Behavioral support for a neural taste theory

The time required for rats to make a behavioral taste discrimination was predicted from neural discharge rates and tested using conditioned aversion. Predictions were based on the hypothesis that the responses evoked from a neural population by two different chemicals must diverge by a certain critical total number of spikes before the chemicals are discriminable. This total could be derived from the responses of all neurons in the population. Behavioral discrimination times generally supported predictions made from second-order (bulbar) neural responses, but were ambiguous concerning predictions based on fourth-order (thalamic) responses. The implications of these results for the possible functions of bulbar and thalamic taste neurons is discussed.     

On the singularity of taste sensations: What is a taste primary?

It is commonly accepted that there are four primary tastes. This conceptualization is notable for its simplicity and usefulness in guiding research and organizing data in the stimulus, receptor, neural and psychophysical aspects of taste. But however strongly defended or widely used, the concept of a primary taste remains undefined, especially in the psychophysical sense. The present research sets forth and tests one psychophysical defining property of a taste primary, that it should appear singular, and remain singular when mixed with other tastes; i.e., several singular tastes, when mixed, should remain separate and distinct. It is shown that mixtures are often judged to be as singular as their separate components, indicating synthesis of new tastes different from the components, and thus the existence of more than the few primary tastes.     

Effect of solution temperature on taste intensity in humans

The dependence of taste intensity upon both molar concentration and solution temperature was investigated by the method of magnitude estimation. For each of the four taste substances (glucose, NaCl, citric acid, quinine sulfate) 4–5 concentrations of solution were evaluated at each of six temperatures (25–50°C). Power functions of the form T = kCⁿ related subjective intensity to molarity at a fixed solution temperature. The exponent n for all tastes but citric acid was unaffected by temperature, suggesting that the growth rate of intensity with concentrations is unaffected within a 25° change. The intercept k varied with temperature for glucose, and was linearly related to temperature for NaCl.     

Comparison times are longer for hedonic than for intensity judgements of taste stimuli

Response times of intensity and hedonic comparisons were determined in a within-subjects experimental design. Forced-choice paired comparisons of orange lemonades with various concentrations of added quinine sulfate were made by 48 subjects. Depending on experimental condition, the subjects had to focus either on intensity or on pleasantness and give their responses as fast as possible. The data showed shorter response times for intensity comparisons than for pleasantness comparisons. Although taste processing may be partially serial and partially parallel, the larger part of the response times and the differences between them may be due to cognitive processing.     

Temperature dependence of human taste responses

Human taste thresholds for NaCl, HCl, Dulcin and QSO₄ were determined at 17, 22, 27, 32, 37, and 42°C using the psychophysical method of forced choice. The thresholds for all four compounds were lowest between 22° and 32° and rose above and below this temperature range. The results favored a physical-chemical basis for the transduction process over an enzymatic explanation. The effect of temperature on the intensity of suprathreshold solutions of QHCl was tested by varying the adapting and stimulus temperatures of solutions presented to one side of the tongue in a matching procedure with a standard on the other side. The temperature effect was found to be independent of the adapting temperature and the standard concentration, and to depend only on the stimulus temperature.     

Sweet taste enhancement through pulsatile stimulation depends on pulsation period not on conscious pulse perception

When aqueous NaCl solutions are tasted at continuously alternating concentrations, overall saltiness ratings exceed those observed for solutions with the same averaged, but non-alternating concentrations. In the present study, this effect is replicated for alternating aqueous sucrose solutions. We tested the hypothesis that enhancement depends on the conscious perception of intensity contrasts. High sucrose pulses were continuously alternated with low sucrose intervals at pulsation periods between 1.5 s and 20 s. Tastant pulsation enhanced sweetness intensity and this enhancement varied between 8 and 14%, peaking for periods from 4.5 s to 6 s (Study 1). This range coincided with the average pulsation period at which perceived taste pulses blended into a continuous stimulus, i.e. the taste fusion period (TFP). When comparing intensity ratings of sucrose solutions at individualized pulse periods of 0.5, 1.0 and 2.0 times TFP to ratings for continuous sucrose solutions of the same net concentration, pulsatile stimuli were perceived as significantly sweeter (p < 0.01; Study 2). However, sweetness intensity enhancement was the same for all pulsation periods. It was shown that sweet taste enhancement peaks at pulsation periods ranging from 0.5 to 2.0 TFP and that the level of conscious pulsation perception does not affect taste enhancement. The results suggest the introduction of enhancement effects at pre-conscious stages of gustatory processing. Further mechanisms that may account for such pre-conscious effects are discussed.     

