Electrophysiology of appetitive taste and appetitive taste conditioning in humans

The present study examines two novel aspects of appetitive processing and conditioning: the electrophysiological response to (a) the appetitive taste of a sweet fluid and (b) appetitively/taste conditioned visual stimuli. Event-related potentials (ERPs) were recorded from 32 subjects while they performed a taste conditioning task, in which two symbols were paired with a sweet or a neutral fluid. The results show a clear P1/N1/P2 complex, and a late positive potential (P3) with maxima at right fronto-central electrode sites, in response to the taste stimuli. Of these components particularly the P3 showed robust differences between the sweet and neutral taste. In addition, the electrophysiological response to the taste conditioned stimuli (CS), also showed the expected differences for both P2 and P3 at frontocentral electrodes. The present data demonstrate that the employed paradigm is a useful methodology to study the electrophysiological responses to unconditioned appetitive stimuli. Since appetitive conditioning is assumed to be disturbed in many psychopathological conditions, such as substance dependence, depression and eating disorders, this paradigm could be employed to get more insight in these conditions in humans.     

Reduced palatability in lithium- and activity-based, but not in amphetamine-based, taste aversion learning

Conditioned taste aversions (CTA) based on lithium chloride (Experiment 1), amphetamine (Experiment 2), and wheel running (Experiment 3) were examined using the analysis of the microstructure of licking to measure the palatability of the taste serving as the conditioned stimulus (CS). Pairing saccharin with amphetamine reduced saccharin intake without reducing the size of licking clusters, initial lick rate, or the distribution of inter-lick intervals (ILIs) within a cluster. By contrast, pairing saccharin with lithium or wheel-running reduced saccharin intake as well as lick cluster size, initial lick rate, and the distribution of ILIs within a cluster. As lick cluster size, initial lick rate, and ILI distribution can be used as indices of stimulus palatability, the current results indicate that taste aversions based on either lithium or activity reduced the palatability of the CS. This suggests that aversions based on both lithium and wheel running involve conditioned nausea to the CS taste. The absence of similar changes in licking microstructure with amphetamine-based CTA is consistent with other evidence indicating this does not involve nausea.     

Reduced palatability in drug-induced taste aversion: II. Aversive and rewarding unconditioned stimuli

Drugs of abuse are known to reduce intake of a taste conditioned stimulus (conditional stimulus, CS), a behavioral response sometimes seen as paradoxical because the same drugs also serve as rewards in other behavioral procedures. In the present study we compared patterns of intake and palatability (assessed using microstructural analysis of licking) for a standard saccharin CS paired with the following: lithium chloride, morphine, amphetamine, or sucrose. We found that morphine and amphetamine, like lithium-induced illness, each suppressed CS intake and caused a reduction in saccharin palatability. Sucrose, a rewarding stimulus, did not reduce the palatability of the saccharin CS. We interpret these finds as evidence that drugs of abuse induce conditioned taste aversions.     

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”.     

Central gustatory lesions and learned taste aversions: unconditioned stimuli

The efficacy of two different unconditioned stimuli (US) in producing conditioned taste aversion (CTA) was tested in rats after bilateral ibotenic acid (IBO) lesions of the gustatory nucleus of thalamus (TTAx) and the medial and lateral parabrachial nuclei (mPBNx, lPBNx). An initial study determined an equivalent dose for the two USs, LiCl and cyclophosphamide (CY), using non-lesioned rats. Subsequently, using a separate set of lesioned animals and their sham controls (SHAM), injections of CY were paired 3 times with one of two taste stimuli (CSs), 0.1 M NaCl for half the rats in each group, 0.2 M sucrose for the other half. After these conditioning trials, the CS was presented twice more without the US, first in a 1-bottle test, then in a 2-bottle choice with water. The acquisition and test trials had 2 intervening water-only days to assure complete rehydration. Two weeks later, the same rats were tested again for acquisition of a CTA using LiCl as the US and the opposite CS as that used during the CY trials. The SHAM and TTAx groups learned to avoid consuming the taste associated with either CY or LiCl treatment. The two PBNx groups failed to learn an aversion regardless of the US.     

