The decrement in conditioned fear with increased trials of simultaneous conditioning is not specific to the simultaneous procedure

Three experiments using a conditioned punishment paradigm with rat subjects examined the possibility that the nonmonotonic acquisition function previously found to characterize simultaneous conditioning was due to the noninformative nature of the conditioned stimulus (CS). In Experiment 1 the suppressive effects of a CS previously presented with an unconditioned stimulus (US) in a simultaneous and forward (informative) manner were compared following 20 and an additional 60 conditioning trials. Excitatory conditioning similarly diminished with increased trials for both the simultaneous and forward procedures. Experiment 2 employed a between-groups design. Simultaneous, forward, and trace conditioning procedures were compared following 20 or 100 trials. Each of the three 100-trial groups showed less resistance to extinction than their 20-trial counterparts. Experiment 3 determined that the decrement in excitatory conditioning for the 100-trial groups was not due to the greater number of US presentations, per se, but rather to the number of CS-US pairings. The nonmonotonic acquisition function observed with all three conditioning procedures indicated that informational factors were not responsible for the decrement observed in simultaneous conditioning. The pattern of results suggested that subjects receiving extended conditioning trials were better able to discriminate between training and testing.     

Mechanisms of blocking by contextual stimuli

The influence of contextual stimuli on the conditioning and performance of responding to a discrete stimulus was examined in the US preexposure paradigm using both context shift manipulations and a measure of context conditioning. Four groups of rats received both repeated exposure to an electric shock US in one context (Context 1), and repeated nonshocked exposure to a second context (Context 2). Two additional groups of rats received exposure to these contexts, but never received shock presentations. Rats exposed to shock learned to escape from the stimuli of Context 1, but did not escape from the stimuli provided by Context 2. Rats not exposed to shock failed to escape from either context. All rats then received a single CER conditioning session in which four pairings of a 3-min noise CS and shock US were presented. Half the rats received those CS-US pairings in the excitatory Context 1, while the remaining rats received those pairings in the neutral Context 2. Finally, half the rats in each of the CER conditioning treatments received extinction test trials of the noise CS in Context 1, while the remaining rats received those test trials in Context 2. Thus, this design factorially manipulated the presence of excitatory or neutral contextual stimuli during both conditioning and testing of a discrete CS. In comparison with the two groups of rats never preexposed to shock alone, attenuation in acquisition of conditioned suppression observed during test trials occurred only when CER conditioning had been administered in the excitatory Context 1, and this effect was manifested when testing occurred in either the excitatory Context 1 or the neutral Context 2. These results support the model of R. A. Rescorla and A. R. Wagner (1972) (in A. H. Black & W. F. Prokasy (Eds.) Classical Conditioning II, pp. 64–99, New York: Appleton-Century-Crofts) which asserts that contextual stimuli and sicrete CSs compete for limited associative strength supportable by a given US.     

On the intentional control of conditioned evaluative responses

The evaluative conditioning (EC) effect is defined as a change in the evaluation of a conditioned stimulus (CS) due to its pairing with a valenced unconditioned stimulus (US). The current research investigated the controllability of EC effects by asking participants to either promote or prevent the influence of CS–US pairings before they provided evaluative ratings of the CS. Experiment 1 showed that instructions to maximize or minimize the influence of CS–US pairings moderated EC effects in line with task instructions. However, this moderation was observed only when participants were able to recall the valence of the US that had been paired with a given CS. When participants failed to remember the valence of the US, significant EC effects emerged regardless of control instructions. Experiment 2 tested whether the influence of CS–US pairings on CS evaluations can be intentionally reversed. The results showed that reversal instructions led to a reverse EC effect when participants were able to recall the valence of the US that had been paired with a given CS, but not when they were unable to recall the valence of the US. Taken together, these results suggest that US valence memory is a necessary precondition for controlling the expression of a conditioned evaluative response, but it is not a necessary precondition for the emergence of EC effects per se.     

