WEBVTT 00:00:00.019 --> 00:00:04.104 Let me tell you about Oliver Sacks, the famous physician, professor and author of unusual 00:00:05.004 --> 00:00:09.173 neurological case studies. We’ll be looking at some of his fascinating research in future 00:00:09.209 --> 00:00:12.216 lessons, but for now, I just want to talk about Sacks himself. Although he possesses 00:00:12.909 --> 00:00:17.690 a brilliant and inquisitive mind, Dr. Sacks cannot do a simple thing that your average 00:00:17.069 --> 00:00:21.094 toddler can. He can’t recognize his own face in the mirror. 00:00:21.094 --> 00:00:26.096 Sacks has a form of prosopagnosia, a neurological disorder that impairs a person’s ability 00:00:26.096 --> 00:00:30.157 to perceive or recognize faces, also known as face blindness. Last week we talked about 00:00:31.057 --> 00:00:35.146 how brain function is localized, and this is another peculiarly excellent example of 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 00:00:41.559 --> 00:00:45.568 friend from a crowd, because the specific sliver of his brain responsible for facial 00:00:45.649 --> 00:00:49.652 recognition is malfunctioning. There’s nothing wrong with his vision. The sense is intact. 00:00:49.949 --> 00:00:54.012 The problem is with his perception, at least when it comes to recognizing faces. Prosopagnosia 00:00:54.579 --> 00:00:59.370 is a good example of how sensing and perceiving are connected, but different. 00:00:59.037 --> 00:01:03.316 Sensation is the bottom-up process by which our senses, like vision, hearing and smell, 00:01:03.649 --> 00:01:08.690 receive and relay outside stimuli. Perception, on the other hand, is the top-down way our 00:01:08.069 --> 00:01:12.142 brains organize and interpret that information and put it into context. So right now at this 00:01:13.042 --> 00:01:17.080 very moment, you’re probably receiving light from your screen through your eyes, which 00:01:17.008 --> 00:01:21.011 will send the data of that sensation to your brain. Perception meanwhile is your brain 00:01:21.083 --> 00:01:24.087 telling you that what you’re seeing is a diagram explaining the difference between 00:01:25.023 --> 00:01:29.070 sensation and perception, which is pretty meta. Now your brain is interpreting that 00:01:29.007 --> 00:01:36.007 light as a talking person, whom your brain might additionally recognize as Hank. 00:01:39.006 --> 00:01:44.006 [Intro] 00:01:44.006 --> 00:01:47.099 We are constantly bombarded by stimuli even though we’re only aware of what our own 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 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 00:01:58.023 --> 00:02:03.024 see ultraviolet and infrared light like a mantis shrimp. I probably can’t even smell 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. 00:02:10.019 --> 00:02:10.067 Yeah. 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 00:02:15.034 --> 00:02:19.034 stuff. So every animal has its limitations which we can talk about more precisely if 00:02:19.034 --> 00:02:24.013 we define the Absolute Threshold of Sensation, the minimum stimulation needed to register 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 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 00:02:31.092 --> 00:02:35.126 absolute threshold of sensation. We have to use a percentage because sometimes I'll play 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 00:02:39.064 --> 00:02:42.108 volume. Why? Because brains are complicated. 00:02:43.008 --> 00:02:46.044 Detecting a weak sensory signal like that beep in daily life isn’t only about the 00:02:46.044 --> 00:02:50.129 strength of the stimulus. It’s also about your psychological state; your alertness and 00:02:51.029 --> 00:02:56.029 expectations in the moment. This has to do with Signal Detection Theory, a model for 00:02:56.029 --> 00:03:01.077 predicting how and when a person will detect a weak stimuli, partly based on context. Exhausted 00:03:01.077 --> 00:03:05.104 new parents might hear their baby’s tiniest whimper, but not even register the bellow 00:03:06.004 --> 00:03:11.013 of a passing train. Their paranoid parent brains are so trained on their baby, it gives 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. 00:03:15.062 --> 00:03:19.113 Conversely, if you’re experiencing constant stimulation, your senses will adjust in a 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 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 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 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 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 00:03:40.072 --> 00:03:44.138 brightness, and yeah, I can tell with my eyeballs that some stars are brighter than others, 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 00:03:49.989 --> 00:03:50.690 brightness. 