0:00:00.019,0:00:04.104 Let me tell you about Oliver Sacks, the famous[br]physician, professor and author of unusual 0:00:05.004,0:00:09.173 neurological case studies. We’ll be looking[br]at some of his fascinating research in future 0:00:09.209,0:00:12.216 lessons, but for now, I just want to talk[br]about Sacks himself. Although he possesses 0:00:12.909,0:00:17.690 a brilliant and inquisitive mind, Dr. Sacks[br]cannot do a simple thing that your average 0:00:17.069,0:00:21.094 toddler can. He can’t recognize his own[br]face in the mirror. 0:00:21.094,0:00:26.096 Sacks has a form of prosopagnosia, a neurological[br]disorder that impairs a person’s ability 0:00:26.096,0:00:30.157 to perceive or recognize faces, also known[br]as face blindness. Last week we talked about 0:00:31.057,0:00:35.146 how brain function is localized, and this[br]is another peculiarly excellent example of 0:00:36.046,0:00:41.145 that. Sacks can recognize his coffee cup on[br]the shelf, but he can’t pick out his oldest 0:00:41.559,0:00:45.568 friend from a crowd, because the specific[br]sliver of his brain responsible for facial 0:00:45.649,0:00:49.652 recognition is malfunctioning. There’s nothing[br]wrong with his vision. The sense is intact. 0:00:49.949,0:00:54.012 The problem is with his perception, at least[br]when it comes to recognizing faces. Prosopagnosia 0:00:54.579,0:00:59.370 is a good example of how sensing and perceiving[br]are connected, but different. 0:00:59.037,0:01:03.316 Sensation is the bottom-up process by which[br]our senses, like vision, hearing and smell, 0:01:03.649,0:01:08.690 receive and relay outside stimuli. Perception,[br]on the other hand, is the top-down way our 0:01:08.069,0:01:12.142 brains organize and interpret that information[br]and put it into context. So right now at this 0:01:13.042,0:01:17.080 very moment, you’re probably receiving light[br]from your screen through your eyes, which 0:01:17.008,0:01:21.011 will send the data of that sensation to your[br]brain. Perception meanwhile is your brain 0:01:21.083,0:01:24.087 telling you that what you’re seeing is a[br]diagram explaining the difference between 0:01:25.023,0:01:29.070 sensation and perception, which is pretty[br]meta. Now your brain is interpreting that 0:01:29.007,0:01:36.007 light as a talking person, whom your brain[br]might additionally recognize as Hank. 0:01:39.006,0:01:44.006 [Intro] 0:01:44.006,0:01:47.099 We are constantly bombarded by stimuli even[br]though we’re only aware of what our own 0:01:47.099,0:01:52.113 senses can pick up. Like I can see and hear[br]and feel and even smell this Corgi, but I 0:01:53.013,0:01:58.014 can’t hunt using sonar like a bat or hear[br]a mole tunneling underground like an owl or 0:01:58.023,0:02:03.024 see ultraviolet and infrared light like a[br]mantis shrimp. I probably can’t even smell 0:02:03.024,0:02:09.119 half of what you can smell. No! No! We have[br]different senses. Mwah mwah mwah mwah mwah. 0:02:10.019,0:02:10.067 Yeah. 0:02:10.067,0:02:14.134 There’s a lot to sense in the world, and[br]not everybody needs to sense all the same 0:02:15.034,0:02:19.034 stuff. So every animal has its limitations[br]which we can talk about more precisely if 0:02:19.034,0:02:24.013 we define the Absolute Threshold of Sensation,[br]the minimum stimulation needed to register 0:02:24.319,0:02:28.840 a particular stimulus, 50% of the time. So[br]if I play a tiny little beep in your ear and 0:02:28.084,0:02:31.092 you tell me that you hear it fifty percent[br]of the times that I play it, that’s your 0:02:31.092,0:02:35.126 absolute threshold of sensation. We have to[br]use a percentage because sometimes I'll play 0:02:36.026,0:02:39.064 the beep and you’ll hear it and sometimes[br]you won’t even though it’s the exact same 0:02:39.064,0:02:42.108 volume. Why? Because brains are complicated. 