Taste

Taste, or gustation, in humans and other terrestrial mammals is an oral chemoreceptive, and also mechanoreceptive and thermoreceptive, sensory system. Gustatory receptor organs occur primarily in specialized epithelial structures, papillae, found in limited and species-characteristic regions of mammalian tongue and palate (Weijnen and Mendelson 1977; Finger and Silver 1987). The taste system normally responds during drinking, biting, licking, chewing, and swallowing.

Taste is a component of a cognitive system, Flavor (Gibson 1966; White 1996), which includes SMELL (especially retronasal olfaction), chemesthesis (common chemical sense), tactile (HAPTIC PERCEPTION) input from oral structures, proprioception from the temporomandibular joints, and, via bone conduction, auditory responses (AUDITION; McBride and MacFie 1990). Laboratory procedures (Cattalanotto et al. 1993) or respiratory disease (Doty 1995) can remove component sensory systems; smell is often a major factor.

Taste responses originate in epithelial-derived receptor cells which have a lifespan of a week or two, all spent within an intraoral cutaneous sensory structure called a taste bud. The receptor cells differentiate from precursor cells, move closer to the epithelial surface, contact one or more sensory neurons and other taste receptor cells, and often extend microvilli which are separated from the outside world only by the secretions of taste bud cells (Beidler 1971). The brief lifespan is presumably demanded by direct but indispensable interactions with the diverse and often intense chemical, thermal, and mechanical energy levels in the oral cavity.

No one biochemical or biophysical mechanism transduces all gustatory stimulus chemicals into biological events. Substantial differences between gustatory chemicals, and the requirement that responses be sensitive and selective over a million-fold concentration span, apparently underlie this diversity. Gustatory chemicals effective in humans range from hydrogen ions to relatively large and complex protein molecules, and encompass most classes and configurations of molecules (Pfaff 1985; Brand, et al. 1993; Beauchamp and Bartoshuk 1997).

Many different gustatory stimuli activate a single taste receptor cell or individual neurons of the afferent nerve that innervates that receptor cell (Simon and Roper 1993). However, there is disagreement concerning the implications of this breadth of responsiveness. The predominant view, known as the basic tastes model, proposes that only the most robust responses have a role in sensory processing, with the others filtered somewhere in the central nervous system. This conceptualization permits assignment of a receptor or neuron to one or two of the test stimuli, and categorization as a "N" best unit, "N" being the gustatory stimulus chemical that evoked the largest response. The theory asserts that direct linkages exist between the identified best-stimulus, the stimulated receptor cell or neuron, and a category of taste perception. However, no human gustatory SINGLE-NEURON RECORDINGS exist (Meiselman and Rivlin 1986; Halpern 1991). Most generalizations are made from laboratory rodents.

The best-stimulus and best-response outcomes provide neurophysiological support for the basic tastes model's claim that gustatory experience depends on four or five distinct, independent classes of "vertically integrated" taste stimuli, receptor cells, afferent neurons, and independent perceptual categories. The corresponding basic or primary gustatory stimuli and perceptual categories are commonly specified as an alkaloid derivative such as a quinine salt bitter, the metallic salt NaCl salty, an acid sour, a saccharide such as sucrose sweet, and monosodium glutamate plus a 5"-ribonucleotide umami.

Psychophysical evidence for the basic tastes model includes a single basic taste sufficing for human descriptions of aqueous solutions of pure chemicals, successful assignment of taste intensity into basic tastes, and inability to discriminate between a number of inorganic and organic acids, or between the sugars fructose, glucose, maltose, and sucrose (Bartoshuk, 1988). Further, in certain laboratory rodents in whom a pharmacological agent, amiloride, selectively blocks gustatory neural responses to small metallic salts, behavioral measures using the same agent show alteration of responses toward NaCl and KCl but not to sucrose.

