In infancy, growth is so rapid and the consequences of neglect are so severe that gains are closely monitored. Length, weight, and head circumference should be measured monthly at first, and every organ should be checked to make sure it functions well.

Weight gain is dramatic. Newborns lose weight in the first three days and then gain an ounce a day for several months. Birthweight typically doubles by 4 months and triples by a year. An average 7-pound newborn will be 21 pounds at 12 months (9,525 grams, up from 3,175 grams at birth).

Physical growth in the second year is slower but still rapid. By 24 months, most children weigh almost 28 pounds (13 kilograms). They have added more than a foot in height—from about 20 inches at birth to about 34 inches at age 2 (from 51 to 86 centimeters). This makes 2-year-olds about half their adult height and about one-fifth their adult weight, four times heavier than they were at birth (see Figure 3.1).

Each of these numbers is a norm, which is a standard, for a particular population. The “particular population” for the norms just cited is North American infants. Remember, however, that genetic diversity means that some perfectly healthy newborns from every continent are smaller or larger than these norms.

At each well-baby checkup, the baby’s growth is compared to that baby’s previous numbers. Often measurements are expressed as a percentile, from 0 to 100, comparing each baby to others the same age. For example, weight at the 30th percentile means that 30 percent of all babies weigh less, and 70 percent weigh more.

For any baby, an early sign of trouble occurs when percentile changes markedly, either up or down. If an average baby moves from, say, the 50th to the 20th percentile, that could be the first sign of failure to thrive, which could be caused by dozens of medical conditions. Pediatricians consider it “outmoded” to blame parents for failure to thrive, but in any case the cause should be discovered, and remedied (Jaffe, 2011, p. 100).

Throughout life, health and growth correlate with regular and ample sleep (Maski & Kothare, 2013). As with many health habits, sleep patterns begin in the first year.

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Newborns sleep about 15 to 17 hours a day. Every week brings a few more waking minutes. For the first two months the norm for total time asleep is 14¼ hours; for the next 3 months, 13¼ hours; for the next 12 months, 12¾ hours. Remember that norms are averages; individuals vary. Parents report that, among every 20 infants in the United States, one sleeps nine hours or fewer per day and one sleeps 19 hours or more (Sadeh et al., 2009).

National averages vary as well. By age 2, the typical New Zealand toddler sleeps 15 percent more than the typical Japanese one (13 hours compared to 11 ) (Sadeh et al., 2010).

Infants also vary in how long they sleep at a stretch. Preterm and breast-fed babies wake up often. Part of this depends on an adult’s perspective. If a night is thought to be from midnight to 5 A.M., many babies occasionally sleep “through the night” at 3 months. But if night is 10 P.M. to 6 A.M., many 1-year-olds don’t sleep all night (C. Russell et al., 2013).

Over the first months, the time spent in each type or stage of sleep changes. Babies born preterm may always seem to be dozing. About half the sleep of full-term newborns is REM (rapid eye movement) sleep, with flickering eyelids and rapid brain waves. That indicates dreaming. REM sleep declines over the early weeks, as does “transitional sleep,” the half-awake stage. At 3 or 4 months, quiet sleep (also called slow-wave sleep) increases markedly.

Sleep varies not only because of biology (age and genes) but also because of culture and caregivers. Babies who are fed formula and cereal sleep longer and more soundly—easier for parents but not necessarily good for the baby. Where babies sleep depends primarily on the baby’s age and the culture, with bed-sharing (in the parents’ bed) and co-sleeping (in the parents’ room) the norm in some cultures, but unusual in others (Esposito et al., 2015).

Parents are soon frustrated if they think their babies will adjust to adult sleep–wake schedules. Infant brain patterns and hunger needs do not allow them to sleep quietly for long stretches. This can create a problem for the entire family: Maternal depression and family dysfunction are more common when infants wake up often at night (Piteo et al., 2013).

Overall, 25 percent of children under age 3 have sleeping problems, according to parents surveyed in an Internet study of more than 5,000 North Americans (Sadeh et al., 2009). Problems are especially common when the baby is the parents’ first child.

