The Price of High Plasticity in the Young Brain

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chapter 4 Physical Development in Infancy and Toddlerhood

Infants acquire new motor skills by building on previously acquired capacities. Eager to explore her world, this baby practices the art of crawling. Once she can fully move on her own, she will make dramatic strides in understanding her surroundings.

chapter outline

·   Body Growth

·   Changes in Body Size and Muscle–Fat Makeup

·   Individual and Group Differences

·   Changes in Body Proportions

·   Brain Development

·   Development of Neurons

·   Neurobiological Methods

·   Development of the Cerebral Cortex

·   Sensitive Periods in Brain Development

·   Changing States of Arousal

· ■  BIOLOGY AND ENVIRONMENT  Brain Plasticity: Insights from Research on Brain-Damaged Children and Adults

· ■  CULTURAL INFLUENCES  Cultural Variation in Infant Sleeping Arrangements

·   Influences on Early Physical Growth

·   Heredity

·   Nutrition

·   Malnutrition

·   Learning Capacities

·   Classical Conditioning

·   Operant Conditioning

·   Habituation

·   Imitation

·   Motor Development

·   The Sequence of Motor Development

·   Motor Skills as Dynamic Systems

·   Fine-Motor Development: Reaching and Grasping

·   Perceptual Development

·   Hearing

·   Vision

·   Intermodal Perception

·   Understanding Perceptual Development

· ■  BIOLOGY AND ENVIRONMENT  “Tuning In” to Familiar Speech, Faces, and Music: A Sensitive Period for Culture-Specific Learning

On a brilliant June morning, 16-month-old Caitlin emerged from her front door, ready for the short drive to the child-care home where she spent her weekdays while her mother, Carolyn, and her father, David, worked. Clutching a teddy bear in one hand and her mother’s arm with the other, Caitlin descended the steps. “One! Two! Threeee!” Carolyn counted as she helped Caitlin down. “How much she’s changed,” Carolyn thought to herself, looking at the child who, not long ago, had been a newborn. With her first steps, Caitlin had passed from infancy to toddlerhood—a period spanning the second year of life. At first, Caitlin did, indeed, “toddle” with an awkward gait, tipping over frequently. But her face reflected the thrill of conquering a new skill.

As they walked toward the car, Carolyn and Caitlin spotted 3-year-old Eli and his father, Kevin, in the neighboring yard. Eli dashed toward them, waving a bright yellow envelope. Carolyn bent down to open the envelope and took out a card. It read, “Announcing the arrival of Grace Ann. Born: Cambodia. Age: 16 months.” Carolyn turned to Kevin and Eli. “That’s wonderful news! When can we see her?”

“Let’s wait a few days,” Kevin suggested. “Monica’s taken Grace to the doctor this morning. She’s underweight and malnourished.” Kevin described Monica’s first night with Grace in a hotel room in Phnom Penh. Grace lay on the bed, withdrawn and fearful. Eventually she fell asleep, gripping crackers in both hands.

Carolyn felt Caitlin’s impatient tug at her sleeve. Off they drove to child care, where Vanessa had just dropped off her 18-month-old son, Timmy. Within moments, Caitlin and Timmy were in the sandbox, shoveling sand into plastic cups and buckets with the help of their caregiver, Ginette.

A few weeks later, Grace joined Caitlin and Timmy at Ginette’s child-care home. Although still tiny and unable to crawl or walk, she had grown taller and heavier, and her sad, vacant gaze had given way to an alert expression, a ready smile, and an enthusiastic desire to imitate and explore. When Caitlin headed for the sandbox, Grace stretched out her arms, asking Ginette to carry her there, too. Soon Grace was pulling herself up at every opportunity. Finally, at age 18 months, she walked!

This chapter traces physical growth during the first two years—one of the most remarkable and busiest times of development. We will see how rapid changes in the infant’s body and brain support learning, motor skills, and perceptual capacities. Caitlin, Grace, and Timmy will join us along the way to illustrate individual differences and environmental influences on physical development.

image1 Body Growth

TAKE A MOMENT…  The next time you’re walking in your neighborhood park or at the mall, note the contrast between infants’ and toddlers’ physical capabilities. One reason for the vast changes in what children can do over the first two years is that their bodies change enormously—faster than at any other time after birth.

Changes in Body Size and Muscle–Fat Makeup

By the end of the first year, a typical infant’s height is about 32 inches—more than 50 percent greater than at birth. By 2 years, it is 75 percent greater (36 inches). Similarly, by 5 months of age, birth weight has doubled, to about 15 pounds. At 1 year it has tripled, to 22 pounds, and at 2 years it has quadrupled, to about 30 pounds.

FIGURE 4.1 Body growth during the first two years.

