[Script Info]
Title: 
[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:00.02,0:00:04.10,Default,,0000,0000,0000,,Let me tell you about Oliver Sacks, the famous\Nphysician, professor and author of unusual
Dialogue: 0,0:00:05.00,0:00:09.17,Default,,0000,0000,0000,,neurological case studies. We’ll be looking\Nat some of his fascinating research in future
Dialogue: 0,0:00:09.21,0:00:12.22,Default,,0000,0000,0000,,lessons, but for now, I just want to talk\Nabout Sacks himself. Although he possesses
Dialogue: 0,0:00:12.91,0:00:17.69,Default,,0000,0000,0000,,a brilliant and inquisitive mind, Dr. Sacks\Ncannot do a simple thing that your average
Dialogue: 0,0:00:17.07,0:00:21.09,Default,,0000,0000,0000,,toddler can. He can’t recognize his own\Nface in the mirror.
Dialogue: 0,0:00:21.09,0:00:26.10,Default,,0000,0000,0000,,Sacks has a form of prosopagnosia, a neurological\Ndisorder that impairs a person’s ability
Dialogue: 0,0:00:26.10,0:00:30.16,Default,,0000,0000,0000,,to perceive or recognize faces, also known\Nas face blindness. Last week we talked about
Dialogue: 0,0:00:31.06,0:00:35.15,Default,,0000,0000,0000,,how brain function is localized, and this\Nis another peculiarly excellent example of
Dialogue: 0,0:00:36.05,0:00:41.14,Default,,0000,0000,0000,,that. Sacks can recognize his coffee cup on\Nthe shelf, but he can’t pick out his oldest
Dialogue: 0,0:00:41.56,0:00:45.57,Default,,0000,0000,0000,,friend from a crowd, because the specific\Nsliver of his brain responsible for facial
Dialogue: 0,0:00:45.65,0:00:49.65,Default,,0000,0000,0000,,recognition is malfunctioning. There’s nothing\Nwrong with his vision. The sense is intact.
Dialogue: 0,0:00:49.95,0:00:54.01,Default,,0000,0000,0000,,The problem is with his perception, at least\Nwhen it comes to recognizing faces. Prosopagnosia
Dialogue: 0,0:00:54.58,0:00:59.37,Default,,0000,0000,0000,,is a good example of how sensing and perceiving\Nare connected, but different.
Dialogue: 0,0:00:59.04,0:01:03.32,Default,,0000,0000,0000,,Sensation is the bottom-up process by which\Nour senses, like vision, hearing and smell,
Dialogue: 0,0:01:03.65,0:01:08.69,Default,,0000,0000,0000,,receive and relay outside stimuli. Perception,\Non the other hand, is the top-down way our
Dialogue: 0,0:01:08.07,0:01:12.14,Default,,0000,0000,0000,,brains organize and interpret that information\Nand put it into context. So right now at this
Dialogue: 0,0:01:13.04,0:01:17.08,Default,,0000,0000,0000,,very moment, you’re probably receiving light\Nfrom your screen through your eyes, which
Dialogue: 0,0:01:17.01,0:01:21.01,Default,,0000,0000,0000,,will send the data of that sensation to your\Nbrain. Perception meanwhile is your brain
Dialogue: 0,0:01:21.08,0:01:24.09,Default,,0000,0000,0000,,telling you that what you’re seeing is a\Ndiagram explaining the difference between
Dialogue: 0,0:01:25.02,0:01:29.07,Default,,0000,0000,0000,,sensation and perception, which is pretty\Nmeta. Now your brain is interpreting that
Dialogue: 0,0:01:29.01,0:01:36.01,Default,,0000,0000,0000,,light as a talking person, whom your brain\Nmight additionally recognize as Hank.
