The Pleasure Instinct Part 3

Few foods inspire such pa.s.sion in people as chocolate. This love affair goes far beyond the typical fondness for sweets: after all, we're not likely to head out into a snowy night, panic-stricken after finding that we are out of lemon-creme pie or bubble gum. There is something special about chocolate that drives us to extraordinary lengths. Chocoholics find nothing strange in spending a small fortune for even a sampler box of champagne truffles. No, a simple sweet addiction is not the same as a chocolate addiction-indeed, many connoisseurs prefer the darkest, most bitter variety.

The history of the chocolate bean is a story riddled with desire that transcends cultural distinctions. The roots of chocolate go back some twenty-six hundred years to the great Olmec and Mayan civilizations that flourished throughout southern Mexico, Belize, Guatemala, and Honduras. Spouted, teapot-shaped vessels have been excavated from towns such as Colha in northern Belize, and found to contain residue of ancient chocolate. The Mayan drink was very different from the watery, sugar-laden version of hot chocolate that dominates modern society. The journals of Spanish conquistadors are filled with descriptions of middle Mayan culture that include the preparation of dried cacao beans ground into a powder and mixed with water, honey, chili pepper, and sometimes maize. The liquid would then be heated and repeatedly poured from one vessel to another to produce a thick head of rich chocolate foam that was the most coveted part of the drink.

A reverence for chocolate was also present in Aztec culture throughout the region. The great Aztec emperor Moctezuma reportedly drank up to fifty flagons of chocolate per day, believing it to have restorative and even aphrodisiac powers. Within this culture, the cacao bean became the primary form of currency, and folklore has it that when the Spanish conquistadors stormed Moctezuma's temple, they found beans in place of gold.

After conquering the Aztecs, Hernando Cortes returned to Spain and brought King Carlos treasures of cacao beans and a recipe for making xocoatl xocoatl, which was sweetened with sugar by members of his court. Today this recipe lives on, and the domesticated cacao tree grows on farmlands near the equator in a number of regions including the Caribbean, Africa, southeastern Asia, and in several South Pacific islands such as Samoa and New Guinea.

Whereas modern processing and distribution technology has made chocolate a more common item on the food landscape, its seductive properties have remained a mystery to science. It has only been in the past few years that neuroscientists and biochemists have begun to get a handle on why we find chocolate so pleasurable.

Chocolate contains more than 350 known compounds, several of which activate three important brain systems that contribute to the experience of pleasure.The first ingredient that gives chocolate its wide fan base is plain old sugar, an underappreciated compound these days. Considering our modern tendency to detest all that is carbohydrate and the epidemic-like rates of diabetes in many subpopulations, it is easy to understand why sugar is seen as something to avoid if you consider yourself a health-conscious individual. But in reasonable doses, sugars have a profound and positive impact on our physiology, most notably in the form of a calming effect. Placing a small amount of liquid sweetened with either glucose or sucrose on the tongue of a crying newborn has an immediate calming effect that can last for several minutes. Sugars, in their varying chemical structures from lactose to sucrose, have been shown to activate the brain's opioid system, a set of circuitry that plays a prominent role in regulating the body's stress response.

In addition to sucrose, chocolate contains small amounts of theobromine (a mild stimulant) and phenylethylamine, a substance that is chemically similar to amphetamine. Once in the brain, each of these ingredients has an effect on the dopamine and noradrenergic neurotransmitter systems, which are implicated in attention and general arousal. These compounds are thought to provide the "boost" we all experience after eating chocolate.

But chocolate gives us more than a mere boost; most people crave the sense of euphoria that lingers long after the treat is gone. A recently discovered trio of chemicals has been identified in chocolate that seems to be at the heart of this feeling of well-being that is familiar to all chocoholics. Anandamide is a chemical messenger in the brain that binds to the same nerve cell receptors that are activated by tetrahydrocannabinol (THC)-that's right, the active compound in marijuana. Anandamide, it turns out, is released in small quant.i.ties during times of stress and provides a calming and a.n.a.lgesic effect; however, it is quickly broken down by naturally produced enzymes, so there is never very much of the substance in the brain under normal circ.u.mstances.The buzz one gets from marijuana is another story altogether-in this case a deluge of THC enters the brain, overwhelming the ability of the enzymes to break it down, so it has a prolonged and more intense effect than the naturally occurring version. The "THC buzz" is essentially an exaggeration or amplification of normal cannibinoid brain system functioning.

The chocolate buzz occurs through a slightly different mechanism. Small amounts of anandamide are present in chocolate (darker chocolates have larger quant.i.ties), but not so much that would activate the brain's cannibinoid system above normal. The key to unraveling this mystery came when two additional anandamide-like compounds were identified in chocolate and found to be present in fairly large quant.i.ties. While these related compounds don't activate THC receptors directly, they increase the effect of naturally occurring anandamide by blocking the enzymes that usually break it down.This means that even small amounts of naturally occurring anandamide or that ingested while eating chocolate will stay in the brain for a prolonged period of time, since it is not metabolized as quickly as normal.The result is that blissed-out feeling of calm that we experience after downing a hot chocolate or going through a few Droste pastilles.

It is easy to see why the depressed and stressed among us self-medicate with chocolate. It quenches the pleasure instinct by activating three key brain transmitter systems that are involved in reward, although they have evolved as adaptations to very different environmental circ.u.mstances.

The sucrose in chocolate is just a "souped-up" version of fructose-a form of sugar that is naturally present in most fruits that were widely available to early hominid hunter-gatherers. Sugars are a critical component of life because they provide metabolic energy in the form of ATP that powers the many biochemical reactions within every cell of our body. For the average hunter-gatherer, fruits were a very good nutritional choice, since ounce for ounce they offer a rich source of energy with virtually no exposure to dangerous horns, teeth, or claws. The only problem is in identifying fruits as a desirable substance to eat.

