Uniquely human - Curious Folks Ask: 162 Real Answers on Amazing Inventions, Fascinating Products, and Medical Mysteries - Sherry Seethaler

Curious Folks Ask: 162 Real Answers on Amazing Inventions, Fascinating Products, and Medical Mysteries - Sherry Seethaler (2009)

Chapter 7. Uniquely human

Odd eats

Why do humans cook food, and when did they start doing this? Do any other animals modify their food intake?

Early humans probably first realized the value of cooked food when they tasted tubers—root vegetables like potatoes and cassava—that had been roasted by a lightning-sparked grass fire. Not only are cooked tubers more delectable, but heat alters the structure of starches and proteins, making them easier to digest and rendering some poisonous vegetables edible.

How long ago humans were able to control fire is still under dispute. By 250,000 years ago, our ancestors could certainly invite the neighbors over for a barbeque. Across Europe and the Middle East, ancient earthen ovens with burned animal bones date from that time. Some anthropologists argue that humans controlled fire almost 2 million years ago. They point to circular areas of scorched earth almost that old discovered in Africa. These “bonfires” contained a mixture of burned wood types, which suggests that they were deliberately set, rather than the remains of a tree struck by lightning.

Wild and domesticated animals can learn to distinguish a food’s nutritional properties based on its appearance, smell, or taste. Animals modify their diets according to how their nutritional needs change as they mature, during pregnancy and lactation, and as a result of disease. Zoopharmacognosy—self-medication by animals—is a particularly intriguing aspect of diet modification.

Ill animals of many different species consume things that normally are not a part of their diets but that have medicinal properties. These include laxatives, antidiarrheals, antibiotics, antiparasitics, and antidotes to toxins they have previously consumed. For example, wild chimpanzees with parasitic infections eat leaves from a shrub commonly known as bitter leaf. The leaves contain several chemicals that can kill parasites that cause malaria and other tropical infections.

Many animals eat soil—a habit known as geophagy. Soil is a source of minerals. Geophagy is also a form of zoopharmacognosy. Soil containing certain types of clay combats diarrhea. By binding to toxic plant compounds, soil makes some plants safer to eat. Soil also can enhance plants’ pharmacological activities by binding to interfering compounds.

A study showed that zoopharmacognosy requires learning; it is not purely instinctive. In the study, lambs were given food laced with one of three chemicals that cause stomachaches. Then they were given a choice of three medicines, each of which would cure a stomachache caused by only one of the chemicals. Only lambs that had prior experience being cured by the appropriate medicine could select it when given the choice of all three.

Sink like a stone

Humans, as primates, typically learn how to swim when they’re old enough to follow instructions from swimmers. Can any other primates swim instinctively? What other mammal species do not swim instinctively?

I remember being surprised as a child to learn that our cat could swim very well, although he much preferred terra firma. Many land mammals can swim, and they tend to use a similar gait in water as they do on land.

It is difficult to know for sure what mammals cannot swim, since many avoid water if given the choice. However, rats, mice, horses, elephants, camels, bears, antelope, skunks, at least some species of bats, and at least one species of armadillo reportedly can swim.

According to the San Diego Zoo’s Associate Curator of Mammals, Karen Killmar, most monkeys can probably swim. This behavior has not been documented in all species but has been seen in many. In contrast, there are no reports of great apes (gorillas, chimpanzees, orangutans) swimming. They have been observed in the wild wading in deep water, but not actually swimming. Most researchers do not believe that these species have swimming as an instinctive behavior.

A word of caution to pet owners: Some dogs do not like to swim, and they may panic in deep water, especially if a steep bank makes it difficult for them to climb out. And, of course, a strong current can fatigue even the best swimmer, canine or otherwise.

Dino breath

Not considering the millions of years dividing their times on Earth, am I mistaken in thinking that humans could not have survived in the oxygen content of the air during the dinosaur era?

Dinosaurs roamed the Earth from about 230 million years ago to about 65 million years ago. Estimates of the oxygen levels during the dinosaurs’ reign differ greatly, but a 2005 study published in the journal Science found that the atmosphere’s oxygen concentration has increased over the last 205 million years from 10 percent to 21 percent.

