8.2 Brain Development

Brains grow rapidly before birth and throughout infancy, as you saw in Chapter 5. By age 2, most neurons have connected to other neurons and substantial pruning has occurred. The 2-year-old’s brain already weighs 75 percent of what it will weigh in adulthood; the 6-year-old’s brain is 90 percent of adult weight. (The major structures of the brain are diagrammed in Figure 8.2).

Connections A few of the dozens of named parts of the brain are shown here. Although each area has particular functions, the entire brain is interconnected. The processing of emotions, for example, occurs primarily in the limbic system, where many brain areas are involved, including the amygdala, hippocampus, and hypothalamus.

Since most of the brain is already present and functioning by age 2, what remains to develop? The most important parts!

The social understanding that develops as the prefrontal cortex matures distinguishes humans from other primates. For example, a careful series of tests given to 106 chimpanzees, 32 orangutans, and 105 human 2½-year-olds found that young children were “equivalent … to chimpanzees on tasks of physical cognition but far outstripped both chimpanzees and orangutans on tasks of social cognition” such as pointing or following someone’s gaze (Herrmann et al., 2007, p. 1365).

As their brains mature, children become better at controlling their emotions. For example, when a stranger greets them, many 2-year-olds are speechless, hiding behind their mothers if possible. Adults may feel equally shy, but they bravely respond. Brain scans of the prefrontal cortex and amygdala (soon described) taken at age 18 may show inhibition, but most inhibited people no longer act in extremely anxious ways (Schwartz et al., 2010). [Lifespan Link: Emotional regulation is further discussed in Chapter 10.]

Speed of Thought

After infancy, some brain growth is the result of proliferation of the communication pathways (dendrites and axons). However, most increased brain weight occurs because of myelination. Myelin (sometimes called the white matter of the brain) is a fatty coating on the axons that speeds signals between neurons (see Figure 8.3).

Faster and Faster Myelination is a lifelong process. Shown here is a cross section of an axon (dark middle) coated with many layers of Schwann cells, as more and more myelin wraps around the axon throughout childhood. Age-related slowdowns in late adulthood are caused by gradual disappearance of myelin layers.

DR. DAVID FURNESS, KEELE UNIVERSITY/SCIENCE SOURCE

Although myelination continues for decades, the effects are especially apparent in early childhood (Silk & Wood, 2011). The areas of the brain that show greatest early myelination are the motor and sensory areas (Kolb & Whishaw, 2013).

Speed of thought from axon to neuron becomes pivotal when several thoughts must occur in rapid succession. By age 6, most children can see an object and immediately name it, catch a ball and throw it, write their ABCs in proper sequence, and so on. In fact, rapid naming of letters and objects—possible only when myelination is extensive—is a crucial indicator of later reading ability (Shanahan & Lonigan, 2010).

Of course, adults must be patient when listening to young children talk, helping them get dressed, or watching them write each letter of their names. Everything is done more slowly by 6-year-olds than by 16-year-olds because the younger children’s brains have less myelination, and slower information processing. However, thanks to myelination, older preschoolers are much quicker than toddlers, who sometimes forget what they were doing before they finish.

The Brain’s Connected Hemispheres

One part of the brain that grows and myelinates rapidly during early childhood is the corpus callosum, a long, thick band of nerve fibers that connects the left and right sides of the brain. Growth of the corpus callosum makes communication between the hemispheres more efficient, allowing children to coordinate the two sides of their brains, and hence both sides of their bodies.

Failure of the corpus callosum to develop results in serious disorders: This failure is one of several possible causes of autism (Frazier et al., 2012).

To understand the significance of the corpus callosum, note that each side of the body and brain specializes, being dominant for certain functions. This is lateralization, literally, “sidedness.” The entire human body is lateralized, apparent not only in right- or left-handedness but also in the feet, the eyes, the ears, and the brain itself. Genes, prenatal hormones, and early experiences all affect which side does what. Lateralization advances with development of the corpus callosum (Kolb & Whishaw, 2013).

