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Young brains learn by losing connections
A baby's nervous system is wired crudely, like a mess of small roads with no highways. Babies flop around like little potato bugs with limbs because the connections are everywhere and none are precise. From birth until about 25, the strong connections get reinforced while the ones that don't serve the person are removed. Powerful events can stamp in after a single trial.
Some circuits are built to resist change: the ones for heartbeat, breathing, and digestion. Others change easily, and through childhood, adolescence, and young adulthood learning can happen with almost no effort. Children go from no language to many words, and build sentences with words they have never heard.
Around 25, give or take a year or two, that changes. Fire together, wire together is the statement of Stanford's Carla Shatz, and it holds in early development. After 25 it doesn't apply in the same way. Change in the adult brain is gated: it takes a series of steps that shift internal state, and deciding to change is not enough.
Why does the infant brain prune connections?
Synapse numbers in the human cortex peak in early childhood and then fall by roughly half by adulthood, according to Peter Huttenlocher's counts from the 1970s and 1980s at the University of Chicago. Connections used often are kept and the rest are removed. This pruning sharpens circuits so they work faster and cost less energy. The process continues through adolescence, especially in the prefrontal cortex.
What does fire together, wire together mean?
The phrase summarizes a principle from Donald Hebb in 1949 that cells active at the same moment strengthen their link. Carla Shatz at Stanford showed that in the developing visual system, spontaneous waves of activity in the retina help sort out which connections stay. The slogan itself was coined by Shatz in a 1992 article. It describes the strengthening step of learning at the level of synapses.
Why do children pick up languages easily?
Patricia Kuhl at the University of Washington found that babies can discriminate the sounds of every language at six months but lose that ability for sounds not in their own language by about twelve months. The brain tunes itself toward the sounds it hears most. Studies of immigrants also show that those who arrive in childhood reach native-like grammar more often than those who arrive after puberty. The effect is called a sensitive period.
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