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.     

Alternative fixed-ratio fixed-interval schedules of reinforcement

Five rats were trained under alternative fixed-ratio fixed-interval schedules, in which food reinforcement was provided for the completion of either a fixed-ratio or a fixed-interval requirement, whichever was met first. Overall response rate and running rate (the rate of responding after the postreinforcement pause) decreased for all subjects as the fixed-ratio value increased. As the proportion of reinforcements obtained from the fixed-ratio component increased and the alternative schedule approached a simple fixed ratio, overall response rate and running rate both increased; conversely, as the proportion of reinforcements obtained from the fixed-interval component increased and the alternative schedule approached a simple fixed interval, response rates decreased. Postreinforcement pause length increased linearly as the average time between reinforcements increased, regardless of the schedule parameters. A break-run pattern of responding was predominant at low- and medium-valued fixed ratios. All subjects displayed at least occasional positively accelerated responding within interreinforcement intervals at higher fixed-ratio values.     

Conjunctive schedules of reinforcement: I. Rate-dependent effects of pentobarbital and d-amphetamine

Key pecking of pigeons was maintained under conjunctive schedules of food presentation in which both a fixed-interval and a fixed-ratio schedule had to be completed before a peck produced food. For two pigeons, pecks on a single key completed both schedule requirements (fixed-interval 3-min, fixed-ratio 50 for one bird, fixed-interval 5-min, fixed-ratio 50 for the second). For two other pigeons, each requirement was scheduled on a separate key. On the two-key schedule, a peck after 5 min on the key scheduling the fixed-interval requirement produced food if at least 10 pecks had occurred on the ratio key (conjunctive fixed-interval 5-min, fixed-ratio 10). When each requirement was scheduled on a separate key, response rates on the fixed-ratio key were generally higher in the early portion of the interval and declined as the interval progressed; responding on the fixed-interval key, once initiated, typically remained at a constant rate throughout the interval. Responding under the single-key schedule was characterized by a high rate early in the interval; this then changed to a lower rate that continued until a peck produced food. For all pigeons, increases in response rates with pentobarbital and d-amphetamine were inversely related to the control rate of responding. When equivalent rates on each key of the two-key schedule were compared, both drugs increased rates on the fixed-ratio key less. Although the effects of both drugs were rate dependent, each drug differentially modified the pattern of responding under the single-key schedule.     

Effects of d-amphetamine and pentobarbital under concurrent fixed-ratio schedules.

Pigeons were studied under a two-key concurrent fixed-ratio schedule of food presentation. During the first five sessions, the fixed-ratio requirements were 30 responses on one key (major key) and 120 responses on the other key (minor key): responding occurred almost exclusively on the major key. When the fixed-ratio requirements were then made equal at 30 responses on both keys, responding continued to predominate on the major key. The asymmetric distribution of responses persisted when the concurrent fixed-ratio fixed-ratio schedule was interrupted with periods during which the major key was associated with extinction while the other key remained associated with a fixed-ratio schedule. Additionally, in some subjects the fixed-ratio requirements were increased. These schedule modifications decreased the asymmetry in responding but did not eliminate it. d-Amphetamine decreased rates on both keys and slightly increased the asymmetric distribution of responses, while pentobarbital reversed the distribution of responses by increasing low rates and decreasing high rates. The pigeons maintained their original asymmetric distribution of responses during the 1 1/2-year-long study, despite schedule alterations and drug administrations.     

Effects of methadone on alternative fixed-ratio fixed-interval performance: latent influences on schedule-controlled responding.