Dialogue: 0,0:01:39.01,0:01:44.01,Default,,0000,0000,0000,,[Intro]
Dialogue: 0,0:01:44.01,0:01:47.10,Default,,0000,0000,0000,,We are constantly bombarded by stimuli even\Nthough we’re only aware of what our own
Dialogue: 0,0:01:47.10,0:01:52.11,Default,,0000,0000,0000,,senses can pick up. Like I can see and hear\Nand feel and even smell this Corgi, but I
Dialogue: 0,0:01:53.01,0:01:58.01,Default,,0000,0000,0000,,can’t hunt using sonar like a bat or hear\Na mole tunneling underground like an owl or
Dialogue: 0,0:01:58.02,0:02:03.02,Default,,0000,0000,0000,,see ultraviolet and infrared light like a\Nmantis shrimp. I probably can’t even smell
Dialogue: 0,0:02:03.02,0:02:09.12,Default,,0000,0000,0000,,half of what you can smell. No! No! We have\Ndifferent senses. Mwah mwah mwah mwah mwah.
Dialogue: 0,0:02:10.02,0:02:10.07,Default,,0000,0000,0000,,Yeah.
Dialogue: 0,0:02:10.07,0:02:14.13,Default,,0000,0000,0000,,There’s a lot to sense in the world, and\Nnot everybody needs to sense all the same
Dialogue: 0,0:02:15.03,0:02:19.03,Default,,0000,0000,0000,,stuff. So every animal has its limitations\Nwhich we can talk about more precisely if
Dialogue: 0,0:02:19.03,0:02:24.01,Default,,0000,0000,0000,,we define the Absolute Threshold of Sensation,\Nthe minimum stimulation needed to register
Dialogue: 0,0:02:24.32,0:02:28.84,Default,,0000,0000,0000,,a particular stimulus, 50% of the time. So\Nif I play a tiny little beep in your ear and
Dialogue: 0,0:02:28.08,0:02:31.09,Default,,0000,0000,0000,,you tell me that you hear it fifty percent\Nof the times that I play it, that’s your
Dialogue: 0,0:02:31.09,0:02:35.13,Default,,0000,0000,0000,,absolute threshold of sensation. We have to\Nuse a percentage because sometimes I'll play
Dialogue: 0,0:02:36.03,0:02:39.06,Default,,0000,0000,0000,,the beep and you’ll hear it and sometimes\Nyou won’t even though it’s the exact same
Dialogue: 0,0:02:39.06,0:02:42.11,Default,,0000,0000,0000,,volume. Why? Because brains are complicated.
Dialogue: 0,0:02:43.01,0:02:46.04,Default,,0000,0000,0000,,Detecting a weak sensory signal like that\Nbeep in daily life isn’t only about the
Dialogue: 0,0:02:46.04,0:02:50.13,Default,,0000,0000,0000,,strength of the stimulus. It’s also about\Nyour psychological state; your alertness and
Dialogue: 0,0:02:51.03,0:02:56.03,Default,,0000,0000,0000,,expectations in the moment. This has to do\Nwith Signal Detection Theory, a model for
Dialogue: 0,0:02:56.03,0:03:01.08,Default,,0000,0000,0000,,predicting how and when a person will detect\Na weak stimuli, partly based on context. Exhausted
Dialogue: 0,0:03:01.08,0:03:05.10,Default,,0000,0000,0000,,new parents might hear their baby’s tiniest\Nwhimper, but not even register the bellow
Dialogue: 0,0:03:06.00,0:03:11.01,Default,,0000,0000,0000,,of a passing train. Their paranoid parent\Nbrains are so trained on their baby, it gives
Dialogue: 0,0:03:11.01,0:03:15.06,Default,,0000,0000,0000,,their senses a sort of boosted ability, but\Nonly in relation to the subject of their attention.
Dialogue: 0,0:03:15.06,0:03:19.11,Default,,0000,0000,0000,,Conversely, if you’re experiencing constant\Nstimulation, your senses will adjust in a
Dialogue: 0,0:03:20.01,0:03:24.04,Default,,0000,0000,0000,,process called sensory adaptation. It is the\Nreason that I have to check and see if my
Dialogue: 0,0:03:24.04,0:03:27.63,Default,,0000,0000,0000,,wallet is there if it’s in my right pocket,\Nbut if I move it to my left pocket, it feels
Dialogue: 0,0:03:27.96,0:03:31.02,Default,,0000,0000,0000,,like a big uncomfortable lump. It’s also\Nuseful to be able to talk about our ability
Dialogue: 0,0:03:31.57,0:03:35.43,Default,,0000,0000,0000,,to detect the difference between two stimuli.\NI might go out at night and look up at the
Dialogue: 0,0:03:35.04,0:03:40.07,Default,,0000,0000,0000,,sky and, well, I know with my objective science\Nbrain that no two stars have the exact same
Dialogue: 0,0:03:40.07,0:03:44.14,Default,,0000,0000,0000,,brightness, and yeah, I can tell with my eyeballs\Nthat some stars are brighter than others,
Dialogue: 0,0:03:45.04,0:03:49.65,Default,,0000,0000,0000,,but other stars just look exactly the same\Nto me. I can’t tell the difference in their
Dialogue: 0,0:03:49.99,0:03:50.69,Default,,0000,0000,0000,,brightness.
