Measuring Pavlovian appetitive conditioning in humans with the postauricular reflex

Despite its evolutionary and clinical significance, appetitive conditioning has been rarely investigated in humans. It has been proposed that this discrepancy might stem from the difficulty in finding suitable appetitive stimuli that elicit strong physiological responses. However, this might also be due to a possible lack of sensitivity of the psychophysiological measures commonly used to index human appetitive conditioning. Here, we investigated whether the postauricular reflex—a vestigial muscle microreflex that is potentiated by pleasant stimuli relative to neutral and unpleasant stimuli—may provide a valid psychophysiological indicator of appetitive conditioning in humans. To this end, we used a delay differential appetitive conditioning procedure, in which a neutral stimulus was contingently paired with a pleasant odor (CS+), while another neutral stimulus was not associated with any odor (CS?). We measured the postauricular reflex, the startle eyeblink reflex, and skin conductance response (SCR) as learning indices. Taken together, our results indicate that the postauricular reflex was potentiated in response to the CS+ compared with the CS?, whereas this potentiation extinguished when the pleasant odor was no longer delivered. In contrast, we found no evidence for startle eyeblink reflex attenuation in response to the CS+ relative to the CS?, and no effect of appetitive conditioning was observed on SCR. These findings suggest that the postauricular reflex is a sensitive measure of human appetitive conditioning and constitutes a valuable tool for further shedding light on the basic mechanisms underlying emotional learning in humans.     

Individual mouse taste cells respond to multiple chemical stimuli

Sensory organs are specialized to detect and decode stimuli in terms of intensity and quality. In the gustatory system, the process of identifying and distinguishing taste qualities (e.g. bitter versus sweet) begins in taste buds. A central question in gustatory research is how information about taste quality is extracted by taste receptor cells. For instance, whether and how individual taste cells respond to multiple chemical stimuli is still a matter for debate. A recent study showed that taste cells expressing bitter-responsive taste receptors do not also express sweet-responsive taste receptors and vice versa. These results suggest that the gustatory system may use separate cellular pathways to process bitter and sweet signals independently. Results from electrophysiological studies, however, reveal that individual taste receptor cells respond to stimuli representing multiple taste qualities. Here we used non-invasive Ca²⁺ imaging in slices of lingual tissue containing taste buds to address the issue of quality detection in murine taste receptor cells. We recorded calcium transients elicited by chemical stimuli representing different taste qualities (sweet, salty, sour and bitter). Many receptor cells (38 %) responded to multiple taste qualities, with some taste cells responding to both appetitive ('sweet') and aversive ('bitter') stimuli. Thus, there appears to be no strict and separate detection of taste qualities by distinct subpopulations of taste cells in peripheral gustatory sensory organs in mice.     

Evidence that oral and nutrient reinforcers differentially condition appetitive and consummatory responses to flavors

MYERS, K. P. AND W. G. HALL. Evidence that oral and nutrient reinforcers differentially condition appetitive and consummatory responses to flavors. PHYSIOL BEHAV 64(4) 493–500, 1998.—Rats tend to increase their intake of a flavor that has previously been paired with either sweet taste or with caloric repletion. However, it is unclear whether such a change in intake is caused by changes in appetitive behaviors such as orienting and approach, or changes in consummatory behaviors and oral responsiveness. Also, it is unclear whether oral reinforcers (sweetness) and postingestive reinforcers (nutrients) lead to the same kinds of behavioral change. In the current experiments, weanling rats with oral and gastric cannulas repeatedly experienced a flavor paired with either sweetness, high caloric density, or neither. Rats were then tested for differences in appetitive olfactory orienting and consummatory oral responsiveness elicited by the flavor. Results suggest that oral reinforcement (sweetness) produces conditioning of appetitive responding to the flavor, while postingestive reinforcement produces conditioning of consummatory responding. A second experiment indicates that these behavioral changes are specific increases in responsiveness conditioned by flavor + unconditioned stimulus (US) pairing, and are unlikely to be nonspecific effects of daily unconditioned stimulus exposure.     