Early in our hominid evolution, the brain opioid system became very important for controlling our eating behavior, mainly in functioning to make sure that certain foods seemed more palatable than others. During this point in the evolutionary history of humans, opioid system activation and the pleasurable sensations that result became a.s.sociated with the consumption of foods that have relatively high concentrations of sugar. This a.s.sociation was strictly in terms of alterations in brain wiring-some hominids evolved changes in their opioid system that made its indirect activation possible through receptors that were sensitive to the presence of sugar. In a very real sense, the opioid system, which until then probably played a role mainly in s.e.xual reproduction, was co-opted by selection factors that made it very cost-effective (energywise) for hominids to be able to find and want to eat sugar-rich fruits. Hominids with a tendency to experience pleasure when eating something sweetened by natural sugar had a clear survival advantage over their peers who were not "afflicted" with this important mutation to opioid system wiring. Similar mutations may have occurred within the dopamine reward system and the cannibinoid system, making the taste of chocolate a "triple threat."

Sugar and Health Modern societies have very different survival pressures, and hence selection factors, than those of early hominids. Not only do we have access to all the fruits we want, we have also perfected the packaging and delivery of refined sugar such as sucrose in a staggering variety of forms that include processed foods and candies. Our opioid systems are awash in a sea of sweet-tasting stimulants, and this has serious consequences for societal health.

Study after study has shown that in humans, the most palatable foods release the highest levels of beta-endorphins into our bloodstream. Endorphins are the brain's natural opioids that are typically released during stress. When this system is rendered inactive by administering an opioid antagonist (drugs that bind to opioid receptors but do not activate them) such as naltrexone or naloxone, subjects report that foods taste less palatable and food consumption is often substantially reduced. Thus there is very clear evidence that the opioid system is involved in the hedonic experience of food.

In the past decade research has shown that obese people often have a different opioid system response compared with non.o.bese individuals. At least two independent studies have found that obese subjects produce up to three times as much beta-endorphin in their blood plasma after consuming a palatable meal when compared to their skinnier counterparts. One interpretation of this finding is that some individuals may be predisposed to obesity because they have hyperactivated opioid systems and literally experience more intense pleasure in response to opioid-system-activating foods than others. It is currently unknown, however, whether this change in opioid system functioning is a cause or a result of obesity.

The opioid system also plays an important role in attachment behaviors. In mouse pups, the response to being separated from their mother consists of ultrasonic vocalizations accompanied by a brief period of hyperactivity until the two are reunited. Mice that have their opioid system blocked by chemical agents or through genetic manipulations fail to display the same plaintive calls as normal mice. They do, however, protest to other events such as sudden changes in temperature or the introduction of an adult male. Hence, attachment behaviors depend on opioid system activation.

The fact that attachment behaviors seem to involve the opioid system is interesting when one considers that breast milk is rich in lactose, a sugar that serves to stimulate the activation of this system. Newborns and infants are innately attracted to the smell (see chapter 5) and taste of breast milk, and they have an uncanny ability to identify milk from their own mother. By the end of the first week of life, newborns prefer the taste of their mother's breast milk over cow's milk. There are many compounds in mother's milk that may account for this preference. Besides being sweetened with the sugar lactose, it is rich in essential fatty acids that we will see are sought out by virtually all humans, from newborns to adults. Additionally, mother's milk contains a number of important immune factors and whole immune cells (one reason that synthesizing human breast milk has not been possible for manufacturers of baby formula) that may be critical in helping the infant identify and develop a preference for its own mother's milk over milk from an unrelated lactating mother.

Hence, newborns that are allowed to breast-feed will naturally self-stimulate their own opioid system, which itself may be a necessary component for the development of normal attachment. The timing of this sequence of behaviors is important, since the ingestion of lactose occurs coincidentally with other forms of stimulation that are known to activate the opioid system, such as the sensation of being touched and the familiar smell of Mom. All of these stimuli activate the opioid system at a highly opportune time for the development of maternal-offspring attachment-during feeding behavior.A biological mechanism such as this, which uses the experience of pleasure to prod newborns toward behaviors that at once maximize both attachment and the intake of nutrition, is likely to have tremendous survival value.

Getting Wired for Taste Modern science has identified five basic food tastes: sweet, sour, bitter, salty, and the latest entry, umami, which is caused by the presence of monosodium glutamate (MSG). The first step in the path toward taste perception begins with the humble taste bud. Under an electron microscope, a taste bud has a shrublike appearance-think rhododendron-that is shaped by forty or so elongated epithelial cells. Each epithelial cell has receptors that preferentially respond to the presence of compounds affiliated with one of the five taste groups. As I eat my lunch of stir-fried vegetables, the natural sugars in the carrots and snow peas that pa.s.s by the approximately five thousand taste buds that line the perimeter of my tongue will activate groups of cells that are most sensitive to sweet-tasting food; salt-sensitive cells will become activated in response to the presence of sodium and pota.s.sium in the veggies and sauce added for seasoning; still other cells will be excited by the MSG.

The collective ensemble of activated cells sends this taste information on to the next stage of processing in the medulla and other nearby brain-stem structures that control the automatic behaviors involved in feeding such as sucking, salivation, and swallowing. From the brain stem, this signal makes its way to the thalamus, and finally branches out to cortical gustatory areas where the conscious perception of taste occurs and to various limbic nuclei where taste information can be integrated with memory, emotions, and motivation centers that regulate our desire to eat.

From the basic physiology and anatomy of the two chemical senses, we know that taste and smell are processed in very different ways. Epithelial cells in the olfactory mucosa respond to thousands of different types of odorants, while those that make up the gustatory system seem to have evolved a preference for basically five dominant taste cla.s.ses. Besides this difference, however, there are remarkable similarities in the evolution of these systems in the human species and their development in the individual.

By the beginning of the second trimester of pregnancy-just about the time when Melissa was regaining her desire to eat and morning sickness was making a thankful exit-Kai's taste buds were beginning to mature. It is probably no coincidence that his first real sucking and swallowing behaviors also started at about this time, since the continued development of his taste buds, and most importantly their anatomical connection into functional taste circuitry in the brain-stem, depend on stimulation. The brain-stem sites mature very early as well and will provide Kai with a complete set of reflexive movement patterns for getting the nutrition he needs-everything from sucking and swallowing behavior to changes in facial expression in response to sweet versus bitter tastes. But it is unlikely that fetuses can consciously perceive tastes at this point in their development, since cortical taste sites are not yet mature. Anencephalic newborns lack most of their cerebral cortex, yet they are capable of the same behaviors. These include tongue protrusions to reject bitter-tasting liquids, and salivation in response to sweets, even though detailed investigations show that these infants have no genuine awareness of such tastes.

