Instrumental performance following reinforcer devaluation depends upon incentive learning

Conditioning an aversion to the reinforcer following instrumental training reduces performance in a subsequent extinction test. Three experiments examined whether this reinforcer-devaluation effect depends upon experience with the devalued reinforcer prior to the extinction test. In Experiments 1 and 2 thirsty rats were trained to press a lever for sucrose solution in a single session. All animals then received an injection of lithium chloride (LiCl) either immediately following the session or after a delay of 6 hr. On the next day either the sucrose solution or water was presented non-contingently either in the operant chamber without the lever present or in a separate drinking cage. In a subsequent extinction test only the animals that had received immediate LiCl and re-exposure to non-contingent sucrose pressed less than those in the delayed-LiCl control groups. Experiment 3 demonstrated that this difference depended, at least in part, on post-conditioning exposure to a contingent reinforcer. Lever pressing and chain pulling were reinforced concurrently with either a sucrose or a sodium chloride solution in a single session immediately before the administration of LiCl. All animals then received non-contingent presentations of one of the reinforcers in the absence of both manipulanda. Finally, performance of both actions was assessed in an extinction test. Re-exposure to a reinforcer produced a relative reduction in the performance of its associated action on test. These results are interpreted as evidence that the instrumental reinforcer devaluation effect depends upon a process of incentive learning.     

Incentive learning and the motivational control of instrumental performance

Hungry rats were trained to press a lever and pull a chain concurrently, with one action being reinforced with a sucrose solution and the other with food pellets. In addition, in the first two experiments all animals experienced non-contingent presentations of the two incentives in the absence of the operant manipulanda while either thirsty or hungry and either before (Experiment 1A) or after (Experiment 1B) the instrumental training. When lever pressing was assessed subsequently in extinction under thirst, the animals pressed at a relatively high rate only if (1) this action had been reinforced with the sucrose solution rather than the food pellets during training and (2) they had received the non-contingent presentations of the sucrose solution and food pellets on days on which they were thirsty rather than hungry. A third experiment demonstrated that non-contingent exposure to the sucrose solution alone, but not to water under thirst was sufficient to bring about this type of motivational control of instrumental performance.     

The effect of the instrumental training contingency on susceptibility to reinforcer devaluation

Two experiments investigated performance of instrumental lever pressing by rats following post-conditioning devaluation of the sucrose reinforcer produced by establishing an aversion to it. In Experiment I rats responded less in an extinction test after being averted from the sucrose following training on a ratio schedule, but not following an equivalent amount of training on an interval schedule. This was true even though the devalued sucrose would not act as an effective reinforcer on either the ratio or interval schedule. Experiment II provided a further investigation of the insensitivity of interval responding to reinforcer devaluation by comparing test performance under simple extinction with responding when the devalued reinforcer was presented on either a response-contingent or non-contingent schedule during the test. Once again simple extinction performance was unaffected by prior reinforcer devaluation. Furthermore, neither non-contingent nor contingent presentations of the devalued reinforcer significantly depressed responding below the level seen in the extinction condition. Ratio, but not interval performance appears to be controlled by knowledge about the instrumental contingency that encodes specific properties of the training reinforcer.     

Free-operant acquisition with delayed reinforcement

The acquisition of free-operant lever pressing by hungry rats was investigated under a schedule in which the first lever press in each second programmed a reinforcer delivery after a fixed delay. In two studies acquisition was observed under programmed delays ranging between 0 and 32 sec, although animals trained with a delay of 64 sec pressed no more than yoked, non-contingent controls. In the final study an attempt was made to enhance sensitivity to the instrumental contingency under a 64-sec delay by exposing the animals to the operant chamber in the absence of the lever prior to each training session. Only animals receiving such exposure pressed significantly more than their non-contingent controls.     

