WEBVTT - Ep115 "What is color? Part 1: Why hunters wear orange" 0:00:05.080 --> 0:00:09.560 Today's episode is about color. What is the very specific 0:00:09.680 --> 0:00:14.400 reason that hunters wear orange? Why do birds and bees 0:00:14.480 --> 0:00:18.079 go to different flowers? Why do most mammals look like 0:00:18.280 --> 0:00:21.640 they've evolved at least in part for moving around at night? 0:00:22.079 --> 0:00:25.200 And what does that have to do with hairless humans 0:00:25.480 --> 0:00:28.000 getting angry? And what does any of this have to 0:00:28.040 --> 0:00:32.960 do with road signs or camouflage or mantis shrimp or 0:00:33.040 --> 0:00:36.479 the sun or the dress that broke the Internet or 0:00:36.640 --> 0:00:43.880 women who can see more colors than you can. Welcome 0:00:43.880 --> 0:00:46.839 to Inner Cosmos with me David Eagleman. I'm a neuroscientist 0:00:46.920 --> 0:00:50.120 and an author at Stanford, and in these episodes we 0:00:50.240 --> 0:00:52.760 look at the world inside us and around us to 0:00:52.880 --> 0:00:56.640 understand why and how our lives look the way they do. 0:01:12.959 --> 0:01:16.440 Today's episode and next week's as well, is about the 0:01:16.680 --> 0:01:22.800 absolutely amazing and often underappreciated topic of color. And if 0:01:22.840 --> 0:01:25.199 I do my job right, this is going to allow 0:01:25.280 --> 0:01:27.360 you to see the world around you with. 0:01:27.440 --> 0:01:28.880 Totally fresh eyes. 0:01:29.240 --> 0:01:32.840 As a neuroscientist, I've always been obsessed with color, what 0:01:33.240 --> 0:01:37.440 purposes it serves, how different people see it differently, why 0:01:37.520 --> 0:01:41.360 some people or some animals see more or fewer colors, 0:01:41.440 --> 0:01:45.080 or why kings and queens always love to wear purple, 0:01:45.560 --> 0:01:49.200 Or why some birds evolve to be red, or why 0:01:49.680 --> 0:01:53.720 blue animals are so rare, or what having to scavenge 0:01:53.760 --> 0:01:56.240 at night two hundred and fifty million years ago means 0:01:56.320 --> 0:01:59.600 for my dog's color vision. Today, in my laboratory, I've 0:01:59.640 --> 0:02:03.040 studied color illusions, and next week I'll show you how 0:02:03.080 --> 0:02:06.040 to see impossible colors that you've never seen before. 0:02:06.560 --> 0:02:07.960 So let's get started. 0:02:08.600 --> 0:02:12.120 Of all the qualities of our experience, color is one 0:02:12.120 --> 0:02:15.200 of the most intimate and vivid. It's tied to our 0:02:15.240 --> 0:02:19.840 emotions and our memories and our reactions. We remember the 0:02:19.919 --> 0:02:24.000 red of a childhood wagon, the green of summer leaves, 0:02:24.040 --> 0:02:29.040 the yellow of a favorite sweater. Color feels fundamental, like 0:02:29.120 --> 0:02:31.760 something that exists out there in the world, waiting for 0:02:31.880 --> 0:02:35.119 us to notice it. But of course it doesn't exist 0:02:35.400 --> 0:02:38.280 in the outside world. The weird thing is that color 0:02:38.880 --> 0:02:43.400 is not a property of light itself. Photons the particles 0:02:43.400 --> 0:02:47.640 of light. They don't carry color. They carry energy defined 0:02:47.680 --> 0:02:53.040 by their wavelength. The sensation of orange or cobalt blue 0:02:53.639 --> 0:02:58.640 or chartruse that only happens inside your head. Color is 0:02:58.720 --> 0:03:03.720 a construct that your brain invents based on electrical signals 0:03:03.760 --> 0:03:08.000 from your eyes. The world reflects and emits different wavelengths 0:03:08.040 --> 0:03:12.000 of light, but it's your brain that assigns those wavelengths 0:03:12.320 --> 0:03:17.080 different experiences, and we give these different names and feelings 0:03:17.120 --> 0:03:22.000 and meanings. So we live in a colorless world until 0:03:22.080 --> 0:03:26.440 a brain comes along to paint it. This episode and 0:03:26.480 --> 0:03:29.400 the next is a journey into that strange truth. We're 0:03:29.440 --> 0:03:33.880 going to explore what color really is and the strange 0:03:34.040 --> 0:03:37.800 and idiosyncratic ways that we and other animals perceive it. 0:03:38.120 --> 0:03:40.480 But we're not going to stop just with the biology. 