Effects of methadone on pigeons' key pecking were examined under four conditions selected to analyze the control of behavior under alternative fixed-ratio fixed-interval schedules. In Condition 1, pigeons pecked under one of three different alternative schedules (alternative fixed-ratio 50 fixed-interval 90 s, alternative fixed-ratio 75 fixed-interval 90 s and alternative fixed-ratio 200 fixed-interval 90 s) each week. In Condition 2, fixed-ratio 50 or fixed-ratio 75 schedules were in effect during baseline sessions, and alternative fixed-ratio 50 fixed-interval 90-s or alternative fixed-ratio 75 fixed-interval 90-s schedules were in effect during sessions in which methadone was administered. In Condition 3, effects of methadone on key pecking maintained under fixed-ratio 50 and fixed-ratio 75 schedules were examined, whereas in Condition 4 the effects of methadone on key pecking under a fixed-interval 90-s schedule as well as fixed-ratio 50 and fixed-ratio 75 schedules were investigated. Control by the fixed-interval contingency was assessed by computing the proportion of total session reinforcers delivered under the fixed-interval schedule. Methadone administration (0.5-4.0 mg/kg) shifted the predominant source of schedule control under the alternative schedule from the fixed-ratio schedule to the fixed-interval contingency. This shift was dependent on methadone dose and fixed-ratio size. Control by the fixed-interval contingency was greatest following extensive exposure to the interval component embedded within the alternative schedule (Condition 1), but was apparent to a lesser degree with even very limited exposure to the alternative fixed-ratio fixed-interval schedule (Condition 2). Interreinforcement intervals comparable to those under fixed-interval schedule were not observed under the fixed-ratio schedules presented alone (Condition 3). Repeated exposure to the fixed-interval contingency outside the context of the alternative fixed-ratio fixed-interval schedule did not engender performance changes under a fixed-ratio schedule which would mimic those of increased fixed-interval contingency control (Condition 4). These data suggest that drug administration can be used to unmask the influence of contingencies that are latent under baseline conditions and reveal influences of both past and present environmental variables.     

Responding under fixed-ratio and multiple fixed-interval fixed-ratio schedules of electric shock presentation

Squirrel monkeys, initially trained under a schedule of electric shock postponement and then under fixed-interval schedules of electric shock presentation, were studied under multiple fixed-interval fixed-ratio and under fixed-ratio schedules of shock presentation. Under the fixed-interval (10-min) component of the multiple schedule, a pause was followed by a gradual increase in responding to a rate maintained until shock presentation; under the fixed-ratio (3-, 10-, or 30-response) component of the multiple schedule, a brief pause was typically followed by a relatively high and uniform rate of responding until shock was presented. When the 60-sec timeout periods, which usually followed shock presentation, were eliminated from the multiple schedule for one monkey, responding was only transiently affected. In the one monkey studied, responding was maintained under a fixed-ratio schedule alone (with timeout periods), but rates of responding were lower than under the fixed-ratio component of the multiple schedule. Characteristic patterns of responding, similar to those engendered under schedules of food presentation or shock termination, can be maintained under fixed-ratio schedules of shock presentation; further, patterns of responding can be controlled by discriminative stimuli in multiple schedules.     

Neurochemical changes correlated with behavior maintained under fixed-interval and fixed-ratio schedules of reinforcement.

Key pecking of 4 pigeons was maintained under a multiple 3-min fixed-interval, 30-response fixed-ratio schedule of food presentation. Only one schedule was in effect during an experimental session, and each was correlated with a different keylight stimulus and location (left vs. right). The different schedule components alternated across days or weeks. Cerebrospinal fluid was collected from chronically implanted intracerebroventricular cannulae following sessions with the different schedules, as well as following sessions in which reinforcement was withheld (extinction), when response-independent food was delivered, and when the experimental chamber was dark and there were no scheduled events. Metabolites of the neurotransmitters serotonin, norepinephrine, and dopamine were assayed in cerebrospinal fluid using high-performance liquid chromatography with electrochemical detection. Compared to the fixed-ratio condition, responding maintained under the fixed-interval schedule resulted in consistently higher levels of the serotonin metabolite 5-hydroxyindoleacetic acid and of the dopamine metabolite homovanillic acid in all pigeons. Levels of 3-methoxy-4-hydroxyphenylethylene glycol, a metabolite of norepinephrine, and dihydroxyphenylacetic acid, another dopamine metabolite, were also higher in 3 of the 4 pigeons following exposure to the fixed-interval schedules when compared to levels of these metabolites after exposure to the fixed-ratio schedule. Extinction of fixed-ratio responding resulted in large increases in 5-hydroxyindoleacetic acid compared to levels of this metabolite under the fixed-ratio schedule, whereas this serotonin metabolite decreased during extinction of responding under the fixed-interval schedule. Control procedures suggested that the neurochemical changes were not related to the rate of responding but were a function of the specific experimental conditions. Distinctive neurochemical changes that accompany schedule-controlled responding show the sensitivity of the neurochemical environment to behavioral contingencies and demonstrate further the profound impact that such contingencies have on biobehavioral processes.     

