Response rates track the history of reinforcement times

When conditioning involves a consistent temporal relationship between the conditioned stimulus (CS) and unconditioned stimulus (US), the expression of conditioned responses within a trial peaks at the usual time of the US relative to the CS. Here we examine the temporal profile of responses during conditioning with variable CS-US intervals. We conditioned stimuli with either uniformly distributed or exponentially distributed random CS-US intervals. In the former case, the frequency of each CS-US interval within a specified range is uniform but the momentary probability of the US (the hazard function) increases as time elapses during the trial; with the latter distribution, short CS-US intervals are more frequent than longer intervals, but the momentary probability of the US is constant across time within the trial. We report that, in a magazine approach paradigm, rats' response rates remained stable as time elapses during the CS when the CS-US intervals were uniformly distributed, whereas their response rates declined when the CS-US intervals were exponentially distributed. In other words, the profile of responding during the CS matched the frequency distribution of the US times, not the momentary probability of the US during the CS. These results are inconsistent with real-time associative models, which predict that associative strength tracks the momentary probability of the US, but may provide support for timing models of conditioning in which conditioned responding is tied to remembered times of reinforcement.     

A role for CS-US contingency in Pavlovian conditioning

Two experiments evaluated the role of conditioned stimulus-unconditioned stimulus (CS-US) contingency in appetitive Pavlovian conditioning in rats. In both experiments, some groups received a positively contingent CS signaling an increased likelihood of the US relative to the absence of the CS. These groups were compared with control treatments in which the likelihood of the US was the same in the presence and absence of the CS. A trial marker served as a trial context. Experiment 1 found contingency sensitivity. There was a reciprocal relationship between responding to the CS and the trial marker. Experiment 2 showed that this result was not stimulus or response specific. These results are consistent with associative explanations and the idea that rats are sensitive to CS-US contingency.     

Mechanisms underlying retarded emergence of conditioned responding following inhibitory training: evidence for the comparator hypothesis

The comparator hypothesis posits that conditioned responding is determined by a comparison at the time of testing between the associative strengths of the conditioned stimulus (CS) and stimuli proximal to the CS at the time of conditioning. The hypothesis treats all associations as being excitatory and treats conditioned inhibition as the behavioral consequence of a CS that is less excitatory than its comparator stimuli. Conditioned lick suppression by rats was used to differentiate four possible sources of retarded responding to an inhibitory CS. These include habituation to the unconditioned stimulus (US), latent inhibition to the CS, blocking of the CS-US association by the conditioning context, and enhanced excitatory associations to the comparator stimuli. Prior research has demonstrated the first three phenomena. Therefore, we employed parameters expected to highlight the fourth one--the comparator process. In Experiment 1, our negative contingency training was shown to produce a conditioned inhibitor that passed inhibitory summation and retardation tests. In Experiment 2 we found transfer of retardation from an inhibitory CS to a novel stimulus when the location where retardation-test training occurred was excitatory, which is indicative of contextual blocking and/or comparator effects. In Experiment 3, extinction of the conditioning context was found to attenuate retardation regardless of whether extinction occurred before or after the CS-US pairings of the retardation test. This indicates that much of the present retardation was due to the comparator process rather than to contextual blocking. Experiment 4 demonstrated that habituation to the US did not contribute to retardation in the present case. Collectively, these studies suggest that retardation following inhibitory training can be explained without recourse to any of the traditional mechanisms of conditioned inhibition.     

Conditioned taste aversion is enhanced when the unconditioned stimulus is presented in a different context

Using a conditioned taste aversion procedure with rats as the subjects, two experiments examined the effect of presenting a conditioned stimulus (CS saccharin solution) in one context followed by an unconditioned stimulus (US LiCl) in a different context. Experiment 1 showed that animals which received the above-mentioned procedure (Group D) showed a more marked conditioned aversion to the CS than animals which were given both the CS and the US in the same context (Group S). Experiment 2 found that in both Group D and Group S, aversion to the CS increased when the subjects were exposed to the conditioned context after the conditioning. These findings supported the argument that the strength of the CS-US association acquired during conditioning is compared with that of the context-US to determine the magnitude of aversion revealed to the CS.     

