The enzymes in snot may help to change the chemical makeup of odors that enter the nose. Credit: ptaxa/iStock

Snot is often what shows up after a hard sneeze. It’s a constant companion of allergies and the common cold. It’s wet, sticky and — to most people — best left up the nose.

But snot, or mucus, also contains many different kinds of proteins. Those proteins may play an important role in something else that happens in the nose: smelling. In a recent study, researchers from Japan’s University of Tokyo showed that proteins in mucus change the makeup of odors before those scents even make it to smell receptors. Smell receptors are also proteins. They stick out from the cells that send signals about a smell to the brain, which identifies the odor.

That means that sticky, wet, gross mucus might have a more glamorous role: It may be important for smelling smells.

It seems natural to assume a connection between smells and snot. After all, the human nose is home to the sense of smell — and is an exit for snot. But “most people and most scientists pay no attention at all to mucus,” neuroscientist Leslie Vosshall told Science News. Vosshall is at Rockefeller University in New York City and was not involved in the recent study.

Scientists suspect that some molecules in mucus carry smells to other parts of the nose, where they can be detected. Other molecules in snot are enzymes, which start chemical reactions. Some enzymes may protect the body by chopping toxic substances — such as inhaled chemicals — into smaller, safer chunks. But until now, scientists did not know whether this chopping action could affect the smell of something.

To learn about smells and mucus, the researchers experimented on mice. They removed mucus from the noses of mice. Then, they mixed in chemicals that have particular odors. One of these chemicals was benzaldehyde, also known as artificial almond oil. After five minutes in mouse snot, the benzaldehyde had broken down into two chemicals — one that had no smell and another that did.

When the researchers inactivated the enzymes, by boiling the mucus, and then tried the same experiment again, the benzaldehyde did not break down.

That part of the experiment showed that the mucus could change the chemical composition of odors. Next, the researchers showed that the mice brains also register this difference. For this part of the project, the scientists “turned off” the mucus chemicals in the mice noses that usually chop up odorous molecules. As a result of this change, the mouse brains reacted differently than they did before — showing that their brains had picked up on the change.

The researchers also used mouse behavior to show that mucus changes the smell of something. For this part of the experiment, they used mice that had been trained to identify certain smells. (In training, the mice had been given treats when they went to those smells. After training, the mice naturally went back to those smells, hoping for more treats.) When the scientists turned off the important molecules in the mouse mucus, the mice were unable to recognize those favorite smells.

Scientists don’t know whether the molecules in mucus work the same way in people. Human mucus does have many of the same proteins as the mucus in mouse noses, so it’s worth investigating. Early studies do suggest that human snot can change odors, so stay tuned. And cover your nose when you sneeze.

POWER WORDS (adapted from the Yahoo! Kids Dictionary)

olfactory Of, relating to, or contributing to the sense of smell.

proteins Fundamental components of all living cells, including many substances, such as enzymes, hormones and antibodies, that are necessary for the proper functioning of an organism.

molecule A group of like or of different atoms held together by chemical forces.

enzyme Any of numerous proteins produced by living organisms that function as biochemical catalysts.

bacteria Single-celled microorganisms that vary in terms of morphology, oxygen and nutritional requirements, and motility. They may be free-living, saprophytic, or pathogenic in plants or animals.

In a new study of visual abilities, researchers asked volunteers to identify the biggest orange circle. Here, each orange circle on the right is a little bit larger than the one on the left. Misleading images usually fooled adults but not children, while

Can you believe your eyes? A recent experiment suggests that the answer to that question may depend on your age.

In the experiment, kids and adults were asked to look at the same visual illusion — a picture that was designed to trick the viewer. The researchers who ran the experiment say that adults were more easily fooled by the illusion, and that the kids, especially those younger than age 7, saw the picture more accurately.

Martin Doherty, a psychologist at the University of Stirling in Scotland, led the team of scientists. A psychologist is a scientist who studies behavior and processes in the brain and may offer counseling to patients. Doherty says that his experiment can tell scientists something about how the human brain develops. In particular, the experiment shows that what the brain does to “see” visual context is a process that develops slowly.

The words “visual context” refer to how a person sees something in relation to the things around it. A baseball may look large when next to a golf ball, for example, but appear small when next to a basketball.

In this experiment, Doherty and his team tested the perception of the participants using pictures of solid orange circles. The researchers showed the same pictures to two groups of people. The first group included 151 children ages 4 to 10, and the second group included 24 adults of ages 18 to 25.

The first group of pictures showed two circles alone on a white background. One of the circles was larger than the other, and the participants were asked to identify the larger one. Four-year-olds identified the correct circle 79 percent of the time. Adults identified the correct circle 95 percent of the time.

Next, both groups were shown a picture where the orange circles, again of different sizes, were surrounded by gray circles. Here’s where the illusion came in — remember the baseballs, golf balls and basketballs.

If an orange circle is surrounded by smaller gray circles, then it appears larger than it really is. If an orange circle is surrounded by larger gray circles, then it appears smaller than it really is.

