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.     

Posttraining handling facilitates memory for auditory-cue fear conditioning in rats

A large number of studies have indicated that stress exposure or the administration of stress hormones and other neuroactive drugs immediately after a learning experience modulates the consolidation of long-term memory. However, there has been little investigation into how arousal induced by handling of the animals in order to administer these drugs affects memory. Therefore, the present study examined whether the posttraining injection or handling procedure per se affects memory of auditory-cue classical fear conditioning. Male Sprague-Dawley rats, which had been pre-handled on three days for 1 min each prior to conditioning, received three pairings of a single-frequency auditory stimulus and footshock, followed immediately by either a subcutaneous injection of a vehicle solution or brief handling without injection. A control group was placed back into their home cages without receiving any posttraining treatment. Retention was tested 24 h later in a novel chamber and suppression of ongoing motor behavior during a 10-s presentation of the auditory-cue served as the measure of conditioned fear. Animals that received posttraining injection or handling did not differ from each other but showed significantly less stimulus-induced movement compared to the non-handled control group. These findings thus indicate that the posttraining injection or handling procedure is sufficiently arousing or stressful to facilitate memory consolidation of auditory-cue classical fear conditioning.     

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.     

Nonassociative learning processes determine expression and extinction of conditioned fear in mice

Freezing to a tone following auditory fear conditioning is commonly considered as a measure of the strength of the tone-shock association. The decrease in freezing on repeated nonreinforced tone presentation following conditioning, in turn, is attributed to the formation of an inhibitory association between tone and shock that leads to a suppression of the expression of fear. This study challenges these concepts for auditory fear conditioning in mice. We show that acquisition of conditioned fear by a few tone-shock pairings is accompanied by a nonassociative sensitization process. As a consequence, the freezing response of conditioned mice seems to be determined by both associative and nonassociative memory components. Our data suggest that the intensity of freezing as a function of footshock intensity is primarily determined by the nonassociative component, whereas the associative component is more or less categorical. We next demonstrate that the decrease in freezing on repeated nonreinforced tone presentation following conditioning shows fundamental properties of habituation. Thus, it might be regarded as a habituation-like process, which abolishes the influence of sensitization on the freezing response to the tone without affecting the expression of the associative memory component. Taken together, this study merges the dual-process theory of habituation with the concept of classical fear conditioning and demonstrates that sensitization and habituation as two nonassociative learning processes may critically determine the expression of conditioned fear in mice.     

Contextual potentiation of acquired behavior after devaluing direct context-US associations

In two experiments, water-deprived rats were exposed to tone-footshock pairings in a particular conditioning context. In Experiment 1, the training parameters were selected on the basis of parametric pilot data so as not to produce direct associations between the conditioning context and footshock. Despite independent evidence of no appreciable direct association between the conditioning context and footshock, animals tested in the conditioning context exhibited more lick suppression to the tone than other animals tested with the tone outside of the conditioning context. Thus, the conditioning context potentiated suppression to the tone in the absence of appreciable fear of the conditioning context. In Experiment 2, training parameters were used that permitted the formation of direct associations between the conditioning context and footshock; however, for half the animals, these associations to the context were then extinguished. With the tone absent, a comparison of extinguished animals tested either in the conditioning context or outside the conditioning context detected no differences in suppression, demonstrating the effectiveness of the extinction manipulation. However, other animals also subjected to extinction of direct context-footshock associations displayed greater suppression to the tone within the conditioning context than comparable animals tested outside the context. Collectively the data indicate that the superior retention-test performance seen within the training context can arise both from the commonly assumed direct associations between the training context and the unconditioned stimulus and from the potentiation by the training context of the associations between the nominal conditioned stimulus and unconditioned stimulus.     

Different importance of the volatile and non-volatile fractions of an olfactory signature for individual social recognition in rats versus mice and short-term versus long-term memory

