CDC Milestone App

As you learned this week, milestones play an important role in the early years of a child’s development. While we know that children grow and develop at their own pace, milestones show us that a child is developing as expected. This week, you will continue to support Paul and Amy and introduce them to an important app.

To prepare for this discussion,

· Read Chapter 5: Physical Development in Infancy and Toddlerhood

· Watch To Walk CDE 119 Full Links to an external site.

· Download Android’s CDC Milestone Tracker Links to an external site. or iPhone’s CDC’s Milestone Tracker Links to an external site. .

· Set up a profile in your CDC Milestone App by using Navigating the CDC Milestone Tracker App guide Links to an external site. .

For this discussion, you will utilize the case study below:

Case Study

Paul and Amy have been loving the first two months of parenthood with their daughter, Charlie. At Charlie’s two-month well-visit, they asked many questions about Charlie’s physical development. Their pediatrician suggested that Paul and Amy download the CDC milestone tracker app so they could keep track of Charlie’s development and see how she is doing with mastering milestones.

In your initial post, assume the role of Paul or Amy and provide a review of the CDC Milestone Tracker app that includes the following:

· Rate how you like using the app on a scale of 1 to 5 (one being you would not recommend and a five being a superior rating). Provide at least two reasons to support your rating.

· Explain at least two elements of the app that you think are helpful for tracking physical development from 2 months to 2 years.

· Discuss how you might use this app to ensure you are providing developmentally appropriate learning activities for children from 2 months to 2 years.

CHAPTER 5 PHYSICAL DEVELOPMENT IN INFANCY AND TODDLERHOOD

Mother

Shang Meng Lei, 9 years, China

This painting captures a mother and her infant affectionately imitating each other’s facial expressions and gestures. During the first year, infants grow rapidly, move on their own, increasingly investigate and make sense of their surroundings, and learn from others.

Reprinted with permission from The International Museum of Children’s Art, Oslo, Norway

WHAT’S AHEAD IN CHAPTER 5

5.1 Body Growth

Changes in Body Size and Muscle–Fat Makeup • Changes in Body Proportions • Individual and Group Differences

5.2 Brain Development

Development of Neurons • Measures of Brain Functioning • Development of the Cerebral Cortex • Sensitive Periods in Brain Development • Changing States of Arousal

■ Biology and Environment: Brain Plasticity: Insights from Research on Children with Brain Injury

■ Cultural Influences: Cultural Variation in Infant Sleeping Arrangements

5.3 Influences on Early Physical Growth

Heredity • Nutrition • Malnutrition • Emotional Well-Being

■ Social Issues: Health: Lead Exposure and Children’s Development

5.4 Learning Capacities

Classical Conditioning • Operant Conditioning • Habituation • Statistical Learning • Imitation

5.5 Motor Development

The Sequence of Motor Development • Motor Skills as Dynamic Systems • Fine-Motor Development: Reaching and Grasping

5.6 Perceptual Development

Hearing • Vision • Object Perception • 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! Threeeee!” 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 toward 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 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 how individual differences and environmental influences affect physical development. ■

5.1 BODY GROWTH

5.1 Describe major changes in body growth over the first two years.

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.

5.1.1 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, averaging 36 inches. Similarly, by age 5 months, 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 5.1 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, very hungry, and sleeping more on the days before a spurt (Lampl, 1993; Lampl & Johnson, 2011).

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 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.

Figure 5.1 Body growth during the first two years. These photos depict the dramatic changes in body size and proportions during infancy and toddlerhood in two children—a boy, Shanwel, 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 how both children added “baby fat” in the early months of life and then slimmed down, a trend that continues into middle childhood.

ALL PHOTOS © LAURA DWIGHT PHOTOGRAPHY

5.1.2 Changes in Body Proportions

As the child’s overall size increases, 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 5.1, 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.

5.1.3 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 in size, as African-American children tend to be (Bogin, 2001).

Children of the same age differ in rate of physical growth; some make faster progress toward 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 European-American and Hispanic children of the same chronological age. And girls are considerably ahead of boys—the reason Timmy’s skeletal age lags behind that of Caitlin and Grace. At birth, the sexes differ by about 4 to 6 weeks, a gap that widens over infancy and childhood (McCormack et al., 2017; Tanner, Healy, & Cameron, 2001). Girls are advanced in development of other organs as well. 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.

5.2 BRAIN DEVELOPMENT

5.2a Describe brain development during infancy and toddlerhood, current methods of measuring brain functioning, and appropriate stimulation to support the brain’s potential.

5.2b Explain how the organization of sleep and wakefulness changes over the first two years.

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.

5.2.1 Development of Neurons

The typical adult 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 5.2). Neurons send messages to one another by releasing chemicals called neurotransmitters, which cross synapses.

The basic story of brain growth concerns how neurons develop and form this elaborate communication system. Figure 5.3 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 page 94 in Chapter 3). 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 (Budday, Steinmann, & Kuhl, 2015; Moore, Persaud, & Torchia, 2016). A surprising aspect of brain growth is programmed cell death, which makes space for these connective structures: As synapses form, many surrounding neurons die—40 to 60 percent, depending on the brain region (Jabès & Nelson, 2014). 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 new 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. At the same time, pruning allows for rearrangement and strengthening of remaining synapses, which fine-tunes neural circuitry and is essential for effective information processing. In all, about 50 percent of synapses are pruned during childhood and adolescence (Jiang & Nardelli, 2016). 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.

Figure 5.2 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 5.3 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, responsible for complex thought (see page 159), 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. 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 auditory and visual areas. (Based on Thompson & Nelson, 2001.)

If few neurons are produced after the prenatal period, what causes the extraordinary 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. Certain types of glial cells also participate directly in neural communication, by picking up and passing on neuronal signals and releasing neurotransmitters. 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 account for the overall increase in size of the brain, from nearly 30 percent of its adult weight at birth to 70 percent by age 2 (Budday, Steinmann, & Kuhl, 2015; Terni, López-Murcia, & Llobet, 2017). Growth is especially rapid during the first year, when the brain more than doubles in size.

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 organization and the activity of its regions using neurobiological techniques.

Figure 5.4 Electroencephalogram (EEG) using the geodesic sensor net (GSN). Interconnected electrodes embedded in the head cap record electrical brain-wave activity in the cerebral cortex.

OLI SCARFF/GETTY IMAGES

5.2.2 Measures of Brain Functioning

Table 5.1 describes major measures of brain functioning. Among these methods, the two most frequently used 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 (see Figure 5.4). As a child processes a particular stimulus, event-related potentials (ERPs) can 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 regions of the cortex, and atypical brain functioning in children at risk for learning and emotional problems (DeBoer, Scott, & Nelson, 2007; Gunnar & de Haan, 2009).

Table 5.1 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. 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 high-quality 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 child 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 activity anywhere in the brain. Also like fMRI, 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 during testing.

Figure 5.5 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.)

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. In positron emission tomography (PET), the child must lie quietly on a scanner bed, and in functional magnetic resonance imaging (f MRI), inside a tunnel-like apparatus. But unlike PET, fMRI does not depend on X-ray photography, which requires injection of a radioactive substance. Rather, when a child 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 5.5a and b).

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. A neuroimaging technique that works well in infancy and early childhood is near infrared spectroscopy (NIRS) (refer again to Table 5.1). 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 5.5c illustrates. But unlike PET and fMRI, which map activity changes throughout the brain, NIRS examines only the functioning of the cerebral cortex.

The measures just reviewed are powerful tools for uncovering relationships between the brain and psychological development. But like all research methods, they have limitations. Even though a stimulus produces a consistent pattern of brain activity, investigators cannot be certain that an individual has processed it in a certain way (Kagan, 2013b). And a researcher who takes a change in brain activity as an indicator of information processing must make sure that the change was not due instead to hunger, boredom, fatigue, or body movements. Consequently, other methods—both observations and self-reports—must be combined with brain-wave and imaging findings to clarify their meaning. Now let’s turn to the developing organization of the cerebral cortex.

5.2.3 Development of the Cerebral Cortex

The cerebral cortex, resembling half a shelled walnut, surrounds the rest of the brain. It is the largest brain structure—accounting for 85 percent of the brain’s weight and containing the greatest number of neurons and synapses. Each of the 20 billion neurons located in the cerebral cortex has, on average, 7,000 synaptic connections, yielding more than 23,000 miles of myelinated neural fibers (Budday, Steinman, & Kuhl, 2015). 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 5.6 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 synaptic growth 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 (Gilmore et al., 2012). Language areas are especially active from late infancy through the preschool years, when language development flourishes (Pujol et al., 2006).

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 complex thought—in particular, consciousness and various “executive” processes, including inhibition of impulses; integration of information; self-regulation of cognition, emotion, and behavior; and 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 (Jabès & Nelson, 2014; Jiang & Nardelli, 2016).

Figure 5.6 The left side of the human brain, showing the cerebral cortex. The cortex is divided into different lobes, each of which contains a variety of regions with specific functions. Some major regions are labeled here.

Lateralization and Plasticity of the Cerebral Cortex

The cerebral cortex has two hemispheres, or sides, that differ in their motor, cognitive, and emotional functions. Some tasks are done mostly by the left hemisphere, others by the right—specialization of the hemispheres is called lateralization. For example, each hemisphere receives sensory information from the side of the body opposite to it and controls only that side (vision is a complicated exception). 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).

Handedness is the most obvious reflection of cerebral lateralization in humans. In longitudinal research on several hundred ethnically diverse U.S. infants, hand preference in reaching for objects was evident for about half between ages 6 and 14 months. An additional one-fourth began to show a hand preference over the second year; the rest as yet showed no preference (Campbell, Marcinowski, & Michel, 2018; Michel et al., 2014). Like adults, most infants who displayed a hand preference preferred the right hand, which is controlled by the left hemisphere. Nevertheless, numerous studies comparing identical and fraternal twins reveal the heritability of left-handedness to be weak (Ooki, 2014; Vuoksimaa et al., 2009). Although many hypotheses about early environmental influences on handedness exist, none is consistently supported by research.

Handedness, however, is not a good indicator of how cognitive functions are organized in the cerebral cortex. The approximately 10 percent of people who are left-handed or to varying degrees mixed-handed usually display the same lateralization pattern for language and spatial abilities as right-handers. For a few non-right-handers, the cortex is less clearly specialized. And a small minority of healthy right- and left-handers display atypical forms of hemispheric specialization (Guadalupe et al., 2014; Mazoyer et al., 2014). These range from lack of lateralization (language governed by both hemispheres) to a reversal of the typical pattern (language in the right hemisphere, spatial abilities in the left).

Handedness—as in this 1-year-old’s preference for using his left hand—is the most visible reflection of cerebral lateralization. Although handedness becomes increasingly apparent with age, the majority of young left- and right-handers show the same lateralization pattern for cognitive functions.

Why, in the overwhelming majority of humans, does lateralization of cognitive and emotional functions occur? A lateralized brain may have evolved because it permits a wider array of functions to be carried out effectively, enabling humans to cope more successfully with changing environmental demands than if both sides processed information in exactly the same way. Studies using fMRI reveal, for example, that the left hemisphere is better at processing information in a sequential, analytic (piece-by-piece) way, useful 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 (Silbereis et al., 2016).

Nevertheless, the popular notion that lateralization means that there are “right-brained” and “left-brained” people is an oversimplification. The two hemispheres communicate and work together to enhance information processing of all kinds, doing so more rapidly and effectively with age.

Researchers study the timing of brain lateralization to learn more about brain plasticity. A 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 the 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.

Early in development, the hemispheres have already begun to specialize for cognitive and emotional processing. The majority of 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 (Hespos et al., 2010).

Nevertheless, research on children and adults who survived brain injuries offers evidence for substantial plasticity in the young brain, summarized in the Biology and Environment box on the following page. Furthermore, early experience greatly influences the organization of the cerebral cortex for language. 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 verbally advanced show greater left-hemispheric specialization for language than their more slowly developing agemates (Bishop et al., 2014; 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 and, therefore, young children’s ability to learn, which is fundamental to their survival. And although the cortex is programmed from the start for hemispheric specialization, experience greatly influences the rate and success of its advancing organization.

5.2.4 Sensitive Periods in Brain Development

Animal studies confirm 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 undergo accelerated synapse elimination and 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 in isolation, the brains of the stimulated animals are larger 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.

Biology and EnvironmentBrain Plasticity: Insights from Research on Children with Brain Injury

In the first few years of life, the brain is highly plastic. It can reorganize areas committed to specific functions in a way that the mature brain cannot. Adults who suffered focal brain injury (damage to a specific part of the brain) in infancy and early childhood show milder cognitive deficits than adults with similar, later-occurring injury (Huttenlocher, 2002). Nevertheless, the young brain is not totally plastic. When it is injured, its functioning is compromised.

Brain Plasticity in Infancy and Early Childhood

In a large study of children with focal injuries to the cerebral cortex that occurred around the time of birth or in the first six months of life, language and spatial skills were assessed repeatedly into adolescence (Stiles, Reilly, & Levine, 2012; Stiles et al., 2008, 2009). All the children had experienced early brain seizures or hemorrhages. fMRI and PET scans 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 with age in their drawing skills—gains that did not occur in individuals who experienced similar brain injuries in adulthood (Stiles, Reilly, & Levine, 2012; Stiles et al., 2008, 2009).

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 (Forsyth, 2014). But early focal injury has far less impact than later injury on both language and spatial skills, revealing the young brain’s plasticity.

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. In telling stories, they produce simpler narratives than agemates who had no early brain injuries. And as the demands of daily life increase, they have difficulty with self-regulation—managing homework and other responsibilities (Anderson, Spencer-Smith, & Wood, 2011; Stiles, Reilly, & Levine, 2012).

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 (Fiori & Guzzetta, 2015; 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 because performing them well requires the collaboration of many regions in the cerebral cortex. In sum, the full impact of an early brain injury may not be apparent for many years, until higher-order skills are expected to develop.

This preschooler, who experienced brain damage in infancy, has been spared massive impairments because of high plasticity of the brain. Here, a teacher encourages her to cut basic shapes to strengthen spatial skills, which remain more impaired than language after early brain injury.

A System of Influences

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 the same time, for reasons not yet fully understood, some children who experienced focal brain injury in infancy or early childhood display lasting cognitive deficits (Anderson, Spencer-Smith, & Wood, 2011; Anderson et al., 2014). Accumulating evidence suggests that age combines with an array of other factors to affect extent of plasticity. These include the site of the injury, the severity of injury, and the availability of diverse environmental supports, including a stimulating home environment, warm parenting, access to intervention services, and high-quality schooling (Dennis et al., 2014).

At older ages, specialized brain structures are in place, but after focal injury the brain can still recover to some degree. When an individual practices relevant tasks, the brain strengthens existing synapses and generates new ones. Nevertheless, when brain injury is widespread, recovery for both children and adults is greatly reduced. And when damage occurs to certain regions—for example, the prefrontal cortex—recovery is also limited. Because of its executive role in thinking and widespread connections throughout the brain, prefrontal abilities are difficult to transfer to other cortical areas. Consequently, early prefrontal lesions typically result in persisting, general intellectual deficits (Pennington, 2015). Clearly, plasticity is a complex process that is not equivalent throughout the brain.

For example, when babies are born with cataracts in both eyes (clouded lenses, preventing clear visual images), those who have corrective surgery within 4 to 6 months show rapid improvement in vision, except for subtle aspects of face perception, which require early visual input to the right hemisphere to develop. The longer cataract surgery is postponed beyond infancy, the less complete the recovery in visual skills (Maurer & Lewis, 2013). If surgery is delayed until adulthood, vision is severely and permanently impaired.