But the experiments added a twist: In some of the pictures, the smaller orange circle was surrounded by even smaller gray circles — making the orange circle appear larger than the other orange circle, which was the real larger one. And the larger orange circle was surrounded by even bigger gray circles — so it appeared to be smaller than the real smaller orange circle.

When young children ages 4 to 6 looked at these tricky pictures, they weren’t fooled — they were still able to find the bigger circle with roughly the same accuracy as before. Older children and adults, on the other hand, did not do as well. Older children often identified the smaller circle as the larger one, and adults got it wrong most of the time.

“When visual context is misleading, adults literally see the world less accurately than they did as children,” Doherty told Science News.

As children get older, Doherty said, their brains may develop the ability to perceive visual context. In other words, they will begin to process the whole picture at once: the tricky gray circles, as well as the orange circle in the middle. As a result, they’re more likely to fall for this kind of visual trick.

Doherty is not the first scientist to study visual context in children, and earlier studies have found that children, just like adults, can be fooled by illusion. Carl Granrud is a psychologist at the University of Northern Colorado in Greeley. He told Science News that Doherty’s findings seem sound, but that they were “somewhat surprising.” He pointed out that in other visual illusion tests, children were fooled, suggesting they had developed the ability to see visual context.

This experiment shows that sometimes, in order to get a sneak peek inside the brain, you have to try to trick it — and see what happens.

POWER WORDS (from the Yahoo! Kids Dictionary)

psychology The science that deals with mental processes and with behavior

optical illusion An image seen with the eyes that is deceptive or misleading.

perceive To become aware of directly through any of the senses, especially through sight or hearing.

context The part of an image that surrounds a particular part of the image and determines its meaning.

Going Deeper:

Studies with mice have shown how the same cells on the tongue that taste sourness also detect fizzy flavors.

What does fizz taste like? In bubbly beverages like soda or champagne, tiny bubbles give the drink a lift — and have a distinct taste. Scientists have long wondered how we taste these bubbles. In a new study on mice, scientists have connected that fizzy-taste sensation to the ability to taste sourness.

Scientists previously thought the taste of bubbles comes from the bubbles bursting on the tongue — but that idea may have to change, says Charles Zuker. A neuroscientist, or a scientist who studies the brain and nervous system, Zuker is now at Columbia University in New York. He and his team of researchers studied the nervous systems of mice to understand how the tongue tastes carbon dioxide, which is the gas that makes up the bubbles.

In the experiment, five different groups of mice were genetically engineered to be missing one taste sensation. (“Genetically engineered” means the researchers were able to turn off the switches for certain tastes by altering the responsible genes.) The mice in one group were bred so that they could not taste sweet. In another group, the mice could not taste sour. In the other three groups, the mice could not taste umami, or salty or bitter.

When the scientists gave carbon dioxide gas to the mice, the nervous systems of the rodents in four groups responded to carbon dioxide. But for mice that could not taste sour, their nervous systems did not show any sign of tasting carbon dioxide.

This tipped off the researchers to the connection between sourness and bubbles. When the scientists turned off the sour taste in the mice genes, they also turned off the ability to taste carbon dioxide.

The scientists then zoomed in on the sour taste. Animals like mice or human beings are able to detect different tastes by using taste buds, located near the surface of the tongue. A taste bud is a group of 50 to 150 cells called taste receptors. (Under a microscope, this bundle of cells looks a little like a big bunch of bananas.) The tips of the taste receptor cells pick up tastes in the mouth, and then send that information to the brain.

When they studied the cells that detect sourness, Zuker and his colleagues found a protein, attached to the sour-sensing cells, that is crucial to the process of tasting carbon dioxide. When carbon dioxide comes into contact with this protein, the protein knocks off particles called protons. These protons, in turn, stimulate the sour cells.

So when a mouse — or person — drinks a fizzy drink, there’s a one-two punch. First, the protein knocks off protons. Second, the protons stimulate the sour-sensing cells —and the brain says, “Hey! That’s a taste!”

That may seem like a lot of work to get from a can of soda to a taste — but the science of the senses is anything but simple. Taste “is a very challenging system to study,” Alexander Bachmanov, a scientist at the Monell Chemical Senses Center in Philadelphia, told Science News. “Everything is very small but very complex.”

POWER WORDS (adapted from the Yahoo! Kids Dictionary)

nervous system The system of cells, tissues and organs that regulates the body’s responses to internal and external stimuli. In vertebrates it consists of the brain, spinal cord, nerves, ganglia and parts of the receptor and effector organs.

neuroscience Any of the sciences, such as neuroanatomy and neurobiology, that deal with the nervous system.

proteins Molecules that contain carbon, hydrogen, oxygen, nitrogen and usually sulfur. Proteins are fundamental components of all living cells and include many substances that are necessary for the proper functioning of an organism.

carbon dioxide A colorless, odorless, incombustible gas formed during respiration, combustion and organic decomposition and used in food refrigeration, carbonated beverages, inert atmospheres, fire extinguishers and aerosols.

Going Deeper:

Some people with synesthesia always see the letters in the alphabet as a certain color. The color of each letter is always the same.

