Comparison of taste qualities elicited by tactile,electrical, and chemical stimulation of single human taste papillae

Taste quality responses elicited by chemical, electrical, and tactile stimulation of 40 single fungiform papillae in four subjects were examined. A comparison of responses revealed differences in taste quality mediation for the three different classes of stimuli. Chemical stimulation elicited multiple taste qualities from a greater proportion of papillae than did either electrical or tactile stimulation. In addition, the chemical data revealed the presence of consistent bitter-sour and sour-salty confusions. Both tactile and electrical stimulation elicited relatively few bitter and sweet responses, as compared with either sour or salty responses. However, correct quality identification in these papillae for bitter and sweet compounds was no different from that for sour or salty compounds. In addition, electrical stimulation elicited a greater proportion of salty responses than did tactile stimulation. Comparison of the taste quality elicited by either tactile or electrical stimulation of a papilla with the taste quality exhibiting the greatest relative chemical sensitivity in the papilla also revealed independence of responding, and it was observed that both electrical and tactile stimulation elicited consistent taste quality responses from papillae in which these taste qualities could not be elicited by any concentration of any chemical test compound. The observed differences in quality judgments are discussed as possibly being the result of cross-modal gustatory associations resulting from nongustatory components of inadequate stimulation.     

Taste of nutrients: Amino acids,vitamins, and fatty acids

Multidimensional scaling techniques were used to determine the range in taste of nutrients for human subjects. The nutrients tested (amino acids, vitamins, and fatty acids) span the traditional sweet, sour, salty, and bitter gustatory quality range. The results also suggest that alkaline, sulfurous, and fatty qualities exist as well; the possibility that these three quality groupings are due to olfactory or tactile rather than gustatory input is discussed.     

Intensity scales for pure tastes and for taste mixtures

In four series of studies, taste intensities of sour, bitter, sweet, and salt were measured by number matching (magnitude estimation), and by noise matching. The two procedures agreed in their estimates of the power-function exponent for each taste. Representative exponents obtained from the studies are 1.0 to 1.1 for sour and bitter and 1.3 to 1.5 for sweet and salt. In a second set of studies each taste was judged against three or more background levels of a second taste. The results suggest that the power-function exponent is unaffected when a second taste is present in the solution. Whether the intercept changes in taste mixtures was not determined in these experiments.     

Magnitude production and magnitude estimation of taste intensity

Magnitude productions of sodium chloride (salty), quinine hydrochloride (bitter), and sucrose (sweet) yielded steeper psychophysical functions than those obtained with magnitude estimation. Hydrochloric acid (sour) produced the opposite effect. The results are discussed with respect to previous findings in taste intensity scaling and to general psychophysical considerations.     

Flavour neophobia in gerbils (Meriones unguiculatus) and hamsters (Mesocricetus auratus)

Flavour neophobia in gerbils and hamsters was studied by comparing the ingestive/pouching behaviour of these animals when presented with either a familiar unflavoured peanut or a novel one that imparted basic flavours such as sweet, salty, sour, or bitter. Neophobia was expressed by an aversion towards the novel nut manifested by greater latency either to ingest it (gerbils) or to deposit it in the cheek pouch (hamsters) relative to that of the unflavoured one. This method, involving reactions to the taste of novel solid food, provides a more sensitive measure than the traditional one measuring intake of a novel flavoured fluid. The results indicated that gerbils and hamsters encountering either a sweet, salty, or sour nut for the first time showed a neophobic reaction. However, there was no apparent sign of neophobia when the animals were tested with a nut flavoured with quinine. The theoretical implications of these findings are discussed.     

Auditory food cue conditioning: Effects of spatial contiguity and taste quality

The effects of spatial location of an auditory stimulus and quality of a potentiating taste on the aversive conditioning of an auditory food cue were investigated. In Experiment 1 rats ate salty food activating a tone from a speaker either in (spatially contiguous with) or displaced from the food and were then made ill. It was found that spatial contiguity during conditioning resulted in avoidance of food with a contiguous or a displaced tone in testing, and spatial displacement during conditioning resulted in avoidance of food only if the tone was also displaced in testing. Experiment 2 was identical, except rats ate salty, bitter, or sweet food with a displaced tone during conditioning and testing. The salty and bitter food groups demonstrated an avoidance of noisy food relative to the sweet food group. These results indicate that spatial contiguity interacts with taste quality in the conditioning of nongustatory food cues.     