Studies of infants placed in orphanages who were later exposed to family rearing confirm the importance of a generally stimulating environment for healthy 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 consistently attained average mental test scores in childhood and adolescence, performing as well as a comparison group of early-adopted British-born children.

This Romanian orphan receives little adult contact or stimulation. The longer he remains in this barren environment, the greater his risk of brain damage and lasting impairments in all domains of development.

But Romanian children who had been institutionalized for more than the first six months showed substantial intellectual deficits (see Figure 5.7). Although they improved in intelligence test scores during middle childhood and adolescence, they remained well 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).

Neurobiological findings indicate that early, prolonged institutionalization leads to a generalized decrease in volume and activity of 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 (Hodel et al., 2015; McLaughlin et al., 2014; Perego, Caputi, & Ogliari, 2016). 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. 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 & Cheatham, 2003; Gunnar et al., 2001). 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. The longer the children spent in orphanage care, the higher their cortisol levels—even 6½ years after adoption. In other research, orphanage children from diverse regions of the world who were adopted after 1 year of age by American families displayed abnormally low cortisol—a blunted physiological response that is also a sign of impaired capacity to manage stress (Koss et al., 2014; Loman & Gunnar, 2010). Persisting abnormally high or low cortisol levels are linked to later learning, emotional, and behavior problems, including both internalizing (fear and anxiety) and externalizing (anger and aggression) difficulties.

Figure 5.7 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. 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.)

Finally, early deprived rearing may also disrupt the brain’s typical response to pleasurable social experiences. After sitting on their mother’s lap and playing an enjoyable game, preschoolers adopted, on average, at age 1½ years from Romanian orphanages had abnormally low urine levels of oxytocin—a hormone released by the brain that evokes calmness and contentment in the presence of familiar, trusted people (Fries et al., 2005). And as we will see in Chapter 7, forming secure attachment relationships with their adoptive parents is more challenging for children who spent their infancy in neglectful institutions.

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 impoverished, her biological mother’s loving care—holding her gently, speaking softly, interacting playfully with her, and breastfeeding—likely prevented irreversible damage to her brain. Besides offering gentle, appropriate stimulation, sensitive adult care helps normalize cortisol production in both typically developing and emotionally traumatized infants and young children (Gunnar & Quevedo, 2007; Tarullo & Gunnar, 2006). Good parenting seems to protect the young brain from the potentially damaging effects of both excessive and inadequate stress-hormone exposure.

In the Bucharest Early Intervention Project, 136 institutionalized Romanian babies were randomized into conditions of either care as usual or transfer between 6 and 31 months of age to high-quality foster families. Specially trained social workers provided foster parents with counseling and support. Periodic follow-ups when the children were between 2½ and 12 years old revealed that the foster-care group exceeded the institutional-care group in intelligence test scores, language development, self-regulation skills, emotional responsiveness, social skills, EEG and ERP assessments of brain development, and adaptive cortisol levels. On nearly all measures, earlier entry into foster care predicted better outcomes (Almas et al., 2016; McLaughlin et al., 2015; Nelson, Fox, & Zeanah, 2014; Troller-Renfree et al., 2018). 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, those that overwhelm children with expectations beyond their current capacities also interfere with the brain’s potential. In recent years, expensive early learning centers as well as “educational” tablets and DVDs aimed at babies have become widespread. Within these contexts, infants are trained with letter and number flash cards and toddlers are given a full curriculum of reading, math, science, art, and more. There is no evidence that these programs yield smarter “super-babies” (Principe, 2011). 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.

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 the 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 development 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 (Belsky & de Haan, 2011). No evidence exists for a sensitive period in the first few years of life 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 depends on ordinary, stimulating experiences—like this infant’s exploration of the textures of a fallen log and pebbly surface. It provides the foundation for experience-dependent brain growth—refinements due to culturally specific learning. But too much emphasis on training at an early age—for example, drill on mastering the ABCs—can interfere with access to everyday experiences the young brain needs to grow optimally.

Tetra Images / Alamy Stock Photo

5.2.5 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 decline. The newborn baby takes round-the-clock naps that total about 16 to 18 hours. The average 2-year-old still needs 12 to 13 hours of sleep, 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 (Galland et al., 2012). Between ages 3 and 5, napping subsides.

These changing arousal patterns are due to brain development, but they are also affected by cultural beliefs and practices and parents’ needs. Dutch parents, for example, view sleep regularity as far more important than 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 & Harkness, 2010; Super et al., 1996). 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. Furthermore, as the Cultural Influences box on the following page reveals, isolating infants and toddlers in a separate room for sleep is rare around the world.

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. But infants who receive more milk or solid foods during the day are not less likely to wake, though they feed less at night (Brown & Harries, 2015). However, babies just a few weeks old—though they typically wake every two hours—have some capacity to resettle on their own. When infants wake up and cry, parents who in the early weeks wait just a few minutes before initiating feeding, granting the baby an opportunity to settle and return to sleep, have infants who are more likely to sleep for longer nighttime periods at 3 months of age (St James-Roberts et al., 2015, 2017). Around 2 to 3 months, most Western infants begin sleeping 4 to 5 hours at a stretch.

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 InfluencesCultural Variation in Infant Sleeping Arrangements

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, 2011, p. 62). And the American Academy of Pediatrics (2016b) has issued a controversial warning that parent–infant bedsharing increases the risk of sudden infant death syndrome (SIDS) and accidental suffocation.

Yet parent–infant cosleeping—in the same room and often in the same bed—is the norm for approximately 90 percent of the world’s population, in cultures as diverse as the Japanese, the rural Guatemalan Maya, the Inuit of northwestern Canada, and the !Kung of Botswana. Japanese and Korean children usually lie next to their mothers throughout infancy and early childhood (Shimizu, Park & Greenfield, 2014; Yang & Hahn, 2002). Bedsharing is also common in U.S. ethnic minority families (McKenna & Volpe, 2007). African-American children, for example, frequently fall asleep with their parents and remain with them for part or all of the night (Buswell & Spatz, 2007).

Cultural values strongly influence infant sleeping arrangements. In one study, researchers interviewed Guatemalan Mayan mothers and American middle-SES mothers about their sleeping practices. Mayan mothers stressed the importance of promoting an interdependent self, explaining that cosleeping builds a close parent–child bond, which is necessary for children to learn the ways of people around them. In contrast, American mothers emphasized an independent self, mentioning their desire to instill early autonomy, prevent bad habits, and protect their own privacy (Morelli et al., 1992).

Over the past several decades, cosleeping has increased in Western nations, including the United States. In a survey of a large, nationally representative sample of U.S. mothers, 20 percent reported routinely bedsharing with their infants (Smith et al., 2016). Parents who support the practice say that it helps their baby sleep, makes breastfeeding more convenient, reduces infant distress, and provides valuable bonding time (McKenna & Volpe, 2007; Tikotzky et al., 2010).

Babies who sleep with their parents breastfeed three times longer than infants who sleep alone (Huang et al., 2013). Because infants arouse to nurse more often when sleeping next to their mothers, some researchers believe that cosleeping may actually help safeguard babies at risk for SIDS. Consistent with this view, SIDS is rare in Asian nations where bedsharing is widespread, including Cambodia, China, Japan, Korea, Thailand, and Vietnam (McKenna, 2002; McKenna & McDade, 2005).

Critics warn that bedsharing will promote sleep and adjustment problems, especially excessive dependency. Yet in Western societies, bedsharing often emerges in response to children’s sleep and emotional difficulties, rather than occurring as a consistent, intentional practice (Mileva-Seitz et al., 2017). A study following children from the end of pregnancy through age 18 years showed that young people who had bedshared in the early years were no different from others in any aspect of adjustment (Okami, Weisner, & Olmstead, 2002).

A more serious concern is that infants might become trapped under the parent’s body or in soft bedding and suffocate. Parents who are obese or who use alcohol, tobacco, or illegal drugs do pose a serious risk to bedsharing babies, as does the use of quilts and comforters or an overly soft mattress (Carpenter et al., 2013).

A 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.

But with appropriate precautions, parents and infants can cosleep safely (Ball & Volpe, 2013). In cultures where cosleeping is widespread, parents and infants usually sleep with light covering on hard surfaces, such as firm mattresses or wooden planks. Alternatively, babies sleep in a cradle or hammock next to the parents’ bed.

To protect against SIDS and other sleep-related deaths, the American Academy of Pediatrics (2016b) recommends that parents roomshare but not bedshare for the first year, placing the baby within easy view and reach on a separate surface designed for infants. Currently, the majority of U.S. mothers of infants—65 percent—report usually roomsharing without bedsharing (Smith et al., 2016). However, some researchers point out that placing too much emphasis on separate sleeping may have risky consequences—for example, inducing tired parents to avoid feeding their babies in bed in favor of using dangerously soft sofas (Bartick & Smith, 2014).

In general, when discussing each infant’s sleep environment with parents, pediatricians are wise to take into account cultural values and motivations (Ward, 2015). Then they can work within that framework to ensure the sleep environment is a safe one.

At the end of the first year, as REM sleep (the state that usually prompts waking) declines, infants approximate an adultlike sleep–wake schedule. But even after they 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 (Armstrong, Quinn, & Dadds, 1994; Scher, Epstein, & Tirosh, 2004; Scher et al., 1995). As Chapter 7 will reveal, around the middle of the first year, infants are forming a clear-cut attachment to their familiar caregiver and begin protesting when she or he 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. In one study, young babies whose mothers were warm, sensitive, and available to them at bedtime slept more during the night (Philbrook & Teti, 2016). In turn, lower infant nighttime distress predicted greater maternal sensitivity in the following months.

Bedtime routines promote sleep as early as the first two years. In a study carried out in 13 Western and Asian nations, over 10,000 mothers reported on their bedtime practices and their newborn to 5-year-olds’ sleep quality (Mindell et al., 2015). Consistently engaging in bedtime routines—for example, rocking and singing in infancy, storybook reading in toddlerhood and early childhood—was associated with falling asleep more readily, waking less often, and getting more nighttime sleep throughout the entire age range (see Figure 5.8).

As noted in Chapter 4 (page 137), restful sleep is vital for infants’ learning and memory. For example, compared to 12-month-olds in a no-nap condition, those given an opportunity to nap imitated more adult actions with toys observed earlier that day (Konrad et al., 2016b). Similarly, 6-month-olds who slept well the previous night remembered more than those who woke often (Konrad et al., 2016a). In addition to supporting memory storage, sleep enhanced toddlers’ ability to solve a novel problem: figuring out how to efficiently navigate a tunnel to reach a caregiver waiting at the other end (Berger & Scher, 2017).

Figure 5.8 Relationship of bedtime routines to night-wakings in infancy/toddlerhood and the preschool years. In a large sample of mothers in 13 Western and Asian nations, the more consistently they used bedtime routines, the less often their infants, toddlers, and preschoolers woke during the night. Findings were similar for ease of falling asleep and amount of nighttime sleep. (From J. A. Mindell, A. M. Li, A. Sadeh, R. Kwon, & D. Y. Goh, 2015, “Bedtime Routines for Young Children: A Dose-Dependent Association with Sleep Outcomes,” Sleep, 38, p. 720. Copyright © 2015 by permission of the Associated Professional Sleep Societies, LLC. Reprinted by permission.)

Ask Yourself

Connect ■ Explain how either too little or too much stimulation can impair cognitive and emotional development in the early years.

Apply ■ Which infant enrichment program would you choose: one that emphasizes gentle talking and touching and social games, or one that includes reading and number drills and classical music lessons? Explain.

Reflect ■ What is your attitude toward parent–infant cosleeping? Is it influenced by your cultural background? Explain.

5.3 INFLUENCES ON EARLY PHYSICAL GROWTH

5.3 Cite evidence that heredity, nutrition, and parental affection all contribute to early physical growth.

Physical growth, like other aspects of development, results from a continuous and complex interplay between genetic and environmental factors. Good nutrition and relative freedom from disease are essential for young children’s healthy development, while environmental pollutants are a threat. The Social Issues: Health box on the following page considers the extent to which one of the most common pollutants, lead, impairs children’s brain growth and cognitive functioning and contributes to behavior problems.

Social Issues: HealthLead Exposure and Children’s Development

Lead is a highly toxic element that, at blood levels exceeding 60 μg/dL (micrograms per deciliter), causes brain swelling, hemorrhaging, disrupted functioning of neurons, and widespread cell death. Before 1980, exposure to lead resulted from use of lead-based paints in housing (infants and young children often ate paint flakes) and from use of leaded gasoline (producing a highly breathable form of lead in car exhaust). Laws limiting the lead content of paint and mandating lead-free gasoline led to a sharp decline in U.S. children’s lead levels, from an average of 15 μg/dL in 1980 to less than 1 μg/dL today (Tsoi, Cheung, & Cheung, 2016).

Yet in areas near certain airports (because some aviation fuels still contain lead), near industries using lead production processes, or where lead-based paint remains in older homes, children’s blood levels are still markedly elevated. Contaminated soil and imported consumer products, such as toys made of leaded plastic, are additional sources of exposure.

In the Flint, Michigan, water crisis of 2014 to 2015, the city began using a new water source without adding corrosion inhibitors, causing lead to leach from aging pipes and contaminate the city’s drinking water. This resulted in blood levels of lead above 5 μg/dL—the level deemed high enough by the U.S. government to warrant immediate efforts to reduce exposure—in as many as 5 percent of the city’s children (Hannah-Attisha et al., 2016).

Over the past quarter century, longitudinal studies of the consequences of lead have been conducted in multiple countries, including Australia, Mexico, New Zealand, the United Kingdom, and the United States. Each tracked children’s lead exposure over an extended time and controlled for factors associated with both lead levels and mental test scores (such as SES, home environmental quality, and nutrition) that might otherwise account for the findings.

Nearly all investigations reported negative relationships between lead exposure and children’s intelligence test scores (Canfield et al., 2003; Hubbs-Tait et al., 2005; Lanphear et al., 2005). Impaired cognitive functioning was evident at all levels of exposure—even small quantities. Some studies reported persisting effects into adulthood (Mazumdar et al., 2011; Reuben et al., 2017). Higher blood levels were also associated with distractibility, overactivity, weak academic performance, childhood behavior problems, and adolescent antisocial behavior (Needleman et al., 2002; Nevin, 2006; Stretesky & Lynch, 2004; Wright et al., 2008).

A public health worker pricks a 5-month-old’s foot to collect blood as part of a program to monitor children’s lead levels in Flint, Michigan. Even at low levels, exposure to lead can impair children’s cognitive functioning and contribute to behavior problems.

Furthermore, in several investigations, cognitive effects were much greater for low-SES than higher SES children (Bellinger, Leviton, & Sloman, 1990; Ris et al., 2004; Tong, McMichael, & Baghurst, 2000). A stressed, disorganized home life seems to heighten lead-induced brain damage. Dietary factors can also magnify lead’s toxic effects. Iron and zinc deficiencies, especially common in low-SES children, increase lead concentration in the blood (Noonan et al., 2003; Wolf, Jimenez, & Lozoff, 2003).

In sum, lead impairs mental development and contributes to behavior problems. Low-SES children are more likely to live in lead-contaminated areas and to experience additional risks that magnify lead-induced damage. Because lead is a stable element, its release into the environment is difficult to reverse. Therefore, in addition to laws that control lead pollution, interventions that reduce the negative impact of lead—through involved parenting, dietary enrichment, better schools, and public education about lead hazards—are vital.

