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, 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).
Although the left and right hemispheres are implicated for different functions, some researchers note that a strict right/spatial and left/language dichotomy is overly simplistic (Vilasboas, Herbet, & Duffau, 2017). Despite lateralization, the two hemispheres interact in a great many complex ways to enable us to think, move, create, and exercise our senses (Efron, 1990; Richmond, Johnson, Seal, Allen, & Whittle, 2016; Springer & Deutsch, 1998). Complex activities such as thinking and problem solving involve communication between both hemispheres of the brain (Turner, Marinsek, Ryhal, & Miller, 2015). The corpus callosum, a collection of 250 to 800 million neural fibers, connects the left and right hemispheres of the brain, permitting them to communicate and coordinate processing (Banich & Heller, 1998). During early childhood, the corpus callosum grows and begins to myelinate, permitting the two halves of the brain to communicate in more sophisticated and efficient ways and to act as one, enabling the child to execute large and fine motor activities such as catching and throwing a ball or tying shoelaces (Banich, 1998; Brown & Jernigan, 2012).
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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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 foreign countries, show deficits in brain volume as well as cognitive and perceptual deficiencies that may persist into adolescence (Hodel et al., 2015; C. A. 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 had a particularly powerful role (Hensch, 2018).
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); 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; Neville & Bavelier, 2001). 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 & Lewis, 2013). All infants around the world need these basic experiences 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, & Nelson, 2017).
A second type of development, experience-dependent brain development, refers to the growth that occurs in response to learning experiences (Bick & Nelson, 2017). For example, experiences such as learning to stack blocks or crawl on a slippery wood floor are unique to individual infants, and they influence what particular brain areas and functions are developed and reinforced. Experience-dependent development is the result of lifelong experiences that vary by individual based on contextual and cultural circumstances (Kolb et al., 2014; Stiles & Jernigan, 2010). Exposure to enriching experiences, such as interactive play with toy cars and other objects that move; hands-on play with blocks, balls, and cups; and stimulating face-to-face play can all enhance children’s development (Kolb, 2018). For example, a longitudinal study that followed more than 350 infants from 5 to 24 months of age found that the quality of mother–infant interactions at 5 months predicted greater brain activity in the prefrontal cortex at 10 and 24 months of age, suggesting that parenting quality may contribute to brain development in infancy (Bernier, Calkins, & Bell, 2016). One the other hand, exposure to deprivation and trauma can have lasting negative effects on brain development (Harker, 2018).
Sleep and Brain Development
Whereas adults sleep approximately 8 hours each day, the typical neonate sleeps about 16 to 18 hours each day. Sleep declines steadily. Six-month-old infants sleep about 12 hours (Figueiredo, Dias, Pinto, & Field, 2016). Infant rats, rabbits, cats, and rhesus monkeys also sleep much longer than adults, suggesting that sleep serves a developmental function (Blumberg, Gall, & Todd, 2014). In adults, sleep is thought to permit the body to repair itself, as indicated by increased cell production and the removal of metabolic wastes during sleep (Tononi & Cirelli, 2014). Sleep is also associated with increases in connections among neurons (Krueger, Frank, Wisor, & Roy, 2016).
One hypothesis for infants’ increased time in sleep is that it provides stimulation and promotes brain development.
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Sleep is associated with adult memory, improving the consolidation of memory, and sleep deficits are associated deficits in attention, memory, and learning (Chambers, 2017; Doyon, Gabitov, Vahdat, Lungu, & Boutin, 2018; R. M. C. Spencer, Walker, & Stickgold, 2017). REM sleep, rapid eye movement sleep, is thought to be particularly important for cognition. Adults’ eyes flutter and move while in REM sleep, and dreaming occurs during REM sleep (Lewis, 2017). Infants spend about half of their sleep time in REM sleep, decreasing to about 20% in adulthood. Given that dreaming happens in REM, one hypothesis for infants’ increased time in sleep is that it provides stimulation and promotes brain development (Friedrich, Wilhelm, Mölle, Born, & Friederici, 2017). Neonates with poor sleep patterns showed poor attention at 4 months and increased distractibility at 18 months of age (Geva, Yaron, & Kuint, 2016). In another