Pavlovian aversive and appetitive odor conditioning in humans: subjective, peripheral, and electrocortical changes

The effects of presynaptic guanosine-5'-O-(3-thio)triphosphate (GTPγS) on GABAergic inhibitory postsynaptic currents (IPSCs) were studied in cultured hippocampal neurons using whole-cell recordings. Inclusion of GTPγS (0.5–1 mM) in the presynaptic electrode reduced both the amplitude and paired-pulse depression of IPSCs, indicating that the probability of GABA-release had been reduced. Presynaptic GTPγS increased the depression of IPSCs by the GABA_B-receptor-agonist baclofen (10 μM), and the effect of baclofen was poorly reversible after washing. Stimulation of the GABAergic neuron at 80 Hz for 1 s was accompanied by tetanic depression of the IPSCs by 52±6% and was followed by post-tetanic potentiation (PTP), reaching a peak value of 71±21% and lasting about 100 s. IPSCs evoked after tetanic stimulation were depressed and PTP was absent when tetanic stimulation was applied within 3 min after starting injection of GTPγS into the presynaptic neuron. At longer times, basal release underlying a single IPSC was depressed. This affected the ratios recorded in response to tetanic stimulations such that tetanic depression was abolished, while PTP increased to 117±34%. In conclusion, GTPγS reduces the probability of GABA-release in both a use- and time-dependent manner, most likely through an inhibitory action on presynaptic Ca²⁺-influx through voltage-gated Ca²⁺ channels or an interaction with small GTP-binding proteins in the nerve terminals. Electronic Publication     

Generalization of appetitive conditioned responses

A stimulus (conditioned stimulus, CS) associated with an appetitive unconditioned stimulus (US) acquires positive properties and elicits appetitive conditioned responses (CR). Such associative learning has been examined extensively in animals with food as the US, and results are used to explain psychopathologies (e.g., substance‐related disorders or obesity). Human studies on appetitive conditioning exist, too, but we still know little about generalization processes. Understanding these processes may explain why stimuli not associated with a drug, for instance, can elicit craving. Forty‐seven hungry participants underwent an appetitive conditioning protocol during which one of two circles with different diameters (CS+) became associated with an appetitive US (chocolate or salty pretzel, according to participants’ preference) but never the other circle (CS?). During generalization, US were delivered twice and the two CS were presented again plus four circles (generalization stimuli, GS) with gradually increasing diameters from CS? to CS+. We found successful appetitive conditioning as reflected in appetitive subjective ratings (positive valence, higher contingency) and physiological responses (startle attenuation and larger skin conductance responses) to CS+ versus CS?, and, importantly, both measures confirmed generalization as indicated by generalization gradients. Small changes in CS‐US contingency during generalization may have weakened generalization processes on the physiological level. Considering that appetitive conditioned responses can be generalized to non‐US‐associated stimuli, a next important step would be to investigate risk factors that mediate overgeneralization.     

Selective expression of electrical correlates of differential appetitive classical conditioning in a feeding network

Electrical correlates of differential appetitive classical conditioning were recorded in the neural network that underlies feeding in the snail Lymnaea stagnalis. In spaced training (15 trials over 3 days), the lips and the tentacle were used as CS+ (reinforced conditioned stimulus) or CS- (nonreinforced conditioned stimulus) sites for behavioral tactile conditioning. In one group of experimental animals, touch to the lips (the CS+ site) was followed by sucrose (the unconditioned stimulus, US), but touch to the tentacle (the CS- site) was not reinforced. In a second experimental group the CS+/CS- sites were reversed. Semi-intact lip-tentacle-CNS preparations were made from both experimental groups and a naive control group. Intracellular recordings were made from the B3 motor neuron of the feeding network, which allowed the monitoring of activity in the feeding central pattern generator (CPG) interneurons as well as early synaptic inputs evoked by the touch stimulus. Following successful behavioral conditioning, the touch stimulus evoked CPG-driven fictive feeding activity at the CS+ but not the CS- sites in both experimental groups. Naive snails/preparations showed no touch responses. A weak asymmetrical stimulus generalization of conditioned feeding was not retained at the electrophysiological level. An early excitatory postsynaptic potential (EPSP) response to touch was only enhanced following conditioning in the Lip CS+/tentacle CS- group but not in the Tentacle CS+/lip CS- group. The results show that the main features of differential appetitive classical conditioning can be recorded at the electrophysiological level, but some characteristics of the conditioned response are selectively expressed in the reduced preparation.     

