Comparison of drug effects on fixed-ratio performance and chain performance maintained under a second-order fixed-ratio schedule.

In one component of a multiple schedule, pigeons were required to complete the same four-response chain each session by responding sequentially on three identically lighted keys in the presence of four successively presented colors (chain performance). Food presentation occurred after five completions of the chain (i.e., after 20 correct responses). Errors, such as responding on the center or right key when the left was designated correct, produced a brief timeout but did not reset the chain. In the other component, responding on a single key (lighted white) was maintained by food presentation under a fixed-ratio 20 schedule. In general, phencyclidine and d-amphetamine produced dose-dependent decreases in the overall response rates in both components. With pentobarbital, overall rate in each component generally increased at intermediate doses and decreased at higher doses. All three drugs produced dose-dependent disruptive effects on chain-performance accuracy. Phencyclidine and pentobarbital increased percent errors at doses that had little or no rate-decreasing effects, whereas d-amphetamine generally increased percent errors only at doses that substantially decreased overall rate. At high doses, all three drugs produced greater disruption of chain performance than of fixed-ratio performance, as indicated by a slower return to control responding, although the effects of d-amphetamine were less selective than those of phencyclidine or pentobarbital.     

Effects of schedule of reinforcement on a pentobarbital discrimination in rats.

The purpose of this study was to determine the effects of the schedule of reinforcement on a pentobarbital discrimination in rats. Five rats were trained to discriminate 10 mg/kg pentobarbital from saline under a multiple fixed-interval 180-s fixed-ratio 20 schedule of reinforcement. During both saline and pentobarbital training sessions, subjects emitted a higher percentage of correct responses under the fixed-ratio component as compared to the fixed-interval component of the multiple schedule. Determination of the pentobarbital dose-response curve under the fixed-ratio component resulted in a steep curve characterized by responding on the saline lever at low doses and on the drug lever at higher doses. Under the fixed-interval component, a graded dose-effect curve was produced, with considerable responding on both levers after intermediate doses of pentobarbital. The administration of phencyclidine and MK-801 resulted in an intermediate level of drug-lever responding for some subjects. Administration of d-amphetamine resulted in saline (nondrug) appropriate responding. The results of this study demonstrate that the schedule of reinforcement is a determinant of drug stimulus control, just as it is a determinant of other drug effects.     

Drug discrimination under a concurrent fixed-interval fixed-interval schedule.

Pigeons were trained to discriminate 5.0 mg/kg pentobarbital from saline under a concurrent fixed-interval (FI) FI schedule of food presentation on which, after pentobarbital administration, responses on one key were reinforced with food under an FI 60-s component and responses on the other key were reinforced under an FI 240-s component. After saline administration, the schedule contingencies on the two keys were reversed. After both pentobarbital and saline, pigeons responded more frequently on the key on which responses had been programmed to produce the reinforcer under the FI 60 component of the concurrent schedule. The schedule was changed to concurrent FI 150 FI 150 s for drug-substitution tests. In each bird, increasing doses of pentobarbital, ethanol, and chlordiazepoxide produced increases in the proportion of responses on the key on which responses had been reinforced under the FI 60 component after pentobarbital administration during training sessions. The proportion of responses on that key was slightly lower for ethanol than for chlordiazepoxide and pentobarbital. At a dose of pentobarbital higher than the training dose, responding decreased on the key that had been reinforced under the FI 60 component during training sessions. Phencyclidine produced less responding on the key programmed under the FI 60-s component than did pentobarbital. Methamphetamine produced responding primarily on the key on which responses had been reinforced under the FI 60-s component after saline administration.     

