Multiple schedules in practical application: Research trends and implications for future investigation

Researchers began studying multiple schedules in basic laboratories, but recent advances have extended research on multiple schedules to a wide variety of socially significant applications, especially during the last decade. Applied researchers have used multiple schedules to (a) promote stimulus control over high‐rate appropriate behaviors, (b) thin the schedule of reinforcement following functional communication training, and (c) obtain stimulus control over problem behaviors maintained by automatic reinforcement. In the current paper, we reviewed 31 studies with 147 applications identified through a search of the applied literature on multiple schedules. Using these studies, we (a) reviewed the empirical literature on multiple schedules, (b) recommended multiple‐schedule procedures that serve as best practice guidelines for applied behavior analysts, (c) identified the generality and boundaries of current knowledge about the effectiveness of multiple schedules, and (d) critically analyzed the literature to provide directions for future multiple‐schedule research.     

Comparison of naturalistic and arbitrary discriminative stimuli during schedule thinning following functional communication training

We replicated and extended research on incorporating naturalistic discriminative stimuli into schedule thinning following functional communication training with three boys with autism spectrum disorder who engaged in severe behavior. Comparing naturalistic to arbitrary discriminative stimuli, two participants demonstrated differentiated communication in fewer sessions when arbitrary stimuli were used, while the third participant mastered the discriminations in a comparable number of sessions. Although previous research has demonstrated success in rapidly thinning the schedule with arbitrary stimuli, we extended this line of research by evaluating the extent to which differentiated communication would maintain during rapid schedule thinning in both naturalistic and arbitrary conditions. Two participants' communication remained differentiated, and in both conditions, during rapid schedule thinning. However, neither discrimination maintained for the third participant. Results are discussed in terms of the existing literature and directions for future research.     

Effects of ratio reinforcement schedules on discrimination performance by Japanese monkeys

In Experiment 1, Japanese monkeys were trained on three conditional position-discrimination problems with colors as the conditional cues. Within each session, each problem was presented for two blocks of ten reinforcements; correct responses were reinforced under continuous-reinforcement, fixed-ratio 5, and variable-ratio 5 schedules, each assigned to one of the three problems. The assignment of schedules to problems was rotated a total of three times (15 sessions per assignment) after 30 sessions of acquisition training. Accuracy of discrimination increased to a moderate level with fewer trials under CRF than under ratio schedules. In contrast, the two ratio schedules, fixed and variable, were more effective in maintaining accurate discrimination than was CRF. With further training, as asymptotes were reached, accuracy was less affected by the schedule differences. These results demonstrated an interaction between the effects of reinforcement schedules and the level of acquisition. In Experiment 2, ratio sizes were gradually increased to 30. Discrimination accuracy was maintained until the ratio reached 20; ratio 30 strained the performance. Under FR conditions, accuracy increased as correct choice responses cumulated after reinforcement.     

Discriminability of frequency of food or stimulus presentations in variable-time schedules

Pigeons responded in a two-alternative forced-choice task in which reinforcement was dependent upon the frequency of events that occurred in an immediately preceding schedule sample. On a given trial the events were either brief food presentations or brief visual and auditory stimulus changes. High levels of stimulus control were obtained by food-presentation schedules only. Discriminative control by frequency or stimulus change was absent. Stimulus control by food frequency was decreased by the imposition of a delay period between the schedule sample and the choice. Moreover, stimulus control by food frequency was related to the ratio of food-presentation schedule pairs when novel schedules were presented in a transfer test.     

