Surprising Facts About How Your Brain Works

Most of what we pick up about the brain arrives as a headline, and headlines rarely come with the evidence. That is a pity, because the real findings are usually stranger than the rumours, and the stories of how they were found are often better still.

Here are a few things worth knowing about the organ reading this sentence. Each comes with the experiment, the patient or the observation behind it, so you can judge it for yourself.

It runs on about 20 watts

Start with the basics. The adult human brain weighs around 1.3 to 1.4 kilograms.

Lift a bag of sugar and you are holding about the weight of the thing doing the lifting.

That is roughly 2 per cent of body mass. Yet the brain burns about 20 per cent of the energy the body uses at rest. In electrical terms it runs on some 20 watts, less than a dim light bulb. Around three quarters of it is water, and most of what remains is fat, much of it wrapped around nerve fibres as insulation.

It also has no reserve tank. Cut its blood supply and consciousness fades within about ten seconds. The brain is expensive to run and it cannot be paused.

One more oddity belongs here. The brain has no pain receptors of its own. This is why the surgeon Wilder Penfield, working in Montreal, could stimulate the exposed brains of patients who were awake. They did not report pain. They reported tingles, music and sudden scraps of memory. Headaches are real, but the brain tissue is not what hurts.

Pain itself is odd, as it happens. A stubbed toe hurts twice because two kinds of nerve fibre carry the news. Thin insulated fibres deliver a sharp first pain at around 15 metres a second. Slower, uninsulated fibres deliver the dull, spreading ache a second or so later. The second wave is still on its way when the first one reaches you.

Your gut has a nervous system of its own

The gut has more neurons than the spinal cord.

Some 500 million of them form the enteric nervous system, a mesh woven through the wall of the oesophagus, stomach and intestines. It can run digestion on its own, even when its links to the brain are cut. In 1998 the American anatomist Michael Gershon gave it a memorable name: the second brain.

It cannot think. It does talk, and the two-way traffic between gut and head is now called the gut–brain axis.

The first good evidence came from an unlucky accident. In the 1820s an army surgeon, William Beaumont, treated a fur trader named Alexis St Martin whose wound healed leaving a hole into his stomach. Beaumont could watch digestion as it happened. He noticed that when St Martin was angry or afraid, what he saw in the stomach changed.

So the phrase "gut feeling" has some basis in anatomy. Your mood reaches your stomach through actual nerves, and signals travel back up the same way.

You see a prediction, not a picture

It feels as though your eyes work like a camera and your brain watches the film. The evidence points somewhere else.

The information your senses deliver is noisy, ambiguous and slightly late. Yet the world appears clear and immediate. The reason, many neuroscientists now argue, is that the brain does not wait for the evidence. It predicts.

The idea is old. Hermann von Helmholtz suggested in 1867 that perception is unconscious inference: a conclusion drawn from incomplete data using past experience. A modern version, called predictive coding, was set out by Rajesh Rao and Dana Ballard in 1999. In this account, higher areas of the brain send predictions down to lower ones, and what travels back up is mainly the mismatch between the guess and the incoming signal.

You can catch this happening. In the flash-lag illusion, a light is flashed exactly level with a moving dot, yet the flash seems to trail behind it. One possible explanation is that the brain projects moving objects forward a little, to make up for its own processing delays.

Note the careful wording: "many neuroscientists now argue" and "possibly". This is a strong and useful theory, not a settled fact. But it does explain why illusions work, why you can read messy handwriting and why you sometimes hear your name in a noisy room when nobody said it.

Memory is rebuilt every time you use it

Memory feels like a recording you replay. It is closer to a story you retell.

In 1932 the Cambridge psychologist Frederic Bartlett asked English students to read a Native American folk tale, "The War of the Ghosts", and to recall it later. With each retelling the unfamiliar details dropped out or turned into familiar ones. Canoes became boats. The supernatural twist was explained away. Bartlett concluded that remembering is reconstruction, with the gaps filled in by expectation.

A vivid example comes from the psychologist Jean Piaget. He clearly remembered his nurse fighting off a kidnapper when he was a baby in Paris. Years later the nurse confessed she had invented the whole story. He had heard it so often that his mind supplied the pictures.

We also know where new memories are made, thanks to one man. On 1 September 1953, at Hartford Hospital in Connecticut, a surgeon tried to cure a 27-year-old's severe epilepsy by removing the inner part of both temporal lobes. The seizures eased. But Henry Molaison, known to science for 55 years only as H.M., could no longer form new memories. He would read the same magazine with fresh interest and greet researchers who had worked with him for decades as polite strangers. His loss showed that making memories depends on a particular part of the brain, the hippocampus and the tissue around it.

Forgetting follows a curve, and sleep works against it

Forgetting is not random. Between 1879 and 1885 the German psychologist Hermann Ebbinghaus memorised lists of meaningless syllables, such as DAX, BOK and YAT, and tested how quickly he lost them. He was his own only subject. The result was psychology's first great graph, the forgetting curve. It also turned out that reviews spaced over time flatten the curve.

Sleep helps too, and not merely by keeping you out of trouble. In 1924 John Jenkins and Karl Dallenbach at Cornell asked two students to learn nonsense syllables, then tested them after hours spent either asleep or awake. They remembered markedly more after sleep. The researchers thought sleep simply sheltered memories from interference. A century of work since points to something more active: during sleep the brain replays the day and moves what matters into long-term storage.

In rats, this replay has been recorded directly, and it runs in fast-forward. A route that took the animal seconds to run can be replayed by its hippocampus within a fraction of a second.

Much of what we believe about the brain is wrong

Few organs attract as much nonsense. In 2012 a team of researchers surveyed teachers in Britain and the Netherlands and found that, on average, they believed about half of the common brain myths put to them. Teachers who knew more about the brain in general were no less likely to believe them.

The best known example is learning styles. More than nine in ten British teachers in that survey agreed that pupils learn better when taught in their preferred style. A scientific review in 2008 had found no good evidence for the idea.

None of this is a criticism of teachers. It shows how well a tidy story travels, and how easily a claim that sounds scientific gets accepted without anyone checking it.

So, a short list to keep:

1. Your brain is about 2 per cent of your weight and uses about 20 per cent of your resting energy.
2. Brain tissue cannot feel pain.
3. Your gut has some 500 million neurons and talks to your head.
4. Perception is probably a running prediction, corrected by the senses.
5. Every memory is rebuilt when you recall it, and can be wrong while feeling certain.
6. Spaced review and a night's sleep both help memories last.
7. When a brain fact sounds neat, ask how anyone knows.

These come from The Brain: How three pounds of tissue make a mind, No. 8 in The Visual Encyclopaedia. It covers the brain in one hundred chapters, from the neuron to sleep, illness and AI, and each chapter explains the mechanism, says how we know and has a detailed labelled plate. It is £9 on Gumroad with a 30-day refund: matsiems.gumroad.com/l/the-brain. You can also read it free online at matsiems.com/books/the-brain.