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New parents “are rarely well-prepared for the degree of sleep disruption a newborn infant engenders.” As a result many become “desperate” and institute patterns that they may later regret (C. Russell et al., 2013, p. 68). But what patterns should they follow? Experts, strangers, and close relatives give conflicting advice. Co-sleeping is one example.

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Prenatal and postnatal brain growth (measured by head circumference) is crucial for later cognition (Gilles & Nelson, 2012). From two weeks after conception to two years after birth, the brain grows more rapidly than any other organ, being about 25 percent of adult weight at birth and almost 75 percent at age 2 (see Figure 3.3). Over the same two years, brain circumference increases from about 14 to 19 inches.

If teething or a stuffed-up nose temporarily slows weight gain, nature protects the brain, a phenomenon called head-sparing. Sadly, head-sparing does not last forever: Prolonged malnutrition (discussed later) affects the brain.

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BRAIN BASICS Findings from neuroscience are discussed in every chapter of this book. We begin here with the basics—neurons, axons, dendrites, neurotransmitters, synapses, and the cortex.

Communication within the central nervous system (CNS)—the brain and spinal cord—begins with nerve cells, called neurons. At birth, the human brain has an estimated 86 billion neurons, far more than any other primate. Especially in the cortex (the brain’s six outer layers (see Figure 3.4) where most thinking, feeling, and sensing occur), humans have more neurons than other mammals (Herculano-Houzel et al., 2014).

The final part of the brain to mature is the prefrontal cortex, the area behind the forehead that is crucial for anticipation, planning, and impulse control. The prefrontal cortex is inactive in early infancy and gradually becomes more efficient in childhood, adolescence, and adulthood, with marked variation from one person to another at every age (Walhovd et al., 2014).

Neurons connect to other neurons via intricate networks of nerve fibers called axons and dendrites (see Visualizing Development below). Each neuron typically has a single axon and numerous dendrites, which spread out like the branches of a tree. The axon of each neuron reaches toward the dendrites of other neurons at intersections called synapses, which are critical communication links within the brain.

Axons and dendrites do not touch at synapses. Instead, the electrical impulses in axons cause the release of chemicals called neurotransmitters, which carry information from the axon of the sending neuron to the dendrites of the receiving neuron.

GROWTH AND PRUNING During the first months and years, rapid growth and refinement in axons, dendrites, and synapses occur, especially in the cortex. Dendrite growth is the main reason that brain weight triples from birth to age 2 (Johnson, 2011).

An estimated fivefold increase in dendrites in the cortex occurs in the 24 months after birth, with about 100 trillion synapses present at age 2. According to one expert, “40,000 new synapses are formed every second in the infant’s brain” (Schore & McIntosh, 2011, p. 502).

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Early dendrite growth is called transient exuberance: exuberant because it is rapid and transient because some is temporary. This expansive growth is followed by pruning. A gardener might prune a rose bush by cutting away some growth to enable more, or more beautiful, roses to bloom. Similarly, unused brain connections atrophy and disappear.

Pruning is beneficial. Indeed, insufficient pruning may be the reason that many toddlers with autism have heads that are larger than average; too many dendrites make them hypersensitive to sights and sounds, unable to tolerate social interaction (Lewis et al., 2013).

As one expert explains it, there is an

exuberant overproduction of cells and connections, followed by a several-year sculpting of pathways by massive elimination of much of the neural architecture.

[Insel, 2014, p. 1727]

Notice the word sculpting, as if an artist created an intricate sculpture from raw marble or wood. Human infants are gifted sculptors, designing their brains for whatever family, culture, or society they happen to be born into, discarding the excess in order to think more clearly.

Experiences sculpt the brain. Some sculpting is called experience-expectant and some is called experience-dependent (Greenough et al., 1987).

Brain development is experience-expectant when it must happen for normal brain maturation to occur. Because they are basic to human development, expectant experiences occur for almost every baby. In deserts and in the Arctic, on isolated farms and in crowded cities, almost all babies have things to see, objects to manipulate, and people to love them. Without such expected experiences, dendrites and specific regions within the brain do not grow.

In contrast, certain facets of brain development are experience-dependent: They result from experiences that differ from one infant to another, resulting in brains that also differ. What specific language is heard, whose faces are seen, or how emotions are expressed—from slight pursing of the lips to throwing oneself on the ground—vary from one home to another.

