Fear Conditioning Procedure Research Articles

Functional Analysis of Genes Implicated in Down Syndrome: 1. Cognitive Abilities in Mice Transpolygenic for Down Syndrome Chromosomal Region-1 (DCR-1)

Down syndrome occurs every 1/1000 births and is the most frequent genetic cause of mental retardation. The genetic substrate of Down syndrome, an extra chromosome 21, was discovered by Lejeune, half-a-century ago, and the chromosome has been fully sequenced, although the gene(s) implicated in the mental retardation observed with the syndrome are still unknown. Observations of patients with partial trisomy of the 21q22.2 fragment suggest that most of the signs of the syndrome, including mental retardation, could be influenced by the region referred to as the Down Minimal Chromosomal Region-1 (DCR-1) for that reason. Using the extensive syntenies between human chromosome 21 and murine chromosome 16, Smith et al. (1995, 1997) developed transpolygenic mice with human chromosome 21 fragments covering the DCR-1. Here, we explored cognitive performances in mice over-expressing the genes carried by these fragments with the Morris water-maze and fear-conditioning procedures. The 152F7 transpolygenic mice had lower performance levels, compared to non-transgenic and other transgenic mice on most measurements in the water-maze. In fear-conditioning, all transgenic mice recorded lower performance levels compared to controls in the altered context stage. The 230E8, 141G6 and 285E6 mice failed to learn or react when the sound used as the conditional stimulus was added. These results showed that the 152F7 region played a crucial role in cognitive impairment, supporting the hypothesis of DYRK-1A gene involvement. However, the data presented here also suggest that other chromosomal regions within the DCR-1 may be involved in specific cognitive functions.

Role of amygdala in conditioned and unconditioned fear generated in the periaqueductal gray

The amygdala and ventral portion of the periaqueductal gray (vPAG) are crucial for the expression of the contextual freezing behavior. However, it is still unclear whether the amygdala also plays a role in defensive behaviors induced by electrical stimulation of the dorsal periaqueductal gray (dPAG). In the present study, rats were implanted with electrodes into dPAG for determination of the thresholds for freezing and escape responses before and after sham or electrolytic lesions in the amygdala. Animals were then submitted to a context fear conditioning procedure. Amygdala lesions disrupted contextual freezing but did not affect defensive behaviors induced by dPAG electrical stimulation. These results indicate that contextual and unconditioned freezing behaviors are mediated by independent neural circuits.

Expression of conditional fear with and without awareness

Conditional responding during simple Pavlovian conditioning is often characterized as a form of implicit memory. The extent to which this type of associative learning is independent of awareness is an issue of continuing debate. Previous studies have demonstrated conditioning in the absence of awareness. However, their results have been questioned based on methodological concerns with postexperimental questionnaires. In the present study, skin conductance response (SCR) and unconditioned stimulus (UCS) expectancy were measured concurrently as participants were exposed to a differential delay fear conditioning procedure in which one tone (CS+) predicted a loud white noise, whereas a second tone (CS-) was presented alone. UCS predictability was varied on a trial-by-trial basis by presenting conditioned stimuli (CSs) at volumes just above or below the perceptual threshold. Differential UCS expectancy (awareness) was observed only on perceived trials, whereas differential SCR developed on both perceived and unperceived trials. Although perceived stimuli elicited larger SCRs, the magnitude of conditioning, indexed by differential conditioned response expression (conditioned SCR to CS+ minus the SCR to CS-), was not influenced by stimulus perception. These data indicate that conditional fear can be expressed when individuals are unaware of fear-eliciting stimuli and suggest that the degree of conditioning is independent of awareness during differential Pavlovian fear conditioning.

Deficits in trace cued fear conditioning in galanin-treated rats and galanin-overexpressing transgenic mice.

