All functions of mind reflect functions of brain.
—Eric Kandel
Studying the human brain is a daunting task. In fact, the human brain is so vastly complex that it would take tens of thousands of pages to do justice to what is known about its structure and function. But how much do we really need to know about the brain to help us in our work as therapists? My belief is that a basic understanding of the nervous system, without getting lost in the details, would be very helpful. With this as our goal, we will move through a thumbnail sketch of the basic structures, functions, and development of the nervous system. Keep in mind that this is a skewed look at the human nervous system biased toward those structures, processes, and theories that will be relevant to the chapters to come.
Neurons
It is impossible, in principle, to explain any pattern by invoking a single quantity.
—Gregory Bateson
The basic unit of the nervous system is the neuron, which receives and transmits signals via chemical transmission and electrical impulses. There are an estimated 100 billion neurons in the brain, with between 10 and 100,000 synaptic connections each, creating limitless networking possibilities (Nolte, 2008; Post & Weiss, 1997). Neurons have fibers called axons covered with myelin, an insulator that enhances the efficiency of communication. Because neurons myelinate as they develop, one way of measuring the maturity of a neural network is to measure its degree of myelinization. Multiple sclerosis—a disease that breaks down myelin—results in a decrease in the efficiency of neural communication, negatively impacting cognition, affect, and movement (Hurley, Taber, Zhang, & Hayman, 1999). The white matter of the brain is white because myelin is white (or at least light in color). Gray matter consists primarily of neural cell bodies.
When a neuron fires, information is carried via an electrical charge that travels down the length of its axon. Neurons communicate with one another across synapses (the spaces between neurons) via chemical messengers called neurotransmitters. The combination of these two complementary processes creates the brain’s electrochemical system. Many neurons develop elaborate branches, called dendrites, which form synaptic connections with thousands of dendrites from other neurons. The relationships formed among these dendrites organize the complex networking of the nervous system.
Glia
Complex, statistically improbable things are by their nature more difficult to explain than simple, statistically probable things.
—Richard Dawkins
Although the focus of neuroscience research is usually on neurons, they make up only half the volume of the cerebral cortex. The other half of our brain is made up of approximately one trillion cells known as glia. One reason we know so much more about neurons is that they are approximately 10 times larger than glial cells. It has long been known that glia play an important supportive role in the construction, organization, and maintenance of neural systems. More recently, it has become apparent that they are also involved in neural network communication and plasticity (Allen & Barres, 2005; Pfrieger & Barres, 1996; Sontheimer, 1995; Vernadakis, 1996). Neural plasticity refers to the ability of neurons to change the way they are shaped and relate to one another as the brain adapts to the environment through time.
Astrocytes, the most abundant kind of glia, have been shown to participate in the regulation of synaptic transmission and to be involved in the coordination and synchronization of synaptic activity (Fellin, Pascual, & Haydon, 2006; Newman, 1982). There now appears to be glial as well as neural transmission. There is also the distinct possibility that astrocytes both shape and modulate synapses (Halassa, Fellin, & Hayden, 2007). Through evolution, the ratio of glial cells to neurons has steadily increased, leading some to believe that our expanding cognitive sophistication is, in part, related to the participation of astrocytes in information processing (Nedergaard, Ransom, & Goldman, 2003; Oberheim, Wang, Goldman, & Nedergaard, 2006). We will revisit this in a later chapter when we discuss Einstein’s glial cells and his exceptional imaginal abilities.
Neurogenesis
What we teach today is part biology and part history…but we don’t always know where one ends and the other begins.
—J. T. Bonner
Neurogenesis, the birth of new neurons via cell division, occurs in the lower regions of the ventricles, the fluid-filled cavities within our brains. Some fish and amphibians, which demonstrate ongoing neurogenesis, possess nervous systems that continue to grow in size throughout life (Fine, 1989). During evolution, it appears that primates may have traded much of their capacity for neurogenesis to continue building existing neural networks in order to retain past learning and develop expert knowledge. In other words, if instead of being replaced, neurons are retained and continually modified through the branching of their dendrites in reaction to new experience, more refined learning may result (Purves & Voyvodic, 1987). Neurons do not appear to have a life span, but die off either as a function of normal apoptosis or because their biochemical environment becomes inhospitable. High levels of cortisol, a lack of blood flow, or the buildup of harmful free radicals can all lead to neuronal death.
