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  • Let me tell you about Oliver Sacks, the famous physician, professor and author of unusual

  • neurological case studies. Well be looking at some of his fascinating research in future lessons,

  • but for now, I just want to talk about Sacks himself. Although he possesses

  • a brilliant and inquisitive mind, Dr. Sacks cannot do a simple thing that your average toddler can.

  • He can’t recognize his own face in the mirror.

  • Sacks has a form of prosopagnosia, a neurological disorder that impairs a person’s ability

  • to perceive or recognize faces, also known as face blindness. Last week we talked about

  • how brain function is localized, and this is another peculiarly excellent example of that.

  • Sacks can recognize his coffee cup on the shelf, but he can’t pick out his oldest

  • friend from a crowd, because the specific sliver of his brain responsible for facial

  • recognition is malfunctioning. There’s nothing wrong with his vision. The sense is intact.

  • The problem is with his perception, at least when it comes to recognizing faces.

  • Prosopagnosia is a good example of how sensing and perceiving are connected, but different.

  • Sensation is the bottom-up process by which our senses, like vision, hearing and smell,

  • receive and relay outside stimuli. Perception, on the other hand, is the top-down way our

  • brains organize and interpret that information and put it into context. So right now at this

  • very moment, youre probably receiving light from your screen through your eyes, which

  • will send the data of that sensation to your brain. Perception meanwhile is your brain

  • telling you that what youre seeing is a diagram explaining the difference between

  • sensation and perception, which is pretty meta. Now your brain is interpreting that

  • light as a talking person, whom your brain might additionally recognize as Hank.

  • We are constantly bombarded by stimuli even though were only aware of what our own

  • senses can pick up. Like I can see and hear and feel and even smell this Corgi,

  • but I can’t hunt using sonar like a bat or hear a mole tunneling underground like an owl or

  • see ultraviolet and infrared light like a mantis shrimp. I probably can’t even smell

  • half of what you can smell. No! No! We have different senses. Mwah mwah mwah mwah mwah.

  • Yeah.

  • There’s a lot to sense in the world, and not everybody needs to sense all the same stuff.

  • So every animal has its limitations which we can talk about more precisely

  • if we define the Absolute Threshold of Sensation, the minimum stimulation needed to register

  • a particular stimulus, 50% of the time. So if I play a tiny little beep in your ear and

  • you tell me that you hear it fifty percent of the times that I play it,

  • that’s your absolute threshold of sensation. We have to use a percentage because sometimes I'll play

  • the beep and youll hear it and sometimes you won’t even though it’s the exact same volume.

  • Why? Because brains are complicated.

  • Detecting a weak sensory signal like that beep in daily life isn’t only about the

  • strength of the stimulus. It’s also about your psychological state; your alertness and

  • expectations in the moment. This has to do with Signal Detection Theory, a model for

  • predicting how and when a person will detect a weak stimuli, partly based on context.

  • Exhausted new parents might hear their baby’s tiniest whimper, but not even register the bellow

  • of a passing train. Their paranoid parent brains are so trained on their baby,

  • it gives their senses a sort of boosted ability, but only in relation to the subject of their attention.

  • Conversely, if youre experiencing constant stimulation, your senses will adjust in a

  • process called sensory adaptation. It is the reason that I have to check and see if my

  • wallet is there if it’s in my right pocket, but if I move it to my left pocket,

  • it feels like a big uncomfortable lump. It’s also useful to be able to talk about our ability

  • to detect the difference between two stimuli. I might go out at night and look up at the sky

  • and, well, I know with my objective science brain that no two stars have the exact same brightness,

  • and yeah, I can tell with my eyeballs that some stars are brighter than others,

  • but other stars just look exactly the same to me. I can’t tell the difference in their brightness.

  • Are you done? Is it time for your to go? Gimme, gimme a kiiiissss. Yes, yes. Okay. Good girl.

  • The point at which one can tell the difference is the difference threshold, but it’s not linear.

  • Like. if a tiny star is just a tiny bit brighter than another tiny star, I can tell.

  • But if a big star is that same tiny amount brighter than another big star, I won’t

  • be able to tell the difference. This is important enough that we gave the guy who discovered

  • it a law. Weber’s Law says that we perceive differences on a logarithmic, not a linear scale.

  • It’s not the amount of change. It’s the percentage change that matters.

  • Alright. How about now we take a more in depth look at how one of our most powerful senses works?

  • Vision. Your ability to see your face in the mirror is the result of a long but

  • lightning quick sequence of events. Light bounces off your face and then off the mirror

  • and then into your eyes, which take in all that varied energy and transforms it into

  • neural messages that your brain processes and organizes into what you actually see,

  • which is your face. Or if youre looking elsewhere, you could see a coffee cup or a

  • Corgi or a scary clown holding a tiny cream pie.

  • So how do we transform light waves into meaningful information? Well, let’s start with the light itself.

  • What we humans see as light is only a small fraction of the full spectrum

  • of electromagnetic radiation that ranges from gamma to radio waves. Now light has all kinds

  • of fascinating characteristics that determine how we sense it, but for the purposes of this topic,

  • well understand light as traveling in waves. The wave’s wavelength and frequency

  • determines their hue, and their amplitude determines their intensity or brightness.