Galanin inhibits the release of several neurotransmitters and produces performance deficits in a variety of spatial and aversive learning and memory tasks. The experiments in this study investigated the role galanin has in emotional learning and memory using a standard delay cued and contextual fear conditioning task. Rats were administered galanin into the lateral ventricles before training, and scored for freezing behavior in the same context and in a novel context with and without an auditory cue (CS) that had been paired previously with an aversive stimulus (US). Galanin-overexpressing transgenic mice were tested in an identical behavioral protocol. The galanin-administered rats and the transgenic mice were not significantly different from their respective controls on this task. A more challenging trace cued and contextual fear conditioning procedure was administered to separate groups of galanin-treated rats and galanin-overexpressing transgenic mice. Subjects were trained with the same CS and US, however, a 2.5-sec delay was inserted between CS offset and US onset. Following the trace conditioning, rats administered galanin and mice overexpressing galanin both exhibited significantly less freezing to the CS in the novel context as compared with their control groups. These results indicate that the observed disruption of cued fear conditioning was specific to the more difficult trace conditioning task. These findings are the first demonstration that galanin impairs performance on an emotional memory task and support the hypothesis that galanin-induced deficits are specific to more difficult cognitive tasks.

Slow late component in conditioned stimulus-evoked potentials from the amygdala after fear conditioning in the rat.

Male Wistar rats were subjected to a differential Pavlovian fear conditioning procedure in which one of two tones (6 or 10 kHz) was followed by an electric shock (CS+) and the other was not (CS-). Before and after fear conditioning, we recorded the evoked potentials elicited by CS+ and CS- from electrodes aimed at the lateral nucleus of the amygdala. Before conditioning, a slow, negative component with peak amplitude around 150 ms was present in the evoked potentials. This component was sensitive to habituation. After fear conditioning, both CS+ and CS- elicited the same late component, albeit with a larger amplitude. This enhancement was temporary: decreasing amplitude was observed in the course of CS test presentations under extinction. Prior research revealed a comparable slow component in the amygdala of the cat under similar experimental conditions. The collective results indicate that the large late component in the amygdala is enhanced by fear conditioning, suggesting that such enhancement reflects the anticipation of a biologically significant event.

Cholinergic modulation of pavlovian fear conditioning: effects of intrahippocampal scopolamine infusion.

Cholinergic neurotransmission has been implicated in the acquisition of a variety of tasks, including Pavlovian fear conditioning. To more precisely define the role of cholinergic modulation in this process, the effect of site-specific cholinergic antagonism was assessed. Male Long-Evans rats were implanted with chronic, bilateral cannulae aimed at the dorsal hippocampus. Infusions of scopolamine hydrobromide (50 microg bilaterally) or phosphate-buffered saline (PBS) were made immediately prior to a signaled Pavlovian fear conditioning procedure. On consecutive days following training, all rats were given independent tests assessing freezing to both the training context and the tone conditional stimulus (CS). Relative to PBS infused controls, rats that received intrahippocampal infusions of scopolamine showed a significant attenuation of contextual freezing but comparable levels of freezing to the tone CS. Neither shock sensitivity nor general activity levels differed between rats infused with scopolamine or PBS. These findings suggest that fear conditioning to context, but not discrete CS, requires intact cholinergic neurotransmission in the hippocampus.

Behavioral and neuropsychological foundations of olfactory fear conditioning

Pavlovian fear conditioning procedures have been a fruitful means of exploring the neural substrates of associative learning. There is now substantial evidence suggesting that many aspects of conditioned fear depend critically upon the integrity of the amygdala and the perirhinal cortex. Recent studies in our laboratory examining the contributions of these areas to olfactory and contextual fear conditioning are reviewed; collectively the results of these studies suggest that the amygdala participates critically in the acquisition and expression of fear conditioned to both an olfactory conditioned stimulus (CS) and to the training context, while the perirhinal cortex contributes to olfactory, but not contextual, fear conditioning. Moreover, it appears that perirhinal cortex may play a prominent role in recognition of the CS following conditioning. These results are discussed in light of the extent to which they replicate and extend previous research examining the contributions of these areas to fear conditioned to auditory and visual CSs.

Arousal-related associative response characteristics of dorsal lateral geniculate nucleus neurons during acoustic Pavlovian fear conditioning.