The traditional wisdom concerning neurogenesis in vertebrates, and especially primates, has been that new neurons are no longer created after early development (Michel & Moore, 1995; Rakic, 1985). Despite considerable evidence to the contrary, this dogma held sway through most of the 20th century. However, research continues to demonstrate that new neurons are formed in the brains of adult birds (Nottebohm, 1981), tree shrews (Gould et al., 1997), primates (Gould, Reeves, Fallah, et al., 1999), and humans (Gould, Reeves, Graziano, et al., 1999). Further, neurogenesis is regulated by environmental factors and experiences such as stress and social interactions (Fowler, Liu, Ouimet, & Wang, 2002).
Humans have maintained the ability to create neurons in areas involved with new learning, such as the hippocampus, the amygdala, and the cerebral cortex (Eriksson et al., 1998; Gould, 2007; Gross, 2000). The importance of these discoveries and the abandonment of the old dogma cannot be underestimated. Nobel-prize-winning neuroscientist Eric Kandel referred to Nottebohm’s discovery of seasonal neurogenesis in birds as having resulted in one of the great paradigm shifts in modern biology (Specter, 2001).
Neural Systems
I believe in God, only I spell it Nature.
—Frank Lloyd Wright
As the brain develops and matures, neurons organize in more and more complex neural networks tailored to carry out the numerous functions of the nervous system. The two most basic divisions of the nervous system are the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS includes the brain and spinal cord, whereas the PNS is comprised of the autonomic nervous system and the somatic nervous system. The autonomic and somatic nervous systems are involved in the communication between the CNS and the sense organs, glands, and the body (including the heart, intestines, and lungs).
The autonomic nervous system has two branches, called the sympathetic and parasympathetic nervous systems. The sympathetic system controls the activation of the nervous system in response to a threat or other form of motivation. The parasympathetic system balances the sympathetic system by fostering conservation of bodily energy, immunological functions, and repair of damaged systems. A third system referred to as the smart vagus operates in parallel to the parasympathetic branch of the autonomic nervous system and is dedicated to fine-tuning bodily reactions, especially in social situations (Porges, 2007). These three systems will be of particular interest in later chapters, when we discuss attachment and the effects of stress and trauma.
Although MacLean’s formulation of the triune brain is seen as too simplistic by most neuroscientists, many still recognize the tripartite division of the brain into the cerebral cortex, the limbic system, and the brainstem. Each layer is thought of as having different responsibilities. The brainstem—the inner core of the brain—oversees the body’s internal milieu by regulating temperature, heart rate, and basic reflexes such as blood flow and respiration. The structure and functions of the brainstem were shaped during our genetic history and are fully formed and functional at birth. The reflexes we see in the newborn who grasps her mother, suckles her breast, and knows to hold her breath when put under water are genetic memories retained from our tree-dwelling ancestors.
The outer layer of the brain, the cerebral cortex, is first organized by, and then comes to organize, our experiences and how we interact with the world. As we grow, the cortex allows us to form ideas and mental representations of ourselves, other people, and the environment. Distinct from the brainstem, the cortex is experience dependent, which means that it is shaped through countless interactions with our social and physical worlds. In this way we grow to adapt to the particular niche into which we are born.
The two halves of the cerebral cortex have gradually differentiated during primate evolution to the point where each has developed areas of specialization, referred to as lateral dominance or specialization. Language is the best-understood example of lateral specialization. The two cerebral hemispheres communicate with each other primarily via the corpus callosum, which consists of long neural fibers that connect the two. Although the corpus callosum is the largest and most efficient mode of communication between the hemispheres in adults, there are a number of smaller cortical and subcortical interconnections between the two halves of the brain (Myers & Sperry, 1985; Sergent, 1986, 1990).
The cortex has been subdivided by neuroanatomists into four lobes: frontal, temporal, parietal, and occipital (Figure 4.1). Each is represented on both sides of the brain and specializes in certain functions: the occipital cortex comprises the areas for visual processing; the temporal cortex for auditory processing, receptive language, and memory functions; the parietal cortex for linking the senses with motor abilities and the creation of the experience of a sense of our body in space; and the frontal cortex for motor behavior, expressive language, executive functioning, abstract reasoning, and directed attention. The term prefrontal cortex is often used to refer to the foremost portion of the frontal lobe. Two additional cortical lobes, the cingulate and insula cortices, are gaining increasing recognition as distinct and important areas of the cortical-subcortical interface. They are involved in the integration of inner and outer experience, linking the rest of the cortex with somatic and emotional experience.
