How Our Senses Work

The Circuitry of Seeing

What makes seeing possible?

Pure and simple: we see with our brains. What we know as sight is actually the ultimate outcome of light's fantastic journey through the brain's visual system. The processing of light begins in our eyes, which are an extension of the brain and the most exposed part of the central nervous system. In the eyes, light is filtered and focused before being translated into electrical impulses and sent on its way along the optic nerve to the brain. The final destination of these impulses is the brain's vision centre, the visual cortex located at the rear of the brain, where the impulses are interpreted as visual images.

How do we recognize what we are seeing?

We don't just see people and things, we recognize them for who and what they are. Most of us take this ability for granted, but it is a very complicated process and one not yet fully understood by scientists.

One theory-called the feature-detection model of vision-suggests that individual cells along the visual pathway are pre-programmed to respond to certain shapes. Cells programmed to recognize different types of curved lines, for instance, might work together to recognize a face.

Feature detection led to speculation (most of it humorous) about the existence of a "grandmother cell," one single cell in your brain imprinted with the image of your grandmother. But the grandmother-cell model implied the brain would need to assign a different cell to everything seen in a lifetime, and this data-storage task would be too great even for the adept human brain. Critics of the feature detection model have also pointed out that the process of assembling all those shapes into an image would be clumsy and take too long.

In recent years, the feature-detection model has been modified by the spatial-frequency theory, which sees images as compositions created by the brain out of variations of light and dark. In this model, the brain takes the differing wavelengths of light and dark reflected by an object and translates them into its own "computer code." The low-frequency wavelengths give the brain a kind of fuzzy outline of the image, while the high frequencies fill in the details.

Do you see the world as it really is?

Studies of the mechanics of the eye have revealed that the images reaching the retina are only two-dimensional, flat like a photograph. Yet the world is three-dimensional, and we perceive it that way. The image received by the eyes is also upside down, yet our brains perceive the world the right way up. We are able to make the necessary corrections, apparently, because the brain expects the physical world to have certain qualities, such as three-dimensionality.

Furthermore, the certainty that up is up and down is down is part of the brain's innate equipment. From the beginning of our conscious lives, the brain looks for visual cues that conform to its inborn understanding of reality.

How can you find a white cow in a snowstorm?

The situation: To a potential buyer, an artist shows a canvas that is completely white.

Buyer's question: "What is it?"

Artist's answer: "A white cow in a snowstorm."

The joke is old, but here's why no buyer should be fooled: Our visual system is expert at recognizing objects against all kinds of backgrounds. A real cow would stick out for a variety of reasons: The white of the cow would be different from the white of the snow. The cow's body would form a distinct outline against any non-snowy background. And if you argue that the cow is in front of a snow-covered hill, then there would either be two brown eyes staring at you or some variation of texture or brightness that, at least for the careful observer, would call attention to the cow's presence.

Every day our visual system distinguishes thousands of objects from very cluttered backgrounds. Whether we're searching for a phone booth in a crowded cityscape, the face of a friend in a crowd, or a mushroom on a forest floor, our visual system can find and focus on the desired object to the virtual exclusion of all others. Such discrimination can fine-tune itself with amazing subtlety. For example, when you look at a face and see the baring of teeth, you not only can tell a smile from a sneer, but also a false smile from a genuine one.

The calculations made by your brain as it sorts through and processes all this information are so complicated that they would be impossible for all of the world's most powerful computers linked together to duplicate. Those calculations and the paths they follow in the brain are so complex, in fact, that very powerful computers are needed for scientists to begin even to theorize about them.

Making Sense of Sounds

How does the brain tell one sound from another?

The auditory, or hearing, centres of the brain are located in the temporal lobes, behind the temples. It is here that the brain "hears" sound, registering it as loud or soft, high or low. What the brain hears as loudness corresponds to the strength of the sound waves picked up by the ears. Pitch relates to the waves' frequency, the number of waves per second. Timbre is the blending of sound waves that each voice or instrument produces.

Our brains' hearing centres interpret the multitude of sound signals we receive, comparing them with one another and grouping them in orderly patterns. This process is all-important in deciphering speech. But before they reach these higher brain centres. the sound signals from each ear are routed through the auditory nerve to the brain stem at the top of the spinal cord. There they pass through a series of relay stations; many signals then cross over to the opposite side of the brain before being collected in the thalamus, which routes them to the hearing centres. At many points along the way, nerve cells can "decide" whether or not to pass on a given signal. This filters out much confusing noise.

