neurological case studies. We’ll 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, you’re 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 you’re 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 we’re 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 you’ll 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 you’re 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 you’re 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,

we’ll 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 we’ve 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 you’re colorblind. About one in

fifty people have some level of color vision deficiency. They’re 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 we’ll 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.