Between the brainstem and the cortex lies a region referred to as the limbic system, which is involved with learning, motivation, memory, and emotion. Because this book focuses on development and psychotherapy, you will notice repeated references to two limbic structures. The first is the amygdala, a key component in neural networks involved in attachment as well as the appraisal and expression of emotion throughout life (Cheng, Knight, Smith, & Helmstetter, 2006; Phelps, 2006; Strange & Dolan, 2004). The other is the hippocampus, which organizes explicit memory and the contextual modulation of emotion in collaboration with the cerebral cortex (Ji & Maren, 2007).
Neurotransmitters and Neuromodulators
Brains exist because the distribution of resources necessary for survival and the hazards that threaten survival vary in space and time.
—John Allman
Recall that, within the nervous system, neurons communicate with each other via chemical messengers called neurotransmitters. Different neural networks tend to utilize different sets of neurotransmitters, which is why certain psychotropic medications impact different symptoms. Chemicals that serve as neurotransmitters include monoamines, neuropeptides, and amino acids. Neuromodulators (e.g., the hormones testosterone, estrogen, cortisol, and other steroids) regulate the effects of the neurotransmitters on receptor neurons. Amino acids are the simplest and most prevalent neuromodulators. Glutamate is the major excitatory amino acid in the brain and central to neural plasticity and new learning (Cowan & Kandel, 2001; Malenka & Siegelbaum, 2001). Interactions with one of its primary receptors, N-methyl-D-aspartate (NMDA), regulates long-term potentiation and long-term depression, thereby shaping the relationship between neurons (Liu et al., 2004; Massey et al., 2004; Zhao et al., 2005).
The monoamines—including dopamine, norepinephrine, and serotonin—play a major role in the regulation of cognitive and emotional processing (Ansorge, Zhou, Lira, Hen, & Gingrich, 2004). All three are produced in different areas of the brainstem and are carried upward via ascending neural networks to the cortex. Dopamine, produced in the substantia nigra and other areas of the brainstem, is a key neurotransmitter in motor activity and reward reinforcement. Too much dopamine can result in mood changes, increased motor behavior, and disturbed frontal lobe functioning, which, in turn, can cause depression, memory impairment, and apathy. Parkinson’s disease results from damage to the substantia nigra and a consequent loss of dopamine. Many believe that schizophrenia is caused by too much dopamine, which overloads sensory processing capabilities and creates hallucinations and delusions.
Norepinephrine, produced in the locus coeruleus and other brain regions, is a key component of the emergency system of the brain and is especially relevant for understanding stress and trauma. High levels result in anxiety, vigilance, symptoms of panic, and a fight-flight response. Norepinephrine also serves to enhance memory for stressful and traumatic events. Serotonin, generated in the raphe nucleus, is distributed widely throughout the brain and plays a role in arousal, the sleep–wake cycle, and the mediation of mood and emotion (Fisher et al., 2006). Popular antidepressant medications such as Prozac and Paxil cause higher levels of available serotonin in the synapses and higher levels of neurogenesis (Encinas, Vaahtokari, & Enikolopov, 2006).
The group of neurotransmitters known as neuropeptides includes endorphins, enkephalins, oxytocin, vasopressin, and neuropeptide-Y. These compounds work together with neuromodulators to regulate pain, pleasure, and reward systems. The endorphins tend to modulate the activity of monoamines, making them highly relevant for understanding psychiatric illnesses. Endogenous endorphins (endorphins produced by the body) serve as an analgesic in states of physical pain. They are also involved with dissociation and self-abusive behavior, as we will discuss in a later chapter on trauma. The relationship between the monoamines and neuropeptides is vitally important to the growth and organization of the brain.
Glucocorticoids/Cortisol
We’re lousy at recognizing when our normal coping mechanisms aren’t working. Our response is usually to do it five times more, instead of thinking; maybe it’s time to try something new.
—Robert Sapolsky
Cortisol, the most important glucocorticoid, is often referred to as the “stress hormone.” It is produced in the adrenal glands in response to a wide variety of everyday challenges. The term glucocorticoid comes from the fact that it was first recognized for its role in glucose metabolism. With further study, however, many more functions of cortisol were uncovered. Glucocorticoid receptors (GRs) are found in almost all of the tissues of our bodies. At normal levels and over short periods, cortisol enhances memory, mobilizes energy, and helps to restore homeostasis after stressful situations. Glucocorticoids stimulate gluconeogenesis and the breakdown of lipids and proteins to make energy available to us for emergencies. If we have to fight or flee, we are going to need energy.
