Journal Issue: Caring for Infants and Toddlers Volume 11 Number 1 Spring/Summer 2001
The Growth of the Brain
In view of recent public excitement over early brain growth, it might have been appropriate to begin this summary of the early years with a discussion of brain development. Instead, this summary began with the growth of the mind and the person because developmental scientists know considerably more about cognitive, socioemotional, and personality growth than they know about brain development. Indeed, developmental neuroscience is a recent addition to the study of the child. Furthermore, processes of brain development are best understood when considered in relation to the pace and timing of concurrent mental, emotional, and social advances of early childhood, because these behavioral achievements provide clues about what is likely to be happening within the brain.
Unfortunately, considerable misunderstanding of early brain development occurs when neurons and synapses are considered independently of the development of thinking, feeling, and relating to others.22 Time-limited windows of opportunity—during which critical stimuli from the environment are necessary for healthy brain development—are exceptional rather than typical, consistent with the gradual course of most features of early development. Brain development is lifelong, not limited to the early years, consistent with the enduring capacities for growth in thinking, feeling, and adapting throughout life. And although the talking, singing, and playing of caregivers are valuable stimulants of early brain development, so also are the caregiver's efforts to provide adequate nutrition; to protect young children from the hazards of drugs, environmental toxins (like lead), and uncontrollable stress; and to obtain early vision and hearing screening. Each of these elements is an important requirement of healthy brain growth.Blooming and Pruning of Brain Connections
Developmental scientists' observations of early development provide other important clues for what to expect in the developing brain.23 For example, the powerful innate capabilities that underlie the newborn's readiness to learn suggest that brain growth begins early and advances quickly during the prenatal months. And indeed it does. Brain development begins within the first month after conception, when the brain and spinal cord begin to take shape within the embryo. By the sixth prenatal month, nearly all of the billions of neurons (nerve cells) that populate the mature brain have been created, with new neurons generated at an average rate of more than 250,000 per minute. Once neurons are formed, they quickly migrate to the brain region where they will function. Neurons become differentiated to assume specialized roles, and they form connections (synapses) with other neurons that enable them to communicate and store information. Neurons continue to form synapses with other neurons throughout childhood. By the moment of birth, the large majority of neurons are appropriately located within an immature brain that has begun to appear and function like its mature counterpart.
Furthermore, given the newborn's hunger for novelty, attention to sensory experience, and preference for social stimulation, significant changes in the brain's neuronal architecture would be expected after birth. This is precisely what occurs, although the manner in which the brain becomes organized (or wired) in the early years is intriguing. Both before and after birth, an initial "blooming" of brain connections occurs: Neurons create far more synapses with other neurons than will ever be retained in the mature brain. This proliferation of synapses creates great potential for the developing brain, but it also makes the young brain inefficient and noisy with redundant and unnecessary neural connections. Consequently, this proliferation is soon followed by a stage of "pruning" when little-used synapses are gradually eliminated to reach the number required for the brain to operate efficiently.
How are synapses selected for retention or elimination? Early experience plays an important role. Stimulating experiences activate certain neural synapses, and this triggers growth processes that consolidate those connections. Synapses that are not activated progressively wither over time. Through this "use it or lose it" principle, therefore, the architecture of the developing brain becomes adapted to the needs of everyday stimulation and experience. The effects of this principle can be observed behaviorally in the early years. Vision, for instance, is an example of this principle. During the early months of life, visual acuity increases because the neural pathways connecting eye to brain become consolidated while infants gaze at the world around them. But if infants experience prolonged visual deprivation (which can result, for example, from congenital cataracts), those pathways will remain unorganized. If the cataracts are removed in childhood, there may still be irreversible deficits in vision because the neural connections were never consolidated. In this respect, therefore, early vision develops according to a sensitive period that begins abruptly (at birth) but very gradually tapers off.
