Schedule-induced wood-chewing in rats and its dependence on body weight

In Experiment 1, 12 rats were exposed to an FT 60 schedule of food reinforcement, followed either by extinction or by a massed-food control condition, in the presence of a wood block. In 9 rats, wood-chewing behavior increased systematically during the FT 60 condition and declined again during extinction or massed food, while the other 3 rats showed virtually no chewing behavior at any stage of the experiment. In Experiment 2, frequency and bout duration of wood-chewing under an FT 60 schedule of food reinforcement declined as body weight increased, in 7 rats. We conclude that wood-chewing qualifies as a schedule-induced behavior, and that it resembles schedule-induced drinking in its dependence on body weight. Unlike drinking, however, induced chewing occupied the middle region of the 60-sec interreinforcement interval, declined markedly within the session, and showed considerable within- and between-subject variability.     

Frustration and the production of schedule-induced polydipsia

In two experiments, the hypothesis that frustration mediates the production of schedule-induced polydipsia was tested. In Experiment I, a group in which reward was reduced from 6 to 2 pellets of food in an operant chamber was found to increase water intake compared to a group maintained at 2 pellets reward. In Experiment II, rats trained to approach food on a partial reinforcement schedule in a runway subsequently showed lower levels of water intake in the operant test for polydipsia than rats given continuous reinforcement during runway training. The results are interpreted as supporting a frustration hypothesis of schedule-induced polydipsia and are discussed within the context of persistence theory.     

Multiple determinants of transfer of evaluative function after conditioning with free-operant schedules of reinforcement

The aim of the four present experiments was to explore how different schedules of reinforcement influence schedule-induced behavior, their impact on evaluative ratings given to conditioned stimuli associated with each schedule through evaluative conditioning, and the transfer of these evaluations through derived stimulus networks. Experiment 1 compared two contrasting response reinforcement rules (variable ratio [VR], variable interval [VI]). Experiment 2 varied the response to reinforcement rule between two schedules but equated the outcome to response rate (differential reinforcement of high rate [DRH] vs. VR). Experiment 3 compared molar and molecular aspects of contingencies of reinforcement (tandem VIVR vs. tandem VRVI). Finally, Experiment 4 employed schedules that induced low rates of responding to determine whether, under these circumstances, responses were more sensitive to the molecular aspects of a schedule (differential reinforcement of low rate [DRL] vs. VI). The findings suggest that the transfer of evaluative functions is determined mainly by differences in response rate between the schedules and the molar aspects of the schedules. However, when neither schedule was based on a strong response reinforcement rule, the transfer of evaluative judgments came under the control of the molecular aspects of the schedule.     

The interaction of conditioned taste aversions and schedule-induced polydipsia: Effects of repeated conditioning trials

In Experiment 1, rats poisoned following schedule-induced saccharin consumption showed a moderate reduction in the schedule-induced consumption of saccharin. With repeated poisoning, schedule-induced saccharin polydipsia was markedly reduced. Acquisition of conditioned aversion under the schedule-induced procedure was significantly slower than acquisition under water deprivation. In addition, recovery of consumption of the previously poisoned solution during extinction was more rapid under schedule-induced polydipsia. Experiment 2 revealed that schedule-induced polydipsia was less sensitive to suppression by conditioned aversions than a prandial drinking condition in which subjects were equally food deprived but were given a mass feeding instead of spaced pellet deliveries, suggesting that the relative insensitivity of schedule-induced polydipsia to conditioned taste aversions is not simply a function of different levels of food deprivation. This relative insensitivity is offered as a partial basis for the occurrence and maintenance of schedule-induced alcohol polydipsia.     

Excitatory and inhibitory effects of free reinforcers

Rats’ responding was maintained on a random-interval 1-min food schedule. In addition, non-contingent pellets were delivered, independently of the animals’ behavior, at either fixed intervals (Experiment 1) or at random intervals (Experiment 2). As the rate of delivery of the periodic and aperiodic free reinforcers increased, the rate of responding decreased. But these free reinforcers, in addition to having this inhibitory effect, had also a local excitatory effect upon responding: lever-pressing increased to a level above its mean rate following the delivery of a free food pellet. The time course of this behavioral aftereffect of free reinforcers, for both fixed and random intervals, was dependent upon the proportion of the interval between successive free food deliveries. The relation of these results to those obtained with response-contingent reinforcement, free punishment, and in schedule-induced phenomena is discussed.     