Cortisol evolved to be useful for periods of brief stress which, when resolved, allow GRs to signal the adrenal glands to shut down production. Prolonged cortisol release, on the other hand, can weaken the immune system by preventing T-Cell proliferation. In fact, the synthetic form of cortisol is called hydrocortisone and is used to treat inflammation and allergies by inhibiting natural immunological responses. Sustained high levels of cortisol disrupt protein synthesis, halt neural growth, and disturb the sodium-potassium balance to the point of neural death. Early and prolonged stress has been correlated with memory deficits, problems with affect regulation, and reduction of volume in brain regions including the hippocampus and amygdala (Buchanan, Tranel, & Adolphs, 2006).
It is believed that sustained high levels of glucocorticoids early in life can have a negative impact on brain development and make a child more vulnerable to subsequent stress. It has been shown that maternal behavior in rats stimulates the development of GRs in the brains of their pups. Greater density of GRs in the brain results in enhanced feedback to the adrenal glands, which serves to shut down cortisol production. This is one of the underlying neurobiological correlates associating maternal attention with resilience and positive coping later in life. The production and availability of these neurochemicals shape all of our experience, from bonding and affect regulation to cognitive processing and our sense of well-being. Regulation of these neurochemicals to control psychiatric symptomatology is the focus of the field of psychopharmacology (Gitlin, 2007; Stahl, 2008).
Genetics and Epigenetics
I am convinced that it will not be long before the whole world acknowledges the results of my work.
—Gregor Mendel
At the forefront of the science of genetics is Abbot Gregor Mendel, who, in the garden of his ancient abbey, discovered many of the principles of inheritance that still hold true. It turns out that his discoveries with pea plants apply to animals and humans because the underlying mechanisms of heredity are similar for all complex life forms. As you probably remember, his findings included dominant and recessive genes and the principles of segregation and independent assortment.
With the benefits of modern technology, Mendel’s observations of the natural world were later understood to be the effects of template genetics, or the way in which genes and chromosomes combine to pass along traits from one generation to the next. We now know that our genetic information is coded in four amino acid bases (adenine, thymine, guanine, and cytosine) that flow from DNA to messenger RNA (mRNA) to protein. Although this understanding was a huge leap forward in our knowledge of the underlying processes of genetic transmission, it accounts for only about 2% of genetic expression. The scientific term for the other 98% of genetic material was “junk,” once thought to be accumulated debris of natural selection. It turns out, however, that some of this junk plays an important role in guiding introns and exons, which help determine whether specific elements of the genetic code get expressed or lie dormant.
Biologist C. H. Waddington coined the term epigenetics by combining the words genetics and epi, Greek for over or above. Epigenesis describes the transformation of cells from their original undifferentiated state during embryonic development into a specific type of cell. Thus, epigenetics is the study of how our genotype is orchestrated into our phenotype. Understanding the elements of epigenetics may help us grasp why identical twins with the same genes may differ in phenotype, that is, why one becomes schizophrenic and the other does not.
This gets us back to the old nature-nurture debate and the question: What do we inherit, and what do we learn from experience? Our best guess is that almost everything involves an interaction between the two. While we inherit a template of genetic material (genotype), what gets expressed (phenotype) is guided by noncoded genetic information that is experience dependent. Experience can include anything from toxic exposure to a good education; high levels of sustained stress to a warm and loving environment; feast to famine. Thus, many more genes are involved with the regulation of what is expressed than with the direct synthesis of protein. So while template genetics may guide the early formation of the brain during gestation, the regulation of gene expression directs its long-term development in reaction to ongoing adaptation to the social and physical worlds. Epigenetics is a term used to describe this change in the phenotypic expression of genes in the absence of a change in the DNA template.
An example of this process of particular relevance to emotional development and psychotherapy is the impact of early stress on the adult brain. Meaney and his colleagues (1991) believe that early environmental programming of neural systems has a profound and long-lasting effect on the hypothalamic-pituitary-adrenal (HPA) axis, which regulates an individual’s responsivity to stress. Research with rats has demonstrated that the stress of early maternal deprivation downregulates the degree of neurogenesis and the response to stress during adulthood (Mirescu, Peters, & Gould, 2004; Karten, Olariu, & Cameron, 2005). Just as important for us, these processes are reversible later in life. As therapists, we attempt to reprogram these neural systems via a supportive relationship and the techniques we bring to bear during treatment. In other words, we are using epigenetics to change the brain in ways that enhance mental and physical well-being.
