PSY 5130 Week 3 Discussion

Learning Objectives

After completing this chapter, you should be able to:

Detail the process of nerve function and course of brain development through the lifespan.

Identify patterns of physical growth and change.

Outline major milestones in motor development.

Specify the physical signs of aging during adulthood, and distinguish between primary and secondary aging.

Describe the role of touch in psychosocial development.

Explain how our sense of smell and taste develop and change.

Compare the onset and consequences of various types of hearing loss.

5Physical Development: Brain and Body

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Outline age-related developments in the visual system.

Prologue

When my son Max was 3 years old he could consistently hit a plastic baseball onto the tall roof of his grandparents’ house. He could throw and catch better than any kid his age. It was easy to see that he had terri�ic hand-eye coordination and would excel in the sport. By high school, however, despite being an outstanding athlete who excelled at basketball, soccer, and other sports, Max could not have lasted a day on the baseball team.

What could have accounted for the change?

The answer is related brain and body development. For Max, genetics and brain maturation led to exceptional hand-eye coordination at a very early age; his use of muscles that facilitated growth of baseball skills supported increased brain expansion in the areas best suited for that sport. Then, for a number of reasons, it gradually became more and more dif�icult for Max to �ind opportunities to play baseball and he became interested in other physical activities, especially basketball. Because of plasticity, his brain began to accommodate basketball skills that the environment was dictating and (literally) pruned areas involved in baseball skills that were no longer being stimulated as before. The question remains whether brain activity stimulated basketball movements or if basketball movements stimulated brain growth— or maybe there is a reciprocal interaction we don’t yet understand.

Throughout the lifespan, hormonal, neuronal, and physical changes of the brain and body are unquestionably governed by programmed genes. However, as you learned with regards to critical and sensitive periods, the environment can have a profound effect on developmental trajectories. In this chapter, we will focus more on the �irst part of the brain and body question and explore the universal aspects of biological and physical growth. In the chapter that follows, we will account for more individual factors that affect health and physical growth and decline.

5.1 Nervous System Development

Every physical and mental action originates with the nervous system. Without it, we would not be able to engage in any processes that de�ine us as human. The mature nervous system consists of the brain and spinal cord, designated the central nervous system (CNS), and neural tissues in the peripheral nervous system that extend away from the CNS into every other part of the body (see Figure 5.1).

Beginning with a simple tube reminiscent of brains from primitive organisms, in a short time the human nervous system becomes extraordinarily complex. Neural development in humans begins when gastrulation occurs in the third week of gestation (see Chapter 3). The mesoderm sends signaling molecules to the ectoderm, which responds by forming the neural plate. This strip of neuronal stem cells will eventually con�igure the entire nervous system. From the neural plate, stem cells migrate and are involved in speci�ic areas of neural circuit generation.

The neural plate begins to fold and form grooves, forming the neural tube. By the end of week four, there are distinct areas that will later form the hindbrain, the midbrain, and the forebrain. These structures will form secondary structures by the end of week 7. The optical vesicle also appears during the fourth week, which will later form the eye and the optic nerve. Part of cell differentiation is dependent on proximity to the neural plate and how the cells become genetically programmed. Initial cell differentiation is expressed independent of experience, as the human genome directs the process. That is, cells are guided by genetic programming to become parts of various systems. Once cells reach their intended destinations, neural activity and experiences become a larger factor in determining emerging neural pathways (Cooper, 2013). The production of functioning neurons commences around post-conception day 42 and will continue for approximately 120 days (Stiles & Jernigan, 2010).

Figure 5.1: The nervous system

The nervous system has two divisions: the central nervous system (the brain and spinal cord) and the peripheral nervous system (all of the nervous tissue located outside the brain and spinal cord).

By the end of the �irst trimester, the fetus will display re�lexes. It has also released the hormones that will determine the outward appearance of genitalia. The outer surface of the brain is still relatively smooth, and lacks visible gyri (ridges) and sulci (depressions). These will develop rapidly during the second trimester (Figure 5.2). Their convolutions allow for greater surface area and are probably the reason human brains are more advanced than any other species (Zilles, Palomero-Gallagher, & Amunts, 2013). However, the absolute number of brain cells is thought to be a factor in relative mammalian intelligence as well (Roth & Dicke, 2005).

Figure 5.2: Major regions of the mature brain

The midbrain, hindbrain, and forebrain (shown here in a mature brain) begin to appear during week four of development. The gyri and sulci (singular gyrus and sulcus) refer to the ridges and depressions of the brain.

During the second trimester additional structures mature and cells continue to be formed. By the end of this period, almost all neurons have been created but are yet to develop most of the connections that occur during the lifespan. Because most of the cells have been generated and structures are in place, the third trimester focuses on further sophistication of structures and systems.

Neurons and Synaptic Development

As is mentioned earlier in this section, the framework for the nervous system begins to form around day 14 of gestation, but its basic building block, the neuron, does not begin development until day 42. There are at least 100 billion neurons in the human brain. Although neurons come in many shapes and sizes, they have a number of common features. Unlike other cells, neurons communicate with each other in an elaborate electrochemical relay system. As depicted in Figure 5.3, information is �irst transmitted by dendrites, structures that receive incoming signals. The message then travels to the soma (cell body). If the signal is to be continued, it travels via the axon. The transmission may be sped up by a myelin sheath, which provides electrical insulation and eventually covers most of the long, threadlike axons. Unmyelinated �ibers conduct impulses in a wave-like, energy intensive, sequential fashion. After myelination (the process of forming the sheath around the nerve), the axon is only exposed at regular gaps in the sheath, called the nodes of Ranvier. The electrical impulse cannot �low through the myelin, so it “jumps” to the next node, which might be a millimeter or more away (Morell & Quarles, 1999). This process speeds transmission of impulses and also saves energy since less surface area of the axonal membrane is used. Therefore, myelination is an important advance, as faster neural processing is necessary to move faster physically and to think in more complex ways.

Figure 5.3: The neuron

The neuron is the basic element of the nervous system. Information is �irst received by the dendrites. The message travels to the cell body (soma). If the message is to be continued, it travels through the axon. Transmission speed is increased when the axon is covered in myelin, which allows the electrical transmission to “jump” from node to node. At the terminal buttons, neurotransmitters are released into the synapse between the sending and receiving neurons.

The timing of myelination is governed by maturation. The myelination of sensory and motor neurons that are essential to early physical development is mostly complete by 40 months, whereas the neurons that are responsible for higher brain functions like reasoning and complex decision making are not myelinated until early adulthood. When experiences are limited, brain growth is similarly restricted. Compared to infants with richer experiences, those raised in less stimulating environments show signi�icant brain differences in structure, weight, and volume (Lawson, Duda, Avants, Wu, & Farah, 2013; Luby, 2015). Not surprisingly, poor nutrition leads to less myelin development and a general reduction in brain size, though early treatment can often reverse these negative effects (Atalabi, Lagunju, Tongo, & Akinyinka, 2010; El-Sherif, Babrs, & Ismail, 2012; Gladstone et al., 2014).

Whether myelinated or not, neurons transmit electrochemical impulses to neighboring neurons (or glands or muscle �ibers) at bulblike structures called terminal buttons. This transmission is achieved without the neurons actually touching each other. Instead, they form a synapse, or gap between the sending and receiving neurons. Every terminal button contains vesicles that release chemicals called neurotransmitters into the synapse (see Figure 5.4). Depending on a number of factors, especially the concentration of the speci�ic neurotransmitter, the receiving neuron will either carry the message forward or not (the “all-or-none” principle). That is why sometimes people can perceive a faint sound or a distant light while at other times they cannot. The chemical messengers have either reached a particular threshold to transmit the sensory information or not.

Figure 5.4: Neural transmission

These neighboring neurons are able to share information using a complex process that involves transferring information as an electrical impulse within the sending neuron and as a chemical message between neurons.

Timing of Growth

At birth the infant brain weighs only about 25% of its adult weight, though the head is proportionately closer to adult size than other body parts; because of increased mass, by 2 years old the weight of the brain will have tripled. A popular theory to explain the rapid postnatal brain growth is based in evolution. Natural selection promoted a large and more sophisticated brain while also providing advantage to an upright gait. The vertical posture changed the position of the pelvis and made for a narrower birth canal that limited fetal brain growth. Therefore, in order to have a large, sophisticated brain, it would need to continue growing after exiting the relatively small birth canal. So instead of a brain that is mostly developed in the womb to allow locomotion and other tasks immediately after birth (like other mammals), humans have relatively undeveloped brains that continue to need plenty of attention.

Variations in synaptogenesis (synaptic growth) correspond to sensitive periods in brain development. Therefore, the rate and timing of synapse and dendrite formation are important to understanding development (Tierney & Nelson, 2009; Twardosz, 2012). At birth, the vast majority of synapses have yet to form, setting the stage for explosive growth. As a new object is seen, a new sound is heard, or a new movement is made, neurons branch and extend their reach to other neurons and form new synapses. Although synaptic development initially unfolds by genetic programming (maturation), experience dictates which synapses receive the most stimulation and make the most connections. Although active changes in the brain are especially noticeable for the �irst 20 years or more, postnatal brain development is particularly concentrated during infancy and early childhood (Kolb, 2009). In just a few years, children become able to think, use language, practice most of the physical skills they will use as adults, and learn social behaviors that will aid their survival.

When brain development peaks, as many as 250,000 neurons are born every minute; by the time a child is 2 years old, some cells may have up to 10,000 connections (Kolb & Gibb, 2011). Note in Figure 5.5 that synapses in the visual cortex that are responsible for sight reach peak production between the 4th and 8th postnatal months. Synapses in the more sophisticated reasoning centers of the prefrontal cortex do not peak until the 15th month; growth in language areas peaks just before infants begin to speak. Later, reasoning centers in the prefrontal cortex do not reach maturity until early adulthood.

In total, our 100 billion neurons establish trillions of synapses, forming a complex yet integrated communication network. If stimulation is lacking during sensitive periods of brain development, prospects for growth, including psychosocial processes, �ine and gross motor behavior, and language, can become limited (Gladstone et al., 2014; Vandersmissen & Peeters, 2015). Therefore, children must be given opportunities for new experiences and shielded from negative environmental effects like malnutrition.

Figure 5.5: Timing of synapse and dendrite formation

The rate and timing of synapse and dendrite formation vary by age and are important to understanding development. Notice, for example, that growth in language areas peaks just before infants begin to speak.

