When the aroma of roast turkey and pumpkin pie fills the house tomorrow, chances are the scent will not only whet the appetite, but also will evoke long-forgotten memories of Thanksgiving past.
"Odor identification and odor memory produce powerful emotions," says Linda Bartoshuk, a psychologist who specializes in taste and smell research at the John B. Pierce Foundation Laboratory in New Haven, Conn. "This is not hard to understand if you know that the same part of the brain that deals with emotion deals with smell."
Unlike the smell of a lemon or of peppermint, "the smell of your grandmother's kitchen on Thanksgiving doesn't quite have a name," says Pierce psychologist William S. Cain. So when you try to identify those aromas wafting through the house, it stimulates the brain and "you start calling up any information you can from your memory."
Your mind conjures up images and emotions stored with the memory of those special smells: The sight of your grandmother ladling out spicy hot cider. The feeling of your family being together. The sound of crackling logs in fireplace.
And later, at the dinner table, smell combines with another sense -- taste -- to produce the varied flavor of the Thanksgiving meal. Take away smell and turkey would be just a textured chunk of food. Remove taste, and the tart cranberry sauce would be no more than a flavorless glob of gelatin. And without both taste and smell, sweet potatoes would be pretty bland stuff.
Ever since Aristotle and other early Greek scientists studied taste and smell some 2,400 years ago, a long list of medical investigators, including Nobel prize winner Linus Pauling, have tackled the mysteries of these two senses.
Being able to detect taste and odors are basic biological traits. "Think of the evolution of life," says Richard Doty, director of the Taste and Smell Center at the University of Pennsylvania. "The ability to detect chemicals was one of the things that evolved." Even simple celled bacteria, Doty says, can detect a variety of chemical stimulae and 'know' what to stay away from."
Olfaction, or the ability to discern odors, also evolved as a protective mechanism, and had evolutionary advantage for those animals that developed it. "The olfactory system acts like a transmitter," Doty says. "It's a primitive function, important in finding a mate, knowing what to ingest, what not to ingest, and memorizing a terrain."
Today, humans rely less on smell and more on the sophisticated development of other sensory systems, including vision and hearing. But newborns still have the capability to use smell as an important tool. Within 24 hours after birth, "a breast-fed baby starts to show a preference for the odor of the mother's breasts compared to those of another lactating mother," Doty says. "They have learned that very early. This is true in many mammals. Mothers also learn the odors of their infants. But if you bottle-feed an infant, babies don't show that preference."
Today, the study of these two important senses is giving medical investigators a new look at the brain and at the nervous system -- the "hard wiring" that connects taste buds in the mouth and olfactory receptors in the nose to the brain.
Unlike all other nerve cells in the body, those serving the taste buds and olfactory bulbs can divide and repair themselves.
"The whole neural apparatus replenishes itself at regular intervals," says University of Pennsylvania's Doty. "The cells smelling today are not the same cells that you smelled with several months ago."
A better understanding of this finding, Doty says, could someday provide clues to prompting other nerve cells -- for example in the brain or spinal cord -- to divide and repair damage.
Among the other information now known about taste and smell:
*Laste is made up of four basic building blocks: sweet, salty, sour and bitter. A theory that there is a fifth basic taste, metalic, is no longer widely believed.
*The ability to taste is inborn and probably remains constant throughout life.
*The ability to smell is also present at birth, but certain qualities of smelling, like knowing when a smell is noxious or pleasant, must be learned.
*Humans are able to detect tens of thousands of smells, but the ability to detect odors begins deteriorating around age 40 and continues to decline throughout life.
Someone who raves about the mouth-watering "taste" of pizza, or chocolate, or turkey, or almost any food, is more likely to be describing the attributes of smelling.
"Most sensory input from eating is smell, not taste," says Bartoshuk. "There's no way that you can identify turkey with a sweet, sour, bitter or salty taste. Smell does that."
Which explains why cold sufferers and people with hay fever often complain about temporarily losing their taste. In fact, says Doty, their taste buds function just fine. It's their blocked nasal passages that are causing the problem.
Taste and smell -- two of the body's five senses -- both begin with a complex series of chemical reactions in the mouth and nose and end with messages received by the brain. After that, any similarities between the two senses end.
"Taste and smell are very different senses," Bartoshuk says. Even the brain pathways for the senses "are totally different," she says. "We only cluster them together because we learn to do that through eating."
Bite into a piece of pumpkin pie and a chain of chemical events begins in the mouth. Substances in saliva, known as enzymes, help break down food into smaller molecules. The diluted food bathes the thousands of taste buds -- tiny, orange-shaped structures located throughout the mouth, not just on the tongue as many high school biology students have been taught to believe.
The notion that the tongue contains specific regions that correspond to tasting each of the four primary tastes is also wrong.
"That idea is just not true, and never has been," says Bartoshuk. This notion grew from an erroneous translation of a German scientists' doctoral dissertation written around the turn of the century. The researcher was actually trying to determine minimum threshholds of taste on various parts of the tongue.
"It got into the literature and into many high school texts when it was misread by people who didn't know German very well," Bartoshuk says. The mistaken theory took on a life of its own.
Taste buds are located on the tongue, the soft part of the roof of the mouth, the pharynx in the throat, the esophagus, and even the epiglottis, which closes off the windpipe during swallowing.