Conditioned stimulus informativeness governs conditioned stimulus-unconditioned stimulus associability

In a conditioning protocol, the onset of the conditioned stimulus ([CS]) provides information about when to expect reinforcement (unconditioned stimulus [US]). There are two sources of information from the CS in a delay conditioning paradigm in which the CS-US interval is fixed. The first depends on the informativeness, the degree to which CS onset reduces the average expected time to onset of the next US. The second depends only on how precisely a subject can represent a fixed-duration interval (the temporal Weber fraction). In three experiments with mice, we tested the differential impact of these two sources of information on rate of acquisition of conditioned responding (CS-US associability). In Experiment 1, we showed that associability (the inverse of trials to acquisition) increased in proportion to informativeness. In Experiment 2, we showed that fixing the duration of the US-US interval or the CS-US interval or both had no effect on associability. In Experiment 3, we equated the increase in information produced by varying the C/T ratio with the increase produced by fixing the duration of the CS-US interval. Associability increased with increased informativeness, but, as in Experiment 2, fixing the CS-US duration had no effect on associability. These results are consistent with the view that CS-US associability depends on the increased rate of reward signaled by CS onset. The results also provide further evidence that conditioned responding is temporally controlled when it emerges.     

The comparator hypothesis of conditioned response generation: manifest conditioned excitation and inhibition as a function of relative excitatory strengths of CS and conditioning context at the time of testing

In the present research water-deprived rats were used in a conditioned lick suppression paradigm to test and further develop Rescorla's (1968) contingency theory, which posits that excitatory associations are formed when a conditioned stimulus (CS) signals an increase in unconditioned stimulus (US) likelihood and that inhibitory associations develop when the CS signals a decrease in US likelihood. In Experiment 1 we found that responding to a CS varied inversely with the associative status of the context in which the CS was trained and that this response was unaltered when testing occurred in a distinctively dissimilar context with a different conditioning history, provided associative summation with the test context was minimized. These results suggest that manifest excitatory and inhibitory conditioned responding is modulated by the associative value of the training context rather than that of the test context. In Experiment 2 it was demonstrated that postconditioning decreases in the associative value of the CS training context reduced the effective inhibitory value of the CS even when testing occurred outside of the training context. Moreover, this contextual deflation effect was specific to the CS training context as opposed to any other excitatory context. Collectively, these studies support the comparator hypothesis, which states that conditioned responding is determined by a comparison of the associative strengths of the CS and its training context that occurs at the time of testing rather than at the time of conditioning. This implies that all associations are excitatory and that responding indicative of conditioned inhibition reflects a CS-US association that is below (or near) the associative strength of its comparator stimulus. It is suggested that response rules which go beyond a monotonic relation between associative value and response strength can partially relieve learning theories of their explanatory burdens, thereby allowing for simpler models of acquisition.     

Learning with arbitrary versus ecological conditioned stimuli: Evidence from sexual conditioning

Laboratory investigations of Pavlovian conditioning typically involve the association of an arbitrary conditioned stimulus (CS) with an unconditioned stimulus (US) that has no inherent relation to the CS. However, arbitrary CSs are unlikely to become conditioned outside the laboratory, because they do not occur often enough with the US to result in an association. Learning under natural circumstances is likely only if the CS has a preexisting relation to the US. Recent studies of sexual conditioning have shown that in contrast to an arbitrary CS, an ecologically relevant CS is resistant to blocking, extinction, and increases in the CS-US interval and results in sensitized responding and stronger second-order conditioning. Although the mechanisms of these effects are not fully understood, these findings have shown that signature learning phenomena are significantly altered when the kinds of stimuli that are likely to become conditioned under natural circumstances are used. The implications of these findings for an ecological approach to the study of learning are discussed.     

The role of contingency in classical conditioning

The assumption that classical conditioning depends on a contingent relation between the conditioned stimulus (CS) and the unconditioned stimulus (US), which was proposed some decades ago as an alternative to the traditional contiguity assumption, still is widely accepted as an empirical generalization, if no longer as a theoretical postulate. The first support for the contingency assumption was provided by experiments in which occasional CS-US pairings produced no response to the CS in random training--i.e., training in which the probability of the US was the same in the presence and absence of the CS. Those early experiments, the results of which too often are taken at face value, are reconsidered along with various later experiments that show conditioning, both of the CS and its context, in random training. The evidence suggests that CS-US contingency is neither necessary nor sufficient for conditioning and that the concept has long outlived any usefulness it may once have had in the analysis of 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.     