These photos depict the dramatic changes in body size and proportions during infancy and toddlerhood in two individuals—a boy, Chris, and a girl, Mai. In the first year, the head is quite large in proportion to the rest of the body, and height and weight gain are especially rapid. During the second year, the lower portion of the body catches up. Notice, also, how both children added “baby fat” in the early months of life and then slimmed down, a trend that continues into middle childhood.

Figure 4.1  illustrates this dramatic increase in body size. But rather than making steady gains, infants and toddlers grow in little spurts. In one study, children who were followed over the first 21 months of life went for periods of 7 to 63 days with no growth, then added as much as half an inch in a 24-hour period! Almost always, parents described their babies as irritable and very hungry on the day before the spurt (Lampl,  1993 ; Lampl, Veldhuis, & Johnson,  1992 ).

One of the most obvious changes in infants’ appearance is their transformation into round, plump babies by the middle of the first year. This early rise in “baby fat,” which peaks at about 9 months, helps the small infant maintain a constant body temperature. In the second year, most toddlers slim down, a trend that continues into middle childhood (Fomon & Nelson,  2002 ). In contrast, muscle tissue increases very slowly during infancy and will not reach a peak until adolescence. Babies are not very muscular; their strength and physical coordination are limited.

Individual and Group Differences

In infancy, girls are slightly shorter and lighter than boys, with a higher ratio of fat to muscle. These small sex differences persist throughout early and middle childhood and are greatly magnified at adolescence. Ethnic differences in body size are apparent as well. Grace was below the growth norms (height and weight averages for children her age). Early malnutrition contributed, but even after substantial catch-up, Grace—as is typical for Asian children—remained below North American norms. In contrast, Timmy is slightly above average, as African-American children tend to be (Bogin,  2001 ).

Children of the same age also differ in rate of physical growth; some make faster progress toward a mature body size than others. But current body size is not enough to tell us how quickly a child’s physical growth is moving along. Although Timmy is larger and heavier than Caitlin and Grace, he is not physically more mature. In a moment, you will see why.

The best estimate of a child’s physical maturity is skeletal age, a measure of bone development. It is determined by X-raying the long bones of the body to see the extent to which soft, pliable cartilage has hardened into bone, a gradual process that is completed in adolescence. When skeletal ages are examined, African-American children tend to be slightly ahead of Caucasian children at all ages, and girls are considerably ahead of boys. At birth, the sexes differ by about 4 to 6 weeks, a gap that widens over infancy and childhood (Tanner, Healy, & Cameron,  2001 ). This greater physical maturity may contribute to girls’ greater resistance to harmful environmental influences. As noted in  Chapter 2 , girls experience fewer developmental problems than boys and have lower infant and childhood mortality rates.

Changes in Body Proportions

As the child’s overall size increases, different parts of the body grow at different rates. Two growth patterns describe these changes. The first is the  cephalocaudal trend —from the Latin for “head to tail.” During the prenatal period, the head develops more rapidly than the lower part of the body. At birth, the head takes up one-fourth of total body length, the legs only one-third. Notice how, in  Figure 4.1 , the lower portion of the body catches up. By age 2, the head accounts for only one-fifth and the legs for nearly one-half of total body length.

In the second pattern, the  proximodistal trend , growth proceeds, literally, from “near to far”—from the center of the body outward. In the prenatal period, the head, chest, and trunk grow first, then the arms and legs, and finally the hands and feet. During infancy and childhood, the arms and legs continue to grow somewhat ahead of the hands and feet.

image2 Brain Development

At birth, the brain is nearer to its adult size than any other physical structure, and it continues to develop at an astounding pace throughout infancy and toddlerhood. We can best understand brain growth by looking at it from two vantage points: (1) the microscopic level of individual brain cells and (2) the larger level of the cerebral cortex, the most complex brain structure and the one responsible for the highly developed intelligence of our species.

Development of Neurons

The human brain has 100 to 200 billion  neurons , or nerve cells that store and transmit information, many of which have thousands of direct connections with other neurons. Unlike other body cells, neurons are not tightly packed together. Between them are tiny gaps, or  synapses , where fibers from different neurons come close together but do not touch (see  Figure 4.2 ). Neurons send messages to one another by releasing chemicals called  neurotransmitters , which cross the synapse.

FIGURE 4.2 Neurons and their connective fibers.

This photograph of several neurons, taken with the aid of a powerful microscope, shows the elaborate synaptic connections that form with neighboring cells.

FIGURE 4.3 Major milestones of brain development.

Formation of synapses is rapid during the first two years, especially in the auditory, visual, and language areas of the cerebral cortex. The prefrontal cortex undergoes more extended synaptic growth. In each area, overproduction of synapses is followed by synaptic pruning. The prefrontal cortex is among the last regions to attain adult levels of synaptic connections—in mid-to late adolescence. Myelination occurs at a dramatic pace during the first two years, more slowly through childhood, followed by an acceleration at adolescence and then a reduced pace in early adulthood. The multiple yellow lines indicate that the timing of myelination varies among different brain areas. For example, neural fibers myelinate over a longer period in the language areas, and especially in the prefrontal cortex, than in the visual and auditory areas.