When tested in the olfactory cued social recognition/discrimination test, rats and mice differ in their retention of a recognition memory for a previously encountered conspecific juvenile: Rats are able to recognize a given juvenile for approximately 45 min only whereas mice show not only short-term, but also long-term recognition memory (≥ 24 h). Here we modified the social recognition/social discrimination procedure to investigate the neurobiological mechanism(s) underlying the species differences. We presented a conspecific juvenile repeatedly to the experimental subjects and monitored the investigation duration as a measure for recognition. Presentation of only the volatile fraction of the juvenile olfactory signature was sufficient for both short- and long-term recognition in mice but not rats. Applying additional volatile, mono-molecular odours to the "to be recognized" juveniles failed to affect short-term memory in both species, but interfered with long-term recognition in mice. Finally immunocytochemical analysis of c-Fos as a marker for cellular activation, revealed that juvenile exposure stimulated areas involved in the processing of olfactory signals in both the main and the accessory olfactory bulb in mice. In rats, we measured an increased c-Fos synthesis almost exclusively in cells of the accessory olfactory bulb. Our data suggest that the species difference in the retention of social recognition memory is based on differences in the processing of the volatile versus non-volatile fraction of the individuals' olfactory signature. The non-volatile fraction is sufficient for retaining a short-term social memory only. Long-term social memory - as observed in mice - requires a processing of both the volatile and non-volatile fractions of the olfactory signature.     

Memory enhancement of classical fear conditioning by post-training injections of corticosterone in rats

There is extensive evidence that post-training administration of the adrenocortical hormone corticosterone facilitates memory consolidation processes in a variety of contextual and spatial-dependent learning situations. The present experiments examine whether corticosterone can modulate memory of auditory-cue classical fear conditioning, a learning task that is not contingent on contextual or spatial representations. Male Sprague-Dawley rats received three pairings of a single-frequency auditory stimulus and footshock, followed immediately by a post-training subcutaneous injection of either corticosterone (1.0 or 3.0mg/kg) or vehicle. Retention was tested 24h later in a novel test chamber and suppression of ongoing motor behavior served as the measure of conditioned fear. Corticosterone dose-dependently facilitated suppression of motor activity during the 10-s presentation of the auditory cue. As corticosterone administration did not alter responding after unpaired presentations of tone and shock, tone alone, shock alone or absence of tone/shock, the findings indicated that corticosterone selectively facilitated memory of the tone-shock association. Furthermore, injections of corticosterone given 3h after training did not alter motor activity during retention testing, demonstrating that corticosterone enhanced time-dependent memory consolidation processes. These findings provide evidence that corticosterone modulates the consolidation of memory for auditory-cue classical fear conditioning and are consistent with a wealth of data indicating that glucocorticoids can modulate a wide variety of emotionally influenced memories.     

Opioid receptors mediate direct predictive fear learning: evidence from one-trial blocking

Pavlovian fear learning depends on predictive error, so that fear learning occurs when the actual outcome of a conditioning trial exceeds the expected outcome. Previous research has shown that opioid receptors, including μ-opioid receptors in the ventrolateral quadrant of the midbrain periaqueductal gray (vlPAG), mediate such predictive fear learning. Four experiments reported here used a within-subject one-trial blocking design to study whether opioid receptors mediate a direct or indirect action of predictive error on Pavlovian association formation. In Stage I, rats were trained to fear conditioned stimulus (CS) A by pairing it with shock. In Stage II, CSA and CSB were co-presented once and co-terminated with shock. Two novel stimuli, CSC and CSD, were also co-presented once and co-terminated with shock in Stage II. The results showed one-trial blocking of fear learning (Experiment 1) as well as one-trial unblocking of fear learning when Stage II training employed a higher intensity footshock than was used in Stage I (Experiment 2). Systemic administrations of the opioid receptor antagonist naloxone (Experiment 3) or intra-vlPAG administrations of the selective μ-opioid receptor antagonist CTAP (Experiment 4) prior to Stage II training prevented one-trial blocking. These results show that opioid receptors mediate the direct actions of predictive error on Pavlovian association formation.     

Fear conditioning and social groups: statistics, not genetics

Humans display more conditioned fear when the conditioned stimulus in a fear conditioning paradigm is a picture of an individual from another race than when it is a picture of an individual from their own race ( Olsson, Ebert, Banaji, & Phelps, 2005 ). These results have been interpreted in terms of a genetic "preparedness" to learn to fear individuals from different social groups ( Ohman, 2005 ; Olsson et al., 2005 ). However, the associability of conditioned stimuli is strongly influenced by prior exposure to those or similar stimuli. Using the Kalman filter, a normative statistical model, this article shows that superior fear conditioning to individuals from other groups is precisely what one would expect if participants perform optimal, Bayesian inference that takes their prior exposures to the different groups into account. There is therefore no need to postulate a genetic preparedness to learn to fear individuals from other races or social groups.     

