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Taste and smell are separate senses anatomically: taste is mediated by receptors in the oral cavity, smell by receptors high in the nasal cavity; their receptors, too, connect to the brain by diﬀerent nerves. In perception, however, they function in intimate connection with each other and also with a third sense, the ‘common chemical sense’ (CCS). The CCS is mediated by receptors in both cavities and in the eyes, throat, and larynx, and gives sensations of irritation and pungency. These three ‘chemical senses’ are indispensable to feeding, sexual, and predatory behavior in most organisms. In human beings they function mainly to promote the intake of healthy substances and to reject harmful ones.
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In everyday perception, sensations aroused via the chemical senses are often blended with one another. Eating and drinking stimulate taste and smell together, but both sensations are localized in the mouth, and in everyday parlance we call the complex ‘taste’ or ﬂavor. Touch, temperature, pungency (as in pepper), even sound (as in crunchy foods) may also participate in the complex ‘taste.’ The neural (brain) mechanism underlying this referral and melding of disparate sensations is largely unknown, but the phenomenon is nonetheless central to understanding eating and drinking.
In addition, substances like salt and vinegar that stimulate taste may simultaneously trigger pungency as well as true taste. Likewise ‘odors,’ e.g., ammonia, may trigger pungency as well as true smell.
To evaluate the separate roles of the chemical senses it is useful to compare their psychological and psychophysical properties in several categories, as follows.
The number of taste qualities is small, the classical view being four: sweet, sour, salty, and bitter. Occasionally others are debated, such as the savory taste of monosodium glutamate (called ‘umani’ in Japan). A complication here is the potential confusion of true taste with other sensations localized in the mouth as mentioned above.
Qualitative discrimination is a hallmark of smell, but the number of qualities is even harder to settle. One can discriminate among hundreds of thousands of odorous compounds, but the roles of quality, intensity, and CCS are hard to disentangle. Descriptive terms for qualities abound, but none of the various schemes for their classiﬁcation commands general acceptance. When it comes to CCS, little, if any, quality discrimination is to be discerned.
2. Hedonic Tone
Tastes and smells are very often experienced as pleasant or unpleasant, even at moderate intensity levels. This characteristic is not surprising given their roles in monitoring intake of foods and airborne substances. Acceptance of sweet and rejection of bitter appear already in neonates, but odor preferences develop over the ﬁrst decade. Compelling as they are, preferences are modiﬁable, dramatically by conditioned aversion (pairing of chemical sensation with nausea) or gradually by repeated experience. Even CCS sensations, initially unpleasant, can become appetizing, as in peppered food concoctions. The contexts in which sensations occur help determine their hedonic tone. For example, sour and bitter alone are unpleasant, but in lemonade or chocolate medium, agreeable.
The strength of tastes, smells, and CCS sensations all increase with increasing stimulus concentration, but in strikingly diﬀerent ways. The following example illustrates these diﬀerences. To ‘double’ subjectively rated taste intensity requires roughly doubling of the concentration. To ‘double’ subjective smell intensity requires 3 to 100 times (or more) increase in concentration, depending on the particular compound. (This property of smell is called psychophysical compression, and no other sense exhibits compression more impressively.) To ‘double’ a CCS sensation (nasal irritation from inhaled CO ) may require only a 50 percent increase in concentration (psychophysical expansion). Thus, as a function of increasing stimulus concentration, odor sensation grows slowly, CCS sensation fast, and taste sensation roughly proportionally to concentration.
4. Absolute Sensitivity
Smell exhibits exquisite sensitivity. That is, justdetectable physical threshold levels are often amazingly low. Some compounds, notably the sulphurous mercaptans, require for detection only about one part per trillion of air. This approximates a theoretical limit of only a few molecules absorbed by a few receptors. Across compounds, however, sensitivity varies by over a trillion-fold. To account for this in terms of chemical structure is a challenge for olfactory science, so far met only for limited families of organic compounds. For the same compound, the detection threshold in CCS (evaluated in persons lacking smell) is higher than in smell, but everyday intensities often stimulate both senses simultaneously.
Taste sensitivity is less remarkable and, like smell, also varies among compounds. Some bitter compounds are notable for high sensitivity. Genetic diﬀerences account for large diﬀerences among persons’ sensitivity to some bitter compounds, such as those known as PTC and PROP.
5. Diﬀerential Sensitivity
In contrast to their impressive absolute sensitivity, taste and smell exhibit poor diﬀerential sensitivity. By this is meant that to make a just-noticeable increase in supra-threshold strength requires a relatively large percentage increase in concentration: estimates range between 15 percent and 60 percent, as compared with 1 percent for vision and 10 percent for hearing. One investigation of factors limiting olfactory discrimination blames random ﬂuctuations in stimulus concentration rather than the sense of smell itself. Discounting stimulus ﬂuctuation, Cain (1988) concluded that smell rivals hearing in diﬀerential sensitivity.