Drug discrimination under two concurrent fixed-interval fixed-interval schedules

Pigeons were trained to discriminate 5.0 mg/kg pentobarbital from saline under a two-key concurrent fixed-interval (FI) 100-s FI 200-s schedule of food presentation, and later tinder a concurrent FI 40-s FI 80-s schedule, in which the FI component with the shorter time requirement reinforced responding on one key after drug administration (pentobarbital-biased key) and on the other key after saline administration (saline-biased key). After responding stabilized under the concurrent FI 100-s FI 200-s schedule, pigeons earned an average of 66% (after pentobarbital) to 68% (after saline) of their reinforcers for responding under the FI 100-s component of the concurrent schedule. These birds made an average of 70% of their responses on both the pentobarbital-biased key after the training dose of pentobarbital and the saline-biased key after saline. After responding stabilized under the concurrent FI 40-s FI 80-s schedule, pigeons earned an average of 67% of their reinforcers for responding under the FI 40 component after both saline and the training dose of pentobarbital. These birds made an average of 75% of their responses on the pentobarbital-biased key after the training dose of pentobarbital, but only 55% of their responses on the saline-biased key after saline. In test sessions preceded by doses of pentobarbital, chlordiazepoxide, ethanol, phencyclidine, or methamphetamine, the dose-response curves were similar under these two concurrent schedules. Pentobarbital, chlordiazepoxide, and ethanol produced dose-dependent increases in responding on the pentobarbital-biased key as the doses increased. For some birds, at the highest doses of these drugs, the dose-response curve turned over. Increasing doses of phencyclidine produced increased responding on the pentobarbital-biased key in some, but not all, birds. After methamphetamine, responding was largely confined to the saline-biased key. These data show that pigeons can perform drug discriminations under concurrent schedules in which the reinforcement frequency under the schedule components differs only by a factor of two, and that when other drugs are substituted for the training drugs they produce dose-response curves similar to the curves produced by these drugs under other concurrent interval schedules.     

Phencyclidine discrimination in the pigeon using color tracking under second-order schedules

Pigeons were trained to track different key colors, depending on whether they had been injected with phencyclidine or saline prior to the session. A second-order schedule was used to generate large numbers of responses prior to the initial food delivery. The procedure offers several advantages over traditional procedures for studying drug discrimination.     

Effects of d-amphetamine on responding under second-order schedules of reinforcement with paired and nonpaired brief stimuli.

Three pigeons were studied under a multiple schedule in which pecks in each component were reinforced according to a variable-interval 120-s second-order schedule with fixed-interval 60-s units. In the first component of the multiple schedule, the completion of a fixed interval produced either food or a 4-s change in key color plus houselight illumination. In the second component an identical schedule was in effect, but the stimulus was a 0.3-s change in key color. Both long and short brief stimuli were not paired with food presentations in Conditions 1 and 3 and were paired with food in Condition 2. There were no consistent differences in response patterns under paired and nonpaired brief-stimulus conditions when the stimulus was a 4-s change in key color accompanied by houselight illumination. However, pairing the 0.3-s key-color change with food presentations resulted in higher indices of curvature and lower response rates in the early segments of the fixed interval than when the stimulus was not paired with food presentations. Low doses of d-amphetamine (0.3 and 1 mg/kg) produced small and inconsistent increases in overall response rates, and higher doses (3 and 10 mg/kg) decreased overall response rates. d-Amphetamine altered response patterns within fixed intervals by decreasing the indices of curvature and increasing response rates in the early segments of the fixed interval. Response rates and patterns under paired and nonpaired brief-stimulus conditions were not differentially affected by d-amphetamine. Thus, evidence for the enhancement of the conditioned reinforcement effects of psychomotor stimulant drugs was not found with the second-order schedules used in the present study.     

Drug discrimination under a concurrent fixed-ratio fixed-ratio schedule.

Pigeons were trained to discriminate 5.0 mg/kg pentobarbital from saline under a two-key concurrent fixed-ratio 10 fixed-ratio 40 schedule of food presentation, in which the fixed-ratio component with the lower response requirement was programmed to reinforce responding on one key after drug administration (pentobarbital-biased key) and on the other key after saline administration (saline-biased key). After responding stabilized, pigeons averaged 98% of their responses on the pentobarbital-biased key during training sessions preceded by pentobarbital, and they averaged 90% of their responses on the saline-biased key during training sessions preceded by saline. In test sessions preceded by doses of pentobarbital, chlordiazepoxide, or ethanol, pigeons switched from responding on the saline-biased key at low doses to responding on the pentobarbital-biased key at higher doses (the dose-response curve was quantal). High doses of phencyclidine produced responding on both keys, whereas pigeons responded almost exclusively on the saline-biased key after all doses of methamphetamine. These and previous experiments using concurrent reinforcement schedules to study drug discrimination illustrate that the schedule of reinforcement is an important determinant of the shape of dose-effect curves in drug-discrimination experiments.     

Drug discrimination under a concurrent schedule.

