The Human Brain

What is the primary function of the human brain?

The textbook answer is that the brain processes information. That is true and almost useless, because it describes a calculator equally well. The more accurate answer has two parts.

First, the brain keeps the body alive. Breathing, heart rate, temperature, hunger, thirst, the sleep and wake cycle. These are handled by the brainstem and hypothalamus, they never pause, and they account for a large share of the organ's running costs. Nothing else the brain does matters if this fails.

Second, the brain predicts. This is the part that changed how neuroscientists talk about the organ over the last two decades. Rather than sitting passively and reacting to what the senses report, the brain constantly generates expectations about what it is about to sense, and then compares those expectations against the incoming signal. What travels up from your eyes and ears is mostly the error, the part the prediction got wrong. It is a far cheaper way to run a body, and it explains why you can be genuinely startled by a stair that is not there.

Everything else, language, planning, empathy, doing your taxes, is built on those two jobs. Thinking is not a separate department. It is prediction running far enough ahead of the present that it stops looking like perception.

What are the main regions of the human brain?

The brain divides into two hemispheres, and each hemisphere divides into four lobes. Add the cerebellum tucked underneath at the back and the brainstem descending into the spinal cord, and you have the six regions visible from the outside.

• Frontal lobe. Planning, decision-making, self-control, voluntary movement, and speech production. The largest lobe and the last to mature.
• Parietal lobe. Touch, temperature, pain, where your limbs are in space, and rough number sense.
• Temporal lobe. Hearing, understanding language, recognising faces, and forming memories. The hippocampus is buried inside it.
• Occipital lobe. Vision, almost exclusively. The smallest lobe, sitting as far from your eyes as it is possible to get.
• Cerebellum. Balance, coordination and timing. About a tenth of the volume, roughly four fifths of the neurons.
• Brainstem. Breathing, heartbeat, swallowing, the switch between sleep and waking. The part that cannot be spared.

Underneath the surface sit the structures people quote most often without being able to place: the hippocampus, which turns experience into lasting memory; the amygdala, which flags what matters before you have consciously noticed it; the thalamus, which relays nearly every sense except smell; the hypothalamus, four grams of tissue running hunger, thirst, temperature and hormones; and the corpus callosum, the 200-million-fibre bridge between the halves. The map at the top of this page has all eleven.

One warning about lobe diagrams. The borders are useful conventions, not real seams. No single lobe performs a whole function on its own, and the interesting work happens in networks that cut straight across the lines.

How many brain cells does a human have?

About 170 billion cells, of which roughly 86 billion are neurons and roughly 85 billion are glial cells.

The 86 billion figure is the one that travels, and it is usually quoted as the number of brain cells, which is wrong by about half. It counts neurons only. Glia are the other half of the organ, and they are not packing material: they insulate axons, clear waste, regulate blood flow and shape signalling directly.

The older number you may remember, 100 billion neurons, was never measured. It circulated for decades as an estimate nobody had checked. In 2009 a team led by Suzana Herculano-Houzel dissolved whole human brains into a uniform suspension and counted the cell nuclei in samples of the resulting soup, a method that does not care how unevenly cells are distributed. The answer came back at 86.1 billion neurons and 84.6 billion non-neuronal cells.

The distribution is the genuinely surprising part. The cerebellum holds about 69 billion of those neurons. The cerebral cortex, the folded outer sheet that does the reasoning and the language and the planning, holds only about 16 billion. Four out of every five of your neurons are working on movement and timing.

How does the human brain process information?

Not the way a computer does, and the difference matters more than the similarities.

A signal arrives as a change in voltage travelling down a neuron's axon, at anything from half a metre per second to 120 metres per second depending on whether that fibre is insulated in myelin. At the end it stops, because neurons do not touch. The message crosses a gap of 20 to 40 nanometres as a puff of chemical, lands on the next cell, and either nudges it closer to firing or further away. A single neuron may be listening to thousands of others at once and fires only when the sum crosses a threshold.