This research determined whether fear-conditioned, acoustic stimuli induce thalamic arousal reflected in associative responses in dorsal lateral geniculate nucleus (dLGN) neurons. Rabbits received a Pavlovian discriminative fear conditioning procedure in which one tone conditioned stimulus (CS +) was always paired with an aversive unconditioned stimulus (US) and another tone (CS-) was never paired with the US. Responses of single dLGN neurons to random CS+ and CS- presentations were then recorded. Nine of 15 recorded neurons demonstrated significantly greater firing during the CS+ versus the CS-. Their spontaneous activity demonstrated tonic firing during increased neocortical arousal and burst firing during decreased neocortical arousal. The results demonstrate that dLGN neurons show associative responses to fear-conditioned, acoustic stimuli and present a model for investigating the neural circuits by which such stimuli affect sensory processing at the thalamic level.

Arousal-related associative response characteristics of dorsal lateral geniculate nucleus neurons during acoustic Pavlovian fear conditioning.

This research determined whether fear-conditioned, acoustic stimuli induce thalamic arousal reflected in associative responses in dorsal lateral geniculate nucleus (dLGN) neurons. Rabbits received a Pavlovian discriminative fear conditioning procedure in which one tone conditioned stimulus (CS +) was always paired with an aversive unconditioned stimulus (US) and another tone (CS-) was never paired with the US. Responses of single dLGN neurons to random CS+ and CS- presentations were then recorded. Nine of 15 recorded neurons demonstrated significantly greater firing during the CS+ versus the CS-. Their spontaneous activity demonstrated tonic firing during increased neocortical arousal and burst firing during decreased neocortical arousal. The results demonstrate that dLGN neurons show associative responses to fear-conditioned, acoustic stimuli and present a model for investigating the neural circuits by which such stimuli affect sensory processing at the thalamic level.

Emotion, memory, and pain

I n recent years, there has been a surge of interest in research on emotional memory [7-10]. Much of this work has involved the use of classical fear-conditioning procedures, in which a relatively neutral stimulus, such as a tone or flashing light, is associatively paired with a noxious event, typically painful electric shocks delivered to the skin. After one or at most a few pairings of the neutral and the noxious stimulis, the neutral stimulus acquires aversive properties and elicits defensive or protective responses when presented alone. The fear-conditioning literature is relevant to the Focus article by Lenz and colleagues in three respects. First, something like fear conditioning is presumably going on in the patients described, in whom thalamic stimulation elicits affective responses and pain sensations, but only if the patient previously had previous experience of pain with a strong affective component. Somehow, the affective and painful experiences come to be associated, such that the recurrence of the pain sensation causes a revival of the affective reaction as well. We have a fairly good understanding about how all this comes about in the case of fear conditioning, but it is not altogether clear how it might arise in these patients. An interesting difference between fear conditioning and the learning that the patients undergo involves the role of the painful stimulus. In fear conditioning, the painful stimulus is what gives the neutral stimulus meaning. But for the patients, it is the painful stimulus that acquires conditioned meaning, if the proposed hypothesis is correct. Second, studies of fear conditioning have revealed the basic components of a neural system involved in the formation of emotional memory. This system crucially involves circuits into and within the amygdala [7,8,10]. Whether the amygdala and its connections might be involved in the affective learning that the

One-trial context fear conditioning as a function of the interstimulus interval

In two experiments, we examined the effects of a wide range of interstimulus intervals (2.5, 15, 45, 120, 135, and 405 sec) on one-trial context fear conditioning with rats. Here, the interstimulus interval (ISI) denotes the time between placement in a conditioning chamber and the onset of a single footshock. On the conditioning day, we observed that the rats’ behavior at the time of shock onset varied systematically across ISI values. On the subsequent test day, we used context-evoked freezing as a measure of context conditioning and found the well-known inverted U-shaped ISI function. We also found that conditioned freezing for the shortest ISI values was concentrated early in the test session, whereas freezing at longer ISIs was distributed more evenly throughout the test session. The freezing results found here are more consistent with the literature on conditioning with punctate cues than are previously described results from one-trial context fear-conditioning procedures.