Chemesthesis and taste: evidence of independent processing of sensation intensity

The ability to perceive taste from temperature alone ("thermal taste") was recently shown to predict higher perceptual responsiveness to gustatory and olfactory stimuli. This relationship was hypothesized to be due in part to individual differences in CNS processes involved in flavor perception. Here we report three experiments that tested whether subjects who differ in responsiveness to thermal taste and/or chemical taste also differ in responsiveness to oral chemesthesis. In experiment 1, subjects identified as 'thermal tasters' (TTs) or 'thermal non-tasters' (TnTs) used the general Labeled Magnitude Scale to rate the intensity of sensations produced on the tongue tip by capsaicin, menthol, sucrose, NaCl, citric acid, and QSO4. TTs rated all four taste stimuli higher than did TnTs, whereas sensations of burning/stinging/pricking and temperature from capsaicin and menthol did not differ significantly between groups. In experiment 2, testing with capsaicin on both the front and back of the tongue confirmed there was no difference in ratings of burning/stinging/pricking when subjects were grouped according to the ability to perceive thermal taste. In experiment 3, subjects were classified as high- or low-tasters according to their ratings of sucrose sweetness rather than thermal taste. No group difference was found for perception of capsaicin even when presented in mixture with sucrose or NaCl. The results are discussed in the context of previous evidence of an association between chemesthesis and sensitivity to the bitter tastant PROP, and in terms of the various peripheral and central neural processes that may underlie intensity perception in taste and chemesthesis.     

Interactions between stimuli with different taste qualities

Adaptation effects were examined between all possible pairs of the following substances: water, NaCl, urea, citric acid, caffeine, and sucrose. Each substance was used both as an adapting solution and as a test stimulus. Adaptation to a stimulus of one quality affects the taste of other stimuli through the addition of a water taste to the usual taste of the second stimulus, rather than by enhancement of the response to the second stimulus, per se, as had previously been thought. That is, water becomes a taste stimulus when it is presented following various adaptation conditions. The taste of the water solvent in the second stimulus adds to the normal taste of the solute in that stimulus. No evidence was found for true interactions among different taste qualities.     

Studies on the perception of taste: Do primaries exist?

The idea that there are four primary tastes is basic to almost all research in gustation. Although this concept guides the formulation and interpretation of studies of taste psychophysics, as well as studies of taste stimuli, receptors, and neural organization, there is a surprising absence of studies designed to determine if this quadripartite view of taste is correct. The present group of three studies is designed to ask this question on a psychophysical level, comparing the taste data with combinations of tones, which appear to remain separate; subjects are asked: (a) whether tastes and tones—including mixtures—are perceived as singular or more-than-one, (b) whether increasing the number of components in a mixture (tastes or tones) results in increases in the perceived complexity, and (c) whether stimuli (tastes or tones) remain identifiable in mixtures with other stimuli. In all three experiments, it appears that the individual tones remained distinct and unaltered in combinations. Taste stimuli combined very differently from tones, in ways that suggested that the original components lost their identity, and sensations other than those in the original components (e.g., “primary tastes”) were synthesized. From this it follows that taste is composed of many sensations, not just the “primary four”.     

Effects of hunger,satiety and glucose load upon taste intensity and taste hedonics

Subjects rated both the taste intensity and taste pleasantness of 4 compounds representing sweet, salty, sour and bitter, respectively. The typical pleasantness ratings appeared to conform to an inverted L shaped function for sweetness (maximum pleasantness at 1.0 M glucose) and for saltiness, and conformed to a negatively sloping function for citric acid and quinine sulfate. These pleasantness functions appeared robust when testing was performed either after an overnight fast, after breakfast or after lunch, respectively. After a satiating glucose load, however, the pleasantness of glucose taste failed to exhibit a breakpoint at 1.0 M glucose, suggesting that a change occurred in hedonic perception of taste after this exceptionally satiating intake. Satiety seems to influence taste pleasantness, but only to a limited degree, and affects only sweet preferences.     

Improved flow-chamber for taste stimulation

Performance criteria are suggested for flow-chambers, and instructions are supplied for optimizing the design of the most popular type of flow-chamber for physiological taste research in accordance with these criteria. A modification of the “standard” design is described which substantially improves performance.     