Associative effects of US preexposure: modulation of conditioned responding by an excitatory training context

In two experiments we examined factors that contribute to retarded emergence of conditioned responding to a conditioned stimulus (CS) trained in a context in which unsignaled unconditioned stimuli (USs) had previously been administered. In both experiments water-deprived rats were used in a conditioned lick suppression task to measure the conditioned response elicitation potential of the CS and the training context. From Experiment 1 we determined that nonreinforced exposure to the excitatory context after US preexposure and prior to CS-US pairings in that context eliminated the conditioned response deficit observed on a subsequent test of the CS. The recovery from the US preexposure deficit was nearly as great in animals that received nonreinforced exposure to the excitatory training context after the CS-US pairings but prior to the ultimate test of the CS. From Experiment 2 we determined that the recovery induced by contextual deflation after CS training was specific to deflation of the context in which the CS was trained as opposed to another excitatory context. In total, these experiments suggest that context-US associations partially mask the expression of a learned CS-US association. These results are discussed in terms of recent models of conditioned response generation.     

Pavlovian second-order conditioned analgesia

Three experiments with rat subjects assessed conditioned analgesia in a Pavlovian second-order conditioning procedure by using inhibition of responding to thermal stimulation as an index of pain sensitivity. In Experiment 1, rats receiving second-order conditioning showed longer response latencies during a test of pain sensitivity in the presence of the second-order conditioned stimulus (CS) than rats receiving appropriate control procedures. Experiment 2 found that extinction of the first-order CS had no effect on established second-order conditioned analgesia. Experiment 3 evaluated the effects of post second-order conditioning pairings of morphine and the shock unconditioned stimulus (US). Rats receiving paired morphine-shock presentations showed significantly shorter response latencies during a hot-plate test of pain sensitivity in the presence of the second-order CS than did groups of rats receiving various control procedures; second-order analgesia was attenuated. These data extend the associative account of conditioned analgesia to second-order conditioning situations and are discussed in terms of the mediation of both first- and second-order analgesia by an association between the CS and a representation or expectancy of the US, which may directly activate endogenous pain inhibition systems.     

Prevention of the degraded-contingency effect by signalling training trials

Presentation of unsignalled unconditioned stimuli (USs) interspersed among Pavlovian excitatory conditioning trials weakens conditioned responding to a target conditioned stimulus (CS; Rescorla, 1968). However, signalling these intertrial USs with another cue (a cover stimulus) has been shown to alleviate this degraded-contingency effect (e.g. Durlach, 1982, 1983). In contrast to signalling the inter-trial USs, the present experiments examined the effect on the degraded-contingency effect of signalling the target CS-US pairings. Experiment 1, using parameters selected to avoid overshadowing, found that consistently presenting a cover stimulus immediately prior to the target CS-US pairings during degraded-contingency training alleviated the degraded-contingency effect. Experiment 2 examined the underlying mechanism responsible for this cover-stimulus effect through posttraining associative inflation of the cover stimulus or the context, and found that inflation of the cover stimulus attenuated responding to the target CS (i.e. empirical retrospective revaluation). The results are discussed in terms of various acquisition- and expression-focused models of acquired responding.     

Conditioning the unconditioned response: modification of the rabbit's (Oryctolagus cuniculus) unconditioned nictitating membrane response

Conditioning-specific reflex modification (CRM) occurs when classical conditioning modifies responding to an unconditioned stimulus (US) in the absence of a conditioned stimulus (CS). Three experiments monitored rabbit nictitating (Oryctolagus cuniculus) membrane unconditioned responses to 5 intensities and 4 durations of periorbital electrical stimulation before and after CS or US manipulation. CRM occurred after 12 days of CS-US pairings but not following unpaired CS/US presentations or restraint. CRM survived CS-alone and CS/US-unpaired extinction of the conditioned response (CR) but not presentations of the US alone, although CRs remained intact. Thus, CRs could be weakened without eliminating CRM and CRM could be weakened without eliminating CRs. Data indicate CRM is a reliable, associative effect that is more than a generalized CR and may not be explained by habituation, stimulus generalization, contextual conditioning, or bidirectional conditioning.     