Left-handed people tend to have thicker corpus callosa than right-handed people do, perhaps because they need to readjust the interaction between the two sides of their bodies, depending on the task. For example, most left-handed people brush their teeth with their left hand because using their dominant hand is more natural, but they shake hands with their right hand because that is what the social convention requires.

Smarter than Most? Beware of stereotypes. Obviously, this student is a girl, Asian, left-handed, and attending a structured school (note the uniform). Each of these four characteristics leads some to conclude that she is more intelligent than other 7-year-olds. But all children have brains with the potential to learn: Specific teaching, not innate characteristics, is crucial.

STOCKBYTE/GETTY IMAGES

Infants and toddlers usually prefer one hand to the other for grabbing spoons and rattles. By age 2 most children have a dominant hand used for scribbling and throwing. Preschool teachers notice that about 1 child in 10 prefers the left hand. Handedness is partly genetic (Goymer, 2007), but many cultures have tried to make everyone right-handed, with some success. When left-handed children were forced to use their right hands, most learned to write right-handedly. However, neurological success was incomplete: Their brains were only partly reprogrammed (Klöppel et al., 2007).

Even today, many cultures endorse the belief that being right-handed is best, an example of the difference-equals-deficit error, explained in Chapter 1. Consider language: In English, a “left-handed compliment” is insincere, and no one wants to have “two left feet” or to be “out in left field.” In Latin, dexter (as in dexterity) means “right” and sinister means “left” (and also “evil”). Gauche, the French word for left, means “socially awkward” in English. Many languages are written from left to right, which is easier for right-handed people.

The design of doorknobs, scissors, baseball mitts, instrument panels, and other objects favor the right hand. (Some manufacturers have special versions for lefties, but few young children know to ask for them.) In many Asian and African cultures, the left hand is used only for wiping after defecation; it is an insult to give someone anything with that “dirty” hand.

Developmentalists advise against switching a child’s handedness, not only because this causes adult–child conflicts and may create neurological confusion but also because left lateralization is an advantage in some professions, especially those involving creativity and split-second actions. A disproportionate number of artists, musicians, and sports stars were/are left-handed, including Jimi Hendrix, Bill Gates, Oprah Winfrey, Lady Gaga, and Justin Bieber. Five of the past six presidents of the United States were/are lefties: Gerald Ford, Ronald Reagan, George H.W. Bush, Bill Clinton, and Barack Obama.

Acceptance of left-handedness is more widespread now than a century ago. More adults in Great Britain and the United States claim to be left-handed today (about 10 percent) than in 1900 (about 3 percent) (McManus et al., 2010). There also seem to be more left-handed men than women, as well as more left-handers in North America than elsewhere.

Astonishing studies of humans whose corpus callosa were severed to relieve severe epilepsy, as well as research on humans and other vertebrates with intact corpus callosa, have revealed how the brain’s hemispheres specialize. Typically, the brain’s left half controls the body’s right side as well as areas dedicated to logical reasoning, detailed analysis, and the basics of language. Then the brain’s right half controls the body’s left side and areas dedicated to emotional and creative impulses, including appreciation of music, art, and poetry. Thus, the left side notices details and the right side grasps the big picture.

This left–right distinction has been exaggerated, especially when broadly applied to people (Hugdahl & Westerhausen, 2010). No one is exclusively left-brained or right-brained (except severely brain-damaged individuals); moreover, the brain is plastic [Lifespan Link: Brain plasticity is discussed in Chapter 1], especially in childhood, so a lost function in one hemisphere is sometimes replaced in the other hemisphere.

Further, both sides of the brain are usually involved in every skill. That is why the corpus callosum is crucial. As myelination progresses, signals between the two hemispheres become quicker and clearer, enabling children to become better thinkers and less clumsy.

To pick an easy example: No 2-year-old has the balance to hop on one foot, but most 6-year-olds can do it—an example of brain balancing. Many songs, dances, and games that young children love involve moving their bodies in some coordinated way—difficult, but fun because of that.