(Adapted from Thompson & Nelson, 2001.)

The basic story of brain growth concerns how neurons develop and form this elaborate communication system.  Figure 4.3  summarizes major milestones of brain development. In the prenatal period, neurons are produced in the embryo’s primitive neural tube. From there, they migrate to form the major parts of the brain (see  Chapter 3  page 82 ). Once neurons are in place, they differentiate, establishing their unique functions by extending their fibers to form synaptic connections with neighboring cells. During the first two years, neural fibers and synapses increase at an astounding pace (Huttenlocher,  2002 ; Moore, Persaud, & Torchia,  2013 ). A surprising aspect of brain growth is  programmed cell death , which makes space for these connective structures: As synapses form, many surrounding neurons die—20 to 80 percent, depending on the brain region (de Haan & Johnson,  2003 ; Stiles,  2008 ). Fortunately, during the prenatal period, the neural tube produces far more neurons than the brain will ever need.

As neurons form connections, stimulation becomes vital to their survival. Neurons that are stimulated by input from the surrounding environment continue to establish synapses, forming increasingly elaborate systems of communication that support more complex abilities. At first, stimulation results in a massive overabundance of synapses, many of which serve identical functions, thereby ensuring that the child will acquire the motor, cognitive, and social skills that our species needs to survive. Neurons that are seldom stimulated soon lose their synapses, in a process called  synaptic pruning  that returns neurons not needed at the moment to an uncommitted state so they can support future development. In all, about 40 percent of synapses are pruned during childhood and adolescence to reach the adult level (Webb, Monk, & Nelson,  2001 ). For this process to advance, appropriate stimulation of the child’s brain is vital during periods in which the formation of synapses is at its peak (Bryk & Fisher,  2012 ).

If few new neurons are produced after the prenatal period, what causes the dramatic increase in brain size during the first two years? About half the brain’s volume is made up of  glial cells , which are responsible for  myelination , the coating of neural fibers with an insulating fatty sheath (called myelin) that improves the efficiency of message transfer. Glial cells multiply rapidly from the fourth month of pregnancy through the second year of life—a process that continues at a slower pace through middle childhood and accelerates again in adolescence. Gains in neural fibers and myelination are responsible for the extraordinary gain in overall size of the brain—from nearly 30 percent of its adult weight at birth to 70 percent by age 2 (Johnson,  2011 ; Knickmeyer et al.,  2008 ).

Brain development can be compared to molding a “living sculpture.” First, neurons and synapses are overproduced. Then, cell death and synaptic pruning sculpt away excess building material to form the mature brain—a process jointly influenced by genetically programmed events and the child’s experiences. The resulting “sculpture” is a set of interconnected regions, each with specific functions—much like countries on a globe that communicate with one another (Johnston et al.,  2001 ). This “geography” of the brain permits researchers to study its developing organization and the activity of its regions using neurobiological methods.

Neurobiological Methods

Table 4.1  describes major measures of brain functioning. The first two methods detect changes in electrical activity in the cerebral cortex. In an electroencephalogram (EEG), researchers examine brain-wave patterns for stability and organization—signs of mature functioning of the cortex. And as the person processes a particular stimulus, event-related potentials (ERPs) detect the general location of brain-wave activity—a technique often used to study preverbal infants’ responsiveness to various stimuli, the impact of experience on specialization of specific brain regions, and atypical brain functioning in individuals with learning and emotional problems (DeBoer, Scott, & Nelson,  2007 ; deRegnier,  2005 ).

Neuroimaging techniques, which yield detailed, three-dimensional computerized pictures of the entire brain and its active areas, provide the most precise information about which brain regions are specialized for certain capacities and about abnormalities in brain functioning. The most promising of these methods is functional magnetic resonance imaging (fMRI). Unlike positron emission tomography (PET), fMRI does not depend on X-ray photography, which requires injection of a radioactive substance. Rather, when an individual is exposed to a stimulus, fMRI detects changes in blood flow and oxygen metabolism throughout the brain magnetically, yielding a colorful, moving picture of parts of the brain used to perform a given activity (see  Figure 4.4a  b , and  c ).