5.3.1 Heredity

Because identical twins are much more alike in body size than fraternal twins, we know that heredity contributes considerably to physical growth (Dubois et al., 2012; Jelenkovic et al., 2016). When diet and health are adequate, height and rate of physical growth are largely governed by heredity. In fact, as long as negative environmental influences such as poor nutrition or illness are not severe, children and adolescents typically show catch-up growth—a return to a genetically determined growth path—once conditions improve. After her adoption, Grace grew rapidly until, at age 2, she was nearly average in size by Cambodian standards. Still, the health of the brain and many internal organs may be permanently compromised by malnutrition. (Recall the consequences of inadequate prenatal nutrition for long-term health, discussed on pages 107–108 in Chapter 3.)

Twin studies reveal that genetic makeup also affects body weight (Elks et al., 2012; Kinnunen, Pietilainen, & Rissanen, 2006; Liu et al., 2015). At the same time, environment—in particular, nutrition—plays an especially important role.

Applying What We Know

Reasons to Breastfeed

NUTRITIONAL AND HEALTH ADVANTAGES

EXPLANATION

Provides the correct balance of fat and protein

Compared with the milk of other mammals, human milk is higher in fat and lower in protein. This balance, as well as the unique proteins and fats contained in human milk, is ideal for a rapidly myelinating nervous system.

Ensures nutritional completeness

A mother who breastfeeds need not add other foods to her infant’s diet until the baby is 6 months old. The milks of all mammals are low in iron, but the iron contained in breast milk is much more easily absorbed by the baby’s system. Consequently, bottle-fed infants need iron-fortified formula.

Helps ensure healthy physical growth

One-year-old breastfed babies are leaner (have a higher percentage of muscle to fat), a growth pattern that persists through the preschool years and that is associated with a reduction in later overweight and obesity.

Protects against many diseases

Breastfeeding transfers antibodies and other infection-fighting agents from mother to baby and enhances functioning of the immune system. Compared with bottle-fed infants, breastfed babies have far fewer allergic reactions and respiratory and intestinal illnesses. Breast milk also has anti-inflammatory effects, which reduce the severity of illness symptoms. Breastfeeding in the first four months (especially when exclusive) is linked to lower blood cholesterol levels in adulthood and, thereby, may help prevent cardiovascular disease.

Protects against faulty jaw development and tooth decay

Sucking the mother’s nipple instead of an artificial nipple helps avoid malocclusion, a condition in which the upper and lower jaws do not meet properly. It also protects against tooth decay due to sweet liquid remaining in the mouths of infants who fall asleep while sucking on a bottle.

Ensures digestibility

Because breastfed babies have a different kind of bacteria growing in their intestines than do bottle-fed infants, they rarely suffer from constipation or other gastrointestinal problems.

Smooths the transition to solid foods

Breastfed infants accept new solid foods more easily than do formula-fed infants, perhaps because of their greater experience with a variety of flavors, which pass from the maternal diet into the mother’s milk.

Sources: American Academy of Pediatrics, 2012; Druet et al., 2012; Owen et al., 2008; Peres et al., 2015; UNICEF, 2017a.

5.3.2 Nutrition

Nutrition is especially crucial for development in the first two years because the baby’s brain and body are growing so rapidly. Pound for pound, an infant’s energy needs are at least twice those of an adult. Twenty-five percent of infants’ total caloric intake is devoted to growth, and babies need extra calories to keep rapidly developing organs functioning properly (Meyer, 2009).

Breastfeeding Versus Formula-Feeding

Babies need both enough food and the right kind of food. In early infancy, breast milk is ideally suited to their needs, and bottled formulas try to imitate it. Applying What We Know above summarizes major nutritional and health advantages of breastfeeding.

Because of these benefits, breastfed babies in poverty-stricken regions of the world are much less likely to be malnourished and 6 to 14 times more likely to survive the first year of life. The World Health Organization recommends breastfeeding until age 2 years, with solid foods added at 6 months. These practices, if widely followed, would save the lives of more than 800,000 infants annually (World Health Organization, 2017d). Even breastfeeding for just a few weeks offers some protection against respiratory and intestinal infections, which are devastating to young children in developing countries. Also, because a nursing mother is less likely to get pregnant, breastfeeding helps increase spacing among siblings, a major factor in reducing infant and childhood deaths in nations with widespread poverty. (Note, however, that breastfeeding is not a reliable method of birth control.)

Yet many mothers in the developing world do not know about these benefits. In Africa, the Middle East, and Latin America, most babies get some breastfeeding, but fewer than 40 percent are exclusively breastfed for the first six months, and one-third are fully weaned from the breast before 1 year (UNICEF, 2017a). In place of breast milk, mothers give their babies commercial formula or low-grade nutrients, such as rice water or highly diluted cow or goat milk. Contamination of these foods as a result of poor sanitation is common and often leads to illness and infant death. The United Nations has encouraged all hospitals and maternity units in developing countries to promote breastfeeding as long as mothers do not have viral or bacterial infections (such as HIV or tuberculosis) that can be transmitted to the baby. Today, most developing countries have banned the practice of giving free or subsidized formula to new mothers.

Partly as a result of the natural childbirth movement, breastfeeding has become more common in industrialized nations, especially among well-educated women. Today, 83 percent of American mothers begin breastfeeding after birth, but nearly one-third stop by 6 months. And despite the health benefits of breast milk, only 50 percent of preterm infants are breastfed at hospital discharge (Centers for Disease Control and Prevention, 2018a). Breastfeeding a preterm baby presents special challenges, including maintaining a sufficient milk supply with artificial pumping until the baby is mature enough to suck at the breast and providing the infant with enough sucking experience to learn to feed successfully. Kangaroo care (see page 129 in Chapter 4) and the support of health professionals are helpful.

Mothers breastfeeding their infants at a health clinic in Senegal offer one another support. Breastfeeding is especially important in developing countries, where it helps protect babies against life-threatening infections and early death.

Breast milk is so easily digestible that a breastfed infant becomes hungry quite often—every 1½ to 2 hours, compared to every 3 or 4 hours for a bottle-fed baby. This makes breastfeeding inconvenient for many employed women. Not surprisingly, mothers who return to work sooner wean their babies from the breast earlier. But mothers who cannot be with their babies all the time can still breastfeed, and they are more likely to do so if their workplace provides supports, such as private places for breastfeeding and onsite or nearby child care (Smith & Forrester, 2013; Spitzmueller et al., 2016). The U.S. Department of Health and Human Services (2014a) advises exclusive breastfeeding for the first 6 months and inclusion of breast milk in the baby’s diet until at least 1 year.

Women who do not breastfeed sometimes worry that they are depriving their baby of an experience essential for healthy psychological development. Yet breastfed and bottle-fed infants in industrialized nations do not differ in quality of the mother–infant relationship or in later emotional adjustment (Jansen, de Weerth, & Riksen-Walraven, 2008; Lind et al., 2014). A growing number of studies report a slight advantage in intelligence test scores for children and adolescents who were breastfed, after controlling for maternal intelligence, SES, and other factors (Bernard et al., 2017; Horta, Loret de Mola, & Victoria, 2015; Luby et al., 2016). Other studies, however, find no cognitive benefits (Walfisch et al., 2013).

Are Chubby Babies at Risk for Later Overweight and Obesity?

From early infancy, Timmy was an enthusiastic eater who nursed vigorously and gained weight quickly. By 5 months, he began reaching for food on his parents’ plates. Vanessa wondered: Was she overfeeding Timmy and increasing his chances of long-term overweight?

Most chubby babies thin out during toddlerhood and early childhood, as weight gain slows and they become more active. But recent evidence does indicate a strengthening relationship between rapid weight gain in infancy and later obesity (Nanri et al., 2017). The trend may be due to the rise in overweight adults, who engage in unhealthy feeding practices in the first year. Their babies, some of whom may be genetically prone to overeat, establish an early pattern of excessive unhealthy food consumption that persists (Llewellyn & Wardle, 2015). Interviews with large, nationally representative samples of U.S. parents revealed that many routinely served their older infants and toddlers French fries, pizza, candy, sugary fruit drinks, and soda (Miles & Siega-Riz, 2017; Siega-Riz et al., 2010). As many as one-fourth ate no fruits or vegetables.

How can parents prevent their infants from becoming overweight children and adults? One way is to breastfeed for the first six months, which is associated with slower early weight gain and 10 to 20 percent reduced obesity risk in later life (Koletzko et al., 2013). Another strategy is for parents to avoid giving babies foods loaded with sugar, salt, and saturated fats.

Look and Listen

Ask several parents of 1- to 2-year-olds to keep a diary of all the foods and drinks they offer their toddler over a weekend. How healthy are the toddlers’ diets? Did any of the parents report heightened awareness of family nutrition as a result of the diary exercise?

Public policies directed at low-income families, where breastfeeding rates are lowest and unhealthy feeding practices are highest, are a vital child health measure. The U.S. Special Supplemental Nutrition Program for Women, Infants and Children (WIC) provides nutrition education and food to low-income mothers and their children from birth to age 5. New mothers who opt to breastfeed receive enhanced food packages with a greater variety of healthy foods for themselves during their baby’s first year—a policy that has nearly doubled the incidence of breastfeeding among low-income women (Whaley et al., 2012).

Finally, once toddlers learn to walk, climb, and run, parents can provide plenty of opportunities for energetic play. And as Chapter 11 will reveal, because excessive television viewing is linked to lack of exercise, unhealthy eating, and overweight in older children, parents should begin limiting time devoted to TV and other screen media in the first two years.

A WIC counselor meets with breastfeeding mothers to provide nutrition education and enhanced food packages—incentives that have nearly doubled the incidence of breastfeeding among low-income mothers.

5.3.3 Malnutrition

In developing countries and war-torn areas where food resources are limited, malnutrition is widespread. Malnutrition contributes to nearly half of worldwide infant and early childhood deaths—about 2.6 million children annually. It is also responsible for growth stunting of nearly one-fifth of the world’s children under age 5 (UNICEF, 2018). The 8 percent who are severely affected suffer from two dietary diseases, marasmus and kwashiorkor.

Marasmus is a wasted condition of the body caused by a diet low in all essential nutrients. It usually appears in the first year of life when a baby’s mother is too malnourished to produce enough breast milk and bottle-feeding is also inadequate. Her starving baby becomes painfully thin and is in danger of dying.

Kwashiorkor is caused by an unbalanced diet very low in protein. The disease usually strikes after weaning, between 1 and 3 years of age. It is common in regions where children get just enough calories from starchy foods but little protein. The child’s body responds by breaking down its own protein reserves, which causes hair loss, skin rash, swelling of the abdomen, ankles, and feet due to fluid retention, and irritable, listless behavior.

Children who survive these extreme forms of malnutrition often grow to be smaller in all body dimensions and suffer from lasting damage to the brain, heart, liver, pancreas, and other organs (Müller & Krawinkel, 2005; Spoelstra et al., 2012). When their diets do improve, they tend to gain excessive weight (Black et al., 2013). A malnourished body protects itself by establishing a low basal metabolism rate, which may endure after nutrition improves. Also, malnutrition may disrupt appetite control centers in the brain, causing the child to overeat when food becomes plentiful.

The baby on the left, of Niger, Africa, has marasmus, a wasted condition caused by a diet low in all essential nutrients. The swollen abdomen of the toddler on the right, also from Niger, is a symptom of kwashiorkor, which results from a diet very low in protein. If these children survive, they are likely to suffer stunted growth and lasting organ damage as well as serious cognitive and emotional impairments.

Children who experienced marasmus or kwashiorkor show poor fine-motor coordination, have difficulty paying attention, often display conduct problems, and show persisting low scores on intelligence tests (Galler et al., 2012; Venables & Raine, 2016; Waber et al., 2014). In one study, these dietary diseases were associated with increased methylation of many genes involved in brain development—epigenetic changes that predicted cognitive impairments decades later, in mature adulthood (Peter et al., 2016).

The passivity and irritability of malnourished children worsen the impact of poor diet. These behaviors may appear even when protein–calorie deprivation is only mild to moderate. They also accompany iron-deficiency anemia, a condition affecting up to half of children younger than age 5 worldwide that interferes with many central nervous system processes (Wang, 2016). Withdrawal and listlessness reduce the nutritionally deprived child’s ability to pay attention, explore, and evoke sensitive caregiving from parents, whose lives are already disrupted by poverty and stressful living conditions (Corapci, Radan, & Lozoff, 2006). These children score lower than their nonanemic counterparts in intellectual development, with iron supplementation alone failing to correct the difference (Lukowski et al., 2010). For this reason, interventions for malnourished children must improve the family situation as well as the child’s diet.

Inadequate nutrition is not confined to developing countries. Because government-supported supplementary food programs do not reach all families in need, an estimated 16 percent of U.S. children suffer from food insecurity—uncertain access to enough food for a healthy, active life. Food insecurity is especially high among single-mother families (30 percent) and low-income ethnic minority families—for example, Hispanic and African American (18 and 21 percent, respectively) (Coleman-Jensen, et al., 2018). Although few of these children have marasmus or kwashiorkor, their physical growth and ability to learn are still affected.

5.3.4 Emotional Well-Being

We may not think of affection as necessary for healthy physical growth, but it is as vital as food. Weight faltering is a term applied to infants and young children whose weight (but not height) is substantially below age-related growth norms and who are withdrawn and apathetic. Although the immediate cause is usually inadequate caloric intake, contributing factors are generally complex. As many as half of such cases involve a disturbed parent–child relationship. Nearly 10 percent of U.S. infants and young children are affected (Black, 2005; Homan, 2016). If undernutrition is allowed to continue, it soon impairs overall growth, including height and head circumference.

Lana, an observant nurse at a public health clinic, became concerned about 8-month-old Melanie, who was 3 pounds lighter than she had been at her last checkup. Lana noted that Melanie kept her eyes on nearby adults, anxiously watching their every move, and rarely smiled at her mother. During feeding and diaper changing, Melanie’s mother sometimes appeared depressed and distant, at other times impatient and hostile. Melanie tried to protect herself by tracking her mother’s whereabouts and, when she approached, avoiding her gaze.

Often intense parental stressors, such as an unhappy marriage, financial strain, or lack of social support; parental psychological disturbance; or poor parenting skills contribute to weight faltering. Most of the time, affected infants and young children are irritable and display abnormal feeding behaviors, such as poor sucking or vomiting, that both interfere with weight gain and lead parents to feel anxious and helpless, which stress the parent–child relationship further (Batchelor, 2008).

In Melanie’s case, her alcoholic father was out of work, and her parents argued constantly. Melanie’s mother had little energy to meet Melanie’s psychological needs. When treated early, by intervening in feeding problems, helping parents with life challenges, and encouraging sensitive caregiving, children show quick catch-up growth. But if the disorder is not corrected promptly, most remain small and show lasting cognitive and emotional difficulties (Crookston et al., 2013; Shields, Wacogne, & Wright, 2012).

Ask Yourself

Connect ■ Explain why breastfeeding can have lifelong, favorable consequences for the development of infants in poverty-stricken regions of the world.

Apply ■ Eight-month-old Shaun is well below average in height and painfully thin. What serious dietary disease does he likely have, and what types of intervention, in addition to dietary enrichment, can help restore his development?

Reflect ■ Imagine that you are the parent of a newborn baby. Describe feeding and other practices you would use in the first two years, and ones you would avoid, to prevent overweight and obesity.

5.4 LEARNING CAPACITIES

5.4 Discuss infant learning capacities, the conditions under which they occur, and the unique value of each.

Learning refers to changes in behavior as the result of experience. Babies come into the world with built-in learning capacities that permit them to profit from experience immediately. Infants are capable of two basic forms of learning, which we introduced in Chapter 1: classical and operant conditioning. They also learn through their natural preference for novel stimulation. And they have an amazing capacity to learn by analyzing streams of perceptual information for consistent patterns. Finally, shortly after birth, babies appear to learn by observing and imitating the facial expressions and gestures of adults. As we will see, however, newborns’ imitative capacity is hotly debated.

