FORUM ASSIGNMENTS: SPRING 2020 Forum 1: Choose topics from Chapters 1, 2 Forum 2: Choose topics from Chapters 3, 5 Forum 3: Choose topics from Chapters 6-8, 11, 12 DUE FEB 16 DUE FEB 23 DUE MAY 3 After reading the assigned chapters for your forum post choose six topics from those chapters to write about. Your post must be a minimum of 1000 words for full credit. You may choose from the two options below. You may use initials or change people’s names to protect their privacy. You must also make 2 thorough and thoughtful responses to other forum posts. Support your statements with examples, experiences, or references. Post directly into the forum, do not attach a Word document or PDF. For your post you may 1) Describe examples from real life of a person affected by the topic or behavior from the chapter. You may describe someone you know or an example you’ve seen in the media and how it relates to what you’ve read in the text. Or 2) Thoroughly describe and expand on a topic or behavior from the chapter. You may use definitions from the textbook and other educational sources with information referenced to avoid plagiarism. You may give your opinion about the topic. FORMATTING EXAMPLE Topic 1 – Smoking during pregnancy, textbook pg. 65 My aunt Cami smoked before getting pregnant and during her pregnancy. Her daughter Lila was born 8 weeks prematurely and weighed 5 lbs. The book states that preterm births and low birth weights are “more common among the offspring of mothers who smoked during pregnancy.” Lila developed asthma in middle childhood. Continue to describe, you could include your opinion as well. Topic 2 – Down Syndrome, textbook pg. 51 My friend Joy has a daughter, Sky, with Down Syndrome. Joy was 24 when she had Sky. According to our book the incidence rate is 1 in 1,900 at age 20, and then jumps to 1 in 300 at age 35. My friend would cringe at the phrase “incident rate” as she doesn’t feel it was a bad thing to have a daughter with Ds, rather her extra chromosome (an extra copy of the 21 st) gives her something extra special. Continue describing the topic. Continue with topics 3, 4, 5, and 6 for a minimum of 1000 words. Forum Rubric – PSY 211 Criteria Excellent (5 pts) Good (4 pts) Average (3 pts) Poor (2 pt) Topic Count 6 topics 5 topics 3 to 4 topics 1 to 2 topics Post Content Post is well-developed, factually correct, reflective and substantive. Post is factually correct; lacks full development of concept or thought. Post is adequate with superficial thought and preparation; doesn’t add substantive information to topic. Posts information that is off-topic, incorrect, or irrelevant to topic. References & Support Uses references to text and personal experience to support topics in post. Incorporates some references from text and personal experience. Uses personal experience, but no references to text. Includes no references or supporting experience. Peer Response Demonstrates analysis of others’ posts; provides further comment and observation. Elaborates on a post with further comment or observation. Posts shallow contribution to discussion. Posts no follow-up response to others. Writing Skill Clear and concise post that is free of grammatical and spelling errors. Contributes valuable information to forum with minor clarity or grammar/spelling errors. Noted issues with weak written expression in addition to grammar and spelling errors. Unorganized post that contains multiple errors and communication that is inappropriate or distracts the reader. Mechanics Minimum of 1000 words, each paragraph begins with a topic and textbook page and topics are in covered chapters. Minimum of 850 words, each paragraph begins with a topic and textbook page and topics are in covered chapters. Minimum of 700 words, paragraphs do not begin with the topic and textbook page. Less than 700 words. chapter 3 PHYSICAL DEVELOPMENT AND BIOLOGICAL AGING chapter outline 1 Body Growth and Change 3 Sleep Learning Goal 1 Discuss major changes in the body through the life span. Learning Goal 3 Summarize how sleep patterns change as people develop. Patterns of Growth Height and Weight in Infancy and Childhood Puberty Early Adulthood Middle Adulthood Late Adulthood Why Do We Sleep? Infancy Childhood Adolescence and Emerging Adulthood Adulthood and Aging 2 The Brain 4 Longevity and Biological Aging Learning Goal 2 Describe how the brain changes through the life span. Learning Goal 4 Explain longevity and the biological aspects of aging. The Neuroconstructivist View Brain Physiology Infancy Childhood Adolescence Adulthood and Aging Life Expectancy and Life Span Centenarians Biological Theories of Aging ©Adam Smith/The Image Bank/Getty Images preview Think about how much you have changed physically and will continue to change as you age. We come into this life as small beings. But we grow very rapidly in infancy, more slowly in childhood, and once again more rapidly during puberty, and then experience another slowdown. Eventually we decline, but many older adults are still physically robust. In this chapter, we explore changes in body growth, the brain, and sleep across the life span. We also examine longevity and evaluate some fascinating theories about why we age, and we explore both physical and physiological aspects of development. 1 Body Growth and Change Patterns of Growth Height and Weight in Infancy and Childhood LG1 Discuss major changes in the body through the life span. Puberty Early Adulthood Middle Adulthood Late Adulthood In life’s long journey, we go through many bodily changes. We grow up, we grow out, we shrink. The very visible changes in height and weight are accompanied by less visible ones in bones, lungs, and every other organ of the body. These changes will help shape how we think about ourselves, how other people think about us, and what we are capable of thinking, doing, and feeling. Are there strict timelines for these changes? Are they set in our genes? Let’s begin by studying some basic patterns of growth and then trace bodily changes from the time we are infants through the time we are older adults. PATTERNS OF GROWTH Two key patterns of growth are the cephalocaudal and proximodistal patterns. The cephalocaudal pattern is the sequence in which the fastest growth in the human body occurs at the top, with the head. Physical growth in size, weight, and feature differentiation gradually works its way down from the top to the bottom (for example, neck, shoulders, middle trunk, and so on). This same pattern occurs in the head area, because the top parts of the head—the eyes and brain—grow faster than the lower parts, such as the jaw. During prenatal development and early infancy, the head constitutes an extraordinarily large proportion of the total body (see Figure 1). In most cases, sensory and motor development proceeds according to the cephalocaudal pattern. For example, infants see objects before they can control their torso, and they can use their hands long before they can crawl or walk. However, one study contradicted the cephalocaudal pattern by finding that infants reached for toys with their feet before using their hands (Galloway & Thelen, 2004). In this study, infants on average first contacted the toy with their feet when they were 12 weeks old and with their hands when they were 16 weeks old. Thus, contrary to long-standing beliefs, early leg movements can be precisely controlled, some aspects of development that involve reaching do not involve lengthy practice, and early motor behaviors don’t always develop in a strict cephalocaudal pattern. The proximodistal pattern is the growth sequence that starts at the center of the body and moves toward the extremities. An example is the early maturation of muscular control of the trunk and arms, compared with that of the hands and fingers. Further, infants use the whole hand as a unit before they can control several fingers. HEIGHT AND WEIGHT IN INFANCY AND CHILDHOOD Height and weight increase rapidly in infancy, then take a slower course during the childhood years. SECTION 2 cephalocaudal pattern The sequence in which the fastest growth occurs at the top of the body—the head—with physical growth in size, weight, and feature differentiation gradually working from top to bottom. proximodistal pattern The sequence in which growth starts at the center of the body and moves toward the extremities. Biological Processes, Physical Development, and Health 87 1/2 1/3 1/4 1/5 1/6 1/7 1/8 2 months 5 months Newborn 2 6 12 25 Fetal age Years FIGURE 1 CHANGES IN PROPORTIONS OF THE HUMAN BODY DURING GROWTH. As individuals develop from infancy through adulthood, one of the most noticeable physical changes is that the head becomes smaller in relation to the rest of the body. The fractions listed refer to head size as a proportion of total body length at different ages. Infancy The average North American newborn is 20 inches long and weighs 7½ pounds. Ninety-five percent of full-term newborns are 18 to 22 inches long and weigh between 5½ and 10 pounds. In the first several days of life, most newborns lose 5 to 7 percent of their body weight. Once infants adjust to sucking, swallowing, and digesting, they grow rapidly, gaining an average of 5 to 6 ounces per week during the first month. Typically they have doubled their birth weight by the age of 4 months and have nearly tripled it by their first birthday. Infants grow about 3/4 inch per month during the first year, increasing their birth length by about 40 percent by their first birthday. Infants’ rate of growth slows considerably in the second year of life (Hockenberry, Wilson, & Rodgers, 2017; Kliegman & others, 2016). By 2 years of age, infants weigh approximately 26 to 32 pounds, having gained a quarter to half a pound per month during the second year; at age 2 they have reached about onefifth of their adult weight. The average 2-year-old is 32 to 35 inches tall, which is nearly one-half of adult height. The bodies of 5-year-olds and 2-year-olds are different from one another. The 5-year-old not only is taller and heavier, but also has a longer trunk and legs than the 2-year-old. What might be some other physical differences between 2- and 5-year-olds? ©Michael Hitoshi/Getty Images RF 88 CHAPTER 3 Early Childhood What is the overall growth rate like in early childhood? As the preschool child grows older, the percentage of increase in height and weight decreases with each additional year (Kliegman & others, 2016). Girls are only slightly smaller and lighter than boys during these years. Both boys and girls slim down as the trunks of their bodies lengthen. Although their heads are still somewhat large for their bodies, by the end of the preschool years most children have lost their top-heavy look. Body fat also shows a slow, steady decline during the preschool years. Girls have more fatty tissue than boys; boys have more muscle tissue (McMahon & Stryjewski, 2012). Growth patterns vary individually (Hockenberry, Wilson, & Rodgers, 2017; Kliegman & others, 2016). Think back to your preschool years. This was probably the first time you noticed that some children were taller than you, some shorter; some were fatter, some thinner; some were stronger, some weaker. Much of the variation is due to heredity, but environmental experiences are also involved. A review of the height and weight of children around the world concluded that two important contributors to height differences are ethnic origin and nutrition (Meredith, 1978). Why are some children unusually short? The culprits are congenital factors (genetic or prenatal problems), growth hormone deficiency, a physical problem Physical Development and Biological Aging that develops in childhood, maternal smoking during pregnancy, or an emotional difficulty (Hay & others, 2017; Krebs & others, 2016). A recent study of children born small for gestational age or short stature revealed that five years of growth hormone treatment in childhood was linked to an increase to close to average height (Ross & others, 2015). Also, a recent review concluded that accurate assessment of growth hormone deficiency is difficult and that many children diagnosed with growth hormone deficiency re-test normal later in childhood (Murray, Dattani, & Clayton, 2016). In sum, the main factors that contribute to children’s height are genetic influences, ethnic