0:03:40.680 --> 0:03:44.120 Color also shows up in language and culture, so next 0:03:44.160 --> 0:03:48.120 week tackles several surprising aspects about that, and along the 0:03:48.160 --> 0:03:53.000 way we'll meet animals with very different visual worlds. Color 0:03:53.080 --> 0:03:57.680 turns out to be a relationship between physics and perception, 0:03:57.840 --> 0:04:01.280 between the wavelengths of light and the circuits of the brain. 0:04:01.400 --> 0:04:06.360 It's part physics, part neuro So today let's look at 0:04:06.520 --> 0:04:09.360 what we see, what we don't see, and how our 0:04:09.360 --> 0:04:16.599 brains fill in the gaps with stories painted in light. Okay, 0:04:16.680 --> 0:04:21.560 so starting at the beginning, light is electromagnetic radiation, which 0:04:21.600 --> 0:04:25.480 is a traveling wave of electric and magnetic fields. Now 0:04:25.520 --> 0:04:28.760 here's the thing. These waves can span a huge range 0:04:28.880 --> 0:04:30.160 of wavelengths. 0:04:30.480 --> 0:04:32.320 You've got radio waves that. 0:04:32.279 --> 0:04:36.960 Are kilometers long, to gamma rays that are subatomic, and 0:04:37.080 --> 0:04:40.960 things even outside of that. But of the entire spectrum, 0:04:41.320 --> 0:04:44.679 our eyes are sensitive to just a very narrow band, 0:04:44.839 --> 0:04:47.440 really narrow, less than a ten. 0:04:47.400 --> 0:04:49.080 Trillionth of the spectrum. 0:04:49.520 --> 0:04:52.520 The human eye is able to pick up on wavelengths 0:04:52.640 --> 0:04:55.960 roughly four hundred to seven hundred nanometers, and this narrow 0:04:56.080 --> 0:05:00.480 little band is what we label visible light. This little 0:05:00.520 --> 0:05:03.520 band includes all the colors of the rainbow, and it's 0:05:03.560 --> 0:05:07.480 the only part of the spectrum that we can directly see. Now, 0:05:07.520 --> 0:05:09.960 what that means is that all the rest of the spectrum, 0:05:10.000 --> 0:05:14.159 microwaves and radio programs and X rays and cosmic rays 0:05:14.160 --> 0:05:17.640 and infrared and ultraviolet, that's all passing right. 0:05:17.520 --> 0:05:18.480 Through your body. 0:05:19.000 --> 0:05:21.520 And this is totally invisible to you because you don't 0:05:21.560 --> 0:05:25.120 have the receptors to pick up on these It's all 0:05:25.200 --> 0:05:29.200 electromagnetic radiation, it's all light. But you can't see these 0:05:29.240 --> 0:05:31.599 other things. So we're just going to concentrate on that 0:05:31.760 --> 0:05:35.440 little bit that you can see, which we call visible light, 0:05:35.560 --> 0:05:39.880 which spans ROYGBIV red, orange, yellow, green, blue, indigo, violet. 0:05:39.960 --> 0:05:43.440 Now here's the important thing which I mentioned before. Electromagnetic 0:05:43.520 --> 0:05:48.960 radiation has no intrinsic color. A photon doesn't know that 0:05:49.080 --> 0:05:53.240 it's red or blue. It just has a certain wavelength. 0:05:53.720 --> 0:05:56.799 When that photon hits the back of your eye, your retina, 0:05:57.360 --> 0:06:02.120 it interacts with particular cells depending on its wavelength, and 0:06:02.160 --> 0:06:06.799 then your brain starts the journey of constructing the experience 0:06:07.240 --> 0:06:14.000 of color, that perceptual quality, that private experience synthesized by 0:06:14.040 --> 0:06:18.840 your neural networks. It's your internal model's response to the 0:06:18.920 --> 0:06:22.680 frequency of the incoming light. So when we look at 0:06:22.720 --> 0:06:26.720 a red apple, what's happening. The surface of the apple 0:06:26.920 --> 0:06:31.960 absorbs most wavelengths of light, but it reflects those around 0:06:31.960 --> 0:06:35.960 seven hundred nanometers, the long waves that we associate. 