When do we begin to hear?

Babies can actually hear in the womb. The sense of hearing is believed to develop in the 20th week of pregnancy, eight weeks earlier than vision. There is even evidence that babies may recognize their mothers' voices before birth. Studies of infants' sucking reflexes showed that newborns suck faster when they hear their mothers speak.

The hearing of newborn babies is somewhat less acute than that of children. but it becomes keener within a few days. It takes longer for their brains to develop the ability to interpret the sounds they hear. At first, they react to any loud noise by flinging their arms and legs about. This startle reflex is tamed as they learn to recognize common sounds. Soon they may smile when they hear a noise that signals their mothers' arrival. By about five months babies can make distinctions between different spoken sounds, an important step on the way to understanding speech.

Why is it pleasurable to listen to music?

In random noise. sounds of many different frequencies and in no particular order are mixed in such a way that we can make no sense of them. Music, however, presents our auditory system with sounds that have been arranged in orderly patterns. We respond to rhythms in the repetitious beat of drums, and pleasing harmonies in the varied pitch of stringed or wind instruments.

The frequencies of notes that sound pleasant when played together are related in specific mathematical ratios. For example. the frequency of middle C is exactly four-fifths that of the E above it; together they form a harmonious chord. Moreover, tests show that people tend to hear a succession of tones that are closely related in pitch as a connected series, even when other tones are interspersed with them. This is apparently how we pick out a melody played by one instrument in an orchestra.

As mothers know, babies respond well to orderly sound. Low notes and repetitious sounds are particularly soothing. In fact, recordings that repeat a tone every second are marketed as high-tech substitutes for mothers' lullabies. And when calming music is played for premature babies in some intensive-care nurseries, research

shows that infants actually gain weight faster.

What's the difference between people who have absolute pitch and those who are tone-deaf?

A few people-probably less than one percent of us-have the ability to hear a single note and immediately recognize it as, for example, middle C. This mysterious faculty seems to be inborn or imprinted in childhood through early exposure to music. Even with intensive training, hardly anyone has been able to develop the same skill later in life.

It is far easier to learn to distinguish notes when we can compare them with one another. Almost everyone can tell that two notes are different in pitch as long as they are far enough apart. Some people who are tone deaf may have trouble singing notes that are unlike those they use in speaking. Others may recognize that two notes are different but be unable to tell whether the second is higher or lower than the first. Both groups can usually improve greatly with practice.

Heredity may play a part in this but, as with absolute pitch, early experience with music is very important. In any case, a musical ear and its opposite, tone deafness, seem to be functions not of the ear but of the brain. Late in his life, composer Maurice Ravel was in an automobile accident that severely injured the left hemisphere of his brain. The accident left him with an unusual form of aphasia (inability to use language): Ravel could no longer play the piano, sing in tune, or write down musical notes. Yet he could still enjoy listening to music and could even hear pieces in his head.

What is the cocktail-party effect?

What we hear depends in large measure on what we can screen out. We have the mysterious ability to tune in to one conversation amid the babble of a crowded room-a phenomenon called the cocktail-parry effect. But even when we have filtered out most distractions, certain sounds we care about will jump out at us. We can't help noticing the sound of our own name, for example, however softly it is spoken. In the same way, a sleeping mother will be wakened by a cry from her baby, and a single wrong note by one player in a huge symphony orchestra will catch the ear of the conductor.

A country dweller who visits the city is often appalled by the relentless clamour of vehicles and people, which an urban cousin no longer hears; and a teenage student can concentrate while loud rock music is playing to the amazement of his parents.

Also, depending on their attitudes towards the source of the noise, people may hear noises as louder or softer. In one study, for example, people living near a military airport found the sound of jets taking off less noisy when they believed that the airport was vital to the national defence.

Getting in Touch

Do people really need to be touched?

Infants and children have a special need for physical contact-to be held and touched. It is now believed that separating a baby from its mother at birth can interfere with the natural bonding between mother and child. Early contact stimulates the baby's development as well as the mother's maternal feelings.