Effect of telencephalon ablation on the reinforcing and eliciting properties of species-specific events in Betta splendens

In male Betta splendens, aggressive behavior is drastically attenuated following telencephalon ablation. Because instrumental training and Pavlovian conditioning experiments with intact fish have suggested that associative factors may play an important role in the performance of agonistic behaviors, the effect of ablation on instrumental learning and Pavlovian conditioning was studied. In Experiment 1, ablation had no effect on the learning of the instrumental tunnel-swimming response reinforced by mirror presentation (i.e., viewing a conspecific), although the mirror presentations in yoked-control groups elicited fewer responses in ablates than in normal and sham-operated control fish. Yoked controls further established that instrumental responding was maintained by the reinforcement contingency and was not merely the result of increased motor activity. Experiment 2 studied Pavlovian conditioning of the components of the agonistic display. Unconditioned fin erection, gill erection, and tail beating (i.e., unconditioned responses, URs) to the mirror US all were less frequent in ablates than in normals or shams. Of these, only gill cover erection showed evidence of true conditioning (i.e., conditioned responses; CRs) in which responses to the conditioned stimulus (CS) are due to the pairings of CS and US (unconditioned stimulus). However, ablates suffered no impairment of conditioned gill erections. Ablates performed fewer fin erections to the CS; however, fin erection responses were not due to CS-US pairings but were attributable to pseudoconditioning. These results are related to hypotheses postulating the involvement of learning mechanisms in ablation-produced deficits and normal aggressive behavior.     

Switching from approach to withdrawal is easier than vice versa

The preparedness theory of classical conditioning proposed by Seligman (1970, 1971) has been applied extensively over the past 40 years to explain the nature and “source” of human fear and phobias. In this review we examine the formative studies that tested the four defining characteristics of prepared learning with animal fear-relevant stimuli (typically snakes and spiders) and consider claims that fear of social stimuli, such as angry faces, or faces of racial out-group members, may also be acquired utilising the same preferential learning mechanism. Exposition of critical differences between fear learning to animal and social stimuli suggests that a single account cannot adequately explain fear learning with animal and social stimuli. We demonstrate that fear conditioned to social stimuli is less robust than fear conditioned to animal stimuli as it is susceptible to cognitive influence and propose that it may instead reflect on negative stereotypes and social norms. Thus, a theoretical model that can accommodate the influence of both biological and cultural factors is likely to have broader utility in the explanation of fear and avoidance responses than accounts based on a single mechanism.     

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.     

Basolateral amygdala noradrenergic activity mediates corticosterone-induced enhancement of auditory fear conditioning

The present experiment examined whether posttraining noradrenergic activity within the basolateral complex of the amygdala (BLA) is required for mediating the facilitating effects of acutely administered glucocorticoids on memory for auditory-cue classical fear conditioning. Male Sprague-Dawley rats received five pairings of a single-frequency auditory stimulus and footshock, followed immediately by bilateral infusions of the beta1-adrenoceptor antagonist atenolol (0.5 microg in 0.2 microl) or saline into the BLA together with a subcutaneous injection of either corticosterone (3.0 mg/kg) or vehicle. Retention was tested 24 h later in a novel test chamber and suppression of ongoing motor behavior served as the measure of conditioned fear. Corticosterone facilitated memory as assessed by suppression of motor activity during the 10-s presentation of the auditory stimulus and intra-BLA administration of atenolol selectively blocked this corticosterone-induced memory enhancement. These findings provide evidence that, as found with other emotionally arousing tasks, the enhancing effects of corticosterone on memory consolidation of auditory-cue fear conditioning require posttraining noradrenergic activity within the BLA.     

Influence of chronic corticosterone and glucocorticoid receptor antagonism in the amygdala on fear conditioning