Early weaning does not accelerate the expression of nursing-related taste aversions

Preweanling rat pups do not display an aversion to a flavor conditioned stimulus (CS) paired with illness if the CS is presented during the act of suckling. In contrast, 20-day-old pups do form such a conditioned taste aversion while suckling (Martin & Alberts, 1979). The dissolution of the nursing-related "blockade" of toxiphobia correlates with the onset of solid food intake. Moreover, prevention of weaning prolongs the blockade; ingestive experience with solid food is necessary for the expression of nursing-related taste aversions in 26-day-old "food naive" pups (Gubernick & Alberts, 1984). The present experiments tested the possibility that premature weaning to solid food might accelerate the onset of nursing-related toxiphobia. Pups were weaned at 13 days of age and ingested only food and water. These prematurely weaned pups received taste aversion conditioning while suckling on Day 16, but showed no aversion to the CS flavor during a later food test. Thus, conditions that lead to early weaning (ingestion of solid food) do not accelerate the onset of taste aversions to mother's milk.     

Conditioned food aversion elicited by the temperature of drinking water as a conditioned stimulus in rats

It is known that taste can act as a conditioned stimulus (CS) for conditioned food aversion. In the present study, in order to examine whether or not the temperature of drinking water can be a CS, we conducted behavioral experiments in Wistar rats. The following results were obtained: (1) The rats subjected to aversive conditioning to 5 or 40 degrees C distilled water could learn to avoid these CSs, but they did not avoid any taste stimuli. (2) The rats subjected to aversive conditioning to 5 or 40 degrees C 0.1 M sucrose developed a generalized avoidance to sucrose at any temperature. (3) When rats familiarized to 25 degrees C 5 mM saccharin-Na (Sacc) were subjected to aversive conditioning to 5 or 40 degrees C Sacc, they avoided the respective CS, but they did not generalize it to any other stimuli even if having the same temperature as the CS. (4) The rats which had undergone transection of the taste nerves (chorda tympani and glossopharyngeal nerves) could acquire the conditioned response to the temperature of the CS. These results suggest that rats can be conditioned to temperature aversion and that the taste nerves are not needed in the formation of this conditioning.     

Stimulus preexposure reduces generalization of conditioned taste aversions between alcohol and non-alcohol flavors in infant rats

Results of 3 experiments showed that infant rats (age 13-17 days) generalize conditioned taste aversions between alcohol and non-alcohol tastes such as a mixture of sucrose and quinine, apple cider vinegar, or coffee. Nonreinforced preexposure to those tastes reduced generalized aversions between them. Generalization between alcohol and sucrose-quinine was reduced not only after preexposure to both tastes, but also when only the nonconditioned taste was preexposed, whereas with alcohol and vinegar, both tastes had to be preexposed to obtain that effect. In no case was generalization reduced when only the to-be-conditioned taste was preexposed. Previous experience with alcohol alone, as well as with similar gustatory stimuli, may enhance subjects' ability to differentiate them during infantile stages in rats.     

The effect of amygdala-kindled seizures on the acquisition of taste and odor aversions

Two experiments were conducted to investigate the effect of amygdala-kindled seizures on the acquisition of fluid aversions using taste, odor, and compound taste-odor cues. In Experiment 1 all subjects were poisoned with lithium chloride 30 minutes after ingesting a novel taste. Experimental subjects were either kindled 15 minutes following ingestion or 15 minutes following LiCl injection. The strength of the taste aversion in the kindled animals was significantly attenuated compared to nonkindled controls. There was a tendency for kindling within the CS-US interval to be more disruptive than kindling that occurred after LiCl injection. The second study explored the effect of kindling after the CS on the development of aversions to an odor cue or a compound taste-odor cue. This study found that kindling after drinking effectively blocked the acquisition of both odor and taste aversions and compounding did not affect the strength of the odor aversion.     

