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  • Homunculus! If you've heard that word, it probably wasn't in the context of psychology.

  • A monster in Dungeons and Dragons, a song by They Might Be Giants, the bad guys in Fullmetal

  • Alchemist, a novel by James Blaylock. It's Latin for 'little man' and in psychology,

  • it refers to a kind of sensory map of the human body, a depiction of what we'd look

  • like if each of our parts grew in proportion to how much we sense with them.

  • Witness!

  • Look at this dude. His ham hands could rip off a car door. I mean, if he could lift them,

  • which he couldn't.

  • Is this what we're supposed to look like on the inside?

  • This freaky thing illustrates the weighted significance of our sensory receptors. His

  • disproportional hands are monstrous, for example, because we primarily touch the world with

  • our hands, not our elbows, so our hands are extremely sensitive. His mouth, meanwhile,

  • is huge because we also have a ton of sensory receptors in our tongues and lips. It's what

  • we use for tasting food and for sucking face.

  • As we continue our exploration of how we both sense and perceive the world, the Homunculus,

  • while kind of freaky, is actually a pretty attractive model for understanding how our

  • bodies interact with the environment. How we smell it with our outsized nose, taste

  • it with our ridiculous Mick Jagger lips, and touch it with our enormous Donkey Kong hands.

  • So join me, as we get to know the hideous little creature within you.

  • [Intro]

  • Last week we talked about the difference between sensation and perception: how sensation is

  • the process by which our senses and brain receive information from the outer world,

  • while perception is how we organize and interpret that information and give it meaning.

  • Like, right now, my sense of hearing is letting me detect sounds, while my brain is processing

  • and interpreting them, allowing me to identify and perceive the individual sounds and determine

  • if they're coming from the radio, or outside or right behind me.

  • Sound moves in waves that vibrate through a medium, like air. And although they function

  • differently that waves of electromagnetic radiation, including what we think of as light,

  • sound waves also can vary in shape. Short waves have a high frequency and a high pitch,

  • like a plucky violin. Long waves have a low frequency and pitch, like a mournful cello.

  • Wave height, or amplitude, determine a sound's loudness, which we typically measure in decibels.

  • And just as light waves become electrical impulses that we register with our sight,

  • so too do our ears turn vibrating air into signals that our brains can decipher.

  • While the human ear might not be as elegant as the jackrabbit's, or as wild as the long-eared

  • bat's, it's actually a pretty incredible organ. Or, organs, since we have two of them, which

  • helps give us directional stereophonic hearing. That's the 3D type of hearing we couldn't

  • experience if we had just one big, freaky ear in the middle of our foreheads.

  • Your outer ear, the part that you can see and pierce and tug on, collects sound waves

  • and funnels through the ear canal and into the middle ear, where they cause your eardrum

  • to vibrate. From there, sound vibrations are amplified by the so-called 'ossicle bones',

  • which also happen to be the most awesomely named bones in your body: the stirrup, the

  • hammer and the anvil. From here, those physical vibrations travel to the inner ear, where

  • they bump into the snail-shaped cochlea, and its surrounding fluids get jostled around,

  • causing some of your 16,000 tiny cochlear hair cells to bend. This motion triggers neighbouring

  • nerve cells that convert that physical energy into electrical impulses zipping up the auditory

  • nerve into the auditory cortex, where the brain is like 'Oh, songbirds!', or 'Elvis!',

  • or 'Vengeance!', or whatever. And you know what goes great with a little

  • bit of rockabilly, or revenge? A nice meal.

  • One of the greatest joys in life is enjoying flavors; whether you prefer casserole or caviar,

  • we all get our tasting done in the same way, starting with our taste buds. Each of our

  • thousands of taste buds contains a sort of pocket-like pore that contains fifty to a

  • hundred hair-like taste receptor cells that read food molecules and report back to the

  • brain. "That chip is salty, that lemon is sour." Now, everybody used to think that our

  • tongues just detected four distinct tastes: sweet, salty, sour and bitter. And you've

  • probably seen the version of this bogus taste map, which incorrectly assigns certain tastes

  • to certain parts of your tongue. But we now recognize a fifth flavour: the savoury, meaty,

  • MSG-y taste, for which there is no English word, it's known as 'umami'.

  • But taste is nothing without smell. Plug your nose, and a bite of cold bacon is just a mouthful

  • of salt. This is a prime example of sensory interaction; the principle that one sense

  • can influence another.

  • I'll get back to smell in just a bit, but I want to take a moment to talk about what

  • happens when those sensory interactions get messy.

  • Let us take a little questionnaire, shall we?

  • Do certain words trigger a strong, specific taste in your mouth? Like, does the word 'kitten'

  • taste like candy canes?

  • Has hearing a sound ever made you see a color? Like does Prince's voice singing Purple Rain

  • actually cause the colour purple to flash before your eyes?

  • Do you ever feel like you're being touched when you smell something?

  • Like does the smell of lilies give you the sensation of touching a cold, metal surface?

  • Most of you said no to all of those questions, but at least one of you out there answered

  • yes. And more likely than not, that person has synesthesia, a rare and fascinating neurological

  • condition where two or more senses get wrapped together.

  • This kind of sensory mix up is involuntary, and it's experienced without forethought in

  • a durable and consistent way. Like, the number seven is always going to taste like coffee,

  • and it's never going to switch to tomato juice.

