Tongue Taste Map

by Cathy Pelletier

What Can We Do?

One leading research institute in the United States that is tackling these questions is the Monell Chemical Senses Center in Philadelphia, PA (www.monell.org). Another group of researchers in Europe is examining aging and food choice, including the degree and nature of sensory changes as we age (http://healthsense.ucc.ie). For the latter study, data on aging and food choice are currently being collected in 10 European countries. Although changes in food choice and preference will undoubtedly be culturally based, this large study will certainly provide us with valuable normative information that may help us in our work with a clinical aging population.

Speech-language pathologists also may be interested in the annual international Food Choice Conference. This meeting brings together researchers from nutrition and sensory science to examine the cognitive, social, cultural, economic, interpersonal, psychological, and sensory determina nts of food selection and intake and its consequences on health and well-being. The peer-reviewed journal, Appetite, publishes abstracts of this conference.

The Tongue Map Myth

When a clinician suspects taste problems in a client, the clinician may first think about conducting a taste test using the "tongue map." This procedure involves swabbing a person’s tongue with various taste qualities (sweet, sour, etc.) and asking the person to name the taste. We are supposed to perceive sweet taste at the tip of our tongue, for example, and bitter at the back, and other tastes should have their place as well.

If you have performed this test, you may be surprised to learn that the tongue map is wrong. It is a mistranslation of an early-1900s German thesis that was disproved in 1974. Unfortunately, it continues to be published in textbooks today.

For the record, we perceive all taste qualities all over our tongue, although there may be increased sensitivity to certain qualities in certain areas. In addition, it is important to remember that our taste system provides information on the intensity and pleasantness (or unpleasantness) of taste as well. What we like and dislike in food can change over time, a fact to which parents of preschoolers and teenagers can attest.

Food preferences can be influenced by many factors, such as physiologic status, food context, familiarity, and environment. Furthermore, the palatability of food does not necessarily predict whether we eat it. Although we may like ice cream better than yogurt, we may eat yogurt as a midafternoon snack if we are trying to watch our weight. So, figuring out what people like to eat—and will consume—is tricky business.

Taste + Smell + Chemical Irritation = Flavor

Flavor is the term used to describe the complex integration of taste, smell, and chemical irritation of foods in the mouth that add to its "mouthfeel," such as carbonation, the burn of chili peppers, or the coolness of menthol. When people talk about how something "tastes," they are really referring t o its flavor.

There are three cranial nerves that house taste buds: the facial, glossopharyngeal, and vagus nerves. Chemical irritation is due to trigeminal stimulation, although all the taste cranial nerves have free nerve endings that perceive irritation. The trigeminal nerve does not innervate any taste buds. Innervation by these four pairs of cranial nerves may explain why taste remains quite robust throughout our lifetime. Studies show taste thresholds may increase only 2–3 fold with aging (there is great variation across people and qualities), with bitter taste loss greater than other taste qualities.

Loss of olfaction with aging appears to be another story, although individual variation also is observed. We can begin to lose our sense of smell by age 40, with significant gradual decrements occurring each decade thereafter, reaching up to 70% loss by age 70. There is some recent evidence that olfactory loss is not uniform, similar to the taste changes observed in aging. Thus, it is important to remember that flavor enhancers (taste or smell) added to foods may not be perceived similarly across all older adults.

When older adults complain that foods don’t seem to "taste" right, it is most likely the loss of smell (which diminishes flavor) that they are describing. Anyone with a head cold knows this sensation. Safety and quality-of-life issues surrounding olfactory decline in aging demand our attention. Older adults living alone may not be able to detect rancid food or a gas leak. Natural gas does not have an odor, so mercaptan (an odorous chemical) is often added to aid detection. However, typical concentration levels of mercaptan are usually below threshold for older adults, so they cannot smell a gas leak if it occurs.

