Factors governing one-trial contextual conditioning

When a rat was placed in a chamber and shortly thereafter received a single footshock, it showed conditional freezing upon re-exposure to that chamber but not a different one (Experiment 1). Experiments 2–4 showed that the probability of this freezing decreased linearly with decreases in the delay between placement in the chamber and shock delivery. With very short delays (e.g., less than 27 sec), there was no freezing. Experiments 2 and 3 demonstrated that a 2-mm pre-exposure to the chamber, 24 h prior to shock delivery, reduced the minimum delay necessary to successfully condition freezing. Experiment 4 demonstrated that shorter delays were successful in conditioning freezing if a salient tone was a component of the contextual stimulus. The changes in freezing caused by delay interval and preexposure did not simply reflect the total time in the context, suggesting that there may be two requirements that place temporal restrictions on the conditioning of the freezing response. One is satisfied by sufficient exposure, whether or not that exposure is contiguous with shock. The second requirement is for a small amount of context exposure that is contiguous with shock.     

Immediate shock,passive avoidance,and potentiated startle: Implications for the unconditioned response to shock

Three experiments were performed to study the immediate-shock freezing deficit, a deficit in freezing in rats that results when electric shock is delivered immediately upon exposure to a novel context. This deficit was accompanied by failures to detect evidence of passive avoidance (Experiment 1) or potentiation of the auditory startle response (Experiment 2). The deficit in freezing was attenuated by preexposure to the shocked context (Experiment 3). The results support the view that fear-related behaviors are activated by signals for shock rather than by shock itself. They also suggest that the immediate-shock freezing deficit is due to a failure to process the to-be-conditioned contextual cues (Fanselow, 1986a, 1990).     

Differential second-order aversive conditioning using contextual stimuli

Two experiments were conducted to determine if contextual stimuli used as S₂ in a higher-order differential conditioning procedure would control the performance of rats. Discrete stimuli were first paired with footshock in a separate training context. During second-order training, a shock-associated discrete stimulus was presented in one of two discriminable observation chambers. Over 4 days of training, subjects engaged in more freezing in the context associated with an excitatory discrete S₁, relative to a context in which no discrete stimulus, or a stimulus that had been explicitly unpaired with shock delivery, was presented. After acquisition of the second-order discrimination, animals were returned to the original training context where they received a “signaled inflation” treatment designed to change the current value of S₁, and the US. This postconditioning manipulation did not selectively affect performance of defensive freezing or conditional analgesia in S₂.     

Overshadowing of situational cues with variable but not fixed intertrial intervals

In two experiments, we examined the conditions under which signaling an unconditioned stimulus (US) with a nominal conditioned stimulus (CS) interferes with the conditioning of situational cues in defensive freezing in the rat. Subjects received footshock USs that were (1) either signaled or unsignaled and (2) either varied or fixed in their temporal location within the conditioning session. Experiment 1, with only one trial per session, yielded no evidence that signaling affected pretrial freezing using either a fixed or variable interval between placement in the context and shock onset. In a test in which no CSs or footshocks were presented, groups that previously had received footshock at a fixed temporal location showed greatest freezing at around that same time. For groups that had received footshocks at various times, freezing declined across the test session. Experiment 2 showed overshadowing of pretrial freezing after more extensive conditioning with many trials per session, but only if the intershock intervals were variable rather than fixed.     

Avoidance,classical, and pseudoconditioning as a function of species-specific defense reactions in high- and low-avoider rat strains

Rats of the Australian High Avoider (AHA) and Australian Low Avoider (ALA) strains and their reciprocal crosses were exposed to 50 trials of one of three shuttlebox procedures. The avoidance group received pairings of a tone and shock. If the animals shuttled during the tone, they avoided the shock. If they waited until the shock came on, they could then escape it. The classical group received pairings of the tone and a brief inescapable shock. If they shuttled during the tone, the tone ceased and they immediately received the shock. If they did not shuttle, they received the brief shock at the termination of the tone. The pseudoconditioning group received the tone and the shock explicitly unpaired. If they shuttled during either the tone or the shock, the stimulus was terminated. There was no acquisition of anticipatory responding under the pseudoconditioning procedure. All groups evidenced an increase in anticipatory responding over trials under the classical procedure. The AHAs acquired the response faster and reached a higher asymptote than did the ALAs. Performance of the two reciprocal crosses fell in between. A similar pattern was observed under the avoidance procedure, albeit at slightly higher response levels. Subsequent studies established that the AHAs acquired a one-way avoidance response quickly, but were impaired on a passive avoidance task, whereas the reverse was the case for the ALAs. The reciprocal crosses were proficient at both tasks. These results suggest that shuttlebox avoidance is largely accounted for by classical conditioning of the predominant defensive response. When that response is compatible with performance on the task, acquisition is rapid (AHAs), and when it is not, acquisition is slow (ALAs).     