Second-order schedules: discrimination of components

Pigeons were exposed to a series of second-order schedules in which the completion of a fixed number of fixed-interval components produced food. In Experiment 1, brief (2 sec) stimulus presentations occurred as each fixed-interval component was completed. During the brief-stimulus presentation terminating the last fixed-interval component, a response was required on a second key, the brief-stimulus key, to produce food. Responses on the brief-stimulus key before the last brief-stimulus presentation had no scheduled consequences, but served as a measure of the extent to which the final component was discriminated from preceding components. Whether there were one, two, four, or eight fixed-interval components, responses on the brief-stimulus key occurred during virtually every brief-stimulus presentation. In Experiment 2, an attempt was made to punish unnecessary responses on the brief-stimulus key, i.e., responses on the brief-stimulus key that occurred before the last component. None of the pigeons learned to withhold these responses, even though they produced a 15-sec timeout and loss of primary reinforcement. In Experiment 3, different key colors were associated with each component of a second-order schedule (a chain schedule). In contrast to Experiment 1, brief-stimulus key responses were confined to the last component. It was concluded that pigeons do not discriminate well between components of second-order schedules unless a unique exteroceptive cue is provided for each component. The relative discriminability of the components may account for the observed differences in initial-component response rates between comparable brief-stimulus, tandem, and chain schedules.     

The effects of morphine on fixed-interval patterning and temporal discrimination

Changes produced by drugs in response patterns under fixed-interval schedules of reinforcement have been interpreted to result from changes in temporal discrimination. To examine this possibility, this experiment determined the effects of morphine on the response patterning of 4 pigeons during a fixed-interval 1-min schedule of food delivery with interpolated temporal discrimination trials. Twenty of the 50 total intervals were interrupted by choice trials. Pecks to one key color produced food if the interval was interrupted after a short time (after 2 or 4.64 s). Pecks to another key color produced food if the interval was interrupted after a long time (after 24.99 or 58 s). Morphine (1.0 to 10.0 mg/kg) decreased the index of curvature (a measure of response patterning) during fixed intervals and accuracy during temporal discrimination trials. Accuracy was equally disrupted following short and long sample durations. Although morphine disrupted temporal discrimination in the context of a fixed-interval schedule, these effects are inconsistent with interpretations of the disruption of response patterning as a selective overestimation of elapsed time. The effects of morphine may be related to the effects of more conventional external stimuli on response patterning.     

INFLUENCES ON COCAINE TOLERANCE ASSESSED UNDER A MULTIPLE CONJUNCTIVE SCHEDULE OF REINFORCEMENT

Under multiple schedules of reinforcement, previous research has generally observed tolerance to the rate‐decreasing effects of cocaine that has been dependent on schedule‐parameter size in the context of fixed‐ratio (FR) schedules, but not under the context of fixed‐interval (FI) schedules of reinforcement. The current experiment examined the effects of cocaine on key‐pecking responses of White Carneau pigeons maintained under a three‐component multiple conjunctive FI (10 s, 30 s, & 120 s) FR (5 responses) schedule of food presentation. Dose‐effect curves representing the effects of presession cocaine on responding were assessed in the context of (1) acute administration of cocaine (2) chronic administration of cocaine and (3) daily administration of saline. Chronic administration of cocaine generally resulted in tolerance to the response‐rate decreasing effects of cocaine, and that tolerance was generally independent of relative FI value, as measured by changes in ED50 values. Daily administration of saline decreased ED50 values to those observed when cocaine was administered acutely. The results show that adding a FR requirement to FI schedules is not sufficient to produce schedule‐parameter‐specific tolerance. Tolerance to cocaine was generally independent of FI‐parameter under the present conjunctive schedules, indicating that a ratio requirement, per se, is not sufficient for tolerance to be dependent on FI parameter.     

An evaluation of the value of choice-making opportunities in single-operant arrangements: simple fixed- and progressive-ratio schedules

The current study compared the effects of choice and no-choice reinforcement conditions on the task responding of 3 children with autism across 2 single-operant paradigm reinforcer assessments. The first assessment employed simple fixed-ratio (FR) schedules; the second used progressive-ratio (PR) schedules. The latter assessment identified the differential strength of choice-making conditions in promoting task responding relative to no-choice conditions for 2 participants; no differential findings were obtained during the FR assessment.     

