Factors affecting taste sensitivity
- Related Topics:
- human eye
- chemoreception
- mechanoreception
- kinesthesis
- touch
Fluids of extreme temperature, especially those that are cold, may produce temporary taste insensitivity. People generally seem to taste most acutely when the stimulus is at or slightly below body temperature. When the tongue and mouth are first adapted to the temperature of a taste solution, sugar sensitivity increases with temperature rise, salt and quinine sensitivity decrease, and acid sensitivity is relatively unchanged. Gustatory adaptation (partial or complete disappearance of taste sensitivity) may occur if a solution is held in the mouth for a period of time. The effect of one adapting stimulus on the sensitivity to another one (cross adaptation) is especially common with substances that are chemically similar and that elicit the same taste quality. Adaptation to sodium chloride will reduce one’s ability to sense the saltiness of a variety of the inorganic salts but will leave undiminished or even enhance such qualities as bitterness, sweetness, or sourness that were part of the taste of the salt before adaptation. Likewise, adaptation by one acid may reduce sensitivity to the sourness of other acids.
Adaptation studies often are complicated by so-called contrast effects; for example, people say that distilled water tastes sweet following their exposure to a weak acid. Water may take on other taste qualities as well; following one’s adaptation to a sour-bitter chemical such as urea, water may taste salty. Adaptation tends to diminish or enhance the effect of a subsequent stimulus depending on whether the two stimuli normally elicit the same or a contrasting taste. Thus, the adapted sweetness of water and all normally sweet-tasting substances are enhanced after one has tasted acid (sour). The bitterness of tea and coffee or the sourness of lemon are masked or suppressed by sugar or saccharin.
The human gustatory difference threshold (for a just noticeable difference in intensity) is approximately a 20 percent change in concentration. For very weak taste stimuli, however, the threshold sensitivity is less.
Food choice
One’s ability to taste is intimately involved with his eating habits or with his rejection of noxious substances. One of the earliest reflex responses of the infant, that of sucking, can be controlled by gustatory stimuli. Sweet solutions are sucked more readily than plain water; bitter, salty, or sour stimuli tend to stop the sucking reflex.
Many animals provide clear examples of beneficially selective feeding behaviour. Laboratory rats, when given an unhampered choice of carbohydrates, proteins, vitamins, and minerals (each in a separate container), show consistent patterns of selection that may be modified by physiological stresses and strains. A rat made salt-deficient by removal of its adrenal glands, for example, will increase its intake of sodium chloride sufficiently to maintain health and growth; normally, such gland removal is fatal in the absence of salt-replacement therapy. Histories of similar effects have been reported in humans, one case being that of a child with an adrenal disorder who kept himself alive by satisfying an intense salt craving.
Among adults, past experience strongly influences eating habits, sometimes to the point that physiological well-being suffers. Food habits and other factors play a significant role in eating behaviour.
Taste alone is not a reliable guide to safety. Poisonous substances often are unpalatable, but not invariably. Lead acetate, sometimes called sugar of lead, once was used as a sweetening agent with disastrous results before its potentially fatal effects were discovered. Many palatable substances, including some synthetic sweeteners, are toxic.
Taste aversions may be established by conditioning, even for substances that have been previously preferred. In one study, a rat tasted saccharin solution three hours before being exposed to enough radiation to become sick. When the animal recovered, it was found to have a strong aversion to the taste of saccharin. Other aversions selectively can be produced by injecting an individual with a nauseating drug before or after a specific taste experience. For example, the medication disulfiram (Antabuse), used in the treatment of alcoholism, reacts with alcohol to produce nausea and vomiting. An unusual finding is that long delays of up to several hours in the time between the presentation of the taste stimulus and the induction of illness do not prevent the conditioning. In most other studies, only brief intervals (perhaps up to minutes in duration) have been found to result in successful conditioning. Positive preferences also are subject to conditioning, as when the tastes of drugs or vitamins become associated with the feelings of well-being they generate.
Smell (olfactory) sense
In humans the olfactory receptors are located high in the nasal cavity. The yellow-pigmented olfactory membrane covers about 2.5 square cm (0.4 square inch) on each side of the inner nose. Olfactory receptors are long thin cells ending in 6 to 12 delicate hairs called cilia that project into and through the mucus that normally covers the nasal epithelium, or lining. The end of each receptor narrows to a fine nerve fibre, which, along with many others, travels through a channel in the bony roof of the nasal cavity and enters either of two specialized structures called olfactory bulbs—stemlike projections under the front part of the brain—to end in a series of intricate basketlike clusters called glomeruli. Each glomerulus receives impulses from about 26,000 receptors and sends them on through other cells, eventually to reach higher olfactory centres at the base of the brain. Fibres also cross from one olfactory bulb to the other.
Odorous molecules are carried to the olfactory region by slight eddies in the air during quiet breathing, but vigorous sniffing brings a surge of air into the olfactory region. Odour sensitivity may be impaired by blocking the nasal passages mechanically, as when membranes are congested by infection.
Pain endings of the trigeminal nerve fibres are widely distributed throughout the nasal cavity, including the olfactory region. Relatively mild odorants, such as orange oil, as well as the more obvious irritants, such as ammonia, stimulate such nerve endings as well as the olfactory receptors.
Olfactory qualities
The vocabulary of odour is rich with names of substances that elicit a great variety of olfactory qualities. One of the best-known published psychological attempts at classification was in 1916 on the basis of more than 400 different scents on human subjects. On the basis of the apparent similarities of perceived odour quality or confusions in naming, it was concluded that there were six main odour qualities: fruity, flowery, resinous, spicy, foul, and burned.
