroy butterfly,
Speaker 1: which looks like the monarch butterfly. Predators who have tasted
Speaker 1: a monarch and regretted it are.
Speaker 2: Likely to avoid both.
Speaker 1: And this is deception through color, using color as a
Speaker 1: kind of costume. And then let's not forget about the
Speaker 1: theatricality of sexual selection.
Speaker 2: Charles Darwin.
Speaker 1: At first he found the peacock's tail to be a
Speaker 1: headache for his theory of natural selection, because it seems
Speaker 1: to hinder survival. But then Darwin realized that traits like
Speaker 1: the peacock's tail could provide an advantage in the competition
Speaker 1: for mates. In other words, it's for sex appeal. Now,
Speaker 1: this apparently scandalized many scientists when he first proposed it,
Speaker 1: but the evidence holds. In many species, the brightest, most
Speaker 1: colorful individuals are auditioning. They're working to signal health or
Speaker 1: genetic quality or dominance. Just think about a male mandrill's
Speaker 1: brilliantly colored face and rump. Those are just for decoration.
Speaker 1: They're billboards of status. I'll also mention that in some species,
Speaker 1: color shifts with mood or with context. So you may
Speaker 1: have seen a cuttlefish who can change their skin pattern
Speaker 1: in seconds, and some fish develop intense coloration only during
Speaker 1: mating season. These color changes serve as social cues. They
Speaker 1: are visually broadcasting their intentions. So what we see is
Speaker 1: that color is one of evolution's most versatile tools. That
Speaker 1: can hide, it can warn, it can deceive, it can seduce,
Speaker 1: and it can do all of this differently depending on
Speaker 1: who is watching and what their color capabilities are. So
Speaker 1: I want to zoom the camera out to see what
Speaker 1: we've covered today. Color isn't something out there in the world.
Speaker 1: It's something our brains create. Photons have no color. It's
Speaker 1: our brain that absorbs the raw wavelengths and transfer forms
Speaker 1: them into experience. This transformation starts with light sensitive cells
Speaker 1: in your retinam goes all the way to your brain,
Speaker 1: and the color you perceive ends up being shaped by context,
Speaker 1: and your brain's best guess becomes your visual reality.
Speaker 2: And we saw how.
Speaker 1: Our color vision is a patchwork of evolutionary trade offs,
Speaker 1: shaped by ancient nocturnal life, and by the need to
Speaker 1: spot fruit, and perhaps by the need to read each
Speaker 1: other's emotions. We looked at how different animals live in
Speaker 1: very different color worlds, and how our narrow window of
Speaker 1: vision was sculpted by sunlight and survival. And we saw
Speaker 1: how evolutionary pressures shaped the palette of our world. Flowers
Speaker 1: evolved colors not for us, but for the eyes of pollinators.
Speaker 1: The natural world is performing for eyes. Now that we
Speaker 1: have the basics down, next week we're going to move
Speaker 1: to deeper levels, include beyond biology into culture and language
Speaker 1: and art. Because once we understand how color is made
Speaker 1: in the brain, it opens up all sorts of new questions.
Speaker 2: Why was purple the color of royalty.
Speaker 1: How do we see colors that are recent ancestors never
Speaker 1: saw in their lives. Why are you unable to imagine
Speaker 1: a new color? Are there, in fact new colors that
Speaker 1: you could see? Could you lose.
Speaker 2: Your color vision? And many other questions.
Speaker 1: So to wrap today, the main lesson we see is
Speaker 1: that color is a construction. It's a useful fiction of
Speaker 1: the brain, a mental model painted on.
Speaker 2: Top of physics.
Speaker 1: But it's also one of the richest and most emotionally
Speaker 1: resonant parts of being alive. So just take a moment
Speaker 1: and look around at the light. What you're seeing is
Speaker 1: what your brain makes of that light. The colors aren't
Speaker 1: out there, they're in here. Your brain isn't just perceiving
Speaker 1: the world, it's actively constructing it. Go to eagleman dot
Speaker 1: com slash podcast for more information and to find further reading.
Speaker 1: Join the weekly discussions on my substack and check out
Speaker 1: on Subscribe to Inner Cosmos on YouTube for videos of
Speaker 1: each episode and to leave comments until next time. I'm
Speaker 1: David Eagleman, and this is Inner Cosmos.