5.4.1 Classical Conditioning

Newborn reflexes, discussed in Chapter 4, make classical conditioning possible in the young infant. In this form of learning, a neutral stimulus is paired with a stimulus that leads to a reflexive response. Once the baby’s nervous system makes the connection between the two stimuli, the neutral stimulus produces the behavior by itself. Classical conditioning helps infants recognize which events usually occur together in the everyday world, so they can anticipate what is about to happen next. As a result, the environment becomes more orderly and predictable. Let’s take a closer look at the steps of classical conditioning.

As Carolyn settled down in the rocking chair to nurse Caitlin, she often stroked Caitlin’s forehead. Soon Carolyn noticed that each time she did this, Caitlin made active sucking movements. Caitlin had been classically conditioned. Figure 5.9 shows how it happened:

Before learning takes place, an unconditioned stimulus (UCS) must consistently produce a reflexive, or unconditioned, response (UCR). In Caitlin’s case, sweet breast milk (UCS) resulted in sucking (UCR).

To produce learning, a neutral stimulus that does not lead to the reflex is presented just before, or at about the same time as, the UCS. Carolyn stroked Caitlin’s forehead as each nursing period began. The stroking (neutral stimulus) was paired with the taste of milk (UCS).

If learning has occurred, the neutral stimulus by itself produces a response similar to the reflexive response. The neutral stimulus is then called a conditioned stimulus (CS), and the response it elicits is called a conditioned response (CR). We know that Caitlin has been classically conditioned because stroking her forehead outside the feeding situation (CS) results in sucking (CR).

If the CS is presented alone enough times, without being paired with the UCS, the CR will no longer occur, an outcome called extinction. In other words, if Carolyn repeatedly strokes Caitlin’s forehead without feeding her, Caitlin will gradually stop sucking in response to stroking.

Figure 5.9 The steps of classical conditioning. This example shows how a mother classically conditioned her baby to make sucking movements by stroking the baby’s forehead at the beginning of feedings.

Young infants can be classically conditioned most easily when the association between two stimuli has survival value. In the example just described, learning which stimuli regularly accompany feeding improves the infant’s ability to get food and survive (Blass, Ganchrow, & Steiner, 1984).

In contrast, some responses, such as fear, are very difficult to classically condition in young babies. Until infants have the motor skills to escape unpleasant events, they have no biological need to form these associations. After age 6 months, however, fear is easy to condition. Return to page 17 in Chapter 1 to review John Watson’s well-known experiment in which he conditioned Little Albert to withdraw and cry at the sight of a furry white rat. Then test your knowledge of classical conditioning by identifying the UCS, UCR, CS, and CR in Watson’s study.

5.4.2 Operant Conditioning

In classical conditioning, babies build expectations about stimulus events in the environment, but they do not influence the stimuli that occur. In operant conditioning, infants act, or operate, on the environment, and stimuli that follow their behavior change the probability that the behavior will occur again. A stimulus that increases the occurrence of a response is called a reinforcer. For example, sweet liquid reinforces the sucking response in young infants. Removing a desirable stimulus or presenting an unpleasant one to decrease the occurrence of a response is called punishment. A sour-tasting fluid punishes the sucking response, causing babies to purse their lips and stop sucking entirely.

Many stimuli besides food can serve as reinforcers of infant behavior. For example, newborns will suck faster on a nipple when their rate of sucking produces interesting sights and sounds, including visual designs, music, or human voices (Floccia, Christophe, & Bertoncini, 1997). As these findings suggest, operant conditioning is a powerful tool for finding out what stimuli babies can perceive and which ones they prefer.

As this baby and a teenage brother imitate each other’s facial expressions and bubbling sounds, the behavior of each reinforces that of the other, sustaining their pleasurable interaction.

As infants’ motor control improves, operant conditioning expands to include a wider range of behaviors and stimuli. For example, researchers have hung special mobiles over the cribs of 2- to 6-month-olds. When the baby’s foot is attached to the mobile with a long cord, the infant can, by kicking, make the mobile move. Under these conditions, it takes only a few minutes for infants to start kicking more forcefully and quickly to produce the reinforcing sights and sounds of the dancing mobile (Merz et al., 2017; Rovee-Collier & Barr, 2001). As Chapter 6 will reveal, operant conditioning with mobiles is frequently used to study young infants’ memory and their ability to group similar stimuli into categories. Once babies learn the kicking response, researchers see how long and under what conditions they retain it when exposed again to the original mobile or to mobiles with varying features.

Operant conditioning also plays a vital role in the formation of social relationships. As the baby gazes into the adult’s eyes, the adult looks and smiles back, and then the infant looks and smiles again. As the behavior of each partner reinforces the other, both continue their pleasurable interaction. In Chapter 7, we will see that this contingent responsiveness contributes to the development of infant–caregiver attachment.

5.4.3 Habituation

At birth, the human brain is set up to be attracted to novelty. Infants tend to respond more strongly to a new element that has entered their environment, an inclination that ensures that they will continually add to their knowledge base. Habituation refers to a gradual reduction in the strength of a response due to repetitive stimulation. Time spent looking at the stimulus, heart rate, respiration rate, and brain activity may all decline, indicating a loss of interest. Once this has occurred, a new stimulus—a change in the environment—causes responsiveness to return to a high level, an increase called recovery. For example, when you walk through a familiar space, you notice things that are new and different—a recently hung picture on the wall or a piece of furniture that has been moved. Habituation and recovery make learning more efficient by focusing our attention on those aspects of the environment we know least about.

Researchers investigating infants’ understanding of the world rely on habituation and recovery more than any other learning capacity. For example, a baby who first habituates to a visual pattern (a photo of a baby) and then recovers to a new one (a photo of a bald man) appears to remember the first stimulus and perceive the second one as new and different from it. This method of studying infant perception and cognition, illustrated in Figure 5.10, can be used with newborns, including preterm infants (Kavšek & Bornstein, 2010). It has even been used to study the fetus’s sensitivity to external stimuli in the third trimester of pregnancy—for example, by measuring changes in fetal heart rate or brain waves when various repeated sounds are presented, followed by a different sound (see page 96 in Chapter 3).

Recovery to a new stimulus, or novelty preference, assesses infants’ recent memory. Think about what happens when you return to a place you have not seen for a long time. Instead of attending to novelty, you are likely to focus on aspects that are familiar: “I recognize that—I’ve been here before!” Like adults, infants shift from a novelty preference to a familiarity preference as more time intervenes between habituation and test phases in research. That is, babies recover to the familiar stimulus rather than to a novel stimulus (see Figure 5.10) (Colombo, Brez, & Curtindale, 2013; Flom & Bahrick, 2010). By focusing on that shift, researchers can also use habituation to assess remote memory, or memory for stimuli to which infants were exposed weeks or months earlier.

With age, babies habituate and recover to stimuli more quickly, indicating that they process information more efficiently (Kavšek & Bornstein, 2010). As the next section on statistical learning, along with our later discussion of perceptual development will illustrate, habituation and recovery have been used to assess a wide range of infant perceptual and cognitive capacities, including speech perception, musical and visual pattern perception, and object perception. But despite the strengths of habituation research, its findings are not clear-cut. When looking, sucking, heart rate, or brain activity declines and recovers, what babies consciously know about the stimuli to which they responded is not always clear. We will return to this difficulty in Chapter 6.

Figure 5.10 Using habituation to study infant perception and cognition. In the habituation phase, infants view a photo of a baby until their looking declines. In the test phase, infants are again shown the baby photo, but this time it appears alongside a photo of a bald-headed man. (a) When the test phase occurs soon after the habituation phase (within minutes, hours, or days, depending on the age of the infants), participants who remember the baby face and distinguish it from the man’s face show a novelty preference; they recover to (spend more time looking at) the new stimulus. (b) When the test phase is delayed for weeks or months, infants who continue to remember the baby face shift to a familiarity preference; they recover to the familiar baby face rather than to the novel man’s face.

Prashant ZI/Adobe Stock Photo

5.4.4 Statistical Learning

Infants are continuously exposed to rich, temporal streams of auditory and visual information as their caregivers speak, sing, and play music and as they view their surroundings. A wealth of research verifies that infants engage in statistical learning, readily detecting the fundamental structure of this complex flow of information by extracting frequently occurring patterns and doing so fairly automatically, without feedback from others.

Most studies of infants’ statistical learning have focused on the speech stream (Saffran, 2017). On average, babies say their first words around age 12 months, so by the end of the first year they must have determined from the flow of speech spoken by their caregivers where one word ends and the next word begins. In an early study, researchers exposed 8-month-olds to an unfamiliar artificial language consisting of sequences of three syllables that always occur together and, thus, form words—for example, pa-bi-ku go-la-tu. At the same time, the language included syllables that less often occur together because they span word boundaries—in the previous word sequence, ku go-la (Saffran, Aslin, & Newport, 1996). When tested, infants’ patterns of attention revealed a clear ability to isolate the perfectly predictable three-syllable words from the statistically less consistent syllable sequences. The infants did so after just a minute or two of exposure to a language they had never heard before.

Infants engage in similar statistical learning when presented with sequences of musical tones and visual shapes (Dawson & Gerken, 2009; Johnson et al., 2009). For example, 9-month-olds prefer to look at pairs of shapes that previously occurred together consistently, rather than pairs of shapes that did not co-occur consistently (Fiser & Aslin, 2002). Furthermore, statistical learning is not limited to temporal streams of information. Infants also use it with visual stimuli in spatial arrangements that are presented to them all at once (Wu et al., 2011).

Even newborns are capable of statistical learning from speech and visual information (Bulf, Johnson, & Valenza, 2011; Teinonen et al., 2009). Statistical learning from speech is also evident in other species, such as rats, who will never acquire language (Toro & Trobalon, 2005). These findings suggest that statistical learning is a built-in, broadly applied capacity that is functional at birth and that operates across sensory modalities, time and space, and species.

As a mother sings to her 11-month-old, the baby uses her capacity for statistical learning to detect melodic and rhythmic regularities in the tune. Statistical learning appears to be a built-in, powerful means of learning that operates across sensory modalities.

Statistical learning helps explain the speed with which infants sift through the immense quantity of stimulation they encounter daily, extracting regularities that they then use to acquire language, build an organized visual world, and solve other tasks (Aslin, 2017). Although a powerful capacity, statistical learning must also be constrained, or young learners would be overwhelmed by the vast number of statistical patterns in their surroundings and retain few of them. Infants do some of this limiting themselves: Research shows that they are biased to attend to information that is neither too simple nor too complex (Kidd et al., 2012). Also, as we will see in this and later chapters, caregivers provide much support for infant statistical learning by appropriately adjusting the complexity of stimulation to which infants are exposed.

5.4.5 Imitation

Babies seem to come into the world with a primitive ability to learn through imitation—by copying the behavior of another person. For example, Figure 5.11 shows a human newborn imitating two adult facial expressions (Meltzoff & Moore, 1977). Newborn imitation extends to certain gestures, such as head and index-finger movements, and has been demonstrated in many ethnic groups and cultures (Meltzoff & Kuhl, 1994; Nagy, Pal, & Orvos, 2014). As the figure reveals, even newborn primates, including chimpanzees (our closest evolutionary relatives), imitate some behaviors (Ferrari et al., 2006; Myowa-Yamakoshi et al., 2004; Paukner et al., 2014).

Nevertheless, some studies have failed to reproduce the human findings (see, for example, Anisfeld, 2005). And because newborn mouth and tongue movements occur with increased frequency to almost any arousing change in stimulation (such as lively music or flashing lights), some researchers argue that certain newborn “imitative” responses are actually mouthing—a common early exploratory response to interesting stimuli (Jones, 2009). Furthermore, imitation is harder to induce in babies 2 to 3 months old than just after birth. Therefore, skeptics believe that newborns’ apparent imitative behaviors are no more than automatic responses to arousing stimuli (such as adult faces) that decline with age, much like reflexes (Keven & Aikins, 2017).

Others claim that newborns imitate a variety of facial expressions and head movements with effort and determination, even after short delays—when the adult is no longer demonstrating the behavior (Meltzoff, 2017; Meltzoff & Williamson, 2013; Paukner, Ferrari, & Suomi, 2011). Furthermore, these investigators argue that imitation—unlike reflexes—does not decline. Rather, they claim, human babies several months old often do not imitate an adult’s behavior right away because they first try to play familiar social games—mutual gazing, cooing, smiling, and waving their arms. But when an adult models a gesture repeatedly, older human infants soon get down to business and imitate (Meltzoff & Moore, 1994). Similarly, imitation declines in baby chimps around 9 weeks of age, when mother–baby mutual gazing and other face-to-face exchanges increase.

According to Andrew Meltzoff, newborns imitate much as older children and adults do—by actively trying to match body movements they see with ones they feel themselves make. With successive tries, they imitate a modeled gesture with greater accuracy (Meltzoff & Williamson, 2013; Nagy, Pal, & Orvos, 2014). Later we will encounter evidence that young babies are remarkably adept at coordinating information across sensory systems.

Figure 5.11 Imitation by human and chimpanzee newborns. The human infants in the middle row imitating (left) tongue protrusion and (right) mouth opening are 2 to 3 weeks old. The chimpanzee imitating both facial expressions is 2 weeks old. (From A. N. Meltzoff & M. K. Moore, 1977, “Imitation of Facial and Manual Gestures by Human Neonates,” Science, 198, p. 75. Copyright © 1977 by AAAS. And from M. Myowa-Yamakoshi et al., 2004, “Imitation in Neonatal Chimpanzees [Pan Troglodytes].” Developmental Science, 7, p. 440. Copyright © 2004 by John Wiley & Sons.)

Scientists have identified specialized cells in motor areas of the cerebral cortex in primates—called mirror neurons—that may underlie early imitation. Mirror neurons fire identically when a primate hears or sees an action and when it carries out that action on its own (Ferrari & Coudé, 2011). Humans have especially elaborate neural mirroring systems, which are believed to be the biological basis of a variety of interrelated, complex social abilities, including imitation, empathic sharing of emotions, and understanding others’ intentions (Simpson et al., 2014). A growing number of studies report that infants age 6 months and older and adults show a similar, unique change in a particular EEG brain-wave pattern when observing a model perform an action and when engaging in the action themselves (Marshall & Meltzoff, 2014). The consistency of these findings suggests a functioning neural mirroring system in the first year of life.

Still, Meltzoff’s view of newborn imitation as a flexible, voluntary capacity remains highly controversial. Some critics staunchly contend that newborns cannot imitate, arguing that that only gradually do babies learn to engage in imitation through rich social experiences (Jones, 2017; Ray & Heyes, 2011). Others who believe that newborns have some ability to imitate use simpler accounts to explain it, such as a natural tendency for newborn perceptions to elicit corresponding actions (Vincini et al., 2017). These investigators agree that months of opportunities to watch others’ responses, to see oneself act, and to engage in imitative games with caregivers are required for infants to become proficient imitators. Consistent with this view, the human neural mirroring system, though possibly functional at birth, undergoes an extended period of development (Ferrari et al., 2013). And as we will see in Chapter 6, the capacity to imitate expands greatly over the first two years.

However limited it is at birth, imitation is a powerful means of learning. Using imitation, infants explore their social world, learning from other people. As they notice similarities between their own actions and those of others, they experience other people as “like me” and learn about themselves (Meltzoff, 2017). By tapping into infants’ ability to imitate, adults can get infants to exhibit desirable behaviors. Finally, caregivers take great pleasure in a baby who participates in imitative exchanges, which strengthen the parent–infant bond.

Ask Yourself

Connect ■ Which learning capacities contribute to an infant’s first social relationships? Explain, providing examples.

