How Does the Brain Work?

Most people think the brain receives information from the world and processes it. That is backwards. The brain spends most of its time generating predictions about what is about to happen, based on everything it has experienced before. The senses are not the main show. They are the fact-checkers.

Incoming sensory data only gets your full attention when it contradicts the prediction. The rest of the time, you are essentially living inside your brain's best guess about reality. This is why optical illusions work: your brain's prediction overrides the raw data from your eyes.

Neuroscientists call this predictive processing, and it explains an enormous range of otherwise puzzling phenomena, from why chronic pain can persist after an injury has healed, to why placebo treatments sometimes produce measurable physical effects.

A neuron is a cell that specialises in sending messages. It receives inputs through branching dendrites, integrates those signals in the cell body, and if the combined signal is strong enough, fires an electrical pulse called an action potential down its axon at speeds up to 120 metres per second.

When that pulse reaches the end of the axon, it triggers the release of chemical messengers called neurotransmitters. These cross the synapse, the tiny gap between neurons, and bind to receptors on the neighbouring cell. Depending on the neurotransmitter, the receiving neuron becomes either more or less likely to fire.

The pattern of which neurons fire, in which sequence, at what frequency, is what produces every thought, feeling, and movement you have ever had. There is no single location for any experience. Everything is a pattern spread across a network.

The cerebral cortex handles everything you think of as conscious: reasoning, language, sensation, voluntary movement. It is divided into four lobes. The frontal lobe manages planning and personality. The parietal lobe integrates sensory information. The temporal lobe processes sound and language. The occipital lobe at the back handles vision.

Deeper inside, the limbic system governs emotion and survival. The amygdala flags threats and stamps emotional weight onto memories. The hippocampus coordinates memory formation. The hypothalamus regulates hunger, thirst, body temperature, and hormone release.

At the base, the cerebellum coordinates movement with extraordinary precision, and the brain stem silently handles heartbeat, breathing, and consciousness itself. Damage here is almost always fatal. The further back and down you go, the more ancient and essential the function.

For most of the twentieth century, scientists believed the adult brain was fixed and unchangeable. This turned out to be completely wrong. The brain rewires itself throughout life in response to experience, a property called neuroplasticity.

Learning something new builds new synaptic connections. Repeating a behaviour makes the supporting neural pathway faster and more efficient, like widening a road into a motorway. Neglecting a skill allows those connections to weaken and eventually disappear.

Even after serious brain injury, neighbouring regions can sometimes take over functions from damaged areas. Stroke patients relearn movement and language because healthy brain tissue gradually assumes responsibility. The brain is not a fixed structure. It is a living architecture that responds to how you use it.

In 1848, a 25-year-old railroad foreman named Phineas Gage was working in Vermont when an accidental explosion drove an iron rod straight through his skull, entering beneath his left cheekbone and exiting through the top of his head. He survived. He sat up, spoke, and walked to the doctor.

But the Phineas Gage who recovered was, according to everyone who knew him, no longer Gage. His prefrontal cortex had been destroyed. He became erratic, unreliable, unable to make plans or follow through on them. His personality had changed completely.

His physician, John Harlow, recorded the case meticulously. It became the first hard evidence that personality, behaviour, and decision-making were located in a specific part of the brain, not distributed vaguely through the body or seated in the heart, as many believed. Gage's skull remains on display at Harvard Medical School.