Dorsal hippocampal involvement in appetitive trace conditioning and interval timing

Involvement of the dorsal hippocampus (DHPC) in acquisition of Pavlovian trace conditioning and interval timing was examined in an appetitive preparation in which presentations of one conditioned stimulus (CS) were immediately followed by food (delay conditioning), and presentations of another CS were followed by food 15 seconds after its termination (trace conditioning). DHPC lesions did not disrupt acquisition of trace conditioning, but they selectively affected the distribution of conditioned responding over the course of the trace CS in the early stage of acquisition. In addition, lesions disrupted accuracy of timing the conditioned response (CR) for both delay and trace CSs: The control subjects showed maximum CR at the time of food delivery, but in the DHPC-lesioned subjects, the maximum CR was at an earlier time point. This timing deficit did not seem to be due to impulsive responding or deficits in response inhibition because, when the early portion of the delay CS was interrupted shortly by an empty interval, the difference in the time of maximum responding between the lesioned and control subjects was eliminated. Thus, although the involvement of the DHPC in appetitive trace conditioning was not found when a gross measure of conditioning was employed, it was revealed when the temporal distribution of conditioned responding was examined on a moment-by-moment basis as in eyeblink trace conditioning studies.     

Impaired conditioned taste aversion learning in APP transgenic mice

Cognition in transgenic mouse models of Alzheimer's disease (AD) has been predominantly characterized in explicit spatial orientation tasks. However, dementia in AD encompasses also implicit memory systems. In the present study a line of transgenic mice (TgCRND8) encoding a double mutated allele of the human amyloid precursor protein (APP) genes was evaluated in an implicit associative learning task of conditioned taste aversion (CTA). CTA is a form of Pavlovian classical conditioning, in which a mouse learns to avoid a novel taste of saccharine (conditioned stimulus) paired with an experimentally induced (systemic injection of lithium chloride) nausea (unconditioned stimulus). In contrast to conditioned non-Tg mice, TgCRND8 APP mice developed weaker aversion against saccharine and quickly increased its consumption in repeated tests. These results indicate that TgCRND8 mice show a significant impairment not only in explicit spatial memory, as has been previously shown [Nature 408 (2000) 979], but also in implicit memory. Control experiments confirmed that TgCRND8 and non-Tg mice had comparable taste sensitivities in response to appetitive as well as aversive tastes. The study suggests that the CTA paradigm can be a sensitive tool to evaluate deficits in implicit associative learning in APP transgenic mouse models of AD.     