Views of the Brain
When considering the abilities and complexities of the brain, one is struck by the incredible efficiency and splendor expressed in gray and white matter.
—Julian Paul Keenan
Throughout most of the history of neurology, the human brain was only examined after injury or death. The location of brain damage during autopsy was linked to the nature and severity of the patient’s clinical symptoms during life. Brain development was studied by examining and comparing the brains of humans and animals at different ages. These brains were compared for size; the number of neurons, synapses, and dendrites; the degree of myelinization; and other aspects of neural maturation.
Newer techniques allow us to examine brain structure in living subjects. Through the use of computerized tomography (CT) and magnetic resonance imaging (MRI), we are able to see two-and three-dimensional pictures of the living brain. Both of these techniques provide a series of cross-sectional images of the brain through its many layers. CT scans do this via multiple X-rays. MRI scans utilize radio waves and a magnetic field to study the magnetic resonance of hydrogen molecules in the water present in different brain structures. In determining brain–behavior relationships, these measures need to be evaluated on the basis of whether they are causes or correlates of the disorder being studied (Davidson, 1999). In their present practical applications, radiologists learn to read these images for the presence and locations of tumors or lesions in order to assist surgeons in their work. These scans have become an indispensable tool in neurology.
The functioning of the brain can also be measured in many ways. Clinical and mental status exams, tests of strength and reflexes, and neuropsychological assessment all require a patient to perform physical or mental operations that are tied to known neurobiological systems. These clinical tests are supplemented by a number of laboratory tests that measure different aspects of brain functioning. The electroencephalograph (EEG) measures patterns of electrical activity throughout the cortex. There are characteristic brainwave patterns in different states of arousal and stages of sleep. Epilepsy or the presence of tumors will demonstrate characteristic alterations of normal electrical functioning, allowing EEGs to be used as diagnostic tools. EEGs can also be used to measure brain development, because neural network organization is characterized by the replacement of local erratic discharges with more widespread and constant wave patterns (Barry et al., 2004; Field & Diego, 2008b; Forbes et al., 2008).
The most exciting new tools in neuroscience are the various brainscanning techniques providing us with a window to the brain in action. Positron emission tomography, single photon emission tomography, and functional magnetic resonance imaging measure changes in blood flow, oxygen metabolism, and glucose utilization, which tell us about the relative activity of different regions of the brain. Using these techniques, neuroscientists can now explore complex activation–deactivation patterns of brain activity in subjects performing a wide range of cognitive, emotional, and behavioral tasks (Drevets, 1998). Most of these newer scanning techniques are still somewhat experimental, and methodological standards regarding their use and interpretation continue to evolve. These methods, and those yet to be developed, will vastly enhance our understanding of the brain. As they grow increasingly more accurate and specific, so too will our knowledge of neural network functioning.
Brain Development and Neural Plasticity
Swiftly the brain becomes an enchanted loom, where millions of flashing shuttles weave a dissolving pattern—always a meaningful pattern—though never an abiding one
—Sir Charles Sherrington
Experience sculpts the brain through selective excitation of neurons and the resultant shaping of neural networks. Paradoxically, the number of neurons decreases with age while the size of the brain increases. The surviving neurons continue to grow from what look like small sprouts into microscopic oak trees. This process of growth and connectivity is sometimes referred to as arborization.
In order for a neuron to survive and grow, it must wire with other neurons in increasingly complex interconnections. Just as we survive and thrive through our relationships with others, neurons survive and grow as a function of how “well connected” they are. Through what appears to be a competitive process referred to as neural Darwinism, cells struggle for connectivity with other cells in the creation of neural networks (Edelman, 1987). Cells connect and learning occurs through changes of synaptic strength between neurons in response to stimulation. Repeated firing of two adjacent neurons results in metabolic changes in both cells, which provides an increased efficiency in their joint activation. In this process, called long-term potentiation (LTP) or Hebbian learning, excitation between cells is prolonged, allowing them to become synchronized in their firing patterns and joint effectiveness (Hebb, 1949). LTP is believed to be a fundamental principle of neuroplastic learning. Underlying LTP is the constant reaching out of small portions of the dendrites in an attempt to connect with adjacent neurons. When these connections are made, neurons synthesize new protein to build more permanent bridges between them.