Inescapable shock interferes with the acquisition of an appetitive operant

This article reports the reinforcer generality of the interference effect resulting from exposure to inescapable shock. In Experiment 1, rats that received inescapable shock showed weak interference with the acquisition of an appetitive operant compared to animals exposed either to escapable or no shock. In Experiment 2, the response-reinforcer contingency was degraded by introducing a 1-sec delay of reinforcement on the appetitive task. Inescapable shock produced much stronger interference with the acquisition of the operant response than in Experiment 1. The results demonstrate reinforcer generality of the debilitating effects produced by inescapable shock.     

Conditioned inhibition of analgesia

Stimuli that predict the occurrence of aversive events come to elicit conditioned analgesia. Experiments 1A and 1B examined the possibility that conditioning can inhibit analgesia when stimuli are paired in a backward fashion with a shock US (Pavlovian CS- s). Analgesia conditioned in response to shock context exposure was reversed during the CS- (light) presentation after four sessions. The ability of the CS- to function as a conditioned inhibitor of analgesia was then evaluated in both summation (Experiment 1A) and retardation-of-acquisition testing (Experiments 1A and 1B). The results support the conclusion that a stimulus presented after shock in a backward fashion comes to be a conditioned inhibitor of analgesia. Experiments 2A and 2B examined the assumption that the results obtained with our pain sensitivity measure (tailflicking in response to radiant heat) reflect changes in responsiveness to painful input, rather than a general motor inhibition or general insensitivity to sensory input. In Experiment 2A, tailflick responding to painful and nonpainful input was compared in animals receiving either morphine or saline. In Experiment 2B, tailflick responding to painful and nonpainful input to the tail was compared in both the shock and a neutral context. In both experiments, only the painful input yielded changes in responsivity. The results support the conclusion that the alterations in pain sensitivity produced by the CS- for shock represents a conditioned inhibition specific to pain.     

Contribution of conditioned opioid analgesia to the shock-induced associative US-preexposure deficit

The modulatory effect of conditioned opioid analgesia on learning in the US-preexposure paradigm was examined in three experiments using water-deprived rats. In Experiment 1, it was found that tailflick latencies increased immediately after the rats were exposed to a context in which footshock had previously been administered. Prolonged nonreinforced exposure to the context attenuated this analgesia. Experiment 2 tested the possibility that the effectiveness of CS-US pairings in an excitatory context might be reduced by a conditioned analgesic response that lessens the perceived intensity of the US. Administration of the opiate antagonist naloxone prior to CS-US pairings in the excitatory context reduced the US-preexposure deficit—that is, the retarded response to the CS—but did not eliminate it, suggesting that part of the observed deficit resulted from conditioned activation of the endogenous opioid system. In Experiment 3, it was found that exposure to the excitatory context immediately prior to a CS-US pairing in an associatively neutral context resulted in a conditioned response deficit, indicating that the analgesia elicited by the excitatory context was sufficient to reduce US effectiveness. In combination with other recent reports, these results suggest that the associative deficit resulting from preexposure to a shock US may, in certain instances, represent the sum of several different associative processes.     

Conditioned fear assessed by freezing and by the suppression of three different baselines

Rats received either forward or backward pairings of an auditory CS and shock. They were then tested for conditioned suppression to the CS while barpressing for food, licking a sucrose solution, or being spontaneously active. Behavior was simultaneously observed using a time-sampling method. In each case, forward-conditioned animals exhibited more freezing than controls, and freezing was reliably correlated with suppression of the baseline. These results suggest that the different loss-of-baseline measures of aversive conditioning reflect the amount of defensive behavior evoked by the CS. They also suggest the utility of freezing as an index of conditioning. Freezing assayed by the time-sampling method was comparable to the more conventional indices of conditioning in sensitivity to the effects of conditioning.     