Overshadowing of a stimulus-reinforcer association by an instrumental response

In two experiments, rats were first exposed to pairings of a clicker and food; they were subsequently, in order to measure the effectiveness of the clicker as a conditioned reinforcer, given the opportunity to press a lever which turned the clicker on. For one group of animals the food originally delivered in the presence of the clicker had been contingent on their performance of an instrumental response (running in a running wheel); for a second the contingency between clicker and food had been purely classical. Although the actual correlation between clicker and food was identical for the two groups, the clicker was a less effective conditioned reinforcer for the first group than for the second. In a third experiment, all animals were initially required to run to obtain food in the presence of the clicker, but one group received additional trials on which food was delivered contingent on running in the absence of the clicker. This group showed less tendency to lever press for the clicker than a second group that had received free food on trials when the clicker was not presented. The results of all three experiments suggest that conditioning to the clicker could be overshadowed if the occurrence of food was more reliably predicted by the execution of an instrumental running response; they thus support the view that instrumental conditioning depends on the establishment of an association between response and reinforcer similar to the association between stimulus and reinforcer underlying classical conditioning.     

Food choice in rats (Rattus norvegicus): the effect of exposure to a poisoned conspecific

Three experiments were conducted to examine the effects of exposure to a poisoned conspecific on subsequent food aversion in rats. In Experiment 1A, rats that had been aversively conditioned to a cocoa-flavored food were exposed to a poisoned conspecific that had eaten the same food. On the subsequent choice test, the animals increased their aversion to that food. These results were reconfirmed in Experiment 1B, in which a cinnamon-flavored food was used as the stimulus. In Experiment 2, subjects were first exposed to a poisoned conspecific and then conditioned to the food which the conspecific had eaten. On the test, they showed no sign of increased aversion to that food.     

Contextual conditioning and reinstatement of extinguished instrumental responding

In three experiments we studied the relationship between contextual conditioning and the reinstatement of extinguished lever pressing that occurs when noncontingent food is introduced following extinction. In all three experiments the non-contingent food was presented off-baseline (with the response levers not present). On subsequent tests, with the response levers present, animals that had been exposed to food showed more reinstatement of lever pressing than control animals. This finding rules out alternative mechanisms for the reinstated responding that rely on the interaction of non-contingent food and responding, such as superstitious reinforcement or the discriminative after-effects of food. In addition, in each experiment we demonstrated that manipulations known to affect contextual conditioning (signalling the food in Experiment 1, context extinction in Experiment 2, and switching contexts in Experiment 3) reduced the reinstatement. These results are consistent with the claim that contextual conditioning is important in controlling instrumental conditioning and closely parallel findings concerning the reinstatement of Pavlovian responsing following extinction.     

Contingency effects with maintained instrumental reinforcement

In three experiments we investigated the effect on the performance of thirsty rats of varying the instrumental contingency between lever pressing and the delivery of a saccharin reinforcer. In Experiment 1, the subjects performed more slowly in a non-contingent condition, in which the momentary probability of reinforcement was unaffected by whether or not the animals pressed, than in a contingent condition in which the reinforcer was never presented except following a lever press. This was true of performance under both random ratio and interval schedules in which the function determining the probability of reinforcement following a lever press remained the same across the contingent and non-contingent conditions. Experiment 2 demonstrated that instrumental performance was less affected when the contingency was degraded by the introduction of free reinforcers if these reinforcers were signalled. In Experiment 3, lever pressing was reinstated to some degree after non-contingent training by giving non-reinforced exposure to the operant chamber in the absence of the lever. These results suggest that free reinforcers depress instrumental behaviour through a performance mechanism engaged by their ability to support conditioning of the contextual cues.     

The role of the instrumental contingency in the motivational control of performance

In four experiments we investigated an irrelevant incentive effect based upon a transition from hunger to thirst. Hungry rats were trained to lever press either for sucrose solution or for food pellets before performance was tested in extinction while they were thirsty. Reinforcer-specific motivational control was found in the first experiment in that the animals pressed the lever more on tests following training with the sucrose solution rather than with food pellets. Moreover, this effect was seen only when testing was conducted following water, but not following food deprivation. The outcome of the remaining experiments suggests that this motivational control is not mediated by the instrumental contingency between lever pressing and the sucrose reinforcer during training. In these studies lever pressing and chain pulling were reinforced concurrently, one with sucrose and the other with food pellets, in order to equate the noninstrumental functions of the incentives. Following this training, lever pressing in extinction under thirst was unaffected by the type of incentive used as its reinforcer during training.     