Effect on subsequent fixed-interval schedule performance of prior exposure to ratio and interval schedules of reinforcement

In three experiments, we examined the effect on the patterns of responding noted on fixed interval (FI) schedules of prior exposure to a range of interval and ratio schedules. Rats leverpressed for food reinforcement on random ratio (RR), random interval (RI), or variable interval (VI) schedules prior to transfer to FI schedules. In Experiment 1, prior exposure to an RR schedule retarded the development of typical FI patterns of responding. Exposure to a yoked RI schedule produced even greater retardation of typical FI performance. This effect was replicated in Experiment 2, using a within-subjects design. Rats responded on a multiple RR-RI schedule prior to a multiple FI-FI schedule. Typical FI performance emerged more slowly in the component previously associated with the RI than with that associated with the RR. In Experiment 3, exposure to an RR schedule retarded the development of FI performance to a greater extent than did exposure to a VR schedule. The latter schedule was programmed to allow the possibility that inhibitory control would develop after reinforcement. These results confirm that ratio schedules independently result in the disruption of FI responding. This effect was not long lasting and cannot be used plausibly to explain species differences in responding to FI schedules. However, it does suggest that temporal control—as manifested by the transfer of inhibitory control from one schedule to another—could facilitate movement between interval schedules.     

Acquisition of schedule-induced polydipsia by rats in proximity to upcoming food delivery

Food-deprived rats that receive intermittent delivery of small amounts of food develop excessive drinking--specifically, schedule-induced polydipsia (SIP). A main characteristic of SIP is its occurrence at the beginning of interfood intervals. The purpose of this study was to demonstrate that SIP can be developed toward the end of interfood intervals, in closer proximity to upcoming than to preceding food delivery. In Experiment 1, two groups were exposed to a fixed-time (FT) 30-sec food schedule with water available during the first or the last 15 sec of each interfood interval. Two additional groups, which had access to water throughout, were exposed to FT 30-sec or FT 15-sec schedules of food presentation. The FT 30-sec group with free access to water developed the highest level of intake; similar and intermediate levels were induced in all the remaining groups. In Experiment 2, three groups of rats were exposed to an FT 90-sec food schedule with water available during the first, the second, or the last 30 sec of each interfood interval. One additional group with access to water throughout was exposed to the FT 90-sec schedule of food presentation. The group with free access to water developed a higher level of consumption than did the other groups, but by the end of training none of the four groups showed statistical differences in polydipsic drinking. Results show that adjunctive drinking can be developed in proximity to upcoming food delivery even with long interfood intervals.     

Resurgence of behavior during extinction depends on previous rate of response

In two experiments, food-deprived rat subjects leverpressed for food in three successive training phases. In the first phase of both experiments, rats were exposed to a multiple schedule, one component of which produced a high rate of response, and the other of which produced a lower rate of response (multiple random ratio [RR], random interval [RI] in Experiment 1, and multiple differential reinforcement of high rate, differential reinforcement of low rate in Experiment 2). Rats were then transferred to a multiple fixed interval (FI; 60-sec, 60-sec) schedule, until the effects of the first phase on response rate were no longer apparent and their response rates did not differ from those of rats responding on a multiple FI 60-sec, FI 60-sec schedule without previously experiencing a multiple RR, RI schedule. During the third stage oftraining, all rats were placed into extinction. During extinction, rates of responding were higher in the component previously associated with the high rate of responding in Phase 1, and they were lower in the component previously associated with low rates of responding in Phase 1. These results suggest that resurgence effects, like other history effects, are controlled by previous rates of responding.     

Taste aversion learning and schedule-induced polydipsia in rats

Experiment 1 sought to determine whether schedule-induced drinking could be abolished by means of a taste aversion. Polydipsic rats were given access to a .4% saccharin solution while they were exposed to an intermittent food schedule. Immediately after the session, they received an intraperitoneal injection of either lithium chloride or sodium chloride. Following a recovery day with water in the experimental chamber, the animals were again exposed to the saccharin solution. The poisoned animals (lithium chloride) drank very little saccharin compared to the control animals (sodium chloride), indicating that they had learned a taste aversion in only one conditioning trial. Experiment 2 established that polydipsic rats can learn a taste aversion despite a long delay between schedule-induced saccharin consumption and poisoning, and that the delay gradient displayed by polydipsic rats is similar to that observed in thirst-motivated rats.     