Dialogue: 0,0:03:50.07,0:03:57.07,Default,,0000,0000,0000,,Are you done? Is it time for your to go? Gimme,\Ngimme a kiiiissss. Yes, yes. Okay. Good girl.
Dialogue: 0,0:03:58.38,0:04:01.45,Default,,0000,0000,0000,,The point at which one can tell the difference\Nis the difference threshold, but it’s not
Dialogue: 0,0:04:02.07,0:04:06.19,Default,,0000,0000,0000,,linear. Like. if a tiny star is just a tiny\Nbit brighter than another tiny star, I can
Dialogue: 0,0:04:06.02,0:04:10.57,Default,,0000,0000,0000,,tell. But if a big star is that same tiny\Namount brighter than another big star, I won’t
Dialogue: 0,0:04:10.74,0:04:14.27,Default,,0000,0000,0000,,be able to tell the difference. This is important\Nenough that we gave the guy who discovered
Dialogue: 0,0:04:14.03,0:04:19.05,Default,,0000,0000,0000,,it a law. Weber’s Law says that we perceive\Ndifferences on a logarithmic, not a linear
Dialogue: 0,0:04:19.00,0:04:23.02,Default,,0000,0000,0000,,scale. It’s not the amount of change. It’s\Nthe percentage change that matters.
Dialogue: 0,0:04:23.07,0:04:28.09,Default,,0000,0000,0000,,Alright. How about now we take a more in depth\Nlook at how one of our most powerful senses
Dialogue: 0,0:04:28.09,0:04:33.12,Default,,0000,0000,0000,,works? Vision. Your ability to see your face\Nin the mirror is the result of a long but
Dialogue: 0,0:04:34.02,0:04:38.09,Default,,0000,0000,0000,,lightning quick sequence of events. Light\Nbounces off your face and then off the mirror
Dialogue: 0,0:04:38.09,0:04:43.10,Default,,0000,0000,0000,,and then into your eyes, which take in all\Nthat varied energy and transforms it into
Dialogue: 0,0:04:43.10,0:04:47.15,Default,,0000,0000,0000,,neural messages that your brain processes\Nand organizes into what you actually see,
Dialogue: 0,0:04:48.00,0:04:51.06,Default,,0000,0000,0000,,which is your face. Or if you’re looking\Nelsewhere, you could see a coffee cup or a
Dialogue: 0,0:04:52.01,0:04:54.06,Default,,0000,0000,0000,,Corgi or a scary clown holding a tiny cream\Npie.
Dialogue: 0,0:04:54.06,0:04:57.14,Default,,0000,0000,0000,,So how do we transform light waves into meaningful\Ninformation? Well, let’s start with the
Dialogue: 0,0:04:58.04,0:05:02.04,Default,,0000,0000,0000,,light itself. What we humans see as light\Nis only a small fraction of the full spectrum
Dialogue: 0,0:05:02.09,0:05:07.11,Default,,0000,0000,0000,,of electromagnetic radiation that ranges from\Ngamma to radio waves. Now light has all kinds
Dialogue: 0,0:05:08.01,0:05:12.04,Default,,0000,0000,0000,,of fascinating characteristics that determine\Nhow we sense it, but for the purposes of this
Dialogue: 0,0:05:12.04,0:05:16.12,Default,,0000,0000,0000,,topic, we’ll understand light as traveling\Nin waves. The wave’s wavelength and frequency
Dialogue: 0,0:05:17.02,0:05:20.12,Default,,0000,0000,0000,,determines their hue, and their amplitude\Ndetermines their intensity or brightness.
