Chapter 4- Infancy and Development

Researchers study the timing of brain lateralization to learn more about brain plasticity.

A highly plastic cerebral cortex, in which many areas are not yet committed to specific functions, has a high capacity for learning. And if a part of the cortex is damaged, other parts can take over tasks it would have handled. But once the hemispheres lateralize, damage to a specific region means that the abilities it controls cannot be recovered to the same extent or as easily as earlier.

If few new neurons are produced after the prenatal period, what causes the dramatic increase in brain size during the first two years?

About half the brain's volume is made up of glial cells, which are responsible for myelination, the coating of neural fibers with an insulating fatty sheath (called myelin) that improves the efficiency of message transfer. Glial cells multiply rapidly from the fourth month of pregnancy through the second year of life—a process that continues at a slower pace through middle childhood and accelerates again in adolescence. Brain development can be compared to molding a "living sculpture." First, neurons and synapses are overproduced. Then, cell death and synaptic pruning sculpt away excess building material to form the mature brain—a process jointly influenced The resulting "sculpture" is a set of interconnected regions, each with specific functions—much like countries on a globe that communicate with one another.

How experience greatly influences brain organization

Another illustration of how early experience greatly influences brain organization comes from studies of deaf adults who, as infants and children, learned sign language (a spatial skill). Compared with hearing adults, these individuals depend more on the right hemisphere for language processing

Stimulation

As neurons form connections, stimulation becomes vital to their survival. Neurons that are stimulated by input from the surrounding environment continue to establish synapses, forming increasingly elaborate systems of communication that support more complex abilities. At first, stimulation results in a massive overabundance of synapses, many of which serve identical functions, thereby ensuring that the child will acquire the motor, cognitive, and social skills that our species needs to survive.

Brain Development

At birth, the brain is nearer to its adult size than any other physical structure, and it continues to develop at an astounding pace throughout infancy and toddlerhood. We can best understand brain growth by looking at it from two vantage points: (1) the microscopic level of individual brain cells and (2) the larger level of the cerebral cortex, the most complex brain structure and the one responsible for the highly developed intelligence of our species.

At birth,

At birth, the hemispheres have already begun to specialize. Most newborns show greater activation (detected with either ERP or NIRS) in the left hemisphere while listening to speech sounds or displaying a positive state of arousal. In contrast, the right hemisphere reacts more strongly to nonspeech sounds and to stimuli (such as a sour‐tasting fluid) that evoke negative emotion (Fox & Davidson, 1986; Hespos et al., 2010). Nevertheless, research on brain‐damaged children and adults offers dramatic evidence for substantial plasticity in the young brain. Among preschoolers with brain injuries sustained in the first year of life, deficits in language and spatial abilities were milder than those observed in brain-injured adults (Akshoomoff et al., 2002; Stiles et al., 2005, 2008). As the children gained perceptual, motor, and cognitive experiences, other stimulated cortical structures compensated for the damaged areas, regardless of the site of injury. Still, mild deficits in complex skills, such as reading, math, and telling stories, were evident in the school years. When healthy brain regions take over the functions of damaged areas, multiple tasks must be done by a smaller-than-usual volume of brain tissue.

Sensitive period in brain development

Both animal and human studies reveal that early, extreme sensory deprivation results in permanent brain damage and loss of functions—findings that verify the existence of sensitive periods in brain development. For example, early, varied visual experiences must occur for the brain's visual centers to develop normally. If a 1‐month‐old kitten is deprived of light for just three or four days, these areas of the brain degenerate. If the kitten is kept in the dark during the fourth week of life and beyond, the damage is severe and permanent (Crair, Gillespie, & Stryker, 1998). And the general quality of the early environment affects overall brain growth. When animals reared from birth in physically and socially stimulating surroundings are compared with those reared under depleted conditions, the brains of the stimulated animals are larger and heavier and show much denser synaptic connections

Human Evidence: Victims of Deprived Early Environments.

