Motor Development

As you learned in Chapter 2, human movement starts well before birth, as the fetus floats weightlessly in amniotic fluid. After birth, the newborn's movements are jerky and relatively uncoordinated, in part because of physical and neurological immaturity and in part because the baby is experiencing the full effects of gravity for the first time. As you will see in this section, the story of how the uncoordinated newborn, a prisoner of gravity, becomes a competent toddler confidently exploring the environment is remarkably complicated.

Reflexes

Newborns start off with some tightly organized patterns of action known as neonatal reflexes. Some reflexes, such as withdrawal from a painful stimulus, have clear adaptive value; others have no known adaptive significance. In the grasping reflex, newborns close their fingers around anything that presses against the palm of their hand. When stroked on the cheek near their mouth, infants exhibit the rooting reflex, turning their head in the direction of the touch and opening their mouth. Thus, when their cheek comes into contact with their mother's breast, they turn toward the breast, opening their mouth as they do. Oral contact with the nipple then sets off a sucking reflex, followed by the swallowing reflex, both of which increase the baby's chance of getting nourishment and ultimately of surviving. These reflexes are not fully automatic; for example, a rooting reflex is more likely to occur when an infant is hungry.

No benefit is known to be associated with other reflexes, such as the tonic neck reflex: when an infant's head turns or is turned to one side, the arm on that side of the body extends, while the arm and knee on the other side flex. It is thought that the tonic neck reflex involves an effort by the baby to get and keep its hand in view (von Hofsten, 2004).

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ELIZABETH CREWS/THE IMAGE WORKS
LAURENT/RAVONISON/BSIP/SCIENCE SOURCE/HOTO RESEARCHERS, INC.
CUSTOM MEDICAL STOCK PHOTO/ALAMY

The presence of strong reflexes at birth is a sign that the newborn's central nervous system is in good shape. Reflexes that are either unusually weak or unusually vigorous may signal brain damage. Most of the neonatal reflexes disappear on a regular schedule, although some—including coughing, sneezing, blinking, and withdrawing from pain—remain throughout life. Persistence of a neonatal reflex beyond the point at which it is expected to disappear can indicate a neurological problem.

Motor Milestones

Infants progress quickly in acquiring the basic movement patterns of our species, shown in Figure 5.9. As you will see, the achievement of each of the major “motor milestones” of infancy, especially walking, constitutes a major advance, and provides new ways for infants to interact with the world.

FIGURE 5.9 The major milestones of motor development in infancy The average age and range of ages for achievement of each milestone are shown. Note that these age norms are based on research with healthy, well–nourished North American infants.

The average ages that Figure 5.9 gives for the development of each of these important motor skills are based on research with Western, primarily North American, infants. There are, of course, tremendous individual differences in the ages at which these milestones are achieved. Of particular interest is the fact that the degree to which motor skills are encouraged varies from one culture to another, and such variation can affect the course of motor development. Indeed, some cultures actively discourage early locomotion. In modern urban China, for example, infants are typically placed on beds and surrounded by thick pillows to keep them from crawling on the dirty floor (Campos et al., 2000). These restrictions make it difficult for infants to develop the muscle strength required to support their upper trunk, which is necessary for crawling. Among the Aché, a nomadic people who live in the rain forest of Paraguay, infants spend almost all of their first 3 years of life being carried by their mothers or kept very close to her because of safety concerns. These infants thus get relatively little opportunity early on to exercise their locomotor skills (Kaplan & Dove, 1987).

FIGURE 5.10 These images depict the footprint paths for a single infant participant who was tested walking naked, wearing lightweight disposable diapers, and wearing bulkier cloth diapers. The most mature walking behavior was seen in the left–most path, in the absence of diapers (Cole et al., 2012).