Comparisons across a wide range of mammalian species has shown that the taste circuitry that projects from the epithelial cells to brainstem sites is highly conserved across very different animals, and hence is likely to have evolved rather early in the evolutionary lineage of hominids. Just as the conscious perception of taste and its integration with brain systems that regulate pleasure are likely to be relatively newer adaptations built on existing brain-stem circuitry, so it is for the developing fetus, who fails to show signs of real taste preferences until about the third trimester, when brain-stem connections to cortical and limbic regions are complete.

At this point in his development, Kai is experiencing all sorts of tastes-sweets, sours, bitters, you name it-all of which are incorporated into the amniotic fluid through Mom's diet. Even before my son is born, he has a sweet tooth.Although he tends to move most in the late evening hours, his fetal gymnastics can be brought on at any time of the day if his mother indulges in a bowl of Haagen-Dazs's Dulce de Leche. And this is not unusual. Before the days of ultrasound, X-ray contrasts were commonly used to a.s.sess fetus health in the final trimester. Studies performed during this time show that fetuses increase their swallowing behavior and movements if a sweet solution such as saccharine is injected into the amniotic fluid, while they decrease their swallowing if a bitter or noxious-tasting substance is injected. These results are consistent with the idea that taste perception and preferences emerge during this developmental period.

Well before Kai has any exposure to the outside world, he is already establis.h.i.+ng taste preferences that will form a lifetime of eating habits. Evidence from both animal and human research indicates that taste variety is remarkably important during this stage of development. For instance, rats born to mothers who have had their salt intake curtailed during the final stages of gestation lack the ability to perceive the substance after birth. Likewise, rats born to mothers who consume diets rich in particular tastes such as apple juice or alcohol show an enhanced preference for the taste after birth when compared to rats born from mothers with a normal diet. Both of these forms of experience-expectant learning also occur in humans. Finally, it is important to note that a very general relations.h.i.+p appears to exist between the experience of taste variety in the womb and acceptance of novel foods after birth. Newborn rats and humans exposed to an increased variety of tastes in utero typically show less fear of novel foods when compared to newborns from mothers who had a more restricted diet.

Much like we saw with smell, although newborns have an innate preference for specific tastes such as sweets and certain fats, they also exhibit an impressive potential for developing novel taste preferences based on what was experienced in the womb. Moreover, these experiments demonstrate that the development of normal taste perception depends critically on experiencing a wide variety of tastes while in the womb, since limited exposure to a taste cla.s.s (for example, salts) can result in a reduced ability to detect and perceive these tastes after birth.

Survival of the Fattest So far we've seen that humans find the consumption of sweets innately pleasurable, and that the evolution of this tendency can be traced to the evolutionary pressure to identify and desire desire high-energy foods (such as fruits and mother's milk) that are rich in natural sugars and relatively plentiful and safe to consume. But what about fats? Why do humans have such an insatiable appet.i.te for fatty foods? high-energy foods (such as fruits and mother's milk) that are rich in natural sugars and relatively plentiful and safe to consume. But what about fats? Why do humans have such an insatiable appet.i.te for fatty foods?

Although many of the ancient Greeks, including Aristotle, considered fat a basic taste cla.s.s, it has only been in the past few years that food scientists and psychologists are willing to accept the idea that fat has a specific taste. Previously, most scientists believed that fat only acted as a food texture or flavor carrier. But this has changed with the discovery that simply putting a fatty food such as cream cheese into your mouth raises blood serum levels of triacylglycerol (TAG), an indicator of blood fat loading, even if the food is never swallowed. Richard Mattes, a food scientist at Purdue University, and his students followed up on this original study by showing that blocking the subjects' ability to smell the cream cheese has no effect on the outcome, suggesting that it is the taste component of a fat that produces this change in blood TAG levels.

These findings are probably no surprise to researchers such as physiologist Adam Drewnowski, who in the early 1980s showed that subjects' rating of the pleasantness of a food is directly related to the relative proportions of sucrose and fat in the samples tested. We all love foods that are laden with sugar, but there is a limit beyond which we find a food to be too sweet. Hedonic preference ratings first rise and then typically decline with increasing sucrose concentration in these experimental studies. This is not, however, what happens with fatty foods. Surprisingly, hedonic preference ratings typically continue to rise with increases in dietary fat content. It is possible, then, that our innate fondness for fats is even more intense than for sweets. And this makes perfect sense from both evolutionary and developmental perspectives.

Let's start with evolution. In the past 2.5 million years, the hominid lineage leading to humans has evolved significantly larger brains relative to body size when compared to other primates. Understanding the reason for this dramatic expansion has been a long-standing question for those concerned with human evolution. Many theories argue that brain expansion followed the development of some key cognitive or behavioral milestone such as the emergence of bipedalism or language or social group formation or toolmaking, and so on. The list is long and varied, but the question remains: Did these new functional capacities result from or cause the dramatic increase in hominid brain size?

Michael Crawford of the Inst.i.tute for Brain Chemistry and Human Nutrition in London has argued that hominid brain expansion is the direct result of dietary s.h.i.+fts that accompanied the migration of h.o.m.o sapiens h.o.m.o sapiens from the open savannas to freshwater and salt.w.a.ter sh.o.r.eline regions, predominantly in the East African Rift Valley some 250,000 years ago. Human babies have combined brain and body fat that accounts for a whopping 22 to 28 percent of their total body weight, a finding that is not seen in any other terrestrial animals. Fats are an indispensable component for building brains.The very foundation of life-the cell membrane-is made from a double layer of lipids that protects and s.h.i.+elds the internal organs of the cell while at the same time permitting the perfect amount of elasticity so the cell can respond to physical changes in the extracellular environment. In human babies, a high level of dietary fat is critical for normal brain development because it provides energy for growth in the form of fatty acids found in triglycerides; contains important chemical precursors to ketone bodies that regulate brain lipid synthesis; and provides a store of long-chain polyunsaturated fatty acids, most notably docosahexaenoic acid (DHA) and arachidonic acid (AA), which are essential for the formation of retinas and synaptic junctions where brain cells communicate. from the open savannas to freshwater and salt.w.a.ter sh.o.r.eline regions, predominantly in the East African Rift Valley some 250,000 years ago. Human babies have combined brain and body fat that accounts for a whopping 22 to 28 percent of their total body weight, a finding that is not seen in any other terrestrial animals. Fats are an indispensable component for building brains.The very foundation of life-the cell membrane-is made from a double layer of lipids that protects and s.h.i.+elds the internal organs of the cell while at the same time permitting the perfect amount of elasticity so the cell can respond to physical changes in the extracellular environment. In human babies, a high level of dietary fat is critical for normal brain development because it provides energy for growth in the form of fatty acids found in triglycerides; contains important chemical precursors to ketone bodies that regulate brain lipid synthesis; and provides a store of long-chain polyunsaturated fatty acids, most notably docosahexaenoic acid (DHA) and arachidonic acid (AA), which are essential for the formation of retinas and synaptic junctions where brain cells communicate.