In a famous experiment, young monkeys were raised in cages and denied physical contact with other animals. Each monkey could choose between two wire "surrogate mothers", one that gave milk but no tactile stimulation and another that had no milk but was covered in soft terry cloth, which the infants could cling to. They preferred the soft "mother" to the milk provider. Many monkeys deprived of touching actually died.

Paediatricians have noticed a similar tendency among human babies: at the turn of the century almost all orphans placed in orphanages in America before they were a year old died, even though they were well fed and given good medical care after being admitted to the orphanage.

A Boston doctor, visiting a German orphanage, saw an elderly woman carrying a sick baby on her hip. When he asked the staff about her, he was told. "That is Old Anna. When we have done everything we can do medically for a baby, and it is still not doing well we turn it over to Old Anna, and she is always successful." Just being picked up and carried apparently made all the difference. When such loving care was tried in the American institutions, infant mortality declined.

More recently, researchers have found that premature babies in intensive- care nurseries gain weight faster if they are gently stroked by nurses for 15 minutes three times a day, or placed in tiny water beds, whose motion simulates the gentle, rocking embrace that full-term babies receive from their mothers. Touching apparently produces its physiological benefits in part by stimulating the secretion of certain brain chemicals necessary for growth and for the body's response to stress.

Why is touch called "the mother of our senses"?

Touch is the earliest of the human senses to develop. It is already functioning during the seventh week of pregnancy, long before the ears or the eyes are fully formed. And it is literally the broadest of the senses, since the skin, where the touch receptors are found, covers the entire body.

At birth, babies use their sense of touch in their first efforts to understand their surroundings. Even after they have learned to recognize things by sight, they often try to confirm what they see through touch, by patting their mothers' faces or reaching out to the sides of their cribs.

Adults also seem to regard touch as fundamental. How often do you find yourself not believing what you see until you have actually felt it? A sign saying "Wet Paint", for example, often serves less as a warning than as an invitation to touch the paint and find out how wet it really is.

Is there a sense of pressure?

What we call our sense of touch ac tually consists of at least four different kinds of sensory receptors. Specialized pressure receptors react to light touch, deep pressure, or vibration; they can detect a touch as delicate as a butterfly's wing, even when it is so fleeting that it lasts only one-tenth of a second. Like most sense receptors, they "adapt" by sending slower, less intense signals as the same stimulus continues. This is one of the reasons why you stop noticing the pressure of your watch on your wrist or the feel of your clothing on your body.

What sensations other than pressure does touch convey?

Receptors of both cold and warmth do not respond directly to cold or warmth but to relative changes in skin temperature. For example, a cool room feels warm to someone coming in from the cold. Our bodies respond to changes in temperature by rerouting the flow of blood. Cold receptors are the least densely packed of the touch receptors; if a cold knife touches certain parts of your body, for example, you may only feel the pressure of its weight, not its temperature.

Pain receptors are of two kinds: some send a quick. jabbing sensation to your brain and others send a slow, aching or burning sensation. Unlike other sensory signals, they remain strong even when they are stimulated for a long time. Unpleasant as this may be, it has the advantage of assuring that you will not ignore their warning of tissue damage. Because painkillers can block this important message from reaching your brain, it is best to use them sparingly and only under the supervision of a doctor.

Sensations of itching and tickling were once thought to be produced by mild stimulation of pain receptors. Now they are believed to be produced by other specialized receptors that trigger their own characteristic reflexes. While you automatically withdraw your hand from a painfully hot stove, for example, you have a different reflex to an itch: you scratch. If the reflexes are distinct, scientists reason, the receptors must also be different.

Why do people react so differently to being tickled?

Tickling apparently has an emotional as well as a physiological effect. It triggers laughter only in social situations, not when you are alone and tickle yourself. Children respond with special intensity to being tickled, but people from all cultures apparently enjoy the sensation as long as it is provoked by someone they like and is not continued too long.

People differ as to where they are most ticklish. The armpits, the sides of the body, the soles of the feet, and the abdomen are the most common areas. Ribs and the backs of knees may also be sensitive to tickling. As people age their response to tickling lessens.

Can touch heal?

Belief in the healing power of touch goes back to ancient times. Up to the 18^th Century in Britain, some people were sure the monarch's "Royal Touch" could cure scrofula, a form of tuberculosis. Faith healing, which customarily involves touching, is based on trust and belief and is accepted by some religions, although its benefits are controversial. Apparent cures may be the result of the placebo effect: some patients feel better when given a pill containing no medicine because they believe it will help.

