1 00:00:00,019 --> 00:00:04,104 Let me tell you about Oliver Sacks, the famous physician, professor and author of unusual 2 00:00:05,004 --> 00:00:09,173 neurological case studies. We’ll be looking at some of his fascinating research in future 3 00:00:09,209 --> 00:00:12,216 lessons, but for now, I just want to talk about Sacks himself. Although he possesses 4 00:00:12,909 --> 00:00:17,690 a brilliant and inquisitive mind, Dr. Sacks cannot do a simple thing that your average 5 00:00:17,069 --> 00:00:21,094 toddler can. He can’t recognize his own face in the mirror. 6 00:00:21,094 --> 00:00:26,096 Sacks has a form of prosopagnosia, a neurological disorder that impairs a person’s ability 7 00:00:26,096 --> 00:00:30,157 to perceive or recognize faces, also known as face blindness. Last week we talked about 8 00:00:31,057 --> 00:00:35,146 how brain function is localized, and this is another peculiarly excellent example of 9 00:00:36,046 --> 00:00:41,145 that. Sacks can recognize his coffee cup on the shelf, but he can’t pick out his oldest 10 00:00:41,559 --> 00:00:45,568 friend from a crowd, because the specific sliver of his brain responsible for facial 11 00:00:45,649 --> 00:00:49,652 recognition is malfunctioning. There’s nothing wrong with his vision. The sense is intact. 12 00:00:49,949 --> 00:00:54,012 The problem is with his perception, at least when it comes to recognizing faces. Prosopagnosia 13 00:00:54,579 --> 00:00:59,370 is a good example of how sensing and perceiving are connected, but different. 14 00:00:59,037 --> 00:01:03,316 Sensation is the bottom-up process by which our senses, like vision, hearing and smell, 15 00:01:03,649 --> 00:01:08,690 receive and relay outside stimuli. Perception, on the other hand, is the top-down way our 16 00:01:08,069 --> 00:01:12,142 brains organize and interpret that information and put it into context. So right now at this 17 00:01:13,042 --> 00:01:17,080 very moment, you’re probably receiving light from your screen through your eyes, which 18 00:01:17,008 --> 00:01:21,011 will send the data of that sensation to your brain. Perception meanwhile is your brain 19 00:01:21,083 --> 00:01:24,087 telling you that what you’re seeing is a diagram explaining the difference between 20 00:01:25,023 --> 00:01:29,070 sensation and perception, which is pretty meta. Now your brain is interpreting that 21 00:01:29,007 --> 00:01:36,007 light as a talking person, whom your brain might additionally recognize as Hank. 22 00:01:39,006 --> 00:01:44,006 [Intro] 23 00:01:44,006 --> 00:01:47,099 We are constantly bombarded by stimuli even though we’re only aware of what our own 24 00:01:47,099 --> 00:01:52,113 senses can pick up. Like I can see and hear and feel and even smell this Corgi, but I 25 00:01:53,013 --> 00:01:58,014 can’t hunt using sonar like a bat or hear a mole tunneling underground like an owl or 26 00:01:58,023 --> 00:02:03,024 see ultraviolet and infrared light like a mantis shrimp. I probably can’t even smell 27 00:02:03,024 --> 00:02:09,119 half of what you can smell. No! No! We have different senses. Mwah mwah mwah mwah mwah. 28 00:02:10,019 --> 00:02:10,067 Yeah. 29 00:02:10,067 --> 00:02:14,134 There’s a lot to sense in the world, and not everybody needs to sense all the same 30 00:02:15,034 --> 00:02:19,034 stuff. So every animal has its limitations which we can talk about more precisely if 31 00:02:19,034 --> 00:02:24,013 we define the Absolute Threshold of Sensation, the minimum stimulation needed to register 32 00:02:24,319 --> 00:02:28,840 a particular stimulus, 50% of the time. So if I play a tiny little beep in your ear and 33 00:02:28,084 --> 00:02:31,092 you tell me that you hear it fifty percent of the times that I play it, that’s your 34 00:02:31,092 --> 00:02:35,126 absolute threshold of sensation. We have to use a percentage because sometimes I'll play 35 00:02:36,026 --> 00:02:39,064 the beep and you’ll hear it and sometimes you won’t even though it’s the exact same 36 00:02:39,064 --> 00:02:42,108 volume. Why? Because brains are complicated. 