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Understanding the Effects of Maltreatment on Brain Development
Series: Issue Briefs|
Child Welfare Information Gateway |
|Year Published: 2009|
How the Brain Develops
What we have learned about the process of brain development has helped us understand more about the roles both genetics and the environment play in our development. It appears that genetics predisposes us to develop in certain ways. But our experiences, including our interactions with other people, have a significant impact on how our predispositions are expressed. In fact, research now shows that many capacities thought to be fixed at birth are actually dependent on a sequence of experiences combined with heredity. Both factors are essential for optimum development of the human brain (Shonkoff and Phillips, 2000).
- The Newborn Brain
- The Growing Baby's Brain
- Plasticity—The Influence of Environment
- Sensitive Periods
- Brain Development in Adolescence
The Newborn Brain
The raw material of the brain is the nerve cell, called the neuron. When babies are born, they have almost all of the neurons they will ever have, more than 100 billion of them. Although research indicates some neurons are developed after birth and well into adulthood, the neurons babies have at birth are primarily what they have to work with as they develop into children, adolescents, and adults.
During fetal development, neurons are created and migrate to form the various parts of the brain. As neurons migrate, they also differentiate, so they begin to "specialize" in response to chemical signals (Perry, 2002). This process of development occurs sequentially from the "bottom up," that is, from the more primitive sections of the brain to the more sophisticated sections (Perry, 2000a). The first areas of the brain to fully develop are the brainstem and midbrain; they govern the bodily functions necessary for life, called the autonomic functions. At birth, these lower portions of the nervous system are very well developed, whereas the higher regions (the limbic system and cerebral cortex) are still rather primitive (ZERO TO THREE, 2009).
Newborns' brains allow babies to do many things, including breathe, eat, sleep, see, hear, smell, make noise, feel sensations, and recognize the people close to them. But the majority of brain growth and development takes place after birth, especially in the higher brain regions involved in regulating emotions, language, and abstract thought. (See Exhibit 1.) Each region manages its assigned functions through complex processes, often using chemical messengers (such as neurotransmitters and hormones) to help transmit information to other parts of the brain and body (Perry, 2000a).
The Growing Baby's Brain
Brain development, or learning, is actually the process of creating, strengthening, and discarding connections among the neurons; these connections are called synapses. Synapses organize the brain by forming pathways that connect the parts of the brain governing everything we do—from breathing and sleeping to thinking and feeling. This is the essence of postnatal brain development, because at birth, very few synapses have been formed. The synapses present at birth are primarily those that govern our bodily functions such as heart rate, breathing, eating, and sleeping.
The development of synapses occurs at an astounding rate during children's early years, in response to the young child's experiences. At its peak, the cerebral cortex of a healthy toddler may create 2 million synapses per second (ZERO TO THREE, 2009). By the time children are 3, their brains have approximately 1,000 trillion synapses, many more than they will ever need. Some of these synapses are strengthened and remain intact, but many are gradually discarded. This process of synapse elimination—or pruning—is a normal part of development (Shonkoff & Phillips, 2000). By the time children reach adolescence, about half of their synapses have been discarded, leaving the number they will have for most of the rest of their lives. Brain development continues throughout the lifespan. This allows us to continue to learn, remember, and adapt to new circumstances (Ackerman, 2007).
Another important process that takes place in the developing brain is myelination. Myelin is the white fatty tissue that insulates mature brain cells by forming a sheath, thus ensuring clear transmission across synapses. Young children process information slowly because their brain cells lack the myelin necessary for fast, clear nerve impulse transmission (ZERO TO THREE, 2009). Like other neuronal growth processes, myelination begins in the primary motor and sensory areas (the brain stem and cortex) and gradually progresses to the higher-order regions that control thought, memories, and feelings. Also, like other neuronal growth processes, a child's experiences affect the rate and growth of myelination, which continues into young adulthood (Shonkoff & Phillips, 2000).
By the age of 3, a baby's brain has reached almost 90 percent of its adult size. The growth in each region of the brain largely depends on receiving stimulation, which spurs activity in that region. This stimulation provides the foundation for learning.
Plasticity—The Influence of Environment
Researchers use the term plasticity to describe the brain's ability to change in response to repeated stimulation. The extent of a brain's plasticity is dependent on the stage of development and the particular brain system or region affected (Perry, 2006). For instance, the lower parts of the brain, which control basic functions such as breathing and heart rate, are less flexible than the higher functioning cortex, which controls thoughts and feelings. While cortex plasticity may lessen as a child gets older, some degree of plasticity remains. In fact, this brain plasticity is what allows us to keep learning into adulthood and throughout our lives.