The insensitivity of schedule-induced polydipsia to conditioned taste aversions: Effect of amount consumed during conditioning

In Experiment 1, animals poisoned following schedule-induced or prandial-induced saccharin consumption subsequently showed identical aversions to saccharin when tested under water deprivation. In Experiment 2, animals conditioned to avoid saccharin to similar levels under water deprivation were differentially affected when saccharin was subsequently presented on the baselines of schedule-induced and prandial-induced drinking. Together, these data indicate that the differential effects observed on schedule-induced and prandial-induced drinking when animals are poisoned following consumption under these two schedules do not reflect the differential acquisition of taste aversions, but instead reflect the differential tendencies to drink induced by the spaced and massed feedings.     

Schedule-induced drinking facilitates schedule-controlled feeding

Four food-deprived rats barpressed with food reinforcement on a 1-min fixed-interval schedule in 7-h sessions with water either present or absent. They all became polydipsic, drinking after many pellets early in sessions but after fewer pellets later in sessions. Pellets were secured at near maximum rates for about 3 h. Thereafter, the three heaviest drinkers obtained more food with water than without it. The results indicate that schedule-induced drinking does not continue indefinitely and that it can be adaptive with regard to feeding.     

Effects of response-independent stimuli on fixed-interval and fixed-ratio performance of rats: a model for stressful disruption of cyclical eating patterns

Binge eating is often associated with stress-induced disruption of typical eating patterns. Three experiments were performed with the aim of developing a potential model for this effect by investigating the effect of presenting response-independent stimuli on rats’ lever-pressing for food reinforcement during both fixed-interval (FI) and fixed-ratio (FR) schedules of reinforcement. In Experiment 1, a response-independent brief tone (500-ms, 105-dB, broadband, noisy signal, ranging up to 16 kHz, with spectral peaks at 3 and 500 Hz) disrupted the performance on an FI 60-s schedule. Responding with the response-independent tone was more vigorous than in the absence of the tone. This effect was replicated in Experiment 2 using a within-subject design, but no such effect was noted when a light was employed as a disrupter. In Experiment 3, a 500-ms tone, but not a light, had a similar effect on rats' performance on FR schedules. This tone-induced effect may represent a release from response-inhibition produced by an aversive event. The implications of these results for modeling binge eating are discussed.     

What is meant by the term “schedule-induced,” and how general is schedule induction?

The term “schedule-induced” implies that the overall frequency of a behavior is greater in the presence of an intermittent schedule of reinforcement than in the absence of such a schedule. Consequently, the occurrence of interreinforcement behavior is not in itself sufficient evidence of schedule induction: a test of induction requires comparison between an intermittent-schedule condition and a nonschedule baseline. The relative merits of different types of nonschedule baseline are examined, and it is concluded that the best test of schedule-induction involves both an extinction and a massed-reinforcer baseline. A working definition of schedule-induction is suggested on this basis. Studies purporting to show schedule induction of activities other than drinking are critically reviewed, and it is concluded that schedule induction may be less general than is usually supposed. It may therefore be more fruitful to seek an explanation of schedule-induced drinking which focuses specifically on the interaction between food and water ingestion in the rat, rather than an explanation involving concepts such as stress, frustration, or arousal.     

Meal size and intermeal interval both regulate schedule-induced water intake in rats

The present studies examined the effects of intermeal interval and pellet magnitude on the development and maintenance of schedule-induced polydipsia using a between-subjects design as contrasted with earlier investigations, which have exclusively employed within-subject procedures. In Experiment 1, pellets were delivered on one of six fixed-time schedules (20, 40, 60, 120,180, and 240 sec) to different groups of rats. Although the typical bitonic function relating water intake to intermeal interval was obtained, the function showed a wide plateau ranging from 60 to 180 sec. Experiment 2 demonstrated that both intermeal interval and pellet magnitude regulate schedule-induced polydipsia. While small-magnitude pellets could support drinking only at relatively short intermeal intervals, increasing pellet magnitude to medium or large resulted in drinking at progressively longer intermeal intervals. Indeed, we observed drinking on a fixed-time 720-sec schedule with large-magnitude pellets. These findings are derivable from the opponent-process theory of acquired motivation.     