origin, and nutrition. Middle and Late Childhood The period of middle and PUBERTY Puberty is a brain-neuroendocrine process occurring primarily in early adolescence that provides stimulation for the rapid physical changes that take place during this period of development (Susman & Dorn, 2013). In this section, we explore a number of puberty’s physical changes and its psychological accompaniments. Sexual Maturation, Height, and Weight Think back to the onset of your puberty. Of the striking changes that were taking place in your body, what was the first to occur? Researchers have found that male pubertal characteristics typically develop in this order: increase in penis and testicle size, appearance of straight pubic hair, minor voice change, first ejaculation (which usually occurs through masturbation or a wet dream), appearance of curly pubic hair, onset of maximum growth in height and weight, growth of hair in armpits, more detectable voice changes, and, finally, growth of facial hair. What is the order of appearance of physical changes in females? First, for most girls, their breasts enlarge or pubic hair appears. Later, hair appears in the armpits. As these changes occur, the female grows in height and her hips become wider than her shoulders. Menarche—a girl’s first menstruation—comes rather late in the pubertal cycle. Initially, her menstrual cycles may be highly irregular. For the first several years, she may not ovulate every menstrual cycle; some girls do not ovulate at all until a year or two after menstruation begins. Marked weight gains coincide with the onset of puberty. During early adolescence, girls tend to outweigh boys, but by about age 14 boys begin to surpass girls. Similarly, at the beginning of the adolescent period, girls tend to be as tall as or taller than boys of their age, but by the end of the middle school years most boys have caught up or, in many cases, surpassed girls in height. As indicated in Figure 2, the growth spurt occurs approximately two years earlier for girls than for boys. The mean age at the beginning of the growth spurt in girls is 9; for boys, it is 11. The peak rate of pubertal change occurs at 11½ years for girls and 13½ years for boys. During their growth spurt, girls increase in height about 3½ inches per year, boys about 4 inches. SECTION 2 Height gain (inches/year) late childhood involves slow, consistent growth (Kliegman & others, 2016). This is a period of calm before the rapid growth spurt of adolescence. During the elementary school years, children grow an average of 2 to 3 inches a year. At the age of 8, the average girl and the average boy are 4 feet 2 inches tall. During the middle and late childhood years, children gain about 5 to 7 pounds a year. What characterizes children’s physical growth in middle and late childhood? The average 8-year-old girl and the average 8-year-old boy ©RubberBall Productions/Getty Images RF weigh 56 pounds. The weight increase is due mainly to increases in the size of the skeletal and muscular systems, as well as the size of some body organs. Muscle mass and strength gradually increase as “baby fat” decreases in middle and late child5.0 hood (Kliegman & others, 2016). 4.5 Changes in proportions are among the most pronounced physical changes in middle and 4.0 late childhood. Head circumference, waist circumference, and leg length decrease in relation to body height (Hockenberry, Wilson, & Rodgers, 2017). 3.5 Females 3.0 Males 2.5 2.0 1.5 1.0 0.5 0 2 4 6 8 10 12 Age (years) 14 16 18 FIGURE 2 PUBERTAL GROWTH SPURT. On average, the peak of the growth spurt during puberty occurs two years earlier for girls (11½) than for boys (13½). How are hormones related to the growth spurt and to the difference between the average height of adolescent boys and that of girls? Source: Tanner, J.M., et al., “Standards from Birth to Maturity for Height, Weight, Height Velocity: British Children in 1965,” Archives of Diseases in Childhood vol. 41, no. 219, 1966, p. 454–471. puberty A brain-neuroendocrine process occurring primarily in early adolescence that provides stimulation for the rapid physical changes that occur in this period of development. menarche A girl’s first menstrual period. Biological Processes, Physical Development, and Health 89 Hypothalamus: A structure in the brain that interacts with the pituitary gland to monitor the bodily regulation of hormones. Pituitary: This master gland produces hormones that stimulate other glands. It also influences growth by producing growth hormones; it sends gonadotropins to the testes and ovaries and a thyroid-stimulating hormone to the thyroid gland. It sends a hormone to the adrenal gland as well. Thyroid gland: It interacts with the pituitary gland to influence growth. Adrenal gland: It interacts with the pituitary gland and likely plays a role in pubertal development, but less is known about its function than about sex glands. Recent research, however, suggests it may be involved in adolescent behavior, particularly for boys. The gonads, or sex glands: These consist of the testes in males and the ovaries in females. The sex glands are strongly involved in the appearance of secondary sex characteristics, such as facial hair in males and breast development in females. The general class of hormones called estrogens is dominant in females, while androgens are dominant in males. More specifically, testosterone in males and estradiol in females are key hormones in pubertal development. FIGURE 3 THE MAJOR ENDOCRINE GLANDS INVOLVED IN PUBERTAL CHANGE hormones Powerful chemical substances secreted by the endocrine glands and carried through the body by the bloodstream. hypothalamus A structure in the brain that is involved with eating and sexual behavior. pituitary gland An important endocrine gland that controls growth and regulates the activity of other glands. gonads The sex glands, which are the testes in males and the ovaries in females. gonadotropins Hormones that stimulate the testes or ovaries. testosterone A hormone associated in boys with the development of the genitals, increased height, and voice changes. estradiol A hormone associated in girls with breast, uterine, and skeletal development. 90 CHAPTER 3 Hormonal Changes Behind the first whisker in boys and the widening of hips in girls is a flood of hormones, powerful chemical substances secreted by the endocrine glands and carried through the body by the bloodstream (Herting & Sowell, 2017). The endocrine system’s role in puberty involves the interaction of the hypothalamus, the pituitary gland, and the gonads (see Figure 3). The hypothalamus, a structure in the brain, is involved with eating and sexual behavior. The pituitary gland, an important endocrine gland, controls growth and regulates other glands; among these, the gonads—the testes in males, the ovaries in females— are particularly important in giving rise to pubertal changes in the body. How do the gonads, or sex glands, work? The pituitary gland sends a signal via gonadotropins (hormones that stimulate the testes or ovaries) to the appropriate gland to manufacture hormones. These hormones give rise to such changes as the production of sperm in males and menstruation and the release of eggs from the ovaries in females. The pituitary gland, through interaction with the hypothalamus, detects when the optimal level of hormones is reached and maintains it with additional gonadotropin secretion (Susman & Dorn, 2013). Not only does the pituitary gland release gonadotropins that stimulate the testes and ovaries, but through interaction with the hypothalamus the pituitary gland also secretes hormones that either directly lead to growth and skeletal maturation or produce growth effects through interaction with the thyroid gland, located at the base of the throat. The concentrations of certain hormones increase dramatically during adolescence (Piekarski & others, 2017). The concentrations of two key hormones increase in puberty, and the changes are very different in boys and girls: ∙ Testosterone is a hormone associated in boys with the development of genitals, increased height, and deepening of the voice. ∙ Estradiol is a type of estrogen associated in girls with breast, uterine, and skeletal development. A recent study documented the growth of the pituitary gland in adolescence and found that its volume was linked to circulating blood levels of estradiol and testosterone (Wong & Physical Development and Biological Aging others, 2014). In one study, testosterone levels increased eighteenfold in boys but only twofold in girls during puberty; estradiol increased eightfold in girls but only twofold in boys (Nottelmann & others, 1987). Thus, both testosterone and estradiol are present in the hormonal makeup of both boys and girls, but testosterone dominates in male pubertal development, estradiol in female pubertal development (Richmond & Rogol, 2007). A recent study of 9- to 17-year-old boys found that testosterone levels peaked at 17 years of age (Khairullah & others, 2014). The same influx of hormones that grows hair on a male’s chest and increases the fatty tissue in a female’s breasts may also contribute to psychological development in adolescence (Wang & others, 2017). In one study of boys and girls ranging in age from 9 to 14, a higher concentration of testosterone was present in boys who rated themselves as more socially competent (Nottelmann & others, 1987). However, a recent research review concluded that there is insufficient quality research to confirm that changing testosterone levels during puberty are linked to mood and behavior in adolescent males (Duke, Balzer, & Steinbeck, 2014). Hormonal effects by themselves do not account for adolescent psychological development (Graber, 2008). For example, in one study, social factors were much better predictors of young adolescent girls’ depression and anger than hormonal factors (Brooks-Gunn & Warren, 1989). Behavior and moods also can affect hormones. Stress, eating patterns, exercise, sexual activity, tension, and depression can activate or suppress various aspects of the hormonal system. In sum, the hormone-behavior link is complex (Susman & Dorn, 2013). Timing and Variations in Puberty In the United States—where children mature up to a year earlier than children in European countries—the average age of menarche has declined significantly since the mid-nineteenth century. Fortunately, however, we are unlikely to see pubescent toddlers, since what has happened in the past century is likely the result of improved nutrition and health, and the rate of decline in age of onset of puberty has slowed considerably in the last several decades. However, some researchers recently have found that the onset of puberty is still occurring earlier in girls and boys (Herman-Giddens & others, 2012; McBride, 2013). Is age of pubertal onset linked to how tall boys and girls will be toward the end of adolescence? A recent study found that for girls, earlier onset of menarche, breast development, and growth spurt were linked to shorter height at 18 years of age; however, for boys, earlier age of growth spurt and slower progression through puberty were associated with being taller at 18 years of age (Yousefi & others, 2013). Why do the changes of puberty occur when they do, and how can variations in their timing be explained? The basic genetic program for puberty is wired into the species (Dvornyk & Waqar-ul-Haq, 2012). However, nutrition, health, family stress, and other environmental factors also affect puberty’s timing (Susman & Dorn, 2013; Villamor & Jansen, 2016). A cross-cultural study of 48,000 girls in 29 countries found that childhood obesity was linked to early puberty (Currie & others, 2012). A recent study found that child sexual abuse was linked to earlier pubertal onset (Noll & others, 2017). For most boys, the pubertal sequence may begin as early as age 10 or as late as 13½, and it may end as early as age 13 or as late as 17. Thus, the normal range is wide enough that, given two boys of the same chronological age, one might complete the pubertal sequence before the other one has begun it. For girls, menarche is considered within the normal range if it appears between the ages of 9 and 15. Psychological Accompaniments of Puberty What are some links between puberty and psychological