Many aspects of the brain are genetically pre-programmed. For example, babies are born with basic reflexes such as grasping and sucking, can mimic adults’ facial expressions, and are pre-equipped to learn language. More recently, researchers have discovered that experience can actually affect the expression of some genes. In fact, experience can affect brain development by changing the volume of neurons in different brain regions and by altering the size and structure of neurons themselves by creating larger cell bodies or more elaborated networks of dendrites. Experience also affects the building of neural connections and how often they fire, in turn affecting the strength of the connection. Both positive and negative experiences can change the chemical makeup of the brain, altering which genes are expressed and how much they are expressed. This alteration process is called epigenetic modification and can be, but is not always, permanent. A rich learning environment and good nutrition might cause positive changes; negative experiences such as toxic stress, drug exposure, and abuse and neglect might cause negative changes.
The brain develops in a predictable sequence, from the most basic functions to the most complex ones. The interaction of genetic inheritance and environment determines how the brain will develop. There are five basic processes that make up brain development. Many of these processes begin before birth, during the prenatal period. Most are not complete until late adolescence.
- Neurogenesis is the process of forming neurons. This is the first process to occur in brain development, and is completed before a baby is born. Neurogenesis mostly occurs prenatally, but can continue throughout life in specific areas of the adult brain.
- Neural migration is the process of organizing the brain by moving neurons to specific areas based on the functions these cells will perform. Migration begins prenatally, but continues for at least 8 to 10 months after a baby is born. By the time an infant is born, most of the neurons are in the positions where they will perform their specific functions. Migration continues into adulthood in some parts of the brain, including the cerebellum and hippocampus. The continuation of migration into adulthood enables certain regions of the brain to maintain plasticity throughout life.
- Myelination is the process of coating the axon of each neuron with a fatty coating called myelin, which protects the neuron and helps it conduct signals more efficiently. Myelination begins in the brain stem and cerebellum before birth, but is not completed in the frontal cortex until late in adolescence. Breast feeding contributes to more rapid myelination in the brain because of the oleic acid found in human milk.
- Synaptogenesis is the process of forming networks of connections in the brain. Synapses begin forming prenatally in the most primitive areas of the brain (for example, the brainstem), but the process continues throughout life in most other areas of the brain. The kinds of connections that are made and strengthened depend on a baby’s specific experiences. Connections used regularly become stronger and branch out into more complex networks.
- Pruning is the process of weeding out unnecessary connections and strengthening the important ones, based on the child’s experiences. Some pruning begins very early in development, but the most rapid pruning happens between about age 3 and age 16. Different areas of the brain undergo pruning during different sensitive periods. Adults can influence pruning by ensuring that young children have a variety of experiences, that are repeated regularly, to strengthen the essential connections in the brain.
Pruning is a process that is more important than was once believed. Experiences during infancy and childhood form the connections that shape the development of the brain. Pruning is a key part of brain development because it eliminates the connections that are not used often enough. Pruning provides room for the most important networks of connections to grow and expand, making the brain more efficient.