Dialogue: 0,0:05:21.02,0:05:26.02,Default,,0000,0000,0000,,For instance a short wave has a high frequency.\NOur eyes register short wavelengths with high
Dialogue: 0,0:05:26.02,0:05:31.03,Default,,0000,0000,0000,,frequencies as blueish colors while we see\Nlong, low frequency wavelengths as reddish
Dialogue: 0,0:05:31.03,0:05:34.08,Default,,0000,0000,0000,,hues. The way we register the brightness of\Na color, the contrast between the orange of
Dialogue: 0,0:05:34.08,0:05:38.10,Default,,0000,0000,0000,,a sherbet and the orange of a construction\Ncone has to do with the intensity or amount
Dialogue: 0,0:05:38.10,0:05:43.13,Default,,0000,0000,0000,,of energy in a given light wave. Which as\Nwe’ve just said is determined by its amplitude.
Dialogue: 0,0:05:44.03,0:05:47.04,Default,,0000,0000,0000,,Greater amplitude means higher intensity,\Nmeans brighter color.
Dialogue: 0,0:05:47.08,0:05:52.10,Default,,0000,0000,0000,,Someone’s just told me that sherbet doesn’t-\Nisn’t a word that exists. His name is Michael
Dialogue: 0,0:05:52.10,0:05:58.13,Default,,0000,0000,0000,,Aranda and he’s a dumbhead. Did you type\Nit into the dictionary? Type it into Google.
Dialogue: 0,0:05:59.00,0:06:02.01,Default,,0000,0000,0000,,Ask Google about sherbet. So sherbet is a\Nthing.
Dialogue: 0,0:06:02.04,0:06:05.62,Default,,0000,0000,0000,,So after taking this light in through the\Ncornea and the pupil, it hits the transparent
Dialogue: 0,0:06:05.95,0:06:11.58,Default,,0000,0000,0000,,disc behind the pupil: the lens, which focuses\Nthe light rays into specific images, and just
Dialogue: 0,0:06:11.06,0:06:15.06,Default,,0000,0000,0000,,as you’d expect the lens to do, it projects\Nthese images onto the retina, the inner surface
Dialogue: 0,0:06:15.06,0:06:19.12,Default,,0000,0000,0000,,of the eyeball that contains all the receptor\Ncells that begin sensing that visual information.
Dialogue: 0,0:06:20.02,0:06:24.12,Default,,0000,0000,0000,,Now your retinas don’t receive a full image\Nlike a movie being projected onto a screen.
Dialogue: 0,0:06:24.26,0:06:28.93,Default,,0000,0000,0000,,It’s more like a bunch of pixel points of\Nlight energy that millions of receptors translate
Dialogue: 0,0:06:28.09,0:06:31.17,Default,,0000,0000,0000,,into neural impulses and zip back into the\Nbrain.
Dialogue: 0,0:06:32.01,0:06:36.01,Default,,0000,0000,0000,,These retinal receptors are called rods and\Ncones. Our rods detect gray scale and are
Dialogue: 0,0:06:36.51,0:06:40.51,Default,,0000,0000,0000,,used in our peripheral vision as well as to\Navoid stubbing our toes in twilight conditions
Dialogue: 0,0:06:41.01,0:06:45.38,Default,,0000,0000,0000,,when we can’t really see in color. Our cones\Ndetect fine detail and color. Concentrated
Dialogue: 0,0:06:45.04,0:06:50.06,Default,,0000,0000,0000,,near the retina’s central focal point called\Nthe fovea, cones function only in well lit
Dialogue: 0,0:06:50.06,0:06:54.10,Default,,0000,0000,0000,,conditions, allowing you to appreciate the\Nintricacies of your grandma’s china pattern
Dialogue: 0,0:06:54.67,0:07:00.07,Default,,0000,0000,0000,,or your uncle’s sleeve tattoo. And the human\Neye is terrific at seeing color. Our difference
Dialogue: 0,0:07:00.01,0:07:04.83,Default,,0000,0000,0000,,threshold for colors is so exceptional that\Nthe average person can distinguish a million
Dialogue: 0,0:07:04.89,0:07:05.74,Default,,0000,0000,0000,,different hues.
Dialogue: 0,0:07:05.07,0:07:09.25,Default,,0000,0000,0000,,There’s a good deal of ongoing research\Naround exactly how our color vision works.
