Stimulated neurons produce more connections.

As the brain develops, neurons reach out randomly to form new connections, only a small number of which take hold. How the brain chooses which connections to keep and which to prune back appears to be governed by which branches have the most electrical activity.  This is a finding that could help to explain not only how early environmental experiences guide brain development, but how those environmentally experiences can be utilised as a treatment principle for brain injured children. 
 
Stephen Smith, professor of molecular and cellular physiology at the Stanford University School of Medicine, immersed 3-day-old zebra fish in a breathable, jelly like substance that kept the fish alive but immobile. The researchers could then focus video cameras on the fish's developing brain to watch how the branches of individual neurons grew and shrank over time. 

It turns out that determining which of the branches will grow follows an age-old axiom: The squeaky neuron gets the grease. "Louder neurons drown out their quieter neighbours," Smith said. 

Working out this seemingly simple rule took some technical finesse. Smith created zebrafish with a few brain cells that made a protein which prevented them from firing their normal electrical signals. These cells were also engineered to produce a protein that glowed green under the appropriate light. 

He looked for green neurons in the immobilized fish to see how their branches fared compared with neighboring neurons that fired normally. The green neurons didn't compete well.

Although the poorly-firing green neurons still formed extensive branching structures, which the researchers call the neuron's arbor, most of those branches eventually receded while neighboring neurons formed a large number of stable connections. When the fish were five days old, the green neurons had a smaller, less complex arbor than those of neighboring neurons

They gave those losing neurons a fighting chance through another molecular twist, managing to silence some neurons near the green, quietly-firing cells. When that was done, the green cells were able to compete successfully and formed longer, more complex arbors. 

Although this work specifically examined the brains of fish, Smith said the same rules likely apply to all neurons, including those in the human brain. 
 
Neurons that fire regularly while learning to recognize a new person's face, for example, will form larger arbors with more connections that help retain that memory for the future. Likewise, neurons stimulated by engaging toys or experiences will probably create larger arbors than similar neurons in less exciting conditions.  This is good news for children on the Snowdrop programme, who are exposed to highly enriched environments, which encourage their brain cells to develop larger arbors with more connections.