Associative regulation of Pavlovian fear conditioning: unconditional stimulus intensity, incentive shifts, and latent inhibition.

Conditional stimuli (CS) associated with painful unconditional stimuli (US) produce a naloxone-reversible analgesia. The analgesia serves as a negative-feedback regulation of fear conditioning that can account for the impact of US intensity and CS predictiveness on Pavlovian fear conditioning. In Experiment 1 training under naloxone produced learning curves that approached the same high asymptote despite US intensity. Shifting drug treatment during acquisition had effects that paralleled US intensity shifts. In Experiment 3 naloxone reversed Hall-Pearce (1979) negative transfer using a contextual CS, indicating that conditional analgesia acquired during the CS-weak-footshock phase retards acquisition in the CS-strong-footshock phase. Experiment 5 used a tone CS in both a latent-inhibition and a negative-transfer procedure. Only negative transfer was blocked by naloxone. Therefore, negative transfer but not latent inhibition is mediated by a reduction of US processing.     

Naloxone and Pavlovian fear conditioning

Previous research has shown that pretreating rats with the opiate antagonist naloxone increases the freezing that follows painful electric shock. Three experiments, using freezing behavior as a dependent variable, were carried out to determine whether the drug might cause this effect by enhancing fear conditioning. Two of the studies employed a differential context fear-conditioning paradigm. Naloxone did not affect freezing behavior during the preshock adaptation period. In Experiment 1, naloxone was found to increase resistance to extinction in the S+ context. In Experiment 2, naloxone was found to increase freezing in the S+ context. This effect was dependent upon administering naloxone during training but not dependent on administering it during testing. The third study employed a generalization paradigm. It was found that naloxone's effect on postshock freezing was dependent on the place of testing; as the contextual cues of the test chamber were changed from those of the conditioning chamber, the effect of naloxone on freezing was reduced. The results of these experiments lend strong support to the hypothesis that naloxone increases freezing by enhancing the conditioning of fear to contextual stimuli associated with shock.     

Deletion of the mu opioid receptor results in impaired acquisition of Pavlovian context fear

The mu opioid receptor may constitute a critical component of a negative feedback system that regulates Pavlovian fear conditioning. We investigated context fear conditioning acquisition and expression in mu opioid receptor knockout mice (on an inbred, C57 genetic background). We discovered that the mu receptor knockout results in an unexpected and significant deficit in context fear acquisition. Mice lacking the mu receptor showed normal fear acquisition when subjected to a 1-day fear conditioning protocol but evinced deficient fear learning when acquisition was conducted across 5 days. The knockout mice showed normal reactivity to footshock in both fear conditioning protocols. Finally, we confirmed the effectiveness of the receptor deletion in the C57 strain by subjecting the mice to a test of morphine analgesia in the hot-plate assay. As has been seen with mixed genetic background, the receptor deletion resulted in a complete lack of analgesic response to 10 mg/kg morphine. Surprisingly, mice with a single copy of the mu receptor gene (heterozygous knockouts) showed intact sensitivity to morphine but a significant deficit in Pavlovian fear conditioning. The results indicate that deletion of the mu receptor gene impairs fear conditioning and that the conditioning and analgesia effects of heterozygous deletion are dissociable. The conditioning deficit seen in this line of mice may be related to impairment in hippocampus function.     

Amygdala and periaqueductal gray lesions only partially attenuate unconditional defensive responses in rats exposed to a cat

Defensive responses to a cat were observed in rats given excitotoxic lesions of the central nucleus of the amygdala (ACe), dorsolateral periaqueductal gray (dlPAG), ventral periaqueductal gray (vPAG), or sham lesions. Rats were placed adjacent to a compartment containing a cat. Sham-lesioned rats avoided the area nearest the cat and preferred the area furthest away from the cat. They also exhibited numerous defensive responses including, climbing, escape from the apparatus, and freezing. Rats with lesions of the ACe reacted like the sham lesioned rats by preferring the area of the apparatus furthest from the cat, however they climbed and escaped significantly less than sham lesioned rats. Avoidance of the area adjacent to the cat was attenuated in rats with lesions of the vPAG. Climbing along the walls of the apparatus was also attenuated in rats with lesions of the vPAG. Escapes from the apparatus were not significantly reduced by lesions of the vPAG and dlPAG. Thus, ACe lesions attenuated climbing and eliminated escapes, but did not impair locomotion of the rat away from the cat.     