Source: From R. A. Thompson and C. A. Nelson, “Developmental science and the media: Early brain development,” American Psychologist, 56(1): 5–15. Copyright . 2001. Reprinted by permission of the American Psychological Association.

The Adaptive Brain

Rate and timing of physical growth in the brain also allows us to better understand the relationship between sensitive periods and neuroplasticity (the ability of the brain to adapt to experience). The younger the brain, the more “uncommitted” areas there are for neuroplasticity to operate. Sometimes another part of the brain will assume functioning; other times, functioning cells migrate to damaged areas. (In the adult brain, much of the research in the treatment of neurodegenerative disorders like spinal cord injuries and Alzheimer’s disease focuses on this knowledge that certain stem cells can become integrated into existing circuits [Lindvall & Kokaia, 2010; Obernier, Tong, & Alvarez- Buylla, 2014]).

To facilitate neuroplasticity during early brain development, there is a massive overproduction of synapses during infancy (as shown in Figure 5.6) before engaging in a process of reduction, called synaptic pruning, in order to create an individual network of connections for each person. This principle of “use it or lose it” serves as a biological foundation for learning, as mentioned in the prologue. Pruning is natural and desirable because brain ef�iciency improves and behaves adaptively. This favoritism allows neurons that receive the most stimulation—and thus are interpreted as the most important—to be given space to grow more elaborate connections. Like synapse formation, timing of pruning varies depending on brain areas. In some instances, pruning is not complete until adolescence or beyond (Selemon, 2013).

Figure 5.6: Neuron growth and pruning

According to scientists, the brain overproduces synapses during early childhood and then goes through a pruning process later. Neurons that receive the most stimulation are favored over those that receive less stimulation.

Source: From Reynolds and Fletcher-Janzen, Eds, Handbook of Clinical Child Neuropsychology, Figure 4, p. 25. Copyright . 2009. Reprinted with kind permission from Springer Science+Business Media B.V.

Not only does the brain adapt to stimulation, but if a part of the brain is damaged before it has begun its major synaptic growth, other cells can take the place of those that are damaged. For example, researchers have surgically removed brain parts of one-day old ferrets that are essential to hearing. Neural pathways that would otherwise have been eliminated through pruning replaced the missing cells and became functional for hearing instead (Sur & Leamey, 2001).

Critical Thinking

Should the knowledge that the reasoning centers of adolescents are not fully mature have an impact on how they are treated when they commit crimes? For further information and discussion, see Aronson (2007), Beckman (2004), Bonnie and Scott (2013), Steinberg (2013), and the case against Christopher Simmons (APA, 2004).

In humans, when either visual or auditory loss occurs without damage to the brain, the area that would have been dedicated to providing sensory information is recruited for other means (Merabet & Pascual-Leone, 2010). In addition, neuroplasticity sometimes produces apparently random effects. For instance, there appears to be a complete absence of schizophrenia among individuals are born blind or lose vision shortly after birth (Silverstein, Wang, & Keane, 2012). For unknown reasons, the loss of some neural pathways apparently provides a protective factor in schizophrenia.

In adults, a well-known example of neuroplasticity has been measured in London cab drivers, who must acquire “the Knowledge” of London streets. London taxi drivers spend 3 to 4 years learning the layout of the city and acquire an exceptional spatial representation of the streets. Not only can experienced cab drivers relate information about various routes, but areas in the brain that are responsible for spatial representation are signi�icantly larger than in London bus drivers, who do not have to learn the Knowledge (Maguire, Woollett, & Spiers, 2006; Woollett & Maguire, 2011).

The Adolescent Brain

Speci�ic kinds of stimulation continue to predict outcomes well beyond the �irst three years. Studies have shown that cognitive stimulation at age 4 predicts thickness of cognitive areas of the brain around 15 years later (Lawson et al., 2013). Although this study showed that stimulation leads to speci�ic growth, we know that maturation provides general growth patterns as well. Gogtay and his colleagues obtained brain scans every 2 years among individuals between 5 and 20 years of age, resulting in a dynamic map of development (Gogtay et al., 2004). Figure 5.7 shows the sophistication of cortical development that is evident throughout childhood and adolescence. Well into adolescence, axons continue to grow and expand connections, supplanting cell bodies in the process. Basic sensory and motor functions mature �irst, coinciding with the basic learning outcomes of infancy. Speech and language areas come next. The areas in the frontal lobe (one of the four major brain divisions, including the parietal, occipital, and temporal lobes) that are related to judgment and the inhibition of impulses are last to develop.

Adolescence also marks a second wave of overproduction of synapses and neural pruning, and the architecture of the prefrontal cortex begins to change rapidly (Hedman, van Haren, Schnack, Kahn, & Hulshoff Pol, 2012). Because these centers are not mature until after adolescence, some researchers have speculated that immature frontal lobe development is linked to the risky behaviors that are indicative of adolescence. This possibility also raises questions about public policy and whether adolescents should be considered more like children or more like adults with regard to forensic examinations, driving, and other adult-like responsibilities. (See especially Bonnie & Scott, 2013, Steinberg, 2013, and Steinberg & Scott, 2003.) For instance, if judgment among teens is developmentally compromised, then there are implications for holding them completely accountable for crimes.

Figure 5.7: Brain development through childhood and adolescence

In an extensive project to map brain development, scientists found that axons (white matter) continued to replace cell bodies (gray matter) well into adolescence.

Source: Image courtesy of Paul Thompson (USC) and the NIMH.

The Mature Brain

After reaching its maximum mass of a little over 3 pounds by the beginning of adulthood, the cortical volume of the brain begins shrinking with age, by approximately 2 grams per year, or 1.9% per decade (DeCarli, Massaro, et al., 2005). Nevertheless, some brain parts become more active. We continue to create new neural pathways and change existing ones in adulthood as we adapt to new experiences. For instance, researchers recorded speci�ic areas of growth and change in the brains of adults who learned a new video game (Kühn, Gleich, Lorenz, Lindenberger, & Gallinat, 2014). It

is thought that this plasticity, along with increased neuron �iring, allows brains of older adults to compensate and maintain their functionality despite the age-related loss of mass (Daselaar et al., 2015; Sale, Berardi, & Maffei, 2014).

It is still unclear why some parts of the brain adapt while others do not, but exploring the reasons why may have important implications for understanding neurodegenerative diseases and the ability for the brain to recover from injury. For instance, evidence indicates that neural growth can be promoted in the hippocampus and other areas, possibly slowing or reversing the effects of memory loss in dementia and other chronic kinds of neurodegeneration (Gladstone et al., 2014; Ho, Hooker, Sahay, Holt, & Roffman, 2013; Regensburger, Prots, & Winner, 2014). In Chapter 6, we will consider the changes in the most common degenerative brain diseases, including information about the diagnosis, prevalence, and treatment of these diseases.

Section Review

Summarize how the transmission of neural signals occurs and outline how brain activity changes with time.

5.2 Patterns of Physical Growth

Because brain volumes of infants are relatively close to adult size, the heads of infants are disproportionately large as well. On their way to adult proportions, the torso and limbs grow faster than the head. This pattern of growth is an example of directionality, one of the general principles of human growth. In this case, the direction is cephalocaudal, literally meaning “head to tail.” Notice from Figure 5.8 that the head represents about 25% of the body length at birth and then decreases with age. During the �irst 2 years, the torso and limbs quickly begin to catch up. By adulthood, the head makes up less than one-seventh of an individual’s height, or about half of the body proportion it held at infancy.

Figure 5.8: Change in body proportion, by age

One representation of the cephalocaudal principle is the change in body proportion by age. The proportion of head-to-body size decreases by about half from infancy to adulthood, and secondary sex characteristics develop through the teenage years until adulthood.

Physical growth also occurs in a proximodistal pattern—from the inside out. The pattern begins in the prenatal environment and continues after birth, as infants learn to move their torsos before their extremities. Babies learn to use their arms to maintain balance before they use their hands and �ingers to reach for an object. This pattern also overlaps the orthogenetic principle, which states that development begins rather globally and undifferentiated, and gradually increases its differentiation. For example, when infants �irst eat, they are only concerned with latching onto a nipple, sucking, and swallowing. Months later, they will orient their heads on their own, move their arms, and reposition their bodies. When infants are offered a bottle, they begin to coordinate actions of arms, hands, and mouth. Still later, children will learn to hold utensils, drink from a glass, and employ different manners of eating. They may learn to vary their posture or language depending on the company or where they are eating. In this way, the concept of eating transitions from a simple view of suck and swallow to one that is highly differentiated.

We also know conclusively that different body systems grow and mature independently. As seen in Figure 5.9, the nervous system matures quite rapidly beginning in childhood, whereas the pattern of growth of overall stature (body size) is a bit more even. And neither the timing nor the rate of sexual maturation mirrors that of either the nervous system or stature, demonstrating relative autonomous development of body systems. This is the principle of independence of systems. These general principles will become quite apparent as we expand on physical growth and development.

Figure 5.9: Independence of systems

This graph illustrates that different body systems grow and mature independently.

Source: Tanner, J. M. (1962) Growth At Adolescence, 2nd ed., Oxford: Blackwell Scienti�ic Publications. John Wiley & Sons.

Weight and Height in Early Childhood

Height is perhaps the most obvious feature of physical maturation. Whether a child is short, tall, or average, doctors measure patterns of development by consistency of growth. The chart in Figure 5.10 is typical of those used by researchers and professionals in the healthcare �ield to gauge normal changes in weight. In this case, it does not matter much which path children follow; it is more important to see that they are following a consistent pattern and that their weight is not �luctuating excessively.

Figure 5.10: CDC weight-for-age percentiles, birth to 36 months

This standard growth chart shows weight-for-age percentiles for children up to 36 months old.

Source: Adapted from Kuczmarski, R. J., Ogden, C. L, Guo, S. S., et al. 2000 CDC growth charts for the United States: Methods and development. National Center for Health Statistics. Vital Health Statistics 11(246). 2002.

Infants grow in length by about 50%, on average, in the �irst year, from a little over 20 inches (51cm) to about 30 inches (76 cm). During the second year, they add another 5 inches (13 cm). Until adolescence, the annual growth in height decreases gradually, as shown in Figure 5.11. Height can vary dramatically in poor countries where adequate nutrition is not available, like parts of India, Indonesia, and Africa. In areas where children receive suf�icient nutrition, most global variations in height are due to genetic factors. For instance, children of European ancestry tend to be slightly taller than Asian children regardless of where the children reside (Deurenberg, Deurenberg-Yap, Foo, Schmidt, & Wang, 2003; Nightingale, Rudnicka, Owen, Cook, & Whincup, 2011).

