Early brain development

Almost all of the neurons in the brain are generated before birth, during the first three months of pregnancy, and the newborn child’s brain has a similar number of neurons to that of an adult. Many more neurons are formed than are needed, and only those which form active connections with other neurons survive. In the first year after birth the infant brain undergoes an intense phase of development, during which excessive numbers of connections between neurons are formed, and many of these excess connections have to be cut back through the process of synaptic pruning that follows. This pruning process is just as important a stage of development as the early rapid growth of connections between brain cells. The process during which large numbers of connections between neurons are formed is called synaptogenesis. For vision and hearing (visual and auditory cortex), there is extensive early synaptogenesis. The density of connections peaks at around 150% of adult levels between four and 12 months, and the connections are then extensively pruned. Synaptic density returns to adult levels between two and four years in the visual cortex. For other areas such as prefrontal cortex (thought to underpin planning and reasoning), density increases more slowly and peaks after the first year. Reduction to adult levels of density takes at least another 10–20 years; hence there is significant brain development in the frontal areas even in adolescence. Brain metabolism (glucose uptake, which is an approximate index of synaptic functioning) is also above adult levels in the early years. Glucose uptake peaks at about 150% of adult levels somewhere around four to five years. By the age of around ten years, brain metabolism has reduced to adult levels for most cortical regions. Brain development consists of bursts of synaptogenesis, peaks of density, and then synapse rearrangement and stabilisation. This occurs at different times and different rates for different brain regions, which implies that there may be different sensitive periods for the development of different types of knowledge. Neuroscience research into early brain development has informed government education policy for children under three years old in many countries including the USA and the United Kingdom. These policies have focused on enriching the environment of children during nursery and preschool years, exposing them to stimuli and experiences which are thought to maximise the learning potential of the young brain.