TABLE 4.1 Methods for Measuring Brain Functioning

METHODDESCRIPTION
Electroencephalogram (EEG)Electrodes embedded in a head cap record electrical brain-wave activity in the brain’s outer layers—the cerebral cortex. Today, researchers use an advanced tool called a geodesic sensor net (GSN) to hold interconnected electrodes (up to 128 for infants and 256 for children and adults) in place through a cap that adjusts to each person’s head shape, yielding improved brain-wave detection.
Event-related potentials (ERPs)Using the EEG, the frequency and amplitude of brain waves in response to particular stimuli (such as a picture, music, or speech) are recorded in multiple areas of the cerebral cortex. Enables identification of general regions of stimulus-induced activity.
Functional magnetic resonance imaging (fMRI)While the person lies inside a tunnel-shaped apparatus that creates a magnetic field, a scanner magnetically detects increased blood flow and oxygen metabolism in areas of the brain as the individual processes particular stimuli. The scanner typically records images every 1 to 4 seconds; these are combined into a computerized moving picture of activity anywhere in the brain (not just its outer layers). Not appropriate for children younger than age 5 to 6, who cannot remain still during testing.
Positron emission tomography (PET)After injection or inhalation of a radioactive substance, the person lies on an apparatus with a scanner that emits fine streams of X-rays, which detect increased blood flow and oxygen metabolism in areas of the brain as the person processes particular stimuli. As with fMRI, the result is a computerized image of “online” activity anywhere in the brain. Not appropriate for children younger than age 5 to 6.
Near-infrared spectroscopy (NIRS)Using thin, flexible optical fibers attached to the scalp through a head cap, infrared (invisible) light is beamed at the brain; its absorption by areas of the cerebral cortex varies with changes in blood flow and oxygen metabolism as the individual processes particular stimuli. The result is a computerized moving picture of active areas in the cerebral cortex. Unlike fMRI and PET, NIRS is appropriate for infants and young children, who can move within limited range.

FIGURE 4.4 Functional magnetic resonance imaging (fMRI) and near-infrared spectroscopy (NIRS).

(a) This 6-year-old is part of a study that uses fMRI to find out how his brain processes light and motion. (b) The fMRI image shows which areas of the child’s brain are active while he views changing visual stimuli. (c) Here, NIRS is used to investigate a 2-month-old’s response to a visual stimulus. During testing, the baby can move freely within a limited range.

(Photo (c) from G. Taga, K. Asakawa, A. Maki, Y. Konishi, & H. Koisumi, 2003, “Brain Imaging in Awake Infants by Near-Infrared Optical Topography,” Proceedings of the National Academy of Sciences, 100, p. 10723. Reprinted by permission.)

Because PET and fMRI require that the participant lie as motionless as possible for an extended time, they are not suitable for infants and young children (Nelson, Thomas, & de Haan,  2006 ). A neuroimaging technique that works well in infancy and early childhood is near-infrared spectroscopy (NIRS), in which infrared (invisible) light is beamed at regions of the cerebral cortex to measure blood flow and oxygen metabolism while the child attends to a stimulus (refer again to  Table 4.1 ). Because the apparatus consists only of thin, flexible optical fibers attached to the scalp using a head cap, a baby can sit on the parent’s lap and move during testing—as  Figure 4.4c  illustrates (Hespos et al.,  2010 ). But unlike PET and fMRI, which map activity changes throughout the brain, NIRS examines only the functioning of the cerebral cortex.

Development of the Cerebral Cortex

The  cerebral cortex  surrounds the rest of the brain, resembling half of a shelled walnut. It is the largest brain structure, accounting for 85 percent of the brain’s weight and containing the greatest number of neurons and synapses. Because the cerebral cortex is the last part of the brain to stop growing, it is sensitive to environmental influences for a much longer period than any other part of the brain.

Regions of the Cerebral Cortex.

Figure 4.5  shows specific functions of regions of the cerebral cortex, such as receiving information from the senses, instructing the body to move, and thinking. The order in which cortical regions develop corresponds to the order in which various capacities emerge in the infant and growing child. For example, a burst of activity occurs in the auditory and visual cortexes and in areas responsible for body movement over the first year—a period of dramatic gains in auditory and visual perception and mastery of motor skills (Johnson,  2011 ). Language areas are especially active from late infancy through the preschool years, when language development flourishes (Pujol et al.,  2006 ; Thompson,  2000 ).

The cortical regions with the most extended period of development are the frontal lobes. The  prefrontal cortex , lying in front of areas controlling body movement, is responsible for thought—in particular, consciousness, inhibition of impulses, integration of information, and use of memory, reasoning, planning, and problem-solving strategies. From age 2 months on, the prefrontal cortex functions more effectively. But it undergoes especially rapid myelination and formation and pruning of synapses during the preschool and school years, followed by another period of accelerated growth in adolescence, when it reaches an adult level of synaptic connections (Nelson,  2002 ; Nelson, Thomas, & de Haan,  2006 ; Sowell et al.,  2002 ).

FIGURE 4.5 The left side of the human brain, showing the cerebral cortex.

The cortex is divided into different lobes, each containing a variety of regions with specific functions. Some major regions are labeled here.

Lateralization and Plasticity of the Cortex.