The number “6” is a bright shade of pink. Listening to a cello smells like chocolate. And eating a slice of pizza creates a tickling sensation on the back of your neck.

If you have experiences like this, you may be one of the special people with an unusual sensory condition called synesthesia (pronounced sin-uhs-THEE-zha).

People with synesthesia experience a “blending” of their senses when they see, smell, taste, touch or hear. Such people have specially wired brains, so that when something stimulates one of the five senses, another sense also responds. This blending can cause people to see sound, smell colors or taste shapes.

Dozens of different sensory combinations exist. In the most common form of synesthesia, numbers, letters or even days of the week appear in their own distinct color.

If you’ve encountered these types of events, you’re not alone. Scientists say as many as one in every 200 people may be a synesthetes, as people with this condition are called. The phenomenon is known to run in families, and may occur more often among women than men. Many famous people have had synesthesia, including Russian writer Vladimir Nabokov and physicist Richard Feynman.

One thing is certain; most synesthetes treasure their unusual ability to take in the world with an additional sense. After all, who wouldn’t want to experience the world in full, glorious color or sound?

“It’s absolutely a positive experience,” says Patricia Lynn Duffy, a synesthete who has talked to hundreds of others with the condition while writing a book on the subject. “If you proposed to take away someone’s synesthetic ability, I think they would say, ‘No, I like it this way.’’’

When people with certain types of synesthesia look at the image on the left, they can easily detect the six figures facing the opposite way, as shown in the image on the right. To these people, the “S” figures appear in a different color than the “2″ figu

What Color is my “i”?

Most synesthetes learn about their amazing gift by accident. They are surprised to learn that everyone does not experience the world as they do.

Though it may sound strange to many people, Duffy says the experiences are not scary. The people who have synesthesia have always experienced life that way.

“For as long as I could remember, each letter of the alphabet had a different and distinct color. This is just part of the way alphabet letters look to me,” says Duffy. “Until I was 16, I took it for granted that everyone shared those perceptions with me.”

Synesthetes do not actively think about their perceptions — they just happen. Some synesthetes report that they see such colors internally, in “the mind’s eye.” Others, such as Duffy, see their visions projected in front of them, like watching an image on a movie screen.

Scientists know that in synesthesia, those colors are real, not just figments of an active imagination. How? Studies show that the colors synesthetes see are highly specific and consistent over time. If the letter “b” is lime green, it will always be lime green.

Studies done in the mid-1990s showed that synesthesia also can be measured by brain-scanning techniques. For synesthetes who perceive colors when hearing words, a certain part of the brain involved with vision is active in response to sound. That type of activity didn’t occur in non-synesthetes.

Making Connections

So how can the sound of a musical instrument lead to color?

Scientists are still trying to discover exactly how information from the senses merge together in the brain. But this much is known:

Messages gathered from the eyes, ears, mouth, nose and nerves involved in the sense of touch travel to the brain for processing. Much of this sensory processing occurs in an area of the brain called the cortex, the outermost part of the brain that organizes and enables us to respond to the incoming messages.

Information from each of senses is first processed in its own special region. It’s then sent on to “higher” regions in the cortex for further processing. At certain points in the brain, these various senses converge.

One theory is that synesthesia may be caused by “cross-wiring” between areas of the brain that process different sensations, such as color, sound or taste. This theory draws on the fact that children are born with many nerve connections between nearby parts of the brain.

“During our first few years of life, our brain makes more connections than it needs, and then eventually prunes some of those away,” says Edward Hubbard, a post-doctoral researcher at the French National Institute for Health and Medical Research who studies what causes synesthesia.

One thing that may happen in synesthesia, Hubbard says, is that some of these connections don’t get pruned away. If so, then people may see specific colors with particular letters because they have extra connections between the brain areas involved in word and color perception.

Last summer, a group of scientists in the Netherlands found direct evidence of these types of extra connections.

The researchers used a method called DTI to scan the brains of 18 people with synesthesia. They also looked at the brains of 18 non-synesthetes.

Using a kind of magnetic resonance imaging called DTI (an example is shown above) to look at the brains of synesthetes and non-synesthetes, scientists showed that synesthetes who see colored letters have higher levels of white matter in three different br

DTI (which stands for diffusion tensor imaging) measures how water flows in the brain. Within certain brain tissues, or nerve fibers, water flows more freely in one direction than the other. This is especially true in a type of nerve fiber, or axon, that carries messages from brain cell to brain cell. Commonly called “white matter,” these axons connect different parts of the brain to each other.

By measuring the water flow through these tissues, the scientists could measure how many of these axons there were in each brain region. Brain regions that are highly connected will have more white-matter axons.

In synesthetes who saw colored letters, the scientists found higher levels of white matter in three different brain regions. One was in the letter and word region of the brain, known as V4. The other highly connected areas were found in brain regions involved in consciousness — the awareness that you’re thinking, feeling, seeing, hearing or any number of other things your brain enables you to do.

“We have lots of things impinging upon our senses, and some of them become conscious and some of them don’t,” says Hubbard. “Activity in this area might make a person more consciously aware of a synesthetic experience.”