Three pigeons were trained to discriminate a 5.0 mg/kg dose of pentobarbital from saline under a two-key concurrent schedule with responding on the key associated with the presession injection, under both stimulus conditions, producing four times as many reinforcers as responding on the other key. This concurrent schedule resulted in approximately 70% responding to the higher reinforcement key under the pentobarbital stimulus and approximately 30% responding to that key under the saline stimulus. During testing, then, the pigeons were able to dose-dependently emit higher (>70%) or lower (<30%) values than were established under the control conditions. Dose-response curves were determined for pentobarbital (twice), methamphetamine, phencyclidine, chlordiazepoxide, and the combination of pentobarbital and the barbiturate antagonist bemegride. The results obtained with pentobarbital and chlordiazepoxide showed that, as the dose increased, pentobarbital-appropriate responding also increased. Methamphetamine produced relatively flat dose--response curves, whereas phencyclidine administration produced inconsistent effects on responding. The combination of the training dose of pentobarbital with increasing doses of bemegride produced a decrease in pentobarbital-appropriate responding. The results also showed that the dose-response curves for pentobarbital and chlordiazepoxide, instead of being all or none, were graded functions of the drug dose.     

Drug effects on responding maintained by stimulus-reinforcer and response-reinforcer contingencies.

The effects of pentobarbital and d-amphetamine were assessed on key pecking by pigeons under conventional single-key multiple schedules and under two-key multiple schedules in which discriminative stimuli appeared on one key (stimulus key) while pecks on a second key (constant key) produced food. Pecks on the stimulus key had no scheduled consequences. A 60-second variable-interval schedule operated in one component of each multiple schedule: either extinction or a 60-second variable-time schedule operated in the alternate component. When the alternate-component schedule was extinction, a high rate of responding was maintained in the variable-interval component of the single-key schedule; responding on both keys was maintained in the variable-interval component of the two-key schedule. Pentobarbital increased responding in the variable-interval component of the single-key schedule and increased stimulus-key, but not constant-key responding in that component of the two-key schedule. When the alternate-component schedule was changed to variable time, responding declined in the variable-interval component of the single-key schedule; stimulus-key responding was no longer maintained under the two-key schedule. Pentobarbital decreased responding in the variable-interval component of both schedules. With an exception, d-amphetamine only decreased responding in the variable-interval component of the single- and two-key schedules both when the alternate-component schedule was extinction and when it was variable time. The results suggest that the effects of pentobarbital, but not d-amphetamine, depend on the nature of the contingency (stimulus-reinforcer, response-reinforcer) that maintains responding.     

Food-paired stimuli as conditioned reinforcers: effects of d-amphetamine.

Seven pigeons were studied in two experiments in which key pecks were reinforced under a second-order schedule wherein satisfaction of variable-interval schedule requirements produced food or a brief stimulus. In the second part of each session, responses produced only the brief stimulus according to a variable-interval schedule (food extinction). For the 4 pigeons in Experiment 1, the response key was red throughout the session. In separate phases, the brief stimulus was either paired with food, not paired with food, or not presented during extinction. d-Amphetamine (0.3 to 10.0 mg/kg) dose-dependently reduced food-maintained responding during the first part of the session and, at intermediate dosages, increased responding during the extinction portion of the session. The magnitude of these increases, however, did not consistently depend on whether the brief stimulus was paired, not paired, or not presented. It was also true that under nondrug conditions, response rates during extinction did not differ reliably depending on pairing operations for the brief stimulus. In Experiment 2, 3 different pigeons responded under a procedure wherein the key was red in the component with food presentations and blue in the extinction component (i.e., multiple schedule). Again, d-amphetamine produced dose-related decreases in responding during the first part of a session and increases in responding in the second part of the session. These increases, however, were related to the pairing operations; larger increases were observed when the brief stimulus was paired with food than when it was not or when it was not presented at all. Under nondrug conditions, the paired brief stimulus controlled higher response rates during extinction than did a nonpaired stimulus or no stimulus. These findings suggest that d-amphetamine can enhance the efficacy of conditioned reinforcers, and that this effect may be more robust if conditioned reinforcers occur in the context of a signaled period of extinction.     

Effects of reinforcement history on responding under progressive-ratio schedules of reinforcement.

The effects of experimental history on responding under a progressive-ratio schedule of reinforcement were examined. Sixteen pigeons were divided into four equal groups. Groups 1 to 3 were trained to peck a key for food under a fixed-ratio, variable-ratio, or differential-reinforcement-of-low-rate schedule of reinforcement. After training, these pigeons were shifted to a progressive-ratio schedule, later were shifted back to their original schedule (with decreased rates of reinforcement), and finally were returned to the progressive-ratio schedule. Pigeons in Group 4 (control) were maintained on the progressive-ratio schedule for the entire experiment. To test for potential "latent history" effects, pigeons responding under the progressive-ratio schedule were injected with d-amphetamine and given behavioral-momentum tests of prefeeding and extinction. Experimental histories affected responding in the immediate transition to the progressive-ratio schedule; response rates of pigeons with variable-ratio and fixed-ratio histories were higher than rates of pigeons with differential-reinforcement-of-low-rate and progressive-ratio-only histories. Pigeons with differential-reinforcement-of-low-rate histories, and to a lesser degree pigeons with variable-ratio and fixed-ratio histories, also had shorter postreinforcement pauses than pigeons with only a progressive-ratio history. No consistent long-term effects of prior contingencies on responding under the progressive-ratio schedule were evident. d-Amphetamine and resistance-to-change tests failed to reveal consistent latent history effects. The data suggest that history effects are sometimes transitory and not susceptible to latent influences.     