The role of trial tracking in rats’ working memory

The experiments reported in the present study tested whether decreasing intertrial intervals (ITIs) intensifies the disruptive effects of increasing retention intervals (RIs) in a delayed conditional discrimination by decreasing the animal’s trial tracking accuracy (Cohen & Armstrong, 1996; Cohen & Roberts, 1996). Rats responded on a fixed ratio (FR) 1 or fixed interval (FI) 10-sec reinforcement schedule at a second light or tone stimulus, S2, when the first light or tone stimulus, S1, had signaled an FI 10-sec or FR 1 schedule, respectively. RIs between S1 and S2 were increased from 3 to 24 sec and never exceeded ITIs that were reduced from 24 to 6 sec. For some rats, the trials were separated from each other by extending the lever at S1 and retracting it at the end of S2 (ITI lever-retracted group). For other, control rats, the lever remained extended throughout the session (lever-extended group, Experiment 1) or was extended and retracted with the onset and offset of each stimulus (RI/ITI lever-retracted group, Experiment 2). The rats under all trial conditions learned to delay leverpressing on the FI 10-sec schedule. Latency to begin leverpressing on the FI 10-sec schedule declined as RIs were increased, but this effect was attenuated in the ITI lever-retracted groups in both experiments, as would be predicted by thetrial tracking hypothesis. Decreasing ITIs from 24 to 6 sec intensified the disruptive effects of increasing RIs from 3 to 6 sec in the RI/ITI lever-retracted group (Experiment 2), as would be predicted by the trial tracking hypothesis.     

Functional differentiation of adjunctive drinking and wheel running in rats

Food-deprived rats were exposed to a schedule in which a brief stimulus was presented approximately once every 60 sec. The first leverpress to occur in the presence of the stimulus always turned it off, and produced a food pellet 50% of the time. When the rats were given concurrent access to water, a running wheel, or both, drinking predominated during intervals initiated by pellet delivery, while running predominated during intervals initiated without food. When allowed to obtain all of their food pellets at the beginning of a session, rats drank less and ran more than when the intermittent schedule was in effect, and most drinking occurred within the first half of the session, while running was distributed throughout the session. Adjunctive drinking and wheel running appear to be functionally different, drinking being schedule-induced and food-bound, running being neither.     

Instrumental responding by rats on free-operant schedules with components that schedule response-dependent reinforcer omission: Implications for optimization theories

In two experiments, we assessed whether rats optimize the number of reinforcers per response. In Experiment 1, one group of rats was trained to leverpress for food reinforcement on a simple variable-interval (VI) 60-sec schedule. For another group, a negative fixed-ratio component was imposed on the VI schedule to produce a conjoint contingency in which reinforcement became available on the VI schedule but was omitted when the ratio criterion was satisfied. In Experiment 2, one group of rats responded on a VI schedule and also received response-independent food. For another group, responding above a certain rate canceled the response-independent food. Despite the negative contingency experienced by the groups in Experiments 1 and 2, responding was maintained at a level at which the number of obtained reinforcers was reduced substantially below the maximum number possible. In addition, in both experiments, the groups that experienced the negative contingency responded at a lower rate than did a yoked control group that experienced the same frequency of reinforcement but lacked the negative component. These results demonstrate that although rats do not optimize their behavior with respect to reinforcement, they are nevertheless sensitive to some aspect of the instrumental contingency in operation.     

The effects of conditioned taste aversions on schedule-induced polydipsia: An analysis of the initiation and postpellet temporal distribution of licking

Animals poisoned following the schedule-induced consumption of saccharin initially continued to drink following spaced pellet deliveries. Neither the initiation of postpellet drinking (i.e., bout, initiation) nor the size and duration of the bouts was effected by the conditioned aversion procedure. With repeated conditioning trials (i.e., repeated pairings of saccharin and LiCl), schedule induced drinking was eventually reduced. The specific components underlying schedule-induced consumption, however, were differentially affected by the aversion training. Specifically, the decrease in schedule-induced drinking was effected primarily by a decrease in licking occurring between 10 and 60 sec after pellet delivery. Bout initiation and licking immediately postpellet (i.e., within the first 10 sec following pellet delivery) were most resistant to suppression and appeared to be responsible for the relative insensitivity of schedule-induced drinking to conditioned taste aversions. The differential effects of taste aversion conditioning on individual components of elicited behavior are discussed.     

Choice responding following multiple schedule training

Pigeons’ choice responding on 10-sec interpolated probes was studied after baseline training on multiple variable-interval variable-interval schedules of food reinforcement. Unreinforced choice following training with three different relative reinforcement rates (Experiment 1), with a 3-ply multiple schedule (Experiment 2), and with three different relative reinforcement durations (Experiment 3) was examined. Least squares lines were fit to choice relative response rate and schedule relative response rate as functions of training relative reinforcement rate; choice slope was significantly greater than schedule slope in all three experiments. This result is counter to the prediction of Herrnstein’s (1970) theory that these slopes should not differ. Luce’s (1959) theory also failed to account for the data. It was concluded that choice responding was controlled by both approach to the stimulus associated with the smaller mean interreinforcer interval or the longer duration, and avoidance of the other stimulus.