The entire frontal lobe continues to develop for many years after early childhood; dendrite density and myelination are still increasing in emerging adulthood (Johnson, 2010). Nonetheless, neurological control advances significantly every year between ages 2 and 6, as is evident in several ways:

• Sleep becomes more regular.
• Emotions become more nuanced and responsive.
• Temper tantrums subside.
• Uncontrollable laughter and tears are less common.

One example of the maturing brain is in the game Simon Says. Players are supposed to follow the leader only when orders are preceded by the words “Simon says.” Thus, if leaders touch their noses and say, “Simon says touch your nose,” players are supposed to touch their noses; but when leaders touch their noses and say, “Touch your nose,” no one is supposed to follow the example. Young children lose at this game because they impulsively do what they see and hear. Older children can think before acting. The prefrontal cortex works!

Such advances can be observed in every child, but might personal experience rather than brain maturation be the reason? A convincing demonstration that something neurological, not experiential, is the primary reason for these changes comes from a series of experiments.

These experiments begin with young children given a set of cards with clear outlines of trucks or flowers, some red and some blue. They are asked to “play the shape game,” putting trucks in one pile and flowers in another. Three-year-olds (and even some 2-year-olds) can do this correctly.

Then children are asked to “play the color game,” sorting the cards by color. Most children under age 4 fail. Instead they sort by shape, as they had done before. This basic test has been replicated in many nations; 3-year-olds usually get stuck in their initial sorting pattern. By age 5 (and sometimes age 4). most children make the switch.

When this result was first obtained, experimenters thought perhaps the children didn’t have enough experience to know their colors; so the scientists switched the order, first playing “the color game.” Most 3-year-olds did that correctly, because most 3-year-olds know colors. Then, when these children were asked to play “the shape game,” they sorted by color! Even with a new set of cards, such as yellow and green or rabbits and boats, 3-year-olds still tend to sort however they did originally, either by color or shape.

Researchers are looking into many possible explanations for this result (Müller et al., 2006; Marcovitch et al., 2010; Ramscar et al., 2013). All agree, however, that something in the brain must mature before children are able to switch from one way of sorting objects to another. [Lifespan Link: Maturation of the prefrontal cortex is also discussed in Chapter 5, Chapter 11, and Chapter 14.]

Neurons have only two kinds of impulses: on–off, or activate–inhibit. Each is signaled by biochemical messages from dendrites to axons to neurons. Both activation and inhibition are necessary for thoughtful adults, who neither leap too quickly nor hesitate too long. A balanced brain is best throughout life: One sign of cognitive loss in late adulthood is that people become too cautious or too impulsive.

Many young children are notably unbalanced. They are impulsive, flitting from one activity to another. That explains why many 3-year-olds cannot stay quietly on one task, even in “circle time” in preschool, where each child is supposed to sit in place, not talking or touching anyone else. Poor impulse control signifies a personality disorder in adulthood but not in early childhood. Few 3-year-olds are capable of sustained attention, as required in primary school.

During the same age period, the see-saw tips in the opposite direction, as some children play with a single toy for hours. Perseveration refers to the tendency to persevere in, or stick to, one thought or action, as evident in the card-sorting study just described (Hanania, 2010).

Many explanations are plausible, but the tendency is unmistakable. Often young children repeat one phrase or question again and again, and often once they start giggling they find it hard to stop. Another example of perseveration occurs when a child has a tantrum when told to stop an activity. (Wise teachers give a warning—“Cleanup in 5 minutes”—which may help.) The tantrum itself may perseverate. Crying may become uncontrollable, because the child is stuck in the emotion that triggered the tantrum.

Impulsiveness and perseveration are opposite manifestations of the same underlying cause: immaturity of the prefrontal cortex. No young child is perfect at regulating attention; impulsiveness and perseveration are evident in every 2-year-old (Else-Quest et al., 2006).

Ashes to Ashes, Dust to Dust Many religious rituals have sustained humans of all ages for centuries, including listening quietly in church on Ash Wednesday—as Nailah Pierre tries to do. Sitting quietly is develop-mentally difficult for young children, but for three reasons she probably will succeed: (1) gender (girls mature earlier than boys), (2) experience (she has been in church many times), and (3) social context (she is one of 750 students in her school attending a special service at Nativity Catholic church).

