Olfactionsmell Theories Of

Several interesting things about olfaction in clude the ideas that much of one's perceptual processing of odors is unconscious, that it is very difficult to recall smells, and that it is difficult to name them. However, curiously, the experience of a particular smell at a particular moment can stimulate numerous memories, often highly emotional, of episodes in which that smell was present [cf., Proust phenomenon/effect - named after the French novelist Marcel Proust (1871-1922), refers to the main character in his novel "Remembrance of Things Past" (1922-32) in which the individual recalls past events evoked by smells; of the various senses, smells are most likely to call up past memories]. Smell seems to act according to two separate modes of action that may result in different perceptual and informational experiences: the "near" experience of flavors in food and the "far" experience of air-borne smells (e.g., insects and some higher animals secrete volatile chemicals called pheromones, whose molecules travel through the air to other members of the species; cf., the Lee-Boot effect - named after the Dutch biologists S. van der Lee and I. M. Boot who reported the effect in 1955, refers to the gradual slowing down of the estrus cycles of a group of female mice that are housed together; if they are then exposed to the odor of a male mouse or to his urine, their cycles begin again; this latter effect, called the Whitten effect - named after the Australian biologist Wesley Kingston Whitten who described it in 1959, is caused by a pheromone in the male's urine; and the Vandenbergh effect - named after the American biologist John G. Vandenbergh, refers to the acceleration of the onset of puberty in female mice caused by a pheromone in the urine of a mature male mouse). Molecules that evoke the sensation of smell may be described as having a specific size, shape, weight, and "vibration frequency" (i.e., atoms in a given molecule move around in a characteristic pattern at predictable speeds that are different for different substances). Also, the particular atoms that make up a molecule (and the number of electrons available for chemical bonding with other molecules on the smell receptors) are likely to be important components of smell stimuli. However, as yet, there is no consensus as to which of these factors is critical. As is the case with other senses, the mechanism by which the stimulus molecules cause an electrical response (the transduction process) in the receptors in the upper nasal passages (the "olfactory epithelium") is still something of a mystery. There are probably at least two classes of transduction mechanisms in olfac-tion/smell: highly selective processes where specific receptor-cell proteins ("specialists") form reversible chemical bonds with specific parts of odorant molecules, and less selective processes where chemicals directly affect the receptor cell membrane anywhere they contact it ("generalist" smell receptors). Several of the current theories of olfaction concerning the more selective ("specialist") mechanisms are the lock-and-key theory (also called the stereochemical theory) - which holds that variously shaped molecules fit into special sites on the receptor membrane like a key fitted into a lock: when a molecule fits into a receptor site, a change in the structure of the cell membrane occurs, allowing ions into or out of the cell and, consequently, generating an electrical current [cf., the Swedish-American Carl Emil Seashore's (1866-1949) law of complementary odors]; the vibra-tion/vibrational theory - maintains that the stimulus molecule ruptures certain chemical bonds in the cell membrane, causing the release of stored-up energy which, in turn, generates an electrical current and action potentials; which bonds are ruptured in which cells depends on the unique vibration frequency of each stimulus molecule; and the gas chroma-tographic model - states that the rate of movement of an odorant molecule across the receptor surface determines the neural coding of its smell; that is, some molecules travel slowly across the receptor surface whereas others travel more rapidly, with the result that each molecule stimulates a different spatiotemporal pattern of receptors [cf., the earlier infrared theory of L. Beck & W. Miles - states that the olfactory sense organ functions, partly at least, through the filtering or absorption of infrared radiation from substances that emit odors. In the Raman effect/shift - named after the Indian physicist C. V. Raman (18881970), there is a shift in the frequency of light caused by the interaction of a photon with the energy level changes of an atom/molecule exposed to the photon; Raman showed that the interaction of vibrating molecules with photons passing through them alters the spectrum of the scattered light, either increasing or decreasing it by a fixed amount; thus, the Raman effect enables the probing of molecular energy levels and became an important spectroscopic technique that is used universally]. The gas chromatographic approach is based on the observation that the same sort of "across-fiber patterns" that are found in the sense of taste seem to be present in the olfactory/smell system, where it is likely that the "code" for smell qualities will be found in these patterns. However, a label-ed-line theory, such as applied to taste, may not be feasible for olfaction because it has many more types of labeled lines where there is no small list of primary smells for olfaction as there is for taste/gustation. Thus, at present, there is no evidence of olfactory fibers falling into groups as the taste fibers seem to display. The stereo-chemical/lock-and-key theory, or steric theory of odor, is based on the identification of seven primary odors, each associated with a particular molecule shape [cf., the German psychologist Hans Henning's (1885-1946) smell theory/smell prism that proposes six primary qualities of olfaction: foul, fruity, burnt, resinous, spicy, and flower]. The seven odors presumed by the steric theory of odor, along with their proposed shapes, and typical stimuli/examples are: camphoraceous (spherically shaped; e.g., camphor or mothballs); eth-ereal (small, flat, thin; e.g., dry-cleaning flu-id); floral (key-shaped; e.g., rose); musky (disk-shaped; e.g., angelica root oil); minty (wedge-shaped; e.g., peppermint candy); pungent (shape unknown; e.g. vinegar); and putrid (shape unknown; e.g., rotten eggs). Among other theoretical systems for identifying and classifying odors are: the Crocker-Henderson smell system - named after the American chemists Ernest C. Crocker (1888-1964) and Lloyd F. Henderson (? - ?), identifies four primary odors (fragrant, acid, burnt, and caprylic); and the Zwaardemaker smell system - named after the Dutch physiologist Hendrik Zwaardemaker (1857-1930), classifies smells on nine primary dimensions/odors: ethereal (e.g., fruits, wine), aromatic (e.g., spices), fragrant (e.g., flowers, vanilla), ambrosiac

