So!
You want to study brain activity?
In order to get accurate and precise data, we’ll need a piece of technology like functional
magnetic resonance imaging, or fMRI. fMRIs are extremely common in modern neuroscience
studies, and with good reason.
This tech can give us information about what kind of activity is happening in different
parts of the brain in response to different tasks or just at rest.
If you’ve heard that a region of the brain “lit up” or was “activated” in response
to being shown an image or hearing a sound, that news probably came from an fMRI study.
Thousands and thousands of experiments have used fMRI, but the journalism surrounding
the actual results of these studies can get, well, sensational.
Media coverage about a specific study from 2008 claimed that scientists proved we can
smell fear.
Not only that, but fear is contagious.
Let’s dig into that, shall we?
The actual study collected sweat samples from volunteers as they jumped out of an airplane.
They also collected saliva before and after the jump to try and detect the stress hormone,
cortisol.
Then on a separate day, they had participants run on a treadmill and collected sweat and
saliva again.
The idea was that skydiving invoked a fear based stress response while the treadmill
invoked a non-fear based stress response which acted as a control.
There were multiple components to the study, but one involved placing separate participants
in an fMRI, exposing them to a vaporized solution which included either sweat collected from
the skydiving conditions, sweat from exercise, or just air and scanning their brains.
When the participants were exposed to the skydiver sweat, the researchers saw increased
activation of their amygdalas, the so called fear center of the brain.
So if you were a journalist reporting on this study, you could reasonably make the connection
that people could smell something in that sweat sample that indicated fear, right?
That’s a heck of a stretch.
Regions associated with vision, goal-directed behavior, and motor control also lit up, not
just the amygdala.
fMRIs work by showing us where blood is flowing in the brain, but they can’t tell you what
someone is thinking.
A more accurate headline would be that a study suggested that humans can signal emotional
stress.
“Fear is contagious” is a bit sensational.
So today, we’re going to learn the regions of the brain, what happens in each one, and
how to correctly interpret a headline that makes a claim about your brain.
As we learned in the last video, the brain is one of the key pieces in our central nervous
system, along with the spinal cord.
It has to interpret and process information it receives from the outside world, and then
come up with responses for it.
When we look at the brain from the side, we can see three big structures.
The first of which is the cerebrum, this enormous round part.
We’re going to go in depth on the different pieces of the cerebrum in a moment, but for
now, you can think of this as the big brain.
And overall, that isn’t a terrible way to remember this structure.
Because on the back side is a structure called the cerebellum, which literally translates
to the little brain.
This is where your body takes in certain sensory information and regulates movements like balance
and coordination, although more recent research shows that the cerebellum might process emotions
and social behavior too.
All in all, about half of your brain’s neurons live in this part of the brain.
Below the cerebrum and cerebellum is the brain stem.
I personally used to think of the brainstem as just an interface for the spinal cord and
brain, but it’s so much more than that.
Overall, it can regulate heart rate and breathing, as well as sleeping.
It also connects most of the cranial nerves, which are involved in everything from facial
sensation to swallowing.
But most of the time when people are interested in which region of the brain does what, they’re
looking at the big brain, the cerebrum.
Alright, check this thing out, this is the standard view of your cerebrum.
Right now, we’re looking at the outermost layer called the cerebral cortex, but if we
were to slice it in half, we’d see deeper structures called subcortical structures,
literally meaning underneath the cortex.
Among all those subcortical structures are big players like the limbic system which helps
you express emotions and the pituitary gland which pumps out a bunch of different hormones.
It also includes a structure that connects the two sides of the brain called the corpus
callosum, a thick band of nerve fibers that lets the two sides of the brain communicate
with each other.
Each side of the cerebrum is called a hemisphere, the good old left brain and right brain.
Now, you might’ve heard that the left brain is your analytical and logic oriented side
while your right side is the creative side, and that you can be a right vs left brained
person.
Sorry, but that’s not actually a thing.
There’s some evidence that each half deals with language differently, but past that,
we’re talking about minor differences at most.
Importantly though, we can say definitively that the left half of the brain interprets
signals from the right half of the body and vice versa.
So the left hand is controlled by the right side of the brain — that kind of thing.
Knowing that, we can finally look at what the different parts of the cerebral cortex
do.