Apply ■ Nine-month-old Byron has a toy with large, colored push buttons on it. Each time he pushes a button, he hears a nursery tune. Which learning capacity is the manufacturer of this toy taking advantage of? What can Byron’s play with the toy reveal about his perception of sound patterns?

5.5 MOTOR DEVELOPMENT

5.5 Describe dynamic systems theory of motor development, along with factors that influence motor progress in the first two years.

Carolyn, Monica, and Vanessa each kept a baby book, filled with proud notations about when their children held up their heads, reached for objects, sat by themselves, and walked alone. Parents are understandably excited about these new motor skills, which allow babies to master their bodies and the environment in new ways. For example, reaching permits babies to find out about objects by acting on them. Sitting and standing give infants a far more expansive visual perspective on their surroundings. And when infants can move on their own, their opportunities for exploration multiply.

Babies’ motor achievements have a powerful effect on their social relationships. When Caitlin crawled at 7½ months, Carolyn and David began to restrict her movements by saying no and expressing mild impatience. When she walked three days after her first birthday, the first “testing of wills” occurred (Biringen et al., 1995). Despite her mother’s warnings, she sometimes pulled items from shelves that were off limits. “I said, ‘Don’t do that!’ ” Carolyn would repeat firmly, taking Caitlin’s hand and redirecting her attention.

At the same time, newly walking babies more actively attend to and initiate social interaction (Clearfield, 2011; Karasik, Tamis-LeMonda, & Adolph, 2011). Caitlin frequently toddled over to her parents to express a greeting, give a hug, or show them an object of interest. Carolyn and David, in turn, increased their expressions of affection, and playful activities. And when Caitlin encountered risky situations, such as a sloping walkway or a dangerous object, Carolyn and David intervened, combining emotional warnings with rich verbal and gestural information that helped Caitlin notice critical features of her surroundings, regulate her motor actions, and acquire language (Karasik et al., 2008; Walle & Campos, 2014). Caitlin’s delight as she worked on new motor skills triggered pleasurable reactions in others, which encouraged her efforts further. Motor, social, cognitive, and language competencies developed together and supported one another.

5.5.1 The Sequence of Motor Development

Gross-motor development refers to control over actions that help infants get around in the environment, such as crawling, standing, and walking. Fine-motor development has to do with smaller movements, such as reaching and grasping. Table 5.2 shows the average ages at which U.S. infants and toddlers achieve a variety of gross- and fine-motor skills. It also presents the age range during which most babies accomplish each skill, indicating large individual differences in rate of motor progress. Also, a baby who is a late reacher will not necessarily be a late crawler or walker. We would be concerned about a child’s development only if many motor skills were seriously delayed.

Table 5.2 Gross- and Fine-Motor Development in the First Two Years

Motor Skill

Average Age Achieved

Age Range in Which 90 Percent of Infants Achieve the Skill

When held upright, holds head erect and steady

6 weeks

3 weeks–4 months

When prone, lifts self by arms

2 months

3 weeks–4 months

Rolls from side to back

2 months

3 weeks–5 months

Grasps cube

3 months, 3 weeks

2–7 months

Rolls from back to side

4½ months

2–7 months

Sits alone

7 months

5–9 months

Crawls

7 months

5–11 months

Pulls to stand

8 months

5–12 months

Plays pat-a-cake

9 months, 3 weeks

7–15 months

Stands alone

11 months

9–16 months

Walks alone

11 months, 3 weeks

9–17 months

Builds tower of two cubes

11 months, 3 weeks

10–19 months

Scribbles vigorously

14 months

10–21 months

Walks up stairs with help

16 months

12–23 months

Jumps in place

23 months, 2 weeks

17–30 months

Walks on tiptoe

25 months

16–30 months

Note: These milestones represent overall age trends. Individual differences exist in the precise age at which each milestone is attained.

Sources: Bayley, 1969, 1993, 2005.

Historically, researchers assumed that motor milestones emerged in a fixed sequence governed by a built-in maturational timetable. This view has long been discredited. Rather, motor skills are interrelated: Each is a product of earlier motor attainments and a contributor to new ones. Also, children acquire motor skills in highly individual ways (Adolph & Robinson, 2013). For example, before her adoption, Grace spent most of her days lying in a hammock. Because she rarely was placed on her tummy and on firm surfaces that enabled her to move on her own, she did not try to crawl. As a result, she pulled to a stand and walked before she crawled! Babies display such skills as rolling, sitting, crawling, and walking in diverse orders rather than in the sequence implied by motor norms (Adolph, Karasik, & Tamis-LeMonda, 2010).

Many influences—both internal and external to the child—combine to support the vast transformations in motor competencies of the first two years. The dynamic systems perspective, introduced in Chapter 1 (see pages 27–29), helps us understand how motor development takes place.

5.5.2 Motor Skills as Dynamic Systems

According to dynamic systems theory of motor development, mastery of motor skills involves acquiring increasingly complex systems of action. When motor skills work as a system, separate abilities blend together, each cooperating with others to produce more effective ways of exploring and controlling the environment. For example, control of the head and upper chest combine into sitting with support. Kicking, rocking on all fours, and reaching combine to become crawling. Then crawling, standing, and stepping are united into walking (Adolph & Robinson, 2015; Thelen & Smith, 1998).

Each new skill is a joint product of the following factors: (1) central nervous system development, (2) the body’s movement capacities, (3) the goals the child has in mind, (4) the child’s perceptual and cognitive capacities, which are essential for planning and guiding actions, and (5) environmental supports for the skill. Change in any element makes the system less stable, and the child starts to explore and select new, more effective motor patterns. Postural control is fundamental to all other motor actions. Control of the head, shoulders, torso, and trunk must be stable enough to allow babies to turn in different directions, sit up, and move their arms and legs purposefully to pursue various goals, such as interacting with the caregiver, retrieving and manipulating objects, or crossing a room, At the same time, infants’ movements are inseparable from their environmental contexts (Adolph & Franchak, 2017). Parental encouragement or restriction of movement and availability of objects and spaces to explore join with infant attributes to influence the form of new skills and how quickly they are acquired.

The broader physical environment also profoundly influences motor development. Infants with stairs in their home learn to crawl up stairs at an earlier age and also more readily master a back-descent strategy—the safest but also the most challenging position because the baby must turn around at the top, give up visual guidance of her goal, and crawl backward (Berger, Theuring, & Adolph, 2007). And if children were reared on the moon with its reduced gravity, they would prefer jumping to walking or running!

When a skill is first acquired, infants must refine it. For example, in trying to crawl, Caitlin often collapsed on her tummy and moved backward. Soon she figured out how to propel herself forward by alternately pulling with her arms and pushing with her feet, “belly-crawling” in various ways for several weeks. As they attempt a new skill, most babies move back and forth between its presence and absence: An infant might roll over, sit, crawl, or take a few steps but not do so again until the following week. And related, previously mastered skills often become less secure. As the novice walker experiments with balancing the body vertically over two small moving feet, balance during sitting may become temporarily less stable (Chen et al., 2007). This variability is evidence of loss of stability in the system—in dynamic systems theory, a necessary transition between a less mature and a more mature stable state.

Look and Listen

Spend an hour observing a newly crawling or walking baby. Note the goals that motivate the baby to move, along with the baby’s effort and motor experimentation. Describe parenting behaviors and features of the environment that promote mastery of the skill.

Motor mastery involves intense practice. In learning to walk, for example, toddlers practice six or more hours a day, traveling the length of 29 football fields! They fall, on average, 17 times per hour but rarely cry, returning to motion within a few seconds (Adolph et al., 2012). Gradually their ability to balance the body on one leg as the other leg swings forward improves, their small unsteady steps change to a longer stride, their feet move closer together, their toes point to the front, and their legs become symmetrically coordinated (Adolph, Vereijken, & Shrout, 2003). As movements are repeated thousands of times, they promote new synaptic connections in the brain that govern motor patterns.

In tackling challenging motor tasks, babies are steadfast problem solvers, taking into account multiple sources of information. They explore ways of adapting to varied surfaces and openings, such as sliding down a steep slope and turning sideways to fit through a narrow doorway (Franchak & Adolph, 2012; Gill, Adolph, & Vereijken, 2009). And when conditions are uncertain—for instance, a ledge that may not be passable—toddlers are more likely to back off when the penalty for error is high (a fall). In these situations, they also place greater weight on caregivers’ advice (Adolph, Karasik, & Tamis-LeMonda, 2010). If the parent says “go,” they usually proceed; if she says “no,” they avoid.

Dynamic systems theory shows us why motor development cannot be genetically determined. Because it is motivated by exploration and the desire to master new tasks and varies with context, heredity can map it out only at a general level. Rather than being hardwired into the nervous system, motor behaviors are softly assembled from multiple components, allowing for different paths to the same motor skill (Adolph & Robinson, 2015; Spencer, Perone, & Buss, 2011).

Dynamic Motor Systems in Action

To find out how infants acquire motor capacities, researchers conduct microgenetic studies (see page 44 in Chapter 1), following babies from their first attempts at a skill until it becomes smooth and effortless. In one such study, researchers held sounding toys alternately in front of infants’ hands and feet, from the time they first showed interest until they engaged in well-coordinated reaching and grasping. As Figure 5.12 illustrates, the infants violated the normative sequence of arm and hand control preceding leg and foot control, shown in Table 5.2 (Galloway & Thelen, 2004). They first explored the toys with their feet—as early as 8 weeks of age, at least a month before reaching with their hands!

Why did the infants reach “feet first”? Because the hip joint constrains the legs to move less freely than the shoulder joint constrains the arms, infants could more easily control their leg movements. Consequently, foot reaching required far less practice than hand reaching. As these findings confirm, rather than following a strict, predetermined pattern, the order in which motor skills develop depends on the anatomy of the body part being used, the surrounding environment, and the baby’s efforts.

Furthermore, in building a more effective dynamic system, babies often use advances in one motor skill to support others. For example, beginning to walk frees the hands for carrying, and new walkers like to fetch distant objects and transport them—often just for the fun of carrying but also to share with their caregivers (Karasik, Tamis-LeMonda, & Adolph, 2011). Observations of new walkers reveal that, surprisingly, they fall less often when carrying objects than when their hands are empty (Karasik et al., 2012). Even though combining walking with carrying is a more attention-demanding task, toddlers integrate object carrying into their emerging “walking system,” using it to improve their balance (see Figure 5.13).

Figure 5.12 Reaching “feet first.” When sounding toys were held in front of babies’ hands and feet, they reached with their feet as early as 8 weeks of age, a month or more before they reached with their hands. This 2½-month-old skillfully explores an object with her foot.

DEXTER GORMLEY/COURTESY OF COLE GALLOWAY, PH.D.

Figure 5.13 New walkers fall less often when carrying objects. Left: When toddlers are first beginning to walk, carrying objects helps them focus attention and steady their balance. Right: An empty-handed new walker easily tips over.

Cultural Variations in Motor Development

Cultural variations in infant-rearing customs affect motor development. To ensure safety and ease toileting while parents work in the fields, mothers in rural northeastern China place infants on their backs in bags of sand (similar to kitty litter) for most of the day, continuing this practice into the second year. Compared with diapered infants in the same region, sandbag-reared babies are greatly delayed in sitting and walking (Mei, 1994). Among the Zinacanteco Indians of southern Mexico and the Gusii of Kenya, adults view babies who walk before they know enough to keep away from cooking fires and weaving looms as dangerous to themselves and disruptive to others (Greenfield, 1992). As a result, Zinacanteco and Gusii parents actively discourage infants’ gross-motor progress.

In contrast, among the Kipsigis of Kenya and the West Indians of Jamaica, babies hold their heads up, sit alone, and walk considerably earlier than North American infants. In both societies, parents emphasize early motor maturity, practicing formal exercises to stimulate particular skills (Adolph, Karasik, & Tamis-LeMonda, 2010). In the first few months, infants are seated in holes dug in the ground, with rolled blankets to keep them upright. Walking is promoted by frequently standing babies in adults’ laps, bouncing them on their feet, and exercising the stepping reflex (see page 135 in Chapter 4) (Hopkins & Westra, 1988; Super, 1981). As parents in these cultures support babies in upright postures and rarely put them down on the floor, their infants usually skip crawling—a motor skill regarded as crucial in Western nations!

Finally, because it decreases exposure to “tummy time,” the current Western practice of having babies sleep on their backs to protect them from SIDS (see page 138 in Chapter 4) delays gross-motor milestones of rolling, sitting, and crawling (Majnemer & Barr, 2005). Regularly exposing infants to the tummy-lying position during waking hours, even for just a few minutes a day, prevents these delays.

The West Indians of Jamaica believe that exercise helps infants grow up strong and physically attractive. This mother “walks” her baby up her body—an activity that contributes to earlier mastery of walking.

5.5.3 Fine-Motor Development: Reaching and Grasping

Of all motor skills, reaching may play the greatest role in infant cognitive development. By grasping things, turning them over, and seeing what happens when they are released, infants learn a great deal about the sights, sounds, and feel of objects. Because certain gross-motor attainments vastly increase infants’ view of their surroundings, they promote manual coordination. When babies sit, and even more so when they stand and walk, they see the panorama of an entire room (Kretch, Franchak, & Adolph, 2014). In these positions, they focus mainly on nearby objects and want to explore them.

Reaching and grasping, like many other motor skills, start out as gross, diffuse activity and move toward mastery of fine movements. Figure 5.14 illustrates some milestones of reaching over the first nine months. Newborns will actively work to bring their hands into their field of vision: In a dimly lit room, they keep their hand within a narrow beam of light, moving the hand when the light beam moves (van der Meer, 1997). Newborns also make poorly coordinated swipes, called prereaching, toward an object in front of them, but because of poor arm and hand control they rarely contact the object. Like newborn reflexes, prereaching drops out around 7 weeks of age, when babies improve in eye movements involved in tracking and fixating on objects, which are essential for accurate reaching (von Hofsten, 2004). Yet these early behaviors suggest that babies are biologically prepared to coordinate hand with eye in the act of exploring.

At about 3 to 4 months, as infants develop the necessary eye, head, and shoulder control, reaching reappears as purposeful, forward arm movements in the presence of a nearby toy and gradually improves in accuracy (Bhat, Heathcock, & Galloway, 2005). By 5 to 6 months, infants reach for an object in a room that has been darkened during the reach by switching off the lights—a skill that improves over the next few months (McCarty & Ashmead, 1999). This indicates that the baby does not need to use vision to guide the arms and hands in reaching. Rather, reaching is largely controlled by proprioception—our sense of movement and location in space, arising from stimuli within the body. When vision is freed from the basic act of reaching, it can focus on more complex adjustments, such as fine-tuning actions to fit the distance and shape of objects.

Reaching improves as depth perception advances, as infants gain greater control of body posture and arm and hand movements, and as they encounter experiences that motivate them to reach. Providing 3-month olds with many reinforcing reaching opportunities—such as nearby toys that move and sound when the baby’s hand makes contact—results in faster development of reaching (Williams & Corbetta, 2016). Interacting with responsive toys leads infants to attend more closely to their hand–toy contacts, which helps them refine their movements into successful reaches.

Figure 5.14 Some milestones of reaching and grasping. The average age at which each skill is attained is given. (Ages from Bayley, 1969; Rochat, 1989.)

As infants gain in reaching experience, they adjust their movements to the demands of reaching tasks. Four-month-olds aim their reaches ahead of a moving object so they can catch it (von Hofsten, 1993). Around 5 months, babies reduce their efforts when an object is moved beyond their reach (Robin, Berthier, & Clifton, 1996). By 7 months, the arms become more independent: Infants reach for an object by extending one arm rather than both (Fagard & Pezé, 1997). During the next few months, infants become more efficient at reaching for moving objects—ones that spin, change direction, and move sideways, closer, or farther away (Fagard, Spelke, & von Hofsten, 2009; Wentworth, Benson, & Haith, 2000).