These findings don’t rule out other possible causes of synesthesia, says Hubbard. Still, he is now working to see if this type of “cross-wiring” occurs in other forms of synesthesia. Other scientists are looking to see whether other parts of the brain are also involved in synesthesia.

Hubbard is also developing better ways to identify the various processing regions of the brain. “Everybody’s brain differs a little bit in its exact organization,” he says.

Duffy notes that these variations in nerve connections occur not only in synesthetes, but in all people.

“Everybody develops a neural pattern that’s kind of unique, just like a fingerprint,” she says. “That’s why no two people are seeing the world in exactly the same way.”

Going Deeper:

This image shows a neuron as it responds to an electrical signal. The blue traces the path of the signal as it moves through synapses to the neuron.

Think back to the first time you rode a bike or the last time you had ice cream for dessert. Now, imagine a perfect summer day. What’s going on in your noggin’ that allows you to remember, dream and think?

Lots. And some of the world’s brainiest scientists are conducting experiments/doing research to figure out how it all works.

The human brain is amazing. It lets you remember the way to your friend’s house, and how to pedal your bike to get there. It can conjure up memories of the fish you saw while snorkeling and remind you to feed your goldfish at home. It even controls stuff you don’t have to think about, such as your heart rate, breathing and blinking.

In recent years, brain-imaging techniques such as functional magnetic resonance imaging (fMRI) have allowed scientists to watch the brain in action. Studies using fMRI show how different parts of the brain do different things, says neuroscientist Sam Wang, who studies the brain at Princeton University.

For example, one part of the brain, called the amygdala (ah-MIGG-duh-luh), handles emotional information, and another part of the brain, the prefrontal cortex, makes plans for the future. Yet another brain system, the cerebellum (SEHR-eh-BELL-um), helps control your movements and balance, while the hypothalamus (HI-poh-THAH-luh-muss) works to control your body’s temperature.

The brain contains other systems, too. Your hippocampus (HIP-poh-CAM-pus), for example, has the job of transferring information between short-term and long-term memory.

By working together, these systems let you think, remember, see, hear, smell, taste and touch. The goal of this teamwork is to get you through life.

Though the human brain is sometimes compared to a computer, it’s not one. It’s actually much more complex, Wang says.

Computers, for example, are designed to record everything perfectly. Rather than recording everything, the brain sorts through all the information taken in through the senses and decides what to hold on to. Because the brain does all this pre-sorting, things such as the pattern in your rug or sound of songbirds outside your window don’t constantly distract you.

This illustration shows how the billions of neurons in your brain are linked by a web of connections. Neurons interact through electrical connections similar to those in a computer.

ktsimage/iStockphoto

The human brain can also do things that are in many ways faster and better than what any computer can do. For instance, you brain enables you recognize your friends — just from the way they walk — even from a distance. Computers can’t do that. Nor can a computer tell the difference between a cat and a dog, even though most toddlers can.

Though your brain is not a computer, they do share something in common: Both brains and computers use electrical signals to transmit information.

All fired up

Your brain doesn’t get its electrical energy from a socket in the wall, the way a computer does. Instead, it creates and sends electrical signals through specialized cells called neurons.

A neuron’s axons and dendrites help it to transmit electrical signals. Dendrites bring information to the body of the neuron, and axons take information away from the cell body.

U.S. National Cancer Institute

Neurons look different from other cells. That’s because neurons have long extensions called dendrites and axons. These work like electrical wires to transmit messages from your brain throughout your whole nervous system. Dendrites bring information to the body of the neuron, and axons take information away from the cell body.

Information is passed along throughout the nervous system from neuron to neuron. The region where the information is transferred from one neuron to another is called the synapse. The synapse is actually a small gap located between two neurons. When information is transferred from one neuron to another, chemicals called neurotransmitters are released from the end of one neuron and travel across the synapse to reach the other neuron. There, these chemicals attach to special structures called receptors, which are located on the receiving neuron. This attachment creates a small electrical response within the receiving neuron.

These electrical signals race up and down the dendrites and axons at super speeds — up to several hundred feet per second. That’s fast enough to help you flee from a wild animal, or pull your hand away from a sizzling hot frying pan.

The human brain contains billions of neurons, and each individual neuron may receive information from thousands of other neurons. To keep the mental machinery running smoothly, the neurons specialize in doing certain tasks.

Sensory neurons, for example, carry messages from your eyes, ears and other sensory organs to your brain. They alert your brain when your nose picks up a whiff of cinnamon rolls coming from the kitchen. Motor neurons carry signals from your brain to your muscles and organs, enabling you to walk, talk, breathe and scramble to the kitchen to grab a hot roll.

Other types of neurons in the brain help in building social relationships. Mirror neurons, for example, are specialized cells that help you show empathy and understanding to others. They fire not only when you take action, but also when you watch others take action.

“Mirror neurons are active when I pick up a cup, and are also active when I watch someone else pick up a cup,” Wang says. “If you’ve ever winced when you watched a TV surgeon slice into a patient, you have your mirror neurons to thank.”