Both DHA and AA are present in abundance in human milk but noticeably absent in cow's milk. Recognizing the importance of these fatty acids for human brain growth and development, many formula manufacturers have begun supplementing their existing recipes with DHA and AA. Human body fat contains more DHA and AA at birth than at any other time during life, and in the newborn approximately 75 percent of its total energy expenditure goes to brain growth. Hence the fatty acids DHA and AA are important for brain development because they serve as an energy supply to fuel cell growth and proliferation, and because they have a molecular structure that is a unique component for building synapses.

Michael Crawford and his colleagues have suggested that since the natural supply of DHA and AA in human newborns is only enough to last the first three months of life or so, the continued supply of these fatty acids must occur through the child's diet. This means that the availability of foods that are natural sources of DHA and AA is a rate-limiting factor on human brain development and would naturally restrict the expansion of hominid brain size throughout evolutionary history. So where do you find rich veins of DHA and AA? Both are part of the larger omega family of fats whose synthesis requires the presence of two essential fatty acids that are not manufactured by the body, and consequently must be obtained through the diet. Alpha-linolenic acid (ALA) is the foundation of the omega-3 family of fatty acids that your body uses to make DHA, and linoleic acid (LA) is the foundation of the omega-6 family that is used to make AA. Both substances emerged in response to evolutionary pressures in plants to efficiently store and access energy reserves. Photoplankton, algae, and green leaves synthesize ALA in their chloroplasts, while flowering, seed-bearing plants store lipids in the form of seed oils loaded with LA.

Crawford's group has argued that the evolution of the hominid brain to the human form we know today would have been impossible unless early h.o.m.o sapiens h.o.m.o sapiens incorporated large amounts of both ALA and LA into their daily diet. They suggest that the most dramatic increase in hominid brain expansion co-occurred with the migration of incorporated large amounts of both ALA and LA into their daily diet. They suggest that the most dramatic increase in hominid brain expansion co-occurred with the migration of h.o.m.o sapiens h.o.m.o sapiens to sh.o.r.eline environments and lacustrine estuaries, where dietary ALA and LA were plentiful. to sh.o.r.eline environments and lacustrine estuaries, where dietary ALA and LA were plentiful.

Whether or not Crawford's hypothesis is correct, two things are absolutely clear: growing brains require significant amounts of both ALA and LA, and these critical ingredients must be obtained through diet. ALA and LA deficiency in animals and humans results in altered structure and function of brain cell membranes and can lead to severe cerebral abnormalities. These anatomical changes have been linked to a number of disorders. For instance, both ALA and LA are involved in the prevention of some aspects of cardiovascular disease (including cerebral vascularization), and reduced levels of the fatty acids have recently been attributed as a cause of stroke, visual deficits, and several neuropsychiatric disorders, including depression, presenile dementia, and most notably Alzheimer's disease. Another study showed that ALA deficiency decreases the perception of pleasure by directly altering the efficacy of sensory organs and by creating abnormal changes in frontal cortex anatomy.

Taken together, these findings provide compelling evidence that many selection factors were operating to ensure that early h.o.m.o sapiens h.o.m.o sapiens with a taste for foods containing ALA or LA would have a survival advantage over their peers who lacked this preference. Nature has solved this problem by giving us not just a "sweet tooth," but also an appet.i.te for fats. Given these findings and the results of Drewnowski's experiments in the early 1980s, it is possible that we may find eating fats even more pleasurable than sweets. with a taste for foods containing ALA or LA would have a survival advantage over their peers who lacked this preference. Nature has solved this problem by giving us not just a "sweet tooth," but also an appet.i.te for fats. Given these findings and the results of Drewnowski's experiments in the early 1980s, it is possible that we may find eating fats even more pleasurable than sweets.

It is known that humans and other animals can discriminate among different dietary fats and have a preference for corn oil, which can be used as a positive reinforcer in conditioning experiments. Corn oil has three major fatty acid components: linoleic acid (52%), oleic acid (31%), and palmitic acid (13%). Recent experiments in rats have shown that LA has an important effect on the physiological responses of the epithelial taste cells that make up taste buds. It appears that when LA binds to these cells, it increases the strength of the electrical signal that they normally send to the brain-stem in response to a food source. For instance, if LA and sucrose are consumed together, the combined signal sent from the taste bud that announces the arrival of food is stronger than would be the case with sucrose alone. This physiological response has a marked impact on food intake regulation. In a series of behavioral experiments, psychologist David Pittman and his students at Wofford College found that in rats, LA acts to increase the intensity of sweet, salty, and sour tastes such that the natural preference or avoidance of each is enhanced.As predicted from the physiological findings, animals preferred the taste of a solution containing LA and sucrose together more so than a solution with sucrose alone. Likewise, when Pittman's rats were given a mixture of LA with salt or citric acid, they consumed less than when the salt or citric acid solution was offered alone.

Linoleic acid is present in a variety of natural vegetable oils, and since it has a direct effect on the physiological responsiveness of epithelial cells, it is likely to be one of several compounds that give fats their pleasurable taste. The fact that LA can be used as a positive reinforcing stimulus in conditioning tasks tells us that humans and animals are motivated to consume foods that contain the substance. Hence, the pleasure we find in eating fats may serve to ensure that enough essential fatty acids are included in our typical diet to promote and maintain normal brain growth and development. At the same time, this pleasure-mediated mechanism provides yet another example of how modern food manufacturing technology, in proliferating the availability of refined sugars and fats, has essentially removed the selection factors that originally led to these important adaptations. In doing so, we are a society vulnerable to a number of disorders, such as obesity and diabetes, that emerge when the pleasure we receive from eating certain foods is filled well beyond the natural limits imposed by the environmental circ.u.mstances of younger hominids.

