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.