Taste potentiation of auditory aversions in rats (Rattus norvegicus): a case for spatial contiguity

An aversion for an auditory stimulus was established in laboratory rats when a tone was spatially and temporally contiguous with a novel taste in a food conditioned stimulus compound followed by toxicosis. The procedure involved varying the location of the tone relative to a novel tasting food. During toxicosis conditioning, one group ate sweet food with a speaker located in the food, two groups ate sweet food with the speaker displaced (near or far) from the food, and a fourth group was presented with a tone without food available. It was found that the potentiation of auditory aversions required both the presence of a novel taste and spatial contiguity between the taste and the tone.     

Bulimia and taste: possible interactions

Potential abnormalities of taste were examined in bulimic subjects who purged by vomiting and in controls. When spatial testing of the tongue and palate was performed by direct local application of sweet, salty, sour, and bitter solutions, bulimics showed a selective spatial loss on the palate. The palate may be affected by purging because vomit is directed toward the roof of the mouth where the palate receptors are located. The data suggest that the acid in vomit damages these receptors. Bulimics and controls did not differ in their basal ratings of intensity or pleasantness of sweet, salty, sour, and bitter stimuli when these were sipped rather than directly applied to the tongue. However, after ingesting a glucose load, controls found sweet taste significantly less pleasant, whereas bulimics did not. The results suggest that bulimics may also have an abnormal experience of satiety.     

Spatial contiguity facilitates Pavlovian second-order conditioning

Two experiments investigated the effects of spatial contiguity upon the formation of second-order conditioning in pigeon subjects. Experiment 1 used an autoshaping procedure to pair two visual stimuli, S2 and S1, after S1 had previously been paired with food. The resulting second-order conditioning of S2 was superior when both stimuli appeared on the same response key within a trial, compared with their appearing on different keys. Experiment 2 found a similar importance of spatial contiguity between S2 and S1 in a conditioned suppression paradigm. In addition, consistently presenting S2 and S1 in the same spatial location produced superior conditioning compared with varying their spatial relation from trial to trial. The design of these experiments was such as to imply that spatial contiguity facilitates performance by improving the formation of association rather than by promoting stimulus generalization or pseudoconditioning. Moreover, the observation of a facilitative effect of spatial contiguity between S1 and S2 in two different paradigms that use qualitatively different unconditioned stimuli and evoke different responses implies some generality for these findings. Consequently, these results suggest that spatially contiguous stimuli are especially associable in Pavlovian conditioning paradigms.     

Acquisition of a spatially defined operant with delayed reinforcement.

Two experiments investigated the role of an immediate, response-produced auditory stimulus during acquisition, via delayed reinforcement, of a response selected to control for possible unprogrammed, operandum-related sources of response feedback. Experimentally naive rats were exposed to a delayed-food reinforcement condition, specifically a tandem fixed-ratio 1 differential-reinforcement-of-other-behavior 30-s schedule. The response was defined as breaking a photocell beam located near the ceiling at the rear of the operant conditioning chamber. In Experiment 1, rates of photobeam breaking by each rat increased from near zero, regardless of the presence or absence of a tone that immediately followed the response initiating the delay interval. Though not essential, the tone facilitated response acquisition and resulted in more efficient response patterns at stability. Experiment 2 demonstrated that photobeam-breaking response rates under the delayed reinforcement contingency exceeded those in a preceding baseline condition in which no food was delivered. In addition, upon introduction of the delayed reinforcement procedure, correspondence between response patterns and the requirements of the reinforcement schedule increased over baseline levels in the absence of a food contingency. Together with a previous report of Lattal and Gleeson (1990), the present results suggest that response acquisition with delayed reinforcement is a robust phenomenon that may not depend on a mechanically defined response or an immediate external stimulus change to mediate the temporal gap between response and reinforcer.     

The role of gender and taste class in the effects of stress on eating.