A pitfall for the expectancy theory of human eyelid conditioning

Two simple eyeblink conditioning experiments with random intermittent reinforcement schedules were performed. In Experiment 1, subjects had to rate their expectancy for an unconditioned stimulus (US) on a seven-level scale prior to each trial. As anticipated, expectancy for US increased with a successive conditioned stimulus (CS) alone, and decreased with successive CS-US pairings. However, Experiments 1 and 2 showed that the frequency of eyeblink conditioned responses (CRs) evolved in a direction opposite to that of expectancy changes: CRs increased, whereas expectancy for US decreased, and vice versa. The possible effect of sensitization on eyeblink response was ruled out by the lack of a run effect in an unpaired control group in Experiment 2. These results tend to disconfirm the expectancy theory of conditioning. Although they were explicitly predicted by the conventional “strength” theory of conditioning, an alternative interpretation is proposed within a cognitive framework.     

Response rate and reinforcement rate in Pavlovian conditioning

Four experiments used delay conditioning of magazine approach in rats to investigate the relationship between the rate of responding, R, to a conditioned stimulus (CS) and the rate, r, at which the CS is reinforced with the unconditioned stimulus (US). Rats were concurrently trained with four variable-duration CSs with different rs, either as a result of differences in the mean CS-US interval or in the proportion of CS presentations that ended with the US. In each case, R was systematically related to r, and the relationship was very accurately characterized by a hyperbolic function, R = Ar/(r +c). Accordingly, the reciprocal of these two variables-response interval, I (= 1/R), and CS-US interval, i (= 1/r) - were related by a simple affine (straight line) transformation, I = mi+b. This latter relationship shows that each increment in the time that the rats had to wait for food produced a linear increment in the time they waited between magazine entries. We discuss the close agreement between our findings and the Matching Law (Herrnstein, 1970) and consider their implications for both associative theories (e.g., Rescorla & Wagner, 1972) and nonassociative theories (Gallistel & Gibbon, 2000) of conditioning. (PsycINFO Database Record (c) 2011 APA, all rights reserved).     

Fear Develops to the Conditioned Stimulus and to the Context during Classical Eyeblink Conditioning in Rats

In classical eyeblink conditioning, non-specific emotional responses to the aversive shock unconditioned stimulus (US), which are presumed to coincide with the development of fear, occur early in conditioning and precede the emergence of eyeblink responses. This two-process learning model was examined by concurrently measuring fear and eyeblink conditioning in the freely moving rat. Freezing served as an index of fear in animals and was measured during the inter-trial intervals in the training context and during a tone conditioned stimulus (CS) presented in a novel context. Animals that received CS-US pairings exhibited elevated levels of fear to the context and CS early in training that decreased over sessions, while eyeblink conditioned responses (CRs) developed gradually during acquisition and decreased during extinction. Random CS-US presentations produced a similar pattern of fear responses to the context and CS as paired presentations despite low eyeblink CR percentages, indicating that fear responding was decreased independent of high levels of learned eyeblink responding. The results of paired training were consistent with two-process models of conditioning that postulate that early emotional responding facilitates subsequent motor learning, but measures from random control animals demonstrate that partial CS-US contingencies produce decrements in fear despite low levels of eyeblink CRs. These findings suggest a relationship between CS-US contingency and fear levels during eyeblink conditioning, and may serve to clarify further the role that fear conditioning plays in this simple paradigm.     

