Sensation & Perception - Crash Course Psychology #5

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Just what is the difference between sensing and perceiving? And how does vision actually work? And what does this have to do with a Corgi? In this episode of Crash Course Psychology, Hank takes us on a journey through the brain to better explain these and other concepts. Plus, you know, CORGI!
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Table of Contents:
Sensation vs. Perception :54
Sense Thresholds 2:11
Neurology of Vision 4:23
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Let me tell you about Oliver Sacks, the famous
physician, professor and author of unusual

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.

[Intro]

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.