Negating data for the basic tastes model include incompatibility with studies utilizing normal foods or many-component mixtures, inability to produce the range of human gustatory experiences by combining the primary or basic taste stimuli, discrimination between stimuli of a basic taste category, cross-adaptation failure within a basic taste, and frequent descriptions of many substances, including basic taste stimuli, using multiple basic taste terms and other words (side tastes).

An alternative theory is the pattern or across fiber model, utilized by a minority of investigators, which posits that relative responsiveness across an array of taste units with broadly overlapping but different sensitivity profiles is the initial sensory event. The number of gustatory response "types" is left unspecified at greater than four. Psychophysical supporting data include cross-adaptation between "basic taste" categories, inability to identify the components of natural and laboratory mixtures, characterization of some aqueous solutions as complex or "more-than-one," and the cited negating data for the basic tastes model. Furthermore, if oral application of amiloride, a pharmacological blocker of an epithelial sodium channel (see supporting data for the basic tastes model, above), is combined in laboratory rats with a procedure in which drinking of NaCl is followed by injection of a mild poison (conditioned taste aversion for NaCl induced by injection of LiCl), subsequent behavioral avoidance generalizes beyond NaCl drinking to nonsodium salts such as KCl and NH₄Cl, which humans and rats normally categorize as quite different from NaCl. On the other hand, all supporting observations for the basic tastes model represent serious difficulties for the pattern model of taste.

A union of the basic tastes and pattern models might be possible following classic "fusion" CONCEPTS, since CATEGORIZATION neither precludes discrimination nor requires that processing reside primarily at the receptor level. Neither model addresses temporal aspects, although taste shows temporal integration over several seconds while stimulus durations of 50 msec are sufficient for taste PSYCHOPHYSICS.

Gustatory neuroanatomy is similar in all mammalian hindbrains. However, rostral organization in rhesus monkey and probably human differs dramatically from New World primates and most other mammals, with more direct brain stem connections and greater cortical representation (Getchell et al. 1991). The profound evolutionary implications remain unexplored.

Study of gustatory psychophysics as well as cognitive aspects offers many opportunities if investigations avoid excessive adherence to theoretical models. Taste is similar in outline to other perceptual systems, while also reflecting its unique role in adaptive behavior.

See also

• COLOR, NEUROPHYSIOLOGY OF
• PAIN
• QUALIA
• SENSATIONS

Additional links

• The ChemoReception Web Home Page

-- Bruce Halpern

References

Bartoshuk, L. M. (1988). Taste. In R. C. Atkinson, R. J. Herrnstein, G. Lindzey, and R. D. Luce, Eds., Steven's Handbook of Experimental Psychology. 2nd ed., vol. 1: Perception and Motivation. New York: Wiley, pp. 461-499.

Beauchamp, G. K., and L. M. Bartoshuk, Eds. (1997). Tasting and Smelling. Handbook of Perception and Cognition. 2nd ed. San Diego: Academic Press.

Beidler, L. M., Ed. (1971). Handbook of Sensory Physiology. Vol. 4, Chemical Senses, part 2. Taste. Berlin: Springer.

Brand, J. G., J. H. Teeter, R. H. Cagan, and M. R. Kare, Eds.. (1993). Chemical Senses. Vol. 1, Receptor Events and Transduction in Taste and Olfaction. New York: Marcel Dekker.

Cattalanotto, F. A., L. M. Bartoshuk, K. M. Östrom, J. F. Gent, and K. Fast. (1993). Effects of anesthesia of the facial nerve on taste. Chemical Senses 18:461-470.

Doty, R. L., Ed. (1995). Handbook of Olfaction and Gustation. New York: Marcel Dekker.

Finger, T. E., and W. L. Silver, Eds. (1987). Neurobiology of Taste and Smell. New York: Wiley.

Getchell, T. V., R. L. Doty, L. M. Bartoshuk, and J. B. Snow, Jr., Eds. (1991). Smell and Taste in Health and Disease. New York: Raven Press.

