Everything you think, feel and remember depends on about three pounds of tissue inside your skull. The human brain contains roughly 86 billion neurons, nerve cells that send electrical and chemical signals to one another across tiny gaps. This level looks at how that machinery works, what we learned from people whose brains were damaged, and how the brain turns raw signals from the senses into the world we experience.

Neurons and messengers

A neuron receives signals through branching dendrites and, if the input is strong enough, fires an electrical pulse down a long fiber called the axon. At the end, it releases chemical messengers called neurotransmitters across a small gap, the synapse, to the next cell. Different neurotransmitters play different roles. Dopamine is involved in movement, motivation and learning from reward; its loss in certain brain areas causes the tremors of Parkinson's disease. Serotonin is linked to mood, sleep and appetite. Many medicines and drugs work by changing how these messengers are released or absorbed. It is tempting to call dopamine "the pleasure chemical," but the reality is more complicated: it seems more involved in wanting and anticipating than in enjoying.

A map of the brain

The brain is not a uniform mass. The brainstem handles breathing and heartbeat. The cerebellum helps coordinate movement and timing. Deep structures such as the amygdala, involved in fear and emotional memory, and the hippocampus, vital for forming new memories, sit beneath the wrinkled outer layer, the cerebral cortex. The cortex is divided into four lobes on each side. The occipital lobes at the back process vision, the temporal lobes handle hearing and parts of language and memory, the parietal lobes process touch and space, and the frontal lobes support planning, self-control and movement.

Lessons from injury

Much of this map was first drawn from accidents. In 1848 a Vermont railway foreman named Phineas Gage was packing explosives when an iron rod shot through his left cheek and out the top of his head. He survived, and his doctor reported that the once reliable Gage became impulsive and profane, so that friends said he was "no longer Gage." Later retellings exaggerated the change, and there is evidence that he recovered enough to work as a stagecoach driver in Chile, but the case helped convince scientists that the frontal lobes matter for personality and self-control. In 1861 the French doctor Paul Broca examined a patient who could understand speech but say only one syllable, "tan." After the man died, Broca found damage in a specific area of the left frontal lobe, now called Broca's area. It was some of the first strong evidence that particular functions depend on particular regions.

A brain that changes

For much of the twentieth century, scientists believed the adult brain was largely fixed. We now know it has considerable neuroplasticity: connections strengthen with use and weaken with disuse, and some regions can take on new jobs after injury. A well known 2000 study found that London taxi drivers, who must memorize thousands of streets, had a larger rear portion of the hippocampus than comparison volunteers, and the difference grew with years on the job. Plasticity is greatest in childhood but continues throughout life, which is one reason rehabilitation after a stroke can work.

Sensation and perception

Psychologists distinguish sensation, the detection of light, sound, pressure and chemicals by our sense organs, from perception, the brain's work of organizing those signals into meaningful objects. Perception is not a camera recording reality. The brain fills gaps, uses context and makes educated guesses based on past experience, a process called top-down processing. Visual illusions reveal these guesses at work: two lines of equal length can look different depending on the arrows at their ends. Each eye even has a blind spot where the optic nerve leaves the retina, and we never notice it because the brain fills in the hole.

The limits of attention

We also notice far less than we think. In a famous 1999 study by Daniel Simons and Christopher Chabris, volunteers watched a video of people passing basketballs and counted the passes. Midway through, a person in a gorilla suit walked through the scene, faced the camera and thumped their chest. About half of the viewers never saw the gorilla. This effect, called inattentional blindness, shows that attention works like a spotlight: what falls outside it can go unseen even when it is in plain view.

Worked example: texting while driving

A driver insists that glancing at a phone is safe because their eyes are still "mostly on the road." What does this level suggest? First, attention is limited: reading a message pulls the spotlight away from the road even when the eyes point forward, and inattentional blindness means a cyclist can be missed in plain sight. Second, perception relies on expectations, so a driver scanning for cars may fail to register an unexpected child. Third, the frontal lobes that manage self-control and planning are the same ones the driver relies on to resist checking the phone at all. The lesson is not that people are careless, but that the brain has real limits, and good habits and good design should respect them.