Taste cells lie within taste buds, which are located in various tongue papillae, hard and soft palate, and root of the tongue. Taste buds also are found in the larynx, but their function is not clearly understood. Fungiform (anterior tongue), circumvallate (posterior tongue), and foliate (lateral rear area of tongue) papillae house taste buds. Filiform papillae are plentiful on the tongue, but they do not hold taste buds.

Fungiform "mushroom-like" papillae look like small red dots on the tip of the tongue to the naked eye. Some people have a lot of fungiform papillae, and others have just a few. Some researchers believe the number of fungiform papillae on your tongue and your ability to perceive a certain bitter compound may be related to your food choice behaviors. For instance, these researchers propose that "supertasters" (those people with many fungiform papillae and an intense response to the bitter compound) are typically picky eaters, with a limited number of acceptable foods they will consume. "Tasters" and "nontasters" have lower numbers of fungiform papillae and a reduced or nonexistent response, respectively, to the bitter compound. These individuals tend to eat a wider variety of foods. Although an anatomic etiology for food selection and intake is intriguing, one should remember that many factors influence food choice.

Taste buds are modified epithelial "skin-like" cells, with a life span of approximately seven days. Olfactory receptors are true nerve cells that live approximately 30 days. The perception of odors can occur via the nose (orthonasal) and the oral cavity (retronasal) while chewing. Ongoing research is investigating the differences of smell perception given these two olfactory routes, and how they may influence food choice and intake. One study identified a specific olfactory deficit in some people, whereby odors entering the nose are perceived correctly but they cannot discriminate flavors via the retronasal route (see Cowart, Halpern, & Varga).

Odors and Odor Memory

While there are four or five basic taste qualities (see sidebar, p. 20), there are literally thousands of odors we can perceive. The search for an odor categorization schema similar to the basic taste qualities has been illusive to date. Odors are usually named according to the object they are associated with, not a general category label. For instance, an odor may smell like a "rose" or "dirty socks," whereas a taste may be in the "sweet" category and, thus, be associated with many foods.

Most of us have probably experienced a time when we smell a certain odor and recall a vivid memory associated with that odor. For instance, a particular odor may bring back a strong remembrance of afternoons spent with your grandmother in her kitchen. The ability of an odor to evoke strong, vivid memories of a past experience is called a Proustian phenomenon, in honor of French writer Marcel Proust who first eloquently described this experience (see Chu & Downes).

We still don’t quite understand the physiology and development of odor memory, although it appears infants are born with the ability to detect a wide variety of odors. Within hours of birth, infants and mothers can recognize each other by smell alone. One study demonstrated that infants appeared to prefer a mother’s unwashed (and thus, more odorous) breast for feeding than a washed breast. Since an infant’s vision is not well developed, the sense of smell may be critical in facilitating the baby’s orientation to the nipple for feeding.

It also has been suggested that early exposure to various odors from the mother during close skin contact may influence the infant’s ability to discriminate flavors. Thus, bottle-fed babies may not experience the same variety of smells experienced during breast-feeding, nor can they experience the diversity of flavors inherent in breast milk. Breast milk itself will change taste at each feeding, reflecting the diet of the mother. It is not certain whether exposure to different flavors from breast milk influences later food choice.

Tube-fed babies obviously do not experience any chemosensory stimulation from foods. One hypothesis is that there is a sensitive period of chemosensory development during infancy that may later influence a child’s food preferences. If so, we may need to think about providing taste and smell experiences along with our oral-motor stimulation in the neonatal unit and encourage close mother-infant skin contact.

Chemosensory Evaluation

Since chemicals interact with our taste and olfactory receptors to produce the sensations of taste and smell, these senses are called the chemosenses. Methods to assess the chemosenses rely on psychophysics, a branch of experimental psychology that quantifies human sensory responses to physical stimuli.

