Differential acquisition,extinction, and reinstatement of conditioned suppression in mice

In animals, the reappearance of conditioned fear responses after extinction has been primarily investigated using single-cue conditioning paradigms. However, a differential paradigm can overcome several of the disadvantages associated with a single-cue procedure. In the present study, the reinstatement phenomenon was assessed in mice using a differential conditioned suppression paradigm. In a first phase, one conditioned stimulus (CS + ) was consistently paired with an unconditioned stimulus (US; footshock) while another CS (CS–) was not, resulting in selective suppression of previously trained instrumental behaviour during the CS + . After the extinction phase, half of the animals (reinstatement group) were presented with unsignalled USs, while the other half were not (control group). A differential return of conditioned responding was observed in the reinstatement group, but not in the control group. The implications of these findings for future conditioning research are discussed.     

Differential acquisition, extinction, and reinstatement of conditioned suppression in mice

In animals, the reappearance of conditioned fear responses after extinction has been primarily investigated using single-cue conditioning paradigms. However, a differential paradigm can overcome several of the disadvantages associated with a single-cue procedure. In the present study, the reinstatement phenomenon was assessed in mice using a differential conditioned suppression paradigm. In a first phase, one conditioned stimulus (CS + ) was consistently paired with an unconditioned stimulus (US; footshock) while another CS (CS-) was not, resulting in selective suppression of previously trained instrumental behaviour during the CS + . After the extinction phase, half of the animals (reinstatement group) were presented with unsignalled USs, while the other half were not (control group). A differential return of conditioned responding was observed in the reinstatement group, but not in the control group. The implications of these findings for future conditioning research are discussed.     

A developmental dissociation in reinstatement of an extinguished fear response in rats

Recently, studies from our laboratory have shown that 16-day-old rats, in contrast to 23-day-old rats, fail to show either ABA renewal or recovery of an extinguished fear response following a pre-test injection of FG7142 [Kim, J. H. & Richardson, R. (2007). A developmental dissociation of context and GABA effects on extinguished fear in rats. Behavioral Neuroscience; Yap & Richardson, unpublished data]. The present study, using freezing as a measure of learned fear, extends these findings by examining whether there is a developmental difference in susceptibility to reinstatement following extinction. 16- and 23-day-old rats were trained to fear a white-noise conditioned stimulus (CS) by pairing it with a shock unconditioned stimulus (US). This fear was subsequently extinguished by non-reinforced presentations of the CS. Some rats received a post-extinction Reminder which consisted of a single presentation of a reduced-intensity US. Experiments 1 and 2 demonstrated that this Reminder was effective in reinstating extinguished fear in 23-day-olds, and that this reinstatement effect was context-specific in rats this age. In contrast, 16-day-old rats failed to show the reinstatement effect in either experiment. The failure to observe a post-extinction reinstatement effect in the 16-day-olds was not due to a general ineffectiveness of the Reminder treatment at this age because it did alleviate spontaneous forgetting in rats this age (Experiment 3). Taken together, the results suggest that fundamentally different processes may mediate extinction early in development compared to later in development.     

Observing response acquisition: preference for unpredictable appetitive rewards obtained under conditions predicted by DMOD

In five E-maze experiments, rats were given a choice between receiving reward and nonreward in a situation where stimuli were correlated with reward outcome (predictable situation) versus one where the stimuli were uncorrelated with reward outcome (unpredictable situation). Preference for the unpredictable situation occurred under the following conditions: (a) small (one 37-mg pellet), immediate rewards; (b) small, delayed (15 s) rewards, if the cues correlated with reward outcome were absent during the delay interval; (c) large (15 pellets), immediate rewards if a difficult discrimination was required; and (d) if the stimulus predicting nonreward was present at the choice point. Preference for the predictable situation was strongest if reinforcement was delayed and large or the stimulus predicting reward was present at the choice point. A weaker preference for the predictable situation occurred if reinforcement was immediate and large and a simple discrimination was required or if reinforcement was large and delayed and the cues that correlated with reward outcome were absent during the delay interval. The results support the predictions of DMOD (Daly modification of the Rescorla-Wagner model), a mathematical model of appetitive learning (Daly & Daly, 1982).     

