7 (and a half) myths about your brain

In the 19^th Century, serious physicists believed that the Universe was filled with an imaginary substance called luminiferous ether, and doctors believed that illnesses were caused by smelly vapours called miasmas. Both of these scientific myths survived for over one hundred years until, eventually, they were vanquished by evidence.

The field of neuroscience likewise has a stable full of myths about the brain that have slowly been eroded by accumulating data. Some survive today, mainly in the media and some popular science books and articles. Neuroscientist David Linden colourfully refers to them as “neurobullsh*t.” They are maintained not by evidence, but by repetition and belief. Here are a few favourites.

Myth #1: You have a lizard in your head

Have you ever heard that your smouldering passions lie deep in ancient parts of your brain, which you supposedly inherited from prehistoric reptiles? Or that your “rational brain”, which sits atop your “lizard brain”, tries to cage your desires to keep them in check? This intuitive story of your inner reptile, safely wrapped in a cloak of rationality, seemingly explains what it means to be a moral, healthy person. It is also one of the most successful errors in all of science. To quote the neuroscientist Barbara Finlay, “Your brain is not a lizard in drag.”

The idea that your mind is a battleground between passion and reason goes all the way back to Ancient Greece. It became a popular lens for charting brain evolution in the mid-20^th Century, as scientists tried to understand brain function by comparing human brains to other animal brains by eye. More recent neuroscience clearly shows, however, that brains don’t evolve in layers like adding icing to an already-baked cake. Instead, the brains of all mammals, and possibly all vertebrates, follow a single manufacturing plan. The only animal with a lizard brain is a lizard.

Myth #2: The left side of your brain is logical and the right side is creative

In general, no part of your brain is exclusively dedicated to artistic endeavours, mathematical reasoning, or any other psychological function. Pretty much every action you take and every experience you have is computed by neurons distributed across your whole brain.

One part of your brain — the cerebral cortex — indeed consists of two halves or hemispheres, but both are intricately connected to many subcortical bits that make up the rest of your brain. So it’s simply not the case that some neurons in the left hemisphere create a computer engineer and some on the right create a poet. A few functions seem to take place mostly in one hemisphere, such as language ability on the left, but this lateralisation develops gradually and in most, but not every, individual.

Read more about the brain:

• The mind-blowing science behind how our brains shape reality
• How your brain creates pain – and what we can do about it

Myth #3: Cortisol is a stress hormone, and serotonin is a happiness hormone

It’s a common belief that your brain screams “I’m stressed” by having cortisol gush through your arteries, and neurons shower serotonin on each other to create a joyful, happy feeling. In reality, no hormone has just one specific psychological purpose (that we know of), and all the chemicals that help to create your mind work in concert.

Cortisol, for example, boosts the amount of glucose in your bloodstream to provide a quick burst of energy for your cells when your brain predicts the need, whether you feel stressed or not. Your brain tells your adrenal glands to let loose some cortisol right before you exercise or awaken in the morning to drag yourself out of bed. Cortisol may be released during stress but it is not a “stress hormone.”

Likewise, serotonin is not a “happiness hormone.” It has many functions. In your body, for example, serotonin regulates how much fat is made. In your brain, serotonin helps keep track of the energy you spend and gain. It allows you to spend energy even if there’s no immediate reward for doing so, which enables you to explore, forage, and be curious. Serotonin also helps other neurons pass information back and forth as they create your thoughts, feelings, perceptions, and actions.

Myth #4: Your eyes see, your ears hear, and your skin feels

Think about the last time you washed your face. Your skin felt the soothing, warm water. Or did it? Your skin actually has no sensors for wetness. So what’s happening here? Your brain is secretly combining several sources of information, including touch, temperature, and your knowledge from past experience, to construct a feeling of being wet.

All of your sensations, in fact, are computed in your brain, not simply detected in the world by your sense organs. You don’t see with your eyes — you see with your brain, based on a combination of what’s in your head and the sense data coming from your retinas.

Likewise, you hear with your brain as it constructs sounds based only in part on the sense data from your ears. Your experiences of smell, taste, and touch are similarly constructions. So is the feeling of your heart beating in your chest when you run up the stairs and your lungs expanding as you take a deep breath.

Myth #5: Your brain reacts to events in the world

As you go through your day, it may seem like your brain is constantly reacting to events around you. You see a cute puppy and you smile. A friend makes an embarrassing remark and you blush. You’re pricked by a vaccine needle and you feel a twinge of pain. But under the hood, your brain’s neurons do not sit idle until the world turns them on, like some cartoonish chain reaction.

Instead, your brain is constantly guessing what might happen in the next moment, and comparing its guesses to the sense data that it receives from the outside world and inside your body. These guesses are the seeds that give rise to your actions and your experiences.

In fact, your brain begins to conjure your actions and experience before receiving sense data from your eyes, ears, nose and so on. Your brain is not reacting to the world — it is forever predicting, like a fortune teller, imagining what your world will be like, how you will act, and who you will be. The information streaming in from your senses can confirm those predictions; or it can adjust them, a process you might know as “learning.” You can’t feel this predictive drama happening. It’s so quick and effortless that you feel like you’re reacting.

Myth #6: Mirror neurons are special cells that create empathy

Several decades ago, some scientists observed neurons that seemed to have a particular kind of symmetry. They increase their activity when you take a particular action, such as waving your hand, and also when you watch others performing a similar action. These neurons were dubbed “mirror neurons” for this seemingly unique behaviour. But in reality, they are just everyday neurons engaged in ordinary, miraculous prediction.

In every moment, your brain’s predictions begin as silent commands to move parts of your body, like adjusting your heart rate, contracting your intestines, gushing some hormones, or raising your arm. Copies of these commands are sent to your sensory systems to become predictions of what you’ll see, hear, and feel if you move.

These commands are sometimes executed and sometimes not, but they turn out to be a critical part of your ability to perceive anything at all, including the actions of other people. So, the same neurons that help you wave hello to a friend enable you to see someone else wiggle their fingers in their air and to understand it as a greeting. It’s not “mirroring,” it’s a normal part of your brain’s predictive process.

Myth #7: Your brain stores memories

A brain doesn’t store memories like a computer stores files, to be retrieved whole when needed. Your brain reconstructs your memories on demand with electricity and swirling chemicals. We call this process “remembering” but it’s really more like “assembling.” And each time a memory is assembled, it might be built with some different neurons. It’s also influenced by your current situation, so each occurrence may differ in its details.

This is one reason why eyewitness testimony in legal trials can be unreliable. Memories are highly vulnerable to reshaping. In one study of convictions that were later overturned by DNA evidence, 70 per cent of the accused were convicted based on eyewitness testimony.

Read more about the memory:

• A complete beginner’s guide to human memory
• What causes déjà vu? The quirky neuroscience behind the memory illusion

Myth #7½: You can’t grow new brain cells

This myth is partly true (so it’s just half a myth). Most areas of the human brain cannot grow new brain cells, but some parts can. One such part is the hippocampus, which is important for learning, remembering, regulating how much you eat, and other biological functions.

Interestingly, many other animals can regrow neurons throughout much of their brains. Why can’t we? Some scientists wonder if it’s a price we pay for living long lives. A long life requires a dependable memory. Your brain needs a way to reassemble past experiences not just from days or weeks ago, but across the span of years. New neurons, like the ones that sprout in your hippocampus, may be for learning new things and making new memories, rather than remembering (reassembling) the past. In a sense, new neurons enable your brain to cultivate your past as a way of charting your future.