Some neurons have very specific tasks. Things and people that you see on a regular basis — your mother, your dog and even your favorite celebrities — all have a group of dedicated neurons that fire specifically in response to them.

By working together, all the various types of neurons help build our thoughts and actions, Wang says. “Thoughts are basically neurons like these acting together, being put together in patterns.”

Hold that thought

So, with all the various neurons racing through the different brain regions, how can a person think straight? Figuring out how the mind gives rise to thoughts, actions and emotions isn’t easy, and scientists are still working to put all the pieces together. Imaging studies such as those using fMRI have provided some clues.

For example, fMRI studies show that the prefrontal cortex acts as a kind of traffic cop, directing signals to and from different brain regions. Information that comes into the brain through eyes travels to the prefrontal cortex before it is distributed to other brain regions for additional processing. The same holds true for information coming from the other senses.

Other fMRI studies show that when people are sitting around just thinking about something, multiple brain regions are activated. When volunteers in a study were asked to imagine that they are looking at something, the parts of their brain that handle visual information lit up. “The same brain regions that are active during direct visual experience are also active by imagining a scene,” Wang says.

Scientists have also found that your memory plays a role in imagining new scenarios. In recent years, researchers have discovered that the brain regions used to store and retrieve memories are activated when envisioning the future. So all those facts and autobiographical data stored in your brain actually help you construct and predict possible future events.

When it comes to learning new information, one thing is certain: Practice makes perfect. When messages travel from neuron to neuron, over and over, the brain creates a connection between the neurons to form a memory. Once this happens, processing and recalling information becomes easier.

This holds true whether you are trying to learn a new language or learn a new dance move, Wang says. “Memory formation requires multiple steps,” he says. “Once an initial idea or motion is laid down, it must be reinforced both by repetitions and recall.”

Allowing time for rest breaks also aids learning. That’s why spacing out your study time works better than trying to cram information all at once. Wang says one possible reason for this is that breaks provide time for information consolidation.

Now that’s something to keep in mind.

Going Deeper:

Identical twins form when a fertilized embryo splits into two embryos early in development. These twins get the same genes from their parents, but such siblings aren’t the same people.

I have a friend who looks just like me. We both have light brown hair that we wear pulled back, often in pigtails. We dress in the same types of sporty clothes. Our glasses have thick rims and a blue tint. We are both journalists, athletes and moms to little kids. Even our husbands look alike, and even they get my friend and me mixed up sometimes. Everywhere we go, strangers ask us if we’re twins.

My friend and I are not even related. But it’s fun to feel like I’m looking in a mirror when I look at her. And the attention we get helps me imagine what life must be like for actual twins.

Being a “pretend twin” is also fun for me because I’ve always wondered what it would be like to have a sibling who seemed just like me, but was actually a different person altogether. And I’m not the only one who is fascinated by these rare pairs. Lots of scientists are, too.

“It’s a unique birth situation,” says Nancy Segal, a psychologist and twin researcher at California State University, Fullerton. She’s the author of two books about twins and a twin herself. “You feel a little bit special.”

Twins offer scientists the perfect opportunity to study what makes people who they are, Segal says. That’s because twins share more in common than ordinary siblings. Yet, twins still end up being different from each other in important ways — both physically and socially.

By probing these similarities and differences, scientists can begin to figure out which qualities we are born with and which ones result from our experiences. In science, these questions lie at the center of a classic debate called “nature vs. nurture.”

“People study twins not because they’re interested in generalizing about twins,” says Matthew McGue, a psychologist at the University of Minnesota, Twin Cities. (The neighboring Minnesota cities of St. Paul and Minneapolis are called the Twin Cities.) Rather, he says, scientists study these people pairs to learn about the human condition.

Fraternal twins, such as the ones shown here, form when two eggs are fertilized and grow together in the womb. These siblings get a different mixture of genes from their mom and dad.

eyecrave/iStockphoto

Seeing double

There are two types of twins.

Identical twins begin life in the womb as a single fertilized egg. The egg begins to grow normally into a single embryo. Then, for unknown reasons, the embryo splits in two. This usually happens during the first two weeks of growth. About nine months later, two babies are born that often look so similar even their parents can have trouble telling them apart.

The second type of twins is called fraternal. These twins develop when two eggs are fertilized. Fraternal twins are much like regular siblings. They just happen to grow together inside their mother. These twins can be boys, girls, or one of each. Identical twins, on the other hand, always belong to the same gender.

What makes twins interesting to scientists is their DNA. This molecule acts like the instruction manual for life. Stretches of DNA are called genes. And genes determine the color of your eyes, how tall you are, which diseases you might be likely to develop and more.

People have billions of cells in their bodies. In every cell, DNA is grouped into 23 pairs of threadlike structures called chromosomes. One chromosome in each pair comes from your mother. The other chromosome in each pair comes from your father.

During reproduction, genes from both parents get scrambled into a new combination of chromosomes in the child. In ordinary siblings and fraternal twins, each person gets a different mixture of genes from mom and dad. That explains why you might have your mother’s mouth and your father’s eyes, while your brother has the opposite combination.

