—
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!
—
—
Transcript Provided by YouTube:
00:00
Let me tell you about Oliver Sacks, the famous physician, professor and author of unusual
00:05
neurological case studies. We’ll be looking at some of his fascinating research in future
00:09
lessons, but for now, I just want to talk about Sacks himself. Although he possesses
00:12
a brilliant and inquisitive mind, Dr. Sacks cannot do a simple thing that your average
00:17
toddler can. He can’t recognize his own face in the mirror.
00:21
Sacks has a form of prosopagnosia, a neurological disorder that impairs a person’s ability
00:26
to perceive or recognize faces, also known as face blindness. Last week we talked about
00:31
how brain function is localized, and this is another peculiarly excellent example of
00:36
that. Sacks can recognize his coffee cup on the shelf, but he can’t pick out his oldest
00:41
friend from a crowd, because the specific sliver of his brain responsible for facial
00:45
recognition is malfunctioning. There’s nothing wrong with his vision. The sense is intact.
00:49
The problem is with his perception, at least when it comes to recognizing faces. Prosopagnosia
00:54
is a good example of how sensing and perceiving are connected, but different.
00:59
Sensation is the bottom-up process by which our senses, like vision, hearing and smell,
01:03
receive and relay outside stimuli. Perception, on the other hand, is the top-down way our
01:08
brains organize and interpret that information and put it into context. So right now at this
01:13
very moment, you’re probably receiving light from your screen through your eyes, which
01:17
will send the data of that sensation to your brain. Perception meanwhile is your brain
01:21
telling you that what you’re seeing is a diagram explaining the difference between
01:25
sensation and perception, which is pretty meta. Now your brain is interpreting that
01:29
light as a talking person, whom your brain might additionally recognize as Hank.
01:39
[Intro]
01:44
We are constantly bombarded by stimuli even though we’re only aware of what our own
01:47
senses can pick up. Like I can see and hear and feel and even smell this Corgi, but I
01:53
can’t hunt using sonar like a bat or hear a mole tunneling underground like an owl or
01:58
see ultraviolet and infrared light like a mantis shrimp. I probably can’t even smell
02:03
half of what you can smell. No! No! We have different senses. Mwah mwah mwah mwah mwah.
02:10
Yeah.
02:10
There’s a lot to sense in the world, and not everybody needs to sense all the same
02:15
stuff. So every animal has its limitations which we can talk about more precisely if
02:19
we define the Absolute Threshold of Sensation, the minimum stimulation needed to register
02:24
a particular stimulus, 50% of the time. So if I play a tiny little beep in your ear and
02:28
you tell me that you hear it fifty percent of the times that I play it, that’s your
02:31
absolute threshold of sensation. We have to use a percentage because sometimes I’ll play
02:36
the beep and you’ll hear it and sometimes you won’t even though it’s the exact same
02:39
volume. Why? Because brains are complicated.
02:43
Detecting a weak sensory signal like that beep in daily life isn’t only about the
02:46
strength of the stimulus. It’s also about your psychological state; your alertness and
02:51
expectations in the moment. This has to do with Signal Detection Theory, a model for
02:56
predicting how and when a person will detect a weak stimuli, partly based on context. Exhausted
03:01
new parents might hear their baby’s tiniest whimper, but not even register the bellow
03:06
of a passing train. Their paranoid parent brains are so trained on their baby, it gives
03:11
their senses a sort of boosted ability, but only in relation to the subject of their attention.
03:15
Conversely, if you’re experiencing constant stimulation, your senses will adjust in a
03:20
process called sensory adaptation. It is the reason that I have to check and see if my
03:24
wallet is there if it’s in my right pocket, but if I move it to my left pocket, it feels
03:27
like a big uncomfortable lump. It’s also useful to be able to talk about our ability
03:31
to detect the difference between two stimuli. I might go out at night and look up at the
03:35
sky and, well, I know with my objective science brain that no two stars have the exact same
03:40
brightness, and yeah, I can tell with my eyeballs that some stars are brighter than others,
03:45
but other stars just look exactly the same to me. I can’t tell the difference in their
03:49
brightness.
03:50
Are you done? Is it time for your to go? Gimme, gimme a kiiiissss. Yes, yes. Okay. Good girl.
03:58
The point at which one can tell the difference is the difference threshold, but it’s not
04:02
linear. Like. if a tiny star is just a tiny bit brighter than another tiny star, I can
04:06
tell. But if a big star is that same tiny amount brighter than another big star, I won’t
04:10
be able to tell the difference. This is important enough that we gave the guy who discovered
04:14
it a law. Weber’s Law says that we perceive differences on a logarithmic, not a linear
04:19
scale. It’s not the amount of change. It’s the percentage change that matters.
04:23
Alright. How about now we take a more in depth look at how one of our most powerful senses
04:28
works? Vision. Your ability to see your face in the mirror is the result of a long but
04:34
lightning quick sequence of events. Light bounces off your face and then off the mirror
04:38
and then into your eyes, which take in all that varied energy and transforms it into
04:43
neural messages that your brain processes and organizes into what you actually see,
04:48
which is your face. Or if you’re looking elsewhere, you could see a coffee cup or a
04:52
Corgi or a scary clown holding a tiny cream pie.
04:54
So how do we transform light waves into meaningful information? Well, let’s start with the
04:58
light itself. What we humans see as light is only a small fraction of the full spectrum
05:02
of electromagnetic radiation that ranges from gamma to radio waves. Now light has all kinds
05:08
of fascinating characteristics that determine how we sense it, but for the purposes of this
05:12
topic, we’ll understand light as traveling in waves. The wave’s wavelength and frequency
05:17
determines their hue, and their amplitude determines their intensity or brightness.