The developing brain's ongoing adaptations are the result of both genetics and experience. Our brains prepare us to expect certain experiences by forming the pathways needed to respond to those experiences. For example, our brains are "wired" to respond to the sound of speech; when babies hear people speaking, the neural systems in their brains responsible for speech and language receive the necessary stimulation to organize and function (Perry, 2006). The more babies are exposed to people speaking, the stronger their related synapses become. If the appropriate exposure does not happen, the pathways developed in anticipation may be discarded. This is sometimes referred to as the concept of "use it or lose it." It is through these processes of creating, strengthening, and discarding synapses that our brains adapt to our unique environment.
The ability to adapt to our environment is a part of normal development. Children growing up in cold climates, on rural farms, or in large sibling groups learn how to function in those environments. But regardless of the general environment, all children need stimulation and nurturance for healthy development. If these are lacking—if a child's caretakers are indifferent or hostile—the child's brain development may be impaired. Because the brain adapts to its environment, it will adapt to a negative environment just as readily as it will adapt to a positive one.
Researchers believe that there are sensitive periods for development of certain capabilities. These refer to windows of time in the developmental process when certain parts of the brain may be most susceptible to particular experiences. Animal studies have shed light on sensitive periods, showing, for example, that animals that are artificially blinded during the sensitive period for developing vision may never develop the capability to see, even if the blinding mechanism is later removed.
It is more difficult to study human sensitive periods. But we know that, if certain synapses and neuronal pathways are not repeatedly activated, they may be discarded, and the capabilities they promised may be diminished. For example, infants have the genetic predisposition to form strong attachments to their primary caregivers. But if a child's caregivers are unresponsive or threatening, and the attachment process is disrupted, the child's ability to form any healthy relationships during his or her life may be impaired (Perry, 2001a).
While sensitive periods exist for development and learning, we also know that the plasticity of the brain often allows children to recover from missing certain experiences. Both children and adults may be able to make up for missed experiences later in life, but it may be more difficult. This is especially true if a young child was deprived of certain stimulation, which resulted in the pruning of synapses (neuronal connections) relevant to that stimulation and the loss of neuronal pathways. As children progress through each developmental stage, they will learn and master each step more easily if their brains have built an efficient network of pathways.
The organizing framework for children's development is based on the creation of memories. When repeated experiences strengthen a neuronal pathway, the pathway becomes encoded, and it eventually becomes a memory. Children learn to put one foot in front of the other to walk. They learn words to express themselves. And they learn that a smile usually brings a smile in return. At some point, they no longer have to think much about these processes—their brains manage these experiences with little effort because the memories that have been created allow for a smooth, efficient flow of information.
The creation of memories is part of our adaptation to our environment. Our brains attempt to understand the world around us and fashion our interactions with that world in a way that promotes our survival and, hopefully, our growth. But if the early environment is abusive or neglectful, our brains will create memories of these experiences that may adversely color our view of the world throughout our life.
Babies are born with the capacity for implicit memory, which means that they can perceive their environment and recall it in certain unconscious ways (Applegate & Shapiro, 2005). For instance, they recognize their mother's voice from an unconscious memory. These early implicit memories may have a significant impact on a child's subsequent attachment relationships.
In contrast, explicit memory, which develops around age 2, refers to conscious memories and is tied to language development. Explicit memory allows children to talk about themselves in the past and future or in different places or circumstances through the process of conscious recollection (Applegate & Shapiro, 2005).
Sometimes, children who have been abused or suffered other trauma may not retain or be able to access explicit memories for their experiences. However, they may retain implicit memories of the physical or emotional sensations, and these implicit memories may produce flashbacks, nightmares, or other uncontrollable reactions (Applegate & Shapiro, 2005). This may be the case with very young children or infants who suffer abuse or neglect.
Brain Development in Adolescence
Studies using magnetic resonance imaging (MRI) techniques, involving brain scans at regular intervals, show that the brain continues to grow and develop into young adulthood (at least to the mid-twenties). Right before puberty, adolescent brains experience a growth spurt that occurs mainly in the frontal lobe, which is the area that governs planning, impulse control, and reasoning. During the teenage years, the brain again goes through a process of pruning synapses—somewhat like the infant and toddler brain (National Institute of Mental Health, 2001). As the teenager grows into young adulthood, the brain develops more myelin to insulate the nerve fibers and speed neural processing, and this myelination occurs last in the frontal lobe. MRI comparisons between the brains of teenagers and the brains of young adults have shown that most of the brain areas were the same—that is, the teenage brain had reached maturity in the areas that govern such abilities as speech and sensory capabilities. The major difference was the immaturity of the teenage brain in the frontal lobe and in the myelination of that area (National Institute of Mental Health, 2001).
Another change that happens during adolescence is the growth and transformation of the limbic system, which is responsible for our emotions. Teenagers may rely on their more primitive limbic system in interpreting emotions and reacting, since they lack the more mature cortex that can override the limbic response (Chamberlain, 2009).
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