The long-term effect of high- and low-rate responding histories on fixed-interval responding in rats

Tell rats were given extended lever-press training on a fixed-interval (FI) 30-s food reinforcement schedule from the outset or following exposure to one or two previous reinforcement schedules. For 4 rats the previots schedule was either fixed-ratio 20, which generated high response rates, or differential-reinforcement-of-low-rate 20 s, which produced low response rates. For 4 additional rats the extended training on FI 30 s was preceded by experience with two schedules: fixed-ratio 20 followed by differential-reinforcement-of-low-rate 20 s; or the same two schedules in the reverse order. Fixed-interval response rates were initially affected by the immediately preceding schedule, but after 80 to 100 sessions, all traces of prior schedule history had disappeared. The results also showed no long-term effect of schedule history on the interfood-interval patterns of responding on the FI 30-s schedule. These results support one of the most central tenets of the experimental analysis of behavior: control by the immediate consequences of behavior.     

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.     

Effects of negative incentive shifts in food reward on rats' consumption of concurrent ethanol solutions

Frustration stress, typically operationalized as the unexpected loss of reinforcement, has been shown to engender substance use. Abrupt reductions in reinforcer magnitude likely also function as frustration stressors. These negative incentive shifts were previously shown to produce tap‐ and sweetened‐water drinking in rats. The purpose of this study was to investigate whether these shifts in food reward would occasion oral ethanol self‐administration. Nine male Long‐Evans rats operated on a two‐component multiple fixed‐ratio schedule with signaled components producing either a large (4 pellets) or small (1pellet) reinforcer. Components were pseudorandomly arranged to present 4 transitions between past and upcoming reinforcer magnitudes: small‐to‐large, small‐to‐small, large‐to‐large, and large‐to‐small (negative incentive shift). Experiment 1 investigated the effects of negative incentive shifts on consumption of concurrent, freely available 10% sucrose, 10% sucrose plus 10% ethanol, and following sucrose fading, 10% ethanol. Experiment 2 entailed continuation of schedule contingencies with a dose manipulation of 4 ethanol concentrations (0, 5, 10, and 20%) to assess dose‐dependent differences in transition‐type control and consumption. A lever‐press extinction condition was then conducted with 10% ethanol availability. In this novel model of frustration stress, negative incentive shifts prompted ethanol self‐administration at each dose investigated, whereas the other transitions did not.     

Effects of shifts in food reinforcement context on rats’ consumption of concurrently available water or sucrose solution

The purpose of this study was to investigate the effects of signaled transitions from relatively rich to lean conditions of food reinforcement on drinking concurrently available water or sucrose‐sweetened water in rats. Past research demonstrated that these negative incentive shifts produce behavioral disruption in the form of extended pausing on fixed‐ratio schedules. Four male Long‐Evans rats operated on a two‐component multiple fixed‐ratio fixed‐ratio schedule. In one manipulation, the ratio was held constant and the components arranged either a large six‐pellet reinforcer (rich) or small one‐pellet reinforcer (lean). In a second manipulation, the components both produced a one‐pellet reinforcer but differed in terms of the ratio requirement, with the rich and lean conditions corresponding to relatively small and large ratios. In both manipulations, components were pseudorandomly presented to arrange four transitions signaled by retractable levers: lean‐to‐lean, lean‐to‐rich, rich‐to‐rich, and rich‐to‐lean (the negative incentive shift). During experimental conditions, a bottle with lickometer was inserted in the chamber, providing concurrent access either to tap water or a 10% sucrose solution. The negative incentive shift produced considerably more drinking than the other transitions in all rats during both manipulations. The level of drinking was not polydipsic; rather, it appears that the negative incentive shift enhanced the value of concurrently available reinforcers relative to food reinforcement.     

Reinforcement value and fixed‐interval performance

The concept of reinforcement value summarizes the effect of different variables, such as reinforcement delay, reinforcement magnitude, and deprivation level, on behavior. In the present set of experiments, we evaluated the effect of reinforcement devaluation on performance under FI schedules. The literature on timing and reinforcement value suggests that devaluation generates longer expected times to reinforcement than the same intervals trained under control conditions. We devalued reinforcement with delay in Experiments 1A, 1B, and 2, and diminished deprivation in Experiments 3A and 3B. Devaluation reduced response rates, increased the number of one‐response intervals, and lengthened postreinforcement pauses, but had inconsistent effects on other timing measures such as quarter life and breakpoint. The results of delayed reinforcement and diminished deprivation manipulations are well summarized as reinforcement devaluation effects. These results suggest that devaluation may reduce stimulus control. In addition, we argue that the process by which delayed reinforcement affects behavior might also explain some effects observed in other devaluation procedures through the concept of reinforcement value.     