First thing, look at all those different dips and ridges, also known as sulci and gyri respectively.
By having all those folds, you increase the surface area available and thus, shove more
brain into your brain.
Those squiggly lines might seem like random bumps, but they help us divide the cerebral
cortex further into different functional centers, or lobes.
The biggest one is the frontal lobe, which is, as you guess, in the front part of our
brain.
This is where we find a bunch of the structures that make us uniquely human, most notably
our enormous prefrontal cortexes which handle higher order functioning and cognition.
Other animals have prefrontal cortexes, but we’re the freaks with massive ones.
The frontal lobe also houses Broca’s area, one of our language processing centers, and
another big deal center of the brain, the primary motor cortex.
The primary motor cortex is a long region that extends over both halves of your brain
like over-ear headphones.
And each moving body part is represented with a little strip of this cortex — parts like
your ankles or toes getting very little space, but pieces with complex motions like your
individual fingers get a lot of space.
Behind the frontal lobe is the parietal lobe, which processes information coming in from
the body’s senses.
It has another cortex called the somatosensory cortex which is split up to represent different
body parts, so the area that represents the face is next to the area that represents the
eyes, and eyelids, and so on.
We see another cool phenomenon in this cortex — our fingertips, tongue, lips which all
have lots of nerve endings get a huge amount of space dedicated to processing their sensory
input.
Below the parietal lobe is the temporal lobe, which literally means “near the temples”.
This is where we’ll find the main area of the brain that processes hearing, called the
auditory cortex.
And that makes enough sense, the ears are like, right there.
The temporal lobe also has a special area called Wernicke’s area that helps it interpret
speech.
Well, I should say “Vern-ick-ee’s” area since it’s German.
Now, harkening back to the days before fMRI studies, experiments made it seem like we
had two speech centers: Broca’s area for speech production and Wernicke’s
area for speech comprehension.
In reality, language is handled in multiple networks around the brain.
Behind the parietal lobe is our final lobe, the occipital lobe, the area where we process
most of our vision.
I know it seems weird that a lobe in the back of your head would interpret signals from
the front of your head, but it be like that sometimes.
Now, here’s where I want to introduce some asterisks to the conversation.
The primary visual cortex, the main spot where we process vision is in the occipital lobe.
But, if we follow an image from the moment it hits our eyes until it’s processed, we
see that it’s not that straightforward.
After light passes through our eyes, it hits special photoreceptor cells in the back of
our eyes called rod cells and cone cells.
Each of those cells contains light sensitive pigment that kicks off a chemical reaction
that converts light into a nervous signal.
Even before your eyes have decoded those photons — whether its a notification on your phone,
or the words in your text message, or your Timotheé Chalamet wallpaper, that image is
processed in part by the eye itself.
From there, different aspects of vision get processed on different pathways.
One of them carries information about shape, motion, and brightness while another carries
information about color and detail.
Then some information goes towards the primary visual cortex while some crosses the optic
chiasma, a little bridge between the optic nerves that connects the left and right pathways.
Then, we have pathways in the brain that tie that visual information with the coinciding
audio information, or smell, or touch.
After all is said and done, after the visual cortex processes the image, it still relays
that information elsewhere.
I’m going into so much detail because I find it so fascinating that all this prep
work has to be done to process one of the main ways we interpret the world, our sight.
It’s a great reminder that the brain is the most complicated piece of anatomy that
exists.
Yes, that skydiving sweat fMRI experiment I mentioned at the beginning showed increased
activity in the amygdala.
But be careful.
When you’re listening to the results of an fMRI study, whether it’s on the news
or if you go the extra mile and find the primary source, consider exactly what part of the
brain is being reported on.
Be sure to differentiate not just the lobe, but individual parts, because as you can see,
it’s really hard to isolate one specific job to a whole lobe of the brain.
Earlier we mentioned Broca’s area, an area named after French surgeon Pierre Broca after
he noticed that two men lost their ability to speak after both of the patients suffered
injuries to the sides of their heads.
To him, that seemed like pretty good evidence that that part of the brain handled speech,
and while it was more complicated than that, we call that area on the brain Broca’s area
in his honor.
Thanks for watching this episode of Seeker Human, I’m Patrick Kelly