Contextual conditioning in infants, but not older animals, is facilitated by CS conditioning

Context conditioning in infant Sprague-Dawley rats (postnatal days [PD] 15, 17, and 19), juveniles (PD 25), adolescents (PD 35), and adults (PD 71-89) was compared when CS conditioning did or did not occur in the context. Degree of CS conditioning within that context was equated across age, and separate groups at each age were given unpaired presentations of the CS and US within that context. Infants conditioned more effectively to context when CS-US pairings occurred in that context than when they did not, juveniles conditioned to context about equally with and without CS-US pairings in the context, and adolescents and adults conditioned less effectively to context when CS and US were paired than when unpaired. Adolescents had significant context conditioning despite CS-US pairings in the context but adults did not, and overall, context conditioning was strongest for adolescents. Supplementary experiments indicated that with more extensive conditioning experience, the infants' pattern of context conditioning became more similar to that of older animals, and with less conditioning experience the pattern of context conditioning by adults became more similar to that of younger animals, but infants never attained the adult pattern of context conditioning nor did adults attain the infant pattern. The potentiation of context conditioning by CS conditioning observed in infants is consistent with previous evidence derived from compound conditioning. Alternative explanations place common emphasis on infant-specific amodal processing. One views potentiation as a result of the greater perceived intensity of the stimulus compound (CS and context, in this case) during conditioning and the lesser generalization decrement in infants than adults when tested with a single element after conditioning with a compound. The other explanation emphasizes consequences of the redundancy inherent in intersensory compounds, within the theory of Bahrick and Lickliter.     

The effects of lesions in the central nucleus of the amygdala on appetitive classical conditioning in rats

Abstract: Lesions in the central nucleus of the amygdala (cAMY) have been known to interfere with the acquisition of fear classical conditioning when footshock is used as an unconditioned stimulus (US). The present study examined whether or not a similar interference would occur with an appetitive US. Five rats with lesions in the cAMY (the cAMY group), and eight unoperated control rats were trained in an appetitive classical conditioning paradigm, which did not include elements of operant learning, using a visual conditioned stimulus (CS) (5 W of light for 10 s duration) paired with a food pellet US (45 mg, cheese flavor). The behavioral index of appetitive conditioning was an increase in rearing approach behavior to the CS after CS and US pairings. During CS and US pairings, the movement of the rat was limited so that this approach behavior could not occur. As a result, all control rats showed an increase in rearing, but the cAMY group did not. These results suggest that the cAMY is critical for appetitive as well as fear classical conditioning.     

Evaluative conditioning can be modulated by memory of the CS–US pairings at the time of testing

Evaluative conditioning (EC) is the valence change of a (typically neutral) stimulus (CS) that is due to the previous pairing with another (typically valent) stimulus (US). It has been repeatedly shown that EC effects are stronger or existent only if participants know which US was paired with which CS. Knowledge of the CS–US pairings is usually measured temporally close to both the conditioning phase and the CS valence measurement phase. Hence, the relation between EC and knowledge about the pairings could indicate either that participants need to become aware of the pairings at some point or that they need to remember them during the CS valence test. We isolated the impact of memory during the CS valence test in a study that encompassed two sessions. During the first session, participants were presented with CS–US pairings. The valence of the CSs was measured in a second session several days later using both a rating scale and an affective priming procedure. Memory for the pairings was measured both during the first and the second session. Using item-based multilevel analysis, we found that EC in the second session was related to memory for the pairings during the second session, but not to the memory for the pairings measured immediately after the learning phase. For the pairs that were remembered during the first session, but not during the second session, no EC effect was found. These results suggest that memory for CS–US pairings during valence measurement can be relevant for EC effects to occur.     

Conditioning the unconditioned response: modification of the rabbit's (Oryctolagus cuniculus) unconditioned nictitating membrane response

Conditioning-specific reflex modification (CRM) occurs when classical conditioning modifies responding to an unconditioned stimulus (US) in the absence of a conditioned stimulus (CS). Three experiments monitored rabbit nictitating (Oryctolagus cuniculus) membrane unconditioned responses to 5 intensities and 4 durations of periorbital electrical stimulation before and after CS or US manipulation. CRM occurred after 12 days of CS-US pairings but not following unpaired CS/US presentations or restraint. CRM survived CS-alone and CS/US-unpaired extinction of the conditioned response (CR) but not presentations of the US alone, although CRs remained intact. Thus, CRs could be weakened without eliminating CRM and CRM could be weakened without eliminating CRs. Data indicate CRM is a reliable, associative effect that is more than a generalized CR and may not be explained by habituation, stimulus generalization, contextual conditioning, or bidirectional conditioning.     