Gibson, J. J. (1966). The Senses Considered As Perceptual Systems. Boston: Houghton Mifflin.

Halpern, B. P. (1991). More than meets the tongue: Temporal characteristics of taste intensity and quality. In H. T. Lawless and B. P. Klein, Eds., Sensory Science Theory and Applications in Foods. New York: Marcel Dekker, pp. 37-105.

McBride, R. L., and H. J. H. MacFie, Eds. (1990). Psychological Basis of Sensory Evaluation. London: Elsevier.

Meiselman, H. L., and R. S. Rivlin, Eds. (1986). Clinical Measurement of Taste and Smell. New York: Macmillan.

Pfaff, D. W., Ed. (1985). Taste, Olfaction, and the Central Nervous System. New York: Rockefeller University Press.

Simon, S. A., and S. D. Roper, Eds. (1993). Mechanisms of Taste Transduction. Boca Raton, FL: CRC Press.

Weijnen, J. A. W. M., and J. Mendelson, Eds. (1977). Drinking Behavior: Oral Stimulation, Reinforcement, and Preference. New York: Plenum Press.

White, B., Ed. (1996). Special issue on flavour perception. Trends in Food Science and Technology 7:386-458.

Further Readings

Arnott, M. L., Ed. (1975). Gastronomy: The Anthropology of Food and Food Habits. The Hague: Mouton.

Beauchamp, G. K., K. Kurihara, Y. Ninomiya, T. Sato, and T. Yamamoto, Eds. (1994). Kirin international symposium on bitter taste. Physiology and Behavior 56:1121-1266.

Bosma, J. F., Ed. (1973). Fourth Symposium on Oral Sensation and Perception. Development in the Fetus and Infant. Bethesda, MD: U.S. Department of Health, Education, and Welfare publication no. NIH-75-546.

Carterette, E. C., and M. P. Friedman, Eds. (1978). Handbook of Perception, vol. 6A, Tasting and Smelling. New York: Academic Press.

Collings, V. B. (1974). Human taste response as a function of locus of stimulation on the tongue and soft palate. Perception and Psychophysics 16:169-174.

Delconte, J. D., S. T. Kelling, and B. P. Halpern. (1992). Speed and consistency of human decisions to swallow or spit sweet and sour solutions. Experientia 48:1106-1109.

Faull, J. R., and B. P. Halpern. (1972). Taste stimuli: Time course of peripheral nerve responses and theoretical models. Science 178:73-75.

Gay, W. I., Ed. (1973). Methods of Animal Experimentation, vol. 4. Environment and the Special Senses. New York: Academic Press.

Halpern, B. P. (1986). Constraints imposed on taste physiology by human taste reaction time data. Neuroscience and Biobehavioral Reviews 10:135-151.

Halpern, B. P. (1998). Amiloride and vertebrate gustatory responses to NaCl. Neuroscience and Biobehavioral Reviews 23:5-47.

Kawamura, Y., and M. R. Kare, Eds. (1987). Umami: A Basic Taste. New York: Marcel Dekker.

Kawamura, Y., K. Kurihara, S. Nicolaïdis, Y. Oomura, and M. J. Wayner, Eds. (1991). Umami: Proceedings of the second international symposium on umami. Physiology and Behavior 49:831-1030.

Kurihara, K., N. Suzuki, and H. Ogawa, Eds. (1994). Olfaction and Taste 11. Tokyo: Springer.

Mathlouthi, M., J. A. Kanters, and G. G. Birch, Eds. (1993). Sweet-taste Chemoreception. London: Elsevier.

Seiden, A. M., Ed. (1997). Taste and Smell Disorders. New York: Thieme.

Weiffenbach, J. M., Ed. (1977). Taste and Development. The Genesis of Sweet Preference. Bethesda, MD: U.S. Department of Health, Education, and Welfare publication no. NIH-77-1068 .

Copyright © 1999 Massachusetts Institute of Technology