6. Reaction Time
Relative to hearing, touch, and vision, whose reaction times to moderate stimulus levels are of the order of one or two tenths of a second, the chemical senses respond sluggishly. Weak odors give reaction times longer than a second, and even strong odors rarely less than half a second. CCS reaction times may be even longer. Reaction times to taste are also slow, of the order of half a second, and appear to be quality-speciﬁc, with bitter giving the longest reaction times.
Most if not all of the senses exhibit adaptation, which means that the detection, magnitude, and quality of a given stimulus depend on prior history of stimulation. The chemical senses are no exception. In general, in a chemical-free world, whether of taste or smell stimuli, sensitivity increases with time, that is to say one can detect progressively weaker stimuli, and suprathreshold stimuli appear progressively more intense. In contrast, as these senses are exposed to taste or smell stimuli, detection suﬀers and supra-threshold sensations decline. An interesting example is the perception of salt. The tongue is normally adapted to saliva, which contains salt. Bathing the tongue with water removes the salt and thereby increases salt sensitivity (i.e., reduces absolute threshold) by about a hundredfold.
When a taste stimulus lies undisturbed on the tongue the sensation will fade with time and often disappear altogether. Smell sensations, too, fade with prolonged exposure, not altogether, but to a steady state of roughly a third of their initial subjective magnitude.
Both smell and taste can exhibit cross-adaptation. This means that adaptation to a given compound can often aﬀect sensitivity to another compound. Sourness exhibits cross-adaptation with the sourness of all compounds that elicit sourness, consistent with the idea that sourness is mediated by a common mechanism. The same is true of saltiness. In contrast, some sweet compounds cross-adapt, but other do not; and some bitter compounds cross-adapt, but others do not. This is consistent with the belief that sweet and bitter have multiple stimuli (and perhaps multiple receptor mechanisms), unlike sourness and saltiness.
As in taste, some smell compounds cross-adapt with each other, others do not, but the data so far suggest no clear-cut underlying receptor mechanisms. Contrary to naive expectation, a pair of similar-smelling compounds may (or may not) exhibit less cross-adaptation than a pair of dissimilar-smelling compounds. And adaptation by one member of a pair on the other may greatly exceed adaptation in the reverse direction.
From time to time it has been suggested that distilled water can have a deﬁnite taste. It is now known that water can take on any quality, depending on the stimulus to which the tongue is previously adapted. For example, after adaptation to salt, distilled water tastes bitter–sour.
8. Stimulus Mixtures
A single chemical compound of the taste or smell laboratory serves only rarely as a stimulus in everyday tasting or smelling. In eating, especially, normal stimuli are mixtures, often comprising thousands of compounds. Such complexes are diﬃcult to study, so scientists have usually employed mixtures of a few components. Even so, the picture is complicated and much remains to be learned.
Two important phenomena can emerge simultaneously when two or more chemical compounds are tasted or sniﬀed in mixture. (a) Masking. A compound may mask another, either totally (if the masker is suﬃciently strong) so as to eliminate any perceptual trace of the second, or partially so as to reduce but not eliminate the perceptual intensity of the second. Masking may be reciprocal so that the second may also mask the ﬁrst. (b) Integration. A compound may add its eﬀect to the eﬀect of another compound so that the mixture of the two is more detectable than either alone, or have somewhat greater overall strength than either alone.
A mixture of taste compounds may be detectable even though all components are below their thresholds measured separately. Contrary to earlier belief, such integration occurs whether the constituents are of like or unlike quality. One can detect taste mixtures even when their components are qualitatively unrecognizable. Supra-threshold mixtures behave diﬀerently. The subjective intensity of a complex is usually less than predicted by simple addition of the intensities of the constituents assessed separately. Nevertheless, much remains to be learned about the rules of integration and masking in such mixtures.
As for taste, the rules governing olfactory mixtures are only partly known. Integration appears to characterize detection of smell in much the way that it does taste, with generous integration across many constituents. Less is known about supra-threshold mixtures but, similar to taste, mixtures smell weaker than the sum of their unmixed components. Also as in taste, smells are able to mask one another, partly or wholly, depending on their relative strengths.
Integration across compounds in the CCS has been shown to be generous. This ﬁnding has important implications for understanding air pollution. A mixture of many compounds that unmixed are all below the threshold of perceived irritation (of the nose, throat, eyes) can nevertheless cause signiﬁcant irritation. Thus, mixtures may constitute a nuisance or danger, even when each constituent by itself is deemed acceptable.
All of the senses undergo changes throughout the life span, the chemical senses included. By middle to older age the ability to detect smells and tastes appears to be slightly to moderately impaired in nearly everybody, and more markedly in smell than in taste. Supra-threshold intensity losses are clearly more pronounced in smell than in taste.
In evaluating aging of the sense of smell one must emphasize the cognitive richness of this sense. Of special interest is the ability to identify and remember, over long stretches of time, a large number of diﬀerent odors. It is not surprising, therefore, that aging takes a toll on odor identiﬁcation and memory, independent of its eﬀect on sensitivity and magnitude.
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- Cain W S 1988 Olfaction. In: Atkinson R C, Herrnstein R J, Lindzey G, Luce R D (eds.) Stevens’ Handbook of Experimental Psychology, 2nd edn. Wiley, New York, pp. 409–59
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