The cerebral cortex has two hemispheres, or sides, that differ in their functions. Some tasks are done mostly by the left hemisphere, others by the right. For example, each hemisphere receives sensory information from the side of the body opposite to it and controls only that side. *  For most of us, the left hemisphere is largely responsible for verbal abilities (such as spoken and written language) and positive emotion (such as joy). The right hemisphere handles spatial abilities (judging distances, reading maps, and recognizing geometric shapes) and negative emotion (such as distress) (Banish & Heller,  1998 ; Nelson & Bosquet,  2000 ). In left-handed people, this pattern may be reversed or, more commonly, the cerebral cortex may be less clearly specialized than in right-handers.

Why does this specialization of the two hemispheres, called  lateralization , occur? Studies using fMRI reveal that the left hemisphere is better at processing information in a sequential, analytic (piece-by-piece) way, a good approach for dealing with communicative information—both verbal (language) and emotional (a joyful smile). In contrast, the right hemisphere is specialized for processing information in a holistic, integrative manner, ideal for making sense of spatial information and regulating negative emotion. A lateralized brain may have evolved because it enabled humans to cope more successfully with changing environmental demands (Falk,  2005 ). It permits a wider array of functions to be carried out effectively than if both sides processed information exactly the same way.

*The eyes are an exception. Messages from the right half of each retina go to the right hemisphere; messages from the left half of each retina go to the left hemisphere. Thus, visual information from botheyes is received by both hemispheres.

Researchers study the timing of brain lateralization to learn more about  brain plasticity . A highly plastic cerebral cortex, in which many areas are not yet committed to specific functions, has a high capacity for learning. And if a part of the cortex is damaged, other parts can take over tasks it would have handled.But once the hemispheres lateralize, damage to a specific region means that the abilities it controls cannot be recovered to the same extent or as easily as earlier.

At birth, the hemispheres have already begun to specialize. Most newborns show greater activation (detected with either ERP or NIRS) in the left hemisphere while listening to speech sounds or displaying a positive state of arousal. In contrast, the right hemisphere reacts more strongly to nonspeech sounds and to stimuli (such as a sour-tasting fluid) that evoke negative emotion (Davidson,  1994 ; Fox & Davidson,  1986 ; Hespos et al.,  2010 ).

Nevertheless, research on brain-damaged children and adults offers dramatic evidence for substantial plasticity in the young brain, summarized in the  Biology and Environment  box on  page 126 . Furthermore, early experience greatly influences the organization of the cerebral cortex. For example, deaf adults who, as infants and children, learned sign language (a spatial skill) depend more than hearing individuals on the right hemisphere for language processing (Neville & Bavelier,  2002 ). And toddlers who are advanced in language development show greater left-hemispheric specialization for language than their more slowly developing agemates (Luna et al.,  2001 ; Mills et al.,  2005 ). Apparently, the very process of acquiring language and other skills promotes lateralization.

In sum, the brain is more plastic during the first few years than it will ever be again. An overabundance of synaptic connections supports brain plasticity, ensuring that young children will acquire certain capacities even if some areas are damaged. And although the cortex is programmed from the start for hemispheric specialization, experience greatly influences the rate and success of its advancing organization.

Sensitive Periods in Brain Development

Both animal and human studies reveal that early, extreme sensory deprivation results in permanent brain damage and loss of functions—findings that verify the existence of sensitive periods in brain development. For example, early, varied visual experiences must occur for the brain’s visual centers to develop normally. If a 1-month-old kitten is deprived of light for just three or four days, these areas of the brain degenerate. If the kitten is kept in the dark during the fourth week of life and beyond, the damage is severe and permanent (Crair, Gillespie, & Stryker,  1998 ). And the general quality of the early environment affects overall brain growth. When animals reared from birth in physically and socially stimulating surroundings are compared with those reared under depleted conditions, the brains of the stimulated animals are larger and heavier and show much denser synaptic connections (Sale, Berardi, & Maffei,  2009 ).

Human Evidence: Victims of Deprived Early Environments.

For ethical reasons, we cannot deliberately deprive some infants of normal rearing experiences and observe the impact on their brains and competencies. Instead, we must turn to natural experiments, in which children were victims of deprived early environments that were later rectified. Such studies have revealed some parallels with the animal evidence just described.

For example, when babies are born with cataracts (clouded lenses, preventing clear visual images) in both eyes, those who have corrective surgery within four to six months show rapid improvement in vision, except for subtle aspects of face perception, which require early visual input to the right hemisphere to develop (Le Grand et al.,  2003 ; Maurer, Mondloch, & Lewis,  2007 ). The longer cataract surgery is postponed beyond infancy, the less complete the recovery in visual skills. And if surgery is delayed until adulthood, vision is severely and permanently impaired (Lewis & Maurer,  2005 ).