Another study found evidence that about 240 million years ago, oxygen concentrations dropped precipitously and rapidly from about 35 percent to about 12 percent. Therefore, based on these two studies, dinosaurs survived in an oxygen concentration as low as about half of today’s levels.

Today, at high or low elevations, 21 percent of the molecules in the air are oxygen, but fewer air molecules (in a specific volume of air) are present at higher elevations. On the Andean and Tibetan plateaus, about 13,000 feet (4 kilometers) above sea level, each breath you take would contain about as many oxygen molecules as it would if taken at sea level back in the time of the earliest dinosaurs.

Humans can survive under these low-oxygen conditions. In fact, some Andean miners live for long periods at nearly 20,000 feet, where there is even less oxygen.

Just because human populations can survive at these lower oxygen concentrations does not mean that oxygen levels had no role in the evolution of mammals. Small mammals coexisted with dinosaurs. However, the Science paper reported that a dramatic increase in the size and diversity of mammals occurred between 100 and 65 million years ago, during a period of relatively high and stable oxygen levels.

The paper’s authors think that the increase in oxygen levels may have facilitated the evolution of large mammals. Larger mammals have fewer blood vessels per unit of muscle than smaller mammals. As a result, larger mammals need higher levels of oxygen in the environment to achieve maximum rates of metabolism.

Dinosaurs may have had lower rates of metabolism, and lower oxygen needs, than mammals. In addition, a group of dinosaurs that includes the brontosaurus is thought to have had a respiratory system similar to that of modern birds, with a series of air sacs that act like bellows to move air through the lungs. The system allows fresh air to flow through the lungs continuously and could have given dinosaurs a survival advantage in low-oxygen conditions.

Modern man

Has the Homo sapiens species stopped evolving due to our scientific progress in overcoming the survival of the fittest, or are we still undergoing small changes that are not easily perceived?

The Homo sapiens species has existed about 200,000 years, but just 10,000 years ago the transition from hunter-gatherer to agricultural societies created significant evolutionary pressures. Specifically, diet changed, and the spread of infectious diseases increased as population densities increased.

By comparing the genomes of various modern individuals, geneticists can determine how quickly our DNA sequences have been changing, and whether the changes are random or result from some sort of evolutionary pressure.

One interesting example is the gene for lactase. Lactase breaks down lactose, the main sugar in milk. A version of the gene that permits adults to digest lactose is prevalent in people of European ancestry and some African populations but is very rare in Southeast Asian and sub-Saharan African populations. The geographic distribution and timing of the gene’s increase in prevalence correspond to the rise of dairy farming.

Other genes that have changed in one or more populations as a result of relatively recent (over the past 10,000 years) evolutionary pressures include genes that play a role in metabolism, taste and smell, fertility, and skin pigmentation.

In developing nations, where AIDS, malaria, and other scourges kill millions every year, genes that provide resistance to disease are under selection pressure. For example, in regions of the world where malaria is or was recently found, certain versions of genes for hemoglobin—the oxygen-carrying protein in the blood—have become prevalent. These versions of the gene provide some resistance to malaria but can cause blood diseases such as sickle cell anemia.

Some scientists have argued that in developed countries, evolutionary pressures have been relaxed to the extent that humans are no longer evolving. However, others argue that we are still evolving, because not everyone makes equal contributions to the next generation. In addition, they predict that changing climate and increasing populations will create renewed evolutionary pressures.

How evolution may impact how future humans look is impossible to predict. Ironically, the most significant changes in our looks compared to those early farmers are not genetic at all. Increases in height can be tied to better nutrition, obesity is linked to diet and sedentary lifestyles, and smaller jaws develop when people eat softer-textured food. Although genetic factors play a role in height, metabolism, and bone structure, these changes have taken place too rapidly to be purely genetic.

World tour

What caused the migration of early humans out of Africa? Did sufficient climatic change occur to make it better to leave than to risk trying to adapt to some sort of environmental change?