Depending on such variations, infant neurons connect in particular ways; some dendrites grow and others disappear (Stiles & Jernigan, 2010). In other words, every baby needs to develop language—that is expectant, and brains are primed for it. But that language could be Tajik, Tamil, Thai, or Twi. That is experience-dependent. Brains adjust accordingly.

IMPLICATIONS FOR CAREGIVERS Most infants develop well within their culture. Head-sparing usually ensures that brains are nourished, and everywhere adults nurture the young. Even strangers are drawn to babies a few weeks old, talking to them and making faces, holding them if the parents allow it. All that is experience-expectant; babies worldwide expect attention and almost always get it.

Playing with a young baby, allowing varied sensations, and encouraging movement (arm waving in the early months, walking later on) are all fodder for brain connections. Severe lack of stimulation (e.g., no talking at all) stunts the brain.

This does not mean that babies require spinning, buzzing, multitextured, and multicolored toys. In fact, such toys may be a wasted purchase since overstimulated babies may cry to avoid bombardment. Infants are fascinated by simple objects and exaggerated expressions. A mouth opening wide or making smacking sounds captures infant attention.

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The slow development of the prefrontal cortex means that infants cannot yet plan, anticipate, or modify their emotions. Unless adults understand this, they might be angry with an infant who does not smile, or stop crying, or sleep when the adult wishes.

If a frustrated caregiver reacts to crying by shaking the baby, that can cause shaken baby syndrome, a life-threatening type of abusive head trauma (Nadarasa et al., 2014). Because the brain is still developing, shaking an infant sharply and quickly stops the crying because blood vessels in the brain rupture and fragile neural connections break. Death is possible; lifelong intellectual impairment is likely.

Every sense functions at birth. Newborns have open eyes, sensitive ears, and responsive noses, tongues, and skin. Indeed, very young babies use all their senses to attend to everything, especially every person (Zeifman, 2013).

Sensation occurs when a sensory system detects a stimulus, as when the inner ear reverberates with sound or the eye’s retina and pupil intercept light. Thus, sensations begin when an outer organ (eye, ear, nose, tongue, or skin) meets anything that can be seen, heard, smelled, tasted, or touched.

Perception occurs when the brain processes a sensation. This happens in the cortex, usually as the result of a message from one of the sensing organs, such as from the eye to the visual cortex. The sight of a bottle, for instance, is conveyed from the retina to the optic nerve to the visual cortex, but it has no meaning unless the infant has been bottle-fed. Similarly, a scrap of paper means nothing to you unless you are looking for something written on just such a scrap. Perceptions require experience and motivation.

Thus, perception follows sensation, when sensory stimuli are interpreted in the brain. Then cognition follows perception, when people think about what they have perceived. The baby might reach out for the bottle, you might examine the paper. The sequence from sensation to perception to cognition requires first that the sense organs function. No wonder the parts of the cortex dedicated to hearing, seeing, and so on develop rapidly. Now specifics.

HEARING The fetus hears during the last trimester of pregnancy; loud sounds trigger reflexes even without conscious perception. Familiar, rhythmic sounds such as a heartbeat are soothing: Sometimes newborns stop crying if they are held so an ear is on the mother’s chest.

Because of early maturation of the language areas of the cortex, even 4-month-olds attend to voices, developing expectations of the rhythm, segmentation, and cadence of spoken words long before comprehension (Minagawa-Kawai et al., 2011). Soon, sensitive hearing combines with the maturing brain to distinguish patterns of sounds and syllables.

Infants become accustomed to the patterns of their native language, such as which syllable is stressed (various dialects differ), whether inflection matters (it is crucial in Chinese), whether certain sound combinations are repeated, and so on. All this is based on very careful listening, including of speech not directed toward them (Buttelmann et al., 2013).

Better are sounds directed to the infant. Thus, a newborn named Emily has no concept that Emily is her name, but she has the brain and auditory capacity to hear sounds in the usual speech range (not some sounds that other creatures can hear) and an inborn preference for repeated patterns and human speech.

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By about 4 months, when her auditory cortex is rapidly creating dendrites, the repeated word Emily is perceived as well as sensed, especially because that sound emanates from interactions with the people she often sees, smells, and touches. Before 6 months, Emily opens her eyes and smiles when her name is called, perhaps babbling in response.