Studies of infants placed in orphanages who were later exposed to ordinary family rearing confirm the importance of a generally stimulating physical and social environment for psychological development. In one investigation, researchers followed the progress of a large sample of children transferred between birth and 3½ years from extremely deprived Romanian orphanages to adoptive families in Great Britain (Beckett et al.,  2006 ; O’Connor et al.,  2000 ; Rutter et al.,  1998  2004  2010 ). On arrival, most were impaired in all domains of development. Cognitive catch-up was impressive for children adopted before 6 months, who attained average mental test scores in childhood and adolescence, performing as well as a comparison group of early-adopted British-born children.

These children in an orphanage in Romania receive little adult contact or stimulation. The longer they remain in this barren environment, the more likely they are to display profound impairments in all domains of development.

But Romanian children who had been institutionalized for more than the first six months showed serious intellectual deficits (see  Figure 4.6 ). Although they improved in test scores during middle childhood and adolescence, they remained substantially below average. And most displayed at least three serious mental health problems, such as inattention, overactivity, unruly behavior, and autistic-like symptoms (social disinterest, stereotyped behavior) (Kreppner et al.,  2007  2010 ).

Biology and Environment Brain Plasticity: Insights from Research on Brain-Damaged Children and Adults

This preschooler, who experienced brain damage in infancy, has been spared massive impairments because of early, high brain plasticity. A teacher guides his hand in drawing shapes to strengthen spatial skills, which are more impaired than language.

In the first few years of life, the brain is highly plastic. It can reorganize areas committed to specific functions in ways that the mature brain cannot. Consistently, adults who suffered brain injuries in infancy and early childhood show fewer cognitive impairments than adults with later-occurring injuries (Holland,  2004 ; Huttenlocher,  2002 ). Nevertheless, the young brain is not totally plastic. When it is injured, its functioning is compromised. The extent of plasticity depends on several factors, including age at time of injury, site of damage, and skill area. Furthermore, plasticity is not restricted to childhood. Some reorganization after injury also occurs in the mature brain.

Brain Plasticity in Infancy and Early Childhood

In a large study of children with injuries to the cerebral cortex that occurred before birth or in the first six months of life, language and spatial skills were assessed repeatedly into adolescence (Akshoomoff et al.,  2002 ; Stiles,  2001a ; Stiles et al.,  2005  2008 ). All the children had experienced early brain seizures or hemorrhages. Brain-imaging techniques (fMRI and PET) revealed the precise site of damage.

Regardless of whether injury occurred in the left or right cerebral hemisphere, the children showed delays in language development that persisted until about 3½ years of age. That damage to either hemisphere affected early language competence indicates that at first, language functioning is broadly distributed in the brain. But by age 5, the children caught up in vocabulary and grammatical skills. Undamaged areas—in either the left or the right hemisphere—had taken over these language functions.

Compared with language, spatial skills were more impaired after early brain injury. When preschool through adolescent-age youngsters were asked to copy designs, those with early right-hemispheric damage had trouble with holistic processing—accurately representing the overall shape. In contrast, children with left-hemispheric damage captured the basic shape but omitted fine-grained details. Nevertheless, the children improved in drawing skills with age—gains that do not occur in brain-injured adults (Akshoomoff et al.,  2002 ; Stiles et al.,  2003  2008 ).

Clearly, recovery after early brain injury is greater for language than for spatial skills. Why is this so? Researchers speculate that spatial processing is the older of the two capacities in our evolutionary history and, therefore, more lateralized at birth (Stiles,  2001b ; Stiles et al.,  2002  2008 ). But early brain injury has far less impact than later injury on both language and spatial skills. In sum, the young brain is remarkably plastic.

The Price of High Plasticity in the Young Brain

Despite impressive recovery of language and (to a lesser extent) spatial skills, children with early brain injuries show deficits in a wide range of complex mental abilities during the school years. For example, their reading and math progress is slow. And in telling stories, they produce simpler narratives than agemates without early brain injuries (although many catch up in narrative skills by early adolescence) (Reilly, Bates, & Marchman,  1998 ; Reilly et al.,  2004 ). Furthermore, the more brain tissue destroyed in infancy or early childhood, the poorer children score on intelligence tests (Anderson et al.,  2006 ).

High brain plasticity, researchers explain, comes at a price. When healthy brain regions take over the functions of damaged areas, a “crowding effect” occurs: Multiple tasks must be done by a smaller-than-usual volume of brain tissue (Stiles,  2012 ). Consequently, the brain processes information less quickly and accurately than it would if it were intact. Complex mental abilities of all kinds suffer into middle childhood, and often longer, because performing them well requires considerable space in the cerebral cortex.

Brain Plasticity in Adulthood

Brain plasticity is not restricted to early childhood. Though far more limited, reorganization in the brain can occur later, even in adulthood. For example, adult stroke victims often display considerable recovery, especially in response to stimulation of language and motor skills. Brain-imaging techniques reveal that structures adjacent to the permanently damaged area or in the opposite cerebral hemisphere reorganize to support the impaired ability (Kalra & Ratan,  2007 ; Murphy & Corbett,  2009 ).