Dialogue: 0,0:07:09.92,0:07:13.83,Default,,0000,0000,0000,,But two theories help us explain some of what\Nwe know. One model, called the Young-Helmholtz
Dialogue: 0,0:07:13.08,0:07:17.09,Default,,0000,0000,0000,,trichromatic theory suggests that the retina\Nhouses three specific color receptor cones
Dialogue: 0,0:07:18.05,0:07:21.15,Default,,0000,0000,0000,,that register red, green and blue, and when\Nstimulated together, their combined power
Dialogue: 0,0:07:22.05,0:07:26.09,Default,,0000,0000,0000,,allows the eye to register any color. Unless,\Nof course you’re colorblind. About one in
Dialogue: 0,0:07:26.09,0:07:30.10,Default,,0000,0000,0000,,fifty people have some level of color vision\Ndeficiency. They’re mostly dudes because
Dialogue: 0,0:07:30.10,0:07:33.15,Default,,0000,0000,0000,,the genetic defect is sex linked. If you can’t\Nsee the Crash Course logo pop out at you in
Dialogue: 0,0:07:34.05,0:07:38.10,Default,,0000,0000,0000,,this figure, it’s likely that your red or\Ngreen cones are missing or malfunctioning
Dialogue: 0,0:07:39.00,0:07:43.29,Default,,0000,0000,0000,,which means you have dichromatic instead of\Ntrichromatic vision and can’t distinguish
Dialogue: 0,0:07:43.29,0:07:45.03,Default,,0000,0000,0000,,between shades of red and green.
Dialogue: 0,0:07:45.00,0:07:48.06,Default,,0000,0000,0000,,The other model for color vision, known as\Nthe opponent-process theory, suggests that
Dialogue: 0,0:07:48.06,0:07:52.12,Default,,0000,0000,0000,,we see color through processes that actually\Nwork against each other. So some receptor
Dialogue: 0,0:07:53.02,0:07:58.03,Default,,0000,0000,0000,,cells might be stimulated by red but inhibited\Nby green, while others do the opposite, and
Dialogue: 0,0:07:58.03,0:08:00.39,Default,,0000,0000,0000,,those combinations allow us to register colors.
Dialogue: 0,0:08:00.70,0:08:03.79,Default,,0000,0000,0000,,But back to your eyeballs. When stimulated,\Nthe rods and cones trigger chemical changes
Dialogue: 0,0:08:04.57,0:08:10.08,Default,,0000,0000,0000,,that spark neural signals which in turn activate\Nthe cells behind them called bipolar cells,
Dialogue: 0,0:08:10.01,0:08:14.94,Default,,0000,0000,0000,,whose job it is to turn on the neighboring\Nganglion cells. The long axon tails of these
Dialogue: 0,0:08:15.01,0:08:19.61,Default,,0000,0000,0000,,ganglions braid together to form the ropy\Noptic nerve, which is what carries the neural
Dialogue: 0,0:08:19.06,0:08:23.09,Default,,0000,0000,0000,,impulses from the eyeball to the brain. That\Nvisual information then slips through a chain
Dialogue: 0,0:08:23.09,0:08:28.12,Default,,0000,0000,0000,,of progressively complex levels as it travels\Nfrom optic nerve, to the thalamus, and on
Dialogue: 0,0:08:29.02,0:08:32.02,Default,,0000,0000,0000,,to the brain’s visual cortex. The visual\Ncortex sits at the back of the brain in the
Dialogue: 0,0:08:32.06,0:08:37.06,Default,,0000,0000,0000,,occipital lobe, where the right cortex processes\Ninput from the left eye and vice versa. This
Dialogue: 0,0:08:37.06,0:08:42.09,Default,,0000,0000,0000,,cortex has specialized nerve cells, called\Nfeature detectors that respond to specific
Dialogue: 0,0:08:42.67,0:08:47.56,Default,,0000,0000,0000,,features like shapes, angles and movements.\NIn other words different parts of your visual
Dialogue: 0,0:08:47.06,0:08:50.08,Default,,0000,0000,0000,,cortex are responsible for identifying different\Naspects of things.