Figure 5.11: Average growth rates and heights of girls and boys in the United States

Growth rates for boys and girls show similar patterns, with girls beginning the adolescent growth spurt, on average, about 2 years earlier than boys. On average, girls are taller than boys during early adolescence. After age 14, though, girls grow, on average, only a little more than 1/2 an inch (1.4 cm), whereas boys grow another 3 1/3 inches (8.5 cm).

Source: Adapted from Ogden, Fryar, Carroll & Flegal, 2004. Advance Data No. 347. National Center for Health Statistics. October 27, 2004.

Adolescent Growth Spurt

Regardless of where healthy children grow up, their bodies eventually undergo a number of physical changes that mark the transition into adulthood. Part of the tremendous change is the sudden growth in height and weight. This development is often referred to as the adolescent growth spurt and can add 5 inches (12.7 cm) or more in a single year. Girls begin the spurt at about age 10 and boys at about age 12 (refer back to Figure 5.11). Therefore, on average, 12-year-old girls are taller than their male counterparts. In addition, because of their earlier accelerated growth, girls on average grow only about 1/2 inch (1.4 cm) after the age of 14 years, whereas boys grow another 3 1/3 inches (8.5 cm). During this growth spurt, there are also considerable adolescent physical changes associated with sexuality, which will be explored in Chapter 12.

Maximum Height and Diminishing Stature

It has been suggested that because of modern advantages in nutrition it is now possible to gain optimum genetic height, which is a function of both genetic and environmental variables (Silventoinen, 2003; Steckel, 2002). It is estimated that, in modern Western societies, about 20% of �inal body height is due to environmental variation, including nutrition and physical stimulation; in settings with fewer resources, environmental variation is responsible for more than 20% of �inal height. In developing countries and among some families in the United States, food variety is limited. For instance, there are areas all over Asia where protein is lacking and rice makes up the majority of every meal. In isolated communities at higher elevations in South America, produce may be at a premium but animal protein plentiful. And in the United States, many inner-city areas lack easy access to fresh produce and children often grow up eating only limited amounts. As a result, children may lack some vitamins and minerals that are essential for growth. Therefore, heritability of height (the proportion due to genetics) increases as a function of advantages in health, nutrition, and medical science.

Short stature varies inversely with both education and social position, so height can often be used as an indicator of the health and welfare of a population. For instance, in the United States the average person is nearly 3 inches (7.6 cm) taller today than when the country was founded in 1776. And during the 20th century, average body height increased throughout the industrialized world. From the 1870s to the 1970s alone, average height in Western European countries increased by 4.3 inches (11 cm) or nearly half of an inch per decade (Hatton & Bray, 2010). On the other hand, as people moved to cities in the 1700s and 1800s, diseases spread more easily and access to food was more inconsistent compared to when more people lived on farms (Komlos, 1998). These factors probably contributed to the �inding that some cohorts occasionally had lower stature than the previous generation. However, overall, �igures indicate that technological development has led to improved health and living conditions, including the ability to transport foods and services.

Section Review

Describe some universal patterns of physical growth, including stature, and how they may be in�luenced by contextual factors.

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As children grow, they develop the ability to control and coordinate their bodies. From rolling over to feeding themselves frozen treats, these milestones are key to motor development.

Critical Thinking

How do changes in motor skills affect the way infants interact with their environment?

5.3 Motor Development and Decline

As babies grow, parents anxiously look for their children to roll over, stand, and walk. Later, pediatricians will ask about catching a ball, using eating utensils, and manipulating a pencil. These normative milestones are important in the study and understanding of motor development (the ability to control and coordinate body movements). By adolescence, many teens can perform physically as well as or better than many adults. But there is tremendous individual variation, including factors related to genetics, culture, and gender that will in�luence how motor development will occur. As we move into middle and late adulthood, deterioration in motor skills is universal, but how we use our bodies throughout the lifespan will contribute substantially to the course of decline. These features of physical growth will be explored next.

Development in Infancy and Childhood

Physical movements are categorized as either gross motor skills or �ine motor skills. Gross motor skills involve large movements of the head, torso, arms, and legs. The �irst signs of gross motor skills related to locomotion occur when children develop the muscle control to roll over at between 2 and 3 months of age (refer to Table 5.1). Interestingly, although infancy is often associated with a crawling baby, it is not unusual for infants to skip the crawling stage and move right into cruising (walking while holding on to furniture) and then walking.

In contrast, �ine motor skills involve more precise dexterity of the hands and �ingers, initially coordinating with vision. Following the proximal-distal pattern, infants begin to integrate gross motor abilities with smaller hand movements at around 4 months of age. A few months later they are able to hold a bottle, but immature brain development will at �irst cause them to have dif�iculty guiding it to their mouths. Toward the end of the �irst year, they will transition from using the whole-hand palmar grasp to picking up cereal and other small objects between the thumb and fore�inger using what is called the pincer grip. Infants will also begin to bang two toys together and can use eating utensils and cups. These activities coincide with greater mobility, as infants delight in scanning for objects, moving toward them, and picking them up with their more advanced hold. At just a few months of age, infants are becoming less dependent on others for stimulation.

The second year brings added coordination between eye and hand movements. Children learn to get water from a faucet and put together and take apart simple toys. Preschoolers can manipulate pencils and crayons and can color within boundaries. They can also use safety scissors to cut out objects from paper. Well before they reach elementary school, most children are able to acquire the skills needed to accurately use a touch screen, computer keyboard, and mouse.

The Brazelton Neonatal Behavioral Assessment Scale (Brazelton & Nugent, 2011), Gesell Developmental Schedules (Gesell, 1925), and the Bayley Scales (Bayley, 1969) are used in various settings to assess normal developmental milestones. Together, they provide a comprehensive battery of instruments and test individual variation in motor and mental skills for children up to 42 months, or 3 1/2 years of age. The general idea of these schedules is that development is maturational and does not change very much within a healthy population. An individual’s speci�ic behaviors can be assessed and then compared to the norm, or average performance, of a similar group. A series of scores signi�icantly below a standard often indicates a disability. Table 5.1 offers examples of milestones that might typically be evaluated.

Table 5.1: Milestones in motor development, ages 0–4 years

Age Behavior Fine (f) or gross (g) motor behavior

Age Behavior Fine (f) or gross (g) motor behavior

0–6 months

Exhibits re�lexes —

Holds head up g

Rolls over g

Will reach and grasp f

Physically pursues objects f + g

Can sit without support g

Stands while holding on to a parent’s hand g

Pulls self to standing position g

6–12 months

Has the skill to crawl (but may not) g

Walks with support g

Stands alone g

Cruises (walks while holding on to furniture) f + g

Grasps with thumb and fore�inger (pincer grip) f

12–18 months

Walks without support g

Throws objects f + g

Ascends steps with help g

18–24 months

Climbs f + g

Turns on faucet to get water f

Dresses self with help f + g

Drinks from a cup f

Jumps g

2–3 years Dresses self (without buttons) f + g

Ascends steps unaided, alternating feet g

Hops irregularly g

Pours liquid from one container to another f

Draws simple �igures (e.g., circles, crosses, stick �igures) f

3–4 years Can run, jump, and ride a tricycle g

Throws and catches a ball f + g

Jumps 12 inches from a climber to the ground g

Puts together simple puzzles f

Strings beads f

Cuts and pastes f

Draws shapes and symbols holding pencil or crayon between thumb and �irst two �ingers

f

If children are exposed to the �ine motor activity necessary for musical instruments like the piano and violin, most 5- year-olds can begin to play. With some practice, the average kindergartener can tie shoes and easily manipulate zippers, snaps, and buttons. Though these children do not yet fully comprehend visual-spatial movement such as the trajectory

Critical Thinking

Consider again the story about Max’s experiences with physical activities, which is provided in the chapter prologue. How would you design a research study that investigates the relationship between early motor activity and later athletic ability?

of a rolling ball in soccer, a bouncing ball in basketball, or a pitched ball in baseball, they can still engage physically in those activities. Because movement is slower and reaction time is thrown off, accommodations like a batting tee (“T- Ball”) are made for younger elementary-school-age children. Table 5.2 includes examples of milestones that might typically be evaluated.

Table 5.2: Milestones of motor development, ages 4–7 years

Age Behavior Fine (f) or gross (g) motor behavior

4–5 years Hops with purpose g

Ties shoes f

Descends stairs, alternating feet g

Prints recognizable letters and numbers f

Walks across a balance beam g

5–6 years Hand dominance usually apparent —

Skips g

Skips rope g

Connects zippers, buttons, and snaps f

Traces accurately f

Copies shapes f

Uses school supplies appropriately f

7 years Physical movement resembles adult movement —

Uses tools f

Can anticipate trajectory of rolling balls —

By late elementary school (10 or 12 years of age), children can throw a ball, run smoothly, hop, jump with purpose, and kick with great agility and skill. They show outstanding coordination dribbling a soccer ball or a basketball. They have great body control on a skateboard or rollerblades. Though they still lag behind adults in strength and speed, 12-year- olds show adult-like hand-eye coordination in most physical activities, quite unlike the 6-year-old bodies they left behind. The advancement of physical skills also depends on brain maturation because more cognitive sophistication is required to coordinate advanced movements. For the most part, by the end of elementary school children can perform the same movements as adults, though without the same skill or strength.

Although there are clear consequences of experience in motor development, inherited traits have been found to have a stronger effect on motor development than quality of life (Puciato, Mynarski, Rozpara, Borysiuk, & Szyguła, 2011). Among children aged 8–16, height and body fat are more highly correlated with speed and strength than social factors. That is, there is evidence that a person’s genotype indeed is a determining factor in the performance of skills that are universal to many physical endeavors.

Development in Adolescence

The adolescent body is decidedly adult-like. After all, puberty marks the transition into an adult body. Physical abilities of many adolescents exceed that of their parents. Notably, peak swimming ability, as measured among athletes in world competitions, occurs between 18 and 21 years of age (König et al., 2014). In contrast, motor ability, strength, speed, and coordination in other physical tasks generally does not peak until the mid to late 20s (depending on the skills and muscles involved). As noted, genotype is a strong determinant in many motor abilities related to speed and strength. However, other than those aspiring to be elite athletes, most ordinary variations in motor abilities do not necessarily have a global impact on development.

