The Pleasure Instinct: Why We Crave Adventure, Chocolate, Pheromones, and Music (6 page)

Chief among these is the development of the neocortex, the vast outer portion of the cerebrum that constitutes about 85 percent of the human brain’s total mass and that is responsible for higher-level cognitive functions such as language, learning, memory, and complex thought. Reptiles and birds possess an anatomically simpler, three-layered version that is sometimes referred to as archicortex (old cortex). Mammals have a six-layered version that is far denser in cell counts per volume and contains a greater diversity of cell types; however, they also possess several three-layered cortical structures, such as the olfactory cortex and hippocampus (collectively referred to as allocortex), which are similar to the archicortex of nonmammals. For these reasons, it is believed by many (but certainly not all) neuroanatomists that the neocortex is unique to mammals and derived from phylogenetically older cortical areas.

This evolutionary ordering of three-layered cortical areas predating the six-layered neocortex is paralleled in the embryological development of humans. By the sixth week of gestation, a human fetus will already have many brain-stem structures partially developed. These will control basic physiological functioning in the newborn—respiration, sleep-wake cycles, thermoregulation, and a host of motivated behaviors. Brain-stem sites are followed by the initial appearance of subcortical regions such as the thalamus and hypothalamus sometime around the tenth week. As we shall see, many of these brain regions play a pivotal role in the generation of motivated behaviors and act as sensory integration sites. By fourteen weeks the allocortical areas start to develop, eventually becoming part of the limbic system, a region responsible for learning, memory, and processing emotional information. By the sixteenth week, cells begin to appear in what will eventually become the neocortex. However, it’s important to note that none of these areas is wired together functionally yet.This occurs in two distinct stages—neurogenesis and synaptogenesis.

Our understanding of how the human brain develops from a smooth sheet of ectoderm
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into the mature adult form has changed radically in just the past ten years. Scientists now have a much deeper appreciation for the way developing brains depend on specific types of stimulation patterns and sensory experiences to activate important genes. These insights have come from studies of developing infants using noninvasive brain imaging techniques, neuropsychological experiments, and an improved understanding of how genes actually work.

Genes have two basic components, a template (or coding) region that provides information about how to make a specific protein, and a regulatory region that determines when a gene is expressed or repressed. The template region is what we usually think of when we hear the phrase “it’s in our genes.” The information in this region is not modified by experience or learning, only through mutations, which are rare and essentially random. The regulatory region, on the other hand—the on/off switch of the gene—is highly sensitive to a host of experiential factors.

A great variety of signaling proteins can bind to the regulatory region of a gene and modulate its subsequent transcription and expression. These signaling proteins are known as transcription regulators. Put simply, when transcription regulators bind to a segment of a gene, they activate its expression and the eventual production of a new protein. Thus they have direct control over whether a gene is turned on or off.

A number of factors influence the way a transcription regulator binds to a gene. Both internal and external stimuli (that is, things we experience) activate signaling pathways that result in alterations of this binding process. Some signaling pathways are differentially activated as a result of the normal developmental process. Others are activated by stress, learning, hormonal changes, or social/experiential interactions.

For example, psychological and physical stress causes the release of the adrenal gland steroid glucocorticoid (also known as cortisol), which circulates in the peripheral and central nervous system (brain and spinal cord). In the brain, this steroid activates a transcription regulator that binds to the regulatory region of several genes, inducing the transcription and expression of new proteins involved in the long-term regulation of the stress response. Hence, social factors such as stress regulate gene expression and the subsequent production of specific proteins. Indeed, all forms of learning are incorporated into our biological makeup in the altered expression of specific genes that encode the production of selective proteins in brain cells.

Gene expression can be extremely selective in targeting the production of proteins unique to a specific type of nerve cell and brain region. A particular experience, say a psychological stressor, will result in the production of a very specific set of proteins, while another experience, for example, learning a new phone number, will result in a different set. These experience-induced changes in gene expression and subsequent protein production are not transmitted from generation to generation genetically. None of these alterations in gene expression is incorporated into the sperm or egg, and therefore are not heritable.All changes in gene expression that result from learning or being exposed to experiential/environmental factors are transmitted culturally rather than genetically, and they clearly have a profound impact on the way brains develop.

The fact that genes have essentially two functional components has important implications for development and the relationship between nature and nurture. Being familiar with the way genes really work makes it easier to see why most biologists have long ago given up on the nature versus nurture debate as a false dichotomy. The genes that code the way brains are built do not contribute to development unless they are transcribed and expressed. Hence, experience is an essential part of development even at the level of the gene.

How do genes build brains? As I write this chapter my wife is four and a half months pregnant, and every day brings new questions about little Kai’s development. The typical adult human brain has about 100 billion nerve cells or neurons. Each neuron connects to thousands of others, resulting in about 10
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(a 1 followed by 15 zeroes) different connections. How, then, do the 25,000 or so genes identified by the Human Genome Project code such a combinatorically large and complicated system? Clearly, since the numerical differences are so great, genetic information does not uniquely specify where every single neuron resides or where each of its thousands of connections will terminate. Instead, genes specify more general rules for neuron development and migration.

At the earliest stages of development, we start off as three primitive cell layers: endoderm, which consists of cells that eventually line our internal organs and vessels; mesoderm, destined to become the major structural components of the body, including bones and muscle groups; and finally, ectoderm, which becomes the central nervous system, skin, hair, and nails. Kai’s entire mental world—his thoughts, emotions, sensations, and perceptions—emerge from this thin sheet of cells. Within the first few days of gestation, the primitive cell layers elongate and fold into a cylindrical tube called the notochord, the progenitor of Kai’s spinal column. This process recapitulates the earliest event in the evolutionary transition from invertebrate to vertebrate forms that occurred more than 600 million years ago.