This rapid development of hearing is the reason newborn hearing is tested, and, if necessary, remediation begins in infancy. By age 5, those who got cochlear implants in the early months are much better at understanding and expressing language than those with identical losses but whose implants were added after age 2 (Tobey et al., 2013).

SEEING By contrast, vision is immature at birth. Although in mid-pregnancy the eyes open and are sensitive to bright light (if a pregnant woman is sunbathing in a bikini, for instance), the fetus has nothing much to see. Newborns are legally blind; they focus only on things quite close to their eyes, such as the face of their breast-feeding mother.

Almost immediately, experience combines with maturation of the visual cortex to improve vision. Indeed, vision improves so rapidly that researchers are hard-pressed to describe the day-by-day improvements (Dobson et al., 2009). By 2 months, infants not only stare at faces, but also, with perception and the beginning of cognition, smile. (Smiling can occur earlier but not because of perception.)

As perception builds, visual scanning improves. Thus, 3-month-olds look closely at the eyes and mouth, smiling more at happy faces than at angry or expressionless ones. They pay attention to patterns, colors, and motion—the mobile above the crib, for instance.

Because of this rapid development, babies should be allowed to see many sights. A crying baby might be distracted by being taken outside to watch passing cars. If possible, cataracts (present in about 1 newborn in 2,000) should be surgically removed in the early months (Medsinge & Nischal, 2015).

Binocular vision (coordinating both eyes to see one image) cannot develop in the womb (nothing is far enough away), so many newborns use their two eyes independently, momentarily appearing wall-eyed or cross-eyed. Normally, between 2 and 4 months, experience allows both eyes to focus on a single thing (Wang & Candy, 2010).

Depth perception (which requires binocular vision) is usually present by 3 months, but understanding depth takes experience. Not until toddlers have experienced crawling and walking do they know whether a surface is best traversed upright, sitting, or crawling (Kretch & Adolph, 2013). The senses and motor skills take time to coordinate.

TASTING AND SMELLING As with vision and hearing, smell and taste function at birth and rapidly adapt to the social world. Infants learn to appreciate what their mothers eat, first through breast milk and then through smells and spoonfuls of the family dinner.

The foods of a particular culture may aid survival: For example, bitter foods provide some defense against malaria, hot spices help preserve food and may prevent food poisoning, and so on (Krebs, 2009). Thus, for 1-year-olds, developing a taste for their family cuisine not only helps them join their community, it may save their lives.

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Notice once again how early experiences sculpt the brain. Taste preferences endure when a person migrates to another culture or when a particular food that was once protective is no longer so. Indeed, one reason for the obesity epidemic is that, when starvation was a threat, families sought high-fat foods. Now their descendants enjoy fried foods, whipped cream, and bacon, and their inborn preferences and cultural habits jeopardize their health.

Adaptation also occurs for the sense of smell. When breast-feeding mothers used a chamomile balm to ease cracked nipples, their babies preferred that smell almost two years later, unlike babies whose mothers used an odorless ointment (Delaunay-El Allam et al., 2010).

As babies learn to recognize each person’s scent, they prefer to sleep next to their caregivers, and they nuzzle into their caregivers’ chests—especially when the adults are shirtless. One way to help infants who are frightened of the bath (some love bathing, some hate it) is for the parent to join the baby in the tub. The smells of the adult’s body mixed with the smell of soap and the touch, sight, and voice of the caregiver are pleasant, making the entire experience comforting.

TOUCH AND PAIN Similarly, the sense of touch is acute in infants. Wrapping, rubbing, massaging, and cradling are soothing. Even when their eyes are closed, some infants stop crying and visibly relax when held securely by their caregivers. In the first year of life, the heart rate slows and muscles relax when babies are stroked gently and rhythmically on the arm (Fairhurst et al., 2014).

In every culture, parents cuddle their newborns, rocking, carrying, and so on. Some touch (gentle of course) seems experience-expectant, essential for normal growth. Beyond that, how much a baby is touched is experience-dependent, varying by culture. In some south Asian nations, daily massage begins soon after birth (Trivedi, 2015).