Once infants can reach, they modify their grasp. The newborn’s grasp reflex is replaced by the ulnar grasp, a clumsy motion in which the young infant’s fingers close against the palm. Still, even 4- to 5-month-olds modify their grasp to suit an object’s size, shape, and texture (rigid versus soft)—a capacity that improves over the second half-year as infants adjust the hand more precisely and do so in advance of contacting the object (Ransburg et al., 2017; Witherington, 2005). Around 4 to 5 months, when infants begin to sit up, both hands become coordinated in exploring objects. Babies of this age can hold an object in front of their eyes with one hand while the other scans it with the tips of the fingers, and they frequently transfer objects from hand to hand (Soska & Adolph, 2014). By the end of the first year, infants use the thumb and index finger in a well-coordinated pincer grasp. Then the ability to manipulate objects greatly expands. The 1-year-old can pick up raisins and blades of grass, turn knobs, and open and close small boxes.

To explore the surface of this uniquely textured ball, a 6-month-old coordinates both hands—and uses her mouth as well!

Between 8 and 11 months, reaching and grasping are well-practiced. As a result, attention is released from the motor skill to events that occur before and after obtaining the object. For example, 10-month-olds easily modify their reach to anticipate their next action. They reach for a ball faster when they intend to throw it than when they intend to push it down a narrow tube (Kayed & Van der Meer, 2009). Around this time, too, infants begin to solve simple problems that involve reaching, such as searching for and finding a hidden toy. And in the second year, they gradually become more skilled at using tools, such as rakes and hooks, to acquire out-of-reach objects (Rat-Fischer, O’Regan, & Fagard, 2012).

Finally, the capacity to reach for and manipulate an object increases infants’ attention to the way an adult reaches for and plays with that same object (Hauf, Aschersleben, & Prinz, 2007). As babies watch what others do, they broaden their understanding of others’ behaviors and of the range of actions that can be performed on various objects, incorporating those possibilities into their own object-related behaviors.

As with other motor milestones, environmental contexts affect infant reaching. In cultures where mothers carry their infants on their hips or in slings for most of the day, babies have rich opportunities to explore with their hands. Among the !Kung of Botswana, infants grasp their mothers’ colorful, beaded necklaces to steady themselves while breastfeeding as the mother moves. While riding along, they frequently swipe at and manipulate the mother’s jewelry and other dangling objects (Konner, 1977). As a result, !Kung infants are advanced in development of reaching and grasping. And because babies of Mali and Uganda spend half or more of their day held in sitting or standing positions, which facilitate reaching, they, too, develop manual skills earlier than Western infants, who spend much of their day lying down (Adolph, Karasik, & Tamis-LeMonda, 2010).

Ask Yourself

Connect ■ Provide several examples of how motor development influences infants’ and toddlers’ social experiences. How do social experiences, in turn, influence motor development?

Apply ■ List features of everyday contexts that support infants’ progress in reaching, grasping, sitting, and crawling. Why should caregivers place young infants in a variety of waking-time body positions?

Reflect ■ Do you favor early, systematic training of infants in motor skills such as crawling, walking, running, hopping, and stair climbing? Why or why not?

5.6 PERCEPTUAL DEVELOPMENT

5.6a Identify changes in hearing and in depth, pattern, object, and intermodal perception during infancy.

5.6b Explain differentiation theory of perceptual development.

In Chapter 4, you learned that the senses of touch, taste, smell, and hearing—but not vision—are remarkably well-developed at birth. Recall that we used the term sensation to talk about these capacities. That term suggests a fairly passive process—what the baby’s receptors detect when exposed to stimulation. Now let’s turn to a related issue: How does perception change over the first year? Perception, in contrast to sensation, is an active process: When we perceive, we organize and interpret what we sense. Because hearing and vision are the focus of nearly all research on perception, our discussion will address only those two senses.

As we review the perceptual achievements of infancy, you may find it hard to tell where perception leaves off and thinking begins. For this reason, the research we are about to discuss provides an excellent bridge to the topic of Chapter 6—cognitive development during the first two years.

5.6.1 Hearing

Like most mothers and many fathers as well, Vanessa often sang to Timmy. As his first birthday approached, she bought several CDs of nursery songs and lullabies and played one each afternoon at naptime. The simplicity, slowed tempo, repetitiveness, and playful or soothing quality of the music attracted Timmy’s attention (Trehub, 2016). Soon Timmy let Vanessa know his favorite tune. If she put on “Twinkle, Twinkle,” he stood up in his crib and whimpered until she replaced it with “Jack and Jill.”

Around 4 months, infants display a sense of musical phrasing. They prefer Mozart minuets with pauses between phrases to those with awkward breaks (Jusczyk & Krumhansl, 1993). At 6 to 7 months, they can distinguish musical tunes on the basis of variations in rhythmic patterns, including beat structure (duple or triple) and accent structure (emphasis on the first note of every beat unit or at other positions) (Hannon & Johnson, 2004). They are also sensitive to features conveying the purpose of familiar types of songs, preferring to listen to high-pitched playsongs (aimed at entertaining) and low-pitched lullabies (used to soothe) (Tsang & Conrad, 2010). By the end of the first year, infants recognize the same melody when it is played in different keys (Trehub, 2001).

These achievements illustrate the greatest change in hearing over the first year: Using the remarkable statistical learning capacity we discussed earlier (see page 175), babies detect increasingly complex, predictable sound patterns, in speech as well as in music.

Speech Perception

Recall from Chapter 4 that newborns can distinguish nearly all sounds in human languages and that they prefer listening to speech over nonspeech sounds and to their native tongue rather than a rhythmically distinct foreign language. Brain-imaging evidence indicates that in young infants, discrimination of speech sounds activates both auditory and motor areas in the cerebral cortex. Furthermore, when researchers used teething toys to restrain the position and movement of 6-month-olds’ tongues so they could not produce certain speech sounds, the infants had difficulty isolating those sounds from the speech stream (Bruderer et al., 2015; Kuhl et al., 2014). While listening to speech, babies seem to generate internal motor plans that help them perceive particular sounds and that prepare them for producing those sounds.

As infants listen to people talk, they learn to focus on meaningful sound variations. ERP brain-wave recordings reveal that around 5 months, infants become sensitive to syllable stress patterns in their own language (Weber et al., 2004). Between 6 and 8 months, they start to “screen out” sounds not used in their native tongue (Curtin & Werker, 2007). Bilingual infants do so in both their native languages, though slightly later, between 8 and 9 months, due to the challenges of processing the sounds of two languages. But once bilingual babies begin distinguishing native from nonnative sounds, they do so more rapidly and effectively than their monolingual agemates. Their richer linguistic experience seems to induce heightened sensitivity to the details of language sounds (Liu & Kager, 2015, 2016; Ramírez et al., 2017). As the Biology and Environment box above explains, this increased responsiveness to native-language sounds is part of a general “tuning” process in the second half of the first year—a possible sensitive period in which infants acquire a range of perceptual skills for picking up socially important information.

Biology and Environment“Tuning in” to Familiar Speech, Faces, and Music: A Sensitive Period for Culture-Specific Learning

To share experiences with members of their family and community, babies must become skilled at making perceptual discriminations that are meaningful in their culture. As we have seen, at first infants are sensitive to virtually all speech sounds, but around 6 months, they narrow their focus, limiting the distinctions they make to the language they hear and will soon learn.

The ability to perceive faces shows a similar perceptual narrowing effect—perceptual sensitivity that becomes increasingly attuned with age to information most often encountered. After habituating to one member of each pair of faces in Figure 5.15, 6-month-olds were shown the familiar face and the novel face side by side. For both pairs, they recovered to (looked longer at) the novel face, indicating that they could discriminate the individual faces of both humans and monkeys equally well. But at 9 months, infants no longer showed a novelty preference when viewing the monkey pair (Pascalis, de Haan, & Nelson, 2002). Like adults, they distinguished only the human faces. Similar findings emerge with sheep faces: Four- to 6-month-olds easily distinguish them, but 9- to 11-month-olds no longer do (Simpson et al., 2011).

This perceptual narrowing effect appears again in musical rhythm perception. Western adults are accustomed to the even-beat pattern of Western music—repetition of the same rhythmic structure in every measure of a tune—and easily notice rhythmic changes that disrupt this familiar beat. But present them with music that does not follow this typical Western rhythmic form—Baltic folk tunes, for example—and they fail to pick up on rhythmic-pattern deviations. In contrast, 6-month-olds can detect such disruptions in both typically Western and nontypically Western melodies. By 12 months, however, after added exposure to Western music, babies are no longer aware of deviations in foreign musical rhythms, although their sensitivity to Western rhythmic structure remains unchanged (Hannon & Trehub, 2005b).

Several weeks of regular interaction with a foreign-language speaker and of daily opportunities to listen to non-Western music fully restore 12-month-olds’ sensitivity to wide-ranging speech sounds and music rhythms (Hannon & Trehub, 2005a; Kuhl, Tsao, & Liu, 2003). Similarly, 12-month-olds who are given additional exposure and testing for discrimination of monkey faces regain their ability to discriminate such faces (Fair et al., 2012). Adults given similar experiences, by contrast, show little improvement in perceptual sensitivity.

Taken together, these findings suggest a heightened capacity—or sensitive period—in the second half of the first year, when infants are biologically prepared to “zero in” on socially meaningful perceptual distinctions. Notice how, between 6 and 12 months, learning is especially rapid across several domains (speech, faces, and music) and is easily modified by experience. This suggests a broad neurological change—perhaps a special time of experience-expectant brain growth (see page 160) in which babies analyze everyday stimulation of all kinds similarly, in ways that prepare them to participate in their cultural community.

Figure 5.15 Discrimination of human and monkey faces. Which of these pairs is easiest for you to tell apart? After habituating to one of the photos in each pair, infants were shown the familiar and the novel face side by side. For both pairs, 6-month-olds recovered to (looked longer at) the novel face, indicating that they could discriminate human and monkey faces equally well. By 9 months, babies lost their ability to distinguish the monkey faces. Like adults, they showed a novelty preference only to human stimuli. (From O. Pascalis et al., 2002, “Is Face Processing Species-Specific During the First Year of Life?” Science, 296, p. 1322. Copyright © 2002 by AAAS. Republished with permission of American Association for the Advancement of Science conveyed through Copyright Clearance Center, Inc.)

Soon after, infants focus on larger speech units that are critical to figuring out meaning. They recognize familiar words in spoken passages and listen longer to speech with clear clause and phrase boundaries (Johnson & Seidl, 2008; Soderstrom et al., 2003). Around 7 to 9 months, infants extend this sensitivity to speech structure to individual words: They begin to divide the speech stream into wordlike units (Jusczyk, 2002; MacWhinney, 2015).

Analyzing the Speech Stream

Applying their statistical learning capacity, older infants rapidly locate words in adult speech by discriminating syllables that often occur together (indicating that they belong to the same word) from syllables that seldom occur together (indicating a word boundary; see page 175 to review). For example, after hearing the English word sequence pretty baby several times in a brief segment of speech (about 60 words), 8-month-olds can distinguish the word-internal syllable pair (pret-ty) from the word-external syllable pair (ty ba) (Saffran & Thiessen, 2003). They prefer to listen to new speech that preserves the word-internal pattern.

Once infants begin locating words, they focus on the words and discover additional statistical cues that signal word boundaries (Thiessen, Kronstein, & Hufnagle, 2012). Seven- to 8-month-olds detect regular syllable-stress patterns—for example, in English and Dutch, that the onset of a strong syllable (hap-py, rab-bit) often signals a new word (Thiessen & Saffran, 2007). By 10 months, babies can detect words that start with weak syllables, such as “surprise,” by listening for sound regularities before and after the words (Kooijman, Hagoort, & Cutler, 2009).

When speaking to babies, parents often use a single word followed by the same word embedded in the speech stream (“Doggie! Pat the doggie.”). This style of communicating helps infants with word discrimination.

Finally, the more rapidly 10-month-olds detect words within the speech stream (as indicated by ERP recordings), the larger their vocabulary at age 2 years (Junge et al., 2012). Parents’ speech to babies, which often contains single-word utterances followed by the same words embedded in the speech stream (“Doggie!” “See the big doggie?”) aids word discrimination (Lew-Williams, Pelucchi, & Saffran, 2011). As we will see in Chapter 6, adults’ style of communicating with infants facilitates analysis of the structure of speech.

5.6.2 Vision

For exploring the environment, humans depend on vision more than any other sense. Although at first a baby’s visual world is fragmented, it undergoes extraordinary changes during the first seven to eight months of life.

Visual development is supported by rapid maturation of the eye and visual centers in the cerebral cortex. Around 2 months, infants can focus on objects about as well as adults can, and their color vision is adultlike by 4 months (Johnson & Hannon, 2015). Visual acuity (fineness of discrimination) increases steadily, reaching 20/80 by 6 months and an adult level of about 20/20 by 4 years (Slater et al., 2010). Scanning the environment and tracking moving objects also improve over the first half-year as infants better control their eye movements and as they build an organized perceptual world, which enables them to scan more thoroughly and systematically (Johnson, Slemmer, & Amso, 2004).

As babies explore their visual field, they figure out the characteristics of objects and how they are arranged in space. To understand how they do so, let’s examine the development of three aspects of vision: depth, pattern, and object perception.

Depth Perception

Depth perception is the ability to judge the distance of objects from one another and from ourselves. It is important for understanding the layout of the environment and for guiding motor activity.

Figure 5.16 shows the visual cliff, designed by Eleanor Gibson and Richard Walk (1960) and used in the earliest studies of depth perception. It consists of a Plexiglas-covered table with a platform at the center, a “shallow” side with a checkerboard pattern just under the glass, and a “deep” side with a checkerboard several feet below the glass. The researchers found that crawling babies readily crossed the shallow side, but most avoided the deep side. They concluded that around the time infants crawl, most distinguish deep from shallow surfaces and steer clear of drop-offs.

The visual cliff shows that crawling and avoidance of drop-offs are linked, but not how they are related or when depth perception first appears. Recent research has looked at babies’ ability to detect specific depth cues, using methods that do not require that they crawl.

Emergence of Depth Perception

How do we know when an object is near rather than far away? Try these exercises to find out. Pick up a small object (such as your cup) and move it toward and away from your face. Did its image grow larger as it approached and smaller as it receded? Next time you take a bike or car ride, notice that nearby objects move past your field of vision more quickly than those far away.

Motion is the first depth cue to which infants are sensitive. Babies 3 to 4 weeks old blink their eyes defensively when an object moves toward their face as though it is going to hit them (Nánez & Yonas, 1994). Binocular depth cues arise because our two eyes have slightly different views of the visual field. The brain blends these two images, resulting in perception of depth. Research in which two overlapping images are projected before the baby, using a computer-assisted device that ensures each eye receives only one image, reveals that sensitivity to binocular cues emerges at about 8 weeks and improves gradually over the next few months (Kavšek, 2013; Kavšek & Braun, 2016). Finally, beginning at 3 to 4 months and strengthening between 5 and 7 months, babies display sensitivity to pictorial depth cues—the ones artists often use to make a painting look three-dimensional. Examples include receding lines that create the illusion of perspective, changes in texture (nearby textures are more detailed than faraway ones), and overlapping objects (an object partially hidden by another object is perceived to be more distant) (Kavšek, Yonas, & Granrud, 2012).

Why does perception of depth cues emerge in the order just described? Researchers speculate that motor development is involved. For example, control of the head during the early weeks of life may help infants notice motion and binocular cues. Around 5 to 6 months, the ability to turn, poke, and feel the surface of objects may promote perception of pictorial cues (Bushnell & Boudreau, 1993; Soska, Adolph, & Johnson, 2010). And as we will see next, one aspect of motor progress—independent movement—plays a vital role in refinement of depth perception.