Pavlovian conditioning to contexts that precede the place where shock occurs as measured by freezing and activity suppression in mice

Three experiments were conducted to demonstrate that the place where an organism has been, before the organism is moved to a place with aversive consequences, can also become aversive through classical conditioning. In Experiment 1, two groups of 8 mice were exposed to three different contexts in succession, with a single shock occurring in the third context. The distal context was a putative 3-min conditioned stimulus (CS) for freezing; the second context was a delay manipulation; and the unconditioned stimulus (US) occurred in the proximal context. The group delayed for 15 sec showed significantly more freezing to the distal CS context than did the group delayed for 3 h. In a second experiment, conditioning to the distal context was demonstrated with a discrimination procedure for 8 more mice by using two different distal contexts as CS+ and CS? for the proximal context with shock. On CS+ days, 3 min of exposure to the distal context was followed within 5 sec by placement in the proximal box where shock occurred, whereas on CS? days, exposure to a second distal context was followed immediately by return to the home cage. Very strong differences in freezing between the CS+ and CS? distal contexts were found in all 8 mice after 14 days of conditioning. In a third experiment, the discriminative procedure was repeated for 9 more mice, with two changes. More objective stabilometertype activity measures were substituted for observed freezing, and, in addition to the CS+ and CS? distal context trials, each mouse was also exposed to a third discriminative distal context, which was followed by 15 min in a delay chamber followed by shock in the proximal context. This discrimination procedure with the activity suppression measure again resulted in significant differences between the contexts. The CS+ context and the context followed by a 15-min delay did not differ, but both of them differed from the CS? context.     

The postshock activity burst

When a rat receives an electric shock delivered to the floor of an enclosure, it reacts with frenzied activity. On shock termination, the activity persists for a brief period of time and then gradually gives way to a period of freezing. Subsequent grid shocks temporarily disrupt freezing, with the length of disruption determined by shock intensity (Experiment 1). The duration of this activity burst depends predominantly on the test shock intensity but not on the training shock intensity. The reverse is true for the probability of freezing, which is positively related to training shock intensity (Experiment 2). Based on this finding, it is argued that the activity burst is a UR, while freezing is a CR. Further support that freezing is a CR is provided by Experiment 3, which demonstrates that a delay, during which the rat is out of the shock-associated context, between the training and testing periods does not disrupt freezing. A topographical analysis of the behaviors making up the activity burst is provided by Experiment 4. The postshock activity burst was composed predominantly of head movement, turning, and rearing.     

One-trial context fear conditioning with immediate shock: The roles of transport and contextual cues

In three experiments, using a total of 120 albino rats, we assessed whether transportation cues might evoke some of the freezing (i.e., defensive immobility) that we see in a context on a day following a footshock given immediately after placement in that context. The results suggested that immediate shock could directly condition strong fear to both simulated and actual transport cues. Although conditioning to transport cues explains some of the freezing that is seen on the test day, it does not explain all of it. We also found evidence that some of the freezing is due to conditioning to permanent features of the context in which the immediate shock is given. The results support a role for transport cues in theories of context conditioning and argue against shock-processing accounts of the conditioning deficit that results from immediate shock.     

Conditioned and unconditioned stress-induced analgesia: Stimulus preexposure and stimulus change

Aversive events such as electric shock lead to analgesia. There have been differing views with regard to whether the aversive event itself can lead to analgesia as a direct unconditioned reaction, or whether the analgesia is instead a reaction to fear conditioned to cues present during shock or to other associative processes initiated by the aversive event. Maier (1989, 1990) has argued that aversive events such as shock lead to both types of analgesia, with the type observed depending on test conditions. Unconditioned analgesia was argued to be present soon after shock, with conditioned analgesia replacing the unconditioned form if the subject is allowed to remain in the shock environment. Consistent with this argument, the experiments reported here show that (1) preexposure to the environment in which shock later occurs has no effect on the analgesia soon after shock, but eliminates the later analgesia; (2) the initial postshock analgesia is unaffected by removing the subject from the shock environment to a different environment, but the later reaction is prevented by such a change; (3) returning the subject to the shock environment after confinement in a nonshock environment rearouses an analgesic reaction; and (4) this rearousal does not occur if the subject has first been confined to the shock environment without shock.     

Species-specific defense reactions in escape-from-fear situations

Five experiments are reported in which rats were given CS-shock pairings and then permitted, in the absence of shock, to perform a hurdle-jump response that led to CS offset. In Experiment 1–4, the animals failed to learn the required response. Although several procedural variations were employed, the experimental groups continued to perform no better than backward-conditioned controls. In Experiment 5, some animals were punished for remaining immobile during a single trial at the start of hurdle-jump training. These animals performed significantly better than nonpunished controls and better than any group in Experiments 1–4. The results support the idea that the major role of the CS in escape-from-fear situations is to activate an innate motivational system which chooses between the species-specific defense reactions of freezing and fleeing.     