Interaction of methadone, reinforcement history, and variable-interval performance.

In the present study, we examined how a reinforcement schedule history that generated high or low rates of responding influenced the effects of acute (Experiment 1) and chronic (Experiment 2) methadone administration. Initially, key-peck responses of pigeons were maintained under a variable-interval 90-s schedule of food presentation, and a methadone dose-response curve was determined with doses of 0.6, 1.2, and 2.4 mg/kg. The pigeons were then exposed, for at least 40 sessions, to either a fixed-ratio 50 schedule or a differential-reinforcement-of-low-rate 10-s schedule, or were given continued exposure to the variable-interval schedule. The methadone dose-response curve was redetermined after all pigeons again were responding under the variable-interval schedule. The effects of two different daily methadone doses (9.0 and 12.0 mg/kg/day) and withdrawal precipitated by naloxone also were assessed. Experience with a fixed-ratio or differential reinforcement of low rate schedule did not result in significantly different response rates under the variable-interval schedule and, in general, the acute effects of methadone did not have differential effects correlated with schedule history. However, for 2 of 4 subjects the rate-decreasing effects of methadone on rates of key pecking were greater following a history of low-rate responding, suggesting a possible interaction between schedule history and effects of methadone. Daily methadone administration under the variable-interval schedule revealed that pigeons with experience under the differential reinforcement of low rate schedule developed more rapid and complete tolerance to the rate-decreasing effects of methadone. Three of the 4 subjects in this group showed rate increases above drug-free baselines during chronic methadone dosing. Pigeons with a history of fixed-ratio responding also developed tolerance to the rate-decreasing effects of methadone but without the subsequent rate increases seen by subjects with low-rate histories. No subjects with variable-interval histories showed complete recovery of drug-free baselines, suggesting that interpolated training under other schedules may attenuate the rate-altering effects of chronically administered drugs. Naloxone (1.0 mg/kg), administered during the chronic methadone phase, resulted in greater disruption of responding by pigeons with a history of low-rate responding, as compared to subjects in the other two groups.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effects of chlorpromazine on fixed-ratio responding: modification by fixed-interval discriminative stimuli.

Effects of chlorpromazine (1 to 100 mg/kg) were assessed on two pigeons' responding under various modifications of a multiple schedule of food delivery. During a fixed-interval component, the first response after 5 min produced food; during the subsequent, fixed-ratio component, the 30th response produced food. Modifications of the schedule entailed changes in stimulus conditions imposed during the fixed-ratio component that did not systematically alter characteristics of performance under non-drug conditions. In the first phase of the experiment, distinctive visual stimuli were correlated with each schedule component (conventional multiple schedule); chlorpromazine produced small decreases in fixed-ratio responding (20% at 30 mg/kg). When each response during the fixed-ratio component produced the stimulus correlated with the fixed-interval schedule (fixed-interval discriminative stimulus) for 1.2 s, effects of chlorpromazine were not different from those under the conventional multiple schedule. Chlorpromazine produced greater decreases in fixed-ratio responding (55% at 30 mg/kg) when either the first response of each fixed ratio changed the stimulus correlated with the fixed-ratio schedule to the fixed-interval discriminative stimulus for the remainder of the fixed-ratio component, or when the fixed-interval discriminative stimulus was presented independently of responding according to a matched temporal sequence. When the fixed-interval discriminative stimulus was present continuously during the fixed-ratio component (mixed schedule), chlorpromazine produced even more substantial decreases in fixed-ratio responding (greater than 80% at 30 mg/kg). Effects of chlorpromazine on fixed-interval responding were also modified by the schedules of fixed-interval discriminative stimulus presentation. The effects of chlorpromazine were a joint function of the stimuli prevailing during the multiple schedule and the degree to which responding influenced these stimuli.     