Based on genetic and fossil evidence, it is widely accepted that humans originated in Africa. Many researchers recognize two major phases of dispersal. The first, Out of Africa 1, began almost 2 million years ago with Homo erectus, the first truly upright-walking human ancestor. The second, Out of Africa 2, began about 100,000 years ago with Homo sapiens, which evolved in Africa between the two phases of dispersal and eventually replaced archaic humans.

Other researchers consider this view simplistic. They generally agree about the early dispersal, but they propose that multiple later dispersals occurred, some of which may have been from Europe and Asia back to Africa. One reason for the uncertainty is that human fossils are rare compared with stone artifacts, and the cultural traits implied by the tools cannot be reliably tied to biological characteristics of the populations that created them.

What caused humans to disperse is unknown. Some explanations focus on unique features of human culture. Other explanations, acknowledging that other successful species also disperse, focus on environmental changes.

One hypothesis is that the first dispersal was caused by one group of humans outcompeting another. This hypothesis is based on the discovery that two technologically distinct populations of humans existed at the time, and only the population of less advanced toolmakers dispersed from Africa, perhaps because they were at a disadvantage on their shared range. A hypothesis based on advances in toolmaking and competition between groups also has been proposed to explain the second dispersal.

Oscillations from wet to dry conditions occurred around the times of the early and late dispersals. The climatic changes were accompanied by the dispersal of other large animals, which may have been followed and exploited by human populations.

Whatever the reason the dispersals began, a factor that probably facilitated them by enhancing humans’ survival is the reduction in zoonotic diseases. These are diseases, such as sleeping sickness, that typically rely on animals for transmission but also affect humans. Zoonotic diseases are especially prevalent in parts of Africa, compared to cooler, drier climates away from the tropics.

Tree house

Why did early man come down from the trees, when he had no protection on the ground from predators such as lions and tigers?

The dominant view among researchers who study human evolution has been that, beginning about 5 million years ago, ground-dwelling humans who could exploit large herds of game on the African savannah arose from tree-dwelling, vegetarian apes. Some maintain that key human traits, including our upright walk and big brains, evolved because of the challenges of life on the open savannah.

Clearly the greater risk of predation and fierce competition for prey are problems with this hypothesis. If it is accurate, the transition from trees to savannah was probably gradual and fueled by climate change. The transition is thought to have taken place at a time when the African continent was becoming more arid, which would have resulted in the fragmentation of forested areas. Hominins—bipedal primates—would have been forced to spend more time on the ground moving between wooded areas and would have needed to exploit resources available in grasslands.

Other researchers make the case that humans could not control large plains until they domesticated riding animals—horses in Asia and camels in more arid regions—and that occurred within the past 10,000 years. Dissatisfaction with the savannah-dwelling hypothesis has recently led to the proposal of two alternatives: the aquarboreal hypothesis and the tectonic hypothesis.

According to the aquarboreal hypothesis, the transition from trees to ground occurred in coastal forests, where hominins could gather wetland plants and shellfish. As forests became more fragmented, hominins later dispersed along coastal areas and rivers. A beachcomber phase that included diving could explain humans’ excellent voluntary breath control, subcutaneous fat layer, and lack of fur. Such traits are unique among primates but are found in dolphins, hippos, and walruses.

According to the tectonic hypothesis, hominins evolved and expanded in the African Rift, which extends from north to south along eastern Africa. The rift is a rugged terrain formed by volcanic activity and plate tectonics—movement and deformation of the Earth’s crust. Within this complex topography, agile bipeds could gain a tactical advantage over faster-moving quadruped prey and find protection from predators. Humans may have dispersed from Africa into Europe and Asia along a virtually continuous line of tectonically active terrain, as suggested by the locations of the earliest and best-documented sites of human occupation outside of Africa.


Many different animals, such as birds, have hundreds of different species. Why aren’t there tens or hundreds of different species of humans?

In Bones, Stones and Molecules (2004), authors David Cameron and Colin Groves comment that researchers who work with the fossil record consider the present era, which has only one Homo species, as a unique time in the history of our lineage. In other words, multiple species of humans probably coexisted at various times in human history.