In identical twins, on the other hand, each person gets exactly the same genes from each parent. Genetically, these siblings are like clones of each other.

Ever since the discovery of DNA in the 1950s, scientists have wondered how important genes really are. Do these microscopic snippets determine whether we like sports, are good at art and everything else about us? (That’s the “nature” side of the debate.) Or are our personalities a result of the way we’re raised and the experiences we have? (That’s the “nurture” side.)

Researchers now know that the answer lies somewhere in the middle. Genes (nature) determine our potential. But the environment (nurture) often determines whether genes are turned on or off.

Identical twins illustrate that concept perfectly. If genes, or nature, alone determined everything about us, you’d expect identical twins to be identical in every way. Despite their similar looks, however, twins often prefer different types of music, friends, clothes and more.

Studying these differences can help scientists figure out what makes us all the same, and what makes us all different.

“Twins,” Segal says, “give us a beautiful natural experiment.”

Tracking twins

At the University of Minnesota, Twin Cities, researchers have been tracking twins for more than 30 years. Starting in the 1970s, scientists there started bringing different types of siblings (and their families) into the lab.

This study includes identical twins, fraternal twins and a third group called virtual twins. This last category includes siblings who are genetically unrelated but are the same age and grew up in the same home. One sibling might be adopted for example, while the other is a biological offspring of the parents.

Researchers chose to compare these groups because twins in all three categories share a very similar environment growing up. But they differ in how similar their genes are. So differences among groups reveal how important genes are in different situations.

For their research, the scientists initially collected a variety of information from each family, using questionnaires, DNA samples, brain wave patterns and more. Every few years since the work began, the scientists have followed up with the same families and repeated many of the same tests.

The Minnesota researchers now have information about more than 10,000 people. From this large set of data, the scientists (along with similar researchers elsewhere) have turned up lots of interesting results.

One finding is that, in many ways, identical twins are far more alike than fraternal twins, even when those identical twins are raised apart. And fraternal twins are more alike than virtual twins.

These similarities are true for a large number of personality traits, such as how outgoing people are, how aggressive they are and what types of decisions they tend to make. These results suggest that genes play important roles in determining our personalities.

Identical twins also tend to become more similar to each other with age. That’s probably because as they get older they have more control over what they do and how they live.

“When you’re a baby, your parent or caretaker completely controls your physical and social environment,” McGue says. “As [identical] twins get older, they create more similar environments for themselves when given a choice.”

Similar but different

Twin studies have also given scientists insight into mental illnesses.

Not long ago, people believed that schizophrenia, autism, depression and other mental illnesses resulted from poor parenting or negative experiences. Then twin studies came along to shake up that view.

Scientists found that if one identical twin has schizophrenia, the other twin has a 50 percent chance of developing the disease. (Schizophrenics often hear voices that don’t exist, among other symptoms.) When one fraternal twin has the same illness, on the other hand, the other one has only a 10 percent chance of having it. The rates are even lower for virtual twins. Other mental illnesses show similar trends in twins.

Those statistics suggest that the genes you’re born with might set you up to develop mental illnesses, if environmental conditions trigger those genes into action.

“Twin studies have had an extraordinary impact on mental illness,” McGue says. “Showing that genetic factors are important has been terribly important.”

Now that scientists know genes are important in these cases, researchers can begin to zero in on exactly which genes are involved in setting people up to develop these diseases. The work might eventually help doctors develop better treatments for such illnesses.

At the same time, researchers want to know why sometimes only one twin develops a serious disease. What is it about the environment that pushes certain genes to turn on or off?

One possibility is that what happens in the womb influences how genes end up behaving many years later. This is something scientists are currently investigating. One day, the work might lead to simple but life-changing advice, like what moms should eat when pregnant.

These days, scientists are using twin studies to investigate everything from the complexities of genetics to the reasons why people vote the way they do.

“Twinning is a mystery,” McGue says. “There’s no end to questions.”

TEACHER’S QUESTIONS

Here are questions related to this article.

Most kids already have a ton of energy, but kids who drink a lot of cola often end up even more wired than usual. The soda’s high sugar content is partly to blame, but cola also usually includes an energy-sparking chemical called caffeine.

Drinking caffeinated soda can give kids a burst of hyperactive energy.

iStockphoto.com

Like cola, coffee is full of caffeine. That’s why many adults drink it first thing in the morning to help them wake up. The chemical is also naturally found in tea, chocolate, and hot cocoa. Because people crave the caffeine kick—and may even become addicted to it—food manufacturers add the chemical to many other sodas as well as to energy drinks and snacks.

Parents and teachers usually try to keep kids away from caffeine. But is this chemical actually bad for your health? The answer is complicated.

Good caffeine, bad caffeine

First the plus side. Some studies have shown that caffeine might help people respond to things more quickly and even run longer. Scientists have also recently found evidence that caffeinated coffee and tea can help protect the heart, brain, and other organs from disease.