Chapter 7.

The Evolution of the Lullaby Is it not strange that sheep's guts should hale souls out of men's bodies?

-William Shakespeare, Much Ado about Nothing

As neither the enjoyment nor the capacity of producing musical notes are faculties of the least use to man in reference to daily habits of life, they must be ranked among the most mysterious with which he is endowed.

-Charles Darwin, The Descent of Man

Several years ago I volunteered at a behavioral clinic and worked with a group of fourteen adolescents who were diagnosed with attention-deficit-hyperactivity disorder (ADHD). It was an amazing thing to see such large differences in symptoms among teens with the same diagnosis and even within the same individual from day to day. We had our share of students who were chock full of what most would consider normal childhood energy, along with others who were clearly different in that their activity levels seemed unending in practically every context.

At our clinical staff meeting one Monday morning, a new intern raised the possibility of adding music therapy to our group sessions. While some of us, I'm sure, were picturing a guitar and perhaps a few harmonicas, she went on to tell us about her friend who teaches African drumming. She argued pa.s.sionately that kids with ADHD respond well to drum sessions because they promote group cooperation and turn-taking.Within two weeks, we had fifteen beautiful instruments-ten kpanlogos kpanlogos, with their warm, earthy ba.s.s tones, and five djembes djembes, offering a high, snappy timbre.

The drums came complete with a colorful music therapist fresh out of UC Berkeley, who visited us every Thursday afternoon. Joachin began a typical drum session by gathering all the students into a circle and starting a very simple rhythm consisting of one beat sounded roughly every second-a "heartbeat." In the first few weeks, just getting all fourteen students to sit in their chairs at the same time was a genuine accomplishment, but by the end of the first month they began to look forward to each session, and the changes in our little community were palpable. At some point during the fifth session, I remember feeling a deep sense of pride at how fast our circle had joined into a synchronized beat that particular day.

Joachin gradually introduced more complicated rhythms that were to be played on top of the heartbeat. Each student had a rhythm to maintain in concert with the entire circle-some had to play menjani menjani, while others had to play aconcon aconcon or something else, and this varied from session to session.Above all, there was no room for solo performances in the circle, and whenever the group's overall rhythm broke down, we would start over amid sighs of frustration.Toward the end of the third month a noticeable improvement in our collective sound became obvious.The sessions began to take on a true communal feel, and at times the group's two or three main rhythms were so synchronized that the sound became almost hypnotic. During these periods I often lost myself in the moment, imagining the millions of other drum circles that have played across time and culture-humans of all kinds joining and celebrating nature's periodicity. One minute I'm drumming with students in a therapeutic residence, the next I'm part of a tribal clan of early hominids living along the Rift Valley. Perhaps we're drumming in preparation for a hunt or to celebrate the arrival of a newborn or a marriage. or something else, and this varied from session to session.Above all, there was no room for solo performances in the circle, and whenever the group's overall rhythm broke down, we would start over amid sighs of frustration.Toward the end of the third month a noticeable improvement in our collective sound became obvious.The sessions began to take on a true communal feel, and at times the group's two or three main rhythms were so synchronized that the sound became almost hypnotic. During these periods I often lost myself in the moment, imagining the millions of other drum circles that have played across time and culture-humans of all kinds joining and celebrating nature's periodicity. One minute I'm drumming with students in a therapeutic residence, the next I'm part of a tribal clan of early hominids living along the Rift Valley. Perhaps we're drumming in preparation for a hunt or to celebrate the arrival of a newborn or a marriage.

Ancient drums have been discovered in almost every part of the world. Their earliest appearance in the archaeological record dates back to about 6000 B.C., excavated from Neolithic Era sites in northern Africa, the Middle East, and South America. Ceremonial drums have been found in these regions, along with wall markings depicting their use in various aspects of social and religious life. Other percussive and even flute-like instruments have been unearthed at h.o.m.o sapiens h.o.m.o sapiens sites throughout Europe and Asia dating as far back as a hundred thousand years. And the music wasn't limited to modern humans-it appears that Neanderthals made music as well. Archaeologists excavating a cave near Idrija in northwestern Slovenia recently found a bear's polished thighbone with four artificial holes drilled into it that were aligned in a straight line on one side. Although we don't have any way of knowing if this sixty-thousand-year-old object was ever used to make sounds or even music, very similar "bone flutes" have been discovered at sites throughout Europe and Asia dating as far back as a hundred thousand years. And the music wasn't limited to modern humans-it appears that Neanderthals made music as well. Archaeologists excavating a cave near Idrija in northwestern Slovenia recently found a bear's polished thighbone with four artificial holes drilled into it that were aligned in a straight line on one side. Although we don't have any way of knowing if this sixty-thousand-year-old object was ever used to make sounds or even music, very similar "bone flutes" have been discovered at h.o.m.o sapiens h.o.m.o sapiens sites and estimated to be forty thousand to eighty thousand years old. sites and estimated to be forty thousand to eighty thousand years old.

The rule is very simple in modern cultures across the world and throughout all of recorded history: wherever there are humans, there is music. No recorded human culture-whether extinct or extant-has ever been without music production. Although what pa.s.sed for a melody in ancient China undoubtedly differs from, say, what a twenty-first-century European might find entertaining, all humans have a faculty for producing and enjoying music. Indeed, given the omnipresence of music production and enjoyment across human civilizations, some researchers consider musicality to be an evolutionary adaptation, perhaps akin to language. But unlike language, which is used to communicate our thoughts to others, music has no clear-cut survival or reproductive consequences. So the question remains:What is the adaptive function of music?

There are generally three schools of thought on the origins of music. The first group views music as an interesting, albeit evolutionarily irrelevant artifact of our sophisticated brains-a form of "auditory cheesecake." The basic idea is that humans evolved a set of sophisticated cognitive, motor, and perceptual skills that have clear survival and/or reproductive value, and the expression of these skills led naturally to the emergence of other by-product abilities, such as art appreciation and musicality. The cheesecake view is overwhelmingly the most popular in mainstream psychological thinking today.