Based on the known fossil data, most scientists propose that archaic humans first dispersed from Africa approximately 1.8 million years ago. Populations settled in different regions and evolved independently.

Modern humans (Homo sapiens) probably emerged in Africa between 250,000 and 150,000 years ago. They later dispersed, and by 40,000 years ago, Homo sapiens occupied most parts of Africa, Asia, Europe, and Australia. At that time, the Neanderthals (Homo neanderthalensis) still occupied parts of Europe and Asia. Homo erectus likely still existed in Indonesia (although the fossil evidence is a bit sketchy).

What happened next has been the subject of much speculation. One hypothesis is that modern humans clashed violently with the indigenous human populations they encountered, eventually eliminating them. Another hypothesis is that some interbreeding occurred, and that, for example, we each have a little Neanderthal in us. Finally, it may be that modern humans were simply more successful at competing for the available resources, and the other human species just died out.

The fossil record is not complete enough to tell us what happened to all early humans. The greatest amount of information is known about the extinction of the Neanderthals, 27,000 years ago. Evidence to suggest that Homo sapiensengaged in mass genocide of Homo neanderthalensis is lacking. Similarly, the available DNA evidence suggests that interbreeding between modern humans and Neanderthals was uncommon—at least, we did not inherit Neanderthal genes.

It is most likely that modern humans drove Neanderthals to extinction by outcompeting them. Modern humans seem to have hunted and gathered over larger areas than the Neanderthals, who tended to remain in the valley systems they had long occupied. Therefore, modern humans were more efficient at exploiting the environment for limited resources. Homo erectus also seems to have become extinct at the same time Homo sapiens appeared in their region, likely also as a result of competition for limited resources.

Living link?

On the Science Channel, I saw Oliver, the “questionable chimpanzee.” I remember seeing him on the news in the past. I was always intrigued with his upright walk, strong manlike shoulders, and wise eyes. I wondered about his relatives. I found out that Oliver had 47 chromosomes, while chimpanzees have 48 chromosomes. Humans have 46 chromosomes. Does that make Oliver a “link” between a chimp and a human?

Starting in the 1970s, Oliver was promoted as a missing link or “humanzee” because of his unusual physical and behavioral traits, as well as the rumor that he had 47 rather than 48 chromosomes. According to reports, his arms and legs were too long, his ears a funny shape, his head too bald, and his face too small for him to be a chimpanzee. He also walked with a locked-knee bipedal (two-legged) gait.

Primatologists who examined Oliver pointed out that chimps vary widely in their physical characteristics. In addition, most of Oliver’s teeth were pulled when he was very young to prevent him from biting people. As a result, the muscles in his lower face and temples, and even the bones in his jaws, remained underdeveloped. His training could account for his bipedal gait.

Despite these explanations, doubts about his karyotype—chromosome number and characteristics—remained. An early genetic test attributed to unidentified “American scholars” depicted 47 chromosomes followed by a question mark and was exploited by Oliver’s owners to promote him as a missing link.

The karyotype mystery was not solved definitively until 1998, when Oliver was moved to a sanctuary. The results of genetic tests published that year in the American Journal of Physical Anthropology showed that Oliver had 48 chromosomes and that his DNA sequence was highly similar to that of the Central African variety of chimpanzee. The researchers narrowed Oliver’s probable birthplace to Gabon by comparing Oliver’s DNA sequence to DNA sequences from other chimpanzees of known origin.

Even if Oliver had 47 chromosomes, this would not make him a missing link. During the production of egg and sperm cells, a process called meiosis separates chromosome pairs and reduces the number of chromosomes by half so that an offspring gets one chromosome of each pair from each parent. Errors can occur during meiosis and result in offspring with more or fewer than the normal chromosome number. For example, people with Down syndrome, caused by an extra 21st chromosome, and those with Klinefelter’s syndrome, caused by an extra X chromosome, have 47 chromosomes.

Tears for fears

Why is it that when you are about to cry, a lump forms in your throat?