Dialogue: 0,0:08:50.08,0:08:53.15,Default,,0000,0000,0000,,A person who can’t recognize human faces\Nmay have no trouble picking out their set
Dialogue: 0,0:08:54.05,0:08:57.13,Default,,0000,0000,0000,,of keys from a pile on the counter. That’s\Nbecause the brains object perception occurs
Dialogue: 0,0:08:58.03,0:09:01.11,Default,,0000,0000,0000,,in a different place from its face perception.\NIn the case of Dr. Sacks, his condition affects
Dialogue: 0,0:09:02.01,0:09:06.04,Default,,0000,0000,0000,,the region of the brain called the fusiform\Ngyrus, which activates in response to seeing
Dialogue: 0,0:09:06.04,0:09:10.09,Default,,0000,0000,0000,,faces. Sacks’s face blindness is congenital,\Nbut it may also be acquired through disease
Dialogue: 0,0:09:10.09,0:09:13.16,Default,,0000,0000,0000,,or injury to that same region of the brain.\NAnd some cells in a region may respond to
Dialogue: 0,0:09:14.01,0:09:18.06,Default,,0000,0000,0000,,just one type of stimulus, like posture or\Nmovement or facial expression, while other
Dialogue: 0,0:09:19.02,0:09:23.04,Default,,0000,0000,0000,,clusters of cells weave all that separate\Ninformation together in an instant analysis
Dialogue: 0,0:09:23.04,0:09:27.83,Default,,0000,0000,0000,,of a situation. That clown is frowning and\Nrunning at me with a tiny cream pie. I’m
Dialogue: 0,0:09:28.15,0:09:30.93,Default,,0000,0000,0000,,putting these factors together. Maybe I should\Nget out of here.
Dialogue: 0,0:09:30.09,0:09:34.10,Default,,0000,0000,0000,,This ability to process and analyze many separate\Naspects of the situation at once is called
Dialogue: 0,0:09:35.00,0:09:39.05,Default,,0000,0000,0000,,parallel processing. In the case of visual\Nprocessing, this means that the brain simultaneously
Dialogue: 0,0:09:39.05,0:09:44.08,Default,,0000,0000,0000,,works on making sense of form, depth, motion\Nand color and this is where we enter the whole
Dialogue: 0,0:09:44.01,0:09:49.17,Default,,0000,0000,0000,,world of perception which gets complicated\Nquickly, and can even get downright philosophical.
Dialogue: 0,0:09:49.96,0:09:54.54,Default,,0000,0000,0000,,So we’ll be exploring that in depth next\Ntime but for now, if you were paying attention,
Dialogue: 0,0:09:54.05,0:09:57.11,Default,,0000,0000,0000,,you learned the difference between sensation\Nand perception, the different thresholds that
Dialogue: 0,0:09:58.01,0:10:02.09,Default,,0000,0000,0000,,limit our senses, and some of the neurology\Nand biology and psychology of human vision.
Dialogue: 0,0:10:02.09,0:10:05.18,Default,,0000,0000,0000,,Thanks for watching this lesson with your\Neyeballs, and thanks to our generous co-sponsors
Dialogue: 0,0:10:06.08,0:10:13.08,Default,,0000,0000,0000,,who made this episode possible: Alberto Costa,\NAlpna Agrawal PhD, Frank Zegler, Philipp Dettmer
Dialogue: 0,0:10:14.01,0:10:14.09,Default,,0000,0000,0000,,and Kurzgesagt.
Dialogue: 0,0:10:14.09,0:10:17.10,Default,,0000,0000,0000,,And if you’d like to sponsor an episode\Nand get your own shout out, you can learn
Dialogue: 0,0:10:17.10,0:10:22.39,Default,,0000,0000,0000,,about that and other perks available to our\NSubbable subscribers, just go to subbable.com/crashcourse.
Dialogue: 0,0:10:23.28,0:10:27.35,Default,,0000,0000,0000,,This episode was written by Kathleen Yale,\Nedited by Blake de Pastino, and our consultant
Dialogue: 0,0:10:27.04,0:10:31.06,Default,,0000,0000,0000,,is Dr. Ranjit Bhagwat. Our director and editor\Nis Nicholas Jenkins, the script supervisor
Dialogue: 0,0:10:31.06,0:10:34.12,Default,,0000,0000,0000,,is Michael Aranda who is also our sound designer,\Nand our graphics team is Thought Cafe.