5.2 Brain Development

As with motor skills, the brains of young children show impressive growth, but are not nearly as developed as they will be later on. By age 2, most neurons are connected to other neurons and substantial pruning has occurred. The 2-year-old’s brain already is 75 percent of adult weight; the 6-year-old’s brain is 90 percent of adult weight. (Figure 5.3 shows the major structures of the brain.)

FIGURE 5.3 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, but many other brain areas are involved, including the amygdala, hippocampus, and hypothalamus.

The Maturing Cortex

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

Although the 2-year-olds of other primates are more developed than human children in some ways (e.g., climb trees better, walk faster), and although many animals have abilities that people lack (e.g., a dog’s sense of smell), young humans have intellectual capacities far beyond those of any other animal. Human brains continue to develop at least until early adulthood (Konner, 2010).

Considered from an evolutionary perspective, human brains have allowed the species to develop “a mode of living built on social cohesion, cooperation and efficient planning…survival of the smartest,” which seems more accurate than survival of the fittest (Corballis, 2011). Those functions of the brain that distinguish us from apes begin in infancy but develop notably after age 2 years, enabling quicker, better coordinated, and more reflective thought (Johnson, 2010; Kagan & Herschkowitz, 2005).

Between the ages of 2 and 6 years, neurological increases are especially notable in the cortex regions, where planning, thinking, social awareness, and language occur. Elephants, crows, chimpanzees, and dolphins have all surprised researchers with their intelligence, but none come close to Homo sapiens in the relative size of the cortex or its capacity for social understanding (Corballis, 2011). For example, researchers gave a series of tests to 106 chimpanzees, 32 orangutans, and 105 human 2½-year-olds. The young humans were similar to chimpanzees on tasks of physical cognition, but scored significantly higher than both chimpanzees and orangutans on social cognition tasks such as pointing or following someone’s gaze (Herrmann et al., 2007).

One part of the cortex in particular is much larger in humans than in any other creature. That is the prefrontal cortex, a brain area right above the eyes that is called the executive of the brain because planning, prioritizing, and reflection occur there. It is the prefrontal cortex that, for instance, allows young children to begin to plan ahead as well as to think about experiences they have had, for instance deciding who they want at their birthday party or what they liked best about a summer trip. The pre-frontal cortex is very limited in infancy, begins to function in early childhood, and continues to develop for many more years (Johnson, 2010).

Speed of Thought

Most of the increases in brain weight after infancy are the result 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 5.4). Although myelination continues for years, the effects are especially apparent in early childhood (Silk & Wood, 2011).

FIGURE 5.4 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 adulthood are caused by gradual disappearance of myelin layers.

DR. DAVID FURNESS, KEELE UNIVERSITY/SCIENCE SOURCE

Speed of thought from axon to neuron becomes pivotal when several thoughts and actions must occur in rapid succession. By age 6, most children can see an object and immediately name it, catch a ball and throw it, and write their ABCs in proper sequence, to name a few accomplishments. 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 experience, which slows information processing.

Impulsiveness and PerseverationThe young child’s inability to speedily combine thoughts is evident when both action and reflection are needed. Neurons have only two kinds of impulses: on–off, or activate–inhibit. Each is signalled by biochemical messages from dendrites to axons to neurons. A balance of activation and inhibition is needed for thoughtful children, who neither leap too quickly nor hesitate too long.

Many young children have not yet found the balance. They are impulsive, going 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 told to sit in place, not talking or touching anyone else. Others persevere in, or stick to, one thought or action, often repeating the behaviours at inappropriate times, called perseveration. Impulsiveness and perseveration are opposite manifestations of a prefrontal cortex that is responsible for action control such as planning, coordinating action sequences, or focusing on mental goals (Perner & Lang, 2002; Verbruggen & Logan, 2008).

Shapes and ColoursPerseveration gradually declines in every child. Consider a series of experiments in which 3-year-olds consistently make a mistake that they will no longer make by age 5. Children are 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 do this correctly. Then they are asked to “play the colour game,” sorting the cards by colour. Most fail. Instead they sort by shape, as they had done before. This basic test has been replicated in many nations; 3-year-olds usually perseverate, getting stuck in their initial sorting pattern.

When this result was first obtained, researchers thought that 3-year-olds might not know colours. To test this possibility, some other 3-year-olds were asked to sort by colour. Most did that correctly. Then, when asked to play “the shape game,” they still sorted by colour. Even with a new set of cards, such as yellow and green or rabbits and boats, 3-year-olds sort as they did originally, either by colour or shape. Most 5-year-olds can make the switch.

Researchers are looking into many possible explanations for this result (Marcovitch et al., 2010; Müller et al., 2006; Yerys & Munakata, 2006). All agree, however, that something in the brain matures between ages 3 and 5 to enable children to switch their way of sorting objects.