characteristics? How do early and late maturation influence adolescents’ psychological development? Body Image One psychological aspect of puberty is certain for both boys and girls: Adolescents are preoccupied with their bodies (SeninCalderon & others, 2017; Solomon-Krakus & others, 2017). At this age you may have looked in the mirror on a daily, and sometimes even hourly, basis to see if you could detect anything different about your changing body. Preoccupation with one’s body image is strong throughout adolescence but it is especially acute during puberty, a time when adolescents are more dissatisfied with their bodies than in late adolescence. A recent study found that an increase in Facebook friends across two years in adolescence was linked to an enhanced motivation to be thin (Tiggemann & Slater, 2017). SECTION 2 Adolescents show a strong preoccupation with their changing bodies and develop images of what their bodies are like. Why might adolescent males have more positive body images than adolescent females? ©age fotostock/SuperStock Biological Processes, Physical Development, and Health 91 Gender Differences Gender differences characterize adolescents’ perceptions of their bodies (Hoffman & Warschburger, 2017). Girls tend to have more negative body images, which to some extent may be due to media portrayals of the attractiveness of being thin while the percentage of girls’ body fat is increasing during puberty (Benowitz-Fredericks & others, 2012). One study found that both boys’ and girls’ body images became more positive as they moved from the beginning to the end of adolescence (Holsen, Carlson Jones, & Skogbrott Birkeland, 2012). Early and Late Maturation Did you enter puberty early, late, or on time? When adolescents mature earlier or later than their peers, they may have different experiences and perceive themselves differently (Lee & others, 2017; Wang & others, 2017). A recent study found that in the early high school years, late-maturing boys had a more negative body image than early-maturing boys (de Guzman & Nishina, 2014). Similarly, in the Berkeley Longitudinal Study conducted half a What are some outcomes of early and late maturation in century ago, early-maturing boys perceived themselves more positively and had adolescence? more successful peer relations than did late-maturing boys (Jones, 1965). The find©Fuse/Getty Images RF ings for early-maturing girls were similar but not as strong as for boys. When the late-maturing boys were in their thirties, however, they had developed a more positive identity than the early-maturing boys had (Peskin, 1967). Perhaps the late-maturing boys had had more time to explore life’s options, or perhaps the early-maturing boys continued to focus on their physical status instead of paying attention to career development and achievement. An increasing number of researchers have found that early maturation increases girls’ vulnerability to a number of problems (Graber, 2013; Hamilton & others, 2014). Early-maturing girls developmental connection are more likely to smoke, drink, be depressed, have an eating disorder, struggle for earlier indeSexuality pendence from their parents, and have older friends (Negriff, Susman, & Trickett, 2011; Verhoef Early sexual experience is one of a & others, 2014). Researchers have found that early-maturing girls tend to engage in sexual intercourse earlier and have more unstable sexual relationships (Moore, Harden, & Mendle, 2014). number of risk factors in adolescent For example, in a recent Korean study, early menarche was associated with risky sexual behavior development. Connect to “Gender in females (Cheong & others, 2015). Another study found that early-maturing girls’ higher level and Sexuality.” of internalizing problems (depression, for example) was linked to their heightened sensitivity to interpersonal stress (Natsuaki & others, 2010). A recent study found that early maturation predicted a stable higher level of depression for adolescent girls (Rudolph & others, 2014). And early-maturing girls are more likely to drop out of high school and to cohabit and marry at younger ages (Cavanagh, 2009). Apparently as a result of their social and cognitive immaturity, combined with early physical development, early-maturing girls are easily lured into problem behaviors, not recognizing how these behaviors might affect their development. Thus, early-maturing adolescents, especially girls, require earlier risk education efforts related to sexual development, risky behaviors, relationships, and Internet safety than their on-time peers (Susman & Dorn, 2013). In sum, early maturation often has more favorable outcomes for boys than for girls, especially in early adolescence. However, late maturation may be more favorable for boys, especially in terms of identity and career development. Research increasingly has found that early-maturing girls are vulnerable to a number of problems. EARLY ADULTHOOD After the dramatic physical changes of puberty, the years of early adulthood might seem to be an uneventful time in the body’s history. Physical changes during these years may be subtle, but they do continue. Height remains rather constant during early adulthood. Peak functioning of the body’s joints usually occurs in the twenties. Many individuals also reach a peak of muscle tone and strength in their late teens and twenties (Candow & Chilibeck, 2005). However, these attributes may begin to decline in the thirties. Sagging chins and protruding abdomens may also appear for the first time. Muscles start to have less elasticity, and aches may appear in places not felt before. Most of us reach our peak levels of physical performance before the age of 30, often between the ages of 19 and 26. This peak of physical performance occurs not only for the average young adult, but for outstanding athletes as well. Different types of athletes, however, reach their peak performances at different ages. Most swimmers and gymnasts peak in their late teens. Golfers and marathon runners tend to peak in their late twenties. In other areas of athletics, peak performance often occurs in the early to mid-twenties. However, in recent 92 CHAPTER 3 Physical Development and Biological Aging years, some highly conditioned athletes—such as Dana Torres (Olympic swimming) and Tom Brady (football)—have stretched the upper age limits of award-winning performances. MIDDLE ADULTHOOD Like the changes of early adulthood, midlife physical changes are usually gradual. Although everyone experiences some physical change due to aging in middle adulthood, the rates of aging vary considerably from one individual to another. Genetic makeup and lifestyle factors play important roles in whether and when chronic diseases will appear (Koenig, Lincoln, & Garg, 2017; Nasef, Mehta, & Ferguson, 2017; Theendakara & others, 2016). Middle age is a window through which we can glimpse later life while there is still time to engage in preventive behaviors and influence the course of aging (Lachman, Teshale, & Agrigoroaei, 2015). Physical Appearance Individuals lose height in middle age, and many gain weight (Haftenberger & others, 2016; Yang & others, 2017). On average, from 30 to 50 years of age, men lose about 1/2 inch in height, then lose another 1/2 inch from 50 to 70 years of age (Hoyer & Roodin, 2009). The height loss for women can be as much as 2 inches from 25 to 75 years of age. Note that there are large variations in the extent to which individuals become shorter with aging. The decrease in height is due to bone loss in the vertebrae. On average, body fat accounts for about 10 percent of body weight in adolescence; it makes up 20 percent or more in middle age. Noticeable signs of aging usually are apparent by the forties or fifties. The skin begins to wrinkle and sag because of a loss of fat and collagen in underlying tissues (Miyawaki & others, 2016). Small, localized areas of pigmentation in the skin produce aging spots, especially in areas that are exposed to sunlight, such as the hands and face. A twin study found that twins who had been smoking longer were more likely to have more sagging facial skin and wrinkles, especially in the middle and lower portion of the face (Okada & others, 2013). The hair thins and grays because of a lower replacement rate and a decline in melanin production. Since a youthful appearance is valued in many cultures, many Americans strive to make themselves look younger. Undergoing cosmetic surgery, dyeing hair, purchasing wigs, enrolling in weight reduction programs, participating in exercise regimens, and taking heavy doses of vitamins are common in middle age. Baby boomers have shown a strong interest in plastic surgery and Botox, which may reflect their desire to take control of the aging process (Solish & others, 2016). Strength, Joints, and Bones The term sarcopenia is given to age-related loss of lean muscle mass and strength (Bianchi & others, 2016; Marzetti & others, 2017). After age 50, muscle loss occurs at a rate of approximately 1 to 2 percent per year. A loss of strength especially occurs in the back and legs. Obesity is a risk factor for sarcopenia (Cruz-Jentoft & others, 2017). Recently, researchers began using the term “sarcopenic obesity” in reference to individuals who have sarcopenia and are obese (Ma & others, 2016; Yang & others, 2017). One study linked sarcopenic obesity to hypertension (Park & others, 2013). Also, in a recent study sarcopenic obesity was associated with a 24 percent increase in risk for all-cause mortality, with a higher risk for men than women (Tian & Xu, 2016). And a research review concluded that weight management and resistance training were the best strategies for slowing down sarcopenia (Rolland & others, 2011). Maximum bone density occurs by the mid- to late thirties. From that point on, there is a progressive loss of bone. The rate of bone loss begins slowly but accelerates during the fifties (Baron, 2012). Women’s rate of bone loss is about twice that of men. By the end of midlife, bones break more easily and heal more slowly (Rachner, Khosia, & Hofbauer, 2011). A recent study found that greater intake of fruits and vegetables was linked to increased bone density in middle-aged and older adults (Qiu & others, 2017). Famous actor Sean Connery as a young adult in his twenties (top) and as a middle-aged adult in his fifties (bottom). What are some of the most outwardly noticeable signs of aging in middle adulthood? (Top): ©Bettmann/Getty Images; (bottom): ©The Life Picture Collection/Getty Images Cardiovascular System Cardiovascular disease increases considerably in middle age (Hulsegge & others, 2017). The level of cholesterol in the blood increases through the adult years (Hasvold & others, 2016). Cholesterol comes in two forms: LDL (low-density lipoprotein) and HDL (high-density lipoprotein). LDL is often referred to as “bad” cholesterol because when the level of LDL is too high, it sticks to the lining of blood vessels, a condition that can lead to atherosclerosis (hardening of the arteries). HDL is often referred to as “good” cholesterol because when it is high and LDL is low, the risk of cardiovascular disease decreases. One study revealed that a higher level of HDL was linked to a higher probability of being alive at 85 years of age (Rahilly-Tierney & others, 2011). SECTION 2 Biological Processes, Physical Development, and Health 93 Members of the Masai tribe in Kenya can stay on a treadmill for a long time because of their active lives. Incidence of heart disease is extremely low in the Masai tribe, which also can be attributed to their energetic lifestyle. Courtesy of the family of Dr. George V. Mann Researchers have found that almost 50 percent of Canadian and American menopausal women have occasional hot flashes, but only one in seven Japanese women do (Lock, 1998). What factors might account for these variations? ©BLOOMimage/Getty Images RF climacteric The midlife transition during which fertility declines. menopause The time in middle age, usually in the late forties or early fifties, when a woman’s menstrual periods have ceased for one year. 94 CHAPTER 3 In middle age, cholesterol begins to accumulate on the artery walls, which are thickening. The result is an increased risk of cardiovascular disease. Blood pressure, too, usually rises in the