37 00:02:43,008 --> 00:02:46,044 Detecting a weak sensory signal like that beep in daily life isn’t only about the 38 00:02:46,044 --> 00:02:50,129 strength of the stimulus. It’s also about your psychological state; your alertness and 39 00:02:51,029 --> 00:02:56,029 expectations in the moment. This has to do with Signal Detection Theory, a model for 40 00:02:56,029 --> 00:03:01,077 predicting how and when a person will detect a weak stimuli, partly based on context. Exhausted 41 00:03:01,077 --> 00:03:05,104 new parents might hear their baby’s tiniest whimper, but not even register the bellow 42 00:03:06,004 --> 00:03:11,013 of a passing train. Their paranoid parent brains are so trained on their baby, it gives 43 00:03:11,013 --> 00:03:15,062 their senses a sort of boosted ability, but only in relation to the subject of their attention. 44 00:03:15,062 --> 00:03:19,113 Conversely, if you’re experiencing constant stimulation, your senses will adjust in a 45 00:03:20,013 --> 00:03:24,037 process called sensory adaptation. It is the reason that I have to check and see if my 46 00:03:24,037 --> 00:03:27,626 wallet is there if it’s in my right pocket, but if I move it to my left pocket, it feels 47 00:03:27,959 --> 00:03:31,020 like a big uncomfortable lump. It’s also useful to be able to talk about our ability 48 00:03:31,569 --> 00:03:35,430 to detect the difference between two stimuli. I might go out at night and look up at the 49 00:03:35,043 --> 00:03:40,072 sky and, well, I know with my objective science brain that no two stars have the exact same 50 00:03:40,072 --> 00:03:44,138 brightness, and yeah, I can tell with my eyeballs that some stars are brighter than others, 51 00:03:45,038 --> 00:03:49,647 but other stars just look exactly the same to me. I can’t tell the difference in their 52 00:03:49,989 --> 00:03:50,690 brightness. 53 00:03:50,069 --> 00:03:57,069 Are you done? Is it time for your to go? Gimme, gimme a kiiiissss. Yes, yes. Okay. Good girl. 54 00:03:58,379 --> 00:04:01,448 The point at which one can tell the difference is the difference threshold, but it’s not 55 00:04:02,069 --> 00:04:06,190 linear. Like. if a tiny star is just a tiny bit brighter than another tiny star, I can 56 00:04:06,019 --> 00:04:10,568 tell. But if a big star is that same tiny amount brighter than another big star, I won’t 57 00:04:10,739 --> 00:04:14,270 be able to tell the difference. This is important enough that we gave the guy who discovered 58 00:04:14,027 --> 00:04:19,050 it a law. Weber’s Law says that we perceive differences on a logarithmic, not a linear 59 00:04:19,005 --> 00:04:23,023 scale. It’s not the amount of change. It’s the percentage change that matters. 60 00:04:23,068 --> 00:04:28,093 Alright. How about now we take a more in depth look at how one of our most powerful senses 61 00:04:28,093 --> 00:04:33,125 works? Vision. Your ability to see your face in the mirror is the result of a long but 62 00:04:34,025 --> 00:04:38,091 lightning quick sequence of events. Light bounces off your face and then off the mirror 63 00:04:38,091 --> 00:04:43,099 and then into your eyes, which take in all that varied energy and transforms it into 64 00:04:43,099 --> 00:04:47,150 neural messages that your brain processes and organizes into what you actually see, 65 00:04:48,005 --> 00:04:51,064 which is your face. Or if you’re looking elsewhere, you could see a coffee cup or a 66 00:04:52,009 --> 00:04:54,055 Corgi or a scary clown holding a tiny cream pie. 67 00:04:54,055 --> 00:04:57,138 So how do we transform light waves into meaningful information? Well, let’s start with the 68 00:04:58,038 --> 00:05:02,043 light itself. What we humans see as light is only a small fraction of the full spectrum 69 00:05:02,088 --> 00:05:07,106 of electromagnetic radiation that ranges from gamma to radio waves. Now light has all kinds 70 00:05:08,006 --> 00:05:12,038 of fascinating characteristics that determine how we sense it, but for the purposes of this 71 00:05:12,038 --> 00:05:16,119 topic, we’ll understand light as traveling in waves. The wave’s wavelength and frequency 72 00:05:17,019 --> 00:05:20,118 determines their hue, and their amplitude determines their intensity or brightness. 