ALL: COURTESY KAREN ADOLPH

In direct contrast, the Kipsigis people in rural Kenya actively encourage the motor development of their infants; for example, they help their babies practice sitting by propping them up in shallow holes dug in the ground to support their backs (Super, 1976). Other groups, in West Africa and the West Indies, institute an aggressive program of massage, manipulation, and stimulation designed to facilitate their infants' motor development (A. Gottlieb, 2004; Hopkins & Westra, 1988).

These widely varying cultural practices can affect infants' development. Researchers have documented somewhat slower motor development in Aché and Chinese infants compared with the norms shown in Figure 5.9; Kipsigis babies and the infants who undergo exercise regimes, on the other hand, are advanced in their motor–skill development compared with North American infants. Even aspects of infant life that we take for granted in our own culture have an effect on motor development. In a recent study, researchers asked whether diapers—a relatively recent cultural invention—have an impact on walking behavior (Cole, Lingeman, & Adolph, 2012; see Figure 5.10). The researchers found that the same infants exhibited more mature walking behavior when tested naked than when tested diapered, despite the fact that these infants—all residents of New York City—were accustomed to wearing diapers and had rarely walked naked. These data beautifully demonstrate that cultural practices that are undertaken in one domain (toileting) can have unforeseen consequences in another domain (walking behavior).

Current Views of Motor Development

Impressed by the orderly acquisition of skills reflected in Figure 5.9, two early pioneers in the study of motor development, Arnold Gesell and Myrtle McGraw, concluded that infants' motor development is governed by brain maturation (Gesell & Thompson, 1938; McGraw, 1943). In contrast, current theorists, many of whom take a dynamic–systems approach (see Chapter 4), emphasize that early motor development results from a confluence of numerous factors that include developing neural mechanisms, increases in infants' strength, posture control, balance, and perceptual skills, as well as changes in body proportions and motivation (Bertenthal & Clifton, 1998; Lockman & Thelen, 1993; von Hofsten, 2004). (Box 5.3 offers a detailed account of a program of research exemplifying this approach.)

Think for a moment about how each of these factors plays a part in infants' gradual transition from newborns unable even to lift their head to toddlers who walk independently, holding their upper body erect while coordinating the movement of their legs that have grown strong enough to support their weight. Every milestone in this transition is fueled by what infants can perceive of the external world and their motivation to experience more of it. The vital role of motivation is especially clear in infants' determined efforts to attempt to walk when they can get around much more efficiently by crawling. Most parents—and many researchers—have the impression that infants derive pleasure from pushing the envelope of their motor skills.

The Expanding World of the Infant

Infants' mastery of each of the milestones shown in Figure 5.9 greatly expands their world: there is more to see when they can sit up, more to explore when they can reach for things themselves, and even more to discover when they can move about on their own. In this section, we consider some of the ways that motor development affects infants' experience of the world.

Reaching

The development of reaching sets off a minirevolution in the infant's life: “once infants can reach for and grasp objects, they no longer have to wait for the world to come to them” (Bertenthal & Clifton, 1998). However, reaching takes time to develop. That is because, as discussed in Chapter 4, this seemingly simple behavior actually involves a complex interaction of multiple, independent components, including muscle development, postural control, development of various perceptual and motor skills, and so on.

Initially, infants are limited to prereaching movements—clumsy swiping toward the general vicinity of objects they see (von Hofsten, 1982). At around 3 to 4 months of age, they begin successfully reaching for objects, although their movements are initially somewhat jerky and poorly controlled, and their grabs fail more often than not.

Earlier, we noted that infants' achievements in motor development pave the way for new experiences and opportunities to learn. A particularly compelling example comes from studies (described in Chapter 4) in which prereaching infants were given Velcro–patched mittens and Velcro–patched toys that allowed them to pick up objects (Needham, Barrett, & Peterman, 2002). The manual exploration of objects made possible by these “sticky mittens” led to the infants' increased interest in objects and the earlier emergence of their ability to reach independently for them. Interestingly, a related study found that the effects of the “sticky mittens” intervention extended beyond objects (Libertus & Needham, 2011). Improved ability to interact with objects gives infants additional opportunities to learn about the social world—namely, how people interact with objects. Such improvement also provides infants with new ways to interact with caregivers through shared play with objects. Together, these factors serve to increase infants' interest in social partners.