SKIP O’ROURKE/TAMPA BAY TIMES/ZUMAPRESS.COM

Over the years of childhood, from ages 2 to 12, brain maturation (innate) and emotional regulation (learned) increase: Most older children can pay attention and switch activities as needed. By early adolescence, children change tasks at the sound of the bell—no perseveration allowed.

Exceptions include children diagnosed with attention-deficit/hyperactivity disorder (ADHD), who are too impulsive for their age. An imbalance between the left and right sides of the prefrontal cortex and abnormal growth of the corpus callosum seem to underlie (and perhaps cause) ADHD (Gilliam et al., 2011).

As with all biological maturation, development of impulse control and behavioral flexibility is related to culture—hence the reason this chapter is called biosocial development, not simply physical development. A study of Korean preschoolers found that they developed impulse control and reduced perseveration sooner than a comparable group of English children (Oh & Lewis, 2008).

This study included the shape–color task: Of the 3-year-olds, 40 percent of Korean children but only 14 percent of British children successfully shifted from sorting by shape to sorting by color. The researchers considered many possible reasons and finally concluded that “a cultural explanation is more likely” (page 96).

Emotions and the Brain

Now that we have considered the prefrontal cortex, we turn to another region of the brain, sometimes called the limbic system, the major system for emotions. Emotional expression and emotional regulation advance during early childhood. [Lifespan Link: Emotional regulation is discussed further in Chapter 10 and Chapter 15.] Crucial to that advance are three parts of the brain—the amygdala, the hippocampus, and the hypothalamus.

The amygdala is a tiny structure deep in the brain, about the same shape and size as an almond. It registers emotions, both positive and negative, especially fear (Kolb & Whishaw, 2013). Increased amygdala activity is one reason some young children have terrifying nightmares or sudden terrors, overwhelming the prefrontal cortex. A child may refuse to enter an elevator or may hide from a nightmare. The amygdala responds to comfort but not to logic. If a child is terrified of, say, a dream of a lion in the closet, an adult should not laugh but might open the closet door and command the lion to go home.

Another structure in the emotional network is the hippocampus, located right next to the amygdala. A central processor of memory, especially memory for locations, the hippocampus responds to the anxieties of the amygdala by summoning memory. A child can remember, for instance, whether previous elevator-riding was scary or fun.

Early memories of location are fragile because the hippocampus is still developing. Nonetheless, emotional memories from early childhood can interfere with expressed, rational thinking: An adult might have a panic attack but not know why.

The interaction of the amygdala and the hippocampus is sometimes helpful, sometimes not; fear can be constructive or destructive (LaBar, 2007). Studies performed on some animals show that when the amygdala is surgically removed, the animals are fearless in situations that should scare them. For instance, a cat without an amygdala will stroll nonchalantly past monkeys—something no normal cat would do (Kolb & Whishaw, 2013).

A third part of the limbic system, the hypothalamus, responds to signals from the amygdala (arousing) and to signals from the hippocampus (usually dampening) by producing cortisol, oxytocin, and other hormones that activate parts of the brain and body (see Figure 8.4). Ideally, this hormone production occurs in moderation (Tarullo & Gunnar, 2006).

A Hormonal Feedback Loop This diagram simplifies a hormonal linkage, the HPA (hypothalamus-pituitary-adrenal) axis. Both the hippocampus and the amygdala stimulate the hypothalamus to produce CRH (corticotropin-releasing hormone), which in turn signals the pituitary gland to produce ACTH (adrenocorticotropic hormone). ACTH then triggers the production of CORT (glucocorticoids) by the adrenal cortex (the outer layers of the adrenal glands, atop the kidneys). Fear may either build or disappear, depending on other factors, including how the various parts of the brain interpret that first alert from the amygdala.