Changes in Adulthood

Strength, stamina, and speed can continue to improve during the 20s. For most of us, biological declines in mobility and potential peak performance have little effect until middle adulthood (Elmenshawy, Machin, & Tanaka, 2015; Schaie, 2005). At that time, we generally begin to compensate for physical changes by increased anticipatory skills and expertise (Krampe & Charness, 2006; Wright, Bishop, Jackson, & Abernethy, 2011). That is, in competitions adults tend to use experience and �inesse to make up for the physical losses of sarcopenia (natural muscle loss) that begin in the early 30s. In everyday tasks, older adults tend to slow down some activities and break up tasks into smaller units, like using a greater number of grocery bags and performing some activities more slowly than previously. Sometimes the convergence of reduced coordination and osteoporosis becomes quite problematic. Compared to uncoordinated toddlers who fall frequently, the elderly who fall have wrists, arms, and hips that are much more fragile and farther from the ground, and thus they suffer bone fractures much more often.

Sex Differences in Motor Development

There is a common assumption among parents in the United States that infant girls are more advanced physically than infant boys. Overall though, it is the result of anecdotal information more than scienti�ic evidence. As depicted in Figure 5.12, small, statistically signi�icant differences sometimes exist, but they vary by country and by behavior (WHO Multicentre Growth Reference Study Group, 2006). Importantly, when there are milestone differences between sexes within a country, it is due to culture-speci�ic behaviors. When data are pooled for all countries and for both sexes, the size of any differences is “too small to justify sex-speci�ic norms” (p. 71).

Figure 5.12: Sex differences in motor development

Statistically signi�icant differences in motor development exist, but they are likely due to cultural differences in the way that boys and girls are treated. Overall, evidence does not justify identifying a separate set of norms for boys and girls.

Source: de Onis, Mercedes (2006). Assessment of sex difference and heterogeneity in motor milestone attainment among populations in the WHO Multicentre Growth Reference Study. Acta Paediatricia, 450, 66–75. (Figure 1 ). Copyright © 2007 John Wiley and Sons. Published by Jon Wiley & Sons.

As children mature, there is no doubt that sex differences in brain development affect motor behaviors and skills. Studies con�irm that physical disparities exist between boys and girls because of physiological and maturational differences (e.g., Eaton & Yu, 1989; Pellegrini & Smith, 1998). Girls perform better at balancing skills like walking on a beam, balancing on one foot, and playing hopscotch. On the other hand, it should come as no surprise that boys generally outperform girls in gross motor skills that require speed or strength. Beginning at about 3 years old, boys on average jump higher and run faster than girls. These differences are generally due to variability in muscle strength. Even from birth, boys are more active than girls.

Perspectives on evolution and neurobiology reveal that the greater activity level of male infants accelerates brain growth of the motor neurons needed for strength and speed. But beginning at an early age boys on average are also conditioned to be more active than girls. Adults treat girls more delicately and use softer language within 24 hours of birth, a pattern that continues during infancy (Beal, 1994; Johnson, Caskey, Rand, Tucker, & Vohr, 2014). Compared to their interactions with boys, mothers cuddle girls more, and they are more emotionally expressive, smile and talk more, and are more responsive to the needs of girls. Boys are given more latitude, whereas girls tend to be more restricted. In this way, boys may learn to be more independent, which translates to greater activity. Regardless of the reasons, boys get more practice using their motor skills, perhaps laying the groundwork for increased strength later.

Physical Norms and Cultural Variations

Recently it has been suggested that there is more diversity than was once thought in the acquisition of motor skills, providing substance for the nature-nurture debate. Karasik, Adolph, Tamis-LeMonda, and Bornstein (2010) argue that traditional developmental scales are based on Western-educated populations. They highlight a number of cultures in which the environment seems to play a larger role in development. For example, some cultures speci�ically target infant muscles that are later necessary for walking. These muscles are massaged and stretched, and infants are engaged in various motor exercises in an effort to get the children walking sooner. This treatment would be an advantage within environments where there are few safe places for children to crawl.

Contemporary environmental variations can affect other kinds of movement as well, even the seemingly benign use of diapers. In a newer study, researchers asked if the relatively new cultural invention of various diapering practices contribute to differences in motor development and walking behavior (Cole, Lingeman, & Adolph, 2012). In many poorer countries where diapering is a luxury, until children are toilet trained it is typical for them to remain naked during the day. Infants who had been accustomed to walking in disposable diapers were documented walking in one of three conditions: naked, in a cloth diaper, and in a disposable diaper. The resultant footprint paths for the three conditions in Figure 5.13 were noticeably different, with the naked condition providing the most mature pattern. This study shows that cross-cultural research that compares locomotion skills may be less reliable if diapering practices are not taken into account. Furthermore, it is not clear whether the contextual differences of diapering lead to signi�icant changes in later development, such as athletic skills or hip injuries among the elderly.

For the most part, accelerating early physical milestones like walking is probably unnecessary in most developed nations. Parents may want to show off that their not-yet-one-year-old is walking, but the fact is that children will learn to walk anyway. The child who was pushed to walk early may simply begin walking at 12 months instead of 12 months and 2 weeks. So while Karasik et al. (2010) explain that “the �ield suffers from long-standing assumptions of universality based on norms established with [Western] populations” (p. 95), a strong case has yet to be made against the continued use of those norms. Whether milestones are representative of and appropriate for non-Western-educated populations appears to be an important question for further research.

Figure 5.13: Environmental context on walking behavior

Footprint paths of a single child in three conditions show that diapers change walking behavior. When children are naked, they demonstrate the most mature gait.

Source: Adapted from Go naked: Diapers affect infant walking, by Whitney G. Cole, Jesse M. Lingeman and Karen E. Adolph. Developmental Science, Volume 15, Issue 6, pages 783–790, November 2012. John Wiley & Sons. . 2012 Blackwell Publishing Ltd.

Section Review

How does motor ability change from infancy through old age? Give a brief outline of the changes in motor behavior that take place across the lifespan, and consider the possible in�luences of gender and culture on such changes.

NEXTNEXT

Eric Raptosh Photography/Blend Images/Superstock

Getting gray hair, wrinkles, and other signs of primary aging are unavoidable because the physical changes are programmed into our bodies.

5.4 Physical Aging in Adulthood

Overall, two key processes in�luence aging processes such as decline in stature. The �irst process includes gradual but inevitable physical changes that occur in adulthood over the years. This type of biological change, or primary aging, is responsible for gray hair, wrinkles, and reduced ef�iciency of the body’s respiratory, circulatory, and digestive systems. Primary aging is unavoidable, regardless of how healthy a person is, since it is programmed into our species. On the other hand, secondary aging results from disease, poor health habits, and environmental hazards. These factors are more individualized, and will be a primary topic of Chapter 6.

Theoretical perspectives on primary aging generally fall into two categories: programmed aging and damage theories. Despite advances in molecular biology and genetics, no single theory exists that adequately explains the limitations of the human lifespan (Kunlin, 2010). Most likely, the interaction among the various theories may ultimately provide the best explanation for why our bodies age.

Programmed Theories of Aging

Programmed theories of aging (also called adaptive theories) suggest that there are biological and genetic limits to how long we can live. From this perspective, our bodies are “programmed” to last for a certain amount of time, based on a biologic timetable. Some people who live longer than others may inherit a cell structure that has more potential to regenerate rather than turn self-destructive (Davidovic et al., 2010; Guarner & Rubio-Ruiz, 2012). We use the term senescence to describe the biological decline brought about by aging. Senescence decreases immune system functioning and increases our vulnerability to infections, which threatens our ability to survive (Castelo-Branco & Soveral, 2014).

But none of these theories taken alone can account for the complexity of aging. In fact, scientists know that genes become unstable, hormones diminish, and immunity weakens as part of the aging process, but a great deal is still unknown about how these changes happen. Researchers would like to better understand programmed aging so they can eventually discover a way to reprogram certain aspects of aging to lower the occurrence of age-related diseases (Goldsmith, 2008).

Programmed Senescence The length or duration of life is called longevity. Every species has a speci�ic longevity that is a part of their cellular makeup. In 1961, Leonard Hay�lick discovered that cells divide a predetermined number of times. Human cells (lung, skin, muscle, heart) divide approximately 50 times and then slowly come to a stop. The cells stay in a period of senescence while they are still alive but no longer divide; eventually they die (Hay�lick & Moorhead, 1961). The number of times a cell can divide before senescence is known as the Hay�lick limit. The cells’ ability to divide only so many times is an explanation for aging and suggests that the human lifespan has an upper limit.

Building on Hay�lick’s discovery, other scientists have found that cells keep track of the number of times they have divided. Chromosomes have structures called telomeres at either end. These have been likened to the tips of shoelaces in the way they hold the ends of the laces together. Each time a cell divides, the telomeres become shorter. After numerous divisions, the telomeres are too short to allow the cell to divide, and the cells reach their Hay�lick limit and begin apoptosis (normal cell death) (Watts, 2011). This is one of the origins of the idea that we have a biological clock that limits the amount of time we will live.

Endocrine Theory Rather than mutating genes, the endocrine theory says that lower hormone levels secreted by the endocrine glands are responsible for the aging process. Our complex endocrine system controls the many different hormones that regulate

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It is common for people who are active throughout their lifetimes to outlive people who are more sedentary.

many of the body’s processes. The amount of hormones decreases as we age. For example, the onset of menopause can result from a natural decline in reproductive hormones such as estrogen. In middle age, as well, growth hormone levels decline (Kunlin, 2010). It is possible that hormones initiate the action of certain genes being switched on or off, a process that may also be impacted by epigenetics.

Immunological Theory Immunological theory claims that the immune system is programmed to decline over time, making us more vulnerable to disease, which promotes mortality. Scientists suggest that the immune system peaks during adolescence, possibly to assure the continuation of our human species through reproduction. The immune system helps protect the body from harmful substances like bacteria and viruses.

Regardless of the reason, as we age, the response of the immune system grows weaker (Castelo-Branco & Soveral, 2014). As yet, we have failed to identify the speci�ic mechanisms by which the destructive processes take place. In addition, we do not have a complete understanding of how they work. And if the immune system were the primary mechanism that in�luences aging, then it is likely that diseases would be more predictable than they are.