At around 7 months, as infants gain the ability to sit independently, their reaching becomes quite stable, and the trajectory of their reaches is consistently smooth and straight to the target (Spencer et al., 2000; Thelen et al., 1993; von Hofsten, 1979, 1991). The achievement of stable sitting and reaching enables infants to enlarge their sphere of action, because they can now lean forward to capture objects previously out of reach (Bertenthal & Clifton, 1998; Rochat & Goubet, 1995). These increased opportunities for object exploration have ramifications for visual perception. For example, consider the difficulty of perceiving 3D objects as whole objects. By their very nature, the front portions of 3D objects block perception of their back portions. Nevertheless, even without X–ray vision, adults readily fill in the nonvisible portions of 3D objects and perceive them as solid volumes. It turns out that having more experience manipulating objects helps infants to become better at this process of 3D object completion. Infants with better sitting and manual skills are better at perceiving complete 3D objects from a limited view than are infants with weaker sitting and manual skills (Soska, Adolph, & Johnson, 2010).

These sources of evidence suggest that there is a great deal of interaction between visual development and motor development. At the same time, infants can perform quite well on some motor tasks in the absence of vision by using auditory or vestibular cues instead. For example, vision is not necessary for accurate reaching: infants in a completely dark room can successfully nab an invisible object that is making a sound (Clifton et al., 1991). In addition, when reaching for objects they can see, infants rarely reach for ones that are too distant, suggesting that they have some sense of how long their arms are (Bertenthal & Clifton, 1998).

With age and practice, infants' reaching shows increasingly clear signs of anticipation; for example, when reaching toward a large object, infants open their fingers widely and adjust their hand to the orientation of the desired object (Lockman, Ashmead, & Bushnell, 1984; Newell et al., 1989). Furthermore, like an outfielder catching a fly ball, infants can make contact with a moving object by anticipating its trajectory and aiming their reach slightly ahead of it (Robin, Berthier, & Clifton, 1996; von Hofsten et al., 1998). Most impressive, 10–month–olds' approach to an object is affected by what they intend to do after they get their hands on it. Like adults, they reach faster for an object that they plan to throw than for one they plan to use in a more precise fashion (Claxton, Keen, & McCarty, 2003). However, as Figure 5.11 illustrates, infants' anticipation skills remain quite limited for some time.

FIGURE 5.11 This right–handed 14–month–old—a participant in research by Rachel Keen and colleagues—is having a hard time getting the applesauce he has been offered into his mouth. As the photo on the left shows, he has been presented the spoon with its handle to his left, but he has grabbed it with his dominant right hand, which makes it extremely difficult to keep the spoon upright on its way to his mouth. A spill ensued.

COURTESY OF RACHEL KEEN

Self–Locomotion

At around 8 months of age, infants become capable for the first time in their lives of self-locomotion, that is, of moving around in the environment on their own. No longer limited to being only where someone else carries or puts them, their world must seem vastly larger to them.

Infants' first success at moving forward under their own power typically takes the form of crawling. (Box 5.4 describes a recent increase in variability in the onset of crawling.) Many (perhaps most) infants begin by belly crawling or using other idiosyncratic patterns of self–propulsion, one of which researchers refer to as the “inchworm belly–flop” style (Adolph, Vereijken, & Denny, 1998). Most belly crawlers then shift to hands–and–knees crawling, which is less effortful and faster. Other styles of crawling also have colorful names: bear crawls, crab crawls, spider crawls, commando crawls, and bum shuffles (Adolph & Robinson, 2013). The broader point is that infants are remarkably good at finding ways to get around prior to their being able to walfk.

Van Gogh's painting First Steps may have been inspired by the joy that most parents feel at seeing their baby walk alone for the first time and the joy the baby feels taking those first steps.