Our bodies instinctively interpret negative emotions such as anger, sorrow, and fear as stress. In the face of a stressful situation, our nervous systems switch from “rest-and-digest” mode to “fight-or-flight” mode. This response is a relic from our pre-civilization days, when stress usually was a result of life-threatening situations.

The switch between modes is a function of the autonomic nervous system (ANS). Because its actions are mostly out of our conscious control, the ANS is also referred to as the involuntary nervous system. It is composed of the sympathetic nervous system, which activates the fight-or-flight response, and the parasympathetic nervous system, which controls opposite but complementary actions to promote recuperation and restart regular body maintenance activities.

To prepare the body to confront or run from danger, the sympathetic nervous system stimulates the adrenal glands to produce adrenalin, dilates the pupils, increases heart rate and blood pressure, and diverts blood from the intestines to make it available to the muscles in the limbs. The cessation of digestion can result in the nausea that often accompanies sorrow.

The sympathetic nervous system also increases air intake into the lungs. To allow more air to enter the lungs, the throat must open. The opening is relatively small during normal breathing. In response to the stress that initiates crying, the glottis—the gap between the vocal cords and the associated muscles in the throat—expands as wide as possible.

In contrast, during the act of swallowing, the airway needs to be closed to keep out food and liquids. As food is pushed down the esophagus, the upper portion of the airway is lifted by the muscles at the back of the throat, the glottis constricts, and the epiglottis—the flap of cartilage lying just beneath the base of the tongue—is closed over the glottis.

The sensation of a lump in the throat is a result of the glottis muscles being told to open and close at the same time. In other words, the glottis is caught in a tug-of-war. The feeling is usually relatively short-lived, but some people under stress experience it for weeks or months. The sensation is called globus syndrome or globus hystericus if medical tests rule out injury or disease as a cause.

Blind dreams

Do people who are born blind dream? If so, what do they see in their dreams?

For most people, dreaming is an intensely visual experience. Visual imagery is nearly always present in the dreams of sighted people, and their dreams are usually in color. Auditory sensations occur in more than half of their dreams, but only a small percentage involve taste, smell, and touch.

Blind people who lost their vision after about age 5 continue to have visual imagery in their dreams. They see new friends, places, and things in their dreams, not simply memories they’ve retained since before they lost their sight. The fact that dreams do not reflect people’s current visual impairment reveals that the dreaming brain does not simply reproduce perceptions (albeit in a new narrative), but actually constructs things that have never been experienced while awake.

In contrast to people who lost their sight later in life, people completely blind since birth (congenitally blind) or shortly thereafter lack the rapid eye movements usually associated with dreaming. However, they do dream, and they tend to describe their dreams in the same visual language as a sighted person. When asked to elaborate, it becomes clear that the “pictures” in their heads have been created through prior experience with other senses. In dream reports made by congenitally blind people, more than half of the sensory references are to touch, smell, and taste. The rest of the sensory references are auditory.

In one study, a congenitally blind woman described a dream about sitting at a table in a nice restaurant. She knew she was at a table through kinesthetic sense—the sense mediated by feedback from organs in the muscles and tendons. She knew it was a nice restaurant because she felt the thick carpet and heard the quiet atmosphere. She had an image of the table because she had previously felt tables. Similarly, a congenitally blind woman in another study described being in a room with a device that looked like an ATM. She said she knew what it looked like from prior experience touching the buttons on an ATM.

Studies that have compared the composition, organization, and themes of the dreams of blind people and sighted people have found few differences. Blind people reported more dreams in which they had a misfortune occur during locomotion or transportation, and dreams about their guide dogs, consistent with the notion that there is continuity between dream content and waking experiences.


We are all aware of the increase in violence in our society. Is the increasing amount of electronic noise in the atmosphere possibly interfering with brain waves?

About two decades ago, descriptions of a new illness started showing up in the medical literature. It was actually a diverse collection of symptoms: skin problems, dizziness, headaches, fatigue, muscle pain, nausea, problems with concentration and memory, depression, and nervousness.