Perceptual judgements of others' tasting experiences: are they enjoying their food?

Motivated by previous work suggesting that infants make stereotypic facial reactions to different tastes, we assessed communicative signals that might enable an adult to interpret the strength, taste, and hedonic value of a liquid flavour another adult is consuming. Four subjects (tasters) were overtly videoed consuming drinks that varied in strength (low, medium, and high concentrations), taste (sweet, sour, bitter, and salty), and hedonic value (taster-rated enjoyment). 26 observers assessed the strength, taste, and taster's enjoyment of the drink from video clips of the tasters. Observers perceived the hedonic value of the drinks to the taster and the drinks' strength based on the tasters' reactions but were generally poor at assessing the specific taste of the drink. For all tastes except the bitter ones, observers might have based their judgements of taste on how much the taster appeared to enjoy the drink. These findings are discussed in terms of communication of food's value.     

Gustatory quality changes as a function of solution concentration

Four selected female Ss were instructed to respond with the qualities of either bitter, salty, sour, sweet, or no taste to solutions of LiCl, KCl, Li₂SO₄, or K₂SO₄ which varied in concentration from 0.0040 to 0.050 M for the first two salts, and 0.0020 to 0.025 F for the last two. Mean percentages of each quality, with sweet combined with the no taste response, when plotted against concentration, gave functions in which one quality predominated within a given concentration range. This quality was superseded by another over a higher concentration range. These results are explained in terms of an inhibition phenomenon.     

The Five Flavors and Taoism: Lao Tzu's Verse Twelve

In verse twelve of the Tao Te Ching, Lao Tzu makes a curious claim about the five flavors; namely that they cause people not to taste or that they jade the palate. The five flavors are: sweet, sour, salt, bitter (these four are the elements of taste in the West, recognized by the science of taste) and spicy or hot as in ‘heat’ (or picante, not caliente). To the Western mind, the claim, ‘The five flavors cause them [persons] to not taste,’ is counterintuitive; on the contrary, the presence of the five flavors in a dish or in a meal would expand or enhance the senses and the palate, i.e., taste would be augmented by the five flavors. So what is the plausible meaning of the Taoistic claim? To answer this question, I look very briefly at the history of the doctrine of the five flavors and the history of Chinese cuisine. Lao Tzu probably has Confucian feasts in mind in making such a claim, but other interpretations are discussed.     

An application of the τ scale of taste: Interaction among the four qualities of taste

A series of 10 standard solutions, spaced equidistant on the τ scale, were prepared for each of the basic tastes, sweet, bitter, sour, and salty. In addition, four other concentrations of each basic taste, plus 96 compound solutions of two different solutes, were prepared as test stimuli. The Ss, four specialists in taste testing and four laymen, were asked to specify the taste of each test stimulus by matching it with the standard series. In this way, both the precision of the matching procedure and the enhancement or masking of one taste by another were measured in terms of τ, which is a nearly logarithmic function of concentration. The average standard deviations of the matchings among the Ss were in the order of 0.3 to 0.5 units on the τ scale, which corresponds to a value between 1 and 2 decilogs of concentration. Among the interactions observed was the mutual masking of sucrose and quinine sulfate.     

Potassium gymnemate and the sweet and bitter taste provoked electrically

The sweetness or bitterness of taste solutions of saccharin and quinine may be blocked, while the sweetness or bitterness evoked by electrical stimulation of the tongue by cathodal polarization is unchanged. Electrical taste stimulation apparently bypasses the most peripheral process by which taste solutions elicit sweet and bitter and thus acts directly on the receptor and/or its afferent nerve terminals.     