Avoidance of timeout from response-independent food: effects of delivery rate and quality

In three experiments, a rat's lever presses could postpone timeouts from food pellets delivered on response-independent schedules. In Experiment 1, the pellets were delivered at variable-time (VT) rates ranging from VT 0.5 to VT 8 min. Experiment 2 replicated the VT 1 min and VT 8 min conditions of Experiment 1 with new subjects. Finally, subjects in Experiment 3 could postpone timeouts from delivery of pellets that differed in quality rather than quantity (unsweetened versus sweetened pellets). In general, response rates and success in avoiding increased as a function of the rate and quality of the pellets. Also, performance efficiency increased as the experiments progressed, that is, the avoidance response occurred later and later in the response-timeout interval. The results support the conclusion that timeout from reinforcement has functional properties similar to those of more commonly studied aversive stimuli (e.g., shock).     

The effects of reinforcer magnitude on timing in rats

The relation between reinforcer magnitude and timing behavior was studied using a peak procedure. Four rats received multiple consecutive sessions with both low and high levels of brain stimulation reward (BSR). Rats paused longer and had later start times during sessions when their responses were reinforced with low-magnitude BSR. When estimated by a symmetric Gaussian function, peak times also were earlier; when estimated by a better-fitting asymmetric Gaussian function or by analyzing individual trials, however, these peak-time changes were determined to reflect a mixture of large effects of BSR on start times and no effect on stop times. These results pose a significant dilemma for three major theories of timing (SET, MTS, and BeT), which all predict no effects for chronic manipulations of reinforcer magnitude. We conclude that increased reinforcer magnitude influences timing in two ways: through larger immediate after-effects that delay responding and through anticipatory effects that elicit earlier responding.     

A pharmacological examination of the resistance-to-change hypothesis of response strength.

The effects of d-amphetamine sulfate, sodium pentobarbital, haloperidol, and cholecystokinin-octapeptide were examined within the context of Nevin's (1974, 1979) resistance-to-change hypothesis of response strength. In three experiments, rats' responding was reinforced by delivery of food under chained random-interval 30-s random-interval 30-s, multiple fixed-interval 30-s fixed-interval 120-s, or multiple random-interval 30-s random-interval 120-s schedules. Each rat received several doses of each drug and changes in response rate were measured. The resistance-to-change hypothesis predicts greater disruption of response rate relative to baseline in the initial component of the chained schedule and in the 120-s component of the multiple schedules. In the chained schedule cholecystokinin-octapeptide produced greater reductions in response rate relative to baseline in the initial component. However, no differences between components were observed with haloperidol or sodium pentobarbital, and high doses of d-amphetamine reduced response rate in the terminal component relatively more than in the initial component. In the multiple schedules either no differences were observed between components or response rate was reduced more relative to baseline in the 30-s component. The data fail to support the notion that drugs may be viewed within the same context as other response disruptors such as extinction, satiation, and the presentation of alternative reinforcement.     

Acquisition of lever-press responding in rats with delayed reinforcement: A comparison of three procedures

The present study examined the acquisition of lever pressing in rats under three procedures in which food delivery was delayed by 4, 8, and 16 seconds relative to the response. Under the nonresetting delay procedure, food followed the response selected for reinforcement after a specified interval elapsed; responses during this interval had no programmed effect. Under the resetting procedure, the response selected for reinforcement initiated an interval to food delivery that was reset by each subsequent response. Under the stacked delay procedure, every response programmed delivery of food t seconds after its occurrence. Two control groups were studied, one that received food immediately after each lever press and another that never received food. With the exception of the group that did not receive food, responding was established with every procedure at every delay value without autoshaping or shaping. Although responding was established under the resetting delay procedure, response rates were generally not as high as under the other two procedures. These findings support the results of other recent investigations in demonstrating that a response not previously reinforced can be brought to strength by delayed reinforcement in the absence of explicit training.