Pavlovian second-order conditioned analgesia

Three experiments with rat subjects assessed conditioned analgesia in a Pavlovian second-order conditioning procedure by using inhibition of responding to thermal stimulation as an index of pain sensitivity. In Experiment 1, rats receiving second-order conditioning showed longer response latencies during a test of pain sensitivity in the presence of the second-order conditioned stimulus (CS) than rats receiving appropriate control procedures. Experiment 2 found that extinction of the first-order CS had no effect on established second-order conditioned analgesia. Experiment 3 evaluated the effects of post second-order conditioning pairings of morphine and the shock unconditioned stimulus (US). Rats receiving paired morphine-shock presentations showed significantly shorter response latencies during a hot-plate test of pain sensitivity in the presence of the second-order CS than did groups of rats receiving various control procedures; second-order analgesia was attenuated. These data extend the associative account of conditioned analgesia to second-order conditioning situations and are discussed in terms of the mediation of both first- and second-order analgesia by an association between the CS and a representation or expectancy of the US, which may directly activate endogenous pain inhibition systems.     

Is evaluative conditioning really resistant to extinction? Evidence for changes in evaluative judgements without changes in evaluative representations

Evaluative conditioning (EC) is defined as the change in the evaluation of a conditioned stimulus (CS) due to its pairing with a positive or negative unconditioned stimulus (US). Although several individual studies suggest that EC is unaffected by unreinforced presentations of the CS without the US, a recent meta-analysis indicates that EC effects are less pronounced for post-extinction measurements than post-acquisition measurements. The disparity in research findings suggests that extinction of EC may depend on yet unidentified conditions. In an attempt to uncover these conditions, three experiments (N = 784) investigated the influence of unreinforced post-acquisition CS presentations on EC effects resulting from simultaneous versus sequential pairings and pairings with single versus multiple USs. For all four types of CS–US pairings, EC effects on self-reported evaluations were reduced by unreinforced CS presentations, but only when the CSs had been rated after the initial presentation of CS–US pairings. EC effects on an evaluative priming measure remained unaffected by unreinforced CS presentations regardless of whether the CSs had been rated after acquisition. The results suggest that reduced EC effects resulting from unreinforced CS presentations are due to judgement-related processes during the verbal expression of CS evaluations rather than genuine changes in the underlying evaluative representations.     

CS duration,amount, and concentration effects in conditioning taste aversions

The relative contributions of the duration, amount, and concentration of a taste CS were assessed in pairings with an ionizing radiation US in two experiments. Increasing the duration of the CS (0.1% saccharin) up to 9 min yielded increasingly better conditioning over a $\text{1}\text{2}\text{-}\text{hr}$ CS-US delay interval. When the concentration of the sacharin was increased, only a few seconds' exposure to the CS was necessary for near asymptotic conditioning. It was observed that the CS duration effect had not been identified earlier because most investigators utilize 10–20 min CS access periods. The role of CS duration in conditioning over a delay was discussed with reference to the probability of the memory of the CS being present at the time of the pairing with the US event.     

Malleability of conditioned associations: path dependence

Using conditioned suppression of barpressing to investigate the stability of a conditioned stimulus-unconditioned stimulus (CS-US) association, we gave water-deprived rats either a few pairings of the CS with a strong footshock US or many pairings with a weak footshock US so that barpress suppression in response to the CS was equated. Experiment 1 established training parameters that yielded this equivalence. Specifically, rapid acquisition to a preasymptotic level of responding with strong shock produced suppression comparable to the asymptotic level reached more slowly with weak shock. Experiment 2 showed that although equivalent performance was obtained from extensive conditioning with a weak shock or limited conditioning with strong shock, only extensive conditioning with weak shock resulted in retarded acquisition of an association between that same CS and a footshock level perceived as midway between the two initial training shock intensities as implied by asymptotic performance in Experiment 1. Experiment 3 demonstrated that the observed retardation in animals given many conditioning trials with weak shock was CS specific. Collectively, these findings indicate that the malleability of learned behavior is not simply a function of initial associative strength but is dependent on path during initial acquisition.     

What you see is what will change: Evaluative conditioning effects depend on a focus on valence

This study investigated whether evaluative conditioning (EC) effects depend on an evaluative focus during the learning phase. An EC effect is a valence change of an originally neutral stimulus (conditioned stimulus or CS) that is due to the former pairing with a positive or negative stimulus (unconditioned stimulus or US). In three experiments, the task focus during the conditioning phase was manipulated. Participants judged CS–US pairings either with respect to their valence or with respect to another stimulus dimension. EC effects on explicit and implicit measures were found when valence was task relevant but not when the non-valent stimulus dimension was task relevant. Two accounts for the valence focus effect are proposed: (1) An additional direct learning of the relation of CS and evaluative responses in the valence focus condition, or (2) a stronger activation of US valence in the valence focus condition compared to the non-valent focus condition.     