  • For instance a short wave has a high frequency. Our eyes register short wavelengths with high

  • frequencies as blueish colors while we see long, low frequency wavelengths as reddish hues.

  • The way we register the brightness of a color, the contrast between the orange of

  • a sherbet and the orange of a construction cone has to do with the intensity or amount

  • of energy in a given light wave. Which as weve just said is determined by its amplitude.

  • Greater amplitude means higher intensity, means brighter color.

  • Someone’s just told me that sherbet doesn’t- isn’t a word that exists. His name is Michael Aranda

  • and he’s a dumbhead. Did you type it into the dictionary? Type it into Google.

  • Ask Google about sherbet. So sherbet is a thing.

  • So after taking this light in through the cornea and the pupil, it hits the transparent

  • disc behind the pupil: the lens, which focuses the light rays into specific images, and just

  • as you’d expect the lens to do, it projects these images onto the retina, the inner surface

  • of the eyeball that contains all the receptor cells that begin sensing that visual information.

  • Now your retinas don’t receive a full image like a movie being projected onto a screen.

  • It’s more like a bunch of pixel points of light energy that millions of receptors translate

  • into neural impulses and zip back into the brain.

  • These retinal receptors are called rods and cones. Our rods detect gray scale and are

  • used in our peripheral vision as well as to avoid stubbing our toes in twilight conditions

  • when we can’t really see in color. Our cones detect fine detail and color.

  • Concentrated near the retina’s central focal point called the fovea, cones function only in well lit conditions,

  • allowing you to appreciate the intricacies of your grandma’s china pattern

  • or your uncle’s sleeve tattoo. And the human eye is terrific at seeing color. Our difference

  • threshold for colors is so exceptional that the average person can distinguish a million different hues.

  • There’s a good deal of ongoing research around exactly how our color vision works.

  • But two theories help us explain some of what we know. One model, called the Young-Helmholtz

  • trichromatic theory suggests that the retina houses three specific color receptor cones

  • that register red, green and blue, and when stimulated together, their combined power

  • allows the eye to register any color. Unless, of course youre colorblind. About one in

  • fifty people have some level of color vision deficiency. Theyre mostly dudes because

  • the genetic defect is sex linked. If you can’t see the Crash Course logo pop out at you in this figure,

  • it’s likely that your red or green cones are missing or malfunctioning

  • which means you have dichromatic instead of trichromatic vision and can’t distinguish

  • between shades of red and green.

  • The other model for color vision, known as the opponent-process theory, suggests that

  • we see color through processes that actually work against each other. So some receptor

  • cells might be stimulated by red but inhibited by green, while others do the opposite,

  • and those combinations allow us to register colors.

  • But back to your eyeballs. When stimulated, the rods and cones trigger chemical changes

  • that spark neural signals which in turn activate the cells behind them called bipolar cells,

  • whose job it is to turn on the neighboring ganglion cells. The long axon tails of these

  • ganglions braid together to form the ropy optic nerve, which is what carries the neural impulses

  • from the eyeball to the brain. That visual information then slips through a chain

  • of progressively complex levels as it travels from optic nerve, to the thalamus,

  • and on to the brain’s visual cortex. The visual cortex sits at the back of the brain in the occipital lobe,

  • where the right cortex processes input from the left eye and vice versa.

  • This cortex has specialized nerve cells, called feature detectors that respond to specific features

  • like shapes, angles and movements. In other words different parts of your visual

  • cortex are responsible for identifying different aspects of things.

  • A person who can’t recognize human faces may have no trouble picking out their set

  • of keys from a pile on the counter. That’s because the brains object perception occurs

  • in a different place from its face perception. In the case of Dr. Sacks, his condition affects

  • the region of the brain called the fusiform gyrus, which activates in response to seeing faces.

  • Sacks’s face blindness is congenital, but it may also be acquired through disease

  • or injury to that same region of the brain. And some cells in a region may respond to

  • just one type of stimulus, like posture or movement or facial expression,

  • while other clusters of cells weave all that separate information together in an instant analysis of a situation.

  • That clown is frowning and running at me with a tiny cream pie.

  • I’m putting these factors together. Maybe I should get out of here.

  • This ability to process and analyze many separate aspects of the situation at once is called parallel processing.

  • In the case of visual processing, this means that the brain simultaneously

  • works on making sense of form, depth, motion and color and this is where we enter the whole

  • world of perception which gets complicated quickly, and can even get downright philosophical.

  • So well be exploring that in depth next time but for now, if you were paying attention,

  • you learned the difference between sensation and perception, the different thresholds that

  • limit our senses, and some of the neurology and biology and psychology of human vision.

  • Thanks for watching this lesson with your eyeballs, and thanks to our generous co-sponsors

  • who made this episode possible: Alberto Costa, Alpna Agrawal PhD, Frank Zegler, Philipp Dettmer and Kurzgesagt.

  • And if you’d like to sponsor an episode and get your own shout out, you can learn

  • about that and other perks available to our Subbable subscribers, just go to subbable.com/crashcourse.

  • This episode was written by Kathleen Yale, edited by Blake de Pastino, and our consultant

  • is Dr. Ranjit Bhagwat. Our director and editor is Nicholas Jenkins, the script supervisor

  • is Michael Aranda who is also our sound designer, and our graphics team is Thought Cafe.

Let me tell you about Oliver Sacks, the famous physician, professor and author of unusual

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