Glucocorticoid receptor activation within the basolateral amygdala (BLA) during fear conditioning may mediate enhancement in rats chronically exposed to stress levels of corticosterone. Male Sprague-Dawley rats received corticosterone (400 microg/ml) in their drinking water (days 1-21), a manipulation that was previously shown to cause hippocampal CA3 dendritic retraction. Subsequently, rats were adapted to the fear conditioning chamber (day 22), then trained (day 23), and tested for conditioned fear to context and tone (day 25). Training consisted of two tone (20s) and footshock (500 ms, 0.25 mA) pairings. In Experiment 1, muscimol (4.4 nmol/0.5 microl/side), a GABAergic agonist, was microinfused to temporarily inactivate the BLA during training. Rats given chronic corticosterone showed enhanced freezing to context, but not tone, compared to vehicle-supplemented rats. Moreover, BLA inactivation impaired contextual and tone conditioning, regardless of corticosterone treatment. In Experiment 2, RU486 (0, 0.3, and 3.0 ng/0.2 microl/side) was infused on training day to antagonize glucocorticoid receptors in the BLA. Corticosterone treatment enhanced fear conditioning to context and tone when analyzed together, but not separately. Moreover, RU486 (3.0 ng/side) selectively exacerbated freezing to context in chronic corticosterone-exposed rats only, but failed to alter tone conditioning. Serum corticosterone levels were negatively correlated with contextual, not tone, conditioning. Altogether, these suggest that chronic corticosterone influences fear conditioning differently than chronic stress as shown previously. Moreover, chronic exposure to corticosteroids alters BLA functioning in a non-linear fashion and that contextual conditioning is influenced more than tone conditioning by chronic corticosterone and BLA glucocorticoid receptor stimulation.     

Implicit attitude formation through classical conditioning

We sought to demonstrate that attitudes can develop through implicit covariation detection in a new classical conditioning paradigm. In two experiments purportedly about surveillance and vigilance, participants viewed several hundred randomly presented words and images interspersed with critical pairings of valenced unconditioned stimuli (USs) with novel conditioned stimuli (CSs). Attitudes toward the novel objects were influenced by the paired USs: In a surprise evaluation task, the CS paired with positive items was evaluated more positively than the CS paired with negative items. This attitudinal conditioning effect was found using both an explicit measure (Experiments 1 and 2) and an implicit measure (Experiment 2). In a covariation estimation task involving the stimuli presented in the conditioning procedure, participants displayed no explicit memory for the pairings.     

The effect of prior acquisition of food aversion on the poisoned‐partner effect

It has been suggested that poisoned conspecifics function as aversive unconditioned stimuli for rats. The present study examined whether the prior acquisition of food aversion could enhance this effect. In phase 1, subjects were given pairings of cocoa‐flavored pellets and a toxin. In phase 2, they were given the cocoa‐flavored pellets and later presented with a poisoned or nonpoisoned conspecific. Subsequent testing involved a choice between the cocoa‐flavored pellets and regular pellets. Prior exposure to a poisoned conspecific prevented extinction of the conditioned aversion. In contrast, exposure to a nonpoisoned conspecific allowed extinction of the conditioned aversion. These findings suggest that a rat's reaction to a stressed conspecific is affected by their prior experience of aversive events.     

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.     

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.     

Fearlessness in juvenile offenders is associated with offending rate

Poor fear conditioning is a correlate of violent offending in adults, but there is no evidence concerning juvenile offenders. Our aim was to compare emotional learning in juvenile offenders and controls and establish whether crime rate is related to seriousness of emotional learning problems. To this end, emotional learning was assessed in 42 juvenile offenders by measuring skin conductance responding (SCR) during fear conditioning. Compared to controls, juvenile offenders showed lower conditioned SCRs to visual stimuli associated with a subsequent aversive stimulus and the magnitude of the SCR during fear acquisition was inversely associated with the number of their recorded offences. These findings suggest that juvenile offenders have impairments in the neural systems that subserve emotional learning. The implication is that using punitive measures to control persistent offenders is unlikely to be effective in an identifiable group of juvenile offenders.     

Pavlovian conditioning in human skilled motor behavior

The role of Pavlovian contingencies in human skilled motor behavior was investigated in three experiments by means of a new conditioning preparation. In Experiment 1 the present method was shown to be appropriate for the study of associative learning. Subjects who experienced a standard delay configuration performed significantly more conditioned responses than subjects who received either backward conditioning or random pairings. Stimulus generalization was shown to be slight in two additional groups. Subsequent experiments examined conditioning with multiple conditioned stimuli (CSs). In particular, in Experiment 2 some reciprocal overshadowing was demonstrated when two conditional stimuli (tone and vibration) were compounded. Experiment 3 investigated blocking. Blocking was less than expected, however. Subjects’ perceptions of the stimuli and reaction time data suggest that a certain proportion had shifted their attention to the added element of the CS compound. Results are discussed in relation to other studies on Pavlovian learning in humans and animals, which are concerned with “stimulus selection.”