Effects of contingency violations on the extinction of a conditioned fear inhibitor and a conditioned fear excitor

Rats were used in a conditioned-suppression paradigm to assess the effects of contingency variations on responding to a conditioned inhibitor (CS-) and a conditioned excitor (CS+). In Experiment 1, various unconditioned stimulus (US) frequencies were equated across the presence and absence of a CS- in the context of either background cues (continuous-trial procedure) or an explicit neutral event (discrete-trial procedure). With both procedures, a CS-alone treatment enhanced inhibition, whereas treatments involving 50% or 100% reinforcement for the CS- eliminated inhibition without conditioning excitation to that CS. The latter outcome also occurred in Experiment 2, with discrete-trial training equating considerably reduced US frequencies for the presence and absence of the CS-. In further evidence that inhibition was eliminated without conditioning excitation to the CS-, Experiment 3 showed that a novel CS did not acquire excitation when 25%, 50%, or 100% reinforcement was equated across the presence and absence of that CS in the context of a discrete-trial event. Using the procedures of Experiment 1, Experiment 4 showed that a CS+ was extinguished by a CS-alone treatment but was substantially maintained by treatments involving 50% or 100% uncorrelated reinforcement. These effects for a CS+ and a CS- implicate CS-US contiguity, rather than contingency, as the factor determining the extinction of a CS.     

Reciprocal inhibition of a laboratory conditioned fear

Electric shock was used to establish a conditioned fear in 48 college Ss, who then received either deep muscle relaxation training, cognitive relaxation training or anxiety relief conditioning. Relaxation was conditioned to a slide of the word NOW and the aversive stimulus was a pure red slide. The compound stimulus NOW on a red background was presented to generate counterconditioning, followed by a re-presentation of pure red as a measure of transfer. GSR and Blood Volume Pulse were recorded for evaluation of the fear reduction generated by each training procedure. All group differences were non-significant. The two relaxation procedures were equally effective, while the Anxiety Relief group manifested consistently smaller response-reduction. Implications and the usefulness of the paradigm were discussed.     

Compounds of conditioned fear stimuli

A differential conditioning procedure was used with rats to establish different levels of suppression to two different stimuli. One stimulus was paired with shock every time it was presented, while the other stimulus cued shock on only 25% of its presentations. When these two component stimuli were subsequently tested as a simultaneous compound, two different types of component-compound relationships were observed. Some subjects showed greater suppression to the compound than they did to either component stimulus, while other subjects showed an intermediate level of suppression to the compound. The difference in type of component-compound relationship appeared to be related to the degree of stability (over trials) of each subject's reaction to the probabilistic (25%) cue.When a similar discrimination was established based on the intensity of the shock, rather than its probability of occurrence, all subjects showed more suppression to the compound than to its individual component stimuli. These results are discussed in terms of their implications for choice of model for characterizing the interaction of conditioned states.     

Forgetting of conditioned fear inhibition

Three experiments assessed retention of conditioned fear and of fear inhibition over long-term retention intervals (35 days in two experiments, 25 days in the third). Although excitatory conditioning was retained well, conditioned inhibition was not. This effect occurred when inhibition was assessed by compounding stimuli on an appetitive baseline and when it was assessed by testing cues on a baseline of unsignalled avoidance responding. Inhibition was also forgotten when the initial conditioning treatment was designed to reduce the role of possible proactive interference processes.     

Timing behavior and conditioned fear

Rats were trained on a two-response timing procedure which required that response B follow response A by at least a minimum specified interval in order to be reinforced with food. Repeated presentation (5 min on, 5 min off) of an auditory warning stimulus terminated by a brief electric shock to the feet (conditioned fear) produced a marked suppression in the frequency of A-to-B response sequences during the warning stimulus. The distribution of A-to-B interresponse times (timing behavior), however, did not change during the warning stimulus.     

Administration of corticosterone after memory reactivation disrupts subsequent retrieval of a contextual conditioned fear memory: dependence upon training intensity