Matching: its acquisition and generalization

Choice typically is studied by exposing organisms to concurrent variable-interval schedules in which not only responses controlled by stimuli on the key are acquired but also switching responses and likely other operants as well. In the present research, discriminated key-pecking responses in pigeons were first acquired using a multiple schedule that minimized the reinforcement of switching operants. Then, choice was assessed during concurrent-probe periods in which pairs of discriminative stimuli were presented concurrently. Upon initial exposure to concurrently presented stimuli, choice approximated exclusive preference for the alternative associated with the higher reinforcement frequency. Concurrent schedules were then implemented that gave increasingly greater opportunities for switching operants to be conditioned. As these operants were acquired, the relation of relative response frequency to relative reinforcement frequency converged toward a matching relation. An account of matching with concurrent schedules is proposed in which responding exclusively to the discriminative stimulus associated with the higher reinforcement frequency declines as the concurrent stimuli become more similar and other operants-notably switching-are acquired and generalize to stimuli from both alternatives. The concerted effect of these processes fosters an approximate matching relation in commonly used concurrent procedures.     

A comparison of token and tandem schedules of reinforcement on response patterns for adolescents with autism

Token schedules of reinforcement are ubiquitous in clinical settings, yet little research has thoroughly evaluated the effects of token schedules on responding in clinical settings. Basic research has shown token schedules of reinforcement produce lower response rates and larger pre‐ratio pauses compared to tandem schedules. The purpose of the current study was to determine whether the same effects are produced with adolescents with autism or related disorders. We examined response patterns under otherwise identical FR token and FR tandem schedules. Tokens suppressed responding for one participant only under high schedule values and for a second participant under common clinical schedule values; no difference in responding occurred between token and tandem schedules for two participants. These results support the systematic evaluation of token schedules of reinforcement in clinical settings. Additional applied research is needed on token schedules to further our understanding of the underlying mechanisms that contribute to the overall effectiveness of token economies.     

Brief-stimulus presentations on multiform tandem schedules

Three experiments examined the influence of a brief stimulus (a light) on the behavior of food-deprived rats whose lever pressing on tandem schedules comprising components of different schedule types resulted in food presentation. In Experiment 1, either a tandem variable-ratio variable-interval or a tandem variable-interval variable-ratio schedule was used. The variable-interval requirement in the tandem variable-ratio variable-interval schedule was yoked to the time taken to complete the variable-ratio component in the tandem variable-interval variable-ratio schedule, and the length of the variable-interval component in the latter schedule was yoked to the variable-ratio component in the former schedule. If a brief stimulus occurred following completion of the first component, then behavior was differentiated in the two components; subjects responded more quickly in the variable-ratio than in the variable-interval component. If the stimulus was removed, then response rate was determined by the nature of the final component. Similar results were obtained in Experiments 2 and 3 with the use of a three-component tandem variable-ratio variable-interval variable-ratio schedule or tandem variable-interval variable-ratio variable-interval schedule. Thus, a brief stimulus that was not explicitly paired with reinforcement engendered behavior typical of the component schedule preceding its presentation.     

Effects consistent with behavioral contrast during a multiple schedule for discrimination training on mands for attention

This study used a multiple schedule to assess the effects of an S+ or S- in the absence of rules on excessive mand rates with three school-aged children with disabilities and assessed possible contrast effects occurring in conjunction with the intervention. Each of the three participants was exposed to the presence or absence of an S- (black baseball cap). When it was worn, mand approach rates decreased for all three participants; conversely, mand rates increased to twice the baseline rates when the experimenter was not wearing the black baseball cap. Generalization probes conducted for one of the three caregivers showed similar changes when the caregiver wore the cap. Results suggest that effects consistent with positive behavioral contrast occurred with all participants.     