0:02:43.008,0:02:46.044 Detecting a weak sensory signal like that[br]beep in daily life isn’t only about the 0:02:46.044,0:02:50.129 strength of the stimulus. It’s also about[br]your psychological state; your alertness and 0:02:51.029,0:02:56.029 expectations in the moment. This has to do[br]with Signal Detection Theory, a model for 0:02:56.029,0:03:01.077 predicting how and when a person will detect[br]a weak stimuli, partly based on context. Exhausted 0:03:01.077,0:03:05.104 new parents might hear their baby’s tiniest[br]whimper, but not even register the bellow 0:03:06.004,0:03:11.013 of a passing train. Their paranoid parent[br]brains are so trained on their baby, it gives 0:03:11.013,0:03:15.062 their senses a sort of boosted ability, but[br]only in relation to the subject of their attention. 0:03:15.062,0:03:19.113 Conversely, if you’re experiencing constant[br]stimulation, your senses will adjust in a 0:03:20.013,0:03:24.037 process called sensory adaptation. It is the[br]reason that I have to check and see if my 0:03:24.037,0:03:27.626 wallet is there if it’s in my right pocket,[br]but if I move it to my left pocket, it feels 0:03:27.959,0:03:31.020 like a big uncomfortable lump. It’s also[br]useful to be able to talk about our ability 0:03:31.569,0:03:35.430 to detect the difference between two stimuli.[br]I might go out at night and look up at the 0:03:35.043,0:03:40.072 sky and, well, I know with my objective science[br]brain that no two stars have the exact same 0:03:40.072,0:03:44.138 brightness, and yeah, I can tell with my eyeballs[br]that some stars are brighter than others, 0:03:45.038,0:03:49.647 but other stars just look exactly the same[br]to me. I can’t tell the difference in their 0:03:49.989,0:03:50.690 brightness. 0:03:50.069,0:03:57.069 Are you done? Is it time for your to go? Gimme,[br]gimme a kiiiissss. Yes, yes. Okay. Good girl. 0:03:58.379,0:04:01.448 The point at which one can tell the difference[br]is the difference threshold, but it’s not 0:04:02.069,0:04:06.190 linear. Like. if a tiny star is just a tiny[br]bit brighter than another tiny star, I can 0:04:06.019,0:04:10.568 tell. But if a big star is that same tiny[br]amount brighter than another big star, I won’t 0:04:10.739,0:04:14.270 be able to tell the difference. This is important[br]enough that we gave the guy who discovered 0:04:14.027,0:04:19.050 it a law. Weber’s Law says that we perceive[br]differences on a logarithmic, not a linear 0:04:19.005,0:04:23.023 scale. It’s not the amount of change. It’s[br]the percentage change that matters. 0:04:23.068,0:04:28.093 Alright. How about now we take a more in depth[br]look at how one of our most powerful senses 0:04:28.093,0:04:33.125 works? Vision. Your ability to see your face[br]in the mirror is the result of a long but 0:04:34.025,0:04:38.091 lightning quick sequence of events. Light[br]bounces off your face and then off the mirror 0:04:38.091,0:04:43.099 and then into your eyes, which take in all[br]that varied energy and transforms it into 0:04:43.099,0:04:47.150 neural messages that your brain processes[br]and organizes into what you actually see, 0:04:48.005,0:04:51.064 which is your face. Or if you’re looking[br]elsewhere, you could see a coffee cup or a 0:04:52.009,0:04:54.055 Corgi or a scary clown holding a tiny cream[br]pie. 0:04:54.055,0:04:57.138 So how do we transform light waves into meaningful[br]information? Well, let’s start with the 0:04:58.038,0:05:02.043 light itself. What we humans see as light[br]is only a small fraction of the full spectrum 0:05:02.088,0:05:07.106 of electromagnetic radiation that ranges from[br]gamma to radio waves. Now light has all kinds 0:05:08.006,0:05:12.038 of fascinating characteristics that determine[br]how we sense it, but for the purposes of this 0:05:12.038,0:05:16.119 topic, we’ll understand light as traveling[br]in waves. The wave’s wavelength and frequency 0:05:17.019,0:05:20.118 determines their hue, and their amplitude[br]determines their intensity or brightness. 0:05:21.018,0:05:26.021 For instance a short wave has a high frequency.