EMG activities of facial and chewing muscles of human adults in response to taste stimuli

To evaluate facial expression patterns induced by different taste stimuli, electromyograms (EMG) of the facial and chewing muscles of 12 healthy university students, aged 20 to 23 years, were analyzed. Sucrose (sweet), NaCl (salty), citric acid (sour), quinine-HCl (bitter), monosodium glutamate (MSG; umami), homogentisic acid (harsh), tannic acid (astringent), and capsaicine (pungent) were the taste stimuli used. Rating scale tests were made to assess the hedonics (pleasantness) of the taste stimuli. EMG responses of the corrugator supercilii, venter frontalis, orbicularis oculi, depressor anguli, and digastricus muscles to capsaicine, tannic, acid, and citric acid showed larger amplitudes than to NaCl, MSG, homogentisic acid, and sucrose. Increases in EMG responses for most facial and chewing muscles for the taste solutions had significant negative correlations to the solutions' hedonic scale values. Most facial and chewing muscles of adult humans therefore showed greater responses to disliked than to preferred or less preferred tastes.     

Perceptual changes in taste mixtures

Three studies were conducted to quantify perceptual changes that occur when sapid chemicals are tasted in mixture solutions. The primary effect when mixing sweetness (glucose or fructose) with salt (NaCl), sour (citric acid), or bitter (quinine sulfate) was to reduce the intensity of each taste in the mixture. The reduction was not equal for the two components, although the overall (total) taste intensity of the mixture appeared to be approximately 50% of the sum of the intensities of the unmixed components. Mixtures of sweet and salt developed an “unblended” or “clashing” taste, in which the components alternated in attempting to dominate the taste percept. Sweet mixed with either sour or bitter blended in almost all proportions. The “flavor” of sweetness in mixtures differed from that of simple sugar sweetness, suggesting that the presence of a second taste modified the qualitative aspect of sweetness. The magnitude of change in sweetness quality depended upon the sugar being rated, and upon the quality and intensity of the second, or modifying, taste.     

Interactions between CO2 oral pungency and taste

Two experiments are reported in which the perceptual interactions between oral pungency, evoked by CO2, and the taste of each of four tastants--sucrose (sweet), quinine sulfate (bitter), sodium chloride (salty), and tartaric acid (sour)--were explored. In experiment 1 the effect of three concentrations of each tastant on the stimulus-response function for perceived oral pungency, in terms of both rate of change (slope) and relative position along the perceived pungency axis, was determined. In experiment 2 the effect of three concentrations of CO2 on the stimulus-response function for the perceived taste intensity of each tastant was examined. Results show that the characteristics of the mutual effects of tastant and pungent stimulus depend on the particular tastant employed. Sucrose sweetness and CO2 oral pungency have no mutual effect; sodium chloride saltiness or tartaric acid sourness and CO2 oral pungency show mutual enhancement; and quinine sulfate bitterness abates CO2 oral pungency, whereas CO2 has a double and opposite effect on quinine sulfate bitterness--at low concentrations of bitter tastant CO2 enhances bitterness, and at high concentrations of bitter tastant CO2 abates bitterness. It is suggested that the perceptual attributes of saltiness and sourness are closer, from a qualitative point of view, to oral pungency than are the attributes of bitterness and sweetness.     

Cross-enhancement of the sour taste on single human taste papillae

The subjective intensity of one taste quality can be increased by prior exposure of the tongue to a different taste quality stimulus. This phenomenon, called cross-enhancement, may be the result of interactions among the physiological mechanisms that code taste quality. Another possible explanation is that the water solvent of the second stimulus acquires a taste after exposure of the tongue to the first stimulus. This water taste could add to the taste of the solute in the second stimulus and result in an increase of its subjective intensity. A third possibility is that taste receptors on the tongue may be sensitized by exposure to a taste stimulus. Using a small number of highly trained subjects, we have demonstrated that sucrose can enhance the intensity of an acid taste on the single papilla. Neither water taste nor sweet taste system activation played any role in the mediation of this enhancement. Through a series of experimentally derived inferential steps, we conclude that this phenomenon depends on the removal of protons from the acid receptors. In addition, we have demonstrated in the single papilla, that suppression of the acid taste when in mixture with sucrose can occur without sweet system activity. We conclude that sugars, through their capacity to bind protons, act to reduce the availability of protons to the acid receptors.     