On the other hand, too much caffeine can make people anxious and unable to sleep. A 2003 survey of more than 200 students in grades seven through nine found that kids who drank a 16-ounce bottle of cola slept less, woke up more often, and felt more tired the next day than kids who drank less caffeine. This is worrisome because sleeping well is an important part of staying healthy (See “Getting Enough Sleep”).

Caffeine can also raise your blood pressure, increase your heart rate, and make you feel more stressed, which may eventually lead to heart disease and other health problems.

“If you feel a lot of pressure at school, caffeine is going to make you feel even more anxious,” says Jim Lane, a psychologist at Duke University Medical Center in Durham, N.C.

Roasted coffee beans, like these, are ground and brewed into steaming cups of coffee. The fragrant beverage is the main source of caffeine for most adults.

Wikipedia

Love it or hate it, caffeine is hard to avoid. Coffee shops crowd city streets and malls. Vending machines offer caffeinated sodas in schools. And even though caffeine-free versions of coffee, tea, and cola are widely available, more than 80 percent of adults consume caffeine regularly in North America, according to a 2004 study, mostly in the form of coffee. And kids today are drinking more and more soda, caffeinated or not.

Some 30 percent of 8-to-13-year-olds surveyed by researchers at the University of Minnesota said that they drink soft drinks every day, according to a study published last year. And more probably would if they could: 95 percent of kids in the survey said they “like” or “strongly like” the taste of soda.

You’re feeling sleepy . . . NOT!

Caffeine works by blocking the effects of a sleep-inducing substance produced by your body called adenosine. The substance accumulates inside you throughout the day.

As adenosine levels rise, you become calm and drowsy. Later, as you sleep, adenosine levels drop. When you wake up, the cycle starts again. By not allowing adenosine to build up, caffeine keeps you feeling fired up—as if you’re ready to face a tiger attack.

Human brains aren’t the only ones that feel the effects of caffeine. These images show how an extreme amount caffeine affects the brain of a tiny creature. In this case, the top picture shows how a spider spins its web before caffeine and after (bottom).

NASA; Wikipedia

Caffeine raises the amount of sugar in your bloodstream, even if there is no sugar in your caffeinated drink. That’s what gives you extra energy. The chemical also increases your blood pressure, which may make you feel as if your chest is pounding. But if you consume too much caffeine, you will probably feel nervous and sick.

Caffeine claims for brains

People say they like caffeine because it makes them feel alert. In experiments, people who are given caffeine say they feel more awake than do people who have been given a caffeinefree pill or beverage instead, says psychologist Peter Rogers of the University of Bristol in England.

The caffeine in cola beverages like this one affects your brain and nervous system in ways that have nothing to do with sugar or other ingredients.

National Cancer Institute/Wikipedia

In other studies, caffeine appears to shorten reaction times: People press a button more quickly after seeing a symbol appear on a computer screen after they’ve had some caffeine.

On the basis of such findings, it’s tempting to conclude that caffeine helps people respond more quickly and pay better attention. However, says Rogers, there is another, more likely, conclusion.

Studies show that the people who do better on tests after taking caffeine tend to be regular caffeine users already. In other words, they are probably addicted to the chemical.

Taking caffeine away from habitual users causes them to have symptoms of withdrawal, such as headaches and sleepiness. It also slows their reaction times. So, when these people are given their daily dose of caffeine, they feel better and perform better on reaction-time tests than they do without it.

Coffee and other caffeinated beverages can be addictive, even for children.

iStockphoto.com

People who aren’t addicted, on the other hand, may feel jittery and more awake after taking caffeine, but the chemical doesn’t improve their performance on reaction-time tests. And regular caffeine users who get caffeine before the tests aren’t any more alert or quicker to react than people who don’t normally use the chemical and haven’t taken any.

Giving athletes a jolt

Caffeine has become popular with exercisers looking for an extra boost of energy. Research shows, however, that caffeine helps only athletes who are already in top condition and only when they are pushing themselves as hard as possible, says Terry Graham, a caffeine researcher at the University of Guelph in Canada.

In one study, Graham challenged nine runners to run on a treadmill at a very fast pace. On average, these athletes were able to run for about 32 minutes without caffeine. With caffeine in their systems, they ran 7 to 10 minutes longer.

Athletes often take caffeine for an extra boost of energy. But the chemical doesn’t necessarily make them faster or stronger.

iStockphoto.com

Though caffeine may help the performance of world-class athletes, it may harm the health of people who are overweight. Graham’s other research has shown that caffeine interferes with the body’s ability to process sugars, which may lead to a disease called type 2 diabetes.

Kids, who tend to be smaller than adults, feel the various effects of caffeine more strongly than adults do. And just like adults, kids and teens can become addicted to the chemical.

A can of caffeinated soda every now and then is probably OK, nutritionists say, but sip carefully!

The following list shows how many milligrams (mg) of caffeine are contained in some popular products. All beverages refer to an 8-ounce (1-cup) serving, unless otherwise noted.