Indeed, in rural India, mothers need to be taught that the newborn’s need for warmth is more important than immediate bathing and massage, since both of those practices are common for infants and may, inadvertently, harm a newborn. Mothers are encouraged to wipe their newborns with a dry cloth and breast-feed immediately—practices that keep the baby warm, use the sense of touch, and reduce the risk of early death (Acharya et al., 2015).

Pain, motion, and temperature are not among the traditional five senses, but they are often connected to touch. Some babies cry when being changed, distressed at the sudden coldness on their skin and by having to lie down, not held by their caregivers.

Scientists are not certain about pain. Some experiences that are painful to adults (circumcision, setting of a broken bone) are much less so to newborns, although that does not prove that newborns are unable to feel pain (Reavey et al., 2014).

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Many young infants sometimes cry inconsolably; digestive pain is the usual explanation. Teething is also said to be painful. However, these explanations also are unproven. Certainly for adults, pain and crying do not always occur together.

For many newborn medical procedures, a taste of sugar right before the event is an anesthetic. One example is that newborns typically cry lustily when their heel is pricked (to get a blood sample, routine after birth), but when an experimental group had a drop of sucrose before the heel stick, they were much less likely to cry than babies in a control group who had no sucrose (Harrison et al., 2010).

Some people imagine that even the fetus feels pain; others say that pain receptors in the brain are not activated until months after birth. Physiological measures (stress hormones, erratic heartbeats, and brain waves) are studied to assess pain in preterm infants, who typically undergo many procedures that would be painful to an adult (Holsti et al., 2011). More research is needed.

Every basic motor skill (any movement ability), from the newborn’s head-lifting to the toddler’s stair-climbing, develops over the first two years.

REFLEXES The sequence of motor skills begins with reflexes, some of which are listed here in italics. Newborns have even more than the 17 on this list.

Other reflexes are not necessary for survival but signify the state of brain and body functions. Among them are the:

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Although the definition of reflexes implies that they are automatic, their strength and duration vary from one baby to another, and cultural responses shape them. Many newborn reflexes disappear by 3 months, but some morph into more advanced motor skills.

Reflexes become skills if they are practiced and encouraged. As you saw in the chapter’s beginning, Mrs. Todd set the foundation for my fourth child’s walking when Sarah was only a few months old. Similarly, some 1-year-olds can swim—if adults have built on the swimming reflex to teach them to paddle in water from the early weeks.

GROSS MOTOR SKILLS Deliberate actions that use many parts of the body, producing large movements, are called gross motor skills. These skills emerge directly from reflexes and proceed in a cephalocaudal (head-down) and proximodistal (center-out) direction.

Infants first control their heads, lifting them up to look around, an early example of cephalocaudal maturation. Then control moves downward—upper bodies, arms, and finally legs and feet. (See At About This Time, which shows age norms for gross motor skills based on a large, representative, multiethnic sample of U.S. infants.)

Sitting develops gradually. By 3 months, most babies can sit propped up in a lap. By 6 months, they can usually sit unsupported. Babies never propped up (as in some institutions for abandoned infants) sit much later.

Note: As the text explains, age norms are affected by culture and cohort. The first five norms are based on babies on five continents [Brazil, Ghana, Norway, United States, Oman, and India] (World Health Organization, 2006). The next three are from a U.S. only source (Coovadia & Wittenberg, 2004; based on Denver II [Frankenburg et al., 1992]). Mastering skills a few weeks earlier or later does not indicate health or intelligence. Being very late, however, is a cause for concern.

Crawling is another example of the head-down and center-out direction of skill mastery. When placed on their stomachs, many newborns reflexively try to lift their heads and move their arms as if they were swimming. As they gain muscle strength, infants wiggle, attempting to move forward by pushing their arms, shoulders, and upper bodies against whatever surface they are lying on.

Usually by 5 months, infants add their legs to this effort, inching forward (or backward) on their bellies. Exactly when this occurs depends partly on how much “tummy time” the infant has had to develop the muscles, and that, of course, is affected by the caregiver’s culture (Zachry & Kitzmann, 2011).