forties and fifties, and high blood pressure (hypertension) is linked with an increased rate of mortality as well as lower cognitive functioning (Kitaoka & others, 2016; Mrowka, 2017). For example, one study revealed that hypertension in middle age was linked to an increased risk of cognitive impairment in late adulthood (23 years later) (Virta & others, 2013). At menopause, a woman’s blood pressure rises sharply and usually remains above that of a man through life’s later years (Taler, 2009). The health benefits of cholesterol-lowering and hypertension-lowering drugs are a major factor in improving the health of many middle-aged adults and increasing their life expectancy (Yusuf & others, 2016). Regular exercise and healthy eating habits also have considerable benefits in preventing cardiovascular disease (Kim & others, 2017; Sallam & Laher, 2016). In a recent study, a high level of physical activity was associated with a lower risk of cardiovascular disease in the three weight categories studied (normal, overweight, and obese) (Carlsson & others, 2016). Lungs There is little change in lung capacity through most of middle adulthood. However, at about the age of 55, the proteins in lung tissue become less elastic. This change, combined with a gradual stiffening of the chest wall, decreases the lungs’ capacity to shuttle oxygen from the air people breathe to the blood in their veins. For smokers, however, the picture is different and bleaker (Kraen & others, 2017). The lung capacity of smokers drops precipitously in middle age. However, if the individuals quit smoking their lung capacity improves, although not to the level of individuals who have never smoked (Williams, 1995). Also, one study found that lung cancer diagnoses were 68 percent lower among men who were the most physically fit than among those who were the least physically fit (Lakoski & others, 2013). Sexuality Climacteric is a term used to describe the midlife transition in which fertility declines. Menopause is the time in middle age, usually in the late forties or early fifties, when a woman has not had a menstrual period for a full year. The average age at which women have their last period is 52. A small percentage of women—10 percent—go through menopause before age 40. Just as puberty has been coming earlier, however, menopause has been coming later (Birren, 2002). Specific causes of the later incidence of menopause have not been documented, but improved nutrition and lower incidence of infectious diseases may be the reasons. In menopause, production of estrogen by the ovaries declines dramatically, and this decline produces uncomfortable symptoms in some women—“hot flashes,” nausea, fatigue, and rapid heartbeat, for example (Avis & others, 2015; Xi & others, 2017). However, crosscultural studies reveal wide variations in the menopause experience (Sievert & Obermeyer, 2012). For example, hot flashes are uncommon in Mayan women (Beyene, 1986), and Asian women report fewer hot flashes than women in Western societies (Payer, 1991). It is difficult to determine the extent to which these cross-cultural variations are due to genetic, dietary, reproductive, or cultural factors. Menopause is not the negative experience for most women that it was once thought to be. One study in Taiwan found no significant effect of menopausal transition on women’s quality of life (Cheng & others, 2007). However, the loss of fertility is an important marker for women. Do men go through anything like the menopause that women experience? In other words, is there a male menopause? During middle adulthood, most men do not lose their capacity to father children, although there usually is a modest decline in their sexual hormone level and activity (Blumel & others, 2014). Testosterone production begins to decline about 1 percent a year during middle adulthood, and this decline can reduce sexual drive (Hyde & others, 2012). Sperm count usually shows a slow decline, but men do not lose their fertility altogether. We will have more to say about the climacteric and the sexual attitudes and behaviors of middle-aged women and men in the chapter on “Gender and Sexuality.” Physical Development and Biological Aging LATE ADULTHOOD Late adulthood brings an increased risk of physical disability, but there is considerable variability in rates of decline in functioning. Let’s explore changes in physical appearance and the cardiovascular system in older adults. Physical Appearance The changes in physical appearance that take place in middle adulthood become more pronounced in late adulthood. Most noticeable are facial wrinkles and age spots. Our weight usually drops after we reach 60 years of age, likely because we lose muscle, which also gives our bodies a more “sagging” look. Recent research indicates that obesity was linked to mobility limitation in older adults (Anson & others, 2017; JafariNasabian & others, 2017). The good news is that exercise and weight lifting can help slow the decrease in muscle mass and improve the older adult’s body appearance (Fragala, Kenny, & Kuchel, 2015; Zhang & others, 2015). One study found that long-term aerobic exercise was linked with greater muscle strength in 65- to 86-year-olds (Crane, Macneil, & Tarnopolsky, 2013). In another study, at-risk overweight and obese older adults lost significant weight and improved their mobility considerably by participating in a community-based weight reduction program (Rejeski & others, 2017). Circulatory System Significant changes also take place in the circulatory system of older adults (Lima, 2017; Wang, Monticone, & Lakatta, 2016). In late adulthood, hypertension becomes even more problematic and the likelihood of a stroke increases. In one analysis, 57 percent of 80-year-old men and 60 percent of 81-year-old women had hypertension, and 32 percent of the men and 31 percent of the women had experienced a stroke (Aronow, 2007). Today, most experts on aging recommend that consistent blood pressure above 120/80 should be treated to reduce the risk of heart attack, stroke, or kidney disease. A rise in blood pressure with age can be linked to illness, obesity, stiffening of blood vessels, stress, or lack of exercise (Cheng & others, 2017; Kramer, 2015). The longer any of these factors persist, the higher the individual’s blood pressure gets. Various drugs, a healthy diet, and exercise can reduce the risk of cardiovascular disease in older adults (Endes & others, 2016; Georgiopoulou & others, 2017). Geriatric nurses can be especially helpful to older adults who experience acute or chronic illness. To read about the work of one geriatric nurse, see the Connecting with Careers profile. developmental connection Cardiovascular Disease and Alzheimer Disease Cardiovascular disease is increasingly recognized as a risk factor in Alzheimer disease. Connect to “Health.” connecting with careers Sarah Kagan, Geriatric Nurse Sarah Kagan is a professor of nursing at the University of Pennsylvania School of Nursing. She provides nursing consultation to patients, their families, nurses, and physicians regarding the complex needs of older adults related to their hospitalization. She also consults on research and the management of patients who have head and neck cancers. Kagan teaches in the undergraduate nursing program, where she directs a course on “Nursing Care in the Older Adult.” In 2003, she was awarded a MacArthur Fellowship for her work in the field of nursing. In Kagan’s own words: I’m lucky to be doing what I love—caring for older adults and families— and learning from them so that I can share this knowledge and develop or investigate better ways of caring. My special interests in the care of older adults who have cancer allow me the intimate privilege of being with patients at the best and worst times of their lives. That intimacy acts as a beacon—it reminds me of the value I and nursing as a profession contribute to society and the rewards offered in return (Kagan, 2008, p. 1). SECTION 2 Sarah Kagan with a patient. ©Jacqueline Larma/AP Images For more information about what geriatric nurses do, see the Careers in Life-Span Development appendix. Biological Processes, Physical Development, and Health 95 Review Connect Reflect LG1 Review What are cephalocaudal and proximodistal patterns? How do height and weight change in infancy and childhood? What changes characterize puberty? What physical changes occur in early adulthood? How do people develop physically during middle adulthood? What is the nature of physical changes in late adulthood? Discuss major changes in the body through the life span. 2 The Brain The Neuroconstructivist View LG2 Connect In this section, you learned that growth spurts in puberty differ for boys and girls. What research methods probably were used to collect such data? Reflect Your Own Personal Journey of Life How old were you when you started puberty? How do you think this timing affected your social relationships and development? Describe how the brain changes through the life span. Brain Physiology Infancy Childhood Adolescence Adulthood and Aging Until recently, little was known for certain about how the brain changes as we grow and age. Today, dramatic progress is being made in understanding these changes (Denes, 2016; Goedert, 2017; Reuter-Lorenz, Festini, & Jantz, 2016). The study of age-related changes in the brain is one of the most exciting frontiers in science (Villeda, 2017). As we saw in “Biological Beginnings,” remarkable changes occur in the brain during prenatal development. Here we consider the changes in the brain from infancy through late adulthood. Before exploring these developmental changes, let’s first explore what is meant by the neuroconstructivist view and examine some key structures of the brain and see how they function. THE NEUROCONSTRUCTIVIST VIEW neuroconstructivist view Developmental perspective in which biological processes and environmental conditions influence the brain’s development; the brain has plasticity and is context dependent; and cognitive development is closely linked with brain development. myelination The process of encasing axons with a myelin sheath, thereby improving the speed and efficiency of information processing. lateralization Specialization of function in one hemisphere or the other of the cerebral cortex. 96 CHAPTER 3 Not long ago, scientists thought that our genes exclusively determine how our brains are “wired” and that the cells in the brain responsible for processing information just maturationally unfold with little or no input from environmental experiences. In that view, your genes provide the blueprint for your brain and you are essentially stuck with it. This view, however, has turned out to be wrong. Instead, researchers have found that the brain has plasticity and its development depends on context (Botvinick, 2017; Vanderwert & others, 2016). The brain depends on experiences to determine how connections are made (Bick & Nelson, 2016; Vogels, 2017). Before birth, it appears that genes mainly direct basic wiring patterns. Neurons grow and travel to distant places awaiting further instructions (Shenoda, 2017). After birth, the inflowing stream of sights, sounds, smells, touches, language, and eye contact help shape the brain’s neural connections. Thus, the dogma of the unchanging brain has been discarded and researchers are mainly focused on context-induced plasticity of the brain over time (de Haan & Johnson, 2016). The development of the brain mainly changes in a bottom-up, top-down sequence with sensory, appetitive (eating, drinking), sexual, sensation-seeking, and risk-taking brain linkages maturing first and higher-level brain linkages such as self-control, planning, and reasoning maturing later (Zelazo, 2013). In the increasingly popular neuroconstructivist view, (a) biological processes (genes, for example) and environmental experiences (enriched or impoverished, for example) influence the brain’s development; (b) the brain has plasticity and is context dependent; and (c) development of the brain and cognitive development are closely linked. These factors Physical Development and Biological Aging constrain or advance the construction of cognitive skills (Cabeza, Nyberg, & Park, 2017; Dietrich & Haider, 2017; Gao & others, 2017; Goldberg, 2017; Westermann, Thomas, & Karmiloff-Smith, 