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Image by Annie Spratt

Your 3-layered brain

The human brain is the most complex object we know of. There are many sub-regions, collectively holding up to 100 billion neurons, all interconnected to one another. While we might get the impression that works to help us solve problems and get through life, the truth is actually the converse. Your brain isn't a single entity - it's more like a team of rivals. Each sub-region has its own view of what is important and attempts to influence other regions with its view. 

To understand this further, we introduce the Triune (3-layered) Brain Model by the neuroscientist Paul Maclean. 



The brain can be thought of as coming in 3 layers. 

  • The bottom of your brain is what we call the Reptilian layer. Why reptilian?

    Short history. Life on earth started off as single-cell organisms, which over many generations, evolved into the different species of animals that we are familiar with today. During the evolutionary process, animals started developing more and more complex brains; but the extent of brain development varied across different species. The first and most primitive version of the brain is what we call the reptilian layer, dealing with more routine functions. Even till today, reptile brains are (unsurprisingly) largely made up of this reptilian layer -   you can't, for example, train a lizard to solve a puzzle. 

    Over time, some species (e.g. mammals) developed a more advanced layer of brain faculty, to deal with more complicated thought and scenarios. Humans evolved even further  - our brains developed to be capable of everything from understanding intricate physics to travel to space, performing brain surgery, to constructing the world wide web where you are now reading this from. More to come. 

    What does the reptilian brain do? Basic, fundamental, regulatory stuff that is necessary for everyday life. For example, when your body temperature rises too high or drops too low, your reptilian brain triggers for you to sweat or shiver. It monitors your blood sugar levels, and when it drops too low, it tells you: "Eat"! It releases hormones which are necessary for everyday life.


  • The second, middle layer of our brain is what we call the limbic system, sometimes referred to as the mammalian part of the brain. What does the limbic system do?

    It causes us to feel emotions- anger, fear, arousal, anxiety, desires.

    Why "mammalian"? This one is easy -  anyone who has a pet dog or cat or hamster or rabbit would know this. Mammals are capable of understanding and expressing more complex emotions, which influences their behaviour. Your dog, for example, can tell if you're pissed-off if you're scolding him/her, if you're happy and pleased with him/her, and act accordingly. If you think about it, this is pretty impressive - you and your dog have never exchanged a single word in each other's language, and yet your dog understands you (somewhat / when it chooses/ when you bribe him with food). 


  • The third and topmost part of the brain, really more of a wrinkled sheet just behind your eyes and forehead, is the cortex. The cortex is the most recently evolved part of your brain. Most animals do have some amount of cortex. But again, we see that mammals tend to have more developed cortices. And only in a small number of animals - ape, whales, dolphins, do we see a really significant and advanced cortex. The most advanced cortex of course... lies in your head as you read this. Your cortex is simply astounding, as you will see more examples of in subsequent chapters. What does the cortex do? Analytical deduction. Information processing. Strategy devising. You get the idea.

Besides how the individual layers works -  what's even more interesting is how each layer affects the other. 

Because it seems that the reptilian brain is the most basic and the cortex is the most advanced, a common mistake is believing that there is some form of top-down hierarchy - cortex moderates the limbic system, which in turn causes moderates the reptilian brain. 

Sure this does happen. You feel an emotional threat - you watch a horror movie and it scares you. 
Layer 2 can cause layer 1 to activate. And your heart beats a bit faster, your blood pressure goes up slightly, and your palms get a bit clammy. The emotion causes physical changes to your body. 

Layer 3 can of course also influence layer 2. You might be reading a book or watching a movie. By itself, a book is simply a jumble of words. But reading it and processing these words, you might feel terribly sad or tremendously amused. Ditto a movie. I had the misfortune of having to accompany some friends to watch the Avengers. It was so terribly boring I fell asleep. Until I was rudely awoken by some fella sobbing because apparently, Spiderman had died. Hey! Spiderman isn't real! But a loss of sleep is, and I felt I should have been the one upset. But in any case, you get the point. Your cortex interpreted something that was cognitively abstract to make it mean something to you emotionally.

Your cortex can also influence your reptilian brain directly.. One day someone you love right now will die and you will have so many things you want to say to them but you will never be able to do so. Your heart starts beating faster. Or you could think about one of the 30 thousand people in another part of the world who die every day from starvation or a lack of clean water. A pure cognitive thought causes a response from your reptilian brain.

This flow of influence, from top to bottom, is obvious. 

But what many don't realise is that the reverse order of influence also happens.

What's an example? When we are hungry (reptilian brain), we make harsher judgements on people. One of the most remarkable case studies - the largest determinant of whether prisoners get parole or not is dependent on the blood sugar levels of the judge. 

When we are tired, we are less charitable and less empathetic (just think of yourself at the end of a long and troubling day, after not having any sleep the night before). Your ancient reptilian brain has no problems yelling at your super-evolved super-smart cortex, telling it how it should be thinking.

Similarly, our mammalian brain can greatly impact our thinking and assessment. You probably have a daily example of this. When you are sexually aroused you are more generous and you don't see flaws. When you are excited, you make impulsive decisions. And when you are angry, you reject criticism and protect yourself. 

The really important thing to note is that we often do not notice these mutual influencing. Because we are only aware of what is conscious to us, we don't notice that our thinking has been influenced by other parts of the brain subconsciously. That in 2 different emotional or physical states, the same person might make very different decisions.

Nor does the influence of one layer on another stop after just one cycle. Let's say you are quarrelling with someone. You are emotional. And then suddenly you remember that this person did something to you some time ago - a cognitive thought. This memory in your cortex causes your heart to beat even faster, you can feel your blood boiling... you are really mad now. Your blood pressure is rising, causing you to be even more emotional. Your voice gets a bit louder, your body language a little more violent, you judge each other harsher, and nothing that is being said is going through. Much of this happens, as I must stress repeatedly, unconsciously. 

This highlights why it's very difficult to pinpoint one specific reason why you think or behave the way you do. 


But it is precisely because our behaviour is so complex, shaped by so many different factors, that our brain is the most complex mechanism we know of in the universe, and that human life is quite so interesting. 

Of course, in reality, this is a very simplified version of how your brain actually works ( a little "primer" - even your super-rational prefrontal cortex has an emotional side). But it is nonetheless a neat model for us to broadly understand yourself. You can find out more about each section of the brain in the links below:


  • Every thought and every action starts and ends with them - Neurons

  • Your rational brain - the Prefrontal Cortex

  • Anger, aggression, anxiety and fear - the Amygdala

  • I find this so disgusting! - your Insula is triggered

  • I feel your pain - your centre of empathy, the Anterior Cingulate

  • I recognise you! - facial recognition, your Fusiform Cortex

  • "If I could only be more rational" - are emotions always bad for decision making?


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