No, You Don’t Have a “Reptilian Brain”

[ ♪ Intro ] You might have come across articles like How
to Make Friends Using Your Reptilian Brain or Overcome Your Reptile Brain to Lose Weight. And if they seemed to make a lot of sense… Well, I’m sorry to be a spoilsport, but the idea that humans have a primitive reptile brain deep down there somewhere is largely
false. It is based on a tiny bit of truth, though,
which might explain why it’s stuck around for half a century. This ‘reptile brain’ nonsense started
in the sixties when neuroscientist Paul MacLean proposed the Triune Brain Theory. He was trying to explain how animal brains
evolved and why some animals, like us, could reason or plan but others couldn’t. MacLean thought that these more complex abilities
came from extra brain layers added on top of a primitive, instinctive brain base, like
a juicy dumpling is wrapped inside a pastry. The brain’s core was called the protoreptilian
complex. Hence, “reptile brain”. And it’s the name MacLean gave to what we
now call the basal ganglia, a group of neurons at the very centre of the brain. As the name suggests, this region supposedly
explained the behavior of reptiles. MacLean thought it was the neural seat of
motor control as well as instinctual behaviors like defending territory. Wrapped around the reptile brain was the paleomammalian
complex or limbic system, which was largely in charge of emotions. And because emotions are important for things
like social relationships, parental care, bonding and empathy, MacLean thought this
system was especially important to early mammals. The final layer to be added on was what MacLean
describes as the neomammalian complex, a.k.a. the neocortex, which is the 6-layered tissue
that makes up the majority of the folded, “brainy”-looking part of our brains called
the cerebral cortex. This, which is much bigger in “higher”
mammals like us and our primate relatives, is what he thought gives us language, reason
and conscious perception of the outside world. And according to this theory, each of these
layers acts as a separate quote “brain”. So when you get angry and lash out at a potential
rival, that’s your reptile brain taking over, but when you ponder the great wonders
of the world, your neomammalian brain is in charge. Which is why some pop-psychology articles
talk about reigning in your reptilian brain, though that’s not really what MacLean was
going for. In some ways, the Triune Brain Theory is kind
of true, but it’s also pretty wrong. MacLean was kind of right that different parts
of the brain have somewhat different tasks, but his groupings weren’t perfect. Those quote “reptilian” basal ganglia
do help us form habits, and they play a big role in controlling voluntary movements. But they’re also involved in emotions and
executive functions like self control. For example one part, the nucleus accumbens,
is a big player in the brain’s reward circuit. As for the limbic system, that paleomammalian
part, it is heavily involved in emotion and bonding, thanks largely to a structure called
the amygdala. But the term “limbic system” is kind of
falling out of favor because neuroscientists can’t really agree what’s actually in
it. Some neuroscientists consider the orbitofrontal
cortex, the part of the brain that helps make decisions, to be in the limbic system, too,
because it’s tightly connected to other parts of it. Or they use a more encompassing term, the
paralimbic system, instead. Others just stick to functional networks within
the entire cerebral cortex, which includes both the limbic system and that neomammalian
neocortex. Speaking of which, MacLean was right that
the neocortex is a big part of our reasoning, speech and cognitive abilities, but as the
majority of the cerebral cortex, it’s kind of got its fingers in all the neural pies. And it’s not required for intelligence. Octopus can perform some really tricky cognitive
tasks like remembering how to get out of a maze without a shred of neocortex. Also, MacLean’s names for these so-called
brains aren’t great either because they don’t really have anything to do with the
animals they’re named after. That ‘reptile brain’ didn’t first appear
in reptiles, and the limbic system isn’t unique to mammals. In fact, most animals with backbones, including
fish and amphibians, have basal ganglia and a limbic system. And “lower” mammals like mice do have
a neocortex. There’s even evidence that other animals
have a neocortex of sorts. Their brains contain the same types of cells
as the ones in our neocortex, they just aren’t arranged in 6 layers. That might explain how birds, which are actually
reptiles, care for their young or have linguistic abilities. What differs between brains isn’t who has
what so much as their relative size and shape. And it’s not even fair to say that these
regions are most developed in those groups of animals. While it’s true that our neocortex is a
lot bigger than a mouse’s, even accounting for body size, some of the structures in primate
basal ganglia and limbic systems are also larger and more complex. So they aren’t just hold overs from our
more primitive ancestors. But perhaps the biggest problem with the Triune
Brain Theory is the idea that these different “brains” work independently, or even linearly. We don’t have three separate brains, we
have one big coordinated one. Brain activity underlying a behavior may start
in one specific area, but it’ll soon spread as other parts help out, an idea known as
distributive processing. And that’s how that neocortex has its fingers
in all the pies. Let’s say some really attractive person
starts hitting on your partner, and that makes you livid. Those feelings of rage might start with activity
in the amygdala, a part of that “limbic system”. But then other parts of the cerebral cortex
including neocortical areas can either increase or decrease the probability that you’ll
flip out based on cues from the environment. And the basal ganglia, particularly the nucleus
accumbens, play a key role in deciding what you actually do with all your anger by integrating
information from both the neocortex and the limbic system. So your jealous rage isn’t your territorial,
protoreptilian complex taking control, it’s multiple parts of your brain working in concert. To cut MacLean some slack though, his theory
reflected the knowledge neuroscientists had at the time. Like many scientific theories, it evolved
and became outdated as scientists got new information through neuroimaging and behavior studies. So, although MacLean was on the right track
in terms of understanding how brain structure relates to function, he got a lot wrong when
it comes to how those structures work together and the evolution of animal brains. And ultimately, that means you don’t need
to worry about trying to control your reptilian brain because it doesn’t exist! Thanks for watching this episode of SciShow! And if you like learning about brains and
how they work, you might want to check out our sister channel SciShow Psych. And while reigning in your inner reptile might
not be the way to keep your emotions in check, you can learn more about how to actually do
that in our Psych episode on controlling emotions. [ ♪ Outro ]

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