Perceptual characteristics of the amiloride-suppressed sodium chloride taste response in the rat

The contribution of amiloride-sensitive membrane components to the perception of NaCl taste was assessed by using a conditioned taste aversion procedure. Eight independent groups of adult rats were conditioned to avoid either 0.1M NaCl, 0.5M NaCl; 0.1M NH4Cl, or 1.0M sucrose while their tongues were exposed either to water or to the sodium transport blocker amiloride hydrochloride. In contrast to rats exposed to water during conditioning, rats exposed to amiloride were unable to acquire a conditioned taste aversion to 0.1M NaCl. Differences in the acquisition of taste aversions between the amiloride- and nonamiloride-treated groups were not apparent when the conditioned stimulus (CS) was 0.5M NaCl, 0.1M NH4Cl, or 1.0M sucrose. Although the magnitude of the 0.5M NaCl aversion was similar between amiloride- and non-amiloride-treated rats, the perceptual characteristics of the CS differed between groups. Analyses of stimulus generalization gradients revealed that amiloride-treated rats generally avoided all monochloride salts after conditioning to 0.5M NaCl but not nonsodium salts or nonsalt stimuli. In contrast, rats not treated with amiloride only generalized the 0.5M NaCl aversion to sodium salts. No differences in generalization gradients occurred between groups when the CS was 0.1M NH4Cl or 1.0M sucrose. These findings suggest that the "salty" taste of NaCl is primarily related to the amiloride-sensitive portion of the functional taste response in rats. Conversely, the portion of the NaCl response insensitive to amiloride appears to have "sour-salty" perceptual characteristics and does not appear to be perceived as being salty.     

Illness-induced taste aversions in normal and bulbectomized hamsters

Baitshyness acquisition and extinction in male Syrian golden hamsters was evaluated using saccharin solution as the target flavor and 75 or 150 mg/kg injections of cyclophosphamide as the illness-inducing agent. Conditioned aversions were obtained in drug-injected animals, but extinction was rapid and complete within twelve days of two-bottle preference testing. A second experiment using the same animals found that bilateral aspiration of the olfactory bulbs disrupted the subsequent acquisition of an aversion to milk. Baitshyness appears to have advantages over other tasks producing avoidance behavior in the hamster, and the hamster may be useful in neural investigations of conditioned taste aversions which have previously concentrated on the rat.     

Gustatory information of umami substances in three major taste nerves

The chorda tympani (CT), glossopharyngeal (GL), and greater superficial petrosal (GSP) nerves, the three major branches of cranial nerves innervating taste buds, respond with considerable differences to various taste stimuli. To examine which nerve is responsible for transmitting umami taste in rats, we conducted electrophysiological and behavioral experiments. In the electrophysiological study, responses to umami substances were compared among these three nerves. The CT and GSP were more responsive to mixtures of monosodium L-glutamate (MSG) and 5'-inosine monophosphate (IMP) than the GL. Synergistic effects by the mixture of MSG and IMP were the most prominent in the CT followed by the GSP, whereas it was negligible in the GL. In the behavioral study, rats with a combined transection of the CT and GSP could not acquire conditioned taste aversions to umami substances. These results suggest that umami taste is conveyed more dominantly via the CT and GSP than the GL in the rat.     

Constructing quality profiles for taste compounds in rats: a novel paradigm

We developed a novel behavioral method, adapted from the work by Morrison (1967), for the assessment of taste quality in rats. Four groups of rats were trained to discriminate a standard stimulus (either NaCl, sucrose, quinine, or citric acid, which are widely thought to represent the four basic human taste qualities of salty, sweet, bitter, and sour, respectively) from the remaining three compounds (each at multiple concentrations). Animals were then tested for generalization to the standard stimuli when test compounds were presented and a quality profile was constructed. Rats generalized novel concentrations of standard stimuli completely to their training concentrations and generalized their responses to mixtures of NaCl and sucrose on the basis of the relative concentrations of the stimulus components. In general, the sugars (at high concentrations), denatonium, tartaric acid, and sodium gluconate generalized to sucrose, quinine, citric acid, and NaCl, respectively. Monosodium glutamate generalized to a mixture of sucrose and NaCl. KCl produced a complex generalization profile with notable quinine and citric acid components. Our procedure supplements the current use of the conditioned taste aversion generalization procedure which has some procedural and interpretive limitations. Although our procedure involves the use of a complex stimulus delivery and response measurement apparatus and requires substantial initial conditioning of animals, once trained, a single cohort of animals can be tested for its response to a substantial number of test stimuli over the course of many months without any ostensible loss of stimulus control.     