Damage Theories

There are a number of damage theories, but they too have limitations. The wear-and-tear theory makes intuitive sense as it compares the body to a machine. Like a new vehicle or other machine, the body can simply wear out. If you buy a new car, eventually you will begin to see signs of damage—the fenders get scratched, the brakes wear out, and the tires lose their tread. The more you use it, the more wear and tear will occur. Likewise, over time the body experiences damages that add up until there is a failure of a critical organ, such as the heart. Comparing a body to a machine makes this theory seem reasonable because the more we use our bodies, the more it seems like “parts” deteriorate. For instance, a common way to describe aching joints is that they are “worn out.” The number of older people who lose cartilage in their joints and undergo joint replacement surgery provides support for this theory.

On the other hand, a limitation of this theory is that it fails to explain why repeated use has the potential to create positive effect by maintaining �lexibility and improving overall health. Adults who stress their joints and organs through exercise increase their overall health. On average, people who are active throughout their lifetimes outlive people who are more sedentary, even when weight is not a consideration (Moore et al., 2012). Pulmonary (lungs) and cardiac (heart) functions improve with more use as well.

Free Radical Theory One speci�ic damage theory involves a by-product of normal cell metabolism. Cells, the basic building block of all life, begin by having pairs of electrons surrounding their atoms. However, through the process of oxidation, the atoms lose one electron, which leaves the atom with an unpaired electron. When an atom has only one electron instead of a pair, it is called a free radical. These unpaired electrons go hunting for mates, damaging cells in the process. In order to

neutralize the oxidation damage, the body naturally produces antioxidants. These scavenger molecules hunt excess free radicals and balance the damage by converting them into less harmful molecules (Rahman, 2007). This process is part of normal cell functioning, but damage occurs when free radicals accumulate and overwhelm antioxidant defenses. Over a lifetime, the cumulative effect of free radicals causes cells to deteriorate, malfunction, and become susceptible to chronic age-related diseases like cancer and Alzheimer’s disease (Indo et al., 2015; Kunlin, 2010). Furthermore, oxidation is aggravated by known health detriments like smoking and air pollution (Rylance et al., 2015).

It has been theorized that one way to slow the cumulative damage is to consume a diet that is rich in multiple types of antioxidants, like berries, broccoli, red wine, and tea. In theory, supplementing your body’s natural antioxidant defenses stops free radicals from doing damage and hence slows the processes of primary aging (Carocho & Ferreira, 2013;

Perspectives: Signs of Physical Aging

Haryonto, Suksmasari, Wintergerst, & Maggini, 2015). Although this theory makes intuitive sense, there are still signi�icant challenges to accepting the idea that limiting free-radical production is essential to reversing the aging process (e.g., Zuo, Zhou, Pannell, Ziegler, & Best, 2015).

Signs of Aging

Once we enter adulthood, observable changes take place no matter what we do. Aging skin loses moisture and fat, making it dryer. It will eventually become thinner, splotchy, and wrinkled. Hair turns gray and thins. These distinctions are apparent as people look in the mirror, but the external signs have relatively little effect on physical health. That is, when comparing people of the same ages who have wrinkles or hair loss versus those who do not, there are no differences in longevity (Schnohr, Nyboe, Lange, & Jensen, 1998). In contrast to what a mirror might re�lect, reduced organ and immune functioning are two areas of biological aging.

Internal Systems Throughout the lifespan, the body continues to change in stature. Around the age of 50, height decreases because of changes in the muscles, bones, and joints. The tendency to become shorter over time occurs among all races and both sexes (Minaker, 2011). On average, men lose 1 inch (3 cm) and women nearly 2 inches (5 cm) before they are 70 years old. Over the 15 to 20 years after age 70, the loss in stature is doubled (Sorkin, Muller, & Andres, 1999). As people get older, the bones in the spine actually shrink in both density and size, and this shrinkage results in height reduction (Sorkin, Muller, & Andres, 1999; Yeoum & Lee, 2011). Conditions like Parkinson’s disease and osteoporosis contribute to more extreme declines in height.

Like other muscles, the heart becomes less ef�icient beginning in middle age. Across every ethnic group, the heart shrinks, changes shape, and takes longer to squeeze and relax, resulting in reduced blood �low (Cheng et al., 2009). And since virtually all tissues and organs depend on adequate blood �low, this change has a strong effect on aging. In addition, in most of the body’s systems, cellular energy production is reduced, which contributes to diminished capacity to repair itself and therefore greater physiological stress and disease (Mangoni & Jackson, 2004; Sonntag, Eckman, Ingraham, & Riddle, 2007). Not all the news is bad, however. Diet, exercise, and other protective factors can mitigate the natural effects of advanced age.

The Skeletal System While deterioration of internal systems has a direct effect on mortality, changes in the skeletal system are not usually life threatening. They can, however, cause secondary aging effects related to movements and cause substantial pain and discomfort. The two most common age-related developments of the skeletal system are osteoporosis and osteoarthritis.

Human bones under a microscope appear full of holes. Instead of having a smooth, solid texture, they look more like a honeycomb (see Figure 5.14). Bones get weaker when the “holes” in the structures become larger. Although doctors consider this process of osteoporosis a disease, it is partly maturational. The loss of bone accelerates the compression of the spinal column, and individuals often develop a hunchback as the spine bends forward. Osteoporosis is the primary reason that hip fractures occur so often among the elderly. There is strong evidence that osteoporosis can be prevented or slowed. Interventions include engaging in regular exercise, consuming adequate amounts of calcium, obtaining enough vitamin D, avoiding smoking, and drinking alcohol only in moderation.

Figure 5.14: Normal/osteoporotic bones under a microscope

Osteoporosis results in less dense, more porous bones (image on right) as compared to healthy bones (image on left).

JACOPIN/BSIP/SuperStock

In contrast to osteoporosis, osteoarthritis (also called degenerative joint disease) is only partly the result of genetics and the “normal” wear and tear of joints. It occurs when the protective soft tissue that protects the ends of bones deteriorates, resulting in pain when bone grinds against bone. Osteoarthritis is often the result of secondary in�luences like repetitive movement, overuse, physical traumas, and the added weight that obese people carry (Hootman, Helmick, Hannan, & Liping Pan, 2011; Murphy & Helmick, 2012). More men than women under 45 have osteoarthritis, probably because of different environmental stressors (e.g., physical careers) on joints. In the older population, it is unclear why more women than men are affected. In the United States alone, over 27 million people have osteoarthritis.

Section Review

Describe some of the changes that the body experiences during adulthood.

5.5 Sensation and Perception: Touch, Smell, and Taste

For centuries, it has been common to talk about �ive senses: vision, hearing, taste, smell, and touch. We also have a somatosensory (body) system dedicated to skin pressure, pain, and temperature. The senses contain receptors that make up what might be called an information highway in the body. Sensation is the activation of nerves by certain stimuli, and perception is the interpretation of the stimuli through the senses.

Visual, auditory, olfactory, and other sensations are already well developed and can be interpreted in 1-month-old infants, but since infants cannot verbally communicate like adults, the most common method of testing what infants can perceive is through the process of habituation (see Figure 5.15). Like anybody else, infants stop paying attention when they get bored with a particular stimulus. At �irst, they attend to novel stimuli, but their attention gradually diminishes. When they �inally stop responding altogether, we say they habituate. For instance, the �irst time newborns are presented with a rattle, they will turn their heads, curious. Over time, they will lose interest until further stimulation no longer causes any response. They have become habituated to the sound and sight of that particular toy. If infants then pay attention to a different rattle that makes a new sound or looks different, we know that they can discriminate among different sounds, colors, or shapes of rattles. Because they habituate to the �irst rattle but pay attention to the second, we know that they have perceived a change. Psychologists and developmentalists can use the process of habituation to understand and explore an infant’s sensory and perceptual capabilities.

Figure 5.15: Habituation and dishabituation

In phase 1, the experimenter waits until the infant becomes habituated to the pattern (uninterested in the stimulus). In phase 2, the experimenter presents either the original stimulus or one that is novel. Infants who have habituated in phase 1 attend to the original stimulus for a shorter period of time compared to the novel one. Infants who did not participate in phase 1 will attend equally to both stimuli. Habituation allows us to know when infants can discriminate between two stimuli.

Later in life, when and how a change in the senses occurs will vary by individual. But for most individuals, senses will begin changing during middle adulthood, with the exception of vision, which may begin to change earlier. These developments are gradual and only noticeable later. Important new research suggests that there is a link between the strength of various senses as we age, and maintaining cognitive functioning (Rogers & Langa, 2010; Velayudhan, 2015).

Psychology in Action: Habituation

If you have children, you know that the coolest toys, the ones children really like, are those that are at someone else’s house. So you go out and purchase one of those cool toys, only to �ind your child is bored with it. When you go back to the other house, your child again �inds that there are cooler toys there. Buying one of those new toys will once again leave you disappointed. Understanding habituation can save you money and some frustration. Like anyone else, children are attracted to novel stimuli. Children become habituated to their own toys, whereas toys that someone else has are new and exciting. So how can you combat this natural process?

One way is to use different containers for toys and activities. When every toy is always available, children habituate to all of them. If, instead, containers of toys are rotated every few weeks, they remain fresh and novel whenever they appear (dishabituation). Many parents make the mistake of constantly buying toys to keep their children stimulated, when they may have enough already.

Touch

We know that touch is important for infants (see Chapter 4). It stimulates growth and showcases the beginning of psychosocial development. Studies with orphans who are deprived of touch have repeatedly shown that reciprocal physical interactions during early infancy and childhood are essential to healthy development (Carlson, Hostinar, Mliner, & Gunnar, 2014). One demonstration of touch occurred when French researchers used the process of habituation to see if 45 full-term neonates could tell the difference between a prism and a cylinder (Streri, Lhote, & Dutilleul, 2000). The objects were �irst placed into the children’s palms; the grasping re�lex caused the neonates to re�lexively grab on to them. Approximately half the neonates were given prisms, and the others were given cylinders. The children would eventually drop the object, but the research team would place it back into the palm. This pattern was repeated through nine trials. By the ninth trial, the children held the object, on average, for less than half the time of the �irst trial. They had begun to habituate.

The second part of the experiment involved placing the other object in the palm after the ninth trial. That is, if the neonate was in the cylinder group, he or she was given a prism, and vice versa. On average, the children held on to the novel stimuli more than twice as long as on the ninth trial with the habituated object, demonstrating a somewhat sophisticated sense of touch. According to the authors, this study provided the �irst experimental evidence of the ability of neonates to discriminate by touch between two different objects.

Beginning in late adulthood we know that advancing age is responsible for reduced sensitivity to touch and other somatosenses, but interpreting how the information is useful is dif�icult. It is anticipated that understanding how accuracy of touch declines in old age will lead to new discoveries in such areas as pain management and stroke recovery, but standardized somatosensory measures have only recently been developed (Dunn et al., 2015; Wickremaratchi & Llewelyn, 2006).