0:06:35.480 --> 0:06:36.840 With the color red. 0:06:37.600 --> 0:06:41.960 Those photons hit your eye and they activate this cascade 0:06:42.040 --> 0:06:46.080 of cells, and your brain interprets that signal as red. 0:06:46.160 --> 0:06:48.559 But it's not the apple that's red, it's your brain 0:06:49.000 --> 0:06:52.320 painting that surface with meaning. So to get everything set 0:06:52.360 --> 0:06:55.719 up for this episode, let's start at the beginning. The retina, 0:06:55.800 --> 0:06:58.120 which is the lawn of cells at the back of 0:06:58.160 --> 0:07:01.279 your eye. This has two types of cells that are 0:07:01.839 --> 0:07:06.560 sensitive to light. These are called photoreceptors. These two types 0:07:06.600 --> 0:07:09.400 of cells are rods and cones. Now you have tons 0:07:09.400 --> 0:07:11.280 of rods, like one hundred and twenty million of them, 0:07:11.560 --> 0:07:13.960 and they're extremely sensitive to light, but they don't give 0:07:13.960 --> 0:07:17.760 you any color information. They're what allows us to see 0:07:17.800 --> 0:07:20.600 in dim light, but all the air information is just 0:07:20.640 --> 0:07:22.600 black and white. It's just telling you how much light 0:07:22.680 --> 0:07:25.600 is there at that spot, whatever the wavelength. But the 0:07:25.800 --> 0:07:31.640 other type of light sensitive cell is a cone, and cones. 0:07:31.320 --> 0:07:33.000 Are at the center of our story today. 0:07:32.800 --> 0:07:36.800 Because these are the microscopic cells that lead to our 0:07:36.840 --> 0:07:41.160 experience of color. Now, first, there are many fewer cones 0:07:41.200 --> 0:07:43.400 than rods. There's only about six million of them, and 0:07:43.440 --> 0:07:46.080 they're all concentrated at the center of your vision, not 0:07:46.200 --> 0:07:47.040 in the periphery. 0:07:47.160 --> 0:07:49.720 And cones come in three flavors. 0:07:49.720 --> 0:07:52.680 You've got those that are most responsive to red, which 0:07:52.720 --> 0:07:53.760 is a long wavelength. 0:07:53.960 --> 0:07:55.280 Then you've got those that are most. 0:07:55.160 --> 0:07:58.720 Responsive to green, which is a medium wavelength, and those 0:07:58.760 --> 0:08:01.040 that are most responsive to the light that we perceive 0:08:01.200 --> 0:08:04.120 as blue. These are short wavelength, so what you are 0:08:04.160 --> 0:08:07.040 picking up on is essentially red, green, blue, And your 0:08:07.080 --> 0:08:10.920 perception of color doesn't come from any one cone, but 0:08:11.000 --> 0:08:13.600 from the pattern of activity. 0:08:13.240 --> 0:08:14.920 Across all three types. 0:08:15.320 --> 0:08:18.840 For example, if both your red and green cones are activated, 0:08:19.320 --> 0:08:22.520 you will perceive yellow. Now what I just described with 0:08:22.600 --> 0:08:26.400 the cones, that's only the very first layer straight away. 0:08:26.560 --> 0:08:29.640 Even before the signals get to your visual cortex, other 0:08:29.760 --> 0:08:33.000 cells begin to compare and contrast the signals. 0:08:33.280 --> 0:08:36.960 So colors get analyzed in opposing pairs. 0:08:37.400 --> 0:08:41.480 Red versus green, blue versus yellow, black versus white. And 0:08:41.520 --> 0:08:46.480 this is why you can't see reddish green or bluish yellow. 0:08:46.840 --> 0:08:50.679 Those combinations cancel out in our visual system. Okay, Then 0:08:50.720 --> 0:08:54.160 the signals get to the visual cortex and an enormous 0:08:54.200 --> 0:08:57.640 amount of further processing takes place. I'm going to skip 0:08:57.640 --> 0:09:00.440 all the details here. I'll post a chap from my 0:09:00.520 --> 0:09:03.160 textbook on the show notes. But the point I want 0:09:03.200 --> 0:09:05.600 to make is that you might wonder, wait, why is 0:09:05.640 --> 0:09:09.280 there so much computation involved here? Why don't you just 0:09:09.320 --> 0:09:12.120 look at the wavelength hitting the retina and have your 0:09:12.200 --> 0:09:16.000 perception that way. Well, it turns out that wouldn't be 0:09:16.120 --> 0:09:21.720 nearly enough because the lighting conditions totally change what's bouncing 0:09:21.760 --> 0:09:24.000 off an object and hitting your eye. But if you're 0:09:24.000 --> 0:09:27.480 going to assign colors to things and that's going to 0:09:27.559 --> 0:09:31.360 carry some sort of information, you need to somehow account 0:09:31.720 --> 0:09:34.439 for the lighting changes. In other words, one of the 0:09:34.520 --> 0:09:39.360 most important tricks that the brain pulls off is color constancy. 