Lesions of rat infralimbic cortex enhance recovery and reinstatement of an appetitive Pavlovian response

The prefrontal cortex (PFC) has a well-established role in the inhibition of inappropriate responding, and evidence suggests that the infralimbic (IL) region of the rat medial PFC (MPFC) may be involved in some aspects of extinction of conditioned fear. MPFC lesions including, but not those sparing the IL cortex increase spontaneous recovery of extinguished conditioned fear when tested 24 h after an initial extinction session. The current experiment extended these findings by use of appetitive rather than aversive conditioning. Ten IL-lesioned and 11 sham-operated rats were trained on a Pavlovian task in which a conditioned stimulus (CS) was followed by food pellets (the unconditioned stimulus or US). IL lesions had no effect on extinction of the conditioned response (CR, magazine entries) during the first extinction session. However, the level of spontaneous recovery between the first extinction session and a second, 24 h later, was increased in IL-lesioned rats relative to sham animals. In contrast, evidence of savings measured between the extinction sessions did not differ between groups. Furthermore, reinstatement of the CR following unsignaled delivery of the US was also increased in IL-lesioned rats.     

Random fluctuations of response rate

A simple model for fluctuating interresponse times is developed and studied. It involves a mechanism that generates regularly spaced excitations, each of which can trigger off a response after a random delay. The excitations are not observable, but their periodicity is reflected in a regular patterning of responses. The probability distribution of the time between responses is derived and its properties are analyzed. Several limiting cases are also examined.This paper was completed while the writer was a visiting summer scientist at the Lincoln Laboratory, Lexington, Mass.     

The structure of response rate

Interresponse time distributions of the terminal rate under fixed-interval and fixed-ratio schedules were examined, using data from three rats in each case. By means of a sequential analysis, the overall interresponse time distributions were separated into orderly components. Consideration of the component distributions suggested that multiple determinants of rate act in succession, not simultaneously, and that probability of reinforcement has an important effect on the probability of occurrence of interresponse times.     

Time allocation and response rate

Food reinforcement for key pecking by three pigeons was arranged by a variable-interval schedule and a device that assigned each reinforcement to one of 10 component response rates corresponding to 10 classes of equally reinforced interresponse times ranging from 1.0 to 6.0 sec in 0.5-sec classes. The overall number of reinforcements per hour was varied from one to more than 60. Overall response rate was a monotonically increasing, negatively accelerated function of the overall number of reinforcements per hour. This function was decomposed into two time-allocation functions: (1) the time allocated to all of the reinforced component response rates as a function of the total reinforcement rate, and, (2) the time allocated to a particular reinforced component response rate as a function of the reinforcement rate for that component. Asymptotic response rate was predicted by combining the asymptotes of the two separate time-allocation functions: virtually all of the time was spent responding, and the percentage of the time spent responding that was allocated to a particular reinforced component response rate roughly equalled the relative reinforcements per hour for that component.     

Differential response patterns to disgust-related pictures

ABSTRACT

In the present study, attentional bias was investigated as a potential predisposing mechanism for the contamination-related subtype of obsessive-compulsive disorder (C-OC disorder). Fifty healthy participants with varying degrees of subclinical C-OC symptoms performed a visual search task to measure differential attentional biases elicited by neutral, disgust-, and fear-specific pictorial material. Participants had to find a target picture within five neutral distractor pictures randomly presented on different locations in an array. The task was to decide whether the array contained an unpleasant target picture or not. In randomly-selected emotional trials, participants were further asked about the content of the picture and the confidence of their answer. The results show that the reaction times significantly differed between the emotional and neutral pictures. Participants were significantly more confident in answering questions referring to fear compared to disgust pictures. This effect was marginally amplified in participants with higher C-OC symptoms. We discuss the results within the framework of the cost and benefit hypothesis, which postulates that disgust evolutionarily elicits stronger uncertainty compared to fear, owing to the ambiguous nature of the stimuli. Increased uncertainty might be an important but underestimated factor for pathological disgust experience, such as in obsessive-compulsive disorder.     