Smell and Taste

Taste and smell are intertwined and contribute to our enjoyment of life by, among other things, stimulating our desire to eat. In nearly all culture, food is also a social experience, steeped in traditions, meaningfulness, and custom. Taste and smell also provide warning signs of danger, such as tasting spoiled food or smelling smoke. Taste and smell receptors are two areas of the nervous system that are regenerative. The lifespan of these nerve cells is limited—taste receptors are replaced as early as every 10 days—so they must constantly reproduce themselves (Hamamichi, Asano-Miyoshi, & Emori, 2006; Gaillard, Rouquier, & Giorgi, 2004). The ability to detect different tastes undergoes only moderate maturational changes over time, though environmental events, like dental procedures or malnutrition, can have more dramatic effects (Su, Ching, & Grushka, 2015).

Development in Infancy

When newborns turn in the direction of one smell over another, it indicates that they can discriminate between the two odors. Although the sense of smell is not as well developed in humans as in other mammals, it appears that neonates can discriminate among odors quite well. If 2- to 4-day-old neonates are exposed to their own or another baby’s amniotic �luid, they prefer their own (Marlier, Schaal, & Soussignan, 1998). And there is convincing evidence that neonates prefer their mother’s smell to that of strangers, including many studies that show breastfed infants are attracted to both the smell of their own mothers and the smell of her milk (e.g., Lipchock, Reed, & Mennella, 2011). Further, neonates who experience pain are calmed when they smell their own mother’s milk compared to another mother’s milk or formula (Nishitani et al., 2009).

Experiments on odors mimic the way that infants orient toward familiar taste. For instance, parents who feed their infants soy-based formula (because of allergies to animal-based formulas) are often concerned when their children initially reject the formula. However, infants readily begin to associate the new formula with hunger relief and soon learn to prefer its taste to other formulas. And Zhang and Li (2007) showed that newborns as young as 90 minutes can discriminate among four primary tastes. Neonates were exposed to sweet, salty, sour, and bitter tastes and then graded on intensity of expression and mouth actions. Over 93% of infants showed no distinct mouth expression when introduced to a sugar solution, compared to only 27% for a salt solution, 3% for a sour solution, and 21% for a bitter solution. For each taste, infants showed a different range of expressions, as shown in Figure 5.16.

Figure 5.16: Infant discrimination of taste

By administering different taste solutions to 90-minute-old babies, Zhang and Li (2007) showed that infants can discriminate among a number of different tastes. Facial changes in response to taste stimuli could be categorized among nine different expressions: Row A represents no distinct mouth action, B is a pursing action, and C is a gaping action. Whereas over 93% of newborns showed no distinct mouth or facial action (A1) when exposed to a sweet solution, nearly 70% exhibited one of the B responses when given the sour solution. Studies like this one show that even newborns have well-developed taste sensitivity.

Source: Used with permission of Zhang & Li (2007).

Changes in Adulthood Although there is evidence that smell and taste change with age, it is not clear exactly how they change. Changes in taste are likely due in part to a shrinking number of taste and odor receptors beginning in early adulthood, as well as the reduction of saliva that would otherwise release food molecules and trigger �lavor; people between 70 and 85 years of age have only about one-third as many taste buds as young adults have (Moller, 2003). A focus of recent research is the �inding that the inability to identify odors is associated with memory for speci�ic events and with cognitive impairment in general. Furthermore, among individuals with speci�ic genetic markers for Alzheimer’s disease, impaired odor identi�ication predicts later dementia, even when symptoms are not yet present (Rahayel, Frasnelli, & Joubert, 2012; Velayudhan et al., 2015).

Section Review

What do we learn by studying smell, taste, and touch? Consider some of the changes that occur to each of these senses as we develop and age.

Blend Images/Superstock

With infant-directed speech, adults and siblings tend to use high-pitched voices and sing-song intonation.

5.6 Sensation and Perception: Hearing

There are several components in the ear working together to allow us to hear and distinguish among different sounds. When sound waves reach the tiny hair-like cells in the inner ear, the hair-like cells respond to the vibrations and initiate neural transmissions. The inner ear contains speci�ic kinds of sensory receptors that allow us to distinguish between different tones, pitch, and volume. Transmissions of sound travel via the auditory nerve to the auditory centers in the brain.

Development of Hearing

The structure of the ear is nearly complete in the 4-month-old fetus. Perhaps that is why auditory processing of newborns appears to be similar to that of adults and fully functioning at birth. Fetuses remember voices, language, rhymes, and melodies, which we will learn more about in Chapter 7. However, in general, sounds need to be louder and higher in pitch than is necessary for adults (Olsho & Gillenwater, 1989; Werner & Gillenwater, 1990). The tendency of adults—and even older siblings—to use the high pitched, sing-song intonation of infant-directed speech might be nature’s way of responding to infant needs.

At birth, infants will startle at loud noises and can be quieted by familiar voices and soft sounds. By 4 months, children notice different sounds of toys and appear to enjoy making gurgling and babbling sounds. Beginning at around 6 months, children orient towards adults who are speaking to them and will understand speci�ic nouns, like “bottle,” “Mommy,” or “sock,” demonstrating an ability to discriminate among sounds. Before long, infants will begin speaking and learn other aspects of language, a topic of Chapter 8.

Changes in Hearing

For most of us, hearing and other senses are taken for granted—until and unless they are impaired. At all ages, ears must be protected from very loud noises. A very loud volume causes the hair-like cells in the ear to begin to split and fray. After a short period of loud noise, the ear returns to normal. If the cells fray severely, as in a bomb blast, or repeatedly, as in the constant use of earbuds at high volume, the person may no longer be able to hear certain tones in the normal range (National Institute of Health, 2015a). Early hearing loss affects many aspects of language and learning, whereas age-related hearing loss often barely receives notice at �irst. The ability to clearly differentiate sounds begins to decline around age 50, likely because of changes in the way the auditory nerve transmits signals to the brain. This next section will consider these and other types of hearing loss.

Early Hearing Loss When hearing of young children is impaired, it can have far-reaching effects. Because of the critical period for language, when children have severe hearing loss before the age of 3, they usually have dif�iculty producing oral language. But even those who experience hearing loss after the age of 3 often experience speech impairments. Early auditory impairment is also associated with dif�iculties in abstract thought, including solving math problems and understanding concepts, which creates academic problems (Marschark, 2003a, 2003b). It is theorized that these cognitive de�icits are due to the ways in which those with hearing impairments process language, but clear evidence about the causal factors behind differences in cognition has remained elusive. Without hearing aids or cochlear implants, children with hearing loss risk psychosocial problems, such as low self-esteem, because of poor communication skills. However, upon receiving hearing aids or cochlear implants, self-esteem sharply rises, even surpassing that of non-hearing impaired peers (Theunissen et al., 2014; Warner-Czyz, Loy, Evans, Wetsel, & Tobey, 2015).

Activity

Cochlear implants can help provide a sense of sound for those who have severe hearing loss or are deaf. Visit the National Institute on Deafness and Other Communication Disorders to learn more about these implants (http://www.nidcd.nih.gov/health/hearing/pages/coch.aspx (http://www.nidcd.nih.gov/health/hearing/pages/coch.aspx) ). Some parents of deaf children are in favor of cochlear implants but others are not. Why might some parents decide to reject this technology?

Noise-Induced Hearing Loss During adolescence, contemporary teens may be particularly vulnerable to hearing problems. Music players, concerts, home theaters, and outdoor power tools have a cumulative effect that can cause sensorineural hearing loss by damaging auditory receptors in the ear, or the neural pathways that lead from the ear to the brain. At present, this type of hearing loss cannot be repaired. How long does it take for loud music or other noise to cause permanent damage to the auditory system? A simple blast of a �irearm or exposure to loud music over just several months can cause permanent hearing damage called noise-induced hearing loss (Harrison, 2008; Segal, Eviatar, Lapinsky, Shlamkovitch, & Kessler, 2003). By the time North American teenagers graduate from high school, up to one out of �ive will have noise-related preventable hearing loss (Sekhar, Clark, Davis, Singer, & Paul, 2014). Evidence from both industrialized and developing countries show similar results elsewhere as well (Beach, Williams, & Gilliver, 2013; Biassoni et al., 2014; Zia et al., 2014).

Age-Related Hearing Loss In contrast to noise-induced hearing loss, age-related hearing loss (AHL), or presbycusis, is a natural occurrence. AHL causes people to have more dif�iculty differentiating sounds, such as listening to one voice in a room full of people talking. In addition, the ability to hear soft sounds, such as a whisper, or higher frequency sounds, such as a certain letter in words, can be a struggle. As a result, older adults may sometimes think that young people are mumbling. These experiences in social settings can lead the hearing impaired to withdraw from activities and affect quality of life (Ciorba, Bianchini, Pelucchi, & Pastore, 2012). Though AHL will minimally af�lict about half of the population by age 65, a signi�icant proportion of adults do not self-report a hearing loss (Gopinath et al., 2009). This �inding highlights the subtle nature of AHL; the majority of people with moderate hearing loss avoid hearing aids (Firman, 2014). Studies consistently �ind that men on average experience earlier hearing loss and a greater degree of it than women, though they each suffer de�icits of slightly different frequencies (e.g., Kim et al., 2010). While hearing aids have improved considerably in recent years, they are still far from perfect in recreating unassisted hearing. That is, AHL typically affects perception differently, yet hearing aids amplify all sounds equally, creating discomfort. With a rapidly aging population, these are important concerns.

Section Review

Describe the various types of hearing loss and how such changes may in�luence the individual affected.

5.7 Sensation and Perception: Vision

Surprisingly, neither the World Health Organization nor the United States has systematically collected prevalence data on typical vision. Smaller studies exist, but they are likely to be biased and unreliable. One survey of 14,213 adults in the United States indicated a bit more than half of adults have correctable refractive errors, meaning that it is “normal” for many people that the lens of the eye does not correctly bend, or refract, light after it enters the eye (Vitale, Ellwein, Cotch, Ferris, & Sperduto, 2008). Refractive errors result in conditions like nearsightedness (myopia) and farsightedness (hyperopia). For them, glasses or contact lenses prevent otherwise serious impairment. In poorer countries, it is estimated that limited access to prescriptive eyewear causes inadequate vision for over 165 million people (Resnikoff, Pascolini, Mariotti, & Pokharel, 2008). Early visual impairments, in particular, pose academic problems, contributing to lifelong consequences.