Other features of early behavioral development may also reflect the brain's early blooming and pruning of connections. Consider language learning. Newborns can discriminate universal speech sounds, but over time their speech perception becomes limited to the sounds of their native language. This change in perception may reflect the initial proliferation of connections in brain regions governing language and their later refinement.24 Neuroscientists offer similar accounts to explain the early development of memory ability,25 the growth of early categorization and thinking skills,26 and early emotional development and emotion management.27 However, the blooming and pruning of brain connections for these capacities takes place on an extended timetable compared to the narrower window of opportunity that exists for vision.
The timetable for brain development thus varies by region, and it continues throughout life. Sensory regions, which govern sight, touch, hearing, and other sensations, undergo their most rapid growth early in life, while the brain areas guiding higher forms of thinking and reasoning experience blooming and pruning of brain connections into early adolescence. Indeed, the recent discovery that the mature adult brain generates new neurons28 raises the possibility that brain development continues into maturity in yet unknown ways.Brain Growth and Experience
At least two forms of brain development occur throughout life.29 The first, called "experience-expectant," describes how common early experiences provide essential catalysts for normal brain development. Without these essential experiences, brain growth goes awry. The dependence of vision on early visual stimulation is one example. Scientists believe that typical experiences of hearing, exposure to language, coordinating vision and movement, and other common early experiences likewise contribute to the young brain's developing organization. The developing brain "expects" and requires these typical human experiences, and relies on them as a component of its growth.
The second form of brain development occurs throughout life. It is called "experience-dependent" and describes how individual experience fosters new brain growth and refines existing brain structures. These experiences can be unique to an individual. For instance, the brain of a musician who plays a stringed instrument differs from the brain of a poet who works with words and abstract ideas because they have exercised different brain regions throughout life.30 In this respect, the experiences that refine brain functioning throughout life are individualized rather than typical. These experiences influence neural connections uniquely in different individuals, as they account for new learning and skills.Vulnerability of the Developing Brain
The foundation for these achievements is established in the early years, however, and the rapid pace and broad scope of early brain growth means that the immature brain is a vulnerable organ. Beginning at conception and continuing after birth, healthy brain development is imperiled by exposure to hazardous drugs, such as alcohol, cocaine, and heroin; viruses, like HIV and rubella; and environmental toxins, like lead and mercury. The brain is also vulnerable prenatally and postnatally to poor diets that lack essential nutrients, such as iron and folic acid. Chronic maternal stress during pregnancy and after birth can also threaten healthy brain development because of stress hormones that have a toxic effect on developing brain structures.31 Stressful experiences of chronic abuse or neglect, as well as head injuries resulting from accidents, also pose significant risks. The greatest dangers to the developing brain arise, of course, from the combined and cumulative effects of these hazards, such as when children in poverty are malnourished, exposed to hazardous drugs or environmental toxins, or experience head injuries. Enduring harm also arises when early problems are undetected and are allowed to endure uncorrected.
Parents and other caregivers contribute to healthy brain development by talking, singing, playing, and reading to a child. These activities are valuable, especially if they are developmentally appropriate and are attuned to a young child's interests. But more significant contributions occur when parents obtain prenatal and postnatal health care; protect children from environmental hazards, dangerous drugs, and viruses; secure appropriate immunizations, and early vision and auditory screenings; and prevent accidents. The continuing efforts of parents to keep stresses manageable and environments safe for secure exploration offer significant protections to the development of healthy brains and minds.The Importance of the Environment
When scientists seriously consider the remarkable achievements of the first years of life, it is unmistakable that early experiences matter. The early childhood years are crucial to the quality of the life course. But parents are concerned about their young children not just as an investment in the future, but also because children are themselves valuable. Parents seek to create every opportunity for healthy, optimal growth because of the excitement of contributing to enhancing the unique qualities that each child possesses. Likewise, practitioners and policymakers should also strive to strengthen the opportunities, and reduce the vulnerabilities, of early development because children merit society's commitment to them.