Conjunctive schedules of reinforcement: III. A fixed-interval adjusting fixed-ratio schedule

Key pecking of three pigeons was studied under a conjunctive schedule that specified both a fixed-interval and an adjusting fixed-ratio requirement. The fixed-interval schedule was 6 min for one pigeon and 3 min for the other two. The size of the ratio requirement was determined within each cycle of the fixed interval by the duration of the pause before responding began. The fixed-ratio value was at maximum at the start of each fixed interval and decreased linearly until the first response occurred (adjusting fixed-ratio schedule). A peck produced food when the number of responses remaining on the fixed-ratio schedule was completed and when the fixed interval had elapsed. If no response occurred during the interval, the fixed-ratio requirement decreased to one and a single response after the interval elapsed produced food. The initial value of the adjusting fixed-ratio schedule was studied over a range of 0 to 900. Increases in the adjusting fixed-ratio schedule to about 300 responses increased both pause duration and running response rate and also modified the pattern of responding from that obtained under the fixed-interval schedule. Higher values of the adjusting fixed ratio generally decreased pause duration and running response rate and also disrupted responding. Interreinforcement time under the conjunctive schedule was increased substantially when the adjusting fixed-ratio size exceeded 300 responses.     

Effects of body weight on discriminative-stimulus control by phencyclidine in the pigeon.

Using a color-tracking procedure with responses reinforced under a second-order schedule, the discriminative-stimulus properties of phencyclidine were studied in pigeons maintained at 70%, 80%, or 90% of their free-feeding weights. The generalization curves for phencyclidine were similar at all three body weights. Generalization curves for pentobarbital, d-amphetamine, and saline were also unrelated to body weight. These data suggest that food deprivation may not influence the discriminative-stimulus properties of drugs in the way that it influences the reinforcing-stimulus properties of drugs. The reason may be that during discrimination training interoceptive stimuli resulting from food deprivation do not become conditioned to the stimulus properties of the drug, because the food-deprivation stimuli are paired equally often with the presence and absence of drug stimuli.     

Drug discrimination under concurrent variable-ratio variable-ratio schedules

Pigeons were trained to discriminate 5 mg/kg pentobarbital from saline under concurrent variable-ratio (VR) VR schedules, in which responses on the pentobarbital-biased lever were reinforced under the VR schedule with the smaller response requirements when pentobarbital was given before the session, and responses on the saline-biased key were reinforced under the VR schedule with the larger response requirements. When saline was administered before the session, the reinforcement contingencies associated with the two response keys were reversed. When responding stabilized under concurrent VR 20 VR 30, concurrent VR 10 VR 40, or concurrent VR 5 VR 50 schedules, pigeons responded almost exclusively on the key on which fewer responses were required to produce the reinforcer. When other doses of pentobarbital and other drugs were substituted for the training dose, low doses of all drugs produced responding on the saline-biased key. Higher doses of pentobarbital and chlordiazepoxide produced responding only on the pentobarbital-biased key, whereas higher doses of ethanol and phencyclidine produced responding only on this key less often. d-Amphetamine produced responding primarily on the saline-biased key. When drugs generalized to pentobarbital, the shape of the generalization curve under concurrent VR VR schedules was more often graded than quantal in shape. Thus, drug discrimination can be established under concurrent VR VR schedules, but the shapes of drug-discrimination dose-response curves under concurrent VR VR schedules more closely resemble those seen under interval schedules than those seen under fixed-ratio schedules. Graded dose-response curves under concurrent VR VR schedules may relate to probability matching and difficulty in discriminating differences in reinforcement frequency.     