73 00:05:21,018 --> 00:05:26,021 For instance a short wave has a high frequency. Our eyes register short wavelengths with high 74 00:05:26,021 --> 00:05:31,029 frequencies as blueish colors while we see long, low frequency wavelengths as reddish 75 00:05:31,029 --> 00:05:34,076 hues. The way we register the brightness of a color, the contrast between the orange of 76 00:05:34,076 --> 00:05:38,097 a sherbet and the orange of a construction cone has to do with the intensity or amount 77 00:05:38,097 --> 00:05:43,132 of energy in a given light wave. Which as we’ve just said is determined by its amplitude. 78 00:05:44,032 --> 00:05:47,037 Greater amplitude means higher intensity, means brighter color. 79 00:05:47,082 --> 00:05:52,096 Someone’s just told me that sherbet doesn’t- isn’t a word that exists. His name is Michael 80 00:05:52,096 --> 00:05:58,130 Aranda and he’s a dumbhead. Did you type it into the dictionary? Type it into Google. 81 00:05:59,003 --> 00:06:02,010 Ask Google about sherbet. So sherbet is a thing. 82 00:06:02,037 --> 00:06:05,616 So after taking this light in through the cornea and the pupil, it hits the transparent 83 00:06:05,949 --> 00:06:11,580 disc behind the pupil: the lens, which focuses the light rays into specific images, and just 84 00:06:11,058 --> 00:06:15,061 as you’d expect the lens to do, it projects these images onto the retina, the inner surface 85 00:06:15,061 --> 00:06:19,115 of the eyeball that contains all the receptor cells that begin sensing that visual information. 86 00:06:20,015 --> 00:06:24,124 Now your retinas don’t receive a full image like a movie being projected onto a screen. 87 00:06:24,259 --> 00:06:28,930 It’s more like a bunch of pixel points of light energy that millions of receptors translate 88 00:06:28,093 --> 00:06:31,172 into neural impulses and zip back into the brain. 89 00:06:32,009 --> 00:06:36,014 These retinal receptors are called rods and cones. Our rods detect gray scale and are 90 00:06:36,509 --> 00:06:40,514 used in our peripheral vision as well as to avoid stubbing our toes in twilight conditions 91 00:06:41,009 --> 00:06:45,380 when we can’t really see in color. Our cones detect fine detail and color. Concentrated 92 00:06:45,038 --> 00:06:50,063 near the retina’s central focal point called the fovea, cones function only in well lit 93 00:06:50,063 --> 00:06:54,102 conditions, allowing you to appreciate the intricacies of your grandma’s china pattern 94 00:06:54,669 --> 00:07:00,070 or your uncle’s sleeve tattoo. And the human eye is terrific at seeing color. Our difference 95 00:07:00,007 --> 00:07:04,826 threshold for colors is so exceptional that the average person can distinguish a million 96 00:07:04,889 --> 00:07:05,740 different hues. 97 00:07:05,074 --> 00:07:09,253 There’s a good deal of ongoing research around exactly how our color vision works. 98 00:07:09,919 --> 00:07:13,830 But two theories help us explain some of what we know. One model, called the Young-Helmholtz 99 00:07:13,083 --> 00:07:17,090 trichromatic theory suggests that the retina houses three specific color receptor cones 100 00:07:18,053 --> 00:07:21,152 that register red, green and blue, and when stimulated together, their combined power 101 00:07:22,052 --> 00:07:26,086 allows the eye to register any color. Unless, of course you’re colorblind. About one in 102 00:07:26,086 --> 00:07:30,099 fifty people have some level of color vision deficiency. They’re mostly dudes because 103 00:07:30,099 --> 00:07:33,153 the genetic defect is sex linked. If you can’t see the Crash Course logo pop out at you in 104 00:07:34,053 --> 00:07:38,100 this figure, it’s likely that your red or green cones are missing or malfunctioning 105 00:07:39,000 --> 00:07:43,289 which means you have dichromatic instead of trichromatic vision and can’t distinguish 106 00:07:43,289 --> 00:07:45,030 between shades of red and green. 107 00:07:45,003 --> 00:07:48,062 The other model for color vision, known as the opponent-process theory, suggests that 108 00:07:48,062 --> 00:07:52,125 we see color through processes that actually work against each other. So some receptor 109 00:07:53,025 --> 00:07:58,034 cells might be stimulated by red but inhibited by green, while others do the opposite, and 110 00:07:58,034 --> 00:08:00,393 those combinations allow us to register colors. 