For ethical reasons, we cannot deliberately deprive some infants of normal rearing experiences and observe the impact on their brains and competencies. Instead, we must turn to natural experiments, in which children were victims of deprived early environments that were later rectified. Such studies have revealed some parallels with the animal evidence just described. In one investigation, researchers followed the progress of a large sample of children transferred between birth and 3½ years from extremely deprived Romanian orphanages to adoptive families in Great Britain (Beckett et al., 2006; O'Connor et al., 2000; Rutter et al., 1998, 2004, 2010). On arrival, most were impaired in all domains of development. Cognitive catch‐up was impressive for children adopted before 6 months, who attained average mental test scores in childhood and adolescence, performing as well as a comparison group of early‐adopted British‐ born children. But Romanian children who had been institutionalized for more than the first six months showed serious intellectual deficits. Although they improved in test scores during middle childhood and adolescence, they remained substantially below average. And most displayed at least three serious mental health problems, such as inattention, overactivity, unruly behavior

Auditory and Visual Cortex

For example, a burst of activity occurs in the auditory and visual cortexes and in areas responsible for body movement over the first year—a period of dramatic gains in auditory and visual perception and mastery of motor skills (Johnson, 2011). Language areas are especially active from late infancy through the preschool years, when language development flourishes

Sum of plasticity

In sum, the brain is more plastic during the first few years than it will ever be again. An overabundance of synaptic connections supports brain plasticity, ensuring that young children will acquire certain capacities even if some areas are damaged.

Neurobiological findings

Neurobiological findings indicate that early, prolonged institutionalization leads to a generalized decrease in activity in the cerebral cortex, especially the prefrontal cortex, which governs complex cognition and impulse control. Neural fibers These children in an orphanage in Romania receive little adult contact or stimulation. The longer they remain in this barren environment, the more likely they are to display profound impairments in all domains of development connecting the prefrontal cortex with other brain structures involved in control of emotion are also reduced

synaptic pruning

Neurons that are seldom stimulated soon lose their synapses, in a process called synaptic pruning that returns neurons not needed at moment to an uncommitted state so they can support future development. In all, about 40 percent of synapses are pruned during childhood and adolescence to reach the adult level

Why does this specialization of the two hemispheres, called lateralization, occur?

Studies using fMRI reveal that the left hemisphere is better at processing information in a sequential, analytic (piece‐by‐piece) way, a good approach for dealing with communicative information—both verbal (language) and emotional (a joyful smile). In contrast, the right hemisphere is specialized for processing information in a holistic, integrative manner, ideal for making sense of spatial information and regulating negative emotion. A lateralized brain is certainly adaptive

Basic story of brain growth

The basic story of brain growth concerns how neurons develop and form this elaborate communication system. In the prenatal period, neurons are produced in the embryo's primitive neural tube. From there, they migrate to form the major parts of the brain (see Chapter 3, page 64). Once neurons are in place, they differentiate, establishing their unique functions by extending their fibers to form synaptic connections with neighboring cells. During the first two years, neural fibers and synapses increase at an astounding pace

cerebral cortex has 2 hemispheres

The cerebral cortex has two hemispheres, or sides, that differ in their functions. Some tasks are done mostly by the left hemisphere, others by the right. For example, each hemisphere receives sensory information from the side of the body opposite to it and controls only that side. For most of us, the left hemisphere is largely responsible for verbal abilities (such as spoken and written language) The right hemisphere handles spatial abilities (judging distances, reading maps, and recognizing geometric shapes) and negative emotion (such as distress)

Development of Cerebral Cortex

The cerebral cortex surrounds the rest of the brain, resembling half of a shelled walnut. It accounts for 85 percent of the brain's weight and contains the greatest number of neurons and synapses. Because the cerebral cortex is the last part of the brain to stop growing, it is sensitive to environmental influences for a much longer period than any other part of the brain.

frontal lobes

The cortical regions with the most extended period of development are the frontal lobes.

Development of Neurons

The human brain has 100 to 200 billion neurons, or nerve cells that store and transmit information, many of which have thousands of direct connections with other neurons. Between them are tiny gaps, or synapses, where fibers from different neurons come close together but do not touch (see Figure 4.2). Neurons send messages to one another by releasing chemicals called neurotransmitters, which cross the synapse.

Prefrontal cortex

The prefrontal cortex, lying in front of areas controlling body movement, is responsible for thought—in particular, consciousness, inhibition of impulses, integration of information, and use of memory, reasoning, planning, and problem‐solving strategies. From age 2 months on, the prefrontal cortex functions more effectively. But it undergoes especially rapid myelination and formation and pruning of synapses during the preschool and school years, followed by another period of accelerated growth in adolescence, when it reaches an adult level of synaptic connections .