Touching does appear to have specific value in relieving pain-as any child knows who has rubbed a bruise after a fall. Scientists think that the nerve signals triggered by gentle rubbing of pressure receptors may interfere with those from pain receptors, thus lessening the impact of pain signals on the brain's cerebral cortex. Massage also seems to have real physical and psychological benefits; it not only relaxes the muscles but induces a sense of well-being reminiscent of the security babies feel when they are held in their parents¡¦ arms.

Why do amputees feel sensation in missing limbs?

The strange phenomenon of "'phantom limb"' is apparently related to an image of the body that persists after the limb or the use of it has been lost. In many cases, the image of the functioning limb has been stored in the brain since early childhood. Almost every amputee has "'felt" pressure in a missing arm or leg when it is actually the stump that is being touched.

Many amputees feel a persistent mild tingling in the lost limb; others may feel severe pain. Usually the problem disappears over time as the patient corrects his or her body image, but sometimes it becomes so annoying that psychotherapy is needed.

Are some parts of the body more sensitive to touch than others?

Touch receptors are widely distributed throughout the skin. They even cluster around the bases of hair follicles; this allows you to feel the wind blowing through your hair. But receptors are most densely packed in the tongue, lips, and fingers as well as in the nipples and external genital organs, whose stimulation leads to sexual pleasure. The areas of the brain that register signals from these pans of the body are disproportionately large. For example, the importance of the human hand in manipulating objects and in communicating with other people is reflected in the relatively huge size of that part of the cortex devoted to processing messages to and from the hand.

You can demonstrate on your own body the various densities of touch receptors by a simple test of two-point pressure thresholds. lf you touch two toothpicks simultaneously to your lip, you will feel pressure from two different points when the toothpicks are only about one twenty-fifth of an inch a part. However, on your back, where receptors are more widely spaced, you will feel as though the pressure of the two toothpicks is coming from only one point until the toothpicks are about two inches apart.

What the Nose Knows

How does the brain perceive odours?

Deep inside the nasal passages is a mat of mucus and raw nerves called the olfactory epithelium. It is only about half the size of a postage stamp, but it is packed with millions of receptor cells. Air-inhaled through the nose brings with it various kinds of molecules. Some are water-soluble and, after being captured by special molecules in the nose, are transported to the receptor cells in the mucous mat. From there, signals identifying the odour are sent along nerves to the brain, where the final perception is made. Some objects, such as glass and metal don't give off water-soluble molecules, so have no smell ¡V any smell such objects have comes from impurities on their surfaces.

From the nerves in the nasal passages the sensation of smell travels directly to the brain's olfactory bulbs, two broad bean-sized organs behind the bridge of the nose. From these organs, smell signals move to parts of the brain where memories are stored, as well as to areas responsible for stimulating the production of hormones that control appetite, body temperature, and sexual urges. Finally, following some as yet unknown pathways, smell sensations reach locations in the brain where they are consciously identified.

Do we actually need a sense of smell?

How smell affects our brains and behaviour is not well understood; but its association with hormone production and sexual urges, for example, leads some scientists to believe that smell plays a greater part in our lives than we now recognize.

There is no question that smell adds to our enjoyment of food and drink, since it accounts for four-fifths of the flavour we experience. Smell also alerts us to the danger of an unseen fire or a gas leak; it attracts us to a fragrant flower or expands our appreciation of a day by the sea.

How keen is the nose?

It takes only 1/25,000,000,000 of a milligram of methyl mercaptan in a millilitre of air for most people to smell it. This chemical, whose odour is essentially that of decaying flesh, is mixed with odourless natural gas to warn of gas leaks. And the smell of a single drop of highly concentrated perfume in a three-room house, or the equivalent of one part in 500,000,000, is detectable by most people. The lowest concentration of a substance that can be detected by human smell is called that substance's detection threshold.

Although the sense of smell can be triggered by just a few molecules, concentrations only 10 to 50 times above that detection threshold often reach maximum intensity for us. That is, we won't sense that a smell is stronger no matter how much greater the concentration gets. The relatively small range between detection and maximum intensities of smell means that our noses are much better at detecting the presence or absence of odours than at differentiating their intensities.

Can our sense of smell be heightened or dulled?