0:09:39.840 --> 0:09:43.559 This is your brain's ability to perceive and objects color 0:09:43.640 --> 0:09:46.760 as stable even when the lighting changes. 0:09:47.000 --> 0:09:48.480 So let's say I'm wearing a white T. 0:09:48.520 --> 0:09:51.880 Shirt that looks white to you out in the sunlight, 0:09:51.960 --> 0:09:55.199 and it also looks white when I'm in the yellowish 0:09:55.280 --> 0:09:58.240 light of an indoor lamp, or when I'm standing at 0:09:58.240 --> 0:10:00.280 a campfire, or when I'm in the. 0:10:00.080 --> 0:10:01.560 Bright lights of a store. 0:10:02.040 --> 0:10:06.640 But technically the wavelengths bouncing off my white T shirt 0:10:06.920 --> 0:10:10.839 are very different in those conditions. Your brain accounts for 0:10:11.160 --> 0:10:14.280 the context by looking at all the other colors in 0:10:14.320 --> 0:10:19.600 the scene and subtracting that to keep your perception stable. 0:10:21.559 --> 0:10:23.680 This is, by the way, in the laboratory why we 0:10:23.720 --> 0:10:27.760 can make so many color illusions. If you manipulate the 0:10:27.880 --> 0:10:32.439 surrounding light and shade, you can make two identical patches 0:10:32.600 --> 0:10:36.560 appear wildly different in color. If you're interested more on this, 0:10:36.600 --> 0:10:39.720 I'm linking a paper I wrote in Nature Reviews Neuroscience 0:10:39.840 --> 0:10:41.400 on visual illusions. 0:10:41.760 --> 0:10:42.480 But here's the. 0:10:42.400 --> 0:10:44.800 Point I want to get to how weird it is 0:10:45.160 --> 0:10:48.040 that we can study all the pieces and parts of 0:10:48.040 --> 0:10:50.559 the brain and the physiology, but that doesn't really tell 0:10:50.600 --> 0:10:54.720 us anything about why you experience the color a particular way. 0:10:54.960 --> 0:10:58.640 Why don't our brains just register something about wavelength like, oh, 0:10:58.679 --> 0:11:02.920 that's four hundred and fifty animeters, instead of experiencing purpleness 0:11:03.000 --> 0:11:07.200 or a yellowness or a greenness. So consider this thought 0:11:07.320 --> 0:11:11.280 experiment called Mary's Room, which was proposed by the philosopher 0:11:11.360 --> 0:11:15.040 Frank Jackson in nineteen eighty two. He was essentially asking 0:11:15.480 --> 0:11:20.040 how our private, subjective experiences like color can be reduced 0:11:20.120 --> 0:11:25.200 to physical information. Here's the setup he proposed. Mary is 0:11:25.240 --> 0:11:28.240 a brilliant scientist who knows everything there is to know 0:11:28.400 --> 0:11:30.280 about this science of color. 0:11:30.640 --> 0:11:33.880 She understands the wavelengths. 0:11:33.160 --> 0:11:36.520 The neural processes, in other words, the physics and the biology. 0:11:36.800 --> 0:11:41.080 But Mary has lived her entire life in a black 0:11:41.120 --> 0:11:45.040 and white room, and she has never actually seen color. 0:11:45.400 --> 0:11:48.560 She reads about red in the sense that it's a 0:11:48.679 --> 0:11:53.400 seven hundred nanimeter wavelength. She understands how it stimulates particular 0:11:53.480 --> 0:11:56.640 cones in the retina, how it's processed in the visual cortex, 0:11:56.920 --> 0:12:02.480 but she's never experienced red. Then one day she steps 0:12:02.520 --> 0:12:08.319 outside and sees a ripe tomato for the first time. Now, 0:12:08.400 --> 0:12:12.640 Jackson's question is does Mary learn something new when she 0:12:13.040 --> 0:12:16.320 sees red for the first time. If she does, then 0:12:16.320 --> 0:12:22.000 there's something about the experience of color, some qualitative first 0:12:22.120 --> 0:12:27.559 person knowledge that isn't captured just by the objective physical facts. 