Participating in a laboratory study of the effects of stress on eating, healthy, nonsmoking men and women were shown a film about industrial accidents (stress condition) or a pleasant travelogue (control condition) and had access to sweet, salty, and bland snack food. Analyses of food consumption data revealed a significant interaction between sex of subject and the stress manipulation: Stress markedly and significantly decreased food consumption by men but resulted in some increased food consumption by women. Across the three food taste categories, men consistently ate less under stress than they did in the control condition. In contrast, women ate nearly twice as much sweet food and more bland food under stress than they did in the control condition, but these effects were not statistically significant. These results indicate that the relationship between stress and eating depends on the sex of the subject and may relate to type of food available. Contrasting generalizations about stress and eating reported in the archived literature can largely be reconciled by considering these interacting variables.     

One-trial overshadowing

Two experiments on conditioned suppression in rats examined the extent to which conditioning to a tone was overshadowed by the presence of a light. When animals received four conditioning trials, a light of moderate intensity significantly overshadowed the tone. On the first trial of conditioning, however, the light had no effect on conditioning to the tone, unless (Experiment I) its intensity was increased, or (Experiment II) it occurred in closer temporal contiguity to reinforcement than the tone. The results establish that overshadowing can occur on the first trial of conditioning, contrary to the predictions of several theories, but suggest that this may be for different reasons from those operating to produce overshadowing over a series of conditioning trials.     

Medial auditory thalamic input to the lateral pontine nuclei is necessary for auditory eyeblink conditioning

Auditory and visual conditioned stimulus (CS) pathways for eyeblink conditioning were investigated with reversible inactivation of the medial (MPN) or lateral (LPN) pontine nuclei. In Experiment 1, Long-Evans rats were given three phases of eyeblink conditioning. Phase 1 consisted of three training sessions with electrical stimulation of the medial auditory thalamic nuclei (MATN) paired with a periorbital shock unconditioned stimulus (US). An additional session was given with a muscimol (0.5 μL, 10 mM) or saline infusion targeting the LPN followed by a recovery session with no infusions. The same training and testing sequence was then repeated with either a tone or light CS in phases 2 and 3 (counterbalanced). Experiment 2 consisted of the same training as Experiment 1 except that muscimol or saline was infused in the MPN during the retention tests. Muscimol infusions targeting the LPN severely impaired retention of eyeblink conditioned responses (CRs) to the MATN stimulation and tone CSs but only partially reduced CR percentage to the light CS. Muscimol infusions that targeted the MPN had a larger effect on CR retention to the light CS relative to MATN stimulation or tone CSs. The results provide evidence that the auditory CS pathway necessary for delay eyeblink conditioning includes the MATN-LPN projection and the visual CS pathway includes the MPN.     

Compatibility between approach/avoidance stimulation and valenced information determines residual attention during the process of encoding

We tested the hypothesis that the activation of the motivational systems of approach or avoidance by body postures and taste influences residual attention during the process of encoding differentially valenced words. In Experiment 1, participants were asked to stand upright or kneel while learning either positive or negative adjectives. To measure participants' differential cognitive capacities, a dual task paradigm was used, including a finger‐dexterity test as a secondary task. We were able to show that participants performed worse on the secondary task compared to a baseline assessed before if there was incompatibility between postures and the valence of the information. In Experiment 2, we replicated the results with bitter and sweet taste instead of body positions. It is our contention that the activation of approach or avoidance systems by bodily states prepares the organism for information of differential valence. Copyright © 2000 John Wiley & Sons, Ltd.     

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.     