As the limbic system develops, young children watch their parents’ emotions closely. If a parent looks worried when entering an elevator, the child may fearfully cling to the parent when the elevator moves. If this sequence recurs often enough, the child may become hypersensitive to elevators, as fear from the amygdala joins memories from the hippocampus, increasing cortisol via the hypothalamus. If, instead, the parent makes elevator riding fun (letting the child push the buttons, for instance), initial feelings of fear subside, and the child’s brain will be aroused to enjoy elevators—even when there is no need to go from floor to floor.

Knowing the varieties of fears and joys is helpful when a teacher takes a group of young children on a trip. To stick with the elevator example, one child might be terrified while another child might rush forward, pushing the close button before the teacher enters. Every experience (elevators, fire engines, subways, animals at the zoo, a police officer) is likely to trigger a range of emotions, without much reflection, in a group of 3-year-olds: A trip needs several adults, ready to respond to whatever reactions the children have.

Cortisol, which is the primary stress hormone, may flood the brain and destroy part of the hippocampus. Does that mean a young child’s life should be stressfree? No, some cortisol is needed for normal development. However, there is “extensive evidence of the disruptive impacts of toxic stress” (Siegel et al., 2013). Too much cortisol early in life may lead to permanent deficits in learning and health, with major depression, post-traumatic stress disorder, and attention-deficit/hyper-activity in childhood and adolescence.

Yet, stress may sometimes be helpful. Ongoing research seeks to discover exactly how and when stress harms the human brain. Emotionally-arousing experiences—meeting new friends, entering school, visiting a strange place—seem beneficial if a young child has someone or something to moderate the stress. Parent support and child personality at age 3 (such as a child who is not too fearful, and thus becomes accustomed to new experiences) are crucial moderators. When past support and experience are in place, cortisol will not be overwhelming during stressful events at age 6.

In an experiment conducted by Teoh and Lamb (2013), brain scans and hormone measurements were taken of 4- to 6-year-olds immediately after a fire alarm. As measured by their levels of cortisol, some children were upset and some were not. Two weeks later, either a friendly or a stern adult questioned them about the event. Those with higher cortisol reactions to the alarm remembered more details than did those with less stress, which suggests that some stress aided memory.

Good Excuse It is true that emotional control of selfish instincts is difficult for young children because the prefrontal cortex is not yet mature enough to regulate some emotions. However, family practices can advance social understanding.

BARBARA SMALLER/THE NEW YORKER COLLECTION/CARTOONBANK.COM

Another study found that children remembered more when they were interrogated by a friendly interviewer (Quas et al., 2004). Generally, a child’s memory is more accurate when an interviewer is warm and attentive. This finding is particularly useful if a child witnesses a crime (Teoh & Lamb, 2013).

Context is always crucial: Stress can facilitate memory and learning if adults are reassuring. Because of individual variations in genes and early childhood, a study of 5- and 6-year-olds exposed to a stressful experience found that cortisol rose dramatically in some children but not at all in others (de Weerth et al., 2013).

In addition to such individual differences, a crucial factor is past history, specifically whether or not the children experienced chronic early stress. A young child who experienced neglect or abuse day after day may become unable to use his or her brain/cortisol connection to adjust to stress later on (Evans & Kim, 2013).

Studies of children who have been maltreated suggest that excessive stress hormones in early childhood may permanently damage the brain, blunting or accelerating emotional responses lifelong (Wilson et al., 2011). Sadly, this topic leads again to research on those adopted Romanian children mentioned in Chapter 7. When they saw pictures of happy, sad, frightened, or angry faces, their limbic systems were less reactive than were those of Romanian children living with their biological parents. Their brains were also less lateralized, suggesting less specialized, less efficient thinking (Parker & Nelson, 2005). Thus early stress had probably damaged their brains.

Romania no longer permits wholesale international adoptions. Nonetheless, as mentioned earlier, some Romanian children are raised in institutions. In one study, several of them were randomly assigned to foster homes at about age 2. By age 4, they were smarter (by about 10 IQ points) than those who remained institutionalized (Nelson et al., 2007). This research suggests that ages 2 to 4 constitute a sensitive time for brain growth, as measured by tests of language and memory.