0:12:28.000 --> 0:12:33.559 And that thought experiment illustrates the difficulty in explaining subjective 0:12:33.640 --> 0:12:38.040 experience just in terms of physical mechanisms. And the weirdest 0:12:38.080 --> 0:12:42.120 part is that the way the brain constructs this subjective 0:12:42.160 --> 0:12:45.520 experience is not necessarily the same for you and me. 0:12:46.240 --> 0:12:49.880 This is why the dress broke the Internet. You remember 0:12:49.920 --> 0:12:53.600 that viral photo that you might have seen as black 0:12:53.640 --> 0:12:56.640 and blue, or you might have seen as white and gold. 0:12:57.080 --> 0:13:01.000 It comes down to the assumptions that eat brain makes 0:13:01.200 --> 0:13:05.000 about the lighting in the photo. Your brain sees this 0:13:05.160 --> 0:13:08.040 little picture of the dress in a shop, and it 0:13:08.080 --> 0:13:13.840 makes dozens of assumptions totally unconsciously. What is the light 0:13:13.920 --> 0:13:17.160 source in the photograph? Is the dress being lit mostly 0:13:17.200 --> 0:13:21.360 by fluorescent lights or by sunlight? Is the dress facing 0:13:21.440 --> 0:13:24.920 a window or is the window behind it? What time 0:13:24.960 --> 0:13:28.480 of day is it, what season is it? The wild 0:13:28.559 --> 0:13:31.720 part is that you just open your eyes and there 0:13:31.720 --> 0:13:34.360 it is. There's the color of the dress. But under 0:13:34.440 --> 0:13:39.320 the hood, your brain is doing an enormous amount of computation, 0:13:40.000 --> 0:13:45.000 asking questions and making assumptions you never have any awareness of. 0:13:45.559 --> 0:13:50.120 And you believe the colors that your brain tells you. 0:13:50.640 --> 0:13:54.160 But illusions like the dress tell us that your head 0:13:54.640 --> 0:13:57.880 may be making those assumptions differently than the head sitting 0:13:57.920 --> 0:14:01.679 next to you, and therefore the color you're seeing isn't 0:14:01.720 --> 0:14:07.040 something true, it's just your brain's result of the computations. 0:14:07.559 --> 0:14:11.040 For more on the dress, listen to episode thirty one. Okay, 0:14:11.600 --> 0:14:14.040 there's a lot more to say about color perception and 0:14:14.080 --> 0:14:16.520 how different people see things differently, and we're going to 0:14:16.559 --> 0:14:19.640 return to that next week, but for now, I want 0:14:19.680 --> 0:14:22.560 to see how this gets even weirder when we compare 0:14:22.560 --> 0:14:27.240 against other species. Because while you think you're just seeing 0:14:27.320 --> 0:14:32.560 what's out there, what you're actually experiencing is one evolutionary 0:14:32.640 --> 0:14:37.640 solution among many. For example, we humans have three types 0:14:37.680 --> 0:14:41.680 of color photoreceptors, these cones, but most other mammals have 0:14:41.720 --> 0:14:45.360 only two types. Now, why do mammals tend to be 0:14:45.480 --> 0:14:48.760 limited in this way? One idea about this is what's 0:14:48.760 --> 0:14:52.280 called the nocturnal bottleneck hypothesis. 0:14:52.400 --> 0:14:54.080 The idea is that the. 0:14:53.960 --> 0:14:58.160 Common ancestor of mammals and reptiles and birds had more 0:14:58.200 --> 0:15:01.360 photoreceptor types, but we he lost them. 0:15:01.600 --> 0:15:01.920 Why. 0:15:02.360 --> 0:15:05.360 It's because two hundred and fifty million years ago, during 0:15:05.360 --> 0:15:09.480 the Mesozoic era, the ancestors of mammals had to become 0:15:09.560 --> 0:15:14.680 mostly nocturnal to avoid getting stomped and eaten by dinosaurs. 0:15:14.760 --> 0:15:19.440 The large predatory dinosaurs dominated the daytime, and so early 0:15:19.520 --> 0:15:23.880 mammals adapted to nocturnal living to avoid them. So these 0:15:23.920 --> 0:15:27.680 many tens of millions of years of nocturnal activity led 0:15:27.720 --> 0:15:32.800 to several adaptations that reflect nighttime living. So, for example, 0:15:33.000 --> 0:15:37.200 mammals developed excellent senses of hearing and smell, which you 0:15:37.280 --> 0:15:40.000 need for navigating and foraging in the dark, and they 0:15:40.040 --> 0:15:44.920 evolved eyes optimized for low light conditions, like large pupils 0:15:45.200 --> 0:15:48.280 and lots of really good rod cells which allow them 0:15:48.320 --> 0:15:51.160 to see better in dim environments. So, in other words, 0:15:51.200 --> 0:15:53.960 when we look at mammalian eyes today, they seem to 0:15:54.000 --> 0:15:58.560 have been shaped during this prolonged period of nighttime activity. 