This is why the environment of a child matters. Because early experiences can enhance or diminish inborn potential, the environment of early experience shapes the opportunities and risks that young children encounter. The environment that influences early growth is multifaceted. The physical environment, for example, provides opportunities for toddlers to safely explore and learn, poses hazards for accidental injury, and enlivens young children's emotions by the barriers it sets to achieving goals. The biological environment (which begins to influence development prenatally) affects the developing brain and body through the quality of early nutrition, health care, immunizations, sensory screening, and protection from dangerous drugs, viruses, and environmental toxins.
The irreducible core of the environment during early development is people. Relationships matter. They provide the nurturance that strengthens children's security and well-being, offer the cognitive challenges to exercise young minds, impart many essential catalysts to healthy brain growth, and help young children discover who they are and what they can do. Remarkably, most of the significant ways that caregivers promote healthy development occur quite naturally during the course of sensitive adult-child interaction. For instance, the "parentese" that facilitates early language, the caregiving routines that promote predictability and memory skills, the patient structuring of an activity to make it manageable for a child, and the protective nurturance that manages a baby's emotions show that when sensitive adults do what comes naturally, their behavior is optimally suited to promoting early cognitive, socioemotional, and neurobiological growth. In a sense, just as children's developing brains intrinsically expect that eyes will see light and ears will hear sound because of their developmental self-organization, so also do children's developing minds and hearts expect that adults will talk in special ways to them and that caregivers will nurture them as they mature. Normal human development draws upon these natural and unrehearsed features of everyday early experience far more than it requires special educational toys, Mozart CDs, or flashcards.
Unfortunately, "doing what comes naturally" does not always support healthy early development when caregivers are depressed, stressed, absent, or otherwise have neither time nor energy to devote to caring for young children. In these circumstances, attachment relationships become insecure, conflict negotiation results in coercion, self-concept is shaped by denigrating evaluations of the child, and young children do not develop the sense of secure self-confidence that is their birthright. Society's commitment to ensuring the healthy development of every child requires far more, therefore, than standing on the sidelines and wishing parents the best in their efforts to benefit their offspring. It requires enabling parents to integrate work and child responsibilities constructively through family-friendly job conditions, welfare reform that does not endanger stable parent-child relationships, affordable and desirable child care arrangements, and wage policies that ensure adequate family incomes. It requires helping parents to obtain the prenatal and postnatal health care that screens children for developmental difficulties before they become severe, guarantees adequate nutrition, and can protect young children from debilitating diseases and hazardous exposures. Society's commitment to ensuring the healthy development of every child begins with the parent-child relationship, and requires that the broader institutions affecting the family stand alongside parents in their efforts to ensure the well-being of young children.
The relationships that matter do not end with the immediate family. They also include the relationships that young children develop and depend upon in child care. Society's commitment to ensuring the healthy development of every child requires far more, therefore, than hoping that market forces make available high-quality, affordable care for young children. It requires equipping care providers with the knowledge and resources required to provide young children the kind of focused, sensitive care that offers essential catalysts to healthy psychological growth. It requires esteeming the relationships between children and caregivers sufficiently that there are incentives (in wages and benefits, the structure of child care work, and public support) for these relationships to provide stable, reliable support for young children. Society's commitment to ensuring the healthy development of each child requires that all the relationships that young children rely upon are valued and supported.
Recognizing that the early years are a period of unique opportunity and vulnerability means that the environments of early childhood should be designed so they facilitate, rather than blunt, the remarkable intrinsic push toward growth that is characteristic of every child. Doing so not only enhances the well-being of young children, but makes a long-term investment in the well-being of all individuals. A society that is concerned with problems of violence and self-control, school readiness, and social civility wisely takes note of the fact that the origins of these social, emotional, and intellectual qualities take shape early in the life course. In committing itself to the well-being of the youngest citizens, society can promote the well-being of all.