PHYSICAL DEVELOPMENT in INFANCY and TODDLERHOOD I. BODY GROWTH A. Physical growth occurs rapidly during the first 2 years of life. The transition from infancy to toddlerhood-the period that spans the second year of life-is marked by the infant's switch from crawling to walking. B. Changes in Body Size 1. During the first 2 years, the body grows more rapidly than at any time after birth. 2. By the end of the first year the infant's length is 50 percent greater than it was at birth, and by 2 years of age it is 75 percent greater. 3. Birth weight has doubled by 5 months of age, tripled by I year, and quadrupled at 2 years. 4. Research indicates that these height and weight gains occur in little growth spurts. 5. In infancy, girls are slightly shorter and lighter than boys. C. Changes in Body Proportions 1. The cephalocaudal trend is an organized pattern of physical growth and motor control that proceeds from head to tail; growth of the head and chest occurs before that of the trunk and legs. 2. The proximodistal trend is a pattern of physical growth and motor control that proceeds from the center of the body outward; growth of the arms and legs occurs before that of the hands and feet. D. Changes in Muscle-Fat Makeup 1. Body fat, which helps the infant maintain a constant body temperature, increases after birth and peaks around 9 months of age. 2. Toddlers become more slender, a trend that continues into middle childhood. 3. Muscle tissue increases very slowly and does not peak until adolescence. 4. Girls have a higher ratio of fat to muscle than boys. E. Early Skeletal Growth 1. Current body size is not an accurate indication of physical maturity. 2. General Skeletal Growth: a. The best way to estimate a child's physical maturity is to use skeletal age, a measure of the body's bone development. b. The epiphyses are the growth centers in the bones where new cartilage cells are produced and gradually harden. c. As growth continues, the epiphyses thin and disappear and no more growth of the bone is possible. d. Girls are considerably ahead of boys in skeletal age. 3. Growth of the Skull: a. Because of the large increases in brain size, skull growth during the first 2 years is very rapid. b. At birth, the bones of the skull are separated by six gaps called fontanels. F. Appearance of Teeth 1. An infant's first tooth usually appears between 4 to 6 months of age. By age 2, the child has 20 teeth. 2. A child who gets her teeth early is likely to be advanced in physical maturity. II. BRAIN DEVELOPMENT A. At birth, the brain is nearer to its adult size than any other physical structure. B. Development of Neurons 1. The human brain has 100 to 200 billion neurons-nerve cells that store and transmit information. 2. Synapses are tiny gaps between neurons where fibers from different neurons come close together but do not touch. 3. Neurons release chemicals that cross the synapse, thereby sending messages to one another. 4.Neurons that are stimulated by input from the surrounding environment continue to establish new synapses. Those that are seldom stimulated soon die off. 5.Myelinization is a process in which neural fibers are coated with an insulating fatty sheath that improves the efficiency of message transfer. 6.Glial cells make up about half of the brain's volume and do not carry messages; their most important function is myelinization. C. Development of the Cerebral Cortex 1.The cerebral cortex is the largest structure of the human brain and accounts for 85 percent of its weight. 2.It contains the greatest number of neurons and synapses, and it is the last of the brain structures to stop growing. 3.Regions of the Cortex: a. The order in which cortical regions develop corresponds to the order in which various capacities emerge in infancy and childhood. b. One of the last regions of the cortex to develop and myelinate are the frontal lobes, which are responsible for thought and consciousness. 4.Lateralization and Plasticity of the Cortex: a. Lateralization is the specialization of functions of the two hemispheres, or sides, of the cortex. b. Lateralization begins early in life and is molded by early experience. c. Brain plasticity is the ability of other parts of the brain to take over functions of a damaged part. d. During the first few years, the brain is more plastic than at any later time of life. III. FACTORS AFFECTING EARLY PHYSICAL GROWTH A. Heredity 1.Catch-up growth is physical growth that returns to its genetically determined path after being delayed by environmental factors. 2.When environmental conditions are adequate, height and rate of physical growth are largely determined by heredity. 3.Weight is also affected by genetic make-up. However, the environment- especially nutrition- does play an important role. B. Nutrition 1.A baby's energy needs are twice as great as those of an adult. 2.Twenty-five percent of an infant's caloric intake is devoted to growth. If a baby's diet is deficient in either quantity or quality of nutrients, growth can be permanently stunted. 3.Breast- versus Bottle-Feeding: a. Today, nearly two-thirds of American mothers breast- feed their babies. b. Breast-feeding offers many nutritional and health advantages over bottle- feeding. c. Breast-fed babies in impoverished regions of the world are less likely to be malnourished and more likely to survive the first year of life. d. Some mothers cannot nurse because of physiological or medical reasons. e. Breast milk is easily digestible and, as a result, breast-fed babies become hungry more often than bottle-fed infants. f. Breast- and bottle-fed youngsters do not differ in psychological adjustment. 