Synthesizing concurrent interval performances

Concurrent schedules may be viewed as consisting of two pairs of stay and switch schedules, each pair associated with one of the alternatives. A stay schedule arranges reinforcers for staying and responding at one alternative, whereas the associated switch schedule arranges reinforcers for switching to the other alternative. In standard concurrent schedules, the stay schedule at each alternative is equivalent to the switch schedule at the other alternative. MacDonall (1999) exposed rats to one pair of stay and switch variable-ratio schedules and varied the response requirements across conditions. Combining results from symmetric pairs produced composite performances that were described by the generalized matching law. This outcome was noteworthy because the data were obtained from performances at two alternatives with contingencies that were functionally unrelated to each other. This result suggests that concurrent performances may consist of two unrelated performances that alternate as behavior moves between alternatives. The purpose of the present experiment was to extend those results to interval schedules. Rats were exposed to pairs of random-interval schedules, and across conditions their mean intervals were varied. When data from appropriately paired conditions were combined, the composite performances were consistent with the generalized matching law. In addition, the results supported two models of concurrent performances that were based on local variables at an alternative (behavior, and stay and switch reinforcers): a modified version of the contingency discrimination model (Davison & Jenkins, 1985) and the local model (MacDonall, 1999).     

Progressive-ratio schedules: effects of later schedule requirements on earlier performances

Four rats were studied with variants of a progressive-ratio schedule with a step size of 6 in which different terminal components followed completion of the 20th ratio: (a) a reversal of the progression, (b) a fixed-ratio 6 schedule, or (c) extinction. Responding in the progressive-ratio components of these schedules was compared to performances under conventional progressive-ratio baselines. Under baseline conditions, postreinforcement pauses increased exponentially as a function of increasing ratio size, whereas running rates showed modest declines. The procedure of linking the progressive-ratio schedule to the reversed progression or to the fixed-ratio component resulted in decreased pausing. Linking the progressive-ratio schedule to the extinction component had the opposite effect, that of producing weakened progressive-ratio performances as evidenced by increased pausing. Subjects whose responses were reinforced on half of the ratios also showed exponential increases; however, pauses were substantially shorter following ratios on which the reinforcer was omitted. The results suggested that progressive-ratio pausing reflects the influence of remote as well as local contingencies.     

Drug discrimination in rats under concurrent variable-interval variable-interval schedules

Eight rats were trained to discriminate pentobarbital from saline under a concurrent variable-interval (VI) VI schedule, on which responses on the pentobarbital-biased lever after pentobarbital were reinforced under VI 20 s and responses on the saline-biased lever were reinforced under VI 80 s. After saline, the reinforcement contingencies programmed on the two levers were reversed. The rats made 62.3% of their responses on the pentobarbital-biased lever after pentobarbital and 72.2% on the saline-biased lever after saline, both of which are lower than predicted by the matching law. When the schedule was changed to concurrent VI 50 s VI 50 s for test sessions with saline and the training dose of pentobarbital, responding on the pentobarbital-biased lever after the training dose of pentobarbital and on the saline-biased lever after saline became nearly equal, even during the first 2 min of the session, suggesting that the presence or absence of the training drug was exerting minimal control over responding and making the determination of dose-effect relations of drugs difficult to interpret. When the pentobarbital dose-response curve was determined under the concurrent VI 50-s VI 50-s schedule, responding was fairly evenly distributed on both levers for most rats. Therefore, 6 additional rats were trained to respond under a concurrent VI 60-s VI 240-s schedule. Under this schedule, the rats made 62.6% of their responses on the pentobarbital-biased lever after pentobarbital and 73.5% of their responses on the saline-biased lever after saline, which also is lower than the percentages predicted by perfect matching. When the schedule was changed to a concurrent VI 150-s VI 150-s schedule for 5-min test sessions with additional drugs, the presence or absence of pentobarbital continued to control responding in most rats, and it was possible to generate graded dose-response curves for pentobarbital and other drugs using the data from these 5-min sessions. The dose-response curves generated under these conditions were similar to the dose-response curves generated using other reinforcement schedules and other species.     