At the center of each taste bud is a specially sensitive cell. Molecules of food attach to receptors on the cell's membrane, and prompt the cell to send out a chemical signal, which is picked up by a nearby nerve cell, which in turn relays the message to the brain. There the message -- such as "we've got sweet food here in the mouth" -- is believed to be processed in the medulla and cortex, Bartoshuk says.
Four basic building blocks of taste convey messages through simple chemical reactions. Scientists now can account for three: Salty taste comes from sodium ions -- not from chloride as scientists once thought. Hydrogen atoms account for a sour taste. And work by a Cornell University chemist indicates that sweet tasting foods probably are detected by a triangular molecule that is recognized by the taste buds.
The chemistry of a bitter taste is not yet known. Researchers speculate that the mechanism for this kind of taste is particularly complex and serves as a protection against poisonous substances in the environment, which often have a bitter taste.
The sense of smell also relies on chemical molecules, which can be sniffed through the nose or can move like a cloud from the mouth up through the nasal cavity and into the olfactory bulbs, just beneath the eyes. From there, odors are processed in many places throughout the brain.
Everyone does not experience taste -- or smell -- alike. Most of these differences are thought to be genetic, but researchers don't yet understand the signficance. Among the distinctive differences in the ability to taste:
*A small percentage of people, perhaps as little as two or three out of a hundred, experience what researchers call the "artichoke effect." Sipping a liquid -- even just plain water -- after eating an artichoke produces a delightfully sweet taste in the mouth. Researchers don't understand why this sensation is produced.
*Diet sodas are consumed by millions of Americans annually, particularly since the introduction of aspartame, an artificial sweetener with no bitter taste. The older artificial sweeteners, saccharin and cyclamates, produce a bitter taste detected by about two of every three consumers, which probably "accounts for the aspartame's widespread popularity today," Bartoshuk said.
*Soups seasoned with monosodium glutamate (MSG) taste about as salty as soups containing an equal amount of table salt, sodium chloride (NaCl). Yet there may be as much as three times the amount of sodium in the MSG as there is in NaCl. Why people don't notice the difference is still not understood.
Taste and smell are associated with both positive and negative feelings. Humans are born with an innate dislike of bitter taste -- a sensation often associated with poisons. Similarly, sweet tastes are universally pleasant, while tasting sour foods and salty foods fall somewhere in the middle.
There are also blends of tastes. Tonic water -- a bitter-sweet taste -- is one example. Cranberry sauce is another. Each is an example of "mixture suppression," Bartoshuk explains. In each case, the bitter taste is countermanded by a sweet taste, produced by the addition of a great deal of sugar. Yet neither tonic water nor cranberry sauce tastes sugary to consumers.
The same type of taste reaction occurs with some Chinese food. Vinegar, soy sauce and sugar are common ingredients in Chinese foods. "Each ingredient alone has a very potent taste," Bartoshuk says. "But together they produce about the same intensity of taste as any one of them has."
The sense of taste seems to stay "robust" throughout life, Bartoshuk says, but the sense of smell grows weaker with age. Older people often complain that food has become tasteless, when in fact their ability to taste has not changed but their ability to smell has weakened. "Probably 95 percent of the people we see in our clinic who complain of a taste malfunction really have a loss of smell," Doty says.
Losing a sense of smell can be a function of aging or the result of a head injury or a brain tumor. Certain diseases also produce changes in smell. In a study of patients with Alzheimer's disease, Doty and his colleagues found that 100 percent had a smell disorder. A similar study of Parkinson's patients revealed that eight out of every 10 also had difficulty smelling. Patients afflicted with Huntington's chorea also showed this problem, as did people suffering from Korsakoff's psychosis.
Why alterations in taste and smell occur is still unknown. Some evidence suggests that aging causes changes in the cells lining the nasal cavity. Viruses and pollution may also take a toll, resulting in "a loss of smell receptors," Doty says.
The primitive senses of taste and smell may hold clues for a host of diseases, from eating disorders to high blood pressure.
Some research suggests that when bulimics disgorge after a meal, they may rob the brain of a signal that indicates calories have been consumed. The brain then sends out a signal to eat more, perhaps creating a vicious cycle, Bartoshuk says.
Another taste-related disorder may affect some people with high blood pressure, who apparently cannot detect salt, Bartoshuk says. That's "clearly a negative trait," because salt can aggravate high blood pressure. The question researchers seek to answer is which came first: a lowered sensitivity to the taste of salt or the raised blood pressure?
One way to answer that question is by studying the possibility of satisfying salt cravings without actually eating salt. "We know we can produce a salty taste without ingesting sodium," Bartoshuk says, by dipping a cotton swab in a salty solution and touching it to the tongue. Further research in this area could lead to a mechanism for satisfying these cravings without actually eating sodium.
What makes certain foods and odors particularly appealing is still a mystery. And unlike other animals, humans have the unique ability to "derive pleasure from smelling something that has no functional value, like a flower," says the Pierce Foundation's Cain. While scientists don't know why humans are the the only animals who are attracted to smells that have nothing to do with reproduction or food, the answer is more than academic.
By studying taste and smell, Bartoshuk says, scientists have the opportunity to probe "the basic . . . systems of behavior and the brain." Much remains to be accomplished, she notes, to bring "the knowledge of the chemical senses up to par with the higher senses of vision and learning."