Differential sample response schedules in the acquisition of conditional discriminations by pigeons

Pigeons were trained on four matching-to-sample tasks with various schedule requirements in effect on the sample key. Differential sample-schedule requirements (a differential-reinforcement-of-low-rates of 3 sec in the presence of one sample and a fixed-ratio 16 in the presence of the other) produced rapid rates of acquisition that did not differ across tasks. Nondifferential sample-schedule requirements (fixed-ratio 1, fixed-ratio 16 or a differential-reinforcement-of-low-rates of 3 sec in the presence of both samples) produced slower rates of acquisition, which depended on the difficulty of the discriminations between samples and between comparisons. Patterns of stimulus and position preferences were influenced both by the comparison stimuli in each task and by the sample-schedule requirements. Detailed analyses of acquisition revealed frequent instances of complete differential sample control of comparison responding at intermediate levels of overall “accuracy”.     

Effects of effort on response rate

Pigeons were trained to peck for food reinforcement, and the minimum effective force for a peck was varied. In a single-key situation, the response rate was unaffected up to a certain force value, beyond which increments caused a proportional decrement in response rate. Transient enhancement and suppression effects were observed following a change in the force requirement. A change from a high force requirement to a low force requirement resulted in a temporary enhancement of response rate above the stable performance level, whereas a change from a low to a high requirement produced a temporary suppression of the rate below the stable rate. In a two-key situation, the response rate on a key with a given force remained unaffected by the force requirement on the other key. When the rate of response was plotted against the required force, the resultant function was remarkably similar to that obtained from the single-key experiment. When the rates of reinforcement on the two keys differed, the decrement in response rate produced by an increase in the force requirement was proportionally greater on the key with the lower rate of reinforcement.     

Adolescent risk-taking is predicted by individual differences in cognitive control over emotional,but not non-emotional,response conflict

While much research on adolescent risk behaviour has focused on the development of prefrontal self-regulatory mechanisms, prior studies have elicited mixed evidence of a relationship between individual differences in the capacity for self-regulation and individual differences in risk taking. To explain these inconsistent findings, it has been suggested that the capacity for self-regulation may be, for most adolescents, adequately mature to produce adaptive behaviour in non-affective, “cold” circumstances, but that adolescents have a more difficult time exerting control in affective, “hot” contexts. To further explore this claim, the present study examined individual differences in self-control in the face of affective and non-affective response conflict, and examined whether differences in the functioning of cognitive control processes under these different conditions was related to risk taking. Participants completed a cognitive Stroop task, an emotional Stroop task, and a risky driving task known as the Stoplight game. Regression analyses showed that performance on the emotional Stroop task predicted laboratory risk-taking in the driving task, whereas performance on the cognitive Stroop task did not exhibit the same trend. This pattern of results is consistent with theories of adolescent risk-taking that emphasise the impacts of affective contextual influences on the ability to enact effective cognitive control.     

Delay or rate of food delivery as determiners of response rate

Pigeons were confronted with two keys: a green food key and a white changeover key. Food became available for a peck to the green key after variable intervals of time (mean = 113 seconds). A single peck on the changeover key changed the color of the food key to red for a fixed period of time during which the timing of the variable-interval schedule in green was suspended and the switching option eliminated and after which the conditions associated with green were reinstated. In Experiment 1 a single food presentation was obtainable during each red-key period after a minimum delay timed from the switch. This delay and the duration of the red-key period were held constant during a condition but varied between conditions (delay = 2.5, 7.5, 15, or 30 seconds; red-period duration = 30, 60, 120, 240, or 480 seconds). In Experiment 2 additional food presentations were scheduled during a 240-second red-key period with the delay to the first food delivery held constant at 30 seconds, and the delays to later food deliveries varied over conditions. Considering the data from both experiments, the rate of switching to red was a decreasing function of the delay to the first food, the delay to the second food, and perhaps the delay to the third food after a switch. There was no clear evidence that the rate of food in the red-key period made an independent contribution. The ordering of response rates among conditions was consistent with the view that each food presentation after a response adds an incremental effect to the rate of the response and that each food presentation's contribution is a decreasing function of its delay timed from the response.     

