Can an adult brain change? In 2004, scientists in Germany asked 12 adults to practice juggling with three balls for three months. Before and after the practice, they took pictures of their brains. The gray matter, the gray outer layer of the brain, grew in an area that sees moving things.

But three months after they stopped, the gray matter became smaller again. The brain seems to work a little like a muscle.

For a long time, people thought the adult brain could not change its structure. A study published in 2004 challenged this idea. Researchers divided 24 adults who had never juggled into two groups, and 12 of them practiced three-ball juggling for three months. Brain scans showed that the jugglers gained gray matter in an area that processes visual motion.

But an MRI scan only shows that a region looks bigger. It cannot tell whether the change comes from new cells, more branches, or blood flow. Three months after the jugglers stopped practicing, the increase had become smaller.

Later studies found similar changes in people around 60, and even after just seven days of training. In the seven-day study, learning something new seemed to matter more than repeating a skill people already knew.

Twenty-four adults who had never juggled took part in a 2004 study published in Nature. Twelve of them spent three months learning a basic three-ball pattern until they could keep it going for at least sixty seconds. Everyone was scanned before training, after training and three months after practice ended.

In the jugglers, gray matter increased in hMT/V5, a region involved in perceiving visual motion. The increase was transient. Once practice stopped, the gain partly reversed, and some of the skill faded with it. For much of the twentieth century, the adult brain had been considered structurally fixed, and this study helped overturn that assumption.

Follow-up work reported comparable changes in people around sixty and changes detectable within seven days. A British study found alterations in white matter after six weeks. These results call for careful reading. MRI estimates volume but cannot distinguish between more neurons, extra dendrites, glial cells, blood vessels or shifts in water content.

A 2013 review also noted that many such studies used small samples and that results were sensitive to analysis choices. In the seven-day study, the changes seem to come from acquiring a new skill rather than repeating a known one.