Welcome to Your Child's Brain: How the Mind Grows From Conception to College (10 page)

Later, you may still be able to learn a new language, but you have to work much harder. As an adult, your brain’s language areas are no longer under construction and their connections are more difficult to modify. Your sensitive period for language has passed.

Fortunately, as we have said before, experience isn’t something that happens passively, even to babies. Your child’s brain has definite preferences about what it should learn at various stages of development. The types of experience that can modify a developing neural circuit are determined by predispositions that are built into the brain as a result of our evolutionary history. In short, children actively seek out the experiences they need.

What do we mean by experience? Your child’s brain is potentially influenced
by any event that can be detected by sensory receptors, transformed into electrical impulses, and transmitted to the brain. (As we will show you in chapters 10–12, all our knowledge of the world comes in the form of these electrical impulses.) Interactions with parents and other caregivers are only part of the rich tapestry of available stimuli. Physical changes in your infant’s brain can also occur when she watches the mobile that hangs over her crib, when she sticks her toes in her mouth, and when sirens pass on the street outside. Later, her universe expands to include social interactions with other children, finding her way around the neighborhood, learning to play sports, going to school, and much more. All these experiences leave their traces in her brain, some very long-lasting and others transient.

Because neural circuits mature at different times, there are a variety of sensitive periods in development, each corresponding to a particular brain function. Sensitive periods are most common in infants and toddlers because the brain is undergoing such dramatic growth at this stage, but they can occur at other times as well. Some sensitive periods occur before birth, such as the maturation of the sense of touch based on the baby’s experience within his mother’s womb (see
chapter 11
). Many occur soon after birth, as when early interactions with caregivers shape the circuits of the brain that respond to stress (see
chapter 26
). Other sensitive periods, such as the one for the grammatical aspects of language learning, continue well into childhood and adolescence.

As we described in
chapter 2
, preprogrammed chemical cues direct axons to their target regions and orchestrate the formation of a large number of synapses. Once those basic elements are in place, experience can influence the further development of the circuit by controlling the activity of those axons and synapses. Synapses that are more effective at activating their target neuron are more likely to be retained and strengthened, through biochemical plasticity pathways in the target cell, while those that are ineffective become weak or disappear. Synaptic activity can also trigger the growth or retraction of axonal or dendritic branches. Cells that fire together, wire together (see
chapter 21
).

Once these plastic changes have occurred, the brain architecture often becomes more difficult to modify in the future, either because the extra axons and synapses are no longer available or because the biochemical pathways that modify synaptic strength change with age. In this way, the brain uses sensory experience to shape the connections within a neural circuit, pruning away the ones that are
unnecessary while maintaining those that are strongest and most active to produce the perceptions and behaviors that are appropriate for that child’s individual environment.

Experience isn’t something that happens passively, even to babies. Your child’s brain has definite preferences about what it should learn at various stages of development.

Unnecessary synaptic connections are pruned throughout childhood. In the primary visual cortex, the total number of synapses increases rapidly from birth to its peak at eight months old, and then declines slowly to age five, as visual ability is maturing (see
chapter 10
). The biggest reduction in synapse number in this region happens sometime between ages five and eleven. (We don’t know exactly when this change occurs because children ages six to ten have not been studied.) In the frontal cortex, synapse density remains high at least through age seven, falls somewhat by age twelve, and reaches adult levels in the middle teenage years (see
chapter 9
). It is not clear what happens between ages seven and twelve.

Synapse elimination has been studied in much more detail in other primates, and the results are roughly consistent with the sparse human data. In rhesus monkeys, an explosive increase in synapse density in the first few months after birth is followed 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
).

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.

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 characteristics like head size and ear shape, which change as the animal grows, it can’t be specified in advance, so this mapping is learned during development.

The owl’s brain learns this map by using visual experience to calibrate the auditory map. To study this process, researchers equip baby owls with prism glasses, which make objects appear to be shifted to one side. At first the animals make a lot of mistakes as they try to move around with the glasses on, but gradually the brain adapts by changing its visual map to reflect the new reality. The auditory space map also shifts in response to prism glasses, even though the auditory information is unchanged.

The shift happens because the neurons that bring in timing and loudness information extend their axon branches to connect with new neurons in a different part of the map. The former connections remain in place, though their synapses are weakened, allowing the owls to return to the old mapping once the prism glasses are removed. This plasticity occurs in a sensitive period, until about seven months of age. In adults, whose sensitive period has ended, it is more difficult to rearrange connections because their axon arbors are limited to a smaller area of the midbrain and thus the wiring is not already in place to carry signals outside the range established in youth.

One of the basic principles of brain development is that the simplest building blocks are finished first. Later, more complex processes build upon earlier ones. For example, the areas of visual cortex that detect edges and shading must become functional before other visual areas can start to interpret these patterns as objects. For this reason, there is not a single sensitive period for vision, but a series of sensitive periods, each requiring experience for the maturation of a different region of the visual brain. If the experience required to complete an early developmental process is not available, the sensitive period is normally extended for a while, resulting in delayed maturation of that brain circuit and all the others that depend on it. Eventually, though, the window of opportunity closes, and any resulting damage may become permanent.

DID YOU KNOW? THE LIMITS OF BRAIN PLASTICITY

Optimistic popular writers have extolled the wonders of neural plasticity. The idea that experience can produce large changes in the brain is encouraging, as it supports the hope that people can learn and grow throughout life, overcoming obstacles along the way. Stories of untapped potential have a nearly unlimited appeal to the American character. But it’s time to step back and take a careful look at the evidence.