Within-compound associations between the context and the conditioned stimulus

Three experiments were conducted to test for the presence of associations between contextual cues and the nominal conditioned stimulus (CS) in fear conditioning. Rats were given tone-footshock pairings and were tested for their fear of the nominal CS, the tone, in a different context. Experiments 1 and 2 demonstrated that rats given nonreinforced exposure to the training context following conditioning were less fearful of the CS. Experiment 3 indicated that additional footshock presentations in the training context following conditioning produced greater fear of the CS. In both procedures postconditioning treatments designed to directly alter only the associative strength of the training context yielded parallel changes in the conditioned response to the CS. These data suggest that within-compound associations are formed between the context and the CS during classical conditioning.     

Differential effects of two ways of devaluing the unconditioned stimulus after Pavlovian appetitive conditioning

Three experiments with rat subjects investigated the effects of two methods of devaluing a food unconditioned stimulus (US) after pairings of an auditory conditioned stimulus (CS) with that US. Experiment 1 found no effect of postconditioning pairings of the food US with lithium chloride (LiCl) on general activity to a tone CS, even though those pairings substantially reduced food consumption. Experiments 2 and 3 compared the effects on conditioned responding of postconditioning pairings of food with LiCl and with high-speed rotation. In these experiments the general activity measure was supplemented by a detailed visual analysis of the rats' behavior. Experiment 2 found that food-rotation pairings had larger effects than food-LiCl pairings on general activity responding and on two detailed behavioral measures but that food-LiCl pairings had larger effects on food consumption and on one behavioral measure. Experiment 3 replicated the findings of Experiment 2 and found that the ability of the CS to serve as a reinforcer for second-order conditioning after US devaluation was reduced more by food-LiCl pairings.     

Evaluative conditioning effects are modulated by the nature of contextual pairings

Across two studies participants completed a learning phase comprised of two types of trials: context pairing trials in which two (valenced or non-valenced) words were identical or opposite to one another and evaluative conditioning (EC) trials in which a CS was paired with a US. Based on the idea that EC occurs because CS-US pairings function as a symbolic cue about the relation between the CS and the US, we hypothesised that the nature of context pairings (identical or opposite) might moderate EC effects. Results indicate that identity-based context pairs led to typical assimilative explicit and implicit effects whereas opposition-based pairs led to attenuated effects. Implications and different accounts of our findings are discussed.     

Associability of a previously conditioned stimulus as a function of qualitative changes in the US

Hall and Pearce (1979) reported retarded manifestation of conditioned responding to repeated CS-strong shock pairings as a result of prior CS-weak shock pairings. These authors suggested that a latent inhibition (LI)-like process reduced the associability of the CS during weak shock conditioning despite excitatory associations between CS and weak shock being formed. The present experiments examined the possibility that the negative transfer from CS-weak shock conditioning in Stage 1 to CS-strong shock conditioning in Stage 2 observed by Hall and Pearce was due at least in part to the similarity between weak and strong shock rather than, or in addition to, a general loss of CS associability. Specifically, the transfer of decreased associability to conditioning with a dissimilar US was investigated. Consistent with Hall and Pearce, who gave multiple training trials in Stage 2, Experiment 1 found the development of conditioned responding owing to a single CS-strong shock pairing in Stage 2 to be retarded by prior CS-weak shock pairings and compared this effect to conventional LI. In Experiment 2, partial attenuation of the negative transfer following CS-weak shock was obtained by substituting ice water immersion for strong shock, that is, by making the strong US qualitatively dissimilar from the weak US. In contrast, conventional LI resulting from preconditioning CS-only exposures was equivalent for strong shock in Experiment 1 and ice water immersion in Experiment 2. A possible mechanism for the sensitivity to qualitative US changes of the observed negative transfer is discussed.     

Signal transduction mechanisms within the entorhinal cortex that support latent inhibition of cued fear conditioning

