The Pleasure Instinct: Why We Crave Adventure, Chocolate, Pheromones, and Music (7 page)

 

 

At four and a half months into our adventure, Melissa—a prepregnancy vegetarian—can now keep her food down, and occasionally sends me out (often very late!) to pick up a Whopper with extra pickles. It is at about this time that the processes of neurogenesis and cell proliferation begin to peak in Kai’s brain.The numbers are staggering. To reach the estimated one hundred billion neurons that comprise a human brain—the majority of which are in place by midgestation—Kai will have to produce an average of five hundred thousand cells per minute during the first four and a half months. During this time of cell proliferation, neurons migrate to their intended locations, and once there begin to grow extensions toward other cells in a process called synaptogenesis.

One hundred billion neurons alone do not make a brain. It’s in the detailed wiring that connects cells together within and between different regions that we find the mechanisms of sight, smell, hearing, touch, and the multitude of unique capacities that make us human. Each neuron has three basic parts—the cell body, which contains most of the metabolic machinery; dendrites, which take in information from other cells; and an axon, which carries information from the cell to target cells with which it will communicate. After neurogenesis, brain cells extend their dendrites and axons, and form synaptic connections with other cells. A synapse is essentially a point of communication between two neurons, and the dialogue is electrochemical. So important is this process to life that its absence, as measured by a lack of significant electrical activity in scalp EEG recordings, is taken as a definition of death.

It’s been estimated that between midgestation and two years of postnatal life—the peak period of synaptogenesis—close to 15,000 synapses are produced on every cortical neuron. This averages to an astounding 1.8 million new synapses formed every second during this period. At present, there are several controversial theories as to how a cell finds its correct target cells. For example, how do the output cells of the retina know they must bypass certain brain regions and terminate their axons in the proper portions of the visual thalamus? And how then do those thalamic cells know to project to specific regions of the primary visual cortex? Although no single theory is consistent with all of the available data, it is widely accepted among neurobiologists that synaptogenesis is a comparatively long process—continuing through gestation and several years of postnatal life—and works primarily through competition.

Nature begins the job.The general wiring scheme is laid down by our genetic programming and follows a very specific developmental sequence of time-dependent growth patterns that resembles that observed during neurogenesis. The first areas to undergo significant synaptogenesis are in the lower brain-stem, followed by the upper brain-stem, the diencephalon, various subcortical regions, the allocortex, and finally neocortical regions. Neuroscientists are only beginning to understand how genes code the basic wiring of the brain, and the ways in which this process depends on experience. Genes, it turns out, direct axons and dendrites only to their approximate target locations. Once these beginner circuits start to function, however, experience and the pleasure instinct take over to fine-tune the connections and shape them into a precise network unique to a child’s environment.

From the end of gestation through early childhood, Kai will produce about twice as many synaptic connections as he will eventually need in his adult brain. Cells make and receive thousands of synapses during this promiscuous period, resulting in a large-scale but rather diffuse communication system between different brain regions. The overproduction of synapses is an evolutionary trade-off—since the small number of genes we possess simply cannot dictate each of the quadrillion or so connections between brain cells, they instead provide general rules for making contacts between areas. Once these diffuse connections are in place, nature takes over and begins to selectively prune certain synapses based on the types of stimulation patterns (that is, experiences) the brain receives. In this way, experience, guided by the pleasure instinct, refines the communication network within the brain, making certain connections stronger while removing others.

Synaptic pruning is a requirement of normal brain development. As I mentioned, it works directly through a process of competition—survival of the fittest. Each of the synaptic connections in Kai’s brain has the potential to survive past this competitive period of pruning, but about half will perish, and along with them certain functional capabilities.The rule is simple—use it or lose it.To survive, the synapse between two cells must be activated consistently.Those synapses that are activated the most have three advantages over less active synapses. First, when activated, they tend to inhibit surrounding synapses (through cellular responses) that are competing for stimulation. Second, the electrochemical dialogue between active cells triggers a series of biochemical reactions that strengthen the synapse, cementing it in place. And third, synapses that are inhibited during this competitive period have an increased likelihood of triggering active processes that weaken and eventually terminate the connection. These fundamental discoveries from neuroscience have profound implications for the way we raise our children and educate our young during the first two decades of life.

While the major work of neurogenesis and synaptogenesis is finished relatively early during development, synaptic pruning occurs very slowly, lasting at least until an individual’s early twenties and perhaps longer. Although it takes decades, synaptic pruning generally follows the same sequence as neurogenesis and synaptogenesis, progressing from lower structures of the brain-stem that tend to be more broadly represented across phylogenetic class, upward toward structures such as the prefrontal cortex that are most unique to primates. One consequence of this sequential development is that the behavioral, perceptual, and cognitive functions that depend on each of these regions also tend to arise through a specific—and culturally independent—sequence.

Once a particular brain region undergoes synaptogenesis and the overproduction of synaptic connections, this marks the onset of abilities regulated by that region, such as the emergence of color vision, sound localization, and language. Although experience influences all stages of development, it is predominantly the long period of synaptic pruning that fixes the overall quality and nuances of these abilities. As synaptic pruning refines certain abilities, for example, a capacity to discern phrase boundaries unique to human languages, the process also results in the sacrifice of alternative synaptic connections that gradually make it difficult to acquire other abilities, such as the perception of sounds that are not normally a part of human experience. Hence, synaptic pruning is essentially a selection process driven by experience.