Plasma corticosterone levels as an index of the strength of illness induced taste aversions

Data are presented which demonstrate the task generality of pituitary-adrenal changes that accompany avoidance conditioning. In two experiments a conditioned aversion to milk was established by pairing it with lithium chloride (LiCl). In Experiment 1 conditioned pituitary-adrenal activation occurred when, in a conflict situation, animals reexperienced milk that had earlier been paired with LiCl. The relationship between the strength of aversion and corticosterone levels was such that animals showing the greatest avoidance showed the largest elevations in plasma corticosterone. In Experiment 2 this behavior/steroid relationship was manipulated. Dexamethasone (DEX) pretreatment on the day of conditioning was used to attenuate the conditioned aversion. Compared to saline (SAL) controls when reexposed to milk, DEX animals showed an attenuated aversion (i.e., drank more) and a smaller conditioned response (i.e., less adrenocortical activity). The reduction of conditioned elevation in corticosterone was not due to any residual effects of DEX at the time of testing (Experiment 3). Plasma levels of corticosterone represent an index for assessing the strength of illness induced conditioned taste aversions.     

Vestibular lesions selectively abolish body rotation-induced,but not lithium-induced,conditioned taste aversions (oral rejection responses) in rats

Pairing a novel taste with provocative vestibular stimulation results in conditioned taste aversions in both rats and humans. Vestibular system involvement in gustatory conditioning was examined in sham-lesioned or labyrinthectomized rats. Three conditioning trials consisted of 30 min access to asaccharin (0.1%) solution followed by 30 min of rotation (70 rpm) or sham rotation. In a taste reactivity test with saccharin, rotated sham-lesioned rats, but not labyrinthectomized rats, exhibited increased oral rejection reactions compared with control rats. When conditioned with lithium chloride, both labyrinthectomized and sham-lesioned rats displayed robust conditioned rejection reactions. The finding that normal vestibular function is necessary in obtaining rotation-induced conditioned taste aversions supports the face and construct validity of a rat model of motion sickness.     

Effects of desglycinamide-lysine vasopressin on a conditioned taste aversion in rats

Vasopressin and other pituitary peptides have been shown to increase resistance to extinction of active and passive shock-avoidance responses. Both paradigms use external cues as warning and punishing stimuli. The present work asked if a variant of vasopressin would have a similar effect on conditioned taste aversions (CTA), which use internal cues as warning and punishing stimuli. Eight groups of rats were defined by factorial combination of Pretreatment during Neophobia and Conditioning (vasopressin vs saline), UCS during Conditioning (pairing saccharin-flavored water with injections of lithium chloride or saline), and Pretreatment during Extinction (vasopressin vs saline). Rats given three saccharin-lithium pairings developed strong aversions. Of the rats with CTAs, those given vasopressin during conditioning, extinction, or both showed increased resistance to extinction on the last four of eight extinction days. Vasopressin had no other effect on drinking of saccharin. Vasopressin thus appears to enhance resistance to extinction of avoidance responses in both external and internal milieus.     

Taste mixture interactions: Suppression, additivity, and the predominance of sweetness