2011). The neuroconstructivist view emphasizes the importance of interactions between experiences and gene expression in the brain’s development, much as the epigenetic view proposes (Erickson & Oberlin, 2017; Gao & others, 2017; Hensch, 2016; Papenberg, Lindenberger, & Backman, 2017). BRAIN PHYSIOLOGY The brain includes a number of major structures. The key components of these structures are neurons—nerve cells that handle information processing. Structure and Function Looked at from above, the brain has two halves, or hemispheres (see Figure 4). The top portion of the brain, farthest from the spinal cord, is known as the forebrain. Its outer layer of cells, the cerebral cortex, covers it like a thin cap. The cerebral cortex is responsible for about 80 percent of the brain’s volume and is critical in perception, thinking, language, and other important functions. Each hemisphere of the cortex has four major areas, called lobes. Although the lobes usually work together, each has somewhat different primary functions (see Figure 5): ∙ Frontal lobes are involved in voluntary movement, thinking, personality, emotion, memory, sustained attention, and intentionality or purpose. ∙ Occipital lobes function in vision. ∙ Temporal lobes have an active role in hearing, language proFrontal lobe cessing, and memory. ∙ Parietal lobes play important roles in registering spatial location, focusing attention, and maintaining motor control. FIGURE 4 THE HUMAN BRAIN’S HEMISPHERES. The two hemispheres of the human brain are clearly seen in this photograph. It is a myth that the left hemisphere is the exclusive location of language and logical thinking or that the right hemisphere is the exclusive location of emotion and creative thinking. ©A. Glauberman/Science Source Parietal lobe Deeper in the brain, beneath the cortex, lie other key structures. These include the hypothalamus and the pituitary gland as well as the amygdala, which plays an important role in emotions, and the hippocampus, which is especially important in memory and emotion. Occipital lobe Neurons As we discussed earlier, neurons process information. Figure 6 shows some important parts of the neuron, including the axon and dendrites. Basically, an axon sends electrical signals away from the central part of the neuron. At tiny gaps called synapses, the axon communicates with the dendrites of other neurons, which then pass the signals on. The communication in the synapse occurs through the release of chemical substances known as neurotransmitters (Beart, 2016; Dang & others, 2017). How complex are these neural connections? In a recent analysis, it was estimated that each of the billions of neurons is connected to as many as 1,000 other neurons, producing neural networks with trillions of connections (de Haan, 2015). As Figure 6 shows, most axons are covered by a myelin sheath, which is a layer of fat cells. Development of this sheath through a process called myelination helps impulses travel faster along the axon, increasing the speed and efficiency with which information travels from neuron to neuron (Cercignani & others, 2017; Tomassy, Dershowitz, & Arlotta, 2016). Myelination also is involved in providing energy to neurons and in facilitating communication (Kiray & others, 2016). To some extent, the type of information handled by neurons depends on whether they are in the left or the right hemisphere of the cortex (McAvoy & others, 2016). Speech and grammar, for example, depend on activity in the left hemisphere in most people; humor and the use of metaphors depend on activity in the right hemisphere (HollerWallscheid & others, 2017; Moore, Brendel, & Fiez, 2014). This specialization of function in one hemisphere of the cerebral cortex or the other is called lateralization. However, most neuroscientists agree that complex functions such as reading or performing music involve both hemispheres. Labeling people as “left-brained” because they are logical thinkers or “right-brained” because they are creative thinkers does not reflect the SECTION 2 Temporal lobe FIGURE 5 THE BRAIN’S FOUR LOBES. Shown here are the locations of the brain’s four lobes: frontal, occipital, temporal, and parietal. developmental connection Brain Development Might some regions of the brain be more closely linked with children’s intelligence than others? Connect to “Intelligence.” Biological Processes, Physical Development, and Health 97 way the brain’s hemispheres work. For the most part, complex thinking is the outcome of communication between the hemispheres of the brain (Nora & others, 2017; Ries, Dronkers, & Knight, 2016). The degree of lateralization may change as people develop through the life span. Let’s now explore a number of age-related changes in the brain. (a) Incoming information Cell body Nucleus Axon Dendrites (b) Outgoing information (c) Myelin sheath (d) Terminal button To next neuron FIGURE 6 THE NEURON. (a) The dendrites of the cell body receive information from other neurons, muscles, or glands through the axon. (b) Axons transmit information away from the cell body. (c) A myelin sheath covers most axons and speeds information transmission. (d) As the axon ends, it branches out into terminal buttons. INFANCY Brain development occurs extensively during the prenatal period. The brain’s development is also substantial during infancy and later (Crone, 2017; Sullivan & Wilson, 2018). Conducting Research and Measuring the Infant’s Brain Activity Among the researchers who are making strides in finding out more about the brain’s development in infancy are: ∙ Martha Ann Bell and her colleagues (Bell, 2015; Bell & Cuevas, 2012, 2014, 2015; Bell, Kraybill, & Diaz, 2014; Bell, Ross, & Patton, 2018; Bell & others, 2018; M. Li & others, 2017; Lusby & others, 2017; Morasch, Raj, & Bell, 2013; Perry & others, 2016; Smith & others, 2016) who are studying brain-behavior links, emotion regulation, temperament, and the integration of cognition and emotion ∙ Charles Nelson and his colleagues (Berens & Nelson, 2015; Bick & Nelson, 2016, 2017; Bick & others, 2015, 2017; Finch & others, 2017; McLaughlin & others, 2014; Moulson & Nelson, 2008; Nelson, 2007, 2013; Nelson, Fox, & Zeanah, 2014; Righi & others, 2014; Vanderwert & others, 2016; Varcin & others, 2016) who are exploring various aspects of memory development, face recognition and facial emotion, and the role of experience in influencing the course of brain development ∙ Mark Johnson and his colleagues (Anzures & others, 2016; Gliga & others, 2017; Johnson, Jones, & Gliga, 2015; Johnson, Senju, & Tomalski, 2015; Johnson & others, 2015; Milosavlijevic & others, 2017; Saez de Urabain & others, 2017; Senju & others, 2016), who are examining neuroconstructivist links between the brain, cognitive and perceptual processes, and environmental influences; studying the development of the prefrontal cortex and its function; and exploring early identification of autism, face processing, and the effects of early social experiences ∙ John Richards and his colleagues (Emberson & others, 2017a; LloydFox & others, 2015; Richards, 2009, 2010, 2013; Richards, Reynolds, & Courage, 2010; Richards & others, 2015; Sanchez, Richards, & Almi, 2012; Xie & Richards, 2016b, 2017) who are examining sustained attention, perception of TV programs, and eye movements FIGURE 7 MEASURING THE ACTIVITY IN AN INFANT’S BRAIN WITH AN ELECTROENCEPHALOGRAM (EEG). By attaching up to 128 electrodes to a baby’s scalp to measure the brain’s activity, researchers have found that newborns produce distinctive brain waves that reveal they can distinguish their mother’s voice from another woman’s, even while they are asleep. Courtesy of Vanessa Vogel Farley 98 CHAPTER 3 Physical Development and Biological Aging Researchers have been successful in using the electroencephalogram (EEG), a measure of the brain’s electrical activity, to learn about the brain’s development in infancy (Hari & Puce, 2017; Perry & others, 2016; Smith & others, 2016) (see Figure 7). For example, a recent study found that higherquality mother-infant interaction early in infancy predicted higher-quality frontal lobe functioning that was assessed with EEG later in infancy (Bernier, Calkins, & Bell, 2016). FIGURE 8 MEASURING THE ACTIVITY OF AN INFANT’S BRAIN WITH MAGNETOENCEPHALOGRAPHY (MEG). This baby’s brain activity is being assessed with a MEG brain-imaging device while the baby is listening to spoken words in a study at the Institute of Learning and Brain Sciences at the University of Washington. The infant sits under the machine and when he or she experiences a word, touch, sight, or emotion, the neurons working together in the infant’s brain generate magnetic fields and MEG pinpoints the location of the fields in the brain. FIGURE 9 FUNCTIONAL NEAR-INFRARED SPECTROSCOPY (fNRIS). This brain- imaging technique is increasingly being used to assess infants’ brain activity as they move about their environment. ©Oli Scarff/Getty Images Researchers are continuing to explore the use of other techniques to assess infants’ brain functioning. Recently Patricia Kuhl ©Dr. Patricia Kuhl, Institute for Learning and Brain Sciences, and her colleagues at the Institute for LearnUniversity of Washington ing and Brain Sciences at the University of Washington have been using magnetoencephalography, or MEG, brain-imaging machines to assess infants’ brain activity. MEG maps brain activity by recording magnetic fields produced by electrical currents and is being used with infants to assess perceptual and cognitive activities such as vision, hearing, and language in infants (Hari & Puce, 2017) (see Figure 8). And researchers also are increasingly using functional near-infrared spectroscopy (fNIRS), which uses very low levels of near-infrared light to monitor changes in blood oxygen, to study Myelin sheath infants’ brain activity (de Haan & Johnson, 2016; Emberson & others, 2017b). (See Figure 9.) Unlike fMRI, which uses magnetic fields or electrical activity, fNIRS is portable and allows the infants to be assessed as they explore the world around them. Axon Changing Neurons At birth, the newborn’s brain is about 25 percent of its adult weight. By the second birthday, the brain is about 75 percent of its adult weight. Two key developments during these first two years involve the myelin sheath (the layer of fat cells that speeds up movement of electrical impulses along the axons) and connections between dendrites. Myelination, the process of encasing axons with a myelin sheath, begins prenatally and continues after birth (see Figure 10). Myelination for visual pathways occurs rapidly after birth, reaching completion in the first six months. Auditory myelination is not completed until 4 or 5 years of age. Some aspects of myelination continue into adolescence and even into emerging adulthood and possibly beyond (Juraska & Willing, 2017). Indeed, the most extensive changes in myelination in the frontal lobes occur during adolescence (Monahan & others, 2016). Dramatic increases in dendrites and synapses (the tiny gaps between neurons across which neurotransmitters carry information) also characterize the development of the brain in the first two years of life (see Figure 11). Nearly twice as many of these connections are made as will ever be used (Huttenlocher & Dabholkar, 1997). The connections that are used become stronger and survive, while the unused ones are replaced by other pathways or disappear. In the language of neuroscience, these connections are “pruned” (Campbell & others, 2012). Figure 12 vividly illustrates the growth and later pruning of synapses in the visual, auditory, and prefrontal cortex areas of SECTION 2 FIGURE 10 A MYELINATED NERVE FIBER. The myelin sheath, shown in brown, encases the axon (white). This image was produced by an electron microscope that magnified the nerve fiber 12,000 times. What role does myelination play in the brain’s development and children’s cognition? ©Steve Gschmeissner/Science Source Biological Processes, Physical Development, and Health 99 FIGURE 11 THE DEVELOPMENT OF DENDRITIC SPREADING. Note the increase in connectedness between neurons over the course of the first two years of life. Leisman, Gerry, “Intentionality and ‘free-will’ from a neurodevelopmental perspective.” Frontiers in Integrative Science, June 27, 2012. Figure 4. Copyright ©2012 by Gerry Leisman. All rights reserved. Used with permission. At birth 1 month 3 months 15 months 24 months the brain (Huttenlocher & Dabholkar, 1997). As shown in Figure 12, “blooming and pruning” vary considerably by brain region in humans (Gogtay & Thompson, 2010). Changing Structures The areas of the brain do not mature uniformly (de Haan & Johnson, 2016). The frontal lobe is immature in the newborn. As neurons in the frontal lobe become myelinated and interconnected during the first year of life, infants develop an ability to regulate their physiological states (such as sleep) and gain more control over their reflexes. Cognitive skills that require deliberate thinking do not emerge until later (Bell, 2015; Bell & Fox, 1992; Bell & others, 2018). At about 2 months of age, the motor control centers of the brain develop to the point at which infants can suddenly reach out and grab a nearby object. At about 4 months, the neural connections necessary for depth perception begin to form. And at about 12 months, the brain’s speech centers are poised to produce one of infancy’s magical moments: when the infant utters its first word. Early Experience and the Brain Children who grow up in a deprived environment may have depressed brain activity (Berens & Nelson, 2015; Bick & others, 2017; Nelson, FIGURE 12 60 SYNAPTIC DENSITY IN THE HUMAN BRAIN FROM INFANCY TO ADULTHOOD. 