More people became convinced that their symptoms were being caused by “electromagnetic pollution.” They believed that they were affected, while others working under the same conditions were not, because they were particularly susceptible to environmental electromagnetic fields. Hence, the mysterious illness was dubbed “electromagnetic sensitivity syndrome.”

Electromagnetic fields are nothing new. Light is a form of electromagnetic radiation, as are the radio waves that bring us our favorite tunes and talk shows and the microwaves that heat up our frozen dinners. However, advancing technologies and rising demand for electricity have steadily increased our exposure to electromagnetic fields.

In response to concerns about the possible health effects of exposure to electromagnetic fields, the World Health Organization (WHO) launched the International EMF Project in 1996 to review studies on the health effects of exposure to these fields.

One concern is the possibility that low-frequency electromagnetic fields could generate currents within the human body. After all, our heartbeat, communication between nerve cells, and the chemical processes that keep our cells alive involve the movement of charged particles. While large electromagnetic fields could stimulate nerves or affect other biological processes, the WHO concludes that the fields we encounter are too small to produce these effects.

An additional concern is that exposure to radiofrequency fields, especially from cell phones, could cause heating of the brain. Even a small amount of heating by radiofrequency fields has been shown to affect brain activity and behavior in animals. However, the majority of scientists consider the fields produced by cell phones too small to heat the brain.

In laboratory studies it has also been difficult to show that the symptoms of electromagnetic sensitivity syndrome have anything to do with exposure to electromagnetic fields. Therefore, our exposure to these fields does not seem to have any significant effects on health or behavior, including violence, but research is ongoing. The WHO acknowledges that if long-term exposure to these fields negatively impacts a small number of people but is harmless to everyone else, the effects would be tricky to detect.

Sentiment sites

What are emotions? Do they reside in specific parts of our brain, or are they the result of our cognitive thinking? Are emotions genetic, or are they learned? Do animals have emotions?

A fully accepted theory of emotion is lacking, but research has provided tantalizing insights. Our experience tells us that emotions have bodily manifestations such as changes in heart rate, irregular breathing, increased or decreased blood flow to the skin or digestive tract, sweating, and trembling. However, it is not quite as simple as saying that because individuals have a certain set of physiological responses, they experience a particular emotion. Cognition also plays an important role.

When provided with an alternative explanation for their physiological responses, people appear to rationalize what they are feeling, rather than automatically experiencing it as an emotion. In one study, people were given an injection of saltwater and either were told that a side effect of the “vitamin shot” was trembling and a pounding heart or were not told anything. The individuals who had been told about the side effect reported experiencing less intense emotions when placed in an anger-provoking or amusing situation.

Several brain regions play a role in our experience of emotions. The hypothalamus controls the autonomic nervous system, which regulates our physiological responses. The amygdala draws our attention to dangers in the environment that might require an emotional response. The hippocampus is involved in learning and memory, including emotional memories. The cortex helps us choose the most appropriate response in an emotional situation.

Babies can express emotions from a very young age, and expressions of emotion are remarkably cross-cultural. In the late 1960s, researchers discovered that members of a tribe in a remote area of Papua New Guinea, who had never been exposed to Western culture, could accurately interpret facial expressions in photographs of Westerners. And Westerners accurately interpreted the expressions of tribe members. Not all aspects of emotional expression are inborn; cultural rules define when such expressions are appropriate. Which situations evoke emotions is also partly learned.

In 1872, Charles Darwin published The Expression of Emotion in Man and Animals, in which he suggested that human expressions of emotion evolved from similar expressions in other animals. One recent study found that chimpanzees shown videos of emotional scenes (a veterinarian pursuing chimps, a chimp getting a treat) could correctly choose a photograph of another chimp expressing the emotion that the scene would evoke. Of course, although other animals produce and interpret what appear to be expressions of emotion, we cannot determine if their subjective experience of emotion is the same as ours.

Feeling groovy

I have read that the cause of pleasure is dopamine in the brain. I have also read that serotonin is the feel-good chemical, and runner’s high is said to be due to endorphins in the brain. Which is the main cause of pleasure, or do they all interact?

