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The early childhood years are a period of rapid change in the brain. During early and middle childhood, the brain forms and refines a complex network of connections in the brain through synaptogenesis, pruning, and myelination.
The process of forming connections is biologically driven, but experiences also promote synapse formation. The brain produces many more synapses than it will ultimately use. Researchers describe this process as synaptic overproduction. This rapid synapse formation continues throughout early childhood.
The process of myelination also continues during early childhood and is the major cause of the increase in a child's brain size. In the first four years of life, the brain increases to 80% of its adult weight of 2.6 - 3.3 pounds (1200-1500 grams).
Pruning is a key process that shapes the brains of young children. Synaptic overproduction causes synapses to develop extremely rapidly. The pruning process refines these connections based on experience. Connections used regularly become stronger and more complex. Connections not used are considered non-essential, and the brain eventually prunes them away to increase efficiency.
As an example, an infant's brain has connections that allow her to hear sounds from all languages in the world. During the early years, the brain strengthens connections for sounds in the languages she hears regularly. Over time, the brain eliminates the connections for other sounds. This is why most adults have trouble distinguishing sounds that are not in our language.