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.     

d-amphetamine and fixed-interval performance: effects of operant history.

Sixteen rats were initially exposed for 50 sessions to either a fixed-ratio 40 or an interresponse-time-greater-than-11-second food reinforcement schedule, then shifted to a fixed-interval 15-second food reinforcement schedule. Animals with fixed-ratio 40 histories lever pressed at much higher rates under the fixed-interval schedule than did animals with inter-response-time-greater-than-11-second histories. This difference persisted across 93 sessions of fixed-interval exposure. The effects of d=amphetamine were assessed after 15 and 59 sessions of fixed-interval exposure. On both occasions, the low-rate responding of animals with interresponse-time-greater-than-11-second histories was typically increased by all doses of the drug, while the high-rate responding of animals with fixed-ratio 40 histories was typically decreased by all doses of the drug. These results suggest that control response rate under the fixed-interval schedule, which may be affected by a history of responding under another schedule, is the primary determinant of the relative effects of d-amphetamine.     

Conjunctive schedules of reinforcement II: response requirements and stimulus effects

Responding of three pigeons was maintained under conjunctive fixed-ratio, fixed-interval schedules where a key peck produced food after both schedule requirements were completed. The individual schedule requirements were then successively removed and reinstated with responding maintained under the following conditions: conjunctive fixed-ratio, fixed-time; fixed-time; and fixed-interval schedules. Patterns of responding changed in accord with the successive removal of the schedule requirements. Compared to the conjunctive fixed-ratio, fixed-interval schedule, pause duration increased and response rate decreased under conjunctive fixed-ratio, fixed-time schedules and under fixed-time schedules alone. Overall mean rates of responding were highest and pause duration lowest under fixed-interval schedules. When changes in the keylight colors were correlated with completion of the fixed-ratio, the end of the fixed-interval, or both of these conditions, the pattern of responding was modified and indicated a greater degree of control by the individual schedules. Although two birds showed large increases in interreinforcement time when they were initially exposed to the conjunctive schedule, when responding stabilized this measure was largely invariant for all birds across most schedule conditions.     

Low-response-rate conditioning history and fixed-interval responding in rats.

Bar presses by one group of rats were conditioned under a differential-reinforcement-of-low-rate reinforcement schedule immediately prior to conditioning under a fixed-interval schedule. In a second group of rats, bar presses were conditioned first under a differential-reinforcement-of-low-rate schedule and then under a fixed-ratio schedule prior to conditioning under a fixed-interval schedule. Low response rates occurred under the fixed-interval schedule only when it was immediately preceded by low-rate conditioning. Otherwise, fixed-interval responding was similar to responding under the fixed-ratio schedule. This finding suggests that responses of laboratory animals are sensitive to immediate history, and, unlike human responses, are relatively insensitive to a history of low-rate conditioning when it is followed by high-rate conditioning.     

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.     

Matching under concurrent fixed-ratio variable-interval schedules of food presentation

Four pigeons were exposed to concurrent fixed-ratio, variable-interval schedules of food presentation. The fixed-ratio requirement was either 25, 50, 75, or 100 responses, with the variable-interval schedule parameter held constant at 4 minutes. A delay time was imposed between a changeover from one schedule to the other and subsequent food availability. The delay time was varied at each ratio requirement over four values; no delay, 0-second delay, 1.5-second delay, and 5.0-second delay. As the fixed-ratio requirement or the delay time increased, a greater proportion of the total responses and time spent responding occurred under the variable-interval schedule relative to the proportion of food deliveries under that schedule. Neither relative overall response rate nor relative time spent responding equalled the relative frequency of food presentation, as would be predicted by a linear “matching” model. Rather, these data were described by power functions with slopes of approximately 1.0 and intercepts greater than 1.0. In the terms of Baum's (1974) analysis, these deviations from linear matching represent bias in favor of responding under the interval schedule. Bias, as reflected in the intercept of the power function, was greater for the ratio of time than the ratio of responses.     

Effects of d-amphetamine and caffeine on schedule-controlled and schedule-induced responding.

