The newborn's capacity to imitate

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

METHOD

DESCRIPTION

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.