A newborn's brain has nearly all the neurons it will ever have, yet it can barely focus its eyes. By age three, that same brain is understanding sentences, recognizing hundreds of faces and running across a room. What changes is less the number of cells than the connections between them. The brain builds itself in response to the world, and it keeps rewiring throughout life.

Building a brain

About three weeks after conception, a flat sheet of cells in the embryo folds into a tube, the neural tube, which becomes the brain and spinal cord. If it fails to close completely, a baby may be born with spina bifida or worse. Taking the vitamin folic acid before and early in pregnancy greatly lowers the risk of neural tube defects, which is why many countries add it to flour. It is one of the simplest and most effective public health measures in neuroscience.

Over the following months, neurons are born in huge numbers and migrate to their places, then send out axons to find their partners. Development is wasteful by design. Many more neurons are made than survive, and in early childhood synapses form far faster than they will be kept. The brain makes a surplus of synapses and then prunes away the ones that are little used. Experience decides which connections survive, a rule often summed up as "use it or lose it." Pruning and the growth of myelin continue through adolescence and into the twenties.

Critical periods

Some kinds of learning depend heavily on timing. In the 1960s Hubel and Wiesel, whose work on vision appeared in the previous level, sewed shut one eye of very young kittens for a few months. When the eye was reopened, it was healthy, but closing one eye early in life caused a lasting loss of vision through that eye, because its connections in the visual cortex had been taken over by the open eye. The same treatment in adult cats had little effect. There is a critical period when the visual system expects input and wires itself accordingly. Other abilities, such as learning the sounds of a language without an accent, show softer "sensitive periods" that fade gradually rather than closing.

How learning changes synapses

In 1949 the Canadian psychologist Donald Hebb proposed a simple rule: when one neuron repeatedly helps to fire another, the connection between them grows stronger. Hebb's idea gave learning a physical basis, and it was confirmed in 1973 when Timothy Bliss and Terje Lømo, working with rabbits, found long-term potentiation in the hippocampus: a lasting increase in a synapse's strength after brief, intense stimulation. Around the same time the American scientist Eric Kandel was studying memory in an unlikely creature. Kandel used the simple nervous system of the sea slug Aplysia, with its large, easily identified neurons, to show how short-term memories rely on temporary chemical changes and long-term memories on new proteins and new synapses. He shared a Nobel Prize in 2000.

Plasticity in adults

Whether the adult human brain grows new neurons is a live dispute. A 1998 study found signs of new neurons in the adult hippocampus. In 2018 two research groups examined human brain tissue and reached opposite conclusions, one finding almost none after childhood and the other finding them into old age. Studies still disagree about how much new neuron growth happens in adults, partly because the tissue is hard to study.

What is not in dispute is that adult connections change. After a stroke, nearby regions can take over some lost functions, especially with intense practice. In constraint-induced movement therapy, developed by Edward Taub, patients have their good arm restrained so they must use the weak one, and many regain function long after recovery was thought to have ended. Plasticity is not always helpful, though: maps that reorganize after injury may contribute to phantom limb pain and ringing in the ears.

Plasticity has also been sold. Brain training games promise sharper minds, but a large 2016 review led by Daniel Simons found that players mostly improve at the trained games themselves, with weak evidence of benefits for everyday thinking. That same year the US Federal Trade Commission settled with the maker of one popular program over its advertising claims.

Worked example: the lazy eye

A four-year-old's teacher notices that he tilts his head to see the board. An eye doctor finds that his left eye turns slightly inward, so the brain has been ignoring its blurry image. This condition, amblyopia, or "lazy eye," affects a few children in every hundred.

The kitten experiments explain both the danger and the remedy. Because the brain is in a period when visual connections are still being sorted, the ignored eye is losing ground in the visual cortex. The doctor prescribes glasses and an eye patch over the strong right eye for a few hours a day. Forcing the brain to rely on the weak eye gives its connections a chance to compete. Treatment works best in early childhood, and although some improvement is now possible in older children and even adults, the window matters. Here, an experiment on cats more than half a century ago helps a child see clearly.