As we shall see, the behaviors that emerge during synaptic pruning—guided by the pleasure instinct—are really forms of self-stimulation that infants, toddlers, children, and adolescents
must
produce to ensure normal brain development given the ecological and environmental context in which that development and growth take place. A central theme of this book is that nature has solved this functional requirement in primates (and perhaps other mammals) by exploiting the early capacities of limbic structures to produce pleasurable sensations, which have been associated through our long evolutionary lineage with optimal forms of brain stimulation.

From culture to culture, it is striking that infants and toddlers pass through the same developmental milestones, many of which can be used to gauge how far their brain has matured. Once a potential capacity arises, they must self-stimulate in ways that encourage the further development of the brain systems that mediate these and related behaviors. We find this process at work in every sensory modality, and the sequence of self-stimulating behaviors that emerge reflects the order in which our primary sensory systems come online during development.As we will learn in the chapters that follow, this developmental progression is an echo of our evolutionary past.

—
Ashley Montagu,
Touching

—
Diane Ackerman,
A Natural History of the Senses

 

 

 

The sign above the door declared that the children inside were “unsalvageable.” By the time the world met him, Izidor had lived at the Hospital for Irrecoverable Children in the northwest mountain town of Sighetu Marmatiei, Romania, for most of his eleven years. Crippled by polio, he was abandoned shortly after birth and warehoused with five hundred other children, all victims of Romanian president Nicolae Ceausescu’s campaign to expand the population of the country. Ceausescu saw the economic problems of his land as ones that stemmed from a shortage of labor. His solution was to institute a strict program of population control, and in 1966 he banned abortion, birth control, and divorce, and decreed that all Romanian women must bear five children apiece. As a result of this program, state-run orphanages were soon overflowing, and by the late 1980s, Romania had about 180,000 abandoned children in a country with a population of only 5 million.

In 1990, the ABC News program
20/20
did a story called “The Shame of a Nation,” about the plight of Romanian orphans. Horrific images of anguished children in tattered clothing, sleeping on urine-soaked mattresses and floors, mobilized an international relief effort that continues today. In the years that followed, parents in Western Europe and America who adopted some of the orphans began to notice an odd syndrome of autistic-like developmental problems in these otherwise healthy children that included self-hugging behavior, rocking, and extreme sensitivity to touch. Behaviors such as these have been observed in institutionalized children for hundreds of years, but it was not until the late nineteenth century that pediatricians began to study seriously the effects of institutionalization on child development.

During the 1800s, more than half of all American infants institutionalized in foundling homes died of a mysterious disease called marasmus (a Greek word meaning “wasting away”) before their first birthday. As late as 1915, the infant mortality rate approached 100 percent in many homes, and during this year the distinguished pediatrician Henry Dwight Chapin presented alarming findings to the American Pediatric Society. His data showed that nearly every foundling home sampled from ten major cities in the United States reported an astonishing 100 percent mortality rate for children before their second birthday. Once Chapin’s findings were made public, reform measures were implemented that, for the most part, called for improved access to breast milk through wet nurses. This strategy decreased the mortality rate somewhat at most institutions, but Chapin and others were stunned to realize that even those foundling homes with first-class medical care and nutrition had mortality rates that were comparable to, and in some cases actually worse than, less-equipped homes. The question was why.

It is important to note that at the time, American physicians and fashionable parents were under the spell of Luther Emmett Holt, the influential professor of pediatrics at Columbia University who recommended a system of child rearing that most parents would probably not feel comfortable with today. He warned against cradling and rocking infants and advised that parents should handle their babies—including newborns—sparingly. Chapin began to suspect that it was just this type of emotional aridity that was contributing to marasmus and high infant mortality rates.

However, it wasn’t until the late 1930s that the causes of marasmus were identified. A clue came from the studies of Chapin and other pediatricians who noted that the disease was just as prevalent or perhaps even more so in the “best” homes and institutions, where medical care and hygiene were superior to those homes with smaller budgets. A few insightful pediatricians noticed that those institutions that occasionally fostered children out to single families on a temporary basis because of budgetary constraints often had fewer cases of the disease. The obvious conclusion was that it was the increased physical contact in the form of gentle handling, rocking, massage, cuddling, and bathing that seemed to benefit the health of the temporarily fostered infants. Before long, the practice of mothering was incorporated into the daily regimen of most institutionalized infants. The pediatric wards of the famed Bellevue Hospital in New York reported a nearly 25 percent decrease in infant mortality in a single year after mothering sessions—in which infants were gently stroked, massaged, and rocked—were formally introduced into their child care curriculum.

A striking similarity exists in the descriptions of nineteenth-century children reared in foundling homes and those institutionalized in modern Romania. The repetitive self-hugging and rocking movements, the extreme sensitivity to tactile stimulation, and the flat affect of many of these children are all characteristics that would have seemed familiar to Izidor. In his autobiography
Abandoned for Life
, he describes the morning-to-evening routine at the Sighetu Marmatiei orphanage: “Most of the time, the children did the same old thing; rock back and forth, sleep or hurt themselves. If they continued hurting themselves, the house nannies either gave the children more medication or put them into straight jackets.” With a ratio of one nanny for every 75 children during the day and one for every 150 children during the evening, there was not much time for the kinds of tactile stimulation—simple, loving touch—that are known to be so crucial for normal psychological and physical development. It is only in the past twenty years or so that scientists have begun to show that pleasurable touch during the early period of life is crucial for normal brain development and why its deprivation can produce autistic-like behaviors.