Incentive learning following reinforcer devaluation is not conditional upon the motivational state during re-exposure

Three experiments analysed the effect of re-exposure to the reinforcer following aversion conditioning on instrumental performance. In the first experiment, groups of hungry and thirsty rats were trained to press a lever for sucrose, which was then followed by a single injection of lithium chloride (LiCl). On the following day, half the animals in each motivational condition received re-exposure to the sucrose solution; the remaining animals were not re-exposed. In a subsequent extinction test animals that had received re-exposure to the sucrose pressed less than animals that were not re-exposed. Moreover, the effect of re-exposure to the sucrose solution was similar following training under hunger and thirst. In the remaining studies, animals were trained to lever-press for sucrose while either hungry or thirsty. They were then injected with LiCl and re-exposed to the sucrose while either hungry or thirsty, i.e. in the same or different motivational state employed during training, or they were not re-exposed. Lever pressing was then tested in extinction in the training motivational state. As in the first experiment, re-exposure to the reinforcer after aversion conditioning enhanced the magnitude of the reinforcer devaluation effect. More importantly, re-exposure to the sucrose produced a comparable effect on instrumental performance, whether re-exposure was given under the same or different motivational state to that employed during training. These results suggest that the instrumental reinforcer devaluation effect depends upon a process of incentive learning, but that this process is not conditional upon the current motivational state of the animal.     

Context Conditioning and Free operant Acquisition under Delayed Reinforcement

Three experiments examined the effect of context conditioning on the acquisition of freeoperant lever pressing by hungry rats when the presentation of the food reinforcer was delayed for 32 sec. The first study replicated the preexposure effect reported by Dickinson, Watt, and Griffiths (1992): Exposure to the contextual cues with the lever withdrawn prior to each instrumental training session enhanced acquisition, an effect that was attenuated by the presentation of non-contingent reinforcement during the preexposure periods. Signalling the non-contingent reinforcers during the preexposure periods with a brief auditory stimulus enhanced acquisition in a second study, suggesting that the non-contingent reinforcement interferes with acquisition through context conditioning. The final study confirmed this conclusion using a within-subject procedure in which pressing different levers was reinforced in two contexts, one of which was also associated with non-contingent reinforcers.     

Signalling and incentive processes in instrumental reinforcer devaluation

We have previously reported that conditioning an aversion to the reinforcer using an isotonic lithium chloride (LiCl) solution following instrumental training reduces performance in a subsequent extinction test only if animals are re-exposed to the reinforcer prior to the test. Rescorla (1992), in contrast, reported an immediate devaluation effect using a hypertonic LiCl solution that did not depend upon re-exposure. In two experiments we examined the effect of using a hypertonic LiCl solution to condition the aversion to the reinforcer on subsequent instrumental performance in extinction, with and without re-exposure. In Experiment 1 thirsty rats were trained to press a lever for a sucrose solution before being injected with 0.6 M LiCl either immediately or after a delay. Half of the immediate and delay groups were then re-exposed to the sucrose in the absence of the lever, with the remainder being exposed to water. Contrary to the previously reported effects of isotonic LiCl, a hypertonic solution induced a reinforcer devaluation effect in all the immediately poisoned animals, which did not depend upon re-exposure to the reinforcer. In Experiment 2 the possibility that this devaluation effect was induced by the discomfort associated with the hypertonicity of the solution was assessed by replicating Experiment 1 but, in addition, using two immediately poisoned groups given the LiCl injection under anaesthesia. In the absence of anaesthesia, the devaluation effect observed without re-exposure to the reinforcer in Experiment 1 was replicated. When the injection was given under anaesthesia, however, a reinforcer devaluation effect was observed only in animals that were re-exposed to the reinforcer prior to the extinction test. These results were interpreted as evidence that a reinforcer devaluation effect induced by pairing the reinforcer with illness depends upon a process of incentive learning, whereas a devaluation effect mediated by learning a signalling relationship between the reinforcer and somatic discomfort does not.     

