Because it is not possible to directly measure the cellular processes involved in human brain development, scientists rely upon a number of reasonable assumptions about the extent to which animal and human brain development are similar. Since some animal and human psychological behaviors are similar, scientists can also make inferences about the neurobiological processes underlying these behaviors. For example, the new nerve cell connections that take place in the hippocampus of rats, tree shrews, primates, and humans during adult life can be inhibited by stressful social experiences (Bremner and Narayan, 1998; McEwen, 1998). Recent technological advances have improved the ability of neuroscientists to study human brain development and functioning directly through the use of noninvasive techniques such as electroencephalographic (EEG) recordings and functional magnetic resonance imaging (FMRI). These new technologies provide a non-invasive window to some of the physical processes that are observed in invasive animal studies. For example, a recent report in the Lancet medical journal presented information about how FMRI had been used to observe the activation of the temporal cortex in fetuses who were played nursery rhymes through earphones placed on their mothers’ wombs, suggesting that the fetus’s brain is already processing information. As more direct observation of brain development in humans is performed using such techniques, our understanding of the process by which human brains form and function is bound to increase significantly, and correlative research from other animals will be confirmed. Already, five general findings have emerged from this growing body of knowledge. These findings have important implications for both parenting and public policy efforts to support brain development during early childhood.
A child’s brain is not mature at birth.
The newborn infant has approximately the same number of “neurons” – or brain cells – as an adult, yet only about 25% of his or her brain’s volume has developed (Blinkov and Glezer, 1968). Neither the connection between the infant’s neurons, nor the supporting cells that insulate them, are fully formed at birth. The infant’s brain cells are connected by approximately 50 trillions synapses. Over the ensuing years, brain volume will quadruple, while synaptic density will increase even more as the number of synapses grows ten fold by adulthood to approximately 500 trillion. Moreover, as these new cells grow and connections are established, each of the new cells is insulated by a lipid material called “myelin,” that promotes more rapid conduction of nerve impulses.
Even more important for brain development is the fact that in the first three years of life the young child’s brain forms double the numbers of synaptic connections (approximately 1000 trillion) that are ultimately present in the adult brain. These “extra” synaptic connections provide an important clue to how the brain is shaped by experience. Beginning at age 3 and continuing over the next decade or more, synapses are selectively eliminated so that by age 15, the number of synapses in the brain has decreased by about half and remains relatively stable throughout the rest of the individual’s life (Huttenlocher, 1984).
The process of selectively eliminating synapses is so essential to creating order in the human brain that some individuals with an overabundance of synapses have serious behavioral or cognitive disorders, as seen in the condition called Fragile-X Syndrome. Studies of monkeys have likewise indicated that cognitive ability reaches mature levels only after the selective elimination of synapses has been completed (Woo et al., 1997).