50 Synaptic density The graph shows the dramatic increase followed by pruning in synaptic density for three regions of the brain: visual cortex, auditory cortex, and prefrontal cortex. Synaptic density is believed to be an important indication of the extent of connectivity between neurons. 40 30 Visual cortex (vision) Auditory cortex (hearing) Prefrontal cortex (reasoning, self-regulation) 20 10 0 birth 100 200 1 year 3 years CHAPTER 3 adult 300 400 500 600 800 1,000 1,500 2,000 3,000 4,000 6,000 8,000 10,000 Age in days (from conception) 100 11 years Physical Development and Biological Aging Front Front Back Back (a) (b) FIGURE 14 FIGURE 13 PLASTICITY IN THE BRAIN’S HEMISPHERES. Michael Rehbein at 14 years EARLY DEPRIVATION AND BRAIN ACTIVITY. These two photographs are PET (positron emission tomography) scans—which use radioactive tracers to image and analyze blood flow and metabolic activity in the body’s organs. These scans show the brains of (a) a normal child and (b) an institutionalized Romanian orphan who experienced substantial deprivation since birth. In PET scans, the highest to lowest brain activity is reflected in the colors of red, yellow, green, blue, and black, respectively. As can be seen, red and yellow show up to a much greater degree in the PET scan of the normal child than that of the deprived Romanian orphan. of age. Following removal of the left hemisphere of Michael’s brain because of uncontrollable seizures, his right hemisphere reorganized to take over the language functions normally carried out by corresponding areas in the left hemisphere of an intact brain. Courtesy of Dr. Harry T. Chugani, Children’s Hospital of Michigan Courtesy of The Rehbein Family CHILDHOOD The brain and other parts of the nervous system continue developing through childhood (Bell & others, 2018; de Haan & Johnson, 2016). These changes enable children to plan their actions, to attend to stimuli more effectively, and to make considerable strides in language development. During early childhood, the brain and head grow more rapidly than any other part of the body. Figure 15 shows how the growth curve for the head and brain advances more rapidly than the growth curve for height and weight. Some of the brain’s increase in size is due to myelination and some is due to an increase in the number and size of dendrites. Some developmentalists conclude that myelination is important in the maturation of a number of SECTION 2 Percent of total postnatal growth Fox, & Zeanah, 2014; Vanderwert & others, 2016). As shown in Figure 13, a child who grew up in the unresponsive and unstimulating environment of a Romanian orphanage showed considerably depressed brain activity compared with a child raised in a normal environment. Are the effects of deprived environments reversible? There is reason to think that at least to some degree and for some individuals the answer is yes (Dennis & others, 2014). The brain demonstrates both flexibility and resilience. Consider 14-year-old Michael Rehbein. At age 7, he began to experience uncontrollable seizures—as many as 400 a day. Doctors said the only solution was to remove the left hemisphere of his brain where the seizures were occurring. Recovery was slow, but his right hemisphere began to reorganize and take over functions that normally occur in the brain’s left hemisphere, 125 including speech (see Figure 14). Neuroscientists note that what wires the brain—or rewires it, in the case of 100 Michael Rehbein—is repeated experience (Nash, 1997). Each time a baby tries to touch an attractive object or gazes intently at a face, tiny bursts of electricity 75 shoot through the brain, knitting together neurons into circuits. The results are some of the behavioral milestones we discuss in this and other chapters. Brain and head 50 Height and weight 25 0 0 2 4 6 8 10 12 Age (years) 14 16 18 20 22 FIGURE 15 GROWTH CURVES FOR THE HEAD AND BRAIN AND FOR HEIGHT AND WEIGHT. The more rapid growth of the brain and head can easily be seen. Height and weight advance more gradually over the first two decades of life. Biological Processes, Physical Development, and Health 101 abilities in children (Galvan & Tottenham, 2016). For example, myelination in the areas of the brain related to hand-eye coordination is not complete until about 4 years of age. Myelination in the areas of the brain related to focusing attention is not complete until the end of middle or late childhood. The brain in early childhood is not growing as rapidly as it did in infancy, yet the anatomical changes in the child’s brain between the ages of 3 and 15 are dramatic. By repeatedly obtaining brain scans of the same children for up to four years, scientists have found that children’s brains experience rapid, distinct growth spurts (Gogtay & Thompson, 2010). The amount of brain material in some areas can nearly double in as little as one year, followed by a drastic loss of tissue as unneeded cells are purged and the brain continues to reorganize itself. The substantial increases in memory and rapid learning that characterize infants and young children are related to myelination and synaptic growth. In a recent study, young children with higher cognitive ability showed increased myelination by 3 years of age (Deoni & others, 2016). These aspects of the brain’s maturation, combined with opportunities to experience a widening world, contribute to children’s emerging cognitive abilities. Consider a child who is learning to read and is asked by a teacher to read aloud to the class. Input from the child’s eyes is transmitted to the child’s brain, then passed through many brain systems, which translate (process) the patterns of black and white into codes for letters, words, and associations. The output occurs in the form of messages to the child’s lips and tongue. The child’s own gift of speech is possible because brain systems are organized in ways that permit language processing. Recently, researchers have found that contextual factors such as poverty and parenting quality are linked to the development of the brain during childhood (Black & others, 2017; Johnson, Riis, & Noble, 2016; Lomanowska & others, 2017). In one study, children from the poorest homes had significant maturational lags in their frontal and temporal lobes at 4 years of age, and these lags were associated with lower school readiness skills (Hair & others, 2015). In another study, higher levels of maternal sensitivity in early childhood were associated with higher total brain volume (Kok & others, 2015). And in a longitudinal study, 11- to 18-year-olds who lived in poverty conditions had diminished brain functioning at 25 years of age (Brody & others, 2017). However, the adolescents from poverty backgrounds whose families participated in a supportive parenting intervention did not show this diminished brain functioning in adulthood. Significant changes in various structures and regions of the brain continue to occur during middle and late childhood (de Haan & Johnson, 2016). In particular, the brain pathways and circuitry involving the prefrontal cortex, the highest level in the brain, continue to increase in middle and late childhood (Johnson, Riis, & Noble, 2016). The brain is hierarchically organized and mainly develops from the bottom up, with sensory areas reaching maturity before the higher-level association areas such as the prefrontal cortex. In one study, researchers found less diffusion and more focal activation in the prefrontal cortex from 7 to 30 years of age (Durston & others, 2006). The activation change was accompanied by increased efficiency in cognitive performance, especially in cognitive control, which involves flexible and effective control in a number of areas. These areas include controlling attention, reducing interfering thoughts, inhibiting motor actions, and being cognitively flexible in switching between competing choices (Diamond, 2013). Developmental neuroscientist Mark Johnson and his colleagues (Johnson, Grossmann, & Cohen-Kadosh, 2009; Johnson, Jones, & Gliga, 2015) have proposed that the prefrontal cortex likely orchestrates the functions of many other brain regions during development. As part of this neural leadership and organizational role, the prefrontal cortex may provide an advantage to neural connections and networks that include the prefrontal cortex. In their view, the prefrontal cortex likely coordinates the best neural connections for solving a problem. ADOLESCENCE prefrontal cortex The highest level of the frontal lobes that is involved in reasoning, decision making, and self-control. 102 CHAPTER 3 Until recently, little research has been conducted on developmental changes in the brain during adolescence. Although research in this area is still in its infancy, an increasing number of studies are under way (Cohen & Casey, 2017; Crone, 2017; Monahan & others, 2016). Scientists now note that the adolescent’s brain is different from the child’s brain, and that in adolescence the brain is still growing (Goddings & Mills, 2017; Steinberg & others, 2017). Physical Development and Biological Aging Prefrontal cortex Corpus callosum Earlier we indicated that connections between neurons This “judgment” region reins in These nerve fibers connect the brain’s become “pruned” as children and adolescents develop. Because intense emotions but doesn’t two hemispheres; they thicken in of this pruning, by the end of adolescence individuals have finish developing until at least adolescence to process information “fewer, more selective, more effective neuronal connections emerging adulthood. more effectively. than they did as children” (Kuhn, 2009, p. 153). And this pruning indicates that the activities adolescents choose to engage in or not to engage in influence which neural connections will be strengthened and which will disappear. Using fMRI brain scans, scientists have discovered that adolescents’ brains undergo significant structural changes (Casey, Galvan, & Somerville, 2016; Cohen & Casey, 2017). These structural changes occur in the corpus callosum, the prefrontal cortex, and the limbic system. The corpus callosum, where fibers connect the brain’s left and right hemispheres, thickens in adolescence, which improves adolescents’ ability to process information (Chavarria & others, 2014). We described advances in the development of the prefrontal cortex—the highest level of the frontal lobes involved in reasoning, decision making, and self-control—earlier. The prefrontal cortex doesn’t finish maturing until the emerging adult years (18 to 25 years old) or later. At a lower, subcortical level, the limbic system, which is the seat of emotions and where rewards are experienced, Limbic system matures much earlier than the prefrontal cortex and is almost Amygdala A lower, subcortical system in the completely developed by early adolescence (Blakemore & Limbic system structure brain that is the seat of emotions and Mills, 2014; Casey, 