Does experience change the actual structure of the brain?
Yes. Brain development is "activity-dependent," meaning that the electrical activity in every circuit—sensory, motor, emotional, cognitive—shapes the way that circuit gets put together.
Like computer circuits, neural circuits process information through the flow of electricity. Unlike computer circuits, however, the circuits in our brains are not fixed structures. Every experience–whether it is seeing one’s first rainbow, riding a bicycle, reading a book, sharing a joke–excites certain neural circuits and leaves others inactive. Those that are consistently turned on over time will be strengthened, while those that are rarely excited may be dropped away. Or, as neuroscientists sometimes say, “Cells that fire together, wire together.” The elimination of unused neural circuits, also referred to as “pruning,” may sound harsh, but it is generally a good thing. It streamlines children’s neural processing, making the remaining circuits work more quickly and efficiently. Without synaptic pruning, children wouldn’t be able to walk, talk, or even see properly.
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In some ways, never. Our brains are continually re-shaping themselves to meet the demands of everyday life, even throughout adulthood.
Brain development is most sensitive to a baby's nutrition between mid-gestation and two years of age.
Genes and environment interact at every step of brain development, but they play very different roles.