A second view is that music has real survival value and has been forged by the same principles of natural selection that have shaped other cognitive abilities such as binocularity, color vision, sound localization, and so forth. A wide range of suggestions for the function of music has been made, most having to do with its ability to bond the social group through coordinating action and ritual. Undoubtedly, music can have a profound influence on the behaviors and emotions of large groups of people-if you need to be convinced, simply visit a local nightclub or ballpark. This view, however, has its problems because it depends on the rather untenable position of invoking group selection to account for the evolution of musicality-a mechanism that has never proven convincing to scholars of mammalian evolution.

Finally, a third school of thought argues that music evolved primarily through mechanisms of s.e.xual selection rather than natural selection. The chief difference between the two, of course, is that natural selection fosters adaptations that increase an organism's likelihood of survival, while s.e.xual selection fosters adaptations that increase the likelihood of successful mating and reproduction. Both mechanisms impact the ultimate scorecard of evolution-how well an organism pa.s.ses on its genes-yet the adaptations that emerge can often be at odds with one another. For instance, the size and color vibrancy of the peac.o.c.k's tail is an important variable in reproductive success. Peahens are attracted to males with the largest and most colorful displays. At the same time, however, this conspicuousness puts "handsome" peac.o.c.ks at a survival disadvantage from a natural selection viewpoint because they are easier to spot by predators, and vibrant, c.u.mbersome tails make them less able to evade an attack. Indeed, adaptations driven by s.e.xual selection often emerge because because they handicap an organism's survival in some way that makes it easier to a.s.sess its true fitness. In this example, the fitness cost of having a large and colorful tail makes the peac.o.c.k an easy target. Those males who have the most conspicuous tails are truly the fittest-the thinking goes-because they can afford both the metabolic cost of growing a large tail and the survival cost a.s.sociated with attracting the attention of predators. In this context, music is seen as just another ornate animal display designed to get the attention of the opposite s.e.x. The proponents of this view argue that music production is a reliable fitness indicator because it signals an ability to maintain a high degree of skill at the cost of diverting energy, attention, and time away from basic survival behaviors. they handicap an organism's survival in some way that makes it easier to a.s.sess its true fitness. In this example, the fitness cost of having a large and colorful tail makes the peac.o.c.k an easy target. Those males who have the most conspicuous tails are truly the fittest-the thinking goes-because they can afford both the metabolic cost of growing a large tail and the survival cost a.s.sociated with attracting the attention of predators. In this context, music is seen as just another ornate animal display designed to get the attention of the opposite s.e.x. The proponents of this view argue that music production is a reliable fitness indicator because it signals an ability to maintain a high degree of skill at the cost of diverting energy, attention, and time away from basic survival behaviors.

Each of these perspectives on the origins of human musicality is based on distinct mechanisms and therefore has unique implications for our relations.h.i.+p with music and why we find it pleasurable. In this chapter I will offer a fourth perspective: that our attraction to music results from a developmental requirement that we experience distinct cla.s.ses of auditory stimulation for normal brain growth and maturation throughout life but particularly during the first two decades. We will find that there are innate constraints on musical sensitivity that transcend cultural differences and provide a core set of features common to all styles and genres. These features have a great deal in common with the singsong of motherese and will offer clues as to why we find pleasure in music as well as many other types of acoustic experiences.

A Universal Grammar Music is said to be the universal language, but exactly what properties, if any, can be found that transcend culture, geography, and time? Most people prefer the musical genre they grew up with, and even the most casual observer must concede that there is tremendous variation in style from generation to generation. Clearly, learning plays a large role in shaping the specific musical idioms we prefer. Research throughout the past decade, however, has begun to show that certain sounds and note combinations have virtually universal effect on the emotions of listeners independent of the culture in which they were born, raised, and live. Moreover, most neurologically normal listeners, no matter where they are from, can agree on what is and is not musical, even when the sequence of tones is novel or drawn from a foreign scale. This has led some theorists to focus on the similarities between music and language development when speculating on the origins of musicality.

Decades ago, the linguist Noam Chomsky set out to understand why all normal children spontaneously speak and understand complex language. He pointed out that all mature speakers of a language can generate and interpret an infinite number of sentences, despite great variation in their levels of formal education. Moreover, in any given language, most native speakers can agree on whether a sentence seems grammatical. Since most speakers have these abilities despite varying levels of formal linguistic training, Chomsky argued that we are all born with an innate knowledge of language. The instinctual set of rules we unconsciously use to make grammatical judgments as well as to produce and interpret sentences is called the universal grammar. Chomsky argued that linguistic development involves the fine-tuning of this grammar toward settings appropriate to the indigenous language.

The composer Leonard Bernstein was the first to apply Chomsky's ideas about language to music. He suggested that all the world's musical idioms conform to a universal musical grammar. This theory was advanced more formally through the work of psychologist Ray Jackendoff and musicologist Fred Lerdahl. They viewed music as being built from a hierarchy of mental structures, all superimposed on the same sequence of notes and derived from a common set of rules. The discrete notes are the building blocks of a piece and differ in how stable they feel to a listener. Notes that are unstable induce a feeling of tension, while those that are stable create a sensation of finality or being settled. Musical styles differ in the emphasis placed on beat interval and pitch, but most genres use notes of fixed pitch.

Pitch is related to the frequency of the sound wave's vibration that is emitted by an instrument, but is perceived musically relative to other notes and the interval separating them rather than in any absolute sense. When a guitar string is plucked it vibrates at several frequencies at once: a dominant frequency called the fundamental and integer multiples known as harmonics, which add fullness and timbre. For example, a note with a vibration of 64 times per second will have overtones at 128 cycles per second, 192 cycles per second, 256 cycles per second, and so forth. The lowest frequency-which is often the loudest-determines the pitch we hear. In this example, the fundamental frequency is 64 cycles per second and corresponds to the second C below middle C.

When a sound wave vibrates faster, say at a fundamental frequency of 128 cycles per second, we perceive the tone as being higher. Since the fundamental frequency of this new tone at 128 cycles per second is related to the other tone at 64 cycles per second by an integer multiple (128 = 64 2), it will sound higher but with the same pitch (a middle C).The interval that separates our two example tones at 64 and 128 cycles per second is called an octave.All primates perceive tones separated by an octave as having the same pitch quality. The pentatonic scale, common to most musical idioms across the globe, is built from having five distinct pitches within an octave. Throw in two additional pitches per octave and you have the seven-tone diatonic scale that forms the foundation of all Western music, from Beethoven to the Beatles.