The neurotransmitters dopamine, serotonin, and endorphins are three of several chemical languages known to play a role in feelings of pleasure and well-being. Nerve cells chatter in more than 100 different dialects, and future research will likely implicate more of these in our brains’ pleasure conversations.

The discovery of reward circuitry in the brain dates to the 1950s, when researchers made a surprising discovery while investigating the effects of electrical brain stimulation on rats’ ability to learn. When electrodes are implanted in a certain region of the brain, rats will press a lever to the point of exhaustion to self-administer electrical stimulation. People given electrical stimulation in the analogous brain region say the experience is intensely pleasurable.

At the core of this reward circuit are nerve cells that originate near the base of the brain, in the ventral tegmental area. They send projections toward the nucleus accumbens, a structure deep beneath the front of the brain. Dopamine is the main neurotransmitter at these connections. Nerve cells using a variety of neurotransmitters connect the reward circuit with brain regions involved in memory and emotion, which influence the reward response.

This system ensures that an organism eats, drinks, and engages in other adaptive behaviors. Addictive drugs hijack it. For example, heroin makes nerve cells churn out more dopamine. Cocaine inhibits the reuptake of dopamine by the nerve cells that release it, preventing dopamine chatter from being quickly silenced.

In depression, a preeminent factor is the reduced activity of nerve cells that communicate using serotonin. Selective serotonin reuptake inhibitors (SSRIs) are the medication of choice for many depressed people.

Unfortunately, the rate of remission with SSRIs is less than 50 percent, and multiple neurotransmitter systems and multiple brain regions have been implicated in depression. Among them are dopamine and the brain’s reward circuitry, consistent with one of the many symptoms of depression, anhedonia—the inability to experience pleasure.

Avid runners talk about the euphoric state they get from long-distance running. Often they cannot abstain from running, even when they are injured. The addictive aspect of running appears to be due to the brain’s natural opium—endorphins—which, during strenuous exercise, is released into a region of the brain that controls mood.

Mad genius

It seems that many famous artists and writers have suffered from bouts of madness. Is there a relationship between creativity and mental illness, or does it just seem that way because odd or tragic characters are more likely to be remembered?

The idea that madness and creativity are linked goes back to antiquity, but it is not without controversy. Some schools of psychological thought consider creativity to be linked with sound mental health. Today, the prevailing view is that creative genius and some mental disorders are linked, but not necessarily directly.

Three sources of evidence have been mined to determine the relationship between mental disorders and creativity. First, historical data, especially biographies of renowned creators, have been analyzed for indications of symptoms associated with various psychopathologies. Second, psychiatric research has examined the incidence of diagnosed mental disorders and treatment in samples of contemporary creators. Third, psychometric studies—standard personality questionnaires—have compared creative and noncreative individuals.

Conclusions from the three types of studies are consistent. People who are highly creative are more likely to have certain mental disorders, especially depression, than otherwise comparable, less creative individuals. The prevalence and intensity of the symptoms varies among different domains of creativity. For people working in the creative arts, the lifetime prevalence of depression is 50 percent, compared to between 20 and 30 percent for people in business, scientists, and important social figures. Within the creative arts, writers of poetry and fiction and visual artists are most likely to suffer from depression.

Because the defining symptoms of depression include lack of interest and energy and difficulty concentrating, it is paradoxical that depression is associated with creative behavior. Indeed, depression does not appear to be the cause of creative productivity. During a depressive episode, creativity is not enhanced, and mood stabilizers have been found to increase, rather than diminish, productivity.

Instead, studies suggest that a personality trait, self-reflective rumination—conscious, recurring thoughts focused on one’s inner feelings—may be the explanation for the paradox. The tendency to ruminate has been shown to increase vulnerability to depression. Rumination has also been shown to enhance creative ability and interest. In other words, depression and creativity happen to be linked because a third factor causes both.

The role of rumination could also explain the lower prevalence of depression among scientific creators versus artistic creators. Original thinking is important in the arts and sciences. In contrast, introspection is less useful for providing ideas that could advance science than it is for providing original content for poetry and other artistic endeavors.