Reactivation of stabilized memories returns them to a labile state and causes them to undergo extinction or reconsolidation processes. Although it is well established that administration of glucocorticoids after training enhance consolidation of contextual fear memories, but their effects on post-retrieval processes are not known. In this study, we first asked whether administration of corticosterone after memory reactivation would modulate subsequent expression of memory in rats. Additionally, we examined whether this modulatory action would depend upon the strength of the memory. We also tested the effect of propranolol after memory reactivation. Adult male Wistar rats were trained in a fear conditioning system using moderate (0.4 mA) or high shock (1.5 mA) intensities. For reactivation, rats were returned to the chamber for 90 s 24h later. Immediately after reactivation, rats were injected with corticosterone (1, 3 or 10mg/kg) or vehicle. One, 7 and 14 days after memory reactivation, rats were returned to the context for 5 min, and freezing behavior was scored. The findings indicated that corticosterone when injected after memory reactivation had no significant effect on recall of a moderate memory, but it impaired recall of a strong memory at a dose of 3mg/kg. Propranolol (5mg/kg) given after the reactivation treatment produced a modest impairment that persisted over three test sessions. Further, the results showed that corticosterone, but not propranolol deficit was reversed by a reminder shock. These findings provide evidence that administration of glucocorticoids following memory reactivation reduces subsequent retrieval of strong, but not moderate, contextual conditioned fear memory likely via acceleration of memory extinction. On the other hand, propranolol-induced amnesia may result from blockade of reconsolidation process. Further studies are needed to determine the underlying mechanisms.     

Scaling task complexity along a single dimension

Time-on-target on pursuit rotor tracking tasks has been shown to decrease for triangle, square, and circle templates, respectively. Since these templates are unrelated geometric shapes they cannot be scaled along a single continuous dimension. For the present study, templates were constructed to be continuously scaled by eccentricity. It was demonstrated that time-on-target on pursuit rotor tracking was a decreasing function of template eccentricity. Also, it was concluded that elliptical templates were preferred over the usual geometric templates because time-on-target relationships among templates can be examined.     

Generalization of conditioned suppression after differential training

In a modified conditioned suppression procedure, clicks at one frequency (danger signal) preceded shocks, while no shocks followed clicks at a different frequency (safe signal). During generalization tests, the maximal response rate was frequently shifted from the safe signal in the direction away from the danger signal, and the minimal response rate was frequently shifted in the opposite direction, away from the safe signal. There was considerable variability in the results from one animal to another. The generalization tests also suggested different generalization functions according to whether the danger signal was a lower or a higher frequency than the safe signal. The results also showed the development of systematic differences in response rate during and after the safe and danger signals, notably a relatively high rate at the beginning of the safe signal and after the danger signal.     

Generalisation of fear and avoidance along a semantic continuum

Directly conditioned fear and avoidance readily generalises to dissimilar but conceptually related stimuli. Here, for the first time, we examined the conceptual/semantic generalisation of both fear and avoidance using real words (synonyms). Participants were first exposed to a differential fear conditioning procedure in which one word (e.g., “broth”; CS+) was followed with brief electric shock [unconditioned stimulus (US)] and another was not (e.g., “assist”; CS–). Next, an instrumental conditioning phase taught avoidance in the presence the CS+ but not the CS–. During generalisation testing, synonyms of the CS+ (e.g., “soup”; GCS+) and CS– (e.g., “help”; GCS–) were presented in the absence of shock. Conditioned fear and avoidance, measured via skin conductance responses, behavioural avoidance and US expectancy ratings, generalised to the semantically related, but not to the semantically unrelated, synonyms. Findings have implications for how natural language categories and concepts mediate the expansion of fear and avoidance repertoires in clinical contexts.     

Contextual control of the extinction of conditioned fear

Rats received 15 pairings of a CS and shock in one context, and then a series of CS-alone trials in a second context. Even though this extinction procedure produced a complete loss of conditioned suppression, when the animals were returned to the site of original conditioning, suppression was renewed to a level comparable to that of animals that had not undergone extinction. Controls indicated that the renewed suppression was not due solely to pseudoconditioning, suggesting that the CS-US association had survived extinction. Renewed suppression was also demonstrated in a third context that was never associated with shock. Loss of suppression did not necessarily depend upon inhibitory conditioning of the extinction context. The data suggest that extinction of conditioned fear is specific to the context in which it occurs. They also suggest the possibility that animals might discriminate episodes in which a CS is reinforced and nonreinforced independently of the excitatory or inhibitory status of cues, like contextual stimuli, that are coincidentally present during those episodes.     

Stimulus generalization along a dimension based on a verbal concept

The present study examined generalization along a dimension based on a verbal concept of occupational status. The status dimension was scaled by students who placed occupation names into five status categories, Category 1 representing highest status and Category 5 the lowest status. In two experiments, key presses by students were occasionally reinforced when a slide showing an occupation name from Status Category 3 was present. For half the subjects, key presses were not reinforced during a name from Category 1; for the other half, presses were not reinforced during a name from Category 5. Occupation names from all status categories were later singly presented. In this generalization test, subjects typically divided the dimension into two parts, responding alike to all names within each part. The results suggest that generalization along a dimension in humans is mediated by a subject's verbal classifications of stimuli.