2015; Cohen & Casey, 2017). The limbic especially involved in emotion. experience of rewards. This system system structure that is especially involved in emotion is the is almost completely developed in early adolescence. amygdala. Figure 16 shows the locations of the corpus callosum, prefrontal cortex, and the limbic system. FIGURE 16 With the onset of puberty, the levels of neurotransmitters THE CHANGING ADOLESCENT BRAIN: PREFRONTAL CORTEX, LIMBIC change (McEwen, 2013). For example, an increase in the neuSYSTEM, AND CORPUS CALLOSUM rotransmitter dopamine occurs in both the prefrontal cortex and the limbic system during adolescence (Ernst & Spear, 2009). Increases in dopamine have been linked to increased risk taking and the use of addictive drugs (Casey, 2015; Casey, Galvan, & Somerville, 2016; Cohen & Casey, 2017; Webber & others, 2017). Researchers have found that dopamine plays an important role in reward seeking during adolescence (Leyton & Vezina, 2014). Let’s further consider the developmental disjunction between the early development of the developmental connection limbic system and the later development of the prefrontal cortex. This disjunction may account Brain Development for an increase in risk taking and other problems in adolescence. To read further about riskDevelopmental social neuroscience taking behavior in adolescence, see the Connecting Development to Life interlude. Many of the changes in the adolescent brain that have been described involve the rapidly and developmental cognitive neuroemerging field of developmental social neuroscience (which involves connections between science are recent developed fields. development, the brain, and socioemotional processes) and developmental cognitive neurosciConnect to “Introduction.” ence (which involves links between development, cognition, and neuroscience) (Cohen & Casey, 2017; Monahan & others, 2016; Steinberg & others, 2017). For example, consider leading researcher Charles Nelson’s (2003) view that, although adolescents are capable of very strong emotions, their prefrontal cortex hasn’t adequately developed to the point at which they can control these passions. It is as if their brain doesn’t have the brakes to slow down their emotions. Or consider this interpretation of the development of emotion and cognition in adolescents: “early activation of strong ‘turbo-charged’ feelings with a relatively un-skilled set of ‘driving skills’ or cognitive abilities to modulate strong emotions and motivations” corpus callosum A large bundle of axon (Dahl, 2004, p. 18). And in the view of leading expert Jay Giedd (2007), biology doesn’t fibers that connects the brain’s left and right make teens rebellious or have purple hair and it does not mean that they are going to do hemispheres. drugs, but it increases their chances of doing such things. Of course, a major issue is which comes first, biological changes in the brain or experi- limbic system The region of the brain where emotions and rewards are experienced. ences that stimulate these changes (Lerner, Boyd, & Du, 2008). Consider a study in which the prefrontal cortex thickened and more brain connections formed when adolescents resisted peer amygdala A part of the brain’s limbic system pressure (Paus & others, 2008). Scientists have yet to determine whether the brain changes that is the seat of emotions such as anger. SECTION 2 Biological Processes, Physical Development, and Health 103 connecting development to life Strategies for Helping Adolescents Reduce Their Risk-Taking Behavior Beginning in early adolescence, individuals seek experiences that create high-intensity feelings (Steinberg & others, 2017). Adolescents like intensity, excitement, and arousal. They are drawn to music videos that shock and bombard the senses. Teenagers flock to horror and slasher movies. They dominate queues waiting to ride highadrenaline rides at amusement parks. Adolescence is a time when sex, drugs, very loud music, and other high-stimulation experiences take on great appeal. It is a developmental period when an appetite for adventure, a predilection for risks, and desire for novelty and thrills seem to reach naturally high levels. While these patterns of emotional changes are present to some degree in most adolescents, it is important to recognize the wide range of individual differences during this period of development (Dahl, 2004, p. 6). The self-regulatory skills necessary to inhibit risk taking often don’t develop fully until later in adolescence or emerging adulthood (Casey, 2015; Casey, Galvan, & Somerville, 2016; Cohen & Casey, 2017; Steinberg & others, 2017). And, as we just saw, this gap between the increase in risk-taking behavior and the delay in selfregulation is linked to brain development in the limbic system (involved in pleasure seeking and emotion) taking place earlier than development of the frontal lobes (involved in self-regulation) (Monahan & others, 2016). It is important for parents, teachers, mentors, and other responsible adults to effectively monitor adolescents’ behavior. In many cases, adults decrease their monitoring of adolescents too early, leaving them to cope with tempting situations alone or with friends and peers. When adolescents are in tempting and dangerous How might developmental changes in the brain be involved in adolescent risk taking? What are some strategies for reducing adolescent risk taking? ©Chris Garrett/Getty Images situations with minimal adult supervision, their inclination to engage in risk-taking behavior combined with their immature self-regulatory skills can make them vulnerable to a host of negative outcomes (Steinberg, 2016; Steinberg & others, 2017). What does the nature-nurture debate suggest about the influence of adults who monitor adolescents’ lives? come first or whether the brain changes are the result of experiences with peers, parents, and others (Monahan & others, 2016; Webber & others, 2017). Once again, we encounter the nature/nurture issue that is so prominent in an examination of development through the life span. Nonetheless, there is adequate evidence that environmental experiences make important contributions to the brain’s development (Steinberg & others, 2017; Zelazo, 2013). In closing this section on the development of the brain in adolescence, a further caution is in order. Much of the research on neuroscience and the development of the brain in adolescence is correlational in nature, and thus causal statements need to be scrutinized. This caution, of course, applies to any period in the human life span. ADULTHOOD AND AGING Changes in the brain continue during adulthood. Most of the research on the brains of adults, however, has focused on the aging brains of older adults. What are some of the general findings about the aging brain? How much plasticity and adaptability does it retain? The Shrinking, Slowing Brain On average, the brain loses 5 to 10 percent of its weight between the ages of 20 and 90. Brain volume also decreases (Liu & others, 2016; 104 CHAPTER 3 Physical Development and Biological Aging Peng & others, 2016). A study found a decrease in total brain volume and volume in key brain structures such as the frontal lobes and hippocampus from 22 to 88 years of age (Sherwood & others, 2011). Another study found that the volume of the brain was 15 percent less in older adults than younger adults (Shan & others, 2005). Also, recent analyses concluded that in healthy aging the decrease in brain volume is due mainly to shrinkage of neurons, lower numbers of synapses, reduced length and complexity of axons, and reduced tree-like branching in dendrites, but only to a minor extent attributable to neuron loss (Fjell & Walhovd, 2010; Penazzi, Bakota, & Brandt, 2016; Skaper & others, 2017). Further, in a recent study, higher global brain volume predicted lower mortality risk in a large population of stroke-free community-dwelling adults (Van Elderen & others, 2016). Some areas of the brain shrink more than others (Moore & Murphy, 2016). The prefrontal cortex is one area that shrinks with aging, and recent research has linked this shrinkage with a decrease in working memory and other cognitive activities in older adults (Hoyer, 2015). The sensory regions of the brain—such as the primary visual cortex, primary motor cortex, and somatosensory cortex—are less vulnerable to the aging process (Rodrique & Kennedy, 2011). A general slowing of function in the brain and spinal cord begins in middle adulthood and accelerates in late adulthood (Yang & others, 2015; Salthouse, 2017). Both physical coordination and intellectual performance are affected. For example, after age 70, many adults no longer show a knee jerk reflex and by age 90 most reflexes are much slower (Spence, 1989). The slowing of the brain can impair the performance of older adults on intelligence tests and various cognitive tasks, especially those that are timed (Lu & others, 2011). Aging also has been linked to a decline in the production of some neurotransmitters. Reduction in acetylcholine is linked to memory loss, especially in people with Alzheimer disease (Kamal & others, 2017). Severe reductions in dopamine are involved in a reduction in motor control in Parkinson disease (da Silva & others, 2017). Historically, as in the research just discussed, much of the focus on links between brain functioning and aging has been on volume of brain structures and regions. Recently, increased emphasis is being given to changes in myelination and neural networks. Research indicates that demyelination (a deterioration in the myelin sheath that encases axons, which is associated with information processing) of the brain occurs with aging in older adults (Callaghan & others, 2014; Cercignani & others, 2017; Rodrique & Kennedy, 2011). The Adapting Brain The human brain has remarkable repair capability (GarciaMesa & others, 2016; Park & Festini, 2017). Even in late adulthood, the brain loses only a portion of its ability to function, and the activities older adults engage in can influence the brain’s development (Erickson & Oberlin, 2017; Espeland & others, 2016; Reuter-Lorenz & Lustig, 2017). For example, an fMRI study found that higher levels of aerobic fitness were linked with greater volume in the hippocampus, which translates into better memory (Erickson & others, 2011). Three areas reflect the adaptiveness of the brain in older adults: (1) the capacity to generate new neurons, (2) dendritic growth, and (3) delateralization. Researchers have found that neurogenesis, the generation of new neurons, does occur in lower mammalian species such as mice (Adlaf & others, 2017; Kask & others, 2015). Also, research indicates that exercise and an enriched, complex environment can generate new brain cells in mice and that stress reduces the cells’ survival rate (Ramirez-Rodriquez & others, 2014) (see Figure 17). For example, in a recent study, mice in an enriched environment learned more flexibly because of adult hippocampal neurogenesis (Garthe, Roeder, & Kempermann, 2016). And one study revealed that coping with stress stimulated hippocampal neurogenesis in adult monkeys (Lyons & others, 2010). Researchers also recently have discovered that if rats are cognitively challenged to learn something, new brain cells survive longer (Shors, 2009). It also is now accepted that neurogenesis can occur in humans (Feng & Liu, 2017; Horgusluoglu & others, 2017). However, researchers have documented neurogenesis only in two brain regions: the hippocampus (Olesen & others, 2017; Stolp & Molnar, 2015), which is involved in memory, and the olfactory bulb (Bonzano & De Marchis, 2017; Bowers & Jessberger, 2016), which is involved in smell. It also is not known what functions these new brain cells perform, and at this point researchers have documented that they last only a few weeks (Nelson, 2006). Researchers currently are studying factors that might inhibit and SECTION 2 neurogenesis The generation of new neurons. Biological Processes, Physical Development, and Health 105 Exercise Enriched Environment FIGURE 17 GENERATING NEW NERVE CELLS IN ADULT MICE. Researchers have found that exercise (running) and an enriched environment (a larger cage and many toys) can cause brain cells to divide and form new brain cells (Kempermann, van Praag, & Gage, 2000). Cells were labeled with a