Once the notochord is formed, it guides the ectoderm layer, which progresses through a series of well-defined stages, first thickening and then folding in on itself to form the neural tube. At about nineteen days into gestation, just about the time when Melissa and I first learn she is pregnant, the earliest form of Kai’s future brain and spinal cord begin to emerge through a process called neurogenesis. During this period, the front end of Kai’s neural tube develops three enlargements, which eventually become the two cerebral hemispheres and the brain-stem. The neural tube then goes through a rapid growth spurt, where the entire cycle from cell division to cell division takes place in about an hour and a half. Some of these precursor cells are destined to become neurons, while others will mature into glial cells, which serve a variety of supportive functions in the brain.

As cell division and replication continue, the three enlargements begin to take on more detail, eventually forming all the major components of Kai’s brain. At two months into gestation he is little more than two inches long, yet all of his major brain structures have begun to take shape, including the elementary forms of the medulla, pons, and midbrain, which combine to form the brain-stem; subcortical structures such as the thalamus, hypothalamus, and basal ganglia; then a bit more slowly, the allocortex; and even more slowly, the neocortical regions. Kai’s brain will develop from the bottom to the top, with lower brain-stem structures such as the medulla maturing first, followed in sequence by the upper brain-stem, subcortical areas, the allocortical regions, and then the neocortex.

It’s often asked why neurogenesis starts at the bottom of the brain and progresses toward the top (or since the brain is three-dimensional, from the inside of the brain to outer regions). A clue can be found if we compare the embryological development of an individual with the evolutionary development of our species. Until roughly four weeks of gestation, the embryo that will become Kai is practically indistinguishable from embryos of many bird, reptilian, and mammalian species. But by the sixth week, he begins to look more and more like a mammal, and by week seven he appears decidedly primate. As a general rule, species of similar phylogenetic forms tend to look alike for longer periods during development. For example, human and chimpanzee embryos share strikingly similar features until about the seventh week of gestation, at which time they begin to diverge in appearance. Human and rabbit embryos, on the other hand, share similar developmental features only until about the fourth week of gestation. Thus, embryological development is highly conserved across different species—those structures appearing earliest in development are most common across species and thought to be derived from phylogenetically earlier forms.

This makes sense when one considers the vast diversity of adult life forms that mature from a single fertilized egg. Evolution is driven by the natural selection of forms that are altered through mutations, and chances are much greater that an offspring will be viable if a mutation occurs at a developmentally later rather than earlier stage. Mutations that occur early in embryological growth tend to have devastating consequences because they alter all subsequent stages of development. Consequently, successful adaptations are most often “added on” or modified from existing structures that were present in earlier phylogenetic forms.

This relationship between ontogeny and phylogeny explains why human brains take so long to develop and why certain regions mature before others. The human brain, particularly certain areas of the neocortex, distinguishes us from other primates and takes significantly longer to develop and mature than other systems, such as those that control respiration and circulation. First to come online are those brain regions that are involved in the essential behaviors common to most mammals—for example, simple reflexive movements and those that eventually control respiration and digestion. Many of these simpler behaviors depend on neurons in the spinal cord or lower brain-stem.

By the end of the first trimester, Kai has a fairly mature brain-stem and diencephalon, which consists of the thalamus and hypothalamus. The thalamus is a critical sensory relay station that takes part in a two-way dialogue with different areas of the neocortex, sending new information through and often being instructed by the neocortex in a top-down manner to filter certain features of sensory information so that it never reaches conscious perception. The hypothalamus is just as important because it will serve as the mediator between the rest of Kai’s brain and his endocrine and immune systems. His first sensations of hunger and satiety, and the emotions that accompany these survival behaviors, will be mediated largely by cells in his newly formed hypothalamus.

Even within a specific sensory modality—sight, for example—those features of vision that are most common across species, such as detecting movement and contrasting light intensity, arise functionally well before other features, such as color vision and depth perception, which are more unique to primates.As mentioned above, this is because the brain regions that control each of these sensory functions arise in a specific sequence laid down by our genetic programming that reflects a gradient from functions common to many species to functions more unique across phylogenetic classes. We start off broad functionally, and with development continue to add those features that are first common to all mammals, then those that are only seen in primates, and finally those that only humans possess.

After he is born, Kai will be able to see things that move long before he’ll be able to see colors. This is because he will emerge from the womb with only brain-stem, thalamus, and the most primitive visual cortical regions connected and working. And as will become clear, these circuits are in a very sensitive state. For the processes of neurogenesis and synaptogenesis to continue the job of connecting his entire visual brain and fine-tuning it, the circuits he has thus far must be stimulated by specific types of visual experience. It is in the service of this developmental necessity that pleasure—driven by the hypothalamus and other limbic structures that come online early—participates to ensure that newborns seek out the forms of sensory experience that optimally stimulate the time-sensitive growth and maturation of their brains.

This can be thought of as a “bootstrapping” mechanism, whereby developmentally early brain systems that support essential survival and regulatory processes contain the functions that ensure the further development of higher-order brain systems involved in more complicated forms of perception and cognition. Just as Kai’s brain will need nutritionally rich sources of metabolic energy to continue normal development, so too will it need to encounter sensory-rich stimulation at specific intervals during maturation. The experience of pleasure thus provides newborns with a general rule for seeking out patterns of stimulation that guide the normal development of all higher cognitive functions, including perception, language, and abstract reasoning—functions that have traditionally been treated as being entirely separate from their more passionate brethren.