Your age, your health, the weather, the altitude, and the humidity are just a few of the factors that influence the perception of smell. The simplest way to heighten the sense of smell is to inhale through your nose and thus draw more air over the receptor cells in the epithelium. On the other hand, clogged nasal passages dull smell because they block the passage of air to the receptors.

Colds are the most frequent cause of a temporary loss of smell. Since smell is 80 percent of a food's flavour, it is not surprising that many cold-sufferers complain of bland food while they are sick. Victims of serious head injuries occasionally lose the sense of smell. The aftermath of a viral infection or exposure to toxic materials can also leave victims in a permanently odourless world. Others are more lucky and gradually regain their sense of smell. About 25 percent of the people who lose their sense of smell also lose interest in sex. Apparently, odour plays an important role in sexual arousal at least with some people.

How many odours can we identify?

Many attempts have been made to classify odours, but no one system is universally accepted. On the basis of psychological tests, it is believed that human beings can detect between 4,000 and 10,000 odours. Each of them is composed of various concentrations of seven primary odours: ethereal (like dry-cleaning fluid); camphoraceous (mothballs); musky; floral (rose); pepperminty; pungent (vinegar); putrid (rotten eggs).

Each primary odour may have a corresponding type of receptor cell in the nose. Depending on the type of receptor activated by an odour, and how strongly, a different series of electrical signals is sent to the brain. There, the signals are identified as a unique odour.

Do women have a better sense of smell than men?

Women are more sensitive than men to a number of smells. The female hormone oestrogen may account for this sensory difference. As oestrogen levels rise and fall during a woman's monthly cycle, her sensitivity to odours rises and falls. Women are most sensitive to smell during ovulation, when oestrogen levels are highest, and less sensitive during menstruation, when oestrogen is lower. The oestrogen levels of pregnant women drop drastically. It is estimated that a woman is 2,000 times more sensitive to smell before her pregnancy than during it.

The "gherkins-and-ice cream" syndrome, in which pregnant women eat unusual combinations of food, is thought to be due at least partly to a dulled sense of smell during pregnancy. Hence pregnant women can tolerate unusual foods and even crave tastes that they found unappetizing before they became pregnant.

Can we become adapted to a smell?

Many odours that at first seem noxious become less noticeable with time, especially if you are exposed to them frequently. In the first second after a smell is encountered, electrical activity between the brain and the smell receptors rises sharply. After a minute or so, activity drops by about half and then levels off; the smell gradually fades from perception. The puzzling thing is that receptors are still being stimulated and signals are still travelling towards the brain. Somewhere in the central nervous system, no one is exactly sure where, steady incoming signals seem to be interpreted as the norm, and the original perception of a smell diminishes.

A similar kind of adaptation happens if a person is exposed to an odour day after day. The musty smell of an antique shop is nearly undetectable to its owner, just as the barnyard odour that seems so sharp to a city dweller is hardly noticed by the farmer. Your home probably also has its own unique aroma, but you notice it only at times when you haven't been home regularly, for instance, when you return from a holiday.

Is it true that odours influence our behaviour?

There are some obvious ways that odours affect what we do and how we feel. The stink of rotten eggs makes us flinch, while the aroma of a pine forest is pleasing. And the smell of baking bread can make our mouths water

in anticipation.

Yet odours may have more subtle influences on human behaviour. Because smells play a role in sexual attraction in animals, some scientists speculate that the same may be true of humans. Moreover in 1970, psychologist Martha McClintock, noted that young women who lived together in college dorms tended to have the same menstrual cycles. A similar effect has been observed in women working together in offices. The cause may be the odour of a chemical in the sweat of women. This odour, researchers suggest, acts as a signal that brings the monthly cycles of a group of women into line with the rhythms of a dominant few.

Does sickness have a smell?

There are doctors who rely on their sense of smell as a diagnostic tool, literally finding our what is wrong with a patient by using their noses. Certain diseases apparently have distinctive odours, caused by a change in metabolic processes associated with the patient's condition. A garlic odour is a sign of arsenic poisoning, and a fruity smell on the breath can mean either a diabetic or someone who is starving. Here are some other illnesses and their distinct smells: German measles smells like plucked feathers; scrofula (a form of tuberculosis) smells like stale beer; typhoid like baking bread; yellow fever like a butcher's shop. Alert surgeons frequently check for bacterial infection by sniffing a patient's bandages. A musty cellar odour can mean an infected wound.