Transfer of effort across behaviors

A series of experiments investigated the effects of required force and number of responses per reinforcer upon the subsequent performance of a second behavior. In the first experiment, a group of rats required to complete five round trips in an alley per food pellet subsequently bar-pressed for food at a greater rate than a group rewarded for each round trip or a control group that did not receive the alley experience. In the second experiment, a group required to apply a 70-g bar-press force subsequently shuttled for food at a greater rate than a group required merely to touch the lever or a control group that did not undergo the lever-press manipulation. The third experiment found that the force effect persisted across all five test sessions and was attributable to differences both in response speed and interresponse time. The fourth experiment found that both the necessary bar-press force and number of bar presses per reward affected subsequent shuttling in extinction. Two alternative interpretations of these results were compared: (a) the degree of accustomed effort per reinforcer becomes a generalized component of instrumental behavior or (b) high effort increases the habituation frustration-produced disruptive responses.     

Suppression of ethanol-reinforced lever pressing by delaying food availability.

When food was initially available to rats under a fixed-interval 26-second schedule and each liquid-reinforced lever press delayed food availability 8 seconds, suppression of liquid-reinforced lever pressing and liquid consumption occurred when the liquid presented was 4, 8, 16, 32, and 0% ethanol. Suppression did not occur in yoked-control animals, which received food coincidentally with experimental animals but were not directly exposed to the delay dependency. After exposure to the food schedule, each ethanol solution served as a reinforcer in the absence of food presentation. Delaying food availability for increasingly long periods (8 to 2048 seconds) suppressed ethanol-reinforced lever pressing and consumption relative to baseline levels, with the maximum decrease being below the level maintained in the absence of food. However, degree of suppression did not increase monotonically with delay length. Liquid-reinforced performance of yoked-control animals indicated that suppression did not result from changes in the sequencing of food presentation alone.     

Enhancement of food aversion by exposure to a poisoned conspecific in Norway rats (Rattus norvegicus)

Two experiments were conducted to determine whether exposure to a poisoned conspecific enhances prior food aversion in rats. In Experiment 1, subjects were serially exposed to two foods, cocoa‐flavored and cinnamon‐flavored ones, and were then poisoned 1 h later. On the next day, they were exposed to a poisoned conspecific that had eaten a cocoa‐flavored food. On the subsequent choice test, subjects had an enhanced aversion to cocoa‐flavored food. The result was replicated in Experiment 2, in which a cinnamon‐flavored food was assigned as a target. The results are discussed in relation to previous findings.     

Irrelevant incentive learning during instrumental conditioning: The role of the drive-reinforcer and response-reinforcer relationships

Four experiments investigated the processes by which a motivationally-induced change in the value of the training reinforcer affects instrumental performance. Initially, thirsty rats were trained to lever press for either a sodium or non-sodium solution. In Experiment I sodium-trained rats responded faster in extinction following the induction of a sodium appetite, but not following either food or water deprivation. Thus, enhanced extinction performance depends upon the relevance of the training reinforcer to the test drive state. The remaining experiments examined the role of the instrumental contingency. Animals received response-contingent presentations of one solution alternated either within (Experiments II and III) or between sessions (Experiment IV) with non-contingent presentations of another solution. Neither procedure yielded convincing evidence that contingent sodium presentations generated more responding in extinction under a sodium appetite than did non-contingent sodium presentations. On the basis of these results, we argue that the instrumental contingency itself does not play a major role in this irrelevant incentive effect.     