Pupil dilation as an implicit measure of appetitive Pavlovian learning

Appetitive Pavlovian conditioning is a learning mechanism of fundamental biological and pathophysiological significance. Nonetheless, its exploration in humans remains sparse, which is partly attributed to the lack of an established psychophysiological parameter that aptly represents conditioned responding. This study evaluated pupil diameter and other ocular response measures (gaze dwelling time, blink duration and count) as indices of conditioning. Additionally, a learning model was used to infer participants’ learning progress on the basis of their pupil dilation. Twenty‐nine healthy volunteers completed an appetitive differential delay conditioning paradigm with a primary reward, while the ocular response measures along with other psychophysiological (heart rate, electrodermal activity, postauricular and eyeblink reflex) and behavioral (ratings, contingency awareness) parameters were obtained to examine the relation among different measures. A significantly stronger increase in pupil diameter, longer gaze duration and shorter eyeblink duration was observed in response to the reward‐predicting cue compared to the control cue. The Pearce‐Hall attention model best predicted the trial‐by‐trial pupil diameter. This conditioned response was corroborated by a pronounced heart rate deceleration to the reward‐predicting cue, while no conditioning effect was observed in the electrodermal activity or startle responses. There was no discernible correlation between the psychophysiological response measures. These results highlight the potential value of ocular response measures as sensitive indices for representing appetitive conditioning.     

Skin Conductance Conditioning to Potentially Phobic Stimuli as a Function of Interstimulus Interval and Delay Versus Trace Paradigm

Because prepared learning has been defined in terms of response acquisition in spite of degraded input, it was expected that differences in resistance to extinction between skin conductance responses conditioned to potentially phobic and neutral stimuli would increase with increased interstimulus interval (ISI) and be larger with a trace than with a delay conditioning paradigm. Twelve groups with 10 subjects each were observed in a differential conditioning experiment manipulating ISI (2, 8, or 16 sec), paradigm (delay versus trace), and fear-relevance of the conditioned stimulus (potentially phobic versus neutral). The results showed highly reliable resistance to extinction of first-interval anticipatory responses to phobic stimuli, and no resistance to extinction of the corresponding responses to the neutral stimuli. This difference did not interact either with the ISI or the paradigm factor. Thus, although underscoring the reliability of the difference in conditioning to potentially phobic and neutral stimuli, the results did not support the specific hypothesis of conditioning to phobic stimuli as being less dependent on the ISI parameters than conditioning to neutral stimuli.     

Saccharin's rewarding, conditioned reinforcing, and memory-improving properties: mediation by isomorphic or independent processes?

Unconditioned reward and conditioned reinforcing effects may reflect an isomorphic motivational process because increased conditioned reinforcing effects were seen with increased amounts of saccharin consumed in taste and place conditioning. Reinforcing effects in place conditioning leveled off as saccharin unconditioned consumption reached maximum amounts of approximately 140 mg/rat. Posttrial consumption, but not intraperitoneal injection, of saccharin significantly enhanced conditioned place and taste preferences as well as conditioned taste aversions. Saccharin's memory-improving effects in both aversive and appetitive conditioning suggest a process separate from the reward-reinforcement process. Independent of effects on blood glucose, the motivational property of saccharin's sweet taste undergoes differential central processing to mediate reward-reinforcement versus memory improvement processes.     

Temporal dissociation of salience and prediction error responses to appetitive and aversive taste

The feedback‐related negativity (FRN), a frontocentral ERP occurring 200–350 ms after emotionally valued outcomes, has been posited as the neural correlate of reward prediction error, a key component of associative learning. Recent evidence challenged this interpretation and has led to the suggestion that this ERP expresses salience instead. Here, we distinguish between utility prediction error and salience by delivering or withholding hedonistically matched appetitive and aversive tastes, and measure ERPs to cues signaling each taste. We observed a typical FRN (computed as the loss‐minus‐gain difference wave) to appetitive taste, but a reverse FRN to aversive taste. When tested axiomatically, frontocentral ERPs showed a salience response across tastes, with a particularly early response to outcome delivery, supporting recent propositions of a fast, unsigned, and unspecific response to salient stimuli. ERPs also expressed aversive prediction error peaking at 285 ms, which conformed to the logic of an axiomatic model of prediction error. With stimuli that most resemble those used in animal models, we did not detect any frontocentral ERP signal for utility prediction error, in contrast with dominant views of the functional role of the FRN ERP. We link the animal and human literature and present a challenge for current perspectives on associative learning research using ERPs.     