Music is governed by a relatively small set of rules-like language-that can be used to generate an infinite variety of compositions. Music also employs recursion. In the same way that a sentence can be lengthened indefinitely by adding modifiers or additional words, so can a musical piece by inserting new or repeating phrasing. And just as language emerges naturally in children without a need for formal linguistic training, so too does music. Indeed, the only requirement for the development of musicality in babies is exposure to music.

As we have seen in earlier chapters, human newborns are far from being blank slates. With regard to the sensations of touch, motion, smell, and taste, they have clear preferences for certain stimulation patterns that are optimally tuned for regulating brain growth and development. The same is true for hearing. Newborns are attracted to music from birth and are sensitive to acoustic properties that are common to all music systems across cultures. By the time an infant is two months old, it will have roughly the same ability to distinguish pitch and timing differences in musical structure as that of listeners with decades of exposure to music. From the very beginning of life, newborns are attracted to specific features of music that are also preferred by adults the world over.

Babies as young as four months old show a stable preference for music containing consonant rather than dissonant intervals (an interval is a sequence of two tones). They also discriminate two melodies apart more easily if both have a consonant interval structure rather than a dissonant structure. Consonant intervals are those where the pitches (the fundamental frequency) of the const.i.tuent tones are related by small integer ratios. For example, intervals such as the "perfect fifth," with a pitch difference of seven semitones, or the "perfect fourth," with a pitch difference of five semitones, have very small integer ratios of 3:2 and 4:3, respectively. Adult listeners from all cultures find these intervals pleasant-sounding, and babies love them. Both adults and four-month-olds prefer these consonant intervals to dissonant intervals such as the tritone, with a pitch difference of six semitones and a large pitch ratio of 45:32. Infants listen contentedly to melodies composed of consonant intervals but show signs of distress when some of the intervals are replaced by dissonant intervals.This effect has been observed in many cultures and in infants with varying levels of music exposure. Hence it appears to result from an innate predisposition toward certain acoustic features that are pleasurable and indeed seem to be shared by most systems of music.

Another interesting feature of auditory processing that is present in infants is their ability to detect transpositions of diatonic melodies across pitch and tempo. Both infants and adults can recognize a tune based on the diatonic scale as the same when it is transposed across pitch, but fail to do so when the melody comes from a nondiatonic scale. Since all primates perceive tones separated by an equal octave as having the same pitch quality, one might predict that the ability to detect transposition of diatonic melodies is also present in our hairier cousins.To date, this experiment has only been performed with rhesus monkeys and, as expected, they exhibit the same effect as human adults and infants. The presence of similar auditory preferences and perceptual abilities among adult listeners and infants from different cultures suggests that certain features that are critical components of music competence exist at birth.

Eenie-Meenie-Miney-Mo That certain auditory biases exist at birth is probably not news to parents. Even those who are uninitiated to this phenomenon learn quickly that their prelinguistic newborn is a capable communicator. Infants communicate with emotional expression, and parents use this to gauge what their child needs. Few stimuli calm an infant and get their attention more effectively than the lullaby sung in a soothing voice. As we saw in earlier chapters, infants recognize their mother's voice from birth, and are calmed when they hear it. Experiments have shown that newborns and infants are highly sensitive to the prosodic cues of speech, which tend to convey the emotional tone of the message. Prosody is exaggerated even more so in the typical singsong style of motherese that dominates parent-infant dialogue during the first year of life.The infant trains its parents in motherese by responding positively to certain acoustic features they provide over others. Motherese and lullabies have so many acoustic properties in common-such as simple pitch contours, broad pitch range, and syllable repet.i.tion-that theorists have argued them to be of the same music genre.

Just as motherese shows up with the same acoustic properties in virtually every culture, so too does the lullaby. Practically everyone agrees on what is and is not a lullaby. Naive listeners can distinguish foreign lullabies from nonlullabies that stem from the same culture of origin and use the same tempo. Of course, infants make the distinction quite readily. Even neonates prefer the lullaby rendition of a song to the nonlullaby rendition performed by the same singer. Although it is tempting to attribute such preferences to experience, studies have shown that hearing infants raised by deaf parents who communicate only by sign language show comparable biases. It appears, then, that from our very first breath, we carry a set of inborn predispositions that make us seek out specific auditory stimuli. These stimuli are common across cultures and appear in many forms of music but are exemplified in the lullaby. Why should this be the case? One might argue that these acoustic features help foster mother-infant communication, but this just pa.s.ses the question along without really answering it. Why do these specific acoustic properties show up in motherese and the lullaby? They arise because the infant trains his or her parents to provide these stimuli through feedback in the form of emotional expressions of approval and calm.The real question is why these types of auditory experiences pacify and bring pleasure to infants (and adults).

In April 2003, scientists from the University of California at San Francisco discovered that newborn rats fail to develop a normal auditory cortex when reared in an environment that consists of continuous white noise. The hallmark of white noise is that it has no structured sound-every sound wave frequency is represented equally. Neurobiologist Michael Merzenich and his student Edward Chang wanted to understand how the environmental noise that we experience every day influences the development of hearing disorders in children. They speculated that perhaps the increase in noise in urban centers over the past several decades might be responsible for the concomitant increase in language impairment and auditory developmental disorders observed in children over the same period.

Their experiment began by raising rats in an environment of continuous white noise that was loud enough to mask any other sound sources, but not loud enough to produce any peripheral damage to the rats' ears or auditory nerves. After several months, the scientists tested how well the auditory cortex of the rats responded to a variety of sounds. They found significant structural and physiological abnormalities in the auditory cortex of the noise-reared rats when compared to rats raised in a normal acoustic environment. Interestingly, the abnormalities persisted long after the experiment ended, but when the noise-reared rats were later exposed to repet.i.tious and highly structured sounds-such as music-their auditory cortex rewired and they regained most of the anatomical and physiological markers that were observed in normal rats.

This finding created a wave of excitement throughout the scientific community because it clearly showed the importance of experience in influencing normal brain development. The developing auditory cortex of all mammals is an experience-expectant organ, requiring specific acoustic experiences to ensure that it is wired properly. As Chang summarized, "It's like the brain is waiting for some clearly patterned sounds in order to continue its development. And when it finally gets them, it is heavily influenced by them, even when the animal is physically older."