Electrical activity can be detected with fine insulated wires inserted into the olfactory bulb. Portions of the olfactory bulb toward the anterior or oral region in the rabbit are found to be more sensitive to water-soluble substances, whereas the more posterior parts of the olfactory bulb are more sensitive to fat-soluble substances. In addition, when very fine electrodes are used, individual cells (mitral cells) are sensitive to different groups of chemicals. Evidence for the existence of only a few primary receptors, however, does not emerge from such studies; a variety of different combinations of sensitivity has been found. Similarly, recordings from the primary receptor nerve fibres reveal different patterns of sensitivity. Electrical recording of this type also shows that olfactory sensitivity can be enhanced by a painful stimulus, such as a pinch on the foot. This appears to be a reflex that serves to enhance the detection of dangerous stimuli in the environment. Different parts of the olfactory neural pathways seem to be selectively tuned to discriminate different classes of olfactory information. For example, the third- and fourth-order olfactory neurons found beyond the olfactory bulb of the rat seem particularly concerned with distinguishing the odour of sexually receptive females. These neurons appear to be especially important in the preference the male rat shows for the smell of urine from the female in heat.
Odourous substances
To be odorous, a substance must be sufficiently volatile for its molecules to be given off and carried into the nostrils by air currents. The solubility of the substance also seems to play a role; chemicals that are soluble in water or fat tend to be strong odorants. No unique chemical or physical property that can be said to elicit the experience of odour has yet been discovered.
Only seven of the chemical elements are odorous: fluorine, chlorine, bromine, iodine, oxygen (as ozone), phosphorus, and arsenic. Most odorous substances are organic (carbon-containing) compounds in which both the arrangement of atoms within the molecule as well as the particular chemical groups that comprise the molecule influence odour. Stereoisomers (i.e., different spatial arrangements of the same molecular components) may have different odours. On the other hand, a series of different molecules that derive from benzene all have a similar odour. It is of historic interest that the first benzene derivatives studied by chemists were found in pleasant-smelling substances from plants (such as oil of wintergreen or oil of anise), and so the entire class of compounds was labelled aromatic. Subsequently, other so-called aromatic compounds were identified that have less-attractive odours.
The scent of flowers and roots (such as ginger) depends upon the presence of minute quantities of highly odorous essential oils. Although the major odour constituents can be identified by chemical analysis, some botanical essences are so complex that their odours can be duplicated only by adding them in small amounts to synthetic formulations.
Odour sensitivity
In spite of the relative inaccessibility of the olfactory receptor cells, odour stimuli can be detected at extremely low concentrations. Olfaction is said to be 10,000 times more sensitive than taste. A threshold value for the odorant ethyl mercaptan (found in rotten meat) has been cited in the range of 1/400,000,000th of a milligram per litre of air. A just-noticeable difference in odour intensity may be apparent when there is a 20 percent increase in odorant strength, but at low concentrations as much as a 100 percent increase in concentration may be required. Temperature influences the strength of an odour by affecting the volatility and therefore the emission of odorous particles from the source; humidity also affects odour for the same reasons. Hunting dogs can follow a spoor (odour trail) most easily when high humidity retards evaporation and dissipation of the odour. Perfumes contain chemicals called fixatives, added to retard evaporation of the more volatile constituents. The temporary anosmia (absence of sense of smell) following colds may be complete or partial; in the latter case, only the odours of certain substances are affected. Paranosmia (change in perceived odour quality) also may occur during respiratory infections. Changes in sensitivity are reported to occur in women during the menstrual cycle, particularly in regard to certain odorants (steroids) related to sex hormones. Olfactory sensitivity also is said to become more acute during hunger.
Adaptation to odours is so striking that the stench of a junkyard or chemical laboratory ceases to be a nuisance after a few minutes have passed. Olfactory adaptation, as measured by a rise in threshold, is especially pronounced for stronger odours. Cross adaptation (between different odours) may take place; thus, eucalyptus oil may be difficult to detect after one becomes adapted to the smell of camphor. Adaptation was long regarded solely as the result of changes in the olfactory receptor; however, the receptor cells in the nose seem to adapt only partially. Rhythmic discharges continue in the olfactory bulb long after one ceases to detect an odour. Apparently, some olfactory adaptation may occur in the brain as well as in the sense organ.
Effects on behaviour
Mammals in the wild state appear to utilize their odour glands for sexual attraction. Rats show a preference for the branch of a maze that has been scented with the odour of a sexually receptive female. It is likely that some rudiments of these effects operate in humans. The most sexually provocative perfumes have a high proportion of musk or a musklike odour. Genuine musk is derived from the sexual glands of the musk deer and is chemically related to human sex hormones; odour sensitivity in humans varies with the menstrual cycle.
Among laboratory animals the secretion of reproductive hormones can be markedly influenced by odour stimulation. This seems to be an innate physiological process rather than the result of learning. When the odour of a strange male is presented to a recently mated female, pregnancy block occurs. The normal hormonal changes following copulation are blocked under these conditions, and the fertilized egg fails to survive. A related study of the periodicity and length of the menstrual cycle in women exposed to the normal odours of men suggests that there may be similar effects among humans. Human behaviour, though it is molded and shaped by custom and culture, has many of its roots in basic sensual appetites.
Carl Pfaffmann