In infancy and childhood, the goal of brain growth is to form neural connections that ensure mastery of essential skills. Animal research reveals that plasticity is greatest while the brain is forming many new synapses; it declines during synaptic pruning (Murphy & Corbett,  2009 ). At older ages, specialized brain structures are in place, but after injury they can still reorganize to some degree. The adult brain can produce a small number of new neurons. And when an individual practices relevant tasks, the brain strengthens existing synapses and generates new ones (Nelson, Thomas, & de Haan,  2006 ).

Plasticity seems to be a basic property of the nervous system. Researchers hope to discover how experience and brain plasticity work together throughout life, so they can help people of all ages—with and without brain injuries—develop at their best.

FIGURE 4.6 Relationship of age at adoption to mental test scores at ages 6 and 11 among British and Romanian adoptees.

Children transferred from Romanian orphanages to British adoptive homes in the first six months of life attained average scores and fared as well as British early-adopted children, suggesting that they had fully recovered from extreme early deprivation. Romanian children adopted after 6 months of age performed well below average. And although those adopted after age 2 improved between ages 6 and 11, they continued to show serious intellectual deficits.

(Adapted from Beckett et al., 2006.)

Neurobiological findings indicate that early, prolonged institutionalization leads to a generalized decrease in activity in the cerebral cortex, especially the prefrontal cortex, which governs complex cognition and impulse control. Neural fibers connecting the prefrontal cortex with other brain structures involved in control of emotion are also reduced (Eluvathingal et al.,  2006 ; Nelson,  2007b ). And activation of the left cerebral hemisphere, governing positive emotion, is diminished relative to right cerebral activation, governing negative emotion (McLaughlin et al.,  2011 ).

Additional evidence confirms that the chronic stress of early, deprived orphanage rearing disrupts the brain’s capacity to manage stress, with long-term physical and psychological consequences. In another investigation, researchers followed the development of children who had spent their first eight months or more in Romanian institutions and were then adopted into Canadian homes (Gunnar et al.,  2001 ; Gunnar & Cheatham,  2003 ). Compared with agemates adopted shortly after birth, these children showed extreme stress reactivity, as indicated by high concentrations of the stress hormone cortisol in their saliva—a physiological response linked to persistent illness, retarded physical growth, and learning and behavior problems, including deficits in attention and control of anger and other impulses. The longer the children spent in orphanage care, the higher their cortisol levels—even 6½ years after adoption. In other investigations, orphanage children displayed abnormally low cortisol—a blunted physiological stress response that may be the central nervous system’s adaptation to earlier, frequent cortisol elevations (Loman & Gunnar,  2010 ).

Appropriate Stimulation.

Unlike the orphanage children just described, Grace, whom Monica and Kevin had adopted in Cambodia at 16 months of age, showed favorable progress. Two years earlier, they had adopted Grace’s older brother, Eli. When Eli was 2 years old, Monica and Kevin sent a letter and a photo of Eli to his biological mother, describing a bright, happy child. The next day, the Cambodian mother tearfully asked an adoption agency to send her baby daughter to join Eli and his American family. Although Grace’s early environment was very depleted, her biological mother’s loving care—holding gently, speaking softly, playfully stimulating, and breastfeeding—may have prevented irreversible damage to her brain.

In the Bucharest Early Intervention Project, about 200 institutionalized Romanian babies were randomized into conditions of either care as usual or transfer to high-quality foster families between ages 5 and 30 months. Specially trained social workers provided foster parents with counseling and support. Follow-ups between 2½ and 4 years revealed that the foster-care group exceeded the institutional-care group in intelligence test scores, language skills, emotional responsiveness, and EEG and ERP assessments of brain activity (Nelson et al.,  2007 ; Smyke et al.,  2009 ). On all measures, the earlier the foster placement, the better the outcome. But consistent with an early sensitive period, the foster-care group remained behind never-institutionalized agemates living with Bucharest families.

In addition to impoverished environments, ones that overwhelm children with expectations beyond their current capacities interfere with the brain’s potential. In recent years, expensive early learning centers have sprung up, in which infants are trained with letter and number flash cards and slightly older toddlers are given a full curriculum of reading, math, science, art, gym, and more. There is no evidence that these programs yield smarter “superbabies” (Hirsh-Pasek & Golinkoff,  2003 ). To the contrary, trying to prime infants with stimulation for which they are not ready can cause them to withdraw, thereby threatening their interest in learning and creating conditions much like stimulus deprivation!

How, then, can we characterize appropriate stimulation during the early years? To answer this question, researchers distinguish between two types of brain development. The first,  experience-expectant brain growth , refers to the young brain’s rapidly developing organization, which depends on ordinary experiences—opportunities to explore the environment, interact with people, and hear language and other sounds. As a result of millions of years of evolution, the brains of all infants, toddlers, and young children expect to encounter these experiences and, if they do, grow normally. The second type of brain development,  experience-dependent brain growth , occurs throughout our lives. It consists of additional growth and refinement of established brain structures as a result of specific learning experiences that vary widely across individuals and cultures (Greenough & Black,  1992 ). Reading and writing, playing computer games, weaving an intricate rug, and practicing the violin are examples. The brain of a violinist differs in certain ways from the brain of a poet because each has exercised different brain regions for a long time.