4.Are Chubby Babies at Risk for Later Overweight and Obesity? a. Only a slight correlation exists between fatness in infancy and obesity at older ages. b. Infant and toddlers can eat nutritious foods freely, without risk of becoming too fat. c. Physical exercise also guards against excessive weight gain. 5.Malnutrition: a. Recent evidence indicates that 40 to 60 percent of the world's children do not get enough to eat. B. Marasmus is a wasted condition of the body usually appearing in the first year of life that is caused by a diet low in all essential nutrients. c. Kwashiorkor is a disease usually appearing between I and 3 years of age that is caused by a diet low in protein. Symptoms include an enlarged belly, swollen feet, hair loss, skin rash, and irritable, listless behavior. d. Children who survive these forms of malnutrition grow to be smaller in all body dimensions and their brains can be seriously affected. C. Emotional Well-Being 1. Nonorganic failure to thrive is a growth disorder usually present by 18 months of age that is caused by lack of parental love. 2. If the disorder is not corrected in infancy, some children remain small and have lasting cognitive and emotional problems. IV. CHANGING STATES OF AROUSAL A. During the first 2 years, the organization of sleep and wakefulness changes and fussiness and crying also decline. B. Over time, infants remain awake for longer daytime periods and need fewer naps. C. Although brain maturation is largely responsible for changes in sleep and wakefulness, the social environment also contributes. Melatonin secretion in the brain is greater at night by the middle of the first year. D. Even after infants sleep through the night, they continue to wake occasionally for the next few years. V. MOTOR DEVELOPMENT A. The Sequence of Motor Development 1. Gross motor development refers to control over actions that help an infant move around in the environment, such as crawling, standing, and walking. 2. Fine motor development involves smaller movements such as reaching and grasping. 3. Although the sequence of motor development is fairly uniform across children, there are large individual differences in rate of motor progress. 4. Motor control of the head precedes control of the arms and trunk which precedes control of the legs . 5. Head, trunk, and arm control appears before coordination of the hands and fingers . B. Motor Skills as Dynamic Systems 1. According to dynamic systems theory of motor development, mastery of motor skills involves acquiring increasingly complex systems of action. 2. Each new skill is a joint product of central nervous system development, movement possibilities of the body, the task the child has in mind, and environmental supports for the skill. 3. When a skill is initially acquired, it is tentative and uncertain. The infant must practice and refine it so that the skill becomes smooth and accurate. 4. Each skill is acquired by revising and combining earlier accomplishments into a more complex system that allows the child to reach a desired goal. C. Dynamic Motor Systems in Action 1. To study infants' mastery of motor milestones, researchers have conducted microgenetic studies, which follow babies from their initial attempts at a skill until it is smooth and automatic. 2. Esther Thelen's findings show that infants are active problem-solvers in acquiring motor skills. They experiment and revise motor actions to fit changing task conditions. D. Cultural Variations in Motor Development 1. Cross-cultural research shows that early movement opportunities and a stimulating environment contribute to motor development. 2.Cultural beliefs vary concerning the necessity and advisability of deliberately teaching motor skills to babies. 3.Early motor skills are due to complex transactions between nature and nurture. E. Fine Motor Development: Voluntary Reaching and Grasping 1. Voluntary reaching plays a vital role in infant cognitive development. 2. Motor skills start out as gross activities and move toward mastery of fine movements. 3. Prehearing is the uncoordinated, primitive reaching movements of newborns. 4. Development of Voluntary Reaching and Grasping: a. Voluntary reaching appears at about 3 months and gradually improves in accuracy. b. Early reaching is controlled by proprioception, our sense of movement and location in space that arises from stimuli within the body. c. The ulnar grasp is a clumsy grasp of young infants, in which the fingers close against the palm. d. The pincer grasp is a well-coordinated grasp that emerges at the end of the first year, involving the oppositional use of the forefinger and thumb. 5.Early Experience and Voluntary Reaching: a. Trying to push infants beyond their current readiness to handle stimulation can undermine the development of important motor skills. b. As infants' and toddlers' motor skills develop, caregivers must devote more energies to protecting them from harm. F. Bowel and Bladder Control 1.Toilet training is best delayed until the end of the second or beginning of the third year. 2. At this time, toddlers can correctly identify and respond to signals from a full bladder or rectum. VI. LEARNING CAPACITIES A. Learning refers to changes in behavior as the result of experience. B. Classical Conditioning 1. Reflexes allow classical conditioning in young infants. 2. The steps for learning by classical conditioning are as follows: a. An unconditioned stimulus produces an unconditioned, or reflexive, respons. b. A neutral stimulus, which does not lead to the reflexive response, is presented at the same time as or just before the UCS. c. The neutral stimulus, now called a conditioned stimulus , produces the reflex, now called a conditioned response . 