[br]Our eyes register short wavelengths with high 0:05:26.021,0:05:31.029 frequencies as blueish colors while we see[br]long, low frequency wavelengths as reddish 0:05:31.029,0:05:34.076 hues. The way we register the brightness of[br]a color, the contrast between the orange of 0:05:34.076,0:05:38.097 a sherbet and the orange of a construction[br]cone has to do with the intensity or amount 0:05:38.097,0:05:43.132 of energy in a given light wave. Which as[br]we’ve just said is determined by its amplitude. 0:05:44.032,0:05:47.037 Greater amplitude means higher intensity,[br]means brighter color. 0:05:47.082,0:05:52.096 Someone’s just told me that sherbet doesn’t-[br]isn’t a word that exists. His name is Michael 0:05:52.096,0:05:58.130 Aranda and he’s a dumbhead. Did you type[br]it into the dictionary? Type it into Google. 0:05:59.003,0:06:02.010 Ask Google about sherbet. So sherbet is a[br]thing. 0:06:02.037,0:06:05.616 So after taking this light in through the[br]cornea and the pupil, it hits the transparent 0:06:05.949,0:06:11.580 disc behind the pupil: the lens, which focuses[br]the light rays into specific images, and just 0:06:11.058,0:06:15.061 as you’d expect the lens to do, it projects[br]these images onto the retina, the inner surface 0:06:15.061,0:06:19.115 of the eyeball that contains all the receptor[br]cells that begin sensing that visual information. 0:06:20.015,0:06:24.124 Now your retinas don’t receive a full image[br]like a movie being projected onto a screen. 0:06:24.259,0:06:28.930 It’s more like a bunch of pixel points of[br]light energy that millions of receptors translate 0:06:28.093,0:06:31.172 into neural impulses and zip back into the[br]brain. 0:06:32.009,0:06:36.014 These retinal receptors are called rods and[br]cones. Our rods detect gray scale and are 0:06:36.509,0:06:40.514 used in our peripheral vision as well as to[br]avoid stubbing our toes in twilight conditions 0:06:41.009,0:06:45.380 when we can’t really see in color. Our cones[br]detect fine detail and color. Concentrated 0:06:45.038,0:06:50.063 near the retina’s central focal point called[br]the fovea, cones function only in well lit 0:06:50.063,0:06:54.102 conditions, allowing you to appreciate the[br]intricacies of your grandma’s china pattern 0:06:54.669,0:07:00.070 or your uncle’s sleeve tattoo. And the human[br]eye is terrific at seeing color. Our difference 0:07:00.007,0:07:04.826 threshold for colors is so exceptional that[br]the average person can distinguish a million 0:07:04.889,0:07:05.740 different hues. 0:07:05.074,0:07:09.253 There’s a good deal of ongoing research[br]around exactly how our color vision works. 0:07:09.919,0:07:13.830 But two theories help us explain some of what[br]we know. One model, called the Young-Helmholtz 0:07:13.083,0:07:17.090 trichromatic theory suggests that the retina[br]houses three specific color receptor cones 0:07:18.053,0:07:21.152 that register red, green and blue, and when[br]stimulated together, their combined power 0:07:22.052,0:07:26.086 allows the eye to register any color. Unless,[br]of course you’re colorblind. About one in 0:07:26.086,0:07:30.099 fifty people have some level of color vision[br]deficiency. They’re mostly dudes because 0:07:30.099,0:07:33.153 the genetic defect is sex linked. If you can’t[br]see the Crash Course logo pop out at you in 0:07:34.053,0:07:38.100 this figure, it’s likely that your red or[br]green cones are missing or malfunctioning 0:07:39.000,0:07:43.289 which means you have dichromatic instead of[br]trichromatic vision and can’t distinguish 0:07:43.289,0:07:45.030 between shades of red and green. 0:07:45.003,0:07:48.062 The other model for color vision, known as[br]the opponent-process theory, suggests that 0:07:48.062,0:07:52.125 we see color through processes that actually[br]work against each other. So some receptor 0:07:53.025,0:07:58.034 cells might be stimulated by red but inhibited[br]by green, while others do the opposite, and 0:07:58.034,0:08:00.393 those combinations allow us to register colors. 