Hindsight bias: how knowledge and heuristics affect our reconstruction of the past

Once people know the outcome of an event, they tend to overestimate what could have been anticipated in foresight. Although typically considered to be a robust phenomenon, this hindsight bias is subject to moderating circumstances. In their meta-analysis, Christensen-Szalanski and Willham (1991) observed that the more experience people have with the task under consideration, the smaller is the resulting hindsight bias. This observation is one benchmark against which the explanatory power of process models of hindsight bias can be measured. Therefore, we used it to put the recently proposed RAFT model (Hoffrage, Hertwig, & Gigerenzer, 2000) to another test. Our findings were consistent with the "expertise effect." Specifically, we observed-using computer simulations of the RAFT model-that the more comprehensive people's knowledge is in foresight, the smaller is their hindsight bias. In addition, we made two counterintuitive observations: First, the relation between foresight knowledge and hindsight bias appears to be independent of how knowledge is processed. Second, even if foresight knowledge is false, it can reduce hindsight bias. We conclude with a discussion of the functional value of hindsight bias.     

Word order variation in spatial descriptions with adverbs

Previous research has shown that in a three-term spatial reasoning task, the second premise of a German premise pair is especially easy to comprehend if (1) the prepositional object rather than the grammatical subject denotes the given entity, and if (2) the term denoting the given entity precedes the term denoting the new entity. Accordingly, the second premise is easiest to comprehend with noncanonical word order--that is, with the prepositional object in preverbal position denoting the given entity (e.g., To the right of the given object is the new subject). This finding is explained in terms of contextual licensing of noncanonical word order. Here, we discuss and tested two alternative accounts of contextual licensing, given-new and partially ordered set relations (Poset). The given-new account claims that noncanonical word order is licensed by the term denoting the given entity preceding the term denoting the new entity. On the Poset account, noncanonical word order is licensed if the preverbal constituent introduces a new entity that stands in a transitive, irreflexive, and asymmetric relation to a given entity. Comprehension times for second premises with spatial adverbs in four different word orders support both accounts of contextual licensing; Poset licensing was stronger than given-new licensing.     

Brief flight to a familiar enclosure in response to a conditional stimulus in rats

The authors observed brief, directed movement to a familiar enclosure in rats to determine whether this behavior is part of a rat's defensive repertoire when exposed to a conditional-fear stimulus. In Experiment 1, upon exposure to the compound conditional-fear stimulus of tone and light, only rats that received paired presentations of the conditional stimuli and shock fled into a small, familiar enclosure where they then froze. Rats that had received unpaired presentations did not enter the enclosure in significant amounts when later tested. In Experiment 2, the authors observed rats' freezing and use of either a familiar or an unfamiliar enclosure when tested with a conditional-fear stimulus. Rats tested with a familiar enclosure entered it more quickly than did rats without prior exposure to the enclosure. Freezing was greatest when both training and testing environments were similar with respect to access to the enclosure. The results of these 2 experiments support the idea that brief, directed flight in rats is a component of the postencounter stage of predatory imminence (M. S. Fanselow & L. S. Lester, 1988) and is compatible with freezing.     

Changes in feeding and foraging patterns as an antipredator defensive strategy: a laboratory simulation using aversive stimulation in a closed economy.

The effects of the risk of electric shock on the meal patterns of rats living in an operant chamber were investigated. Rats could obtain food by working on a response lever that provided reinforcement according to chained fixed-ratio continuous reinforcement schedules that allowed the animals control over meal size. Using a two-compartment operant chamber with a safe nesting area and manipulanda area with a grid floor, shock could be correlated with responding on the schedule. Shocks (less than or equal to 1.25 per hour) were scheduled to occur randomly throughout the day, independent of the rat's behavior. Shock caused a reorganization of meal patterns such that the animals took less frequent but larger meals. This pattern reduced the time the animals spent at risk without compromising caloric balance. Similar changes in feeding pattern were obtained in both hooded and albino rats. Exposure to shock in a separate chamber did not produce these behavioral modifications. The magnitude of shock-induced alterations of meal patterns was greater with chained fixed-ratio 90 continuous reinforcement than with chained fixed-ratio 10 continuous reinforcement. Additionally, the rats seemed to be able to reduce food intake but increase caloric efficiency, such that the reduced food intake did not have deleterious effects on maintenance of body weight. These behavioral modifications reduced the number of shocks received from that which would have been expected if meal pattern changes had not occurred. We suggest that this technique may provide a useful laboratory simulation of the impact that the risk of predation has on foraging behavior.     

Conditioned fear-induced opiate analgesia on the Formalin test: Evidence for two aversive motivational systems

Fear, conditioned to apparatus cues associated with electric shock, was measured by recording freezing. Pain reactivity was simultaneously measured by recording the paw-licking and paw-lifting response to Formalin injected into a paw. Stimuli associated with shock produced freezing and inhibited the responses to Formalin; whereas various control stimuli did neither. These results indicate that one of the responses to fear is analgesia. The opiate antagonist naltrexone reversed this analgesia, suggesting that the analgesia has an opiate nature. The results were interpreted in terms of a two-aversive-motivational-systems model. One system, the pain system, is instigated by tissue-damaging stimuli and produces recuperative behavior that functions to promote healing. The other system, the fear system, is triggered by conditioned fear stimuli. It produces species-specific defensive reactions and also inhibits the pain system via analgesia.