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. 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 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 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 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 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 00:04:19.005 --> 00:04:23.023 scale. It’s not the amount of change. It’s the percentage change that matters. 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 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 00:04:34.025 --> 00:04:38.091 lightning quick sequence of events. Light bounces off your face and then off the mirror 00:04:38.091 --> 00:04:43.099 and then into your eyes, which take in all that varied energy and transforms it into 00:04:43.099 --> 00:04:47.150 neural messages that your brain processes and organizes into what you actually see, 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 00:04:52.009 --> 00:04:54.055 Corgi or a scary clown holding a tiny cream pie. 00:04:54.055 --> 00:04:57.138 So how do we transform light waves into meaningful information? Well, let’s start with the 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 00:05:02.088 --> 00:05:07.106 of electromagnetic radiation that ranges from gamma to radio waves. Now light has all kinds 00:05:08.006 --> 00:05:12.038 of fascinating characteristics that determine how we sense it, but for the purposes of this 00:05:12.038 --> 00:05:16.119 topic, we’ll understand light as traveling in waves. The wave’s wavelength and frequency 00:05:17.019 --> 00:05:20.118 determines their hue, and their amplitude determines their intensity or brightness. 00:05:21.018 --> 00:05:26.021 For instance a short wave has a high frequency. Our eyes register short wavelengths with high 00:05:26.021 --> 00:05:31.029 frequencies as blueish colors while we see long, low frequency wavelengths as reddish 00:05:31.029 --> 00:05:34.076 hues. The way we register the brightness of a color, the contrast between the orange of 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 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. 00:05:44.032 --> 00:05:47.037 Greater amplitude means higher intensity, means brighter color. 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 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. 00:05:59.003 --> 00:06:02.010 Ask Google about sherbet. So sherbet is a thing. 00:06:02.037 --> 00:06:05.616 So after taking this light in through the cornea and the pupil, it hits the transparent 00:06:05.949 --> 00:06:11.580 disc behind the pupil: the lens, which focuses the light rays into specific images, and just 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 00:06:15.061 --> 00:06:19.115 of the eyeball that contains all the receptor cells that begin sensing that visual information. 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. 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 00:06:28.093 --> 00:06:31.172 into neural impulses and zip back into the brain. 00:06:32.009 --> 00:06:36.014 These retinal receptors are called rods and cones. Our rods detect gray scale and are 00:06:36.509 --> 00:06:40.514 used in our peripheral vision as well as to avoid stubbing our toes in twilight conditions 00:06:41.009 --> 00:06:45.380 when we can’t really see in color. Our cones detect fine detail and color. Concentrated 00:06:45.038 --> 00:06:50.063 near the retina’s central focal point called the fovea, cones function only in well lit 00:06:50.063 --> 00:06:54.102 conditions, allowing you to appreciate the intricacies of your grandma’s china pattern 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 00:07:00.007 --> 00:07:04.826 threshold for colors is so exceptional that the average person can distinguish a million 00:07:04.889 --> 00:07:05.740 different hues. 00:07:05.074 --> 00:07:09.253 There’s a good deal of ongoing research around exactly how our color vision works. 