Figure 5.16 The visual cliff. Plexiglas covers the deep and shallow sides. By refusing to cross the deep side and showing a preference for the shallow side, this infant demonstrates the ability to perceive depth.

Mark Antman / TopFoto

Independent Movement and Depth Perception

At 6 months, Timmy started crawling. “He’s fearless!” exclaimed Vanessa. “If I put him down in the middle of my bed, he crawls right over the edge. The same thing happens by the stairs.” Will Timmy become wary of the side of the bed and the staircase as he becomes a more experienced crawler? Research suggests that he will. Infants with more crawling experience (regardless of when they started to crawl) are far more likely to avoid the deep side of the visual cliff (Campos et al., 2000). And only after many weeks of crawling will infants hesitate at the edge of a real drop-off that has no safety glass to protect them from falling.

From extensive everyday experience, babies gradually figure out how to use depth cues to detect the danger of falling. But because the loss of body control that leads to falling differs greatly for each body position, infants must undergo this learning separately for each posture (Adolph & Franchak, 2017). In one study, 9-month-olds, who were experienced sitters but novice crawlers, were placed on the edge of a shallow drop-off that could be widened (Adolph, 2002, 2008). While in the familiar sitting position, infants avoided leaning out for an attractive toy at distances likely to result in falling. But in the unfamiliar crawling position, they headed over the edge, even when the distance was extremely wide! And newly walking babies will step repeatedly over a risky drop-off (Kretch & Adolph, 2013a). They will also careen down slopes and over uneven surfaces without making necessary postural adjustments (Adolph et al., 2008; Joh & Adolph, 2006). Thus, they fall frequently.

Infants must learn to use depth cues to detect the danger of falling separately for each posture—sitting, crawling, and walking. This novice crawler proceeding headlong over a short staircase risks tumbling awkwardly to the landing. With more crawling experience, he will discover that backing down is more secure.

Even experienced crawlers and walkers encounter new depth-at-an-edge situations that require additional learning. When researchers encouraged crawling and walking babies to cross bridges varying in width over drop-offs (with an adult following alongside to catch infants if they began to fall), most avoided crossing impossibly narrow bridges. And the greater their experience, the narrower the bridge both crawlers and walkers attempted to cross. Nevertheless, walkers perceived the likelihood of falling from a narrow bridge more accurately than crawlers. While crossing, crawlers could not easily see and adjust the placement of their hind limbs to prevent falls. In contrast, experienced walkers had figured out how to turn their body to accommodate the narrow passageway (see Figure 5.17) (Kretch & Adolph, 2013b). As infants and toddlers discover how to avoid falling in different postures and situations, their understanding of depth expands.

Independent movement promotes other aspects of three-dimensional understanding. For example, seasoned crawlers are better than their inexperienced agemates at remembering object locations, finding hidden objects, and recognizing the identity of a previously viewed object from a new angle (Campos et al., 2000; Schwarzer, Freitag, & Schum, 2013). Why does crawling make such a difference? Compare your own experience of the environment when you are driven from one place to another with what you experience when you walk or drive yourself. When you move on your own, you are much more aware of landmarks and routes of travel, and you take more careful note of what things look like from different points of view. The same is true for infants.

Figure 5.17 An experienced walker crosses a narrow bridge over a drop-off. This 14-month-old has figured out how to turn his body sideways to accommodate the narrow passageway. (From K. S. Kretch & K. E. Adolph, 2013b, “No Bridge Too High: Infants Decide Whether to Cross Based on the Probability of Falling Not the Severity of the Potential Fall,” Developmental Science, 16, p. 338. © 2013 Blackwell Publishing Ltd. Reprinted by permission of John Wiley and Sons, Inc., conveyed through Copyright Clearance Center, Inc.)

Pattern Perception

Even newborns prefer to look at patterned rather than plain stimuli (Fantz, 1961). As they get older, they prefer more complex patterns. For example, 3-week-olds look longest at black-and-white checkerboards with a few large squares, whereas 8-week-olds prefer those with many small squares. Because of their poor vision, very young babies cannot resolve the small features in the complex checkerboard, which appears blurred. Around 2 months, when detection of fine-grained detail has improved, infants can detect the features of complex patterns and spend more time looking at them (Gwiazda & Birch, 2001).

In the early weeks of life, infants respond to the separate parts of a pattern. They stare at single high-contrast features, generally on the edges, and have difficulty shifting their gaze away toward other interesting stimuli (Hunnius & Geuze, 2004a, 2004b). At 2 to 3 months, when scanning ability has improved, infants thoroughly explore a pattern’s internal features, pausing briefly to look at each part (Bronson, 1994).

Once babies can take in all aspects of a pattern, they integrate the parts into a unified whole. Around 4 months, infants are so good at detecting pattern organization that they perceive subjective boundaries that are not really present. For example, they perceive a square in the center of Figure 5.18a, just as you do (Kavšek, 2009). Older infants carry this sensitivity to subjective form further. For example, 9-month-olds look much longer at an organized series of moving lights that resembles a human being walking than at an upside-down or scrambled version (Proffitt & Bertenthal, 1990). At 12 months, infants can detect familiar objects represented by incomplete drawings, even when as much as two-thirds of the drawing is missing (see Figure 5.18b) (Rose, Jankowski, & Senior, 1997). As these findings reveal, infants’ increasing knowledge of objects and actions supports pattern perception.

Figure 5.18 Subjective boundaries in visual patterns. (a) Do you perceive a square in the middle of the figure? By 4 months of age, infants do, too. (b) What does the image, missing two-thirds of its outline, look like to you? By 12 months, infants detect a motorcycle. After habituating to the incomplete motorcycle image, they were shown an intact motorcycle figure paired with a novel form. Twelve-month-olds recovered to (looked longer at) the novel figure, indicating that they recognized the motorcycle pattern on the basis of very little visual information. (Based on Kavšek, 2009; Rose, Jankowski, & Senior, 1997.)

Face Perception

Infants’ tendency to search for structure in a patterned stimulus applies to face perception. Newborns prefer to look at photos and simplified drawings of faces with features arranged naturally (upright) rather than unnaturally (upside down or sideways) (see Figure 5.19a and b) (Cassia, Turati, & Simion, 2004; Simion et al., 2001). They also track a facelike pattern moving across their visual field farther than they track other stimuli (Johnson, 1999). And although they rely more on outer features (hairline and chin) than inner features to distinguish real faces, newborns prefer photos of faces with eyes open and a direct gaze (Farroni et al., 2002; Turati et al., 2006). Yet another amazing capacity is their tendency to look longer at both human and animal faces judged by adults as attractive—a preference that may be the origin of the widespread social bias favoring physically attractive people (Quinn et al., 2008; Slater et al., 2010).

Figure 5.19 Early face perception. Newborns prefer to look at the photo of a face and the simple pattern resembling a face over the upside-down versions (a and b). (c) When the complex drawing of a face on the left and the equally complex, scrambled version on the right are moved across newborns’ visual field, they follow the face longer. But if the two stimuli are stationary, infants show no preference for the face until around 2 months of age. (From Cassia, Turati, & Simion, 2004; Valenza et al., 1996; Johnson, 1999.)

Some researchers claim that these behaviors reflect a built-in capacity to orient toward members of one’s own species, just as many newborn animals do (Slater et al., 2011). Others assert that newborns simply prefer any stimulus in which the most salient elements are arranged horizontally in the upper part of a pattern—like the “eyes” in Figure 5.19b. Indeed, newborns do prefer patterns with these characteristics over other arrangements (Cassia, Turati, & Simion, 2004; Simion et al., 2001). Still other researchers argue that from birth on, infants are exposed to faces more often than to other stimuli—experiences that promote newborns’ early sensitivity to faces and facelike stimuli (Johnson, 2011).

Despite newborns’ responsiveness to faces, they cannot discriminate a complex facial pattern from other, equally complex patterns (see Figure 5.19c). But from repeated exposures to their mother’s face, they quickly learn to prefer her face to that of an unfamiliar woman, though they mostly attend to its broad outlines (Bushnell, 2001). Around 2 months, when infants can combine pattern elements into an organized whole, they prefer a complex drawing of the human face to other equally complex stimulus arrangements (Dannemiller & Stephens, 1988). And they clearly prefer their mother’s detailed facial features to those of another woman (Bartrip, Morton, & de Schonen, 2001).

Around 3 months, infants readily make fine distinctions among the features of different faces—for example, between photographs of two strangers, even when the faces are moderately similar (Farroni et al., 2007). At 5 months, infants perceive emotional expressions as meaningful wholes. They treat positive faces (happy and surprised) as different from negative ones (sad and fearful) (Bornstein & Arterberry, 2003). And by 7 months, they discriminate among a wider range of facial emotional expressions, including happiness, surprise, sadness, fearfulness, and anger (Safar & Moulson, 2017; Witherington et al., 2010).

Experience with particular faces influences face processing, leading babies to form group biases at a tender age. As early as 3 months, infants prefer and more easily discriminate among female faces than among male faces (Liu et al., 2015; Ramsey-Rennels & Langlois, 2006). The greater time spent with female adults explains this effect, as infants with a male primary caregiver prefer male faces. Furthermore, 3-month-olds exposed mostly to members of their own race prefer to look at the faces of members of that race, and between 6 and 9 months their ability to discriminate other-race faces weakens (Fassbender, Teubert, & Lohaus, 2016; Kelly et al., 2007, 2009). This own-race bias is absent in infants who have frequent contact with members of other races or who view picture books of other-race faces, and it can be reversed in older infants through exposure to racial diversity (Anzures et al., 2013; Heron-Delaney et al., 2011). Notice how early experience promotes perceptual narrowing with respect to gender and racial information in faces, as occurs for species information, discussed in the Biology and Environment box on page 185.

Exposure to racial diversity in her child-care center means that this baby is unlikely to have developed a preference for faces of her own race. When infants have limited social experiences, group biases emerge early.

Although key aspects of face identification develop in infancy, it continues to improve throughout childhood (Stiles et al., 2015). In line with these findings, despite a right-hemispheric bias for processing faces by the middle of the first year, infants and children show more broadly distributed neural activity to faces than do adolescents and adults (Otsuka et al., 2007; Turati & Quadrelli, 2017). Not until late adolescence does rapid, accurate discrimination of highly similar faces reach an adultlike level of proficiency.

5.6.3 Object Perception

Research on pattern perception involves only two-dimensional stimuli, but our environment is made up of stable, three-dimensional objects. Do young infants perceive a world of independently existing objects—knowledge essential for distinguishing among the self, other people, and things?

Size and Shape Constancy

As we move around the environment, the images that objects cast on our retina constantly change in size and shape. To perceive objects as stable and unchanging, we must translate these varying retinal images into a single representation.

Size constancy—perception of an object’s size as the same, despite changes in the size of its retinal image—is evident in the first week of life. To test for it, researchers habituated infants to a small cube at varying distances from the eye in an effort to desensitize them to changes in the cube’s retinal image size and direct their attention to the object’s actual size. When the small cube was presented together with a new, large cube—but at different distances so they cast retinal images of the same size—all babies recovered to (looked longer at) the novel large cube, indicating that they distinguished objects on the basis of actual size, not retinal image size (Slater et al., 2010).

Perception of an object’s shape as stable, despite changes in the shape projected on the retina, is called shape constancy. Habituation research reveals that it, too, is present within the first week of life, long before babies can actively rotate objects with their hands and view them from different angles (Slater & Johnson, 1999).

Both size and shape constancy seem to be built-in capacities that assist infants in detecting a coherent world of objects. Yet they provide only a partial picture of young infants’ object perception.

Perception of Object Identity

At first, infants rely heavily on motion and spatial arrangement to distinguish objects. When researchers strategically use these cues, even newborn babies can bind together separate elements in a visual display and perceive a unified object. As Figure 5.20 reveals, they realize that a moving rod whose center is hidden behind a moving rectangular box is a complete rod rather than two rod pieces (Valenza & Bulf, 2011). Like size and shape constancy, perception of object unity appears to be a built-in property of the human perceptual system.

Nevertheless, infants have much to learn about the cues signifying boundaries between objects in their visual field, including shape, color, and pattern. When two objects varying in these cues are touching and either stand still or move in unison, babies younger than 4 months have difficulty distinguishing them. As infants become familiar with many objects and as gains in scanning assist them in integrating each object’s features into a unified whole, they rely more on shape, color, and pattern and less on motion (Johnson, 2011; Slater et al., 2010). By 4½ months, they can discriminate two touching objects on the basis of their features in very simple, easy-to-process situations. And prior exposure to one of the test objects enhances 4½-month-olds’ ability to discern the boundary between two touching objects—a finding that highlights the role of experience (Dueker, Modi, & Needham, 2003; Needham, 2001).

Figure 5.20 Testing newborn infants’ ability to perceive object unity. (a) Supported in front of a screen, infants were habituated to a gray rod moving back and forth behind a white rectangular box with ends highlighted by two incomplete circular shapes. The box plus circular shapes also moved back and forth in unison, but out of phase with the rod. Instead of typical continuous motion, the researchers used stroboscopic motion (a rapid series of images of the rod and box along their brief movement paths), which attracts newborns’ attention and adjusts to their weak ability to track moving stimuli. (b) Following habituation, the infants viewed a side-by-side display of a complete rod and a broken rod with a gap corresponding to the location of the box. Each stimulus moved with the same stroboscopic motion as the rod in the habituation phase. Infants recovered to (looked longer at) the broken rod than the complete rod. Their novelty preference suggests that they perceived the rod behind the box in the first display as a single unit. (From E. Valenza & H. Bulf, 2011, “Early Development of Object Unity: Evidence for Perceptual Completion in Newborns,” Developmental Science, 14, p. 801. Copyright © John Wiley & Sons, adapted by permission.)

In everyday life, objects frequently move in and out of sight, so infants must keep track of their disappearance and reappearance to perceive their identity. Habituation research, in which a ball moves back and forth behind a screen, reveals that at age 4 months, infants first perceive the ball’s path as continuous (Johnson et al., 2003). Between 4 and 5 months, infants can monitor more intricate paths of objects. As indicated by their future-oriented eye movements (looking ahead to where they expect an object to reappear from behind a barrier), 5-month-olds even keep track of an object that travels on a curvilinear course at varying speeds (Rosander & von Hofsten, 2004). Again, experience—the opportunity to track a moving object along a fully visible path of movement just before testing—enhances predictive eye tracking (Johnson & Shuwairi, 2009).

From 4 to 11 months, infants increasingly use featural information to detect the identity of an object traveling behind a screen. At first, they need strong featural cues—a change in two features (size and shape, or shape and color)—to signify that a disappearing object is distinct from an emerging object. Later in the first year, change in a single feature is sufficient (Bremner et al., 2013; Wilcox & Woods, 2009). And as before, experience—in particular, physically manipulating the object—boosts older infants’ attention to its surface features.

In sum, perception of object identity is mastered gradually over the first year. We will consider a related attainment, infants’ understanding of object permanence—awareness that an object still exists when hidden—in Chapter 6.

5.6.4 Intermodal Perception

Our world provides rich, continuous intermodal stimulation—simultaneous input from more than one modality, or sensory system. In intermodal perception, we make sense of these running streams of light, sound, tactile, odor, and taste information, perceiving them as integrated wholes. We know, for example, that an object’s shape is the same whether we see it or touch it, that lip movements are closely coordinated with the sound of a voice, and that dropping a rigid object on a hard surface will cause a sharp, banging sound.