Experience-expectant brain growth occurs early and naturally, as caregivers offer babies and preschoolers age-appropriate play materials and engage them in enjoyable daily routines—a shared meal, a game of peekaboo, a bath before bed, a picture book to talk about, or a song to sing. The resulting growth provides the foundation for later-occurring, experience-dependent development (Huttenlocher,  2002 ; Shonkoff & Phillips,  2001 ). No evidence exists for a sensitive period in the first five or six years for mastering skills that depend on extensive training, such as reading, musical performance, or gymnastics. To the contrary, rushing early learning harms the brain by overwhelming its neural circuits, thereby reducing the brain’s sensitivity to the everyday experiences it needs for a healthy start in life.

Experience-expectant brain growth occurs naturally, through ordinary, stimulating experiences. This toddler exploring a mossy log enjoys the type of activity that best promotes brain development in the early years.

Changing States of Arousal

Rapid brain growth means that the organization of sleep and wakefulness changes substantially between birth and 2 years, and fussiness and crying also decline. The newborn baby takes round-the-clock naps that total about 16 to 18 hours (Davis, Parker & Montgomery,  2004 ). Total sleep time declines slowly; the average 2-year-old still needs 12 to 13 hours. But periods of sleep and wakefulness become fewer and longer, and the sleep–wake pattern increasingly conforms to a night–day schedule. Most 6- to 9-month-olds take two daytime naps; by about 18 months, children generally need only one nap. Finally, between ages 3 and 5, napping subsides (Iglowstein et al.,  2003 ).

These changing arousal patterns are due to brain development, but they are also affected by cultural beliefs and practices and individual parents’ needs (Super & Harkness,  2002 ). Dutch parents, for example, view sleep regularity as far more important than the U.S. parents do. And whereas U.S. parents regard a predictable sleep schedule as emerging naturally from within the child, Dutch parents believe that a schedule must be imposed, or the baby’s development might suffer (Super et al.,  1996 ; Super & Harkness,  2010 ). At age 6 months, Dutch babies are put to bed earlier and sleep, on average, 2 hours more per day than their U.S. agemates.

Motivated by demanding work schedules and other needs, many Western parents try to get their babies to sleep through the night as early as 3 to 4 months by offering an evening feeding—a practice that may be at odds with young infants’ neurological capacities. Not until the middle of the first year is the secretion of melatonin, a hormone within the brain that promotes drowsiness, much greater at night than during the day (Sadeh,  1997 ).

Furthermore, as the Cultural Influences box on the following page reveals, isolating infants to promote sleep is rare elsewhere in the world. When babies sleep with their parents, their average sleep period remains constant at three hours from 1 to 8 months of age. Only at the end of the first year, as REM sleep (the state that usually prompts waking) declines, do infants move in the direction of an adultlike sleep–waking schedule (Ficca et al.,  1999 ).

Even after infants sleep through the night, they continue to wake occasionally. In studies carried out in Australia, Israel, and the United States, night wakings increased around 6 months and again between 1½ and 2 years and then declined (Armstrong, Quinn, & Dadds,  1994 ; Scher, Epstein, & Tirosh,  2004 ; Scher et al.,  1995 ). As  Chapter 6  will reveal, around the middle of the first year, infants are forming a clear-cut attachment to their familiar caregiver and begin protesting when he or she leaves. And the challenges of toddlerhood—the ability to range farther from the caregiver and increased awareness of the self as separate from others—often prompt anxiety, evident in disturbed sleep and clinginess. When parents offer comfort, these behaviors subside.

LOOK AND LISTEN

Interview a parent of a baby about sleep challenges. What strategies has the parent tried to ease these difficulties? Are the techniques likely to be effective, in view of evidence on infant sleep development?

Cultural Influences Cultural Variation in Infant Sleeping Arrangements

This Vietnamese mother and child sleep together—a practice common in their culture and around the globe. Hard wooden sleeping surfaces protect cosleeping children from entrapment in soft bedding.

Western child-rearing advice from experts strongly encourages nighttime separation of baby from parent. For example, the most recent edition of Benjamin Spock’s Baby and Child Care recommends that babies sleep in their own room by 3 months of age, explaining, “By 6 months, a child who regularly sleeps in her parents’ room may feel uneasy sleeping anywhere else” (Spock & Needlman,  2012 , p. 62). And the American Academy of Pediatrics ( 2012 ) has issued a controversial warning that parent–infant bedsharing may increase the risk of sudden infant death syndrome (SIDS).

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