Slow extinction of conditioned responding following exposure to two bouts of massed shock

Extinction of rats’ conditioned defensive freezing responses in a context associated with two bouts of massed shock (3 sec) separated by a long unreinforced interbout interval was slower than that in a context associated with distributed shock (60 sec). Resistance to extinction following two bouts of massed shock depended on the rats’ remaining undisturbed in the conditioning context during the long unreinforced interbout interval. Slow extinction of freezing was attributed to either the summation of temporal conditioning at the early and late session times or the formation of an association between the early and late bouts of shock. Importantly, the effects of the two bouts of massed shock could not be explained by what is known about the reinforcing effectiveness of massed shock.     

Activity during prior shock determines subsequent shock-elicited fighting in the rat

Two experiments investigated the relationship between activity during shock and the magnitude of subsequent impairment of shock-elicited fighting in the rat. Different levels of intra-shock activity were engendered in two ways. In Experiment 1, differing temporal forms of inescapable shock were employed to produce markedly different levels of activity. In Experiment 2, a passive-escape procedure was used to explicitly reinforce nonmovement during shock relative to a yoked, inescapable shock control. Results indicated that relative to the performance of subjects not previously shocked, fighting impairment was produced only by those prior treatments that promoted reduced intrashock activity. Since one of the prior shock treatments involved inescapable shock but the other did not, these findings may be viewed as strong support for the notion that behavior during shock, rather than uncontrollability, is the critical determinant of the observed impairment effects. There was some suggestion in both studies that shock treatments that resulted in sustained or increased intrashock activity tended to produce augmentation of fighting. Both inhibitory and facilitative effects of prior shock exposure are discussed in terms of an interacting response theory of shock treatment effects.     

Effects of handling and context preexposure on the immediate shock deficit

Theimmediate shock deficit refers to the failure of a shock to become associated with contextual stimuli when the shock is presented simultaneously with the rat’s placement in a context. The basic procedure consists of a presentation of the shock as soon as the animal is placed in an observation chamber. Handling of the animal, which immediately precedes the shock, and the novelty of the chamber in which the immediate shock is delivered are potential variables that might be responsible for this associative deficit. In Experiment 1, handling reduced context conditioning but was not responsible for the immediate shock deficit. Experiment 2 revealed that the novelty of the chamber was not a significant factor. These results discount the possibility that handling and the novelty of the chamber are responsible for the deficit produced by the immediate shock. It is suggested that immediate shock could be employed as a control procedure for the study of context conditioning.     

Differences in polysubstance use among youth in the child welfare system: Toward a better understanding of the highest-risk teens

The current study extended limited prior work on polysubstance use among youth in the child welfare system (CWS) by addressing their potentially greater risk of engaging in polysubstance use, the causes of interpersonal variation in use, and changes in use over time, particularly at later points of involvement in the CWS. Using longitudinal data from the National Survey of Child and Adolescent Well-Being (n = 1,178), a series of time-invariant and time-varying demographic and contextual factors were explored to assess their role both overall and at unique points of involvement in the CWS. A series of unconditional and conditional curve-of-factor models were estimated and results indicated that time-invariant characteristics of ethnicity and gender were not related to polysubstance use. Time-variant characteristics of age and placement were associated with polysubstance use and highlighted the dynamic nature of age as a risk factor. Out-of-home placement was protective against later substance use for youth who had been removed from contexts with their original caretaker where there were higher levels of reported violence. Our results suggest that in the child welfare population, the modeling of multiple substances rather than a single substance in isolation is more informative because it yields information on the confluence of behaviors that tend to occur and evolve together.     

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.     

Interference with avoidance behavior as a function of qualitative properties of inescapable shocks

Temporal form (continuous vs. pulsating) and shock source (alternating current vs. direct current) were factorially combined to produce four shock treatments. The effects of inescapable presentations of these stimuli on subsequent avoidance response acquisition were measured in dogs (Experiment 1) and in rats (Experiment 2) and revealed an interaction of shock variables. Initially, all groups that received ac shock showed impaired performance for the pulsating and continuous shock conditions; groups that received dc continuous shock were also impaired, while those that received dc pulsating shock were not. While this pattern of interference persisted for dogs, it was transient in rats, with only the ac continuous-shock group continuing to be impaired. Mean avoidance performance were positively related to mean activity levels during inescapable shocks for the dc shock groups but not for the ac shock groups.