Vision in Infancy and Childhood

Even though sight is highly developed in humans, it is the least developed of the senses at birth. The overall structure of the eye is mostly complete when the fetus is 4 months old, but the retinas (where the visual receptors are located) are not fully developed. Neonates can see at least 12 inches (30 cm), which is about the distance from the breast to a mother’s face. By 12 weeks postnatal, color perception may be so well developed that infants begin to show preferences for certain colors over others; by 30 weeks, they can discriminate between the slight variations of one hue (Yang, Kanazawa, & Yamaguchi, 2013; Zemach & Teller, 2007). Though some controversy exists, infant vision is thought to become similar to that of an adult as early as 6 months (Cavallini et al., 2002).

Though newborn vision is not sharp, infants can perceive shapes and patterns. Robert Fantz (1961) famously demonstrated that even 2-week-old babies prefer to look at patterns rather than plain stimuli. Infants are initially interested in simple contrasts like a bull’s eye, and by their third month, they begin to prefer more complex patterns (Brennan, Ames, & Moore, 1966). When given a choice among a number of objects, infants will stare longest at a human face (see Figure 5.17). Evolutionary psychology suggests that a built-in preference for faces allows infants to read the environment, increasing their chances for survival.

Figure 5.17: Infant visual perception

Robert Fantz famously demonstrated that infants prefer to look at more complex patterns, with human faces being most preferred.

In a famous experiment, Eleanor Gibson and Richard Walk (1960) constructed a “visual cliff ” to investigate whether or not infants had depth perception, or the ability to perceive distance and see in three dimensions. They built an elevated glass table, with one side consisting of a checkerboard pattern and the other a sheet of clear glass that gave the illusion of a cliff. Infants aged 6–12 months were placed on the edge of the “cliff ” between the checkerboard and the perceived drop. Then their mothers tried to coax them over the cliff. If the infants refused to crawl over the clear glass, it was hypothesized that they could see that the “drop” was dangerous because they perceived depth. With few exceptions, the infants would not crawl over to their mothers, indicating that infants do indeed have depth perception. Developmentalists do not know the precise age at which infants acquire this skill, but the visual cliff demonstrated that humans attain the ability before they are able to crawl.

Other kinds of visual perception are dif�icult to de�ine. Typically, the operational de�inition of normal distance vision is described as 20/20. This ratio refers to the ability to discriminate objects (usually letters or symbols) at 20 feet compared to the average person at 20 feet. A ratio of 20/40 means that you can see clearly at 20 feet what others see at 40 feet; if you have 20/15 vision, it means you perceive objects better than most people. Before children can accurately identify objects, large scale testing is complicated. When we know clearly that vision is compromised, it is dif�icult to know how many children suffer because there are so many ways to de�ine visual impairment. There are legal de�initions for blindness (vision of less than 20/200 after using corrective lenses) and partial sightedness (visual acuity between 20/70 and 20/200 after correction), but those de�initions refer only to distance vision. Other children have dif�iculty with near vision that severely affects reading, writing, and learning. This contrast of the legal and practical applications of the term visual impairment is therefore problematic.

Vision in Adulthood

Vision typically remains somewhat consistent from middle childhood until about 40, when age-related changes become noticeable (Weale, 2003). A variety of gradual changes in vision take place as we age. The lenses of the eyes—the tissue responsible for focusing images—change shape and become less elastic. Muscle �lexibility needed for focusing diminishes. Lenses become less transparent, so less light enters the eyes, resulting in more dif�iculty seeing print material in low light conditions. Adults in their early 40s may not notice these age-related changes when in bright light conditions, but eventually everyone needs corrective lenses when reading smaller print like food labels (Strenk, Strenk, & Koretz, 2005). This age-related loss of near vision is called presbyopia. Older adults will �ind it easier to see when lights are brighter, so menus in dimly lit restaurants can be especially challenging when vision is less acute.

In addition to the normal changes of presbyopia, more than half of adults in the United States over the age of 60 will develop a cataract, or a gradual clouding of the lens of the eye (Gohdes, Balamurugan, Larsen, & Maylahn, 2005). People with cataracts may have more dif�iculty viewing screen media, reading, or driving. Lights may appear to have a halo around them or produce excessive glare. This makes driving at night, for example, more challenging. Worldwide, cataracts are the leading cause of blindness because they are not often treated in the developing world (Bourne et al., 2013). In countries with available health care, surgery to remove the cloudy part of the lens has become somewhat routine, and most patients report renewed eyesight and independence within a week. Nevertheless, cataracts are still the second leading cause of blindness in the United States. Although cataracts appear to be caused by the effects of primary aging, secondary aging factors such as smoking, diabetes, and exposure to the ultraviolet rays of the sun can fuel their progression (Mukesh et al., 2006).

The leading cause of blindness in the United States is age-related macular degeneration (AMD). The macula is responsible for our sharpest central vision. With AMD, the macula becomes dried out and thin, leading to deterioration in the middle of the visual �ield and a dramatic loss of sharp vision right where we most need it. Everyday events like reading and immediately recognizing faces or objects become dif�icult. Scientists do not have a clear idea of what causes AMD or how to prevent it. During the last decade we have seen great progress in the treatment of some forms of AMD (with injections into the vitreous of the eye), but there is no cure. It is most common in those over 60, Caucasians, those with a family history, and in many studies found to be more common among women than men (Yonekawa, Miller, & Kim, 2015). Secondary aging factors like smoking promote onset as well. Several genes have been identi�ied that affect risk, but as yet none have been found to have a one-to-one relationship (Naj et al., 2013).

In 1992, a placebo-controlled study called the Age-Related Eye Disease Study (AREDS) was launched to investigate whether certain dietary supplements, including antioxidants and zinc, can affect eye diseases like AMD (AREDS, 2001). In the original study and a later follow-up (AREDS2, 2013), it was determined that a speci�ic supplement formulation successfully reduced the progression from early stage AMD to advanced AMD. Because the AREDS formula speci�ically targets age-related disease, it was hypothesized that its effect might be more global. Several follow-up studies have investigated various other age-related diseases such as cataracts and smoking-related lung cancer, but have delivered mixed results (e.g., Chew et al., 2015; Hammond, Fletcher, Roos, Witttwer, & Schalch, 2014; The AREDS2 Research Group, 2013). Nevertheless, the AREDS formula is becoming a standard protocol to delay progression of AMD (Chew, 2015).

Glaucoma also affects a signi�icant proportion of the population. It occurs when increased pressure in the eyeball leads to permanent damage to the nerve that sends visual signals to the brain. Put simply,

B. BOISSONNET/BSIP/Superstock

People suffering from glaucoma experience tunnel vision, or clear central vision with poor peripheral vision, due to nerve impulses that no longer properly transmit information.

the pressure in the eye squishes nerve impulses so that they no longer properly transmit information. The visual �ield in glaucoma is the reverse of macular degeneration: central vision is clear and peripheral vision is blurry. Although scientists do not completely understand its causes, damage to the eye as a result of sports injuries or other traumas increases its prevalence. If recognized early enough, doctors can usually easily treat glaucoma. Treatment does not restore lost vision, but it can prevent further loss. Screening takes only a moment and consists of a puff of air directed at the eye to measure pressure, in the same way that you would push on a ball to check in�lation pressure. Experts recommend yearly testing for glaucoma beginning at age 40.

Beginning in early adulthood, it is also normal to develop �loaters, particles from the inner lining of the eye that �loat around in the liquid center of the eyeball. You may notice them while looking at a white wall or at the blue sky. Floaters are sometimes annoying, but typically do not impair vision.

Section Review

Describe how the visual system develops throughout the lifespan, and identify some of the impairments that affect vision.

Summary & Resources

Chapter Summary Both maturational and environmental factors are instrumental in physical growth and decline. For most people, obtaining adequate nutrition and avoiding environmental toxins will set maturational processes in motion. These include the adolescent growth spurt, obtaining maximum height, and experiencing menopause, among other developments. Variations in development between the sexes become more pronounced during adolescence, but in general differences in physical abilities among boys and girls are much larger within groups than between groups.

Depending on the system, at some point in early adulthood most physical processes begin to diminish; sensory declines appear later. However, most declines do not affect us at �irst and are not noticeable until much later. Scientists have tried to account for the biological mechanisms in aging, but no one theory has done a complete job of accounting for the many changes we see. Nevertheless, outward signs of aging, like those that occur with hair and skin, are universal. Development associated with hearing, vision, and other senses begin universally, but are affected greatly by cultural opportunity, including preventive care and treatment in later life. While topics like stature and sensation and perception that were introduced in this chapter are strongly associated with primary aging, next we will look at factors such as nutrition and activity, which are under more individual control.

Summary of Key Concepts Nervous System Development

The neuron is the basic building block of the nervous system. Neurons communicate with each other by releasing neurotransmitters into the synaptic gap that exists between sending and the receiving neurons. Like other parts of the body, every part of the brain does not grow at a consistent rate. Synaptic growth is stimulated through both maturation and experience. Brain development follows a prescribed maturational pattern. For the brain to respond optimally to stimulation, synapses multiply at a tremendous rate and then go through a pruning process. Though plasticity is an essential process in initial development of the nervous system, it remains an important feature throughout the lifespan. Substantial evidence shows that male and female brains are different, but that �inding must be tempered by environmental circumstances that lead to behavioral differences. Although brain development slows considerably after childhood, we continue to form new connections throughout the lifespan.

Patterns of Physical Growth

A number of principles guide and direct growth. Maturation progresses from top to bottom, inside to outside, and is at �irst global and undifferentiated. Standards for height and weight during childhood allow doctors and researchers to compare individual differences to group norms. The universal adolescent growth spurt physically transforms children into adults. Growth throughout the lifespan is signi�icantly impacted by available nutrients; low stature is a typical outcome of poor nutrition during childhood.

Motor Development and Decline

Gross motor skills involve large movements of the head, torso, arms, and legs. Fine motor skills involve more precise dexterity of the hands and �ingers. The Brazelton Neonatal Behavioral Assessment Scale, Gesell Developmental Schedules, and the Bayley Scales seek to provide standards to measure abilities and potential disabilities in movement. By middle childhood, most children can perform the same tasks as an adult, but with less skill. Adolescence marks a time of transition when children are able to perform motor tasks as well as their parents —and often better. There is unequivocal evidence that sex differences exist in motor behaviors and skills. Girls perform better at balancing skills and boys do better at tasks that require strength and speed.