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 00:07:13.083 --> 00:07:17.090 trichromatic theory suggests that the retina houses three specific color receptor cones 00:07:18.053 --> 00:07:21.152 that register red, green and blue, and when stimulated together, their combined power 00:07:22.052 --> 00:07:26.086 allows the eye to register any color. Unless, of course you’re colorblind. About one in 00:07:26.086 --> 00:07:30.099 fifty people have some level of color vision deficiency. They’re mostly dudes because 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 00:07:34.053 --> 00:07:38.100 this figure, it’s likely that your red or green cones are missing or malfunctioning 00:07:39.000 --> 00:07:43.289 which means you have dichromatic instead of trichromatic vision and can’t distinguish 00:07:43.289 --> 00:07:45.030 between shades of red and green. 00:07:45.003 --> 00:07:48.062 The other model for color vision, known as the opponent-process theory, suggests that 00:07:48.062 --> 00:07:52.125 we see color through processes that actually work against each other. So some receptor 00:07:53.025 --> 00:07:58.034 cells might be stimulated by red but inhibited by green, while others do the opposite, and 00:07:58.034 --> 00:08:00.393 those combinations allow us to register colors. 00:08:00.699 --> 00:08:03.786 But back to your eyeballs. When stimulated, the rods and cones trigger chemical changes 00:08:04.569 --> 00:08:10.080 that spark neural signals which in turn activate the cells behind them called bipolar cells, 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 00:08:15.009 --> 00:08:19.610 ganglions braid together to form the ropy optic nerve, which is what carries the neural 00:08:19.061 --> 00:08:23.094 impulses from the eyeball to the brain. That visual information then slips through a chain 00:08:23.094 --> 00:08:28.121 of progressively complex levels as it travels from optic nerve, to the thalamus, and on 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 00:08:32.061 --> 00:08:37.063 occipital lobe, where the right cortex processes input from the left eye and vice versa. This 00:08:37.063 --> 00:08:42.092 cortex has specialized nerve cells, called feature detectors that respond to specific 00:08:42.669 --> 00:08:47.560 features like shapes, angles and movements. In other words different parts of your visual 00:08:47.056 --> 00:08:50.081 cortex are responsible for identifying different aspects of things. 00:08:50.081 --> 00:08:53.154 A person who can’t recognize human faces may have no trouble picking out their set 00:08:54.054 --> 00:08:57.133 of keys from a pile on the counter. That’s because the brains object perception occurs 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 00:09:02.011 --> 00:09:06.042 the region of the brain called the fusiform gyrus, which activates in response to seeing 00:09:06.042 --> 00:09:10.089 faces. Sacks’s face blindness is congenital, but it may also be acquired through disease 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 00:09:14.006 --> 00:09:18.064 just one type of stimulus, like posture or movement or facial expression, while other 00:09:19.018 --> 00:09:23.035 clusters of cells weave all that separate information together in an instant analysis 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 00:09:28.149 --> 00:09:30.930 putting these factors together. Maybe I should get out of here. 00:09:30.093 --> 00:09:34.104 This ability to process and analyze many separate aspects of the situation at once is called 00:09:35.004 --> 00:09:39.051 parallel processing. In the case of visual processing, this means that the brain simultaneously 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 00:09:44.008 --> 00:09:49.167 world of perception which gets complicated quickly, and can even get downright philosophical. 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, 00:09:54.054 --> 00:09:57.106 you learned the difference between sensation and perception, the different thresholds that 00:09:58.006 --> 00:10:02.087 limit our senses, and some of the neurology and biology and psychology of human vision. 00:10:02.087 --> 00:10:05.185 Thanks for watching this lesson with your eyeballs, and thanks to our generous co-sponsors 00:10:06.085 --> 00:10:13.085 who made this episode possible: Alberto Costa, Alpna Agrawal PhD, Frank Zegler, Philipp Dettmer 00:10:14.008 --> 00:10:14.086 and Kurzgesagt. 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 00:10:17.099 --> 00:10:22.388 about that and other perks available to our Subbable subscribers, just go to subbable.com/crashcourse. 00:10:23.279 --> 00:10:27.350 This episode was written by Kathleen Yale, edited by Blake de Pastino, and our consultant 00:10:27.035 --> 00:10:31.057 is Dr. Ranjit Bhagwat. Our director and editor is Nicholas Jenkins, the script supervisor 00:10:31.057 --> 00:10:34.116 is Michael Aranda who is also our sound designer, and our graphics team is Thought Cafe.