Most of what is known about taste interactions has come from studies of binary mixtures. The primary goal of this study was to determine whether asymmetries in suppression between stimuli in binary mixtures predict the perception of tastes in more complex mixtures (e.g., ternary and quaternary mixtures). Also of interest was the longstanding question of whether overall taste intensity derives from the sum of the tastes perceived within a mixture (perceptual additivity) or from the sum of the perceived intensities of the individual stimuli (stimulus additivity). Using the general labeled magnitude scale together with a sip-and-spit procedure, we asked subjects to rate overall taste intensity and the sweetness, sourness, saltiness and bitterness of approximately equi-intense sucrose, NaCl, citric acid and QSO₄ stimuli presented alone and in all possible binary, ternary and quaternary mixtures. The results showed a consistent pattern of mixture suppression in which sucrose sweetness tended to be both the least suppressed quality and the strongest suppressor of other tastes. The overall intensity of mixtures was found to be predicted best by perceptual additivity. A second experiment that was designed to rule out potentially confounding effects of the order of taste ratings and the temperature of taste solutions replicated the main findings of the first experiment. Overall, the results imply that mixture suppression favors perception of sweet carbohydrates in foods at the expense of other potentially harmful ingredients, such as high levels of sodium (saltiness) and potential poisons or spoilage (bitterness and sourness).     

Perception of temporal order and the identification of components in taste mixtures

The capacity of humans to identify the components of taste mixtures is limited to 3 for most people [D.G. Laing, C. Link, A. Jinks, I. Hutchinson, The limited capacity of humans to identify the components of taste mixtures and taste-odor mixtures. Perception 31 (2002) 617-635]. Here, the hypothesis that temporal processing differences have a major role in limiting capacity is investigated. Thirty two subjects were trained to identify sucrose, sodium chloride and citric acid at FIVE concentration levels and were then required to indicate (1) which tastant was perceived first and (2) the identity of each component, in binary and ternary mixtures. Within each mixture set, e.g. sucrose-citric acid, the concentrations of components were adjusted to provide some conditions where intensities and time differences in processing the individual components were minimized. With binary mixtures, changes in intensity resulted in identification of only one component when the differences were largest, and both being identified when intensities were similar. In contrast, subjects found it difficult to indicate which component was perceived first when the intensities were similar and easy when they were substantially different. The most profound effects occurred with ternary mixtures. With each of the three sets, there were several where subjects could not indicate which component was perceived first, and in 13/18 mixtures not all components were identified above chance. Indeed, the most common number of components identified was 2. However, although perception of order was lost in a number of mixtures, this did not preclude identification of one or all three components. This result contrasts with the finding with odors where total loss of order resulted in loss of identity of all components in ternary mixtures [A. Jinks, D.G. Laing, Temporal processing reveals a mechanism for limiting the capacity of humans to analyze mixtures. Cognitive Brain Res. 8 (1999) 311-325]. Thus, loss of identity with taste mixtures seems to occur to a lesser degree. Indeed suppression rather than temporal effects may be the main cause of the loss of identity of components with ternary taste mixtures.     

Odor of taste stimuli in conditioned "taste" aversion learning

The present research addresses whether rats can express odor aversions to the odor of taste stimuli. In Experiment 1, saccharin or salt were either mixed in distilled water, so the rats could taste and smell them, or presented on disks attached to the tubes' metal spouts so the rats could only smell them. Aversions were established to taste stimuli under both conditions. The results of Experiment 2 indicate that conditioning was to the odor of the tastes when they were presented on disks in Experiment 1, hence both taste and odor aversions were established by means of "taste" stimuli. Taste aversion learning thus may more properly be termed flavor aversion learning, with flavor referring to both taste and odor components.     

Oxytocin and vasopressin secretion in response to stimuli producing learned taste aversions in rats

Administration of lithium chloride, copper sulfate, and apomorphine to rats each stimulated the secretion of oxytocin (OT) and, to a much lesser degree, arginine vasopressin. These agents are assumed to cause visceral illness in rats because of their effectiveness in promoting the acquisition of learned taste aversions. CuSO4 had a greater effect on plasma OT levels when administered ip rather than iv, whereas LiCl did not, results suggesting that LiCl probably stimulates OT secretion by central chemoreceptor activation whereas CuSO4 acts predominantly by local peritoneal irritation. A causal role for circulating OT in the acquisition of learned taste aversions was not found. These and other findings suggest that peripheral levels of OT may represent a quantifiable marker of visceral illness in rats and therefore might be useful in the interpretation of behavioral studies in which learned taste aversions are produced, provided that other stimuli of neurohypophyseal secretion are absent.