Why do smells create some of our strongest memories?

One of the memory centres of the brain, the hippocampus, is closely connected with the sense of smell. Smell signals make just one stop here - in the olfactory bulbs - before making their way straight to the brain. This nearly direct connection may account for the sometimes surprisingly vivid memories that can be stirred by odours. If you were upset by your first experience of school on a long ago autumn day, the smell of fallen leaves can bring back the experience in excruciating detail. If the smell of blooming honeysuckle accompanied your first kiss, the same smell, even a lifetime later, may take you back in time and place. When the painter Marc Chagall returned to Russia, his original homeland, for the first rime in half a century, it was the overpowering scent of wild violets that brought back to him most vividly memories of his youth. Holding two wilted bouquets, he said; "Smell smell. No other flowers have that smell. I haven¡¦t known it in 50 years."

Smell and memory play a more direct role in what foods you like to eat. If a certain food once made you sick, the mere whiff of it can make your stomach queasy. On the happier side, a certain kind of food can be irresistible because of your pleasant associations with it in the past.

The title of smelliest fruit belongs to the durian of east Asia, say most Westerners who sample it. But to Asian gourmets, the spiky, seven-pound, sewer-scented fruit is a delicacy-not just for its sweet raspberry-flavoured, custard-like insides, but for its aroma, which they have learned to love.

What does it mean to have acquired tastes?

Most of us have had the experience of hating a certain taste on our first try, then later growing to like it. Known as an acquired taste, this liking generally develops after repeated exposure to a particular taste. When the tongue is stimulated on a regular basis by a bitter food or drink, its sensitivity to that bitterness drops. Your first taste of strong coffee may have been shocking, for example, but it soon becomes a friendly, familiar taste for many people. However, the food for which you have had to acquire a liking may have tasted just fine from the start to somebody else. A recent study has shown that some people have a genetic inability to taste a bitter chemical that is present in coffee, broccoli, and cabbage; and there may be genetic explanations for food likes and dislikes.

Furthermore, some people seem actually to like tastes that others of us find too bitter or sour, for reasons that go back to childhood and to the kinds of meals that were served in the home. The influence of family, both genetic and environmental is profound in matters of taste, whether they are natural or acquired.

How keen is the sense of taste?

Bitterness can be detected in a solution as weak as one part per 2 million, sourness one part per 130,000, and saltiness one part per 400. It takes much more sweetness to register a sweet sensation ¡V one part per 200.

However, taste buds can be tricked. After you brush your teeth, the usually sweet taste of orange juice seems bitter because of the chemicals left behind by your toothpaste. Conversely, certain chemicals in artichokes make almost anything you put in your mouth for a few minutes afterwards seem sweet.

Did You Know ¡K ?

¡P On your tongue are about 10,000 taste receptors. They are called taste buds, but "taste hairs'' would be a more accurate name in that these receptors project like hairs from the walls of the tiny trenches that run between the bumps on your tongue. When you eat, the receptors send signals to the brain, which translates the signals into combinations of sweet, bitter, salty, and sour tastes.

¡P Newborn babies have few taste buds. But soon after birth more buds begin to grow, and by early childhood they cover the top and some of the bottom of the tongue, as well as areas in the cheeks and throat. Since young children have many more taste buds blooming in their mouths than adults, they frequently find foods to be too bitter or spicy. Adults, on the other hand, often seek out bitter or spicy foods because of a declining number of taste buds. In children and adults, each taste bud lives a matter of days before it is replaced.

¡P Different parts of your tongue are sensitive to different tastes. The four primary tastes - sweet, bitter, salty, and sour - are each associated with a specific area on your tongue. The tip of your tongue is most sensitive to sweet and salty tastes, while sour seems to register most strongly on the sides of the tongue. Far to the rear, grouped in a V-shape, are most of the receptors for bitter tastes.

¡P The taste buds account for less than 20 percent of the flavour of food. The sense of smell, with its own separate receptors, mostly determines what we experience as taste. The temperature and texture of food also contribute to its overall flavour. Oddly, though one's sensitivity to saltiness and bitterness seems to increase as food cools, sensitivity to sweetness increases with heat. For instance, a piece of chocolate may have very little taste when cold, taste fine at room temperature, but seem unpleasantly sweet when hot and half-melted.