Palatability shifts in taste and flavour preference conditioning

Changes in palatability of tastes and flavours as a result of flavour preference conditioning were examined. In Experiment 1, when tastes were paired with glucose in a reverse-order differential conditioning paradigm, rats acquired conditioned preferences for CS + and displayed more hedonic responses to CS + than to CS - in a postconditioning taste reactivity test. In Experiment 2, rats that received oral infusions of flavours as CSs during a reverse-order conditioning procedure expressed both palatability shifts and conditioned preferences for CS + . Rats that received a forward conditioning procedure acquired a preference for CS + , but the palatability of CS + was unchanged. In Experiment 3, hungry rats drank mixtures of a flavour CS and a calorific or sweet tasting reinforcer in a long-exposure conditioning paradigm. When tested hungry, rats preferred CS + whether they had acquired flavour-calorie or flavour-taste associations. However, CS + became more palatable only for rats that acquired flavour-calorie associations. These results suggest that acquisition of flavour preferences, as measured by 2-bottle tests, may not always be accompanied by enhanced palatability.     

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.     

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.     

Associative transfer and stimulus selection in classical conditioning of the rabbit's nictitating membrane response to serial compound CSs

Four experiments were conducted to determine whether in conditioning to a serial compound, CS1-CS2-UCS, there are (a) associative mechanisms operating to extend conditioning beyond the bounds of a CS-UCS contiguity gradient and (b) stimulus selection processes acting to attenuate the potency of CS-UCS contiguity. In Experiments 1 and 2, the CS2-UCS interval was held at .35 sec while the CS1-UCS interval was varied across groups from .75 to 2.75 sec. CS1 test trials revealed substantial CR acquisition at all CS1-UCS intervals. Moreover, Experiment 2 indicated that when the contribution of cross-modal generalization from CS2 to CS1 was factored out, there still remained a substantial level of conditioning, which Experiment 3 indicated was attributable to an associative mechanism like higher-order or sensory conditioning. The observation of CR acquisition at CS1-UCS intervals of 4.75, 8.75, and 18.75 sec in Experiment 4 suggested that serial compound training yields conditioning to CSs located well beyond the single CS contiguity gradient for the rabbit's nictitating membrane response. Experiments 1 and 2 also indicated the presence of stimulus selection processes because, at the shorter CS1-UCS intervals (.75 and 1.25 sec), the levels of test-trial responding to CS2 fell below those observed to the less contiguous CS1.     

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.     

Glutamate receptors in the medial geniculate nucleus are necessary for expression and extinction of conditioned fear in rats

Auditory fear conditioning requires anatomical projections from the medial geniculate nucleus (MGN) of the thalamus to the amygdala. Several lines of work indicate that the MGN is a critical sensory relay for auditory information during conditioning, but is not itself involved in the encoding of long-term fear memories. In the present experiments, we examined whether the MGN plays a similar role in the extinction of conditioned fear. Twenty-four hours after Pavlovian fear conditioning, rats received bilateral intra-thalamic infusions of either with NBQX (an AMPA receptor antagonist; Experiment 1) or MK-801 (an NMDA receptor antagonist; Experiment 1), anisomycin (a protein synthesis inhibitor; Experiment 2) or U0126 (a MEK inhibitor; Experiment 3) immediately prior to an extinction session in a novel context. The next day rats received a tone test in a drug-free state to assess their extinction memory; freezing served as an index of fear. Glutamate receptor antagonism prevented both the expression and extinction of conditioned fear. In contrast, neither anisomycin nor U0126 affected extinction. These results suggest that the MGN is a critical sensory relay for auditory information during extinction training, but is not itself a site of plasticity underlying the formation of the extinction memory.     

Constraints on Pavlovian conditioning of the pigeon: Relative conditioned reinforcing effects of red-light and tone CSs paired with food