Effects of oral chemical irritation on taste

Oral irritation was induced by rinses with capsicum oleoresin and with piperine, constituents of red and black pepper, respectively. The perceived intensities of two concentrations of each of four tastants representing the four classical taste qualities were evaluated after rinsing with these irritants. Comparing taste intensity after rinses with capsicum and after control rinses with emulsifying agents or water, there were significant decrements in taste intensity of citric acid and quinine, and on one concentration of sucrose, but no effect on salt. The effects of piperine were more broad, with significant decrements in perceived intensity relative to emulsion controls for all substances.     

Sweet taste intensity is enhanced by temporal fluctuation of aroma and taste, and depends on phase shift

Pulsatile stimulation enhances taste intensity compared to continuous stimulation with stimuli of the same net tastant concentration. In the present work, we studied the effects of pulsatile delivery of aroma and taste on their combined contribution to taste intensity. Effects on taste perception were evaluated for aroma and taste pulsation and the aroma pulse-taste pulse phase shift. High-concentration sucrose pulses were alternated with water rinses every 2.5 s. Four different aroma (isoamyl acetate) versions were presented: (1) no aroma, (2) continuous aroma (3) aroma pulses in-phase and (4) aroma pulses out-of-phase with taste pulses. Aroma-taste combinations were evaluated for sweetness intensity by a 15-member trained panel using time-intensity analysis. Sweetness intensity was enhanced by pulsatile stimulation of sucrose or isoamyl acetate. In addition, taste enhancement by aroma and tastant pulses was additive if both were presented out-of-phase which resulted a sweetness intensity enhancement by more than 35% compared to a continuous sucrose reference of the same net sucrose concentration. Aroma-induced sweetness enhancement can be explained by cross-modal aroma-taste integration. Amplification of aroma-taste integration by pulsatile stimulation may be attributed to a potentiated afferent input of aroma and taste information prior to aroma-taste integration. Alternative mechanisms include the importance of swallowing on aroma-taste integration.     

Sweet taste of dilute NaCl: Psychophysical evidence for a sweet stimulus

Dilute NaCl tastes sweet. This sweetness could result from coding confusions in the nervous system such that weak NaCl produces neural signals resembling those for sweeteners like sucrose. On the other hand, an analysis of the structural chemistry of water-salt interactions suggests that water shells organized around cations may actually provide a sweet stimulus indistinguishable (to the receptor molecules) from more conventional sweet stimuli. In the present experiment, the sweetness of weak NaCl was abolished in two ways: by adaptation to sucrose and by topical application of Gymnema sylvestre. Since these two operations also abolish the sweet taste of a variety of conventional sweetners, the sweet taste of NaCl appears to result from the presence of a sweet stimulus.     

Effects of stimulation duration on electrogustometric thresholds

The ability to detect low electrical currents presented to the lingual surface is widely used to assess taste function in humans. Despite this fact, the influence of stimulus duration on electrical taste thresholds is not well established. In this study, we evaluated the effects of current duration (0.5, 1.0, and 1.5 s) on electrogustometric detection thresholds for two regions of the anterior tongue in 24 college students. Anodal stimulation was produced using a stainless steel 12.5-mm(2) electrode, and thresholds were determined using a single-staircase procedure. A non-monotonic function for the threshold values was observed for both tongue regions across the stimulus durations, with the 1.0-s duration stimulus resulting in a lower threshold value (i.e., higher sensitivity) than either the 0.5- or 1.5-s durations, which did not differ in magnitude from one another. These data suggest that stimulus-duration-related enhancement of lingual detection threshold sensitivity to electric currents disappears at some point after 1 s.     

Conditioned taste aversions: generalization to taste mixtures

Rats were trained to take their daily water ration within a 30-min session, during which the number of licks per 10-sec presentation of a drinking tube could be recorded. During one of these sessions, one of three stimuli (sucrose, NaCl or HCl) was presented, followed by the administration of cyclophosphamide to produce a conditioned taste aversion. When tested with mixtures of the conditioned stimulus (CS) with the other two stimuli and also with quinine hydrochloride, the animals avoided mixtures containing the CS in proportion to its concentration in the mixture. Although the natural preferences and aversions for these stimuli interacted somewhat with the learned taste aversions, rats responded to the presence of a CS in a mixture and did not generalize to other stimuli not containing the CS. Thus, the generalization of conditioned taste aversions provides a good measure of the behavioral similarities among gustatory stimuli.     