Fear develops to the conditioned stimulus and to the context during classical eyeblink conditioning in rats

In classical eyeblink conditioning, non-specific emotional responses to the aversive shock unconditioned stimulus (US), which are presumed to coincide with the development of fear, occur early in conditioning and precede the emergence of eyeblink responses. This twoprocess learning model was examined by concurrently measuring fear and eyeblink conditioning in the freely moving rat. Freezing served as an index of fear in animals and was measured during the inter-trial intervals in the training context and during a tone conditioned stimulus (CS) presented in a novel context. Animals that received CS-US pairings exhibited elevated levels of fear to the context and CS early in training that decreased over sessions, while eyeblink conditioned responses (CRs) developed gradually during acquisition and decreased during extinction. Random CS-US presentations produced a similar pattern of fear responses to the context and CS as paired presentations despite low eyeblink CR percentages, indicating that fear responding was decreased independent of high levels of learned eyeblink responding The results of paired training were consistent with two-process models of conditioning that postulate that early emotional responding facilitates subsequent motor learning, but measures from random control animals demonstrate that partial CS-US contingencies produce decrements in fear despite low levels of eyeblink CRs. These findings suggest, a relationship between CS-US contingency and fear levels during eyeblink conditioning, and may serve to clarify further the role that fear conditioning plays in this simple paradigm.     

Effect of signaling intertrial unconditioned stimuli in autoshaping

Context-unconditioned-stimulus (US) associations have been suggested as the mediator of the response decrement that occurs when extra USs are added to the intertrial intervals (ITIs) of an otherwise standard Pavlovian conditioning situation. The present autoshaping experiments were concerned with the effect of signaling those extra USs, since such signaling might be expected to lessen their ability to condition the context. Experiments 1 and 2 showed that signaling the ITI USs did reduce their detrimental effects on responding to the conditioned stimulus (CS). To determine whether that reduction was due to an impact of signaling on the target-CS-US association or on performance to the target-CS, Experiment 3 examined responding to differentially trained CSs in a common context, as well as responding to identically trained CSs in differentially trained contexts. Whether the CS was tested in a context of relatively high or low associative strength, more responding occurred to the CS trained with signaled, as compared with unsignaled, ITI USs; further, there was more responding to that CS in the more highly valued context. The pattern of results suggests that contextual value does interact with CS-US learning and may also affect performance to the CS.     

The decrement in conditioned fear with increased trials of simultaneous conditioning is not specific to the simultaneous procedure

Three experiments using a conditioned punishment paradigm with rat subjects examined the possibility that the nonmonotonic acquisition function previously found to characterize simultaneous conditioning was due to the noninformative nature of the conditioned stimulus (CS). In Experiment 1 the suppressive effects of a CS previously presented with an unconditioned stimulus (US) in a simultaneous and forward (informative) manner were compared following 20 and an additional 60 conditioning trials. Excitatory conditioning similarly diminished with increased trials for both the simultaneous and forward procedures. Experiment 2 employed a between-groups design. Simultaneous, forward, and trace conditioning procedures were compared following 20 or 100 trials. Each of the three 100-trial groups showed less resistance to extinction than their 20-trial counterparts. Experiment 3 determined that the decrement in excitatory conditioning for the 100-trial groups was not due to the greater number of US presentations, per se, but rather to the number of CS-US pairings. The nonmonotonic acquisition function observed with all three conditioning procedures indicated that informational factors were not responsible for the decrement observed in simultaneous conditioning. The pattern of results suggested that subjects receiving extended conditioning trials were better able to discriminate between training and testing.     

Differential effects of two ways of devaluing the unconditioned stimulus after Pavlovian appetitive conditioning

Three experiments with rat subjects investigated the effects of two methods of devaluing a food unconditioned stimulus (US) after pairings of an auditory conditioned stimulus (CS) with that US. Experiment 1 found no effect of postconditioning pairings of the food US with lithium chloride (LiCl) on general activity to a tone CS, even though those pairings substantially reduced food consumption. Experiments 2 and 3 compared the effects on conditioned responding of postconditioning pairings of food with LiCl and with high-speed rotation. In these experiments the general activity measure was supplemented by a detailed visual analysis of the rats' behavior. Experiment 2 found that food-rotation pairings had larger effects than food-LiCl pairings on general activity responding and on two detailed behavioral measures but that food-LiCl pairings had larger effects on food consumption and on one behavioral measure. Experiment 3 replicated the findings of Experiment 2 and found that the ability of the CS to serve as a reinforcer for second-order conditioning after US devaluation was reduced more by food-LiCl pairings.