The most basic application is the use of a scale to measure intensity response. Of course, different scaling techniques may produce different response patterns, making interpretation of the results challenging, to say the least. Hedonic testing attempts to quantify how well we like or dislike a food. These assessments must take into consideration many variables, such as the type and number of samples tested, type of scale used, method of sampling, and normal individual sensory variability. People are commonly confused by the difference between liking and preference tests. We may indicate a preference between two products, but that does not necessarily mean we like either one. It is important to understand the interpretation of these two tests.

Results from psychophysical studies provide us information about the normal range of human chemosensory thresholds, our ability to discriminate between certain tastes or smells, and the quality of these sensations given different intensities. They also allow investigation of various sensory phenomena, such as adaptation and mixture suppression. Adaptation is what happens when you put your foot in a hot bath. Initially, your foot feels very hot, but with continuous exposure to the stimulus, the intensity of the response gradually declines and it doesn’t seem so hot anymore.

Adaptation also can occur with tastes and odors. Mixture suppression is the term applied to the perceived decreased intensity of mixed tastants or odors, compared to their intensity when unmixed, given the same concentration levels. The mechanism for mixture suppression is most likely the central nervous system, although there is some evidence that certain mixtures may rely on the peripheral nervous system.

Mixture suppression is one way we can try to mask the bitterness in many medications. Sensory scientists investigate various methods to block taste receptors or disguise the bitter taste with the addition of other ingredients. There are now companies devoted to improving medication flavor (and consumption) by adding flavors agreeable to the patient, no matter if the patient is a child, adult, or pet (www.flavorx.com/faq.asp). It appears that bubblegum flavor is a favorite with children (grape and watermelon are close seconds), but peanut butter is the winner with dogs, according to the FLAVORx company. The flavoring adds approximately $4 to the prescription with this company.

Simple mixtures usually are used in sensory studies because the complexity of food systems makes it difficult to control all the variables inherent in a food product. Thus, commercially available food products are rarely used in sensory evaluation studies. Not only are food products complex, there can be great variability in a purchased food item. Food products change chemically and qualitatively over time as they sit on the shelf. Additionally, we do not know exactly what is in the product. Food companies consider product recipes proprietary information. So, while the list of ingredients is on the label, the concentration level of each ingredient is not listed. Food companies regularly attempt to lower their production costs by using different processing techniques or altering ingredient concentrations. Of course, their goal is to lower production costs without the consumer noticing a difference. Therefore, simple mixtures using chemicals typically are used in sensory studies so the stimuli can be carefully controlled and replicated in future studies.

Etiologies & Treatments

The taste system is remarkably stable over time, with reported partial taste loss infrequent. In fact, studies that have investigated whole-mouth taste intensity after one side of the tongue was anesthetized showed no decline in whole-mouth taste intensity. It was hypothesized that cranial nerve disinhibition may occur when another nerve is blocked or cut. This may explain the lack of taste complaints received from stroke patients, especially those with obvious unilateral oral sensorimotor deficits.

If taste alterations are noticed, they are usually due to medications. A decrease in medication concentration or substitution of another medication may alleviate the taste symptoms. There are several classes of medications that appear to adversely affect the taste system, but carefully controlled clinical studies to elucidate the mechanisms for these problems have not been conducted. The heterogeneous medical problems and histories posed by patients who are typically also on multiple medications make it difficult to control all pertinent variables in a study.

The olfactory system appears much more susceptible than taste to problems. Most of these problems occur because of nasal and sinus infections, which can be successfully treated with medications. However, trauma to the olfactory system may permanently damage the sense of smell. Many patients with head injuries complain of smell loss, and there is no direct treatment. There is great interest in this area of research, however, since olfactory neurons have the capacity to replace themselves every 30 days. While spontaneous recovery is therefore possible, the mechanisms involved in this process are not understood.

Cathy A. Pelletier is a visiting assistant professor in the department of communication sciences and disorders at Syracuse University, Syracuse, NY. She is a certified SLP with extensive experience in the diagnosis and treatment of dysphagia. She recently obtained her doctorate from Cornell University in food science, specializing in sensory evaluation. Contact her by e-mail at capellet@syr.edu.

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