Latent inhibition is a phenomenon by which pre-exposure to a conditioned-stimulus (CS), prior to subsequent pairings of that same CS with an unconditioned-stimulus (US), results in decreased conditioned responding to the CS. Previous work in our laboratory has suggested that the entorhinal cortex is critically involved in the establishment of latent inhibition of cued fear conditioning. Furthermore, utilizing systemic pharmacology, we have demonstrated a role for of NMDA receptors, protein kinase A (PKA), and mitogen activated protein kinase (MAPK, also known as ERK) in latent inhibition of cued fear conditioning, but until now, where these cell signaling cascades are critically activated during latent inhibition of cued fear was unknown. Here, we use direct drug infusion to demonstrate that cell signaling via NMDA receptors, the cAMP/PKA pathway, and the MAPK pathway within the entorhinal cortex are critically involved in latent inhibition of cued fear conditioning. In the present study, CS pre-exposed mice received 20 CS pre-exposures 24h prior to two pairings of the same CS with a 0.53 mA foot shock US, while control animals receive no pre-exposure to the CS. The NMDA antagonist APV (0.25 or 2.5 microg/side), the cAMP inhibitor Rp-cAMP (1.8 or 18.0 microg/side), or the MAPK inhibitor U0126 (0.1 or 1.0 microg/side) were directly infused into the entorhinal cortex prior to pre-exposure. All three drugs produced dose-dependent disruptions in latent inhibition of cued fear conditioning. Importantly, none of the drugs had any effect on cued fear conditioning when administered on training day, suggesting that the effects of each of the drugs were specific to CS pre-exposure. These results are discussed in relation to the potential mechanisms of plasticity that support latent inhibition of cued fear conditioning.     

Learning strategies during fear conditioning

This paper describes a model of fear learning, in which subjects have an option of behavioral responses to impending social defeat. The model generates two types of learning: social avoidance and classical conditioning, dependent upon (1) escape from or (2) social subordination to an aggressor. We hypothesized that social stress provides the impetus as well as the necessary information to stimulate dichotomous goal-oriented learning. Specialized tanks were constructed to subject rainbow trout to a conditioning paradigm where the conditioned stimulus (CS) is cessation of tank water flow (water off) and the unconditioned stimulus (US) is social aggression from a larger conspecific. Following seven daily CS/US pairings, approximately half of the test fish learned to consistently escape the aggression to a neutral chamber through a small escape hole available only during the interaction. The learning curve for escaping fish was dramatic, with an 1100% improvement in escape time over 7 days. Fish that did not escape exhibited a 400% increase in plasma cortisol and altered brain monoamine response to presentation of the CS alone. Elevated plasma cortisol levels represent classical fear conditioning in non-escaping fish, while a lack of fear conditioning and a decreased latency to escape over the training period in escapers indicates learned escape.     

Olfactory fear conditioning paradigm in rats: Effects of midazolam,propranolol or scopolamine

In rodents, fear conditioned responses are more pronounced toward olfactory stimulus, since olfaction is a dominant sense in these subjects. The present study was outlined to investigate if the association between coffee odor (CS1) and electrical footshock (US) would be an effective model for the study of fear-induced behavior and whether compounds used in humans for emotional-related disorders such as midazolam, propranolol, or scopolamine, applied during the different stages of fear conditioning (acquisition, consolidation and expression), affect the defensive responses to both, the olfactory CS1, and the context (CS2) where the CS1 had been presented (second order conditioning). The results revealed that five pairings between coffee odor (CS1) and electrical footshock (US) were able to elicit consistent defensive responses and a second order conditioning to the context (CS2). Midazolam (0.375–0.5 mg/kg; i.p.) treatment was able to interfere with the CS1–US association and with the consolidation of the aversive information. The propranolol (5–10 mg/kg; i.p.) treatment interfered with the CS1–US association, with the retention of fear memory and with the CS1–CS2 association. Propranolol also attenuated the expression of conditioned fear responses when applied before the CS1 test session. Scopolamine (0.6–1.2 mg/kg; i.p.) treatment impaired the acquisition of CS1–US and CS1–CS2 associations, and also disrupted the expression of conditioned fear responses when injected prior to the CS1 test session. These findings have pointed out the usefulness for the olfactory fear conditioning paradigm to investigate drug effects on the acquisition, consolidation and expression of fear conditioned responses.     

Unconditioned stimulus devaluation effects in nutrient-conditioned flavor preferences

Experiments with different temporal relations between the conditioned stimulus (CS) and the unconditioned stimulus (US) in conditioning assessed whether US devaluation effects can be obtained after nutrient-conditioned flavor preference learning. One flavor (CScarb) was paired with a carbohydrate, Polycose; a 2nd flavor (CSprot) was paired with a protein, casein; and a 3rd flavor (CS-) was presented by itself. Following conditioning, one of the nutrients was devalued through pairings with lithium chloride in the absence of the CS flavors. In a subsequent 2-bottle test, rats preferred CScarb over CSprot; however, this preference was smaller when the carbohydrate was devalued than when the protein was devalued. Results suggest that CS flavors are able to form associations with the sensory features of nutrient USs under a wide variety of circumstances.