chemical marker that becomes integrated into the DNA of dividing cells (red). Four weeks later, they were also labeled to mark neurons (nerve cells). As shown here, both the running mice and the mice in an enriched environment had many cells that were still dividing (red) and others that had differentiated into new nerve cells (orange). Courtesy of Dr. Fred Gage, The Salk Institute for Biological Studies FIGURE 18 THE DECREASE IN BRAIN LATERALIZATION IN OLDER ADULTS. Younger adults primarily used the right prefrontal region of the brain (top left photo) during a recall memory task, whereas older adults used both the left and right prefrontal regions (bottom two photos). Courtesy of Dr. Roberto Cabeza 106 CHAPTER 3 promote neurogenesis, including various drugs, stress, and exercise (Choi, Lee, & Lee, 2016; Niwa & others, 2016; Tharmaratnam & others, 2017; Zhou & others, 2017). They also are examining how the grafting of neural stem cells to various regions of the brain, such as the hippocampus, might increase neurogenesis (Noguchi & others, 2015; Otsuki & Brand, 2017; Q. Zhang & others, 2017). And increasing attention is being given to the possible role neurogenesis might play in reversing the course of neurodegenerative diseases such as Alzheimer disease, Parkinson disease, and Huntington disease (Choi, Lee, & Lee, 2016; Ma & others, 2017; Sarlak & Vincent, 2016; Zheng & others, 2017). Dendritic growth can occur in human adults, possibly even in older adults (Eliasieh, Liets, & Chalupa, 2007). One study compared the brains of adults at various ages (Coleman, 1986). From the forties through the seventies, the growth of dendrites increased. However, in people in their nineties, dendritic growth no longer occurred. This dendritic growth might compensate for the possible loss of neurons through the seventies but not in the nineties. Lack of dendritic growth in older adults could be due to a lack of environmental stimulation and activity. Changes in lateralization may provide one type of adaptation in aging adults (Hong & others, 2015). Recall that lateralization is the specialization of function in one hemisphere of the brain or the other. Using neuroimaging techniques, researchers have found that brain activity in the prefrontal cortex is lateralized less in older adults than in younger adults when they are engaging in cognitive tasks (Cabeza, 2002; Cabeza & Dennis, 2013; Rossi & others, 2005; Sugiura, 2016). For example, Figure 18 shows that when younger adults are given the task of recognizing words they have previously seen, they process the information primarily in the right hemisphere, while older adults doing the same task are more likely to use both hemispheres (Madden & others, 1999). The decrease in lateralization in older adults might play a compensatory role in the aging brain (Hong & others, 2015). That is, using both hemispheres may improve the cognitive functioning of older adults. Support for this view comes from another study in which older adults who used both brain hemispheres were faster at completing a working memory task than their counterparts who primarily used only one hemisphere (Reuter-Lorenz & others, 2000). However, the decrease in lateralization may be a mere by-product of aging; it may reflect an age-related decline in the brain’s ability to specialize functions. In this view, during childhood the brain becomes increasingly differentiated in terms of its functions; as adults become older, this process may reverse. Support for the dedifferentiation view is found in the higher intercorrelations of performance on cognitive tasks in older adults than in younger adults (Baltes & Lindenberger, 1997). Physical Development and Biological Aging connecting with research The Nun Study The Nun Study, directed by David Snowdon, is an intriguing, ongoing investigation of aging in 678 nuns, many of whom have lived in a convent in Mankato, Minnesota (Neltner & others, 2016; Pakhomov & Hemmy, 2014; Snowdon, 1997, 2002, 2003; Tyas & others, 2007; White & others, 2016). Each of the 678 nuns agreed to participate in annual assessments of her cognitive and physical functioning. The nuns also agreed to donate their brains for scientific research when they die, and they are the largest group of brain donors in the world. Examination of the nuns’ donated brains, as well as those donated by others, has led neuroscientists to believe that the brain has a remarkable capacity to change and grow, even in old age. The Sisters of Notre Dame in Mankato lead an intellectually challenging life, and brain researchers believe this contributes to their quality of life as older adults and possibly to their longevity. Findings from the Nun Study so far include the following: Positive emotions early in adulthood were linked to longevity (Danner, Snowdon, & Friesen, 2001). Handwritten autobiographies from 180 nuns, composed when they were 22 years of age, were scored for emotional content. The nuns whose early writings had higher scores for positive emotional content were more likely to still be alive at 75 to 95 years of age than their counterparts whose early writings were characterized by negative emotional content. erate declines in intellectual skills than those who had spent most of their lives in service-based tasks, a finding supporting the notion that stimulating the brain with intellectual activity keeps neurons healthy and alive (Snowdon, 2002). This study and other research provide hope that scientists will discover ways to tap into the brain’s capacity to adapt in order to prevent and treat brain diseases (Alexopoulos & Kelly, 2017; Liu & others, 2017). For example, scientists might learn more effective ways to improve older adults’ cognitive functioning, reduce Alzheimer disease, and help older adults recover from strokes (Lovden, Backman, & Lindenberger, 2017; Sperling, 2017). Even when areas of the brain are permanently damaged by stroke, new message routes can be created to get around the blockage or to resume the function of the damaged area, indicating that the brain does adapt. (a) (b) (a) Sister Marcella Zachman (left) finally stopped teaching at age 97. Now, at 99, she helps ailing nuns exercise their brains by quizzing them on vocabulary or playing a card game called Skip-Bo, at which she deliberately loses. Sister Mary Esther Boor (right), also 99 years of age, is a former teacher who stays alert by doing puzzles and volunteering to work the front desk. (b) A technician holds the brain of a deceased Mankato nun. The nuns donate their brains for research that explores the effects of stimulation on brain growth. (Both): ©James Balog Although the Nun Study’s results are intriguing, an order of nuns is in some ways a self-selected group whose members may come to share many social and environmental characteristics through long years of living together. How might future researchers account for any potential biases in such studies? What kinds of mental activities can slow the changes in the brain that occur with age? To read about how one group of researchers is seeking to answer this question, see the Connecting with Research interlude. SECTION 2 Biological Processes, Physical Development, and Health 107 Review Connect Reflect LG2 Describe how the brain changes through the life span. Review What are the major areas of the brain, and how does it process information? How does the brain change during infancy? What characterizes the development of the brain in childhood? How can the changes in the brain during adolescence be summarized? What is the aging brain like? Connect What types of brain research technology can be used to study 3 Sleep LG3 Why Do We Sleep? infants that cannot be used to study them before they are born? Which types can be used on adults but not infants? How might these differences in research tools affect our understanding of how the human brain functions across the life span? Reflect Your Own Personal Journey of Life If you could interview the Mankato nuns, what questions would you want to ask them? Summarize how sleep patterns change as people develop. Infancy Childhood Adolescence and Emerging Adulthood Adulthood and Aging Sleep restores, replenishes, and rebuilds our brains and bodies. What purposes does sleep serve in people’s lives? How do sleep patterns change across the life span? WHY DO WE SLEEP? Sleep that knits up the ravelled sleave of care . . . Balm of hurt minds, nature’s second course. Chief nourisher in life’s feast. —WILLIAM SHAKESPEARE English playwright, late 16th and early 17th century A number of theories have been proposed about why we sleep. From an evolutionary perspective, all animals sleep, and this sleep likely is necessary for survival. Thus, sleep may have developed because animals needed to protect themselves at night. A second perspective is that sleep is restorative, with sleep replenishing and rebuilding the brain and body after the day’s activities. In support of this restorative function, many of the body’s cells show increased production and reduced breakdowns of proteins during sleep (Frank, 2017; Picchioni & others, 2014). Further, a current hypothesis is that sleep is essential to clearing out waste in neural tissues, such as metabolites and cerebrospinal fluid (Aguirre, 2016; Xie & others, 2013). A third perspective is that sleep is critical for brain plasticity (Sterpenich, Ceravolo, & Schwartz, 2017; Walker & Robertson, 2016). For example, neuroscientists recently have argued that sleep increases synaptic connections between neurons (Areal, Warby, & Mongrain, 2017; Cirelli & Tononi, 2015). These increased synaptic connections during sleep have been linked to improved consolidation of memories (Gui & others, 2017; Pace-Schott & Spencer, 2015). Further, a recent research review concluded that not only can sleep improve memory, but losing just a few hours of sleep a night has negative effects on attention, reasoning, and decision making (Diekelmann, 2014). In sum, sleep likely serves a number of important functions, with no one theory accounting for all of these functions. Let’s now turn our attention to how sleep functions at different points in the human life span. INFANCY How much do infants sleep? Can any special problems develop regarding infants’ sleep? The Sleep/Wake Cycle When we were infants, sleep consumed more of our time than it does now (Lushington & others, 2014). Newborns sleep 16 to 17 hours a day, although some 108 CHAPTER 3 Physical Development and Biological Aging sleep more and others less—the range is from a low of about 10 hours to a high of about 21 hours per day. A research review concluded that infants 0 to 2 years of age slept an average of 12.8 hours out of the 24, within a range of 9.7 to 15.9 hours (Galland & others, 2012). A study also revealed that by 6 months of age the majority of infants slept through the night, awakening their parents only one or two nights per week (Weinraub & others, 2012). Although total sleep remains somewhat consistent for young infants, their sleep during the day does not always follow a rhythmic pattern. An infant might change from sleeping several long bouts of 7 or 8 hours to three or four shorter sessions only several hours in duration. By about 1 month of age, most infants have begun to sleep longer at night. By 6 months of age, they usually have moved closer to adult-like sleep patterns, spending their longest span of sleep at night and their longest span of waking during the day (Sadeh, 2008). The most common infant sleep-related problem reported by parents is nighttime waking (Hospital for Sick Children & others, 2010). Surveys indicate that 20 to 30 percent of infants have difficulty going to sleep and staying asleep at night (Sadeh, 2008). One study revealed that maternal depression during pregnancy, early introduction of solid foods, infant TV viewing, and child-care attendance were related to shorter duration of infant sleep (Nevarez & others, 2010). And a recent study found that nighttime wakings at 1 year of age predicted lower sleep efficiency at 4 years of age (Tikotzky & Shaashua, 2012). REM Sleep A much greater amount of time is taken up by REM (rapid eye movement) sleep in infancy than at any other point in the life span (Funk & others, 2016). Figure 19 shows developme...
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