It is easy to imagine the brain as a set of labeled boxes, one for fear, one for language, one for faces. The truth is messier and more interesting. Different regions really do specialize, but almost nothing happens in one place alone. This level builds on the basic map of lobes and structures and asks how the parts cooperate.

From the bottom up

The brain grew in evolution roughly from the bottom up, and its oldest parts keep us alive. At the base, the medulla, part of the brainstem, sets the rhythm of breathing and adjusts heart rate and blood pressure without any conscious effort. Above it sits the midbrain, which helps steer the eyes and orient us toward sudden sounds.

Deep in the center of the brain is the thalamus, a pair of egg-shaped structures. The thalamus relays most incoming sensory signals on to the cortex, and it also filters them, deciding to a degree which ones reach awareness. Smell is the one major sense that partly bypasses it. Just below lies the hypothalamus, no bigger than an almond, yet it is a master regulator. The hypothalamus controls body temperature, hunger and thirst, and the release of hormones through the pituitary gland just beneath it. When you shiver, feel parched or wake at dawn, the hypothalamus is involved.

Wrapped around the thalamus are the basal ganglia, a group of linked structures. The basal ganglia help select which action to perform and form habits, releasing one movement while holding back competing ones. When they fail, movement suffers in characteristic ways: too little movement in Parkinson's disease, and unwanted jerking movements in Huntington's disease, an inherited disorder.

Two hemispheres

The cortex is split into left and right hemispheres, and an odd rule governs them: each side of the brain mainly controls and senses the opposite side of the body. A stroke on the left side of the brain weakens the right arm and leg. The two halves are joined by the corpus callosum, a thick band of some 200 million axons.

In the 1960s, surgeons cut the corpus callosum in a few patients to stop severe epilepsy from spreading. Roger Sperry and his student Michael Gazzaniga then tested these "split-brain" patients by flashing pictures to only one hemisphere at a time. When an object was shown only to the right hemisphere, the patient could not name it but could pick it out with the left hand, which the right hemisphere controls. The reason was that, for most people, the speaking machinery lives on the other side: language depends mainly on the left hemisphere in most people, including about 95 percent of right-handers. Sperry shared a Nobel Prize in 1981. Strikingly, the patients seemed ordinary in daily life; the split showed up mainly in careful laboratory tests.

The cortex as a network

The front of the frontal lobe, the prefrontal cortex, is especially large in humans. It holds goals in mind, weighs options and keeps impulses in check. But it works through constant traffic with regions far away. Modern brain scanning has revealed large networks that rise and fall together: one for focusing attention on a task, and a "default mode network" that becomes more active when the mind wanders, recalls the past or imagines the future. Specialization is real, but a mental act is a pattern across many regions, closer to a chord than a single note.

Two popular myths

You may have heard that people use only ten percent of their brains. This is false. Brain scans show nearly all regions active at some point over a day, and damage to almost any area causes some problem. The brain is also expensive tissue: about 2 percent of body weight using roughly 20 percent of the body's energy at rest, so evolution would hardly maintain a mostly idle organ.

Another myth sorts people into logical "left-brained" and creative "right-brained" types. The hemispheres do differ in their strengths, but in a 2013 University of Utah study of more than a thousand people, researchers found no sign that individuals rely more on one whole hemisphere. Creativity and logic both draw on both sides.

Worked example: reading a stroke

A 68-year-old man is eating breakfast when his coffee cup slips from his right hand. His wife notices the right side of his mouth drooping, and when he tries to say something, the words come out jumbled. What does the map of the brain predict?

Right-sided weakness points to the left hemisphere, because of crossed control. Trouble with words adds evidence, because language depends mainly on the left side. Together they suggest a blocked artery feeding the left side of the cortex. That reasoning is why public health campaigns teach the signs of stroke through the word FAST: Face drooping, Arm weakness, Speech difficulty, Time to call emergency services. Every minute counts, because a blocked artery starves millions of neurons of oxygen, and clot-dissolving treatment works best within a few hours. Here a century of brain mapping becomes something a family member can use in the kitchen.