Synergism of a compound unconditioned stimulus in taste aversion conditioning

Anti-lymphocyte serum (ALS) and lithium chloride have both previously been used successfully as unconditioned stimuli in taste aversion conditioning paradigms in rats. This report substantiates those findings but shows that when the two stimuli are given as a compound unconditioned stimulus in association with saccharin flavoured drinking solution, the conditioned taste aversion response following a second exposure to saccharin alone is more profound than that following conditioning with either ALS or LiCl alone. These results demonstrate synergism between the two stimuli when given together.     

Effects of taste aversion conditioning on the primary antibody response to sheep red blood cells and Brucella abortus in the albino rat

It has been shown that use of the saccharin/cyclophosphamide taste aversion paradigm produced conditioned immunosuppression as well as saccharin avoidance after two post-conditioning exposures to the aversive stimulus [3,18]. In the present study the effects of saccharin/cyclophosphamide conditioning on the primary humoral antibody response to two antigens: sheep red blood cells (SRBC), a T-cell dependent antigen and Brucella abortus (B. abortus), a T-cell independent antigen, were examined. In addition the effects of a third exposure to the aversive stimulus, saccharin, on conditioned immunosuppression were examined. The results indicate that the target cell of taste aversion conditioning, in relation to immune dysfunction, could be the T-lymphocyte and that conditioned immunosuppression is dependent on the presence of the behavioral response. These results constitute a replication of previous studies [3,18] and provide evidence which indicates that behaviorally conditioned immunosuppression is a consequence of the parameters of taste aversion conditioning.     

Conditioned changes in appetitive and consummatory responses to flavors paired with oral or nutrient reinforcement among adult rats

Recent studies of the behavioral organization of conditioned flavor preferences have suggested the involvement of at least two separate learning systems-an appetitive response system sensitive to the oral hedonic properties of the reinforcer, and a consummatory response system sensitive to its nutrient properties. However, these prior studies were conducted with weanling rats, that differ from adults in terms of their prior experience with food, their learning competencies, and the peculiar ontogenetic constraints on their behavior. It is, therefore, unknown whether flavor preference behaviors are similarly organized in adult rats. In this experiment, adult rats were trained to associate a specific CS flavor with either the sweet taste or the postingestive nutrient effects of sucrose. Conditioned appetitive orienting and consummatory oral responding to the CS flavors were then measured. Unlike weanling rats, adult rats exhibited both conditioned appetitive behavior and conditioned consummatory behavior in response a CS that was previously paired with either oral hedonic or nutrient reinforcement. These results suggest a set of important developmental changes in the neurobehavioral mechanisms of flavor preference learning in the postweaning period.     

Contingent and non-contingent actions of sucrose and saccharin reinforcers: effects on taste preference and memory

Preference curves were generated by comparing 14 concentrations each of sucrose and saccharin in a 20-minute test in which rats were presented with a choice of a sweet solution and water. The most preferred concentration and one concentration above and one below the most preferred for both substances were studied further. The sucrose and saccharin solutions were contingently paired with novel flavors in a conditioned taste preference (CTP) paradigm. All of the sweet solutions enhanced the animals' subsequent conditioned taste preferences for the flavors. The lack of difference between the effects of the solutions in this paradigm suggest that they had similar rewarding values and that CTP's are established mainly on the basis of taste cues. In another experiment, post-training ingestion of sucrose solutions, injection of glucose and, to a much lesser extent, ingestion of saccharin solutions retroactively and non- contingently improved retention of a previously formed, classically conditioned association. The results indicated that this effect was mainly due to the post- ingestional effects of the sucrose solutions, although taste factors also had a slight influence. This series of experiments parallels previous findings with self-stimulation as the reinforcer. The results support the hypothesis that reinforcers have a dual action on behavior: the elicitation of affective states that, when paired with environmental stimuli, can influence future behavior towards those stimuli; and a non-contingent retroactive enhancement of retention of previously formed associations.     

Lithium chloride and immunomodulation in taste aversion conditioning.