Through LTP, cell assemblies organize into functional neural networks that are stimulated through trial-and-error learning. This is only one small piece of a vastly complex set of interactions involving the connection, timing, and organization of firing within and between billions of interconnected neurons in the CNS (Malekna & Siegelbaum, 2001). Early in development, there is an initial overproduction of neurons that gradually decreases through the process of pruning, or apoptosis. Neural Darwinism applies to both the survival of neurons and the synaptic connections among them. Synapses that are formed may be subsequently eliminated if they become inactivated or inefficient (Purves & Lichtman, 1980). In fact, elimination of synaptic connections in the cortex continues shaping neural circuitry through adolescence and into adulthood (Cozolino, 2008; Huttenlocher, 1994).
In contrast to the brainstem and limbic system, the cortex is immature at birth and continues to develop throughout adulthood. Because of this developmental timing, brainstem reflexes organize much of the infant’s early behaviors and the behavior of a newborn is dominated by subcortical activity. The neonate will orient to the mother’s smell, seek the nipple, gaze into her eyes, and grasp her hair. A good example of a brainstem reflex is the Moro reflex, by which the infant reaches out with open hands and legs extended, putting the infant into a position conducive to grasping and holding (Eliot, 1999). The child’s eyes reflexively orient to the mother’s eyes and face and a baby’s first smiles are controlled by brainstem reflexes to attract caretakers. In fact, children born with a genetic malformation that results in having only a brainstem are still able to smile (Herschkowitz, Kegan, & Zilles, 1997). These reflexes enhance physical survival and jump start the attachment process by connecting parent and child, while enhancing their bond.
As anyone who has been pregnant can tell you, babies begin to engage in spontaneous activity of the arms and hands well before birth. While the baby is practicing using its arms and legs, parents-to-be grow increasingly excited as these signs of activity grow in frequency and strength. After birth, newborns continue to move all parts of their bodies, allowing them to discover their hands and feet as they pass in front of their faces. Although these movements may look random, they are the brain’s best guess at which movements will eventually be needed. These reflexive movements jump start the organization of motor networks to build the skills the child will need later on (Katz & Shatz, 1996).
Through months and years of trial-and-error learning, these best guesses become shaped into purposeful and intentional behaviors that are reflected in the organization of underlying neural networks (Shatz, 1990). As sensory systems develop, they provide increasingly precise input to guide neural network formation for more complex patterns of behavior. As positive and negative values are connected with certain perceptions and movements—such as the appearance of the mother and reaching out to her—emotional networks will integrate with sensory and motor systems. In the development of these and other systems, we find the sequential activation of reflexive and spontaneous processes priming neural development, which comes to be shaped by ongoing experience.
Cortical Inhibition and Conscious Control
He who conquers others is strong; he who conquers himself is mighty.
—Lao-Tzu
The gradual attenuation of neonatal reflexes and spontaneous behavior corresponds with rising levels of cortical activity and involvement in behavior. As the cortex develops, vast numbers of top-down neural networks connect it with subcortical areas. These top-down networks provide the pathways for inhibiting reflexes and bringing the body and emotions under increasing cortical control. An example of this is the development of the fine motor movements between the thumb and forefinger that are required to hold a spoon. Primitive grasping reflexes allow only for the spoon to be held in a tight fist, rendering it useless as a tool. The developing cortex enables the grasping reflex to be inhibited, while cortical networks dedicated to finger sensitivity and hand–eye coordination mature. Thus, a vital aspect of the development of the cortex is inhibitory—first of reflexes, later of spontaneous movements and even later of emotions and inappropriate social behavior.
Only through repeated trial-and-error learning are early clumsy movements slowly shaped into functional skills. Children and their brains intuitively know this and will resist being held back or helped too much. When we attempt to help, a child’s impatient protest of “Let me do it!” reflects instinctual wisdom of the importance of trial-and-error learning in the growth of neural networks. This makes for many years of messes and boo-boos. Another good example of the process of brain maturation is our ability to swim. The newborn’s brainstem reflex to hold its breath and paddle when dropped into water is lost (inhibited by higher brain circuitry) just weeks after birth. The skills involved with swimming need to be relearned as cortically organized skills in years to come. Motor networks need to be taught body movements, as breathing becomes timed and synchronized with each stroke.