Most 8- to 10-month-olds can lift their midsections and crawl (or creep, as the British call it) on “all fours,” coordinating the movements of their hands and knees. Crawling depends on experience, not just maturation. Some normal babies never do it, especially if the floor is cold, hot, or rough, or if they always lie on their backs. It is not true that babies must crawl to develop normally.

All babies find some way to move (inching, bear-walking, scooting, creeping, or crawling) before they can walk, but many resist being placed on their stomachs. Heavier babies master gross motor skills later than leaner ones because practice and balance is harder when the body is heavy (Slining et al., 2010).

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As soon as they are able, babies take some independent steps, falling frequently at first, about 32 times per hour. They persevere because walking is much quicker than crawling, and it has other advantages—better sight lines and free hands (Adolph & Tamis-LeMonda, 2014).

Once toddlers take those first unsteady steps, they practice obsessively, barefoot or not, at home or in stores, on sidewalks or streets, on lawns or in mud. They “immediately go more, see more, play more, and interact more” (Adolph & Tamis-LeMonda, 2014, p. 191).

FINE MOTOR SKILLS Small body movements are called fine motor skills. The most valued fine motor skills are finger movements, enabling humans to write, draw, type, tie, and so on. Movements of the tongue, jaw, lips, teeth, and toes are fine movements, too.

Actually, mouth skills precede hand skills by many months (newborns can suck; chewing precedes drawing by a year or more). Since every culture encourages finger dexterity, children practice finger movements, and adults teach how to use spoons, or chopsticks, or markers. By contrast, mouth skills such as spitting or biting are not praised. (Only other children admire blowing bubbles with gum.)

Regarding hand skills, newborns have a strong reflexive grasp but lack control. During their first 2 months, babies excitedly stare and wave their arms at objects dangling within reach. By 3 months, they can usually touch such objects, but because of limited eye–hand coordination, they cannot yet grab and hold on unless an object is placed in their hands.

By 4 months, infants sometimes grab, but their timing is off: They close their hands too early or too late. Finally by 6 months, with a concentrated, deliberate stare, most babies can reach, grab, and grasp almost any object that is of the right size. Some can even transfer an object from one hand to the other.

Almost all can hold a bottle, shake a rattle, and yank a sister’s braids. Toward the end of the first year and throughout the second, finger skills improve as babies master the pincer movement (using thumb and forefinger to pick up tiny objects) and self-feeding (first with hands, then fingers, then utensils) (Ho, 2010). (See At About This Time.)

As with gross motor skills, fine motor skills are shaped by culture and opportunity. For example, when given “sticky mittens” (with Velcro) that facilitate grabbing, infants master hand skills sooner than the norm. Their perception advances as well (Libertus & Needham, 2010; Soska et al., 2010). As you remember, experience leads to perception and cognition.

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COMBINING SENSES AND SKILLS All senses and motor skills expand the baby’s cognitive awareness, with practice advancing both skill and understanding (Leonard & Hill, 2014). In the second year, grasping becomes more selective, as experience sculpts the brain. Toddlers learn when not to pull at a sister’s braids or an adult’s earrings or glasses. (Wise adults, however, remove such accessories before holding a baby.)

Data from World Health Organization, 2006.

The age at which walking occurs is a better predictor than simple chronological age of a child’s verbal ability, perhaps because walking children elicit more language from caregivers than crawling ones do (Walle & Campos, 2014). The correlation could go in the opposite direction as well: Walkers see their caregivers more, so they talk more (Adolph & Tamis-LeMonda, 2014, p. 191).

Overall, babies perceive their most important experiences with several senses and skills, in dynamic systems. Breast milk, for instance, is a mild sedative, so the infant literally feels happier at the breast, connecting that pleasure with taste, touch, smell, and sight. But first, a motor skill is needed: The infant must actively suck at the nipple (an inborn reflex, which becomes more efficient with practice).

Because of brain immaturity, cross-modal perception (using several senses to understand the same experience) is particularly common in young infants. Synesthesia (when a sensation is perceived with more than one sense, as when a sound has a color) is also more common in early infancy, because the various parts of the brain are less distinct (Ozturk et al., 2013).

Remember the dynamic systems of senses and motor skills: If one aspect of the system lags behind, the other parts may be affected as well. On the other hand, young walkers are thrilled to grab dozens of objects they could not explore before—-caregivers beware.

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