The auditory cortex of rats and humans-indeed, all mammals-progresses through a very specific set of timed developmental changes. As we have seen in the other sensory systems, this development depends on genes to program the overall structure, but requires the organism to experience environmentally relevant stimuli at specific times to fine-tune the system and trigger the continued developmental progression. Genes don't just magically turn on. In most cases they wait for an internal or environmental promoter to trigger their expression. And the details of development are not in the genes but rather in the patterns of gene expression.

The primate auditory system develops a bit differently from the sensory systems of touch, smell, and taste that we have considered thus far. The peripheral anatomical structures of the auditory system begin to form very early in development, yet the system matures rather slowly as a whole. For instance, by the time Kai had been in Melissa's womb for about four weeks, he already had the beginnings of ears on either side of his embryonic head. Cells were also forming in what will become his cochlea, the sh.e.l.l-shaped organs in each ear that transduce acoustic sound waves into electrical impulses that the brain uses to communicate. By about the twenty-fifth week of gestation, Kai had most auditory brain-stem nuclei in place that will be used to process features of acoustic information such as sound localization and pitch discrimination. But these cells depend on stimulation for continued growth, maturation, and being able to form synaptic connections with their higher cortical target sites.

It is probably no surprise to readers by now that it is precisely at this time-when the brain most needs auditory stimulation-that fetuses begin to hear their first sounds. We know this for two reasons. First, it is at this age that fetuses first show signs of what is called an auditory-evoked potential. Preterm babies are given a battery of tests. Among these is a painless test that involves placing a small headphone over their ears and attaching three electrodes to their scalp to measure their brain's response to auditory stimuli. When a brief clicking noise is played, preterm babies younger than about twenty-seven weeks show little or no electrical response following the stimulus-their brain is not mature enough to register the sound, and they show no sign of hearing it. It's not until after twenty-seven weeks or so that preterm infants show the first evidence of a brain response to auditory stimuli, and not so coincidentally, the first signs of actually hearing sounds.

These results are consistent with observations using ultrasound technology to monitor fetal movements in response to tones played on their mother's stomach. At Kai's sixteen-week ultrasound, he showed no response to auditory stimulation in the form of tones played near Melissa's stomach, or either of our voices.The story had changed by his thirty-week ultrasound. Not only did he appear less embryonic, he also altered his movements whenever we made a loud noise.The most reliable change was a complete halt of his ongoing movement when his mother spoke. My paternal observations are consistent with real experiments showing that fetuses start and stop moving in response to auditory stimuli, and even blink their eyes in reaction to loud sounds heard in the womb.

Throughout the last trimester, Kai's brain was taking in sounds and using them to stabilize and fine-tune his developing auditory system.Although many sounds can pa.s.s through to the womb, he was especially sensitive to those that changed with dramatic pitch contours. This is because even fetal brains show adaptation to unchanging stimuli. A tone that is repeatedly played at the same pitch and amplification is responded to fully at first, but becomes less and less interesting over time. This is mirrored by physiological responses measured from the brain such as auditory-evoked potentials. Evoked potentials become smaller and smaller in preterm babies if the same old boring stimulus is played over and over again.The brain simply begins to habituate, and the stimulus becomes less salient.

Continuous and slowly changing sounds-those that exhibit exaggerated pitch contour and wide pitch variation (exactly like those heard in motherese and in lullabies)-keep the baby and its brain in an attendant state. Fetuses show far less behavioral habituation to music that sounds like motherese than to repet.i.tive tones of the same exact pitch. Likewise, preterm infants older than thirty weeks do not exhibit a decline in their auditory-evoked potential if they are stimulated with sounds that change slightly in pitch rather than stay the same. The sounds of motherese and lullabies are born from acoustic features that are the perfect forms of stimulation to ensure that a fetus's experience-expectant brain will continue to develop normal auditory circuitry and perceptual skills that will help it survive after birth.

The auditory system is not the only part of the brain that benefits from sound stimulation. Research has shown that fetuses older than thirty weeks can distinguish different phonemes such as ba ba versus versus bi bi, suggesting that prenatal experience may be critical to the development of language areas.There is also evidence that auditory stimulation while still in the womb promotes the development of limbic structures such as the hippocampus and the amygdala that support memory and emotional development. Indeed, it is now clear that the sounds a fetus hears in its third trimester can be remembered years later and even influence behavior as late as two years after birth. One researcher, for example, found that infants whose mothers watched a particular soap opera during pregnancy were calmed when they heard the show's theme song, whereas babies whose mothers did not watch the show had no reaction to the song.

Now that Kai is finally born, he is awash in a sea of acoustic information, but not all of these sounds are novel. He is certainly familiar with his mother's voice and to a lesser extent my own. Many of the sounds that Melissa experienced in her final trimester were likely heard by Kai, and although most were not repeated enough to consolidate into long-term memories, they undoubtedly had a significant impact on his auditory development thus far. Kai, like all primates, will continue to need auditory stimulation for decades to come. Normal development of auditory circuitry continues well into the late teens, resulting in steady improvement in many functions such as pitch discrimination and sound localization. Mammals that are denied this stimulation suffer from a range of abnormalities. For example, rats that are raised in an acoustic environment with a restricted frequency range are unable to hear outside this range as adults.This impacts their ability to discriminate sounds that have pitch variation that overlaps with this frequency range. Deprivation also disrupts their ability to localize sounds-an impairment that could prove costly if approached by a predator.

The fact that all primates have auditory perceptual skills that are facilitated by diatonic scale structure, while not true for all mammals, gives us a rough idea of when our faculty for music may have emerged in our evolutionary lineage. Some Old World primates may have evolved auditory circuitry that had improved function relative to competing primates-such as increased pitch discrimination and sound localization-that gave them a distinct survival advantage. As we've seen with modern experimental studies, the successful development of this circuitry depended on the organism experiencing certain forms of auditory stimulation. Clearly, not all primate species have satisfied this demand in the same way. In hominids, natural selection has forged this adaptation by linking these optimal forms of auditory stimulation to the activation of evolutionarily ancient pleasure circuits that are seen in all mammals. These circuits were most likely an earlier adaptation that fostered reproduction. Natu