The human brain is remarkably adaptive, especially during its early formation and into the adolescent years. About three weeks after conception, a thin sheet of cells forms in the embryo and folds into what will become the brain and spinal cord. In the weeks that follow, brain cells multiply, specialize, and move into position, laying down the brain’s basic structure.
As pregnancy continues and into early infancy, the brain grows at an astonishing pace. Connections between brain cells form by the hundreds of thousands every second. By about six months after conception, the brainstem can support vital functions such as breathing and heart rate. In the final months before birth, areas involved in planning, judgment, and self-control begin a period of rapid development that will continue into adulthood.
At birth, the brain contains roughly 90 billion neurons. What changes most after birth is how these cells connect. During infancy and early childhood, the number of connections surges in response to experience, reaching a peak at about twice the level seen in adults.
Pruning
Because the brain cannot maintain every possible connection, it gradually trims away those that are rarely used while strengthening the ones that are used often. This process, known as neural pruning, helps the brain become more efficient and better suited to its environment. Early experiences therefore have a powerful influence on developing abilities, though they do not lock in intelligence or limit future learning.
Some experiences—especially hearing language, seeing the world, and learning to move—are particularly important during early “sensitive periods,” when the brain is most adaptable. As neuroscientist David Eagleman has noted, becoming who we are depends not only on what grows in the brain, but on what is carefully pruned away.
Learning
Through the dual processes of making neural connections and pruning those that aren’t heavily used, the brain shapes itself to function in the environment into which the individual is born. There is a window of time in which certain skills can be acquired and after that window closes the learning in that area becomes more difficult or limited. However, studies by Piaget and others reveal that some higher order thinking skills, such as the concepts or ratio and proportion, cannot be learned by most people until the early teenage years. Throughout adolescence and early adulthood, we have the best opportunity to learn and build understanding of concepts. Sometime between ages 20 and 30 our fluid intelligence, including mental acuity and working memory, begin to deteriorate, but our huge number of existing synaptic connections (called crystallized intelligence) compensate for a slight decline in the speed at which we can form new neural connections. So, while the young brain can form new connections at a prodigious rate, the older brain can problem solve by drawing upon its previously acquired neural connections.