Concurrent performances: synthesizing rate constancies by manipulating contingencies for a single response

An earlier experiment scheduled variable-interval reinforcement for pigeons' pecks on one key, and variable-interval reinforcement alternating with extinction, in a multiple schedule, for pecks on a second key. During the second key's extinction component, first-key pecking was relatively slow and continuous, rarely interrupted by second-key pecking; during the variable-interval component, first-key pecking was frequently interrupted by second-key pecking. When changeover delays operated, so that reinforced pecks on one key could not follow closely upon changeovers from the other key, rapid first-key pecking between interruptions compensated sufficiently for the time lost in second-key pecking that the overall rate of first-key pecking remained roughly constant across the alternating multiple-schedule components. The present experiments duplicated, on a single key, the temporal pattern of first-key pecking generated in the earlier experiments: components of continuous key availability were alternated with components of interrupted key availability. Approximately constant overall rates of responding were observed with a single-key equivalent of a changeover delay scheduled after interruptions and with manipulations of the on-off durations of the interruption cycle. Rate constancies in the original concurrent situation presumably depended on analogous contingencies that operated upon the concurrent responses, rather than on any constant “reserve” of responses.     

Investigations of the reinstatement of extinguished fear

In two experiments, fear was conditioned to the situational cues in one compartment of a hurdle-jumping apparatus and was then extinguished. Subsequently, either one shock (Experiment 1) or three or nine shocks (Experiment 2) were given in a situation distinctively different from that in which conditioning and extinction had taken place. Although some associative strength between the situational cues and fear was shown to have remained after extinction, in neither experiment did the postextinction-shock treatment increase the fear elicited by these cues: Escape-from-fear performance was no better in the shocked groups than in control groups given no additional shock. Thus, the nonassociative hypothesis which postulates that inflating the value of the representation of the UCS with shock-alone presentations can reinstate the extinguished fear of a stimulus was not supported. Rather, the results showed that, after extinction, an increase in fear of a simulus depended on further conditioning to that stimulus. The data also indicated that the nonvisual components of the situational cues predominated over the visual component.     

The effect of response force on avoidance rate

In a Sidman-avoidance schedule of counter losses for two human subjects, the loss-to-loss and response-to-loss intervals were 20 sec. The avoidance response was a vocal response that was louder than a minimum vocal requirement. This requirement was set at 80 db, 95 db, or 110 db. In addition to vocal responses meeting the minimum requirement, all responses exceeding a threshold of 75 db or louder were recorded. The rate of both above-threshold and avoidance responses decreased as the response-force requirement increased. Thus, high response-force requirements produced an effect on avoidance responding similar to its effect on positively reinforced responding.     

The effect of informative events on response rate

A single group of pigeons received two different training conditions presented on different response keys. Responding during the first component of each condition was reinforced, according to a fixed interval schedule, by gaining access to the second component. In the uninformative condition the second component consisted on every trial of the illumination of the response key for 10 sec, the key was then darkened and food presented with a probability of 0.5. In the informative condition half of the trials at the conclusion of the first component resulted in the key being darkened and no additional events were presented. On the remaining trials the second component was similar to that for the uninformative condition. The results from the first two stages revealed that responding during the first component was faster in the informative than uninformative condition when trials were presented separately. In the final stage, when the trials were presented simultaneously, the rate of responding during the first component was eventually similar in the two conditions, but subjects preferred the second component of the informative condition. These results suggest that events which are informative, or perhaps unpredictable, can support a higher response rate than those which are uninformative, or predictable.     

Effects of reinforcement history on response rate and response pattern in periodic reinforcement

Several researchers have suggested that conditioning history may have long-term effects on fixed-interval performances of rats. To test this idea and to identify possible factors involved in temporal control development, groups of rats initially were exposed to different reinforcement schedules: continuous, fixed-time, and random-interval. Afterwards, half of the rats in each group were studied on a fixed-interval 30-s schedule of reinforcement and the other half on a fixed-interval 90-s schedule of reinforcement. No evidence of long-term effects attributable to conditioning history on either response output or response patterning was found; history effects were transitory. Different tendencies in trajectory across sessions were observed for measures of early and late responding within the interreinforcer interval, suggesting that temporal control is the result of two separate processes: one involved in response output and the other in time allocation of responding and not responding.