  • And we're still not sure what causes this phenomenon. One idea suggests that the rogue

  • development of new neural connections may override normal boundaries that typically

  • separate the senses. Another theory suggests that all babies are born with synesthesia

  • and experience mixed sense until the brain matures and creates separate sense channels,

  • unless they don't, in which case you grow up to be a synesthete. Yet another theory

  • links the condition to wonky neurochemistry, in which the neurotransmitters associated

  • with one function turn up way over in a different part of the brain.

  • Just another example of how the mind is still extremely mysterious.

  • So, back to the comparatively boring topic of smells that only smell... unlike our wave

  • detecting senses of sight and hearing, out taste and smell are chemical senses. We differentiate

  • the smells of spring lilacs, grilled cheese, and gasoline when airborne molecules travel

  • up the nose and reach the five to ten million receptor cells at the top of each nasal cavity;

  • and yes, that means when you smell poop, there's poop particles in your nose.

  • These receptors send information to the brain's olfactory bulb, then zips it on to the primary

  • smell cortex and parts of the limbic system responsible for emotion and memory. Unlike

  • our five different taste receptors or two types of retinal receptors, we don't have

  • specifically differentiated smell receptors, rather, odour receptors come together in different

  • combinations.

  • So, just like pressing different keys on your keyboard can allow you to form tons of different

  • words, so too can these distinct combinations of activated smell receptors communicate some

  • ten thousand unique smells.

  • But, how we feel about a smell, and our perception of it, is often tangled up in our experiences

  • with that scent. If our beloved grandma baked gingerbread every time we visited, those memories

  • may make you partial to the smell of gingerbread. Even if we can't immediately name the odor,

  • our brains are amazing at storing and recognizing old scents by their associations. Which is

  • why you may suddenly feel happy when you walk into a bakery, even before you realize you're

  • thinking of your grandma.

  • The emotional power of smell partly has to with how our sense circuitry connects to the

  • brain's limbic system, right next to our emotional registry, the amygdala, and our memory keeper,

  • the hippocampus. That's why scents can be so intimately tied with our feelings and memories.

  • And how a whiff of Nag Champa can immediately transport to back to eating chips in your

  • freshman dorm room.

  • Smelling, hearing, tasting, seeing: all fantastic, but if there's one thing that popular music

  • taught us, besides a bunch of really terrible relationship advice, it's that when it comes

  • to our senses, we're all about touching. We've got songs about magic touches and golden touches,

  • invisible and human touches. Songs about touching ourselves, touching me, touching you, and

  • even what you can't touch.

  • Touch is extremely important, especially during early development. Baby monkeys that are allowed

  • to see, hear and smell, but not touch their mamas become extremely distraught. That's

  • just a mean experiment. Premature human babies gain weight faster if they're held and massaged,

  • and some studies indicated that children that didn't receive enough physical attention as

  • infants are at higher risk for emotional, behavioral, and social problems as they grow.

  • Your sense of touch is actually a combination of four distinct skin sensations: pressure,

  • warmth, cold, and pain. If you touch various spots of your skin with something soft like

  • this anglerfish, you'll feel that you sense different amounts of softness on different

  • parts of your body... just put that up there...

  • The same goes for a warm mug, or an ice cube, or a needle point. You'll sense that some

  • spots are more or less sensitive to each of the four distinct sensations. Other skin sensations,

  • like tickles, itches, and the experience of wetness, are just variations on those four

  • different sensations.

  • Ultimately, your sense of touch joins forces with sensors in your bones, joints, and tendons

  • to provide your personal kinesthesis: the way that body senses its own movement and

  • positioning. You use your kinesthetic sense whenever you walk, dance, swim, or hula hoop.

  • It's what the cops are testing drunk people for when they ask them to touch their noses

  • with their eyes closed. This sense allows you to detect changes in the position of your

  • body without relying on other senses, which is why you can still cha-cha, backstroke,

  • and hula hoop with your eyes closed and your ears plugged.

  • The partner sense to your kinesthesis is your vestibular sense, which monitors your head's

  • position and your balance. This sense of equilibrium is ruled by the pretzel-shaped, semicircular

  • canals and the fluid-filled vestibular sacs that connect those canals to the cochlea in

  • your inner ear. So, if you spin around a bunch and suddenly stop, it'll take a minute for

  • that inner ear fluid to return to normal, which is what makes you feel dizzy. That moving

  • fluid is actually fooling your brain into thinking your body is still spinning. It's

  • a good example of how even our normal functioning senses can fool us. Understanding exactly

  • how we get fooled helps us understand how our sensual perception system works, which

  • is exactly what we're going to be talking about next time.

  • For now, hopefully you realized that your homunculus is actually kinda beautiful, in

  • its own way, because you learned how your sense of hearing, taste, smell, and touch

  • work.

  • And thanks for watching, especially to all our Subbable Subscribers, who make this whole

  • channel possible. If you'd like to sponsor an episode of Crash Course: Psychology, get

  • a copy of one of our Rorschach prints, and even be animated into an upcoming episode,

  • just go to subbable.com/crashcourse.

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

  • our consultant in Dr. Ranjit Bhagwat. Our director and editor is Nicholas Jenkins. Michael

  • Aranda is our sound designer, and our graphics team is Thought Café.

Homunculus! If you've heard that word, it probably wasn't in the context of psychology.

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