0:08:00.699,0:08:03.786 But back to your eyeballs. When stimulated,[br]the rods and cones trigger chemical changes 0:08:04.569,0:08:10.080 that spark neural signals which in turn activate[br]the cells behind them called bipolar cells, 0:08:10.008,0:08:14.937 whose job it is to turn on the neighboring[br]ganglion cells. The long axon tails of these 0:08:15.009,0:08:19.610 ganglions braid together to form the ropy[br]optic nerve, which is what carries the neural 0:08:19.061,0:08:23.094 impulses from the eyeball to the brain. That[br]visual information then slips through a chain 0:08:23.094,0:08:28.121 of progressively complex levels as it travels[br]from optic nerve, to the thalamus, and on 0:08:29.021,0:08:32.025 to the brain’s visual cortex. The visual[br]cortex sits at the back of the brain in the 0:08:32.061,0:08:37.063 occipital lobe, where the right cortex processes[br]input from the left eye and vice versa. This 0:08:37.063,0:08:42.092 cortex has specialized nerve cells, called[br]feature detectors that respond to specific 0:08:42.669,0:08:47.560 features like shapes, angles and movements.[br]In other words different parts of your visual 0:08:47.056,0:08:50.081 cortex are responsible for identifying different[br]aspects of things. 0:08:50.081,0:08:53.154 A person who can’t recognize human faces[br]may have no trouble picking out their set 0:08:54.054,0:08:57.133 of keys from a pile on the counter. That’s[br]because the brains object perception occurs 0:08:58.033,0:09:01.111 in a different place from its face perception.[br]In the case of Dr. Sacks, his condition affects 0:09:02.011,0:09:06.042 the region of the brain called the fusiform[br]gyrus, which activates in response to seeing 0:09:06.042,0:09:10.089 faces. Sacks’s face blindness is congenital,[br]but it may also be acquired through disease 0:09:10.089,0:09:13.160 or injury to that same region of the brain.[br]And some cells in a region may respond to 0:09:14.006,0:09:18.064 just one type of stimulus, like posture or[br]movement or facial expression, while other 0:09:19.018,0:09:23.035 clusters of cells weave all that separate[br]information together in an instant analysis 0:09:23.035,0:09:27.834 of a situation. That clown is frowning and[br]running at me with a tiny cream pie. I’m 0:09:28.149,0:09:30.930 putting these factors together. Maybe I should[br]get out of here. 0:09:30.093,0:09:34.104 This ability to process and analyze many separate[br]aspects of the situation at once is called 0:09:35.004,0:09:39.051 parallel processing. In the case of visual[br]processing, this means that the brain simultaneously 0:09:39.051,0:09:44.080 works on making sense of form, depth, motion[br]and color and this is where we enter the whole 0:09:44.008,0:09:49.167 world of perception which gets complicated[br]quickly, and can even get downright philosophical. 0:09:49.959,0:09:54.540 So we’ll be exploring that in depth next[br]time but for now, if you were paying attention, 0:09:54.054,0:09:57.106 you learned the difference between sensation[br]and perception, the different thresholds that 0:09:58.006,0:10:02.087 limit our senses, and some of the neurology[br]and biology and psychology of human vision. 0:10:02.087,0:10:05.185 Thanks for watching this lesson with your[br]eyeballs, and thanks to our generous co-sponsors 0:10:06.085,0:10:13.085 who made this episode possible: Alberto Costa,[br]Alpna Agrawal PhD, Frank Zegler, Philipp Dettmer 0:10:14.008,0:10:14.086 and Kurzgesagt. 0:10:14.086,0:10:17.099 And if you’d like to sponsor an episode[br]and get your own shout out, you can learn 0:10:17.099,0:10:22.388 about that and other perks available to our[br]Subbable subscribers, just go to subbable.com/crashcourse. 0:10:23.279,0:10:27.350 This episode was written by Kathleen Yale,[br]edited by Blake de Pastino, and our consultant 0:10:27.035,0:10:31.057 is Dr. Ranjit Bhagwat. Our director and editor[br]is Nicholas Jenkins, the script supervisor 0:10:31.057,0:10:34.116 is Michael Aranda who is also our sound designer,[br]and our graphics team is Thought Cafe.