Regular brewed coffee: 135 mg
Red Bull (8.5 oz): 80 mg
Black tea: 40-70 mg
Java Water: 62 mg
Starbucks Coffee Ice cream (1 cup): 40-60 mg
Espresso (1 oz): 30-50 mg
Green tea: 25-40 mg
Mountain Dew and Diet Mountain Dew: 37 mg
Diet Coke: 34 mg
Hershey’s Special Dark Chocolate Bar (1 bar – 1.5 oz): 31 mg
Pepsi: 28 mg
Diet Pepsi: 27 mg
Coca-Cola Classic: 26 mg
Snapple Iced Tea: 24 mg
Jolt gum (1 piece): 20 mg
Hershey’s Milk Chocolate Bar (1 bar – 1.5 oz): 10 mg
Hot cocoa: 5 mg
Chocolate milk: 5 mg
Decaffeinated coffee: 5 mg
Decaffeinated black tea: 4 mg

Going Deeper:

Additional Information

Questions about the Article

Word Find: Feel the Buzz

Your nose is a living sensor that responds to the chemicals in pizza that give this food its distinctive aroma. Your brain recognizes this combination of odors almost instantly.

Dogs have a much better sense of smell than people do. To make it easier for people to detect and identify odors, chemists have invented electronic noses.

NASA

How does your brain do it? It processes the mixture of chemicals that make up a smell as a pattern and then matches that pattern to one that you’ve already stored in your brain. The particular mix of compounds that gives fresh bread, tomato, garlic, and cheese their aroma, for example, means pizza.

But compared with dogs, people aren’t very good at identifying smells. So, chemists are designing sensors—electronic noses—that help people do this job better.

Electronic noses can go where human noses shouldn’t. For example, some electronic noses can sense substances that would be harmful to humans. Other electronic noses can sense chemicals that people can’t detect at all. Researchers have even built an electronic nose to send into space.

Color patterns

Scientists have been designing and building electronic noses for more than 20 years.

An electronic nose developed by NASA researchers can detect hazardous gases in the air on spacecraft.

NASA

One such device looks a bit like a computer chip covered with neat rows of dots. Each dot contains a chemical dye.

“We use anywhere from 20 to 36 different dyes that change color depending on what chemical they’re exposed to,” says Ken Suslick. He’s a chemist at the University of Illinois at Urbana-Champaign.

Some of the dyes are made of materials that change color to show how acidic or basic a chemical is. If you’ve ever used litmus paper, you know how this works. This paper contains a dye that turns red for an acid, such as lemon juice, or blue for a base, such as baking soda.

To use their nose-on-a-chip, Suslick and his coworkers expose it to chemicals that they’re interested in. The chips can detect chemicals in liquids as well as in solids. A scanner detects any color changes that occur after exposure.

The resulting color pattern is like a chemical fingerprint. Each pattern is unique to a single odor or mixture of odors, Suslick says.

Connecting the dots

To find out what the colored dots mean, chemists need a reference library that contains the patterns created by compounds responsible for the smells of different substances.

Each spice and herb has a distinctive smell and produces its own fingerprint of colored dots.

ChemSensing

Your brain already holds such a library. You collect smells all your life, and whenever you sense an odor, your brain tries to connect it with one that’s already familiar to you.

“So, if you smell something, you can almost hear the gears in your brain clicking, saying, ‘Gee, what does that smell like?’” Suslick says. “And you’re sort of going through a list in your head. That’s the library.”

When your brain makes a match, it identifies the smell.

To build a library for his electronic nose, Suslick has exposed the chip to many substances and recorded the resulting patterns of colored dots. With such a collection of patterns in his library, he can then compare the colors produced by a known substance with what he sees for an unknown material.

Ken Suslick’s nose-on-chip can detect different brands and types of soda.

ChemSensing

Chemicals in space

Electronic noses can also alert people to hazards. On the space shuttle or in the International Space Station, for example, a chemical leak could mean a problem with the spacecraft or danger for the crew. Detecting such leaks promptly is essential.

So, NASA researchers are working on the design and testing of an electronic nose, which they call the ENose. They hope that ENose will one day monitor the inside of a spacecraft to make sure that there aren’t any chemical leaks, says Amy Ryan. She’s a chemist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

Amy Ryan holds an early version of ENose.

NASA

ENose uses a set of four chips, each of which has eight sensors. Each sensor consists of a thin plastic film that expands or contracts, depending on the compounds in the air. The reactions of the individual films create a pattern. Like the human brain, the ENose is programmed to recognize these patterns and smells.

Once it’s in place on a spacecraft, ENose will run 24 hours a day, 7 days a week, monitoring the air to make sure that dangerous substances, such as mercury, or coolants, such as Freon, aren’t present in the cabin.

NASA tested an early version of the device for 6 days on a space shuttle mission in 1998, Ryan says. Now, they’re gearing up for a 6-month test on the International Space Station, planned for 2008.

Although the sensors are finished, Ryan is still working on the chemical library and the software that will keep the ENose running in space. She won’t be on the space station with the ENose, so the ENose will send information from the space station to her computer in California.

But even this sensitive space sensor can’t compete with a dog’s amazing ability to detect and identify smells. Electronic noses still have a long way to go to top a dog’s sniffer.

Going Deeper:

Additional Information

Questions about the Article

Word Find: Electronic Nose