Lithium chloride has been used in many studies of conditioning to induce taste aversion behaviour, and in some experiments investigating conditioning effects on immunity it has been used on the assumption that it is immunologically neutral. The studies reported here, however, indicate that LiCl is not immunologically neutral and when used to endow a UCS with noxious properties to enhance the behavioural response in taste aversion conditioned immunosuppression, it may antagonize the residual immunosuppression following initial UCS administration and also the conditioned immunosuppression occurring after CS reexposure. Therefore, conclusions drawn from studies of behaviourally conditioned immunomodulation where LiCl is used as part of either the CS or UCS may require reevaluation.     

Duodenal pain and spinal morphine induce conditioned taste aversion in rats

Conditioned taste aversion (CTA) is a behavioural response essential to the survival of an individual. The combination of taste and odour of most foods provides a strong conditioned stimulus (CS) for an animal to respond in an appropriate way to any harmful unconditioned stimuli (US) that follow. The most widely used conditioned stimuli are drinkable sweet solutions, such as saccharin and sucrose. CTA-like responses are also found for environmental unconditioned stimuli, but these usually take longer training. In the present study, the aversive nature of a duodenal distention with an implanted balloon catheter was studied in freely moving rats using either CTA against a sucrose solution, or a light-dark passive avoidance (PA) paradigm. In addition, the effect of spinal morphine on CTA and the cardiovascular response to duodenal distention were studied. CTA could be induced by a single, but long-lasting 20-minute duodenal distention, which did not induce PA behaviour in a light-dark box. Spinal infusion of morphine alone induced CTA, suggesting that the model is unsuitable to investigate spinal pharmacological modulation of visceral pain. Spinal morphine did reduce the cardiovascular response to duodenal distention, strengthening its validity as a visceral pain model. Since CTA is a complicating factor in the field of chemotherapy in cancer patients and spinal morphine causes nausea and vomiting in humans, CTA may also complicate spinal drug treatment or anaesthesia.     

Modulation of body temperature through taste aversion conditioning.

Injection of rats with bacterial lipopolysaccharide (LPS) results in an initial fall in body temperature followed by a fever. Lithium chloride (LiCl) injection induces a fall in body temperature without subsequent fever production. When these substances were incorporated as unconditioned stimuli in a taste aversion conditioning paradigm, using saccharin flavour as the conditioning stimulus, these differential effects on body temperature were reenlisted on reexposure to saccharin alone 7 days after conditioning. The changes in body temperature on reexposure were similar in direction and kinetics as on the conditioning day although reduced in magnitude. The finding of a true conditioned effect in these studies is in contrast to the paradoxical or compensatory conditioned body temperature responses described elsewhere using different conditioning models. This apparent conflict may be explained on the basis of different unconditioned stimuli acting on efferent versus afferent arms of a negative feedback system. Since body temperature changes often occur in association with immune responses, these findings may have implications to behavioural conditioning of immunity, the outcome of which may reflect the indirect effects on immunity of inadvertent conditioning of thermoregulatory changes.     

Disconnection of the basolateral amygdala complex and nucleus accumbens impairs appetitive pavlovian second-order conditioned responses

There is considerable evidence that the basolateral complex of the amygdala (ABL) is involved in learning about the motivational value of otherwise neutral stimuli. The authors examined the role in this function of the ABL and one of its major efferent structures. the nucleus accumbens. Male Long-Evans rats received either sham, ipsilaterally, or contralaterally placed unilateral lesions of the ABL and accumbens and were trained in an appetitive Pavlovian second-order conditioning task. Sham-lesioned and ipsilaterally lesioned rats acquired the task normally, but contralaterally lesioned rats, in which the ABL and accumbens were functionally disconnected, failed to acquire second-order conditioned responses (although they did acquire second-order conditioned orienting responses). The results suggest that the ABL and accumbens are part of a system critical for processing information about learned motivational value.