Infant, Child & Adolescent Brain
The amazing capabilities of the human brain arise from astoundingly intricate communication among billions of interacting cells. Understanding the processes by which brain cells form, become specialized, travel to their appropriate locations, and connect with each other in increasingly elaborate adaptive networks is the central challenge of developmental neurobiology.
Advances in the study of brain development have become increasingly relevant for medical treatments. For example, several diseases that scientists once thought were purely adult disorders are now being considered from a developmental perspective. Schizophrenia might actually occur because pathways in the brain and connections to it formed incorrectly in early life. Other research suggests that genes that influence brain development could also play a role in a person’s susceptibility to autism spectrum disorders. And regeneration following brain injury is now considered a realistic possibility, thanks to expanding knowledge of how neurons form connections during early development.
Knowing how the brain was first constructed is an essential step toward understanding its later ability to reorganize in response to external influences or injuries. As the brain develops from the embryo to the adult, unique attributes evolve during infancy and childhood that will influence people’s differences in learning ability as well as their vulnerability to specific brain disorders. Neuroscientists are starting to discover general principles that underlie these intricate developmental processes.
THE FIRST YEARS OF LIFE
What does a human baby’s brain look like after its three trimesters of development in the womb? After birth, the baby’s brain continues to grow and develop. The average brain-weight of a newborn human baby is about 370 grams (or 13 ounces), just slightly less than a pound. Compare that to the average weight of an adult brain: 3 pounds, with about 86 billion neurons. The newborn baby’s brain is the product of 40 weeks of brain development, and its rapid development continues after birth.
Gennatas, et al. The Journal of Neuroscience, 2017.
Researchers use MRI scans to study how your age and sex affect the size and shape of your brain. They found distinct differences in the density, volume, mass, and thickness of the gray matter in the brains of young people and adolescents.
NIH.
The brain goes through many changes during adolescence, including the maturation of the cerebral cortex — the outer layer of the brain that is important for reasoning and abstract thinking. This image shows how this area develops during this time, with the blue color indicating areas that are more mature.
How fast does an infant’s brain grow? Immediately after birth, the growth rate of the whole brain is about 1 percent per day. The rate slows as the baby ages, reaching about 0.4 percent per day by 3 months after birth. By the time a baby is 90 days old, its overall brain volume is 64 percent larger than it was at birth, with the fastest-growing brain region, the cerebellum, more than double its volume at birth. Not only is the cerebellum the brain region with the most neurons, but it helps with learning motor skills and movements — highly important for babies learning to grab things and eat food. The overall increase in brain volume is the result of a large number of brain cells growing, multiplying (proliferating), maturing (differentiating), and migrating to different brain regions. During the first three months of life, the number of neurons in the cortex increases by 23–30 percent. The dendrites and axons of these neurons grow longer and make many connections, or synapses (synaptogenesis), which also makes the brain bigger. Adding even more to the brain volume, cells known as glia grow, multiply, and provide myelination (by oligodendrocytes) — in fact, the brain’s white matter looks white due to all the myelin-wrapped nerve fibers in those areas. By the time a child is 5 years old, the brain has reached about 90 percent of its adult size, which still leaves plenty of room to grow during childhood, adolescence, and early adulthood.
The number of connections between neurons (synaptic density) increases rapidly during the first couple years of life, so that a 2-year-old’s brain has 50 percent more synapses than an adult brain, although it is only about 80 percent the size of an adult brain. That’s far too many synapses for the brain to maintain, as synapses use energy and resources. Therefore, during early childhood, the brain begins to reduce the number of synapses and fine-tune the connections — this synaptic pruning process is shaped by toddlers’ experiences as they grow. Just as pruning rose bushes gets rid of the dying or weaker branches so that nutrients go to the newer branches and enable new roses to bloom and flourish, synaptic pruning allows weaker connections to diminish while stronger synapses that are activated more often will grow and stabilize.
EXPERIENCE SHAPES THE BRAIN
Are the brains of human babies similar to the brains of other baby animals such as kittens and ducklings? Compared to other animals, humans are actually born with less developed brains, and human brains take longer to mature. Squirrel monkeys, for example, reach their adult brain size at 6 months old. Rather than developing more fully in the womb or egg, human brains grow and develop extensively after birth. One advantage is that our developing brains are more easily shaped by environment and experience, which helps us adapt appropriately to the surrounding environment.
A baby’s early life experiences — seeing parents’ faces, hearing their voices, and being held in its parents’ arms — provide important sensory inputs that shape connections between neurons. During these critical periods of development, inputs from sensory, motor, and even emotional aspects of life experiences affect how the brain develops and adapts to the given environment. Both genes and environment exert strong influences during critical periods, forming neural circuits that affect learning and behavior. Part of shaping these connections involves neuronal cell death and synaptic pruning, which occur in the embryo and in early postnatal life. Interestingly, changes in neural connections during critical periods coincide with high rates of learning, such as a toddler learning to run or to speak multiple languages.