Out of body

I have had three out-of-body experiences. Is there a relationship between the alternate-universe theory and out-of-body, or are they independent of each other?

Despite their New Age mystique and association with certain substances consumed by hippies, out-of-body experiences are reported by many people, particularly those who suffer from migraines and neurological conditions. Studies of out-of-body experiences have recently been published in highly respected journals, including Science and Nature.

These studies do not tie the phenomenon to alternate universes, though. The existence of multiple, parallel universes is predicted by the complicated mathematics of quantum physics. Even if these alternate universes exist, physicists say it is impossible to access or even perceive them.

Instead, the study of out-of-body experiences is the purview of psychologists and neuroscientists. Out-of-body experiences have been accidentally induced in patients undergoing focused electrical stimulation of the brain during epilepsy treatment. For example, one patient described an instantaneous feeling of lightness and the sensation that she was floating above the bed during the electrical stimulation of a region of her brain called the angular gyrus.

The angular gyrus is on the surface of the brain, toward the rear, and it is a region that receives input about vision, hearing, and touch. The angular gyrus is also close to the vestibular cortex, which processes sensory information to maintain the sense of balance. The brain stimulation research suggests that out-of-body experiences may be caused by the dissociation of information coming simultaneously from two or more senses.

This hypothesis is supported by recent studies that used head-mounted video displays to give people visual information that placed them in a different location. The visual information on its own did not give people the feeling of being outside their bodies. But when they saw their virtual body being touched at the same time as their real body was being touched, they felt as if the virtual body was their own body. Then, when a hammer was swung so that it appeared to hit the virtual body, measurements of skin conductance—a measurement of stress—indicated that the hammer was registered as a threat, even though it posed no real danger.

The studies show that information from the senses can modify the brain’s representation of the physical body. In addition to shedding light on out-of body experiences, the research provides insight into consciousness, since the feeling of being within one’s physical body is a foundation of the concept of self.

Musical mind

Your out-of-body experience answer must have been reassuring to anyone who’s had the experience and was disbelieved when they described the event. I’m writing about a different kind of “out-of-body” experience—music playing in one’s head. In addition to a variety of music, I also hear noisy motors, a distant train, or nonmusical drumming. Have you come across any information on this kind of auditory hallucination?

Everyone has had an earworm—a snippet of a song playing repeatedly in their head like a broken record. Someone’s ringtone, a visit to Disneyland, or even the mere mention of an annoying tune (I promise I’ll refrain) is enough to set it off. What distinguishes a musical hallucination from an earworm is that a musical hallucination appears to originate from outside your head.

People are often afraid to admit that they are hearing things for fear they will be thought of as mentally ill. But auditory hallucinations seem to be fairly common in mentally sound people who have ear problems. In one study, researchers interviewed 32 people who had lost hearing in both ears and discovered that all had experienced musical hallucinations.

These hallucinations are a form of tinnitus—usually a more generic buzzing or ringing in the ears. Changes in fluid levels in the inner ear (with or without hearing loss), as occur in Ménière’s disease, can cause auditory hallucinations in some people, as can many drugs, including alcohol, blood pressure medication, and even aspirin.

Musical hallucinations are considered to be analogous to Charles Bonnet syndrome, in which visually impaired people see things that are not there, and phantom limb syndrome, in which amputees have sensations that seem to be in their missing limb. The common link among these syndromes is sensory deprivation. According to one explanation (Release Theory), normal sensory input suppresses the nerve circuits in which sensory memories are stored. When these circuits are no longer inhibited, previously recorded perceptions are “released” and re-experienced.

Depending on the cause, musical hallucinations can be reduced with a hearing aid (if hearing loss is involved), controlling the body’s retention of fluids (if changes in fluid volume in the inner ear are involved), or changing medications in consultation with a physician. One case report described a woman who could “think” down the volume of her musical hallucinations.

Perhaps it is even possible to harness the hallucinations for good. Some researchers believe that musicians are predisposed to musical hallucinations. Famous composers, including Beethoven and Schumann, experienced them.