(e.g., musk), alliaceous (e.g., garlic), empy-reumatic (e.g., coffee), hircine (e.g., rancid fat, cheese), foul (e.g., belladonna), and nauseous (e.g., rotten meat, feces). The phenomenon of adaptation is among the "laws" of olfaction (other "laws" of smell include fusion, compensation, selectivity, and modulation; cf., Boring, Langfeld, & Weld, 1939). The perceived intensity of an odor is affected by adaptation where only a brief period of exposure is sufficient to render an odor undetectable. In practical terms, adaptation may be beneficial to workers in an animal laboratory, an animal slaughter-house, or a zoo, but non-beneficial to coal miners who might need to detect an increase in the intensity of a potentially lethal gas. See also ADAPTATION, PRINCIPLES/LAWS OF; GUSTATION/ TASTE, THEORIES OF. REFERENCES

Henning, H. (1915). Der geruch. Zeitschrift fur Psychologie, 73, 161-257. Seashore, C. E. (1923). Introduction to psychology. New York: Macmillan. Boring, E. G., Langfeld, H., & Weld, H.

(1939). Introduction to psychology. New York: Wiley. Beck, L., & Miles, W. (1947). Some theoretical and experimental relationships between infrared absorption and olfaction. Science, 106, 511. Amoore, J., Johnson, J., & Rubin, M. (1964).

The stereochemical theory of olfaction. Scientific American, 210, 4249.

Pfaffman, C. (Ed.) (1969). Olfaction and taste. New York: Rockefeller University Press. Mozell, M. (1970). Evidence for a chromatographic model of olfaction. Journal of General Physiology, 56, 4663.

Amoore, J. (1982). Odor theory and odor classification. In E. theimer (Ed.), Fragrance chemistry - the science of the sense of smell. New York: Academic Press. Engen, T. (1982). Perception of odors. New

York: Academic Press. Vandenbergh, J. G. (Ed.) (1983). Pheromones and reproduction in animals. New York: Academic Press.

Gibbons, B. (1986). The intimate sense of smell. National Geographic, 170, 324-361.

Getchell, T., & Getchell, M. (1987). Peripheral mechanisms of olfaction: Biochemical and neurophysiology. In T. Finger & W. Silver (Eds.), Neurobiology of taste and smell. New York: Wiley.

Gilbert, A., & Wysocki, C. (1987). The smell survey: Results. National Geographic, 172, 514-525.


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