Free Welcome to Your Child's Brain: How the Mind Grows From Conception to College by Sam Wang, Sandra Aamodt

by an initially gradual and later accelerating decline over the years of childhood. Adult levels of synapse density are reached after puberty. Although the increase is similar across animals, the decline occurs on somewhat different schedules in different individuals, supporting the idea that environmental events influence synapse elimination.
    In all areas of the cortex studied in monkeys, synapse development follows a similar time course. It is not clear whether this principle of uniform synapse development also applies to children. Brain scans of developing gray matter , where all synapses are found, suggest that frontal regions reach their final volume somewhat later than the visual cortex. However, because of the ages missing from the human synapse counts and variability between individuals, the evidence in support of this position is incomplete. In any event, brain energy measurements in children suggest that the differences in developmental timing among various cortical areas are relatively minor and that synapse elimination continues throughout childhood (see Did you know? Brain food ).
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    DID YOU KNOW? BRAIN FOOD

    As kids grow like weeds (and after all, dandelions are weeds), their brains are burning like torches. It’s expensive enough to support your mature brain, which uses 17 percent of the body’s total energy though it accounts for only 3 percent of the body’s weight. But that’s nothing compared to the cost of building your child’s brain. The brain has nearly reached its full adult size at age seven, but it still contains connections that will be removed later as the child’s experiences help sculpt the mature brain. Synapses use most of the brain’s energy, so maintaining these extra connections is costly. From ages three to eight, children’s brain tissue uses twice as much energy as adult brain tissue. A five-year-old child weighing forty-four pounds (twenty kilograms) requires 860 calories per day, and fully half of that energy goes to the brain.

    Researchers examine the brain’s energy use with an imaging technique called positron emission tomography (a PET scan) that detects radiolabeled glucose, a sugar that is the main fuel for neurons (see figure). (The addition of radioactive atoms allows glucose to be traced through the brain or body.) In the first five weeks after birth, energy use is highest in the somatosensory and motor cortex, thalamus, brainstem, and cerebellum, the most mature parts of the brain at birth, which are responsible for basic functions like breathing, movement, and the sense of touch. At two or three months, energy use increases in the temporal, parietal, and occipital lobes of the cerebral cortex and the basal ganglia , which control vision, spatial reasoning, and action, among other things. From six to twelve months, parts of the frontal cortex increase their energy use, as babies begin to regulate their own behavior. The amount of energy that the brain uses continues to increase until age four and then begins to decline around age nine, reaching adult values sequentially in various areas as they mature, until the pattern becomes fully adultlike in the late teens.
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    To look at the details of how synaptic changes result from experience during a sensitive period, we turn to research in laboratory animals. Barn owls hunt in the dark and must localize sounds accurately to locate their prey. They do this by comparing the difference in the time of sound arrival between the two ears, since a sound coming from the left side will reach the left ear before it reaches the right ear, and vice versa. The more difficult calculation of whether sounds come from above or below is determined from loudness differences created by the shape of the outer ear. An area in the owl’s midbrain receives information about discrepancies in timing and loudness and uses it to form a map of where sounds must be coming from. Because the incoming information depends on individual