Recent Pavlovian conditioning experiments presented all possible CS-US combinations of red-light and tone CSs and food and shock USs to separate groups of pigeons. Pigeons receiving shock USs demonstrated conditioned head raising followed by prancing to both CSs, but CRs were acquired more rapidly to tone than to red light. Although pigeons receiving food USs rapidly acquired a conditioned response of pecking to the red-light CS, there was no evidence of conditioned responding in groups receiving tone-food pairings. This outcome left open the possibility that Pavlovian pairings of tone and food may have resulted in association formation that was not revealed in performance. The present series of experiments attempted to reveal that association, using an indirect method of assessment, conditioned reinforcement. Experiment 1 demonstrated that both red light and tone paired with food became positive conditioned reinforcers, suggesting that an association between tone and food was formed in the same number of Pavlovian conditioning trials that previously failed to yield any direct evidence of conditioning. Experiment 2, which presented fewer conditioning trials, revealed that the tone-food association was formed less rapidly than the red light-food association. Experiment 3 demonstrated that the observed outcomes were not attributable to unconditioned, rather than conditioned, reinforcing effects of the Css.     

Interval, blocking, and marking effects during the development of schedule-induced drinking in rats

Schedule-induced drinking (SID) can occur when food-deprived rats are given access to water while receiving pellets on an intermittent reinforcement schedule. These conditions can increase water intake excessively. The possible role of adventitious reinforcement of postpellet drinking was assessed by testing whether response-reinforcer contiguity, the relative predictiveness of a response, and whether it is marked are important in the development of SID. Rats exposed to a short interpellet interval acquired SID most rapidly, with this acquired drinking response maintained when animals were transferred to a longer interpellet interval, thus indicating an easy-to-hard effect (Experiment 1). Further experiments demonstrated that a stimulus (a brief-flashing house light) occurring prior to pellet delivery could block the acquisition of SID (Experiment 2), while a lick-contingent tone, intended to increase the associability of this response, produced more rapid acquisition of SID (Experiment 3). Analysis of lick distributions revealed that licking became concentrated in the first half of an interpellet interval only after several sessions. Overall, the results indicated that similar factors affect the acquisition of both SID and instrumental conditioning with delayed reinforcement, as is consistent with a superstitious conditioning account of SID development.     

Restoration of the orienting response to a light by a change in its predictive accuracy

In three experiments the orienting response (OR) to a light was reduced by presenting rats with a serial conditioning schedule in which the light was always followed by a tone, and the tone was intermittently paired with food. The original strength of the OR was then restored by changing the nature of the serial conditioning: the light was followed either by the tone and food, or by nothing. Experiments 1 and 2 demonstrated that this change resulted in an increase in the OR relative to control groups, which continued to receive the original training. Experiment 3 revealed a similar effect using a within-subjects design. A further finding from the first two experiments was that the recovery of the OR was accompanied by an enhancement in the conditionability of the light. Overall, the results suggest that the accuracy with which the light signals the events that immediately follow it is an important determinant of the strength of the OR it elicits.     

Role of the hippocampus in blocking and conditioned inhibition of the rabbit's nictitating membrane response.

Bilateral aspiration of the dorsal hippocampus produced a disrupttion of blocking of the rabbit's nictitating membrane response in Kamin's two-stage paradigm (Experiment 1) but had no effect on the formation of a Pavlovian conditioned inhibitor (Experiment 2). The results of Experiment 1 indicated that normal animals and those with cortical lesions given conditioning to a light-plus-tone conditioned stimulus (CS) gave conditioned responses (CRs) to both the light and the tone during nonreinforced presentations of each (test phase). If, however, compound conditioning was preceded by tone acquisition, only the tone elicited a CR during testing; that is, blocking was observed. In rabbits with hippocampal lesions, however, CRs were given to both the light and the tone during testing whether or not compound conditioning was preceded by tone acquisition. The data from Experiment 2 showed that rabbits with hippocampal lesions could discriminate as well as normal rabbits and those with cortical lesions between a light (CS+) and light plus tone (CS-). In addition, when the inhibitory tone was subsequently paired with the unconditioned stimulus in retardation testing, animals in all three lesion conditions acquired the CR at the same rate. Thus, it appears that hippocampal lesions do not disrupt conditioned inhibition. The results of these experiments were taken as support for the view that the hippocampus is responsible for "tuning out" stimuli that have no adaptive value to the organism.     

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.