3. Extinction is a decline in the CR as a result of presenting the CS enough times without the UCS. 4. For a baby to learn easily, the association between a UCS and a CS must have survival value. C. Operant Conditioning 1 . Operant conditioning is a form of learning in which a spontaneous behavior is followed by a stimulus that changes the probability that the behavior will occur again. 2. A reinforcer is a stimulus that increases the occurrence of a response. 3. The removal of a desirable stimulus or the presenting of an unpleasant one to decrease the occurrence of a response is called punishment. 4. Operant conditioning allows researchers to determine what stimuli babies perceive and which ones they prefer. 5. Sudden infant death syndrome , a major cause of infant mortality, may result when problems in brain function prevent babies from learning certain lifesaving responses. D. Habituation and Dishabituation 1. Habituation refers to a gradual reduction in the strength of a response due to repetitive stimulation. 2. Dishabituation is a recovery of responsiveness after a new stimulus appears. E. Imitation 1. Imitation is learning by copying the behavior of another person. 2. Research indicates newborns can imitate adult facial expressions and get to know people by matching behavioral states with them. VII. PERCEPTUAL DEVELOPMENT A. Hearing 1. During the first year, babies start to organize sounds into complex patterns. 2. By 6 months of age, babies "screen out" sounds that are not useful in their language community. 3. Older infants can also detect clauses and phrases in sentences. B. Vision 1. By 3 months, infants can focus on objects and discriminate colors about as well as adults can. 2. By 11 months, visual acuity reaches a near-adult level. 3. Depth Perception: a. Depth perception is the ability to judge the distance of objects from one another and from ourselves. b. Research indicates that around the time that infants crawl, most distinguish deep and shallow surfaces and avoid dangerous-looking drop-offs. c. The Emergence of Depth Perception - 1) Motion is the first type of depth cue to which infants are sensitive. 2) Binocular depth cues arise because our two eyes have slightly different views of the visual field. Binocular sensitivity emerges between 2 and 3 months. 3) Pictorial depth perception, involving such cues as changes in texture and overlapping objects, develops around the middle of the first year. d. Independent Movement and Depth Perception - 1) Investigators have concluded that avoidance of heights is "made possible by independent locomotion." 2) Researchers believe that crawling may promote a new level of brain organization by strengthening certain synaptic connections in the cortex. 4. Pattern Perception: a. Even newborns prefer to look at patterned as opposed to plain stimuli. b. Contrast Sensitivity - 1) Contrast sensitivity accounts for infants' early pattern preferences. 2) The principle states that if babies can detect a difference in contrast between two patterns, they will prefer the one with more contrast. c. Combining Pattern Elements - 1) Once babies can detect all aspects of a pattern, they combine pattern elements and integrate them into a unified whole. 2) By 4 months, infants can perceive subjective boundaries that are not really present. d. Perception of the Human Face - 1) The perception of faces is not an innate perceptual capacity. 2) A baby's tendency to search for structure in a patterned stimulus is quickly applied to face perception. 3) By 3 months, infants can discriminate between the photos of two moderately similar strangers. They can also recognize their mother's face in a photo. 4) Perception of the human face supports infants' earliest social relationships, C. Object Perception 1.Size and Shape Constancy: a. To accurately perceive objects, we must translate changing retinal images into a single representation. b. Size constancy is the perception that an object's size is the same, despite changes in its retinal image size. c. Shape constancy is perception that an object's shape is stable, despite changes in the shape projected on the retina. d. Both of these perceptual capacities appear to be innate and assist babies in detecting a coherent world of objects. 2.Perception of Objects as Distinct, Bounded Wholes: a. The movement of objects relative to one another and to their background enables infants to construct a visual world of separate objects. b. At first, motion and spatial arrangements help infants identify objects. Then, as babies visually track moving objects, they pick up additional information about an object's boundaries, such as its distance from their eye, shape, and color. D. Intermodal Perception 1.Intermodal perception combines information from more than one modality, or sensory system. 2.Recent evidence indicates that babies perceive the world in an intermodal method from the beginning. For example, newborn behaviors suggest that they expect sight, sound, and touch to go together. VIII. UNDERSTANDING PERCEPTUAL DEVELOPMENT A. Eleanor and James Gibson's differentiation theory states that perceptual development involves the detection of increasingly fine-grained, invariant features in the environment. B. Invariant features are features that remain stable in a constantly changing perceptual world. C. Some researchers disagree, believing instead that babies impose meaning on what they perceive, constructing categories of objects and events in the surrounding environment. D. Many researchers combine viewpoints, regarding infant development as proceeding from a perceptual to a cognitive emphasis over the first year of life.
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