What makes this a brain rather than a very slow circuit board is that the connections change. Every time two neurons fire together, the link between them strengthens slightly. That is the physical basis of learning, and it is why the shape of the tissue reflects what a person has spent their life doing.

On top of all this sits the prediction loop. Your senses take in an enormous volume of raw data, and conscious awareness handles a famously tiny slice of it. Estimates for how much information reaches conscious processing per second range from around 10 to 50 bits, against millions arriving at the receptors. The gap is not a failure. It is the compression that makes acting in real time possible at all.

When does the human brain fully develop?

It does not, in the sense people mean. There is no age at which the brain is finished.

The 25 figure has a real origin. The prefrontal cortex, which handles planning and impulse control, is the last area to become fully myelinated, and that process runs into the mid-twenties. Somewhere along the way this became a hard deadline, and then a legal talking point, and then a fact everyone knows.

The largest study of human brain development to date, a 2022 analysis pooling more than 120,000 scans across the lifespan, gives a much less tidy picture. Total cortical grey matter volume peaks at about 5.9 years of age. Cortical thickness peaks even earlier, before age two. Total white matter volume, the insulated wiring, does not peak until about 28.7 years. Subcortical grey matter peaks around 14.4 years. Every measure has its own schedule, and none of them cluster at 25.

The useful version of the truth is less dramatic and more encouraging. Different systems mature at different rates, the ones handling long-range planning finish late, and structural change in response to what you do continues for the rest of your life.

What does a real human brain actually look like?

Softer than almost anyone expects. A fresh, unfixed human brain has roughly the consistency of firm jelly or a very soft cheese, and lifted out of the skull it slumps and deforms under its own weight. The rubbery, self-supporting specimens in teaching labs have been hardened in formalin for weeks. That preservation also drains the colour and stiffens the tissue considerably, so what most people picture as a real brain is a chemically altered version of one.

Living, it is pinkish grey and glossy, wrapped in membranes and threaded with fine red vessels. The grey matter of the cortex is the outer layer, only two to four millimetres thick. Slice into it and the inside is paler and creamier, and that is the white matter, whose colour comes from the fatty myelin sheathing the connecting fibres.

Size: around 1,300 to 1,400 grams in an adult, roughly the weight of a large cabbage or a small laptop. It floats in cerebrospinal fluid, which is why it weighs closer to 50 grams in situ than 1,400, and why a brain out of its fluid is so much heavier and more fragile than it looks.

One more thing that surprises people who see one. There is no visible seam between the parts you have read about. The lobes are named after the skull bones above them, not after any boundary the tissue actually has.

What all of this means for learning

This is the part we care about most, because it is the part we build for.

Three findings on this page are not trivia. They are the mechanics of how anything gets into your head and stays there.

Connections change with use, and that is all learning physically is. When two neurons fire together repeatedly the link between them strengthens. There is no separate storage compartment; the wiring that does the processing is the wiring that holds the memory. Which is why the London taxi drivers ended up with measurably more grey matter in the posterior hippocampus, and why the effect scaled with years on the job rather than appearing overnight.

Spacing beats cramming, because strengthening a connection takes time. The same total minutes spread across a week produce more durable retention than the same minutes in one sitting. This is one of the oldest and most replicated findings in the whole of psychology, and it is still the single most ignored one.

Retrieving beats rereading. Pulling a fact out of your head fires the pathway. Looking at it again mostly exercises your visual system. Testing yourself feels harder and less productive than review, and it produces substantially better retention, which is exactly the kind of result that loses to intuition.

The thing this page will not tell you

That you can train your brain, in the sense the phrase is usually sold. The largest review of the commercial brain-training literature found that practising these programs makes you better at the practised tasks, with weak evidence of transfer to related tasks and essentially none to everyday cognitive performance. You get better at the game.

The honest version is duller: there is no exercise that upgrades general cognition. There is only learning specific things, using methods that respect how the tissue actually works. If you want to know more about a subject, the route is to learn about that subject, spaced out, with recall.

That is the whole design brief for what people now call brainmaxing, and it is what we built NerdSip around: short courses, a question at the end of every lesson, and the material coming back later instead of being read once and filed.