Infants perceive input from different sensory systems in a unified way by detecting amodal sensory properties, information that is not specific to a single modality but that overlaps two or more sensory systems, such as rate, rhythm, duration, intensity, temporal synchrony (for vision and hearing), and texture and shape (for vision and touch). Consider the sight and sound of a bouncing ball or the face and voice of a speaking person. In each event, visual and auditory information are conveyed simultaneously and with the same rate, rhythm, duration, and intensity.

Even newborns are impressive perceivers of amodal properties. After touching an object (such as a cylinder) placed in their palms, they recognize it visually, distinguishing it from a different-shaped object (Sann & Streri, 2007). And they require just one exposure to learn the association between the sight and sound of a toy, such as a rhythmically jangling rattle (Morrongiello, Fenwick, & Chance, 1998).

This toddler exploring a tambourine readily detects amodal relations in the synchronous sounds and visual appearance of its metal jingles.

Within the first half-year, infants master a remarkable range of intermodal relationships. Three- to 5-month-olds can match faces with voices on the basis of lip–voice synchrony, emotional expression, and even age and gender of the speaker. Around 6 months, infants can perceive and remember the unique face–voice pairings of unfamiliar adults (Flom, 2013). And around 7 months, they readily learn associations between arbitrary speech sounds and object motions (Marcus, Fernandes, & Johnson, 2012).

How does intermodal perception develop so quickly? Young infants seem biologically primed to focus on amodal information. Their detection of amodal relations—for example, the common tempo and rhythm in sights and sounds—precedes and provides the basis for detecting more specific intermodal matches, such as the relation between a particular person’s face and the sound of her voice or between an object and its verbal label (Bahrick, 2010).

Intermodal sensitivity is crucial for perceptual development. In the first few months, it enables babies to notice meaningful correlations between sensory inputs and rapidly make sense of their surroundings. As a result, inexperienced perceivers notice a unitary event, such as a hammer’s tapping, without being distracted by momentarily irrelevant aspects of the situation, such as the hammer’s color or orientation.

In addition to easing perception of the physical world, intermodal perception facilitates social and language processing. For example, as 3- to 4-month-olds gaze at an adult’s face, they initially require both vocal and visual input to distinguish positive from negative emotional expressions (Flom & Bahrick, 2007). Only later do infants discriminate positive from negative emotion in each sensory modality—first in voices (around 5 months), later (from 7 months on) in faces (Bahrick, Hernandez-Reif, & Flom, 2005). Furthermore, in speaking to infants, parents often provide temporal synchrony between words, object motions, and touch—for example, saying “doll” while moving a doll and having it touch the infant (Gogate & Bahrick, 2001). This greatly increases the chances that babies will remember the association between the word and the object.

Look and Listen

While watching a parent and infant playing, list instances of parental intermodal stimulation and communication. What is the baby likely learning about people, objects, or language from each intermodal experience?

In sum, intermodal stimulation fosters all aspects of psychological development. When caregivers provide many concurrent sights, sounds, and touches, babies process more information and learn faster (Bahrick, 2010). Intermodal perception is yet another fundamental capacity that assists infants in their active efforts to build an orderly, understandable world.

5.6.5 Understanding Perceptual Development

Now that we have reviewed the development of infant perceptual capacities, how can we put together this diverse array of amazing achievements? Widely accepted answers come from the work of Eleanor and James Gibson. According to the Gibsons’ differentiation theory, infants actively search for invariant features of the environment—those that remain stable—in a constantly changing perceptual world. In pattern perception, for example, young babies search for features that stand out and orient toward faces. Soon they explore a stimulus more thoroughly, noticing stable relationships among its features, detecting patterns, such as complex designs and individual faces. Similarly, infants analyze the speech stream for regularities, detecting words, word-order sequences, and—within words—syllable-stress patterns. The development of intermodal perception also reflects this principle (Bahrick & Lickliter, 2012). Babies seek out invariant relationships—first, amodal properties, such as common rate and rhythm, in a voice and face, later more detailed associations, such as unique voice–face matches.

The Gibsons described their theory as differentiation (where differentiate means “analyze” or “break down”) because over time the baby detects finer and finer invariant features among stimuli. In addition to pattern perception and intermodal perception, differentiation applies to depth and object perception: Recall how in each, sensitivity to motion precedes detection of fine-grained features. So one way of understanding perceptual development is to think of it as a built-in tendency to seek order and consistency—a capacity that becomes increasingly fine-tuned with age (Gibson, 1970; Gibson, 1979).

Acting on the environment contributes to perceptual differentiation. According to the Gibsons, perception is enhanced by the discovery of affordances—the action possibilities that a situation offers an organism with certain motor capabilities (Gibson, 2003). Infants constantly look for ways in which the environment affords possibilities for action. By exploring their surroundings, they figure out which objects can be grasped, squeezed, bounced, or stroked and which surfaces are safe to cross or present the possibility of falling. And from handling objects, babies become more aware of a variety of observable object properties (Perone et al., 2008). As a result, they differentiate their world in new ways.

To illustrate, recall how infants’ changing capabilities for independent movement affect their perception. When babies crawl, and again when they walk, they gradually realize that a sloping surface affords the possibility of falling (see Figure 5.21). With added practice of each skill, they hesitate to crawl or walk down a risky incline. Experience in trying to keep their balance on various surfaces makes crawlers and walkers more aware of the consequences of their movements. Crawlers come to detect when surface slant places so much body weight on their arms that they will fall forward, and walkers come to sense when an incline shifts body weight so their legs and feet can no longer hold them upright.

Figure 5.21 Babies’ changing motor skills transform the way they perceive surfaces. Left: A 12-month- old who has just begun to walk proceeds feet-first down a steep incline, unaware of the high risk of falling. Right: An 18-month-old with extensive experience walking knows that it’s best to sit and scoot down the incline.

Infants do not transfer their learning about slopes or drop-offs from crawling to walking because the affordances for each posture are different (Adolph, Kretch, & LoBue, 2014). Learning takes time because newly crawling and walking babies cross many types of surfaces in their homes each day. As they experiment with balance and postural adjustments to accommodate each, they perceive surfaces in new ways that guide their movements. As a result, they act more competently.

As we conclude this chapter, it is only fair to note that some researchers believe that babies do more than make sense of experience by searching for invariant features and action possibilities: They also impose meaning on what they perceive, constructing categories of objects and events in the surrounding environment (Johnson & Hannon, 2015). We have seen the glimmerings of this cognitive point of view in this chapter. For example, older babies interpret a familiar face as a source of pleasure and affection and a pattern of blinking lights as a human being walking. This cognitive perspective also has merit in understanding the achievements of infancy. In fact, many researchers combine these two positions, regarding infant development as proceeding from a perceptual to a cognitive emphasis over the first year of life.

Ask Yourself

Connect ■ According to differentiation theory, perceptual development reflects infants’ active search for invariant features. Provide examples from research on hearing, pattern perception, object perception, and intermodal perception.

Apply ■ Ben, age 13 months, has just started to walk. Using the concept of affordances, explain why he is likely to step over risky drop-offs.

Reflect ■ Are young infants more competent than you thought they were before you read this chapter? List the capacities that most surprised you.

Summary

5.1 Body Growth (p. 153)

5.1 Describe major changes in body growth over the first two years.

Height and weight gains are greater during the first two years than at any other time after birth. Body fat develops quickly during the first nine months, whereas muscle development is slow and gradual.

Body proportions change as growth follows the cephalocaudal and proximodistal trends.

Assessments of skeletal age reveal that girls are ahead of boys in physical maturity, and African-American children tend to be ahead of European-American and Hispanic children.

5.2 Brain Development (p. 155)

5.2a Describe brain development during infancy and toddlerhood, current methods of measuring brain functioning, and appropriate stimulation to support the brain’s potential.

At birth, the brain is nearer its adult size than any other physical structure. Neurons rapidly form synapses and release neurotransmitters to send messages to one another. During the peak period of synaptic growth in any brain region, many surrounding neurons die through programmed cell death. Neurons that are seldom stimulated lose their synapses in a process called synaptic pruning. Glial cells, responsible for myelination, multiply rapidly through the second year, contributing to large gains in brain weight.

Measures of brain functioning include those that detect changes in electrical activity in the cerebral cortex (EEG, ERPs), neuroimaging techniques (PET, fMRI), and NIRS, which uses infrared light.

The cerebral cortex is the largest, most complex brain structure and the last to stop growing. Its regions develop in the general order in which various capacities emerge in the growing child. The frontal lobes, including the prefrontal cortex (responsible for complex thought) have the most extended period of development. The hemispheres of the cerebral cortex develop specialized functions, a process called lateralization. Brain plasticity, which decreases with age, enables other parts of the brain to take over functions of damaged areas.

Stimulation of the brain is essential during sensitive periods. Prolonged early deprivation like that experienced by infants in impoverished orphanages, can disrupt brain growth and interfere with the brain’s capacity to manage stress, with long-term psychological consequences.

Appropriate early stimulation promotes experience-expectant brain growth through ordinary experiences. No evidence exists for a sensitive period in the first few years for experience-dependent brain growth, which relies on specific learning experiences. In fact, environments that overwhelm children with inappropriately advanced expectations also interfere with the brain’s potential.

5.2b Explain how the organization of sleep and wakefulness changes over the first two years.

Infants’ changing arousal patterns are primarily affected by brain growth, but the social environment also plays a role. Periods of sleep and wakefulness become fewer but longer over the first two years, conforming to a night–day schedule. Most parents in Western nations try to get their babies to sleep through the night much earlier than parents throughout most of the world, who are more likely to cosleep with their babies. Bedtime routines help promote sleep.

5.3 Influences on Early Physical Growth (p. 166)

5.3 Cite evidence that heredity, nutrition, and parental affection all contribute to early physical growth.

Twin and adoption studies reveal that heredity contributes to body size and rate of physical growth.

Breast milk is ideally suited to infants’ growth needs. Breastfeeding protects against disease and prevents malnutrition and infant death in poverty-stricken areas of the world.

Most infants and toddlers can eat nutritious foods freely without risk of becoming overweight. However, the relationship between rapid weight gain in infancy and obesity at older ages is strengthening, perhaps because of a rise in unhealthy early feeding practices.

Marasmus and kwashiorkor are dietary diseases caused by malnutrition that affect many children in developing countries and, if prolonged, can permanently stunt body growth and brain development. Weight faltering illustrates the importance of parental affection and early emotional well-being for normal physical growth.

5.4 Learning Capacities (p. 172)

5.4 Discuss infant learning capacities, the conditions under which they occur, and the unique value of each.

Classical conditioning helps infants associate events that usually occur together in the everyday world. Infants can be classically conditioned most easily when the pairing of an unconditioned stimulus (UCS) and a conditioned stimulus (CS) has survival value.

In operant conditioning, infants act on their environment and their behavior is followed by either reinforcers, which increase the occurrence of a preceding behavior, or punishment, which either removes a desirable stimulus or presents an unpleasant one to decrease the occurrence of a response. In young infants, interesting sights and sounds and pleasurable caregiver interaction serve as effective reinforcers.

Habituation and recovery reveal that at birth, babies are attracted to novelty. Novelty preference (recovery to a novel stimulus) assesses recent memory, whereas familiarity preference (recovery to the familiar stimulus) assesses remote memory.

Infants have a built-in capacity for statistical learning, the ability to extract frequently occurring patterns—in speech, music, and visual shapes—from the complex flow of information in their surroundings.

Although hotly contested, the capacity for imitation, a powerful means of learning that contributes to the parent–infant bond, may be present at birth, as reflected in the apparent ability of newborns to imitate adults’ expressions and gestures. Scientists have identified specialized cells called mirror neurons that may underlie early imitation.

5.5 Motor Development (p. 177)

5.5 Describe dynamic systems theory of motor development, along with factors that influence motor progress in the first two years.

According to dynamic systems theory of motor development, children acquire new motor skills by combining existing skills into increasingly complex systems of action. Each new skill is a joint product of central nervous system development, the body’s movement capacities, the child’s goals and perceptual and cognitive capacities, and environmental supports for the skill.

Cultural values and infant-rearing customs contribute to the emergence and refinement of early motor skills.

During the first year, infants perfect their reaching and grasping. The poorly coordinated prereaching of the newborn gradually becomes more flexible and accurate, and the clumsy ulnar grasp is transformed into a refined pincer grasp.

5.6 Perceptual Development (p. 184)

5.6a Identify changes in hearing and in depth, pattern, object, and intermodal perception during infancy.

Infants organize sounds into increasingly complex patterns. In the middle of the first year, as part of the perceptual narrowing effect, they become more sensitive to the sounds of their own language. Infants’ capacity for statistical learning enables them to detect speech regularities for which they will later learn meanings.

Rapid maturation of the eye and visual centers in the cerebral cortex supports the development of focusing, color discrimination, and visual acuity during the first few months. The ability to scan the environment and track moving objects also improves.

Motion is the first depth cue to which infants are sensitive, followed by sensitivity to binocular and then to pictorial depth cues. Experience in crawling enhances depth perception, but babies must learn to use depth cues for each body position in order to avoid drop-offs.

Newborns prefer to look at patterned rather than plain stimuli. Once able to take in all aspects of a pattern, infants integrate its parts into a unified whole. With age, they prefer more complex, meaningful patterns.

Newborns prefer to look at photos and simplified drawings of faces with features arranged naturally. They quickly learn to prefer their mother’s face to that of an unfamiliar woman, and at 3 months, they make fine-grained distinctions among the features of different faces. At 5 months, they perceive emotional expressions as meaningful wholes. Early experience promotes perceptual narrowing with respect to gender and racial information in faces.

From birth, size and shape constancy and perception of object unity help babies detect a coherent world of objects. At first, infants depend on motion and spatial arrangement to identify objects. After 4 months of age, they rely more on shape, color, and pattern. Soon they can monitor increasingly intricate paths of objects, and they look for featural information to detect the identity of moving objects.

From the start, infants are capable of intermodal perception—combining information across sensory modalities. Detection of amodal sensory properties, such as common rate, rhythm, or intensity, or texture and shape, provides the basis for detecting many intermodal matches.

5.6b Explain differentiation theory of perceptual development.

According to differentiation theory, perceptual development involves detecting increasingly fine-grained invariant features in a constantly changing perceptual world. Perceptual differentiation is guided by discovery of affordances—the action possibilities that a situation offers the individual.

IMPORTANT TERMS AND CONCEPTS

affordances (p. 192)

amodal sensory properties (p. 191)

brain plasticity (p. 160)

cephalocaudal trend (p. 155)

cerebral cortex (p. 158)

classical conditioning (p. 172)

conditioned response (CR) (p. 172)

conditioned stimulus (CS) (p. 172)

differentiation theory (p. 192)

dynamic systems theory of motor development (p. 179)

experience-dependent brain growth (p. 163)

experience-expectant brain growth (p. 163)

glial cells (p. 157)

habituation (p. 174)

imitation (p. 176)

intermodal perception (p. 191)

kwashiorkor (p. 170)

lateralization (p. 159)

marasmus (p. 170)

mirror neurons (p. 177)

myelination (p. 157)

neurons (p. 155)

neurotransmitters (p. 156)

operant conditioning (p. 173)

perceptual narrowing effect (p. 185)

pincer grasp (p. 183)

prefrontal cortex (p. 159)

prereaching (p. 182)

programmed cell death (p. 156)

proximodistal trend (p. 155)

punishment (p. 173)

recovery (p. 174)

reinforcer (p. 173)

shape constancy (p. 190)

size constancy (p. 190)

statistical learning (p. 175)

synapses (p. 155)

synaptic pruning (p. 156)

ulnar grasp (p. 183)

unconditioned response (UCR) (p. 172)

unconditioned stimulus (UCS) (p. 172)

weight faltering (p. 171)

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