0:15:58.720 --> 0:16:02.120 And here's the key for today day since detailed color 0:16:02.200 --> 0:16:06.880 vision is less useful at night, mammals lost their capacity 0:16:07.240 --> 0:16:12.320 for trichromatic vision, in other words, seeing three primary colors. Instead, 0:16:12.640 --> 0:16:16.920 they got a deeper reliance on senses better suited for 0:16:17.000 --> 0:16:19.800 nocturnal activity, like better hearing and smell. So this is 0:16:19.840 --> 0:16:24.760 presumably why modern mammals retain nocturnal traits even after the 0:16:24.800 --> 0:16:29.480 extinction of dinosaurs, which allowed them to diversify into daytime niches. 0:16:29.840 --> 0:16:32.440 In other words, evolutionary pressures. 0:16:31.960 --> 0:16:35.960 Can shape an entire class of animals, leaving a long 0:16:36.080 --> 0:16:40.280 lasting imprint on their physiology and behavior even after the 0:16:40.360 --> 0:16:42.040 environmental conditions change. 0:16:42.160 --> 0:16:43.000 So just as an. 0:16:42.880 --> 0:16:47.040 Example, my dog and your dog, they are di chromatic. 0:16:47.120 --> 0:16:49.680 They only have two types of cones, and their vision 0:16:49.720 --> 0:16:53.360 resembles red green color blindness in humans. They see a 0:16:53.440 --> 0:16:58.440 world of muted blues and yellows, and they can't distinguish 0:16:58.840 --> 0:17:02.400 reds from greens. This is the same with cats, with horses, 0:17:02.440 --> 0:17:06.679 with rodents, and over ninety percent of mammals. Okay, so 0:17:06.720 --> 0:17:10.400 the nocturnal bottleneck hypothesis suggests why a lot of mammals 0:17:10.800 --> 0:17:15.840 have not so great color vision, but some mammals, like humans, 0:17:16.320 --> 0:17:20.959 have evolved better color vision. We have become trichromatic again. 0:17:21.600 --> 0:17:24.560 Now why would it make sense to regain the ability 0:17:24.680 --> 0:17:28.879 to distinguish red and green. Well, one common argument is 0:17:28.920 --> 0:17:32.520 that if you can distinguish red and green, then you 0:17:32.600 --> 0:17:36.760 can see a ripe fruit against a tree canopy at 0:17:36.800 --> 0:17:39.000 a distance, and that's really useful. 0:17:39.200 --> 0:17:42.000 But maybe it's something even deeper than that. 0:17:42.440 --> 0:17:44.879 So let me tell you one of my favorite hypotheses 0:17:44.880 --> 0:17:49.359 about why we humans re evolved red green color vision. 0:17:49.440 --> 0:17:53.080 This comes from my colleague Mark Changizi, and it flips 0:17:53.119 --> 0:17:54.159 the usual story. 0:17:53.920 --> 0:17:54.440 On its head. 0:17:54.480 --> 0:18:00.000 It suggests something deeply social. He argues that we didn't 0:18:00.080 --> 0:18:02.240 evolve red green vision to forage better. 0:18:02.600 --> 0:18:07.000 We evolved to read each other better. So here's the idea. 0:18:07.200 --> 0:18:11.040 Human skin is packed with tiny blood vessels just beneath 0:18:11.040 --> 0:18:15.360 the surface, and when your emotions shift, when you're embarrassed 0:18:15.760 --> 0:18:20.439 or you're angry, or you're afraid or you're aroused, blood 0:18:20.480 --> 0:18:25.399 flow changes. Your skin flushes red or it pails. It 0:18:25.440 --> 0:18:28.320 happens in fractions of a second, and we pick up 0:18:28.359 --> 0:18:47.879 on this in other faces without even realizing it. So 0:18:48.000 --> 0:18:52.800 Tchannghisi suggests that our color vision system is tuned to 0:18:53.000 --> 0:18:57.720 detect these subtle changes in oxygenation of the hemoglobe and 0:18:57.800 --> 0:19:00.879 under the skin. The spacing of our red and green 0:19:01.000 --> 0:19:06.200 cone sensitivities is almost perfect for distinguishing these tiny shifts 0:19:06.280 --> 0:19:09.159 in skin tone. In other words, we're not just seeing 0:19:09.520 --> 0:19:11.920 red and green on apples and trees. We're seeing it 0:19:12.280 --> 0:19:18.000 in faces, in ears, in emotional states, and those social 0:19:18.040 --> 0:19:22.520 signals is why CHANGESI suggests we evolved