The effects of d-amphetamine and caffeine were studied on rates and patterns of lever pressing and schedule-induced licking under fixed-interval schedules of food pellet presentation. In addition, the effects of caffeine were studied on lever pressing and licking under a multiple fixed-ratio fixed-interval schedule. Caffeine reduced mean overall rates of licking at lower doses than it reduced mean overall rates of pressing under the fixed-interval schedules, but the effects of caffeine on both licking and lever pressing depended largely on the control rate of responding. d-Amphetamine reduced mean overall rates of lever pressing and licking at about the same dose, but the effects of d-amphetamine also were a function of the control rate of responding.     

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.     

Effects of cocaine on performance under fixed-interval schedules with a small tandem ratio requirement

Daily administration of cocaine often results in the development of tolerance to its effects on responding maintained by fixed-ratio schedules. Such effects have been observed to be greater when the ratio value is small, whereas less or no tolerance has been observed at large ratio values. Similar schedule-parameter-dependent tolerance, however, has not been observed with fixed-interval schedules arranging comparable interreinforcement intervals. This experiment examined the possibility that differences in rate and temporal patterning between the two types of schedule are responsible for the differences in observed patterns of tolerance. Five pigeons were trained to key peck on a three-component multiple (tandem fixed-interval fixed-ratio) schedule. The interval values were 10, 30, and 120 s; the tandem ratio was held constant at five responses. Performance appeared more like that observed under fixed-ratio schedules than fixed-interval schedules. Effects of various doses of cocaine given weekly were then determined for each pigeon. A dose that reduced responding was administered prior to each session for 50 days. A reassessment of effects of the range of doses revealed tolerance. The degree of tolerance was similar across components of the multiple schedule. Next, the saline vehicle was administered prior to each session for 50 days to assess the persistence of tolerance. Tolerance diminished in all subjects. Overall, the results suggested that schedule-parameter-dependent tolerance does not depend on the temporal pattern of responding engendered by fixed-ratio schedules.     

Drugs and punished responding II: d-amphetamine-induced increases in punished responding

The effects of d-amphetamine on punished responding were studied in two experiments. In Experiment I, pigeons responded under a multiple fixed-ratio 30 response fixed-interval 5-min schedule of food presentation with 60-sec limited holds in both components. Each response was punished with electric shock, the intensity of which was varied systematically. In Experiment II, another group of pigeons responded under a multiple fixed-interval 5-min fixed-interval 5-min schedule of food presentation with 40-sec limited holds. Each response was punished with shock during one component, and every thirtieth response was punished in the other component. d-Amphetamine increased overall rates of punished responding only rarely under any of the punishment conditions; however, response rates within the fixed-interval when rates were low were increased by d-amphetamine when the shock intensity was low (Experiment I), or when responses produced shock intermittently (Experiment II). The data suggest that the effects of d-amphetamine on punished responding depend on the control rate of responding, the punishment intensity, the punishment frequency, and the schedule of food presentation.     

Food-deprivation level alters the effects of morphine on pigeons'' key pecking.

Four pigeons pecked response keys under a multiple fixed-ratio 30 fixed-interval 5-min schedule of food presentation. Components alternated separated by 15-s timeouts; each was presented six times. Pigeons were maintained at 70%, 85%, and greater than 90% of their free-feeding weights across experimental conditions. When response rates were stable, the effects of morphine (0.56 to 10.0 mg/kg) and saline were investigated. Morphine reduced response rates in a dose-dependent manner under the fixed-ratio schedule and at high doses under the fixed-interval schedule. In some cases, low doses of morphine increased rates under the fixed-interval schedule. When pigeons were less food deprived, reductions in pecking rates occurred at lower doses under both schedules for 3 of 4 birds compared to when they were more food deprived. When pigeons were more food deprived, low doses of morphine increased rates of pecking in the initial portions of fixed intervals by a greater magnitude. Thus, food-deprivation levels altered both the rate-decreasing and rate-increasing effects of morphine. These effects may share a common mechanism with increased locomotor activity produced by drugs and with increased drug self-administration under conditions of more severe food deprivation.