Choice in a self-control paradigm: Quantification of experience-based differences

Previous quantitative models of choice in a self-control paradigm (choice between a larger, more-delayed reinforcer and a smaller, less-delayed reinforcer) have not described individual differences. Two experiments are reported that provide additional quantitative data on experience-based differences in choice between reinforcers of varying sizes and delays. In Experiment 1, seven pigeons in a self-control paradigm were exposed to a fading procedure that increased choices of the larger, more-delayed reinforcer through gradually decreasing the delay to the smaller of two equally delayed reinforcers. Three control subjects, exposed to each of the small-reinforcer delays to which the experimental subjects were exposed, but for fewer sessions, demonstrated that lengthy exposure to each of the conditions in the fading procedure may be necessary in order for the increase to occur. In Experiment 2, pigeons with and without fading-procedure exposure chose between reinforcers of varying sizes and delays scheduled according to a concurrent variable-interval variable-interval schedule. In both experiments, pigeons with fading-procedure exposure were more sensitive to variations in reinforcer size than reinforcer delay when compared with pigeons without this exposure. The data were described by the generalized matching law when the relative size of its exponents, representing subjects' relative sensitivity to reinforcer size and delay, were grouped according to subjects' experience.     

Bidirectional Instrumental Conditioning

Three experiments examined bidirectional instrumental conditioning by training hungry rats to push a pole in one direction for food pellets and in the other for either a sugar or a starch solution. In the first study we examined whether the animals learned about the actionreinforcer relations using a specific satiety procedure. Prefeeding one type of reinforcer before an extinction test selectively depressed the performance of the action that had been paired with this reinforcer during training. The second experiment investigated the sensitivity of the bidirectional actions to variations in the action-reinforcer contingencies. When the instrumental contingency was degraded by presenting unpaired reinforcers, the animals pushed less in the direction that was paired with the reinforcer type that was the same as the non-contiguous one. A third study revealed that increasing the rate of reinforcement for one action enhanced its rate of performance without significantly affecting the performance of the other action. We conclude that the effects of reinforcer devaluation, the action-outcome contingency, and the rate of reinforcement are not mediated by Pavlovian associations between the manipulandum and the reinforcer.     

Overshadowing of a context aversion by a novel incentive in operant conditioning

Two experiments examined the processes underlying the suppression of instrumental behaviours by lithium in rats, as reported by Meachum (1988 and this issue). Experiment 1 examined whether presenting a novel sucrose solution prior to lithium chloride administration would overshadow aversion learning to either the stimuli of the operant chamber or to familiar food pellets. After lever pressing had been established, and in the absence of responding, animals received free deliveries of a novel sucrose solution, familiar food pellets, or both, or they were exposed to only the cues of the operant chamber, prior to lithium injections. Lever pressing for food pellets was then assessed. It was found that the animals receiving the novel sucrose, either alone or with the familiar food pellets, pressed more for pellets than either the group receiving only food pellets or the group exposed to only the context. In addition, there was no appreciable difference in the response rates between the context-only group and the group that received the familiar food pellets. These outcomes were interpreted in terms of the novel sucrose overshadowing aversion learning to the context. Experiment 2 investigated whether in fact aversive contextual conditioning could be obtained using the present parameters. This was accomplished by directly manipulating the contexts. In this experiment animals were trained to lever press in two distinctive contexts. Subsequently, one context was paired with the novel sucrose, and the second was experienced in the absence of reinforcement prior to toxicosis. During a subsequent non-reinforced test it was found that responding in the context paired with the novel sucrose was considerably higher than responding in the context that was experienced alone. These findings stand in contrast to the taste-mediated contextual potentiation observed when a consumatory response is used to assess aversive contextual conditioning.     

Variations in the sensitivity of instrumental responding to reinforcer devaluation

In five experiments hungry rats were trained to make a lever press response for a sucrose reinforcer. That sucrose was subsequently devalued by conditioning a food-aversion to it, and the ability of the rats to integrate knowledge about the instrumental contingency with that gained from aversion training was assessed in an extinction test. Experiment I showed successful integration following limited but not extended instrumental training. Experiment II suggested that the crucial factor was the spacing of training; successful integration was seen after massed but not distributed training. The third experiment implicated distributed experience with the reinforcer, rather than distributed response practice, in failures of integration. Experiment IV showed that if the distribution of food-aversion learning was dissimilar to that of instrumental training then a failure of integration could result; this finding was able to account for the distribution of training effects seen in previous studies, but not the effect of extended training. Experiment V replicated the extended training effect seen in Experiment I, and provided evidence that this may reflect the degree of exposure to the reinforcer rather than the extent of response practice.