INTO ADOLESCENCE
What’s going on in the typical teenage brain? It’s no surprise that many changes are happening during adolescence, in the body as well as the brain. But what’s amazing is the brain’s capacity to learn during these teenage years. The teen brain is like a big ball of clay, ready to change and be molded by new experiences — but it is also very messy. During this time, more synaptic pruning occurs, with stronger connections beating out weaker ones in a process called competitive elimination. At the same time, the brain is improving its connections, with neurons extending their dendritic branches and myelination of axons increasing, especially in the frontal lobes.
In exploring how the brain changes during the aging process, scientists are particularly interested in longitudinal studies, which track human subjects over extended periods of time. Longitudinal studies are especially important because they can reveal how early life events and environment can affect outcomes later in life, like education or risk for disease. These studies are also helpful for understanding how a healthy brain changes between early childhood and adolescence. Adolescence can be thought of as a second “critical period” as the more complex functions of the brain develop and can be influenced by environment and experience.
Images of the adolescent brain obtained by magnetic resonance imaging (MRI) show an increase in white matter volume, especially in the corpus callosum — a large bundle of myelinated fibers that connects the brain’s right and left cerebral hemispheres. The growth of the corpus callosum may explain enhanced learning capacity in adolescence, due to the increasing connections. Enhanced connections, changes in the brain’s reward systems, and changes in the balance between frontal and limbic brain regions can all contribute to teenage behaviors such as increased risk taking and sensation seeking — also aspects of an enhanced learning ability.
Unfortunately, this can be a double-edged sword, as the associated risk taking and sensation seeking also increase the risk of addiction. Some regard addiction as a type of acquired learning disorder, pointing to the overlap between brain regions involved in addiction and those supporting learning, memory, and reasoning. Frequent drug use during adolescence is associated with damage to brain regions important for cognitive functions such as memory, attention, and executive functioning. Studies using MRI to measure brain volume and a technique called diffusion tensor imaging (DTI) to study quality of white matter show that alcohol and other drugs of abuse may cause significant changes in gray and white matter in adolescents. Compared to a healthy adolescent brain, adolescents who used alcohol had reduced gray matter volume and reduced white matter integrity. Another study used fMRI to measure brain activity and showed that binge drinking (alcohol) during adolescence was associated with lower brain activity, less sustained attention, and poorer performance on a working memory task.
When do we become adults? The definition of adulthood varies with the context — social, judicial, educational. Neuroscience research indicates that human brains continue to develop until we are about 30 years old. Different brain regions show different rates of growth and maturation. For example, MRI studies show that the gray matter density of most brain regions declines with age; however, gray matter density increases in the left temporal lobe (important for memory and language) until age 30. Brain development in 20-somethings also includes changes in where myelination occurs. Remember that myelination is important for efficiently conducting electrical signals along axons, and myelin protects axons from damage. Earlier in life, more myelination is found in the visual, auditory, and limbic cortices. Closer to 30, the frontal and parietal neocortices become more myelinated, which helps with working memory and higher cognitive functions.
These frontal lobe regions are the last brain regions to develop, gaining more myelin later in life. The frontal lobe is important for executive functioning, which includes attention, response inhibition, emotion, organization, and long-range planning. The late maturation of the frontal lobe might explain characteristics of a “typical teenager,” such as a short attention span, blurting out whatever comes to mind, and forgetting to do homework. However, none of this means that the teenage brain is broken. It is simply experiencing a critical period of development that also opens the brain to millions of new learning opportunities.
PLASTICITY
Plasticity is the ability of the brain to modify itself and adapt to environmental challenges, including sensory inputs. Without plasticity, critical periods would not exist because the brain could not respond to environment and experience. Plasticity is not unique to humans, but our brains’ capacity to adapt is a defining attribute of human beings. Plasticity has been categorized as experience-expectant or experience-dependent.
Experience-expectant plasticity refers to integrating environmental stimuli into normal developmental patterns. Being exposed to certain common or universal environmental experiences — for example, hearing language, seeing faces, or being held — during limited critical, or sensitive, periods of development is essential for healthy brain maturation. An example comes from the bird world; finches that do not hear adult songs before sexual maturation will not learn to sing as well as other members of their species. In this case, the environmental stimuli are the sounds of adult songs, which shape the normal development of the bird’s ability to sing accurately.
Experience-dependent plasticity describes continuing changes in the organization and specialization of a person’s brain regions as a result of life experiences that are not universal or anticipated. These include skills that develop throughout life, with no critical or optimal period for their acquisition. For example, not everyone will play the violin, but violinists often show greater cortical development in the brain region associated with the fingers of their left hand. Using an exciting technology called two-photon imaging, scientists can observe living neurons in animals with a microscope and track their growth after various experiences. The results of these studies indicate that experience-dependent plasticity occurs not only during critical periods but also during adulthood — apparently, our brains are always changing in response to our experiences.
Recent insights into brain development hold considerable promise for new treatments of neurological disorders, traumatic brain injury, and learning disabilities, and could help us understand aging as well. If scientists can design an approach to manipulating adult plasticity — whether with drugs or with therapies that involve rewiring neural circuits — it might be possible to correct problems that result from mistimed critical periods or similar dysfunctions. A better understanding of normal brain function during each developmental stage could be the key to finding age-specific therapies for many brain disorders. 