111 00:08:00,699 --> 00:08:03,786 But back to your eyeballs. When stimulated, the rods and cones trigger chemical changes 112 00:08:04,569 --> 00:08:10,080 that spark neural signals which in turn activate the cells behind them called bipolar cells, 113 00:08:10,008 --> 00:08:14,937 whose job it is to turn on the neighboring ganglion cells. The long axon tails of these 114 00:08:15,009 --> 00:08:19,610 ganglions braid together to form the ropy optic nerve, which is what carries the neural 115 00:08:19,061 --> 00:08:23,094 impulses from the eyeball to the brain. That visual information then slips through a chain 116 00:08:23,094 --> 00:08:28,121 of progressively complex levels as it travels from optic nerve, to the thalamus, and on 117 00:08:29,021 --> 00:08:32,025 to the brain’s visual cortex. The visual cortex sits at the back of the brain in the 118 00:08:32,061 --> 00:08:37,063 occipital lobe, where the right cortex processes input from the left eye and vice versa. This 119 00:08:37,063 --> 00:08:42,092 cortex has specialized nerve cells, called feature detectors that respond to specific 120 00:08:42,669 --> 00:08:47,560 features like shapes, angles and movements. In other words different parts of your visual 121 00:08:47,056 --> 00:08:50,081 cortex are responsible for identifying different aspects of things. 122 00:08:50,081 --> 00:08:53,154 A person who can’t recognize human faces may have no trouble picking out their set 123 00:08:54,054 --> 00:08:57,133 of keys from a pile on the counter. That’s because the brains object perception occurs 124 00:08:58,033 --> 00:09:01,111 in a different place from its face perception. In the case of Dr. Sacks, his condition affects 125 00:09:02,011 --> 00:09:06,042 the region of the brain called the fusiform gyrus, which activates in response to seeing 126 00:09:06,042 --> 00:09:10,089 faces. Sacks’s face blindness is congenital, but it may also be acquired through disease 127 00:09:10,089 --> 00:09:13,160 or injury to that same region of the brain. And some cells in a region may respond to 128 00:09:14,006 --> 00:09:18,064 just one type of stimulus, like posture or movement or facial expression, while other 129 00:09:19,018 --> 00:09:23,035 clusters of cells weave all that separate information together in an instant analysis 130 00:09:23,035 --> 00:09:27,834 of a situation. That clown is frowning and running at me with a tiny cream pie. I’m 131 00:09:28,149 --> 00:09:30,930 putting these factors together. Maybe I should get out of here. 132 00:09:30,093 --> 00:09:34,104 This ability to process and analyze many separate aspects of the situation at once is called 133 00:09:35,004 --> 00:09:39,051 parallel processing. In the case of visual processing, this means that the brain simultaneously 134 00:09:39,051 --> 00:09:44,080 works on making sense of form, depth, motion and color and this is where we enter the whole 135 00:09:44,008 --> 00:09:49,167 world of perception which gets complicated quickly, and can even get downright philosophical. 136 00:09:49,959 --> 00:09:54,540 So we’ll be exploring that in depth next time but for now, if you were paying attention, 137 00:09:54,054 --> 00:09:57,106 you learned the difference between sensation and perception, the different thresholds that 138 00:09:58,006 --> 00:10:02,087 limit our senses, and some of the neurology and biology and psychology of human vision. 139 00:10:02,087 --> 00:10:05,185 Thanks for watching this lesson with your eyeballs, and thanks to our generous co-sponsors 140 00:10:06,085 --> 00:10:13,085 who made this episode possible: Alberto Costa, Alpna Agrawal PhD, Frank Zegler, Philipp Dettmer 141 00:10:14,008 --> 00:10:14,086 and Kurzgesagt. 142 00:10:14,086 --> 00:10:17,099 And if you’d like to sponsor an episode and get your own shout out, you can learn 143 00:10:17,099 --> 00:10:22,388 about that and other perks available to our Subbable subscribers, just go to subbable.com/crashcourse. 144 00:10:23,279 --> 00:10:27,350 This episode was written by Kathleen Yale, edited by Blake de Pastino, and our consultant 145 00:10:27,035 --> 00:10:31,057 is Dr. Ranjit Bhagwat. Our director and editor is Nicholas Jenkins, the script supervisor 146 00:10:31,057 --> 00:10:34,116 is Michael Aranda who is also our sound designer, and our graphics team is Thought Cafe.