Fixed-ratio discrimination: effects of intermittent reinforcement

Four pigeons had discrimination training that required the choice of a left side-key after completing a fixed-ratio 10 on the center key, and a right side-key choice after fixed-ratio 20. Correct choices were reinforced on various fixed-interval, fixed-ratio, random-interval, and random-ratio schedules. When performance was examined across successive 15-second intervals (fixed-interval and fixed-ratio schedules) accuracy was high in the first 15-second interval, decreased in one or several of the next 15-second intervals, and then increased again as reinforcement was approached. When performance was examined across correct trials on fixed-interval and fixed-ratio schedules, accuracy was lowest immediately after reinforcement, followed by a systematic increase in accuracy as the number of correct choices increased. These patterns were due primarily to errors on fixed-ratio 20 trials. Systematic accuracy patterns did not occur on random-interval or random-ratio schedules. The results indicate that when choice patterns differed on fixed-interval and fixed-ratio schedules, the differences were due to the method of data analysis.     

Reporting contingencies of reinforcement in concurrent schedules

Five pigeons were trained on concurrent variable-interval schedules in which two intensities of yellow light served as discriminative stimuli in a switching-key procedure. A conditional discrimination involving a simultaneous choice between red and green keys followed every reinforcer obtained from both alternatives. A response to the red side key was occasionally reinforced if the prior reinforcer had been obtained from the bright alternative, and a response to the green side key was occasionally reinforced if the prior reinforcer had been obtained from the dim alternative. Measures of the discriminability between the concurrent-schedule alternatives were obtained by varying the reinforcer ratio for correct red and correct green responses across conditions in two parts. Part 1 arranged equal rates of reinforcement in the concurrent schedule, and Part 2 provided a 9:1 concurrent-schedule reinforcer ratio. Part 3 arranged a 1:9 reinforcer ratio in the conditional discrimination, and the concurrent-schedule reinforcer ratio was varied across conditions. Varying the conditional discrimination reinforcer ratio did not affect response allocation in the concurrent schedule, but varying the concurrent-schedule reinforcer ratio did affect conditional discrimination performance. These effects were incompatible with a contingency-discriminability model of concurrent-schedule performance (Davison & Jenkins, 1985), which implies a constant discriminability parameter that is independent of the obtained reinforcer ratio. However, a more detailed analysis of conditional discrimination performance showed that the discriminability between the concurrent-schedule alternatives decreased with time since changing over to an alternative. This effect, combined with aspects of the temporal distribution of reinforcers obtained in the concurrent schedules, qualitatively predicted the molar results and identified the conditions that operate whenever contingency discriminability remains constant.     

Signalled and unsignalled percentage reinforcement of performance under a chained schedule

Pigeons were trained to peck a key under a chained fixed-ratio 15 fixed-interval 25-sec schedule of food presentation. In Experiment 1, blocks of sessions in which 100%, 75%, 50%, and 25% of the sequences ended with food presentation were conducted. When food presentation was omitted, a timeout of equal duration replaced it. As the frequency of food presentation decreased so did the frequency of completing the chained schedule. In Experiment 2, 75%, 50%, or 25% of the sequences terminated with food presentation and outcomes were signalled, i.e., completion of the fixed ratio resulted in either a stimulus correlated with the fixed-interval 25-sec schedule or a stimulus correlated with extinction. As the frequency of food presentation decreased, the number of sequences completed per session increased for two pigeons and remained high for a third. In Experiments 3 and 4, assessments of the effects of signalling the outcome of the chained schedule were made with response-independent presentation of events at the end of the sequence. Again, signalling the outcome of the chained schedule led to more chains being completed per session than did not signalling the outcome. Stimuli differentially paired with food presentation have powerful behavioral effects that may be attributed to the potency of these stimuli as conditioned reinforcers.     

Conditional discrimination with ambiguous stimuli.

Five pigeons learned a two-key conditional discrimination. When background color on both keys was red, pecks on the key with a horizontal line produced food. When the color was green, pecks on the key with a vertical line produced food. During part of the experiment, color was presented on only one of the keys. It was found that accuracy was higher when color was combined with the line stimulus correlated with nonreinforcement. In another part of the experiment, color was presented on both keys but a line was present only on one. Accuracy was higher when the line accompanied the nonreinforced option than when the line accompanied the reinforced option. Superior performance when the combined stimuli were displayed on the nonfood key may be explained by the association of different components of the compound stimuli with reinforcement or as the result of rules pigeons follow in solving conditional discriminations.     

Factors affecting conditional discrimination learning by pigeons

In Experiment 1 (within subjects) and Experiment 2 (between subjects) it was shown that the sequential training of pigeons on a color discrimination and then on its reversal, each in a different floor-tilt/texture context, failed to produce conditional control of discriminative performance by those contexts. Daily alternation between the two problems (with correlated contexts) was successful, however. In each of these experiments conditional control was better reflected in generalization test performance in extinction than during sessions of training with reinforcement.