Infants and Children in Context. Tara L. Kuther

to stimulation from the outside world, the number of synapses initially rises meteorically in the first year of life, and the dendrites increase 500% by age 2 (Schuldiner & Yaron, 2015). Cortical thickness peaks by 2 years of age, but surface area continues to develop throughout childhood (Gilmore, Knickmeyer Santelli, & Gao, 2018). Toddlers have more synapses than at any other point in life (see Figure 4.8). This explosion in connections in the early years of life means that the brain makes more connections than it needs, in preparation to receive any and all conceivable kinds of stimulation (Schuldiner & Yaron, 2015). Those connections that are used become stronger and more efficient, while those unused eventually shrink, atrophy, and disappear. This loss of unused neural connections is a process called synaptic pruning, which can improve the efficiency of neural communication by removing “clutter”—excess unused connections. Little-used synapses are pruned in response to experience, an important part of neurological development that leads to more efficient thought (Lyall et al., 2015).

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      Figure 4.8 Synaptogenesis From Birth to Age 2

      Source: Gilmore et al. (2018).

      Another important process of brain development is myelination, in which glial cells produce and coat the axons of neurons with a fatty substance called myelin. Myelination begins prenatally but accelerates after birth (Gilmore et al., 2018). Myelination contributes to advances in neural communication because axons coated with myelin transmit neural impulses more quickly than unmyelinated axons (Lebel & Deoni, 2018). With increases in myelination, infants and children process information more quickly. Their thought and behaviors become faster more coordinated, and complex (Chevalier et al., 2015). Myelination proceeds most rapidly from birth to age 4, first in the sensory and motor cortex in infancy, and continues through childhood into adolescence and early adulthood (Qiu, Mori, & Miller, 2015).

      The Cerebral Cortex

      The wrinkled and folded outermost layer of the brain is known as the cortex. The cortex comprises about 85% of the brain’s mass and develops throughout childhood and some parts mature into early adulthood.

      The cortex is composed of different structures with differing functions, located across four lobes. The various parts of the brain work together; however, as shown in Figure 4.9, each lobe is specialized to a certain extent. The four lobes progress on different developmental timetables. The sensory and motor areas tend to develop first (for example, the visual cortex regions of the occipital lobe). The frontal lobe, specifically a part called the prefrontal cortex, develops throughout infancy, childhood, and adolescence, maturing into early adulthood (Hodel, 2018; Tamnes et al., 2017). The prefrontal cortex is the part of the brain responsible for higher thought, such as planning, goal setting, controlling impulses, and using cognitive skills and memory to solve problems.

      In addition, the cortex is composed of two hemispheres that are joined by a thick band of neural fibers known as the corpus collosum. Although all four lobes appear on both hemispheres, the hemispheres are not identical. Over childhood, the right and left hemispheres become specialized to carry out different functions, a process known as lateralization (Duboc, Dufourcq, Blader, & Roussigné, 2015). For most people, language is governed by the left hemisphere. Each hemisphere of the brain (and the parts of the brain that comprise each hemisphere) is specialized for particular functions and becomes more specialized with experience.

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      Figure 4.9 The Human Brain

      Lateralization (“of the side” in Latin) begins before birth and is influenced both by genes and by early experiences (Young, 2016). For example, in the womb, most fetuses face toward the left, freeing the right side of the body, which permits more movement on that side and the development of greater control over the right side of the body (Previc, 1991). In newborns, the left hemisphere tends to have greater structural connectivity and efficiency than the right—more connections and pathways, suggesting that they are better able to control the right side of their bodies (Ratnarajah et al., 2013). Newborns tend to have slightly better hearing from their right ear (Ari-Even Roth, Hildesheimer, Roziner, & Henkin, 2016). Infants generally display a hand preference, usually right, and their subsequent activity makes the hand more dominant because experience strengthens the hand and neural connections and improves agility. In this way, one hemisphere becomes stronger and more adept over the course of childhood, a process known as hemispheric dominance. Most adults experience hemispheric dominance, usually with the left hemisphere dominating over the right, making about 90% of adults in Western countries right-handed (Duboc et al., 2015).

      Experience and Brain Development

      Stimulation and experience are key components needed to maximize neural connections and brain development throughout life, but especially in infancy. Much of what we know about brain development comes from studying animals. Animals raised in stimulating environments with many toys and companions to play with develop brains that are heavier and have more synapses than do those who grow up in standard laboratory conditions (Berardi, Sale, & Maffei, 2015). Likewise, when animals raised in stimulating environments are moved to unstimulating standard laboratory conditions, their brains lose neural connections. This is true for humans, too. Infants who are understimulated, such as those who experience child maltreatment, or who are reared in deprivation, such as in poor understaffed orphanages in developing countries, show deficits in brain volume as well as cognitive and perceptual deficiencies that may persist into adolescence (Hodel et al., 2015; Nelson et al., 2016; Sheridan & McLaughlin, 2014). In this way, infancy is said to be a sensitive period for brain development, a period in which experience has a particularly powerful role (Hensch, 2018).

      The brain develops in response to experiences that are unique to each individual, such as playing with specific toys or participating in social interactions.

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      The powerful role that experience plays in brain development can be categorized into two types. First, the brain depends on experiencing certain basic events and stimuli at key points in time to develop normally (Bick & Nelson, 2017; Hensch, 2018); this is referred to as experience-expectant brain development. Experience-expectant brain development is demonstrated in sensory deprivation research with animals. If animals are blindfolded and prevented from using their visual system for the first several weeks after birth, they never acquire normal vision because the connections among the neurons that transmit sensory information from the eyes to the visual cortex fail to develop; instead, they decay (DiPietro, 2000). If only one eye is prevented from seeing, the animal will be able to see well with one eye but will not develop binocular vision, the ability to focus two eyes together on a single object. Similarly, human infants born with a congenital cataract in one eye (an opaque clouding that blocks light from reaching the retina) will lose the capacity to process visual stimuli in the affected eye if they do not receive treatment. Even with treatment, subtle differences in facial processing may remain (Maurer, 2017). Deprivation of sound has similar effects on the auditory cortex (Mowery, Kotak, & Sanes, 2016). Brain organization depends on experiencing certain ordinary events early in life, such as opportunities to hear language, see the world, touch objects, and explore the environment (Kolb, Mychasiuk, & Gibb, 2014; Maurer, 2017). All infants around the world need these basic experiences during specific times in development, known as sensitive periods, to develop normally, and it is difficult to repair errors that are the result of severe deprivation and neglect (Berardi et al., 2015; McLaughlin, Sheridan,