Scopolamine Impairs Acquisition and Facilitates Consolidation of Fear Conditioning: Differential Effects for Tone vs Context Conditioning

Cholinergic antagonism impacts selected learning tasks. To understand where scopolamine exerts its action, learning tasks differentially sensitive to hippocampus and amygdala lesions were used. Hippocampal lesions prevent context fear conditioning without effect on tone conditioning. These lesions also produce a time-dependent retrograde deficit in context conditioning. The amygdala is necessary for both tone and context conditioning. To examine the possibility that cholinergic antagonism mimics hippocampal damage or amygdala damage, rats were given scopolamine (1 mg/kg) either before or after fear conditioning. In the fear conditioning procedure, rats received tone-footshock or context-footshock pairings. Evidence of conditioning to the tone and the context was provided by observation of freezing. When given prior to training, scopolamine blocked fear conditioning to the tone in a dose-dependent fashion but had no effect on context conditioning. The impairment of tone conditioning did not occur with methlyscopolamine, indicating the central action of the drug. Rats given scopolamine immediately following fear conditioning, tested later in a drug-free state, froze more to the tone than rats given a control injection. The effect of scopolamine on freezing to the context was not reliable. The present results suggest that scopolamine′s impact on fear conditioning is mediated by some mechanism other than impaired hippocampal or amygdala functioning.

Purkinje cell responses in the anterior cerebellar vermis during Pavlovian fear conditioning in the rabbit

Extracellular single-unit recordings of Purkinje cells in the anterior cerebellar vermis (ACV) of the rabbit found evidence of short-latency (20-30 ms) differential responses to discriminatively-conditioned auditory stimuli during Pavlovian fear conditioning procedures. These differential unit responses appeared to be a function of learning as differential ACV Purkinje cell responses were not observed in naive (untrained) animals. Some of these evoked neurophysiological responses were also correlated with the behavioral conditioned autonomic response. (CAR); a conditioned bradycardiac response. These electrophysiological data, coupled with previous lesion results, suggest that the ACV is part of an important neural circuit for Pavlovian conditioned bradycardia.

A laboratory demonstration of persistent human avoidance

A study is reported with represents, to the authors' knowledge, the first laboratory demonstration of extreme human avoidance responding in extinction following a fear conditioning procedure. Responding in extinction was also found to be enhanced by a serial CS procedure, and support was obtained for the conservation of anxiety hypothesis upon which the serial CS hypothesis is based. Skin conductance, dynomometer, and self-report data supported the contention that fear is the motivating source for avoidance maintenance. The procedure used should provide a valuable paradigm for investigating the underlying principle responsible for maintaining persistent avoidance responding.

Ontogenesis of trace conditioning in young rats: dissociation of associative and memory processes.

Hooded rats from 15 to 30 days of age were trained with a Pavlovian trace fear conditioning procedure in order to study the development of their capacity to learn associations between events separated in time. For both the auditory and visual systems, the associative processes necessary to learn about temporally contiguous events emerged earlier during ontogenesis than the processes necessary to integrate events separated in time. Furthermore, the ability of the rats to integrate events separated by increasingly long intervals continued to improve as they got older. We suggest that the emergence of the capacity to integrate temporally separate events reflects the maturation of memory processes that retain representations of stimulus events over time, and that these processes continue to mature for a considerable period after the basic associative processes have become functional.

Effects of naltrexone on learning and performance of conditional fear-induced freezing and opioid analgesia

In the presence of Pavlovian conditional aversive stimuli rats engage in defensive freezing and exhibit a decreased sensitivity to painful stimulation. Administration of the opioid antagonist naltrexone (7 mg/ml/kg) prior to training in a context fear conditioning procedure using footshock caused an increase in freezing but did not affect suppression of formalin-induced recuperative behaviors relative to controls during a test session 24 hr later. The same dose of naltrexone given during the test session reversed conditional analgesia without altering the level of freezing. These results suggest that full expression of endogenous opioid analgesia during training is not necessary for conditional responding.