05:21
For instance a short wave has a high frequency. Our eyes register short wavelengths with high
05:26
frequencies as blueish colors while we see long, low frequency wavelengths as reddish
05:31
hues. The way we register the brightness of a color, the contrast between the orange of
05:34
a sherbet and the orange of a construction cone has to do with the intensity or amount
05:38
of energy in a given light wave. Which as we’ve just said is determined by its amplitude.
05:44
Greater amplitude means higher intensity, means brighter color.
05:47
Someone’s just told me that sherbet doesn’t- isn’t a word that exists. His name is Michael
05:52
Aranda and he’s a dumbhead. Did you type it into the dictionary? Type it into Google.
05:59
Ask Google about sherbet. So sherbet is a thing.
06:02
So after taking this light in through the cornea and the pupil, it hits the transparent
06:05
disc behind the pupil: the lens, which focuses the light rays into specific images, and just
06:11
as you’d expect the lens to do, it projects these images onto the retina, the inner surface
06:15
of the eyeball that contains all the receptor cells that begin sensing that visual information.
06:20
Now your retinas don’t receive a full image like a movie being projected onto a screen.
06:24
It’s more like a bunch of pixel points of light energy that millions of receptors translate
06:28
into neural impulses and zip back into the brain.
06:32
These retinal receptors are called rods and cones. Our rods detect gray scale and are
06:36
used in our peripheral vision as well as to avoid stubbing our toes in twilight conditions
06:41
when we can’t really see in color. Our cones detect fine detail and color. Concentrated
06:45
near the retina’s central focal point called the fovea, cones function only in well lit
06:50
conditions, allowing you to appreciate the intricacies of your grandma’s china pattern
06:54
or your uncle’s sleeve tattoo. And the human eye is terrific at seeing color. Our difference
07:00
threshold for colors is so exceptional that the average person can distinguish a million
07:04
different hues.
07:05
There’s a good deal of ongoing research around exactly how our color vision works.
07:09
But two theories help us explain some of what we know. One model, called the Young-Helmholtz
07:13
trichromatic theory suggests that the retina houses three specific color receptor cones
07:18
that register red, green and blue, and when stimulated together, their combined power
07:22
allows the eye to register any color. Unless, of course you’re colorblind. About one in
07:26
fifty people have some level of color vision deficiency. They’re mostly dudes because
07:30
the genetic defect is sex linked. If you can’t see the Crash Course logo pop out at you in
07:34
this figure, it’s likely that your red or green cones are missing or malfunctioning
07:39
which means you have dichromatic instead of trichromatic vision and can’t distinguish
07:43
between shades of red and green.
07:45
The other model for color vision, known as the opponent-process theory, suggests that
07:48
we see color through processes that actually work against each other. So some receptor
07:53
cells might be stimulated by red but inhibited by green, while others do the opposite, and
07:58
those combinations allow us to register colors.
08:00
But back to your eyeballs. When stimulated, the rods and cones trigger chemical changes
08:04
that spark neural signals which in turn activate the cells behind them called bipolar cells,
08:10
whose job it is to turn on the neighboring ganglion cells. The long axon tails of these
08:15
ganglions braid together to form the ropy optic nerve, which is what carries the neural
08:19
impulses from the eyeball to the brain. That visual information then slips through a chain
08:23
of progressively complex levels as it travels from optic nerve, to the thalamus, and on
08:29
to the brain’s visual cortex. The visual cortex sits at the back of the brain in the
08:32
occipital lobe, where the right cortex processes input from the left eye and vice versa. This
08:37
cortex has specialized nerve cells, called feature detectors that respond to specific
08:42
features like shapes, angles and movements. In other words different parts of your visual
08:47
cortex are responsible for identifying different aspects of things.
08:50
A person who can’t recognize human faces may have no trouble picking out their set
08:54
of keys from a pile on the counter. That’s because the brains object perception occurs
08:58
in a different place from its face perception. In the case of Dr. Sacks, his condition affects
09:02
the region of the brain called the fusiform gyrus, which activates in response to seeing
09:06
faces. Sacks’s face blindness is congenital, but it may also be acquired through disease
09:10
or injury to that same region of the brain. And some cells in a region may respond to
09:14
just one type of stimulus, like posture or movement or facial expression, while other
09:19
clusters of cells weave all that separate information together in an instant analysis
09:23
of a situation. That clown is frowning and running at me with a tiny cream pie. I’m
09:28
putting these factors together. Maybe I should get out of here.
09:30
This ability to process and analyze many separate aspects of the situation at once is called
09:35
parallel processing. In the case of visual processing, this means that the brain simultaneously
09:39
works on making sense of form, depth, motion and color and this is where we enter the whole
09:44
world of perception which gets complicated quickly, and can even get downright philosophical.
09:49
So we’ll be exploring that in depth next time but for now, if you were paying attention,
09:54
you learned the difference between sensation and perception, the different thresholds that
09:58
limit our senses, and some of the neurology and biology and psychology of human vision.
10:02
Thanks for watching this lesson with your eyeballs, and thanks to our generous co-sponsors
10:06
who made this episode possible: Alberto Costa, Alpna Agrawal PhD, Frank Zegler, Philipp Dettmer
10:14
and Kurzgesagt.
10:14
And if you’d like to sponsor an episode and get your own shout out, you can learn
10:17
about that and other perks available to our Subbable subscribers, just go to subbable.com/crashcourse.
10:23
This episode was written by Kathleen Yale, edited by Blake de Pastino, and our consultant
10:27
is Dr. Ranjit Bhagwat. Our director and editor is Nicholas Jenkins, the script supervisor
10:31
is Michael Aranda who is also our sound designer, and our graphics team is Thought Cafe.
—
This post was previously published on YouTube.
—
Photo credit: Screenshot from video.