Cortical inhibition and descending control are also central to affect regulation. The rapidly changing and overwhelming emotions displayed by very young children reflect this lack of control. As the middle portions of the frontal cortex expand and extend their fibers down into the limbic system and brainstem, children gradually gain increasing capacity to regulate their emotions and find ways to gain soothing, first through others, and eventually by themselves. When these systems are damaged or developmentally delayed, we witness symptoms related to deficits in attention, emotional regulation, and impulse control.
We see the changes in motor control and posture as a child moves from being able to sit upright without help at about 6 months, to crawling at about 9 months, and then to walking without help by about 1 year. At 2 years, a child will walk up and down stairs; by 3 she can peddle a tricycle. As these skills are shaped, so too are the brain systems dedicated to balance, motor control, visual–spatial coordination, learning, and motivation that control them. The growth, development, and integration of neural networks continue to be sculpted by environmental demands. In turn, neuronal sculpting is reflected in increasingly complex patterns of behavior and inner experience.
Sensitive Periods
The principal activities of brains are making changes in themselves.
—Marvin L. Minsky
The brain continues to grow as long as we continue to learn, essentially until the day we die. Early brain development is highlighted by periods of exuberant neural growth and connectivity called sensitive periods triggered by the interaction of genes and experience. These sensitive periods are times of rapid learning during which thousands of synaptic connections are made each second (Greenough, 1987; ten Cate, 1989). The timing of sensitive periods varies across neural systems, which is why different abilities appear at different ages.
The most widely recognized sensitive period is the development of language. At 24 months, an average child understands and uses about 50 words; this increases to 1,000 words by 36 months (Dunbar, 1996). The extent of neural growth and learning during sensitive periods results in early experience having a disproportionate impact on our brains, minds, and experiences. As we learn of the brain’s ability to create new neurons and retain plasticity throughout life, the importance of sensitive periods takes on new meaning. The question for therapists is: How amenable are these established structures to modification? This is a topic we will come back to again and again in later chapters.
The growth of neurons and the development of increasingly complex neural networks require large amounts of energy. Patterns of increasing glucose metabolism during the first year of life proceed in phylogenic order, meaning that the development of more primitive brain structures precedes those which evolve later (Chugani, 1998; Chugani & Phelps, 1991). Early sensitive periods account for the higher level of metabolism in the brains of infants compared to adults. Ever notice how warm a baby’s head is? It has been estimated that in rats’ brains, 250,000 synaptic connections are formed every second during the first month after birth (Schuz, 1978). Just imagine what the number must be for humans.
Networks dedicated to individual senses develop before the association areas that connect them to one another (Chugani, Phelps, & Mazziotta, 1987). The growth and coordination of the different senses parallel what we also witness in such behavioral changes as hand–eye coordination and the ability to inhibit incorrect movements (Bell & Fox, 1992; Fischer, 1987). As the cerebral cortex matures, a child at 8 months is able to distinguish faces and compare them to his or her memory of other faces. It is around this period that stranger anxiety and separation anxiety develop. As the brain matures, we witness increasing cortical activation and the establishment of more efficient neural circuitry firing in increasingly synchronous patterns.
Although both the left and right cerebral hemispheres are developing at very high rates during the early years of life, the right hemisphere appears to have a relatively higher rate of activity and growth during the earliest years (Chiron et al., 1997). During this time, vital learning in the areas of attachment, emotional regulation, and self-esteem are organized in neural networks biased toward the right hemisphere. Somewhere around age 3, this pattern of asymmetrical growth shifts to the left hemisphere.
Summary
The maturation and sculpting of so much of the cortex after birth allows for highly specific environmental adaptations. The caretaker relationship is the primary means by which physical and cultural environments are translated to infants. It is within the context of these close relationships that networks dedicated to feelings of safety and danger, attachment, and the core sense of self are shaped. The first few years of life appear to be a particularly sensitive period for the formation of these networks. It may be precisely because there is so much neural growth and organization during sensitive periods that early interpersonal experiences may be far more influential than are those occurring later. The fact that they are preconscious and nonverbal makes them difficult to discover and more resistant to change. Because these neural networks are sculpted during early interactions, we emerge into self-awareness preprogrammed by unconsciously organized hidden layers of neural processing. The structure of these neural networks organizes core structures of our experience of self.