Ontogenesis of learning: VI. Learned and unlearned responses to visual stimulation in the infant hooded rat.

A Pavlovian fear conditioning procedure was used to examine the ontogeny of the hooded rats' learned responses to visual stimulation. The data suggests a dissociation in the emergence of the processes required to detect visual events and those necessary for learning an association between visual stimulation and shock. Pups did not condition to a visual conditioned stimulus (CS) paired with a shock unconditioned stimulus (US) until they were 17 days old, even though 15-day-olds were clearly able to detect the visual CS. Although 15-day-olds failed to condition to the visual CS, they conditioned successfully when an auditory CS was paired with shock. Thus, the 15-day-olds' failure to condition to the visual CS was not due to a performance deficit or to a general ineffectiveness of shock as an US. These data suggest that the components of the visual system that mediate detection and those required for associative learning mature sequentially.

The reign of error: Psychiatry, authority, and law : by Lee Colemen. Beacon Press (25 Beacon Street, Boston, Massachusetts, 02108), 1984, 300 pp., hardcover —$18.95.

Since the seminal ‘Little Albert’ study by Watson and Rayner (1920), fear conditioning has become one of the most commonly used paradigms for studying the etiology of anxiety-related disorders. In a fear conditioning procedure, a (neutral) conditioned stimulus (CS) is paired with an aversive unconditioned stimulus (US), resulting in fear-related conditioned responses (CRs) to the CS. Whereas fear conditioning research initially focused on observable elements in the environment (i.e., CSs, USs, and their contingency) and their effects (i.e., CRs), subsequent research indicated that attention should also be given to unobservable mental events (e.g., intrusive memories of aversive outcomes) to more fully account for the symptomatology of anxiety disorders. In this paper, we review the research relating to four major research questions on the relationship between mental imagery and fear conditioning: (1) Can mental imagery substitute for actual stimulus administration? (2) Can mental imagery inflate CRs? (3) Can fear conditioning result in the installment of mental images as CRs (i.e., intrusions)? (4) Can mental imagery-based interventions reduce CRs? For all these research questions, tentative confirmatory evidence has been found and these findings corroborate contemporary conditioning theories. Nonetheless, we point to several open questions and methodological issues that require further research.

Autoanalgesia: acquisition, blockade and relationship to opiate binding : W.T. Chance, A.C. White, G.M. Krynock and J.A. Rosecrans, Europ. J. Pharmacol., 58 (1979) 461–468

Acquisition of autoanalgesia (behaviorally activated antinociception) was assessed across 7 consecutive days by shocking rats 10 sec after the determination of their tail-flick latencies. Thus the effect of conditioned fear on antinociception was being investigated, since each shock preceded the subsequent tail-flick test by 24 h. Autoanalgesia was acquired by the second fear-conditioning trial. Although pretreatment with naltrexone or diazepam had no effect, spinal cord transection at the thoracic level effectively obviated autoanalgesia. Investigations of opiate and opioid binding indicated significantly less binding in the fear-conditioned rats as well as an inverse relationship between binding and antinociception. These changes in binding are suggestive of partial mediation of autoanalgesia by an endogenous opiate peptide that is released by the fear-conditioning procedure.

Autoanalgesia: Acquisition, blockade and relationship to opiate binding

Acquisition of autoanalgesia (behaviorally activated antinociception) was assessed across 7 consecutive days by shocking rats 10 sec after the determination of their tail-flick latencies. Thus the effect of conditioned fear on antinociception was being investigated, since each shock preceded the subsequent tail-flick test by 24 h. Autoanalgesia was acquired by the second fear-conditioning trial. Although pretreatment with naltrexone or diazepam had no effect, spinal cord transection at the thoracic level effectively obviated autoanalgesia. Investigations of opiate and opioid binding indicated significantly less binding in the fear-conditioned rats as well as an inverse relationship between binding and antinociception. These changes in binding are suggestive of partial mediation of autoanalgesia by an endogenous opiate peptide that is released by the fear-conditioning procedure.