THE METROPOLITAN MUSEUM OF ART/ART RESOURCE, NY

When infants first begin walking independently, at around 11 to 12 months, they keep their feet relatively wide apart, which increases their base of support; they flex slightly at the hip and knee, thereby lowering their center of gravity; they keep their hands in the air to facilitate balance; and they have both feet on the ground 60% of the time (as opposed to only 20% for adults) (Bertenthal & Clifton, 1998). As they grow stronger and gain experience, their steps become longer, straighter, and more consistent. Practice is vital to infants' gradual mastery over their initially weak muscles and precarious balance (Adolph, Vereikjen, & Shrout, 2003). And practice they do: Adolph and colleagues (2012) found that their sample of 12– to 19–month–olds in New York City averaged 2,368 steps (and 17 falls) per hour!

The everyday life of the newly mobile crawler or walker is replete with challenges to locomotion—slippery floors, spongy carpets, paths cluttered with objects and obstacles, stairs, sloping lawns, and so on. Infants must constantly evaluate whether their developing skills are adequate to enable them to travel from one point to another. Eleanor Gibson and her colleagues found that infants adjust their mode of locomotion according to their perception of the properties of the surface they want to traverse (Gibson et al., 1987; Gibson & Schmuckler, 1989). For example, an infant who had promptly walked across a rigid plywood walkway would prudently revert to crawling in order to get across a water bed. Box 5.5 summarizes a program of research on the early development of locomotion and other forms of motor behavior in infancy, focusing specifically on the integration of perception and locomotion.

BOX 5.4: a closer look “GANGWAY—I’M COMING DOWN”

The interdependence of different developmental domains is beautifully illustrated by a rich and fascinating series of experiments conducted over five decades. This work started with a landmark study by Eleanor Gibson and Richard Walk (1960) that addressed the question of whether infants can perceive depth. It has culminated in research linking depth perception, locomotion, cognitive abilities, emotion, and the social context of development.

To answer the depth–perception question, Gibson and Walk used an apparatus known as the “visual cliff.” As the photo shows, the visual cliff consists of a thick sheet of plexiglass that can support the weight of an infant or toddler. A platform across the middle divides the apparatus into two sides. A checked pattern right under the glass on one side makes it look like a solid, safe surface. On the other side, the same pattern is far beneath the glass, and the contrast in the apparent size of the checks makes it look as though there is a dangerous drop–off—a “cliff”—between the two sides.

Gibson and Walk reported that 6– to 14–month–old infants would readily cross the shallow side of the visual cliff. They would not, however, cross the deep side, even when a parent was beckoning to them to come across it. The infants were apparently unwilling to venture over what looked like a precipice—strong evidence that they perceived and understood the significance of the depth cue of relative size.

Karen Adolph, who had been a student of Gibson, has conducted extensive research on the relation between perception and action in infancy. Adolph and her colleagues have discovered surprising discontinuities in infants' learning what they can and cannot accomplish with their developing locomotor and postural skills (Adolph, 1997, 2000; Adolph, Eppler, & Gibson, 1993; Adolph et al., 2003; Eppler, Adolph, & Weiner, 1996). This research exemplifies our theme of mechanisms of change, in which variation and selection produce developmental change.

As a way of studying the relation between early motor abilities and judgment, the investigators asked parents to try to entice their infants to lean over or crawl across gaps of varying widths in an elevated surface or to crawl or walk down sloping walkways that varied in how steep they were. Some of these tasks were possible for a given infant; the baby would have no trouble, for example, negotiating a slope of a particular steepness. Others, however, were impossible for that infant. Would the babies identify which tasks were which? (An experimenter always hovered nearby to catch any infant who misjudged his or her prowess.)