the ability in 0:19:22.560 --> 0:19:26.680 the first place to track the emotion of the other. Now, 0:19:26.680 --> 0:19:30.040 this potentially helps explain something I mentioned that most mammals 0:19:30.359 --> 0:19:34.840 don't have trichromatic vision, but about two thirds of primates do, 0:19:35.280 --> 0:19:39.600 and these animals, including us, tend to have less fur 0:19:39.800 --> 0:19:44.840 on their faces. Less fur means more exposed skin. More 0:19:44.880 --> 0:19:48.639 skin means more visible blood flow, and more reason to 0:19:48.840 --> 0:19:52.200 evolve a visual system that can pick up on the 0:19:52.320 --> 0:19:56.600 language of color as it plays out in living flesh. 0:19:56.920 --> 0:19:59.840 So maybe red green vision isn't about navigating the jungle, 0:20:00.119 --> 0:20:03.080 about navigating each other. By the way, you might wonder 0:20:03.160 --> 0:20:06.280 does this apply to people with more pigment in their skin, 0:20:06.600 --> 0:20:10.480 Because in people with darker skin, melanin absorbs more light 0:20:10.520 --> 0:20:13.520 and that can make those blood float changes harder to see. 0:20:13.680 --> 0:20:16.600 But the signals are still there, They're just more subtle. 0:20:16.800 --> 0:20:20.359 Even with darker skin. Changes in coloration can be seen 0:20:20.920 --> 0:20:23.320 in eras where the skin is thinner, like the lips 0:20:23.320 --> 0:20:23.560 and the. 0:20:23.520 --> 0:20:25.400 Eyelids and the palms and the cheeks. 0:20:25.840 --> 0:20:29.000 So we see the world differently than most of our 0:20:29.040 --> 0:20:32.919 mammalian cousins, and understanding this sort of thing can clarify 0:20:33.160 --> 0:20:36.399 a lot of the world around you. For example, why 0:20:36.440 --> 0:20:40.439 do hunters wear bright orange vests? You might correctly assume 0:20:40.480 --> 0:20:43.200 it so they can spot one another, right, But why 0:20:43.200 --> 0:20:47.200 don't they wear chartreuse or bright yellow or any other 0:20:47.400 --> 0:20:51.280 equally detectable color. Well, as we just saw, the ability 0:20:51.320 --> 0:20:56.600 to detect reddish colors was lost during the Mesozoic, and 0:20:56.640 --> 0:20:59.320 the great apes, including humans, regained it. 0:21:00.160 --> 0:21:03.160 Deer did not, so deer. 0:21:03.000 --> 0:21:06.720 Can only see two ranges of color, blue and yellow green. 0:21:07.240 --> 0:21:10.000 Deer have great smell and hearing, and they've got great 0:21:10.119 --> 0:21:14.320 night vision, but they're not sensitive to red or orange light. 0:21:14.680 --> 0:21:18.760 And that's why hunters wear blaze orange, which is easily 0:21:18.800 --> 0:21:22.160 spotted by the other humans but not by the deer. 0:21:22.600 --> 0:21:25.320 This color that helps hunters spot each other at a 0:21:25.359 --> 0:21:29.960 distance makes them nearly invisible to their prey. In this case, 0:21:30.040 --> 0:21:35.760 our evolutionary divergence from other species our trichromatic vision, and 0:21:35.760 --> 0:21:40.960 their dichromatic becomes a tactical advantage. We can design ourselves 0:21:41.280 --> 0:21:44.400 to be invisible to them. And by the way, this 0:21:44.440 --> 0:21:48.399 is why you see nature red birds. It's the same idea. 0:21:48.840 --> 0:21:52.280 The red birds can be there against the green leaves 0:21:52.640 --> 0:21:55.600 and they don't have to worry about getting spotted because 0:21:55.680 --> 0:21:59.520 their predators are red green colorblind. And it turns out 0:21:59.560 --> 0:22:03.919 its getically easier to express red than green, So expressing 0:22:04.000 --> 0:22:07.159 red feathers is a perfectly good way to go. Now, 0:22:07.200 --> 0:22:10.360 let's come back to the deer hunters. Note that they 0:22:10.440 --> 0:22:15.000 never wear blue jeans. Why not because deer are much 0:22:15.080 --> 0:22:18.000 more sensitive to blue even than we are to them. 0:22:18.280 --> 0:22:22.080 Blue jeans shine like a warning beacon, so the hunters 0:22:22.119 --> 0:22:26.719 wear camouflage or earthstone pants to blend in. Okay, So 0:22:26.840 --> 0:22:30.600 back to humans with their three types of photoreceptors. It 0:22:30.640 --> 0:22:34.600 turns out that's most humans. A small fraction of humans 0:22:34.840 --> 0:22:38.520 have some forms of color blindness where they're missing one 0:22:38.600 --> 0:22:42.240 type of color photoreceptor, or two types, or all three, 0:22:42.600 --> 0:22:45.320 and you can guess what their experience is. They can 0:22:45.359 --> 0:22:49.040 distinguish fewer and fewer colors. What you might not know 0:22:49.440 --> 0:22:53.560 is that a small fraction of the human female population. 