Additive summation following intradimensional discrimination training

Rats were exposed to intradimensional composite stimuli presented on the response lever that varied in both light intensity and flicker rate. For all subjects, pressing the lever was reinforced when it was illuminated at a high intensity and flickered at a low rate (I + f) or when it was illuminated at a low intensity and flickered at a high rate (i + F). For half the subjects, lever responding was not reinforced when it was illuminated at a low intensity and flickering at a low rate (i + f). For the remaining subjects, lever presses were not reinforced when the lever was illuminated at a high intensity and flickered at a high rate (I + F). When the composite stimulus composed of the light intensity and flicker rates that had been associated only with reinforced responding was displayed (I + F for half the subjects and i + f for the remaining subjects), it controlled the highest response rate of all stimuli (additive summation). The results demonstrated that similar attentional processes control intra- and interdimensional composite-stimulus discriminations in a manner consistent with Weiss' (1972) analysis of summation.     

Successive discrimination training with equated reinforcement frequencies: failure to obtain behavioral contrast

In two experiments, pigeons were trained on two-component multiple schedules in which responding in one component (S₁) was always maintained by a variable-interval schedule. In Experiment I, low response rates were reinforced in the second (S₂) component for six master subjects. This schedule was adjusted to equate reinforcement frequencies in the two components. These subjects were compared to yoked partners, for which reinforcement in the S₂ component was made available on a variable-interval schedule whose value was determined by the master subjects. A similar procedure was used in Experiment II, where the S₂ schedule for master subjects made reinforcers contingent on the absence of responding. No evidence was found in either experiment for a behavioral contrast effect in the S₁ component attributable to response reduction in the S₂ component. A reliable contrast effect was obtained from a group of pigeons given extinction conditions in the S₂ component, which was compared to a group maintained throughout on a multiple variable-interval schedule. The results suggest that previous indications of behavioral contrast in similar situations were probably caused by uneven reinforcement distributions or reflect uncontrolled fluctuations in response rates.     

The effectiveness of and preschoolers' preferences for variations of multiple-schedule arrangements

Multiple schedules of reinforcement have been used to teach children to recruit attention only when it is available, thereby minimizing disruptive requesting during instructional activities. This procedure involves alternating periods of continuous reinforcement (CRF) with periods of extinction and correlating each period with a distinct and continuous discriminative stimulus. The present study evaluated the effectiveness of and children's preferences for multiple schedules in which (a) two different stimuli, one correlated with reinforcement (S+) and another correlated with extinction (S-), were presented; (b) only an S+ was presented (i.e., no stimulus was correlated with extinction), and (c) neither an S+ nor an S- was presented (i.e., a mixed schedule). S+/S- and S+ arrangements were similarly effective for 7 children, but 3 preferred the S+/S- condition and 4 preferred the S+ condition. Correlational analyses suggested that children who responded more effectively given the S- (discrimination indexes were relatively high) preferred the S+/S- condition, whereas children who responded less effectively given the S- preferred the S+ condition. The implications of these findings for arranging multiple schedules for social responses are discussed.     

Rats' performance on variable-interval schedules with a linear feedback loop between response rate and reinforcement rate

Three experiments investigated whether rats are sensitive to the molar properties of a variable-interval (VI) schedule with a positive relation between response rate and reinforcement rate (i.e., a VI+ schedule). In Experiment 1, rats responded faster on a variable ratio (VR) schedule than on a VI+ schedule with an equivalent feedback function. Reinforced interresponse times (IRTs) were shorter on the VR as compared to the VI+ schedule. In Experiments 2 and 3, there was no systematic difference in response rates maintained by a VI+ schedule and a VI schedule yoked in terms of reinforcement rate. This was found both when the yoking procedure was between-subject (Experiment 2) and within-subject (Experiment 3). Mean reinforced IRTs were similar on both the VI+ and yoked VI schedules, but these values were more variable on the VI+ schedule. These results provided no evidence that rats are sensitive to the feedback function relating response rate to reinforcement rate on a VI+ schedule.