Multiple sensitivity to chemical stimuli in single human taste papillae.

The sensitivities of 15 human fungiform papillae were tested using a 5-alternative forced-choice procedure. Subjects were aked to recognize which of the following stimuli was presented to a papilla on each of 250 trials: 5.0 M NaCl, 0.5 N citric acid, 1.0 M quinine hydrochloride, and distilled H2O. Solution droplets were delivered to individual papillae from 0.5 mm diameter platinum wire loops. Based on each subject's responses to distilled H2O, corrections were made for individual response biases. Of the papillae tested, 33 percent responded to all four compounds, 33 percent to three, none to only two, 20 percent to only one, and 13 percent to none of the chemical stimuli. These results are contradictory to earlier work, in which it was suggested that taste quality is encoded by chemically specific papillae, but are consistent with the electrophysiological data suggesting multiple sensitivity of mammalian gustatory receptor cells and first-order neurons. The data suggested that the narrow range of sensitivity reported by von Békésy [2] was determined by the reciprocal relationship between the size of the stimulated area and the concentration necessary to elicit a threshold sensation.     

Lick triggered intracranial stimulation interferes with retrieval of conditioned taste aversion

The effect of brain stimulation on retrieval of conditioned taste aversion (CTA) to 0.15 M LiCl was investigated in 15 overtrained rats. The photoelecttrically detected licks at two drinking spouts delivering water or salt solutions, respectively, were used to trigger single electrical pulses, applied through implanted electrodes to basolateral amygdala or lateral hypothalamus. Current intensities not interfering with licking increased the number of licks required for recognition of the aversive fluid after spout switching from 1 or 2 to more than 5 and occasionally blocked gustatory discrimination completely. Discrimination was disrupted both by amygdalar and lateral hypothalamic stimulation of the same intensity, but amygdalar stimulation was more effective: interference was obtained from all amygdalar (n=21) but from only half of hypothalamic (n=17) stimulation points. The disruptive effects were cumulative. They were almost independent of the lick stimulus delay (1 to 100 ms) and often outlasted the period of stimulation for several min. The interfering effects of electrical stimulation are in accordance with the results of lesion studies indicating that the amygdala and the lateral hypothalamus participate in CTA retrieval, probably by mediating stimulus recognition and/or interruption of consummatory behavior.     

The neuropeptides CCK and NPY and the changing view of cell-to-cell communication in the taste bud

The evolving view of the taste bud increasingly suggests that it operates as a complex signal processing unit. A number of neurotransmitters and neuropeptides and their corresponding receptors are now known to be expressed in subsets of taste receptor cells in the mammalian bud. These expression patterns set up hard-wired cell-to-cell communication pathways whose exact physiological roles still remain obscure. As occurs in other cellular systems, it is likely that neuropeptides are co-expressed with neurotransmitters and function as neuromodulators. Several neuropeptides have been identified in taste receptor cells including cholecystokinin (CCK), neuropeptide Y (NPY), vasoactive intestinal peptide (VIP), and glucagon-like peptide 1 (GLP-1). Of these, CCK and NPY are the best studied. These two peptides are co-expressed in the same presynaptic cells; however, their postsynaptic actions are both divergent and antagonistic. CCK and its receptor, the CCK-1 subtype, are expressed in the same subset of taste receptor cells and the autocrine activation of these cells produces a number of excitatory physiological actions. Further, most of these cells are responsive to bitter stimuli. On the other hand, NPY and its receptor, the NPY-1 subtype, are expressed in different cells. NPY, acting in a paracrine fashion on NPY-1 receptors, results in inhibitory actions on the cell. Preliminary evidence suggests the NPY-1 receptor expressing cell co-expresses T1R3, a member of the T1R family of G-protein coupled receptors thought to be important in detection of sweet and umami stimuli. Thus the neuropeptide expressing cells co-express CCK, NPY, and CCK-1 receptor. Neuropeptides released from these cells during bitter stimulation may work in concert to both modulate the excitation of bitter-sensitive taste receptor cells while concurrently inhibiting sweet-sensitive cells. This modulatory process is similar to the phenomenon of lateral inhibition that occurs in other sensory systems.