0:22:54.080 --> 0:22:56.600 They have a fourth type of cone. They are called 0:22:56.920 --> 0:23:00.960 tetrachromatic instead of the typical chromatic vision. 0:23:01.000 --> 0:23:03.439 With three types, these women can. 0:23:03.320 --> 0:23:07.840 Distinguish something like one hundred million shades of color, so 0:23:07.880 --> 0:23:11.840 for them, a sunset contains hues that the rest of 0:23:11.920 --> 0:23:15.320 us have no capacity to see and no words for. 0:23:15.880 --> 0:23:18.359 By the way, it's only women because the mutation in 0:23:18.400 --> 0:23:21.399 the photoreceptor is on one of the X chromosomes and 0:23:21.440 --> 0:23:23.560 not on the other. And it's not just the rare 0:23:23.640 --> 0:23:27.560 human female who's tetrachromatic. A whole lot of birds and 0:23:27.600 --> 0:23:32.080 reptiles and insects are tetrachromats, and in animals, the fourth 0:23:32.119 --> 0:23:36.520 cone type often extends into the ultraviolet, so bees, for example, 0:23:36.600 --> 0:23:41.280 can see ultraviolet patterns on flowers that are completely invisible 0:23:41.280 --> 0:23:44.920 to us. These patterns help them locate the nectar. They're 0:23:44.960 --> 0:23:48.600 like little runways on the flower pedal that don't look 0:23:48.680 --> 0:23:52.760 like anything to us. And then there's the mantis shrimp. 0:23:52.840 --> 0:23:59.200 This little crustacean has sixteen types of photoreceptors. These photoreceptors 0:23:59.200 --> 0:24:04.720 can detect ultraviolet and polarized light, and presumably colors we 0:24:04.880 --> 0:24:08.720 can't imagine, but are useful in their niche. Of course, 0:24:08.760 --> 0:24:11.720 it's impossible for us to know exactly what the experience 0:24:11.880 --> 0:24:15.200 of the shrimp is because it depends what its brain 0:24:15.320 --> 0:24:18.040 is doing with that data. But what all these examples 0:24:18.080 --> 0:24:22.879 show is that color vision is about usefulness. Millions of 0:24:22.960 --> 0:24:27.080 years of evolution tends to equip species with the ability 0:24:27.160 --> 0:24:32.600 to perceive what enhances survival and reproduction. Our color vision 0:24:32.760 --> 0:24:36.159 is not what a mantis shrimps is, but it was 0:24:36.200 --> 0:24:40.400 good enough to spot ripe fruit, and detect emotional signals 0:24:40.400 --> 0:24:45.600 and flushed skin and navigate a multicolored environment. Okay, now 0:24:45.640 --> 0:24:47.920 I want to talk about the birds and the bees. 0:24:47.640 --> 0:24:48.119 For a moment. 0:24:48.480 --> 0:24:51.800 Here's the thing. We usually think of a flower as 0:24:51.840 --> 0:24:55.720 just a flowersome color and nice scent, But every petal, 0:24:55.800 --> 0:24:59.239 every hue of the flower, every tiny detail is a 0:24:59.320 --> 0:25:03.840 carefully c afted advertisement. It's a billboard designed to say hey, 0:25:03.840 --> 0:25:09.000 I'm open for business to a very specific clientele. And 0:25:09.119 --> 0:25:13.320 the best part about this botanical dating game is how 0:25:13.400 --> 0:25:17.480 birds and bees have entirely different preferences when it comes 0:25:17.520 --> 0:25:21.600 to the colors that they find attractive. So imagine you 0:25:21.640 --> 0:25:25.120 walk into a store and some of the products are 0:25:25.160 --> 0:25:26.399 completely invisible to you. 0:25:26.960 --> 0:25:29.439 That's what happens in the floral world. 0:25:29.680 --> 0:25:32.680 So let's start with the bees. When a bee flies 0:25:32.720 --> 0:25:37.760 into your garden, it's not seeing the same spectrum of colors. 0:25:37.320 --> 0:25:37.920