There is some controversy over whether or not Western norms for motor behavior are always appropriate. Maturation as dictated by genetics predicts the order of motor sophistication, though some cultural variability may exist.

Physical Aging in Adulthood

Scientists distinguish between primary (biological) aging and secondary aging, which results from disease, poor health habits, and environmental hazards. Programmed theories of aging propose that there are biological and genetic limits to how long we can live; the body can simply wear out. There are both external (hair, skin) signs of aging and internal signs that can be measured. Physical stresses and the aging skeletal system cause osteoporosis and osteoarthritis, bone diseases that are common to later adulthood.

Sensation and Perception: Touch, Smell, and Taste

A sensation occurs when various sense organs are stimulated, resulting in a response that is interpreted by the brain. The interpretation or awareness of the sensation is called perception. The �ive commonly discussed senses are all well developed before the end of infancy; most of us do not notice any changes that are due to primary aging until we reach our 40s. The process of habituation allows us to assess infant sensation and perception. Neonates have well-developed senses. They can discriminate among odors and tastes and show preferences for certain sounds and voices. Senses are affected by aging. A marked deterioration in the sense of smell is less noticeable, but it also appears to be an indication for later brain diseases.

Sensation and Perception: Hearing

Children with hearing loss are often disadvantaged in multiple areas. Without proper accommodations and interventions, negative outcomes may be worsened. Increased environmental noise has contributed to growing incidence of noise-induced hearing loss. Presbycusis is a natural condition that is a result of aging.

Sensation and Perception: Vision

Though vision is the least developed sense at birth, it quickly matures to adult levels. Robert Fantz famously demonstrated that infants have a clear preference for pictures of human faces over other static stimuli. Gibson’s visual cliff con�irmed that crawling infants are able to perceive distance and see in three dimensions. Presbyopia, or the loss of near vision, is common to all adults beginning in their 40s. In late adulthood, cataracts, macular degeneration, and glaucoma become common, all of which can cause blindness. Though cataracts are the leading cause of blindness in the world, the loss of vision can usually be fully restored with proper treatment.

Critical Thinking and Discussion Questions 1. Much of what we have learned about early neural circuits, including migration and differentiation, has come via

animal studies. On what basis can we make conclusions regarding similar processes in the human nervous system?

2. How would you design a research study that investigates the relationship between early motor activity and later athletic ability?

3. With regards to habituation, explain what happens when you clean out a junk drawer or look at photos you have not seen in many years.

4. Figure 5.12 depicts differences in normative gross motor behavior of infants in several countries. Norway is especially noticeable. What are some possible reasons for the cultural differences?

5. Your older relative does not want to go to a large social gathering because she has a dif�icult time with auditory discrimination. She tells you that she no longer wants to pretend that she is listening, as she has done in the

past. What advice would you give her? 6. What does it mean to “perceive”? What does it mean to “sense”? If a person had a visual, auditory, or other

impairment, how would you know if the problem was one of sensation or perception? 7. Compare and contrast major causes of vision loss in industrialized countries versus those that are developing.

How are they different? What kind of efforts would you place on prevention versus treatment? How do you think future prevalence rates will change?

Additional Resources Web Resources

Centers for Disease Control and Prevention: preventing hearing loss http://www.cdc.gov/niosh/topics/noise/ (http://www.cdc.gov/niosh/topics/noise/) Centers for Disease Control and Prevention: hip fractures among older adults http://www.cdc.gov/homeandrecreationalsafety/falls/adulthipfx.html (http://www.cdc.gov/homeandrecreationalsafety/falls/adulthipfx.html) National Institutes of Health, National Eye Institute: facts about cataracts https://nei.nih.gov/health/cataract/cataract_facts (https://nei.nih.gov/health/cataract/cataract_facts) National Institutes of Health, National Eye Institute: facts about glaucoma https://nei.nih.gov/health/glaucoma/glaucoma_facts (https://nei.nih.gov/health/glaucoma/glaucoma_facts) National Institutes of Health, National Eye Institute: facts about age-related macular degeneration https://nei.nih.gov/health/maculardegen/armd_facts (https://nei.nih.gov/health/maculardegen/armd_facts)

Further Research

Gogtay, N., Giedd, J. N., Lusk, L., Hayashi, K. M., Greenstein, D., Vaituzis, A. C., . . . Thompson, P. M. (2004). Dynamic mapping of human cortical development during childhood through early adulthood. Proceedings of the National Academy of Sciences of the United States of America, 101(21), 8174–8179. Retrieved from http://doi.org/10.1073/pnas.0402680101 (http://doi.org/10.1073/pnas.0402680101) Juvenile Justice Center. (2004). Cruel and unusual punishment: The juvenile death penalty. Retrieved from http://www.americanbar.org/content/dam/aba/publishing/criminal_justice_section_newsletter/crimju st_juvjus_Adolescence.authcheckdam.pdf (http://www.americanbar.org/content/dam/aba/publishing/criminal_justice_section_newsletter/crimjust_juvjus_Adolescen ce.authcheckdam.pdf) Juvenile Law Center. (2015). Roper v. Simmons ten years later: Recollections and re�lections on the abolition of the juvenile death penalty. Retrieved from http://www.jlc.org/blog/roper-v-simmons-ten-years-later- recollections-and-re�lections -abolition-juvenile-death-penalty (http://www.jlc.org/blog/roper-v-simmons-ten- years-later-recollections-and-re�lections-abolition-juvenile-death-penalty) WHO Multicentre Growth Reference Study Group. (2006). Assessment of sex difference and heterogeneity in motor milestone attainment among populations in the WHO Multicentre Growth Reference Study. Acta Paediatricia, 450, 66–75. Retrieved from http://www.who.int/childgrowth/standards/Difference_motor_development .pdf?ua=1 (http://www.who.int/childgrowth/standards/Difference_motor_development.pdf?ua=1)

Key Terms

adolescent growth spurt A period of rapid growth in height and weight that occurs during puberty.

age-related hearing loss (AHL) The loss of hearing due strictly to biological aging. Also called presbycusis.

age-related macular degeneration (AMD) Deterioration in the middle of the visual �ield (the macula), resulting in a dramatic loss of sharp vision.

antioxidants

Chemicals that neutralize free radicals and prevent them from causing damage to the cells.

axon A projection of a nerve cell; it sends signals indicating stimulation.

blindness Commonly de�ined as vision of less than 20/200 after using corrective lenses.

cataract Clouding of the lens of the eye.

central nervous system (CNS) Consists of the brain and spinal cord.

cephalocaudal A description of developmental growth that follows a head-to-toe pattern.

cochlear implants An electronic device that is inserted into the ear. It attempts to mimic sensory experiences and send nerve impulses to the brain rather than amplify sounds.

damage theories Theories of aging that suggest that the body experiences damages that add up until there is a failure of a critical organ. Also known as error theories.

degenerative joint disease See osteoarthritis.

dendrite A projection of a nerve cell; it receives signals from sending neurons.

depth perception The ability to perceive distance and see in three dimensions.

directionality The general principles of growth, including cephalocaudal and proximodistal patterns.

dishabituation The reappearance of a response to a stimulus that previously showed habituation.

�ine motor skills Movements using precision, usually with the hands and �ingers. Contrast with gross motor skills.

free radical The unstable electron that is in an atom that has only one electron instead of a pair; an accumulation of them can cause cellular damage.

glaucoma Increased pressure in the eyeball, which leads to permanent damage to the nerve that sends visual signals to the brain.

gross motor skills Large body movements using the head, torso, arms, and legs. Contrast with �ine motor skills.

habituation A decrease in response to a stimulus after its repeated presentation.

independence of systems A principle of growth that suggests different body systems mature independently.

infant-directed speech A high-pitched, sing-song intonation that is often directed at infants.

motor development Changes in the ability to control large and small body movements. See also �ine motor skills and gross motor skills.

myelin sheath A fatty, insulating substance that coats axons in order to speed transmission of neural signals.

neural plate A strip of neuronal stem cells.

neuron A nerve cell that transmits impulses in the nervous system.

neuroplasticity The ability of the brain to physically adapt as the result of experience.

neurotransmitters Specialized chemicals used to relay signals from one neuron to another.

nodes of Ranvier The gaps in myelinated axons that allow electrical transmission to jump across myelin and thus speed transmission of nerve impulses.

noise-induced hearing loss Permanent loss of hearing due to environmental exposure to various sounds.

orthogenetic principle States that development begins globally and undifferentiated, and gradually increases its differentiation.

osteoarthritis A disease that occurs when the protective soft tissue that protects the ends of bones deteriorates, resulting in pain when bone grinds against bone. Also called degenerative joint disease.

osteoporosis A loss of bone mass resulting in thinner, weaker bones.

palmar grasp A whole-hand grasp using the palm and all �ive �ingers.

partial sightedness Commonly de�ined as visual acuity between 20/70 and 20/200 after correction.

perception The interpretation of sensory stimuli.

peripheral nervous system Parts of the nervous system that extend beyond central nervous system to other parts of the body.

pincer grip A grip using the thumb and fore�inger for �ine motor manipulation of objects like cereal and pencils.

presbycusis The loss of hearing due strictly to biological aging. Also called age-related hearing loss.

presbyopia Age-related loss of near vision.

primary aging Gradual, inevitable physical changes that occur because of genetics and biology.

programmed theories of aging These theories suggest that there are biological and genetic limits to how long we can live. That is, our bodies are “programmed” to last for a certain amount of time, based on a biologic timetable, then become susceptible to conditions that promote decline.

proximodistal A description of development that identi�ies growth as a pattern that begins from the inside of the body and proceeds outward toward the extremities.

sarcopenia Natural muscle loss.

secondary aging Negative physical change as a result of disease, poor health habits, and environmental hazards.

senescence Unavoidable physical decline brought about by biological aging that is programmed into our species.

sensation The activation of speci�ic nerves as a result of certain stimuli.

soma The body of a neuron.

somatosensory A system of senses dedicated to skin pressure, pain, temperature, and other sensations that result from touch.

synapse The space between two neurons into which neurotransmitters are released.

synaptic pruning The process of reducing the number of underused or weak synapses, which thereby strengthens others.

synaptogenesis Literally, synaptic growth, but ordinarily refers to an explosion of synaptic connections that occurs during the �irst few years.

terminal buttons Bulblike structures at the end of axons. Vesicles in terminal buttons contain neurotransmitters.