Conditioned aversions and their memories in 5-day-old rats during suckling

Association learning during suckling was investigated. Five-day-old rats equipped with tongue cannulae placed either 2 mm rostral or 4-6 mm caudal to the intermolar eminence received sweet or salty solutions while suckling. This ingestion was followed by lithium chloride toxicosis. Pups with anterior cannulae took in considerably less fluid than control pups when tested 5 or 16 days later. A series of control groups demonstrated that this acquired aversion was associative in nature. Pups with posterior cannulae did not form the association. The failure of 5-day-old rats with posterior cannulae to form associations while suckling is not due to the prevention of conditioning by the act of suckling per se. Rather, the failure rests in the fluid's not reaching anterior taste receptors when injected into the posterior oropharynx, where the nipple normally empties its contents. These findings are discussed in terms of the transfer of information obtained during suckling prior to weaning, to feeding and drinking during and after weaning.     

As bitter as a trombone: synesthetic correspondences in nonsynesthetes between tastes/flavors and musical notes

In parallel to studies of various cases of synesthesia, many cross-modal correspondences have also been documented in nonsynesthetes. Among these correspondences, implicit associations between taste and pitch have been reported recently (Crisinel & Spence, 2009, 2010). Here, we replicate and extend these findings through explicit matching of sounds of varying pitch to a range of tastes/flavors. In addition, participants in the experiment reported here also chose the type of musical instrument most appropriate for each taste/flavor. The association of sweet and sour tastes to high-pitched notes was confirmed. By contrast, umami and bitter tastes were preferentially matched to low-pitched notes. Flavors did not display such strong pitch associations. The choice of musical instrument seems to have been driven primarily by a matching of the hedonic value and familiarity of the two types of stimuli. Our results raise important questions about our representation of tastes and flavors and could also lead to applications in the marketing of food products.     

Gustatory cross adaptation: Sourness and bitterness

In five experiments, Sa were presented with a variety of sour and bitter compounds after the tongue was rinsed with distilled H₂0, QHCl, urea, or citric acid. All the acids tested were significantly less sour following adaptation to citric acid than after adaptation to distilled H₂O. The taste of these acids was not affected by rinsing the tongue with QHCl or urea. QHCl adaptation markedly reduced the bitterness of some compounds, while having little effect on others, including urea and citric acid. Both urea and citric acid had smaller but reliable effects on the bitterness of QHCI. These apparently incompatible results do not seem to be the result of a simple verbal confusion between sourness and bitterness. Some compounds were not affected by any of the adapting conditions. The coding mechanisms for the sourness of acids appears to be relatively simple, while that for bitterness is more complex.     

Bitter struggle for survival: Evolved bitterness embodiment of survival motivation

In four experiments, we tested the hypothesis that survival motivation is grounded in the physical experience of bitter taste. Chinese (Experiment 1) and non-Chinese participants (Experiment 2) who tasted a bitter drink were quicker than the control participants who drank plain water in responding to survival-related words in a lexical decision task. Chinese participants who chewed bitter lotus root were more likely to discount the future than those who chewed sour lemon (Experiment 3). Finally, surprise retention tests revealed that experiencing a bitter rather than sweet taste led to higher retention of words processed for survival rather than mating scenarios (Experiment 4). These findings support our prediction that the taste of bitterness embodies survival motivation because both are adaptations to harsh environment.     

Individual gustatory reaction times to various groups of chemicals that provoke basic taste qualities

Reaction times (RTs) to four groups of substances that provoke different taste qualities were measured. Measurements for all substances with the same taste, equalized in perceived intensity and provoking a very strong taste, were made concurrently for each subject. The comparisons were made on the individual level. No significant differences in RTs to substances with the same taste quality were found. When the factor of perceived intensity is kept constant, no effect of the stimulus chemical composition on RTs seems to be present. RTs to stimuli with different tastes differ significantly, the shortest being to salt and the longest to bitter. The difference in RTs for sour and sweet substances is small, and the subjects were not all alike in terms of the order of RTs with respect to these stimuli.