Connecting Hemispheres

One part of the brain that grows and myelinates rapidly during early childhood is the corpus callosum, a long, thick band of nerve fibres that connects the left and right sides of the brain. Growth of the corpus callosum makes communication between hemispheres more efficient, allowing children to coordinate the two sides of the brain or body. Failure of the corpus callosum to mature results in serious disorders; this is one of several possible causes of autism (Frazier & Hardan, 2009), discussed in Chapter 7.

The two sides of the body and of the brain are not identical. 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; the brain’s right half controls the body’s left side, with areas dedicated to emotions, creativity, and appreciation of music, art, and poetry. This left–right distinction has been exaggerated, however (Hugdahl & Westerhausen, 2010), as both sides of the brain are involved in almost every skill. Nonetheless, each side specializes, being dominant for certain functions—the result of lateralization, literally, “sidedness,” which advances with maturation of the corpus callosum (Boles et al., 2008). Lateralization is genetic and present at birth, but practice and time are needed before children can efficiently coordinate hands, feet, ears, and so on.

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

STOCKBYTE/GETTY IMAGES

Some research a decade ago found that, relatively speaking, the corpus callosum was thicker in females than in males, a finding that led to speculation about women’s superior emotional understanding. However, research using more advanced techniques now finds that this sex difference is far from universal. Some individual males and females have notably thicker corpus callosa than others, but gender does not seem relevant (Savic, 2010).

Although gender does not seem to affect thickness of the corpus callosum, handedness might. Left-handed people tend to have thicker corpus callosa than right-handed people, perhaps because they need to vary 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 that is easier, but they shake hands with their right hand because that is what the social convention requires. Left-handed children need to learn when to use their non-dominant hand—with scissors that are not specially designed for left-handed people, for instance.

Often cultures assume everyone should be right-handed, an example of the difference-equals-deficit error. For example, many letters and languages are written from left to right, which is easier for right-handed people. In some Asian and African cultures, it is an insult to give someone anything with the left hand. Fortunately, acceptance of left-handedness is more widespread now than a century ago. About 13 percent of adults in Canada and 10 percent of adults in Great Britain and the United States now identify themselves as left-handed, compared to only 3 percent in 1900 (McManus et al., 2010).

Developmentalists advise against switching a left-handed child, not only because this causes adult–child conflicts and brain confusion, but also because left-handed people may have an advantage in creativity and rapid use of the entire brain. Dan Aykroyd, Michelangelo, Marshall McLuhan, Marie Curie, Jimi Hendrix, Celine Dion, Paul McCartney, Chris Bosch, and Sidney Crosby, as well as four of the past six U.S. presidents (Ronald Reagan, Jimmy Carter, Bill Clinton, and Barack Obama) were or are lefties. However, this link of left-handedness and creativity is still up for debate and does not discount that right-handed individuals are also creative.

Emotions and the Brain

Now that we have considered the prefrontal cortex and the corpus callosum, we turn to the major brain region for emotions, sometimes called the limbic system. Emotional expression and emotional regulation advance during early childhood (more about that in the next chapter), and crucial to that advance are three major areas of the limbic system—the amygdala, the hippocampus, and the hypothalamus.

The amygdala is a tiny structure deep in the brain, named after the Greek word for almond because it is about the same shape and size. It registers emotions, both positive and negative, especially fear (Kolb & Whishaw, 2008). Increased amygdala activity is one reason some young children have terrifying nightmares or sudden terrors, overwhelming the prefrontal cortex and disrupting reason. Children may refuse to enter an elevator, or will hide when they hear thunder. Specifics depend on the child’s innate temperament as well as on past social experiences (Tarullo et al., 2011).

Another structure in the brain’s limbic system, the hippocampus, is 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. Memories of location are fragile in early childhood because the hippocampus is still developing.

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 will stroll nonchalantly past monkeys—something no normal cat would do (Kolb & Whishaw, 2008). With humans, if the amygdala is less connected to the other parts of the brain, young children are likely to be depressed, presumably because emotions are more overwhelming when the rest of the brain is disengaged (Luking et al., 2011).

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 and other hormones that activate parts of the brain and body (see Figure 5.3). Ideally, this hormone production occurs in moderation (Tarullo & Gunnar, 2006).

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.

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As the limbic system develops, young children watch their parents’ emotions closely (the social referencing described in Chapter 4). 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’s amygdala may become hypersensitive to elevators, as fear joins the hippocampus in remembering a specific location, and the result is increased cortisol. If, instead, the parent seems calm and makes elevator riding fun (letting the child push the buttons, for instance), the child will overcome initial feelings of fear, and the child might run to the elevator, happily pushing buttons.

Knowing the varieties of fears and joys is helpful if 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, 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.