The photos below show how infants behaved on slopes when beckoned by an adult (usually their mother). In their first weeks of crawling, infants (averaging around 8½ months in age) unhesitatingly and competently went down shallow slopes. Confronted with slopes that were too steep to crawl down, the babies typically paused for a moment, but then launched themselves headfirst anyway (requiring the experimenter to catch hold of them). With more weeks of crawling practice, the babies got better at judging when a slope was simply too steep and should be avoided. They also improved at devising strategies to get down somewhat steep slopes, such as turning around and cautiously inching backward down the slope.

This infant is refusing to cross the deep side of the visual cliff, even though his mother is calling and beckoning to him from the other side.

COURTESY OF PROFESSOR JOSEPH J. CAMPOS, UNIVERSITY OF CALIFORNIA, BERKELEY

Integrating perceptual information with new motor skills. Researcher Karen Adolph will need to rescue the newly crawling young infant on the left, who does not realize that this slope is too steep for her current level of crawling expertise. In contrast, the experienced walker on the right is judiciously deciding that the slope is too steep for him to walk down.

BOTH: COURTESY OF KAREN ADOLPH

However, when the infants started walking, they again misjudged which slopes they could get down using their new mode of locomotion and tried to walk down slopes that were too steep for them. In other words, they failed to transfer what they had learned about crawling down slopes to walking down them. Thus, infants apparently have to learn through experience how to integrate perceptual information with each new motor behavior they develop. With experience comes increased flexibility, allowing access to multiple strategies for solving previously intractable problems, including laboratory–created challenges such as descending impossibly steep slopes or crossing narrow bridges with wobbly handrails (Adolph & Robinson, 2013).

Infants' decisions in such situations also depend on social information. Infants who are close to being able to make it down a relatively steep walkway can be rather easily discouraged from trying to do so by their mother telling them, “No! Stop!” Conversely, enthusiastic encouragement from a parent can lead an inexperienced crawler or walker to attempt a currently too–steep slope. Thus, the child uses both perceptual and social information in deciding what to do. In this case, the information is obtained through social referencing, the child's use of another person's emotional response to an uncertain situation to decide how to behave (see Chapter 10).

A key finding of Adolph's research is that infants have to learn from experience what they can and cannot do with respect to each new motor skill that they master. Just like the new crawlers and walkers who literally plunge ahead when put atop a sloping walkway, an infant who has just developed the ability to sit will lean too far out over a gap in a platform in an attempt to snag an out–of–reach toy and would fall over the edge if not for the ever–present reseacher–catcher. And, like the experienced crawlers and walkers who pause to make a prudent judgment about whether or not to try a descent, an infant who has been capable of sitting unsupported for some time can judge whether the gap is too wide to lean across and will stay put if it appears to be so. These highly consistent findings across a variety of motor skills have made a very important contribution to our understanding of how infants learn to interact successfully with their environment.

The challenge that young children experience in integrating perceptual information in the planning and execution of actions sometimes results in quite surprising behaviors, especially when children fail to meet the challenge. A particularly dramatic example of failure in the integration of perception and action is provided by scale errors (Brownell, Zerwas, & Ramani, 2007; DeLoache, Uttal, & Rosengren, 2004; Ware et al., 2006). In this kind of error, very young children try to do something with a miniature replica object that is far too small for the action to be at all possible. Toddlers will attempt, in all seriousness, to sit in a tiny, dollhouse–sized chair or to get into a small toy car (see Figure 5.12). In committing a scale error, the child momentarily fails to take into account the relation between his or her own body and the size of the target object. These errors are hypothesized to result from a failure to integrate visual information represented in two different areas of the brain in the service of action. With development, the incidence of scale errors diminishes, although even adults make a variety of action errors (e.g., putting a cup of water into the cupboard instead of the microwave or trying to squeeze into a too tight pair of pants).

FIGURE 5.12 Scale errors These three children are making scale errors, treating a miniature object as if it were a much larger one. The girl on the left has just fallen off the toy slide she was trying to go down; the boy in the middle is persistently trying to get into a very small car; and the boy on the right is attempting to sit in a miniature chair. (From DeLoache et al., 2004)

COURTESY OF JUDY DELOACHE