Equal Reinforcement Rates Research Articles

Factors controlling the micro-structure of human free-operant behaviour: Bout-initiation and within-bout responses are effected by different aspects of the schedule

Two experiments examined factors controlling human free-operant performance in relation to predictions based on the nature of bout-initiation and within-bout responding. Overall, responding was higher for a random ratio (RR) than a random interval (RI) schedule, with equal rates of reinforcement. Bout-initiation rates were not different across the two schedules, but within-bout rates were higher on the RR schedule. Response cost reduced overall rates of responding, but tended to suppress bout-initiation responding more than within-bout responding (Experiments 1 & 2). In contrast, reinforcement magnitude increased all forms of responding (Experiment 2). One explanation consistent with these effects is that bout-initiation responses are controlled by overall rates of reinforcement through their impact on the context (i.e. are stimulus-driven), but that within-bout responses are controlled by response reinforcement (i.e. are goal-directed). These current findings are discussed in the light of these theoretical suggestions.

The effect of reinforcement probability on time discrimination in the midsession reversal task.

We examined how biasing time perception affects choice in a midsession reversal task. Given a simultaneous discrimination between stimuli S1 and S2, with choices of S1 reinforced during the first, but not the second half of the trials, and choices of S2 reinforced during the second, but not the first half of the trials, pigeons show anticipation errors (premature choices of S2) and perseveration errors (belated choices of S1). This suggests that choice depends on timing processes, on predicting when the contingency reverses based on session duration. We exposed 7 pigeons to a midsession reversal task and manipulated the reinforcement rate on each half of the session. Compared to equal reinforcement rates on both halves of the session, when the reinforcement rate on the first half was lower than on the second half, performance showed more anticipation and less perseveration errors, and when the reinforcement rate on the first half was higher than on the second half, performance showed a remarkable reduction of both types of errors. These results suggest that choice depends on both time into the session and the outcome of previous trials. They also challenge current models of timing to integrate local effects.

Signaled alternative reinforcement and the persistence of operant behavior.

Differential reinforcement of alternative behavior (DRA) is a treatment designed to eliminate problem behavior by reinforcing an alternative behavior at a higher rate. Availability of alternative reinforcement may be signaled, as with Functional Communication Training, or unsignaled. Whether or not alternative reinforcement is signaled could influence both the rate and persistence of problem behavior. The present study investigated whether signaling the availability of alternative reinforcement affects the rate and persistence of a concurrently available target response with pigeons. Three components of a multiple concurrent schedule arranged equal reinforcement rates for target responding. Two of the components also arranged equal reinforcement rates for an alternative response. In one DRA component, a discrete stimulus signaled the availability of response-contingent alternative reinforcement by changing the keylight color upon reinforcement availability. In the other DRA component, availability of alternative reinforcement was not signaled. Target responding was most persistent in the unsignaled DRA component when disrupted by satiation, free food presented between components, and extinction, relative to the signaled DRA and control components. These findings suggest the discrete stimulus functionally separated the availability of alternative reinforcement from the discriminative stimuli governing target responding. These findings provide a novel avenue to explore in translational research assessing whether signaling the availability of alternative reinforcement with DRA treatments reduces the persistence of problem behavior.

An evaluation of resistance to change with unconditioned and conditioned reinforcers.

Several reinforcer-related variables influence a response's resistance to change (Nevin, 1974). Reinforcer type (i.e., conditioned or unconditioned) is a reinforcer-related variable that has not been studied with humans but may have clinical implications. In Experiment 1, we identified unconditioned and conditioned reinforcers of equal preference. In Experiments 2, 3, and 4, we reinforced participants' behavior during a baseline phase using a multiple variable-interval (VI) 30-s VI 30-s schedule with either conditioned (i.e., token) or unconditioned (i.e., food; one type of reinforcement in each component) reinforcement. After equal reinforcement rates across components, we introduced a disruptor. Results of Experiments 2 and 3 showed that behaviors were more resistant to extinction and distraction, respectively, with conditioned than with unconditioned reinforcers. Results of Experiment 4, however, showed that when prefeeding disrupted responding, behaviors were more resistant to change with unconditioned reinforcers than with conditioned reinforcers.

Computational substrates of social value in interpersonal collaboration.

Decisions to engage in collaborative interactions require enduring considerable risk, yet provide the foundation for building and maintaining relationships. Here, we investigate the mechanisms underlying this process and test a computational model of social value to predict collaborative decision making. Twenty-six participants played an iterated trust game and chose to invest more frequently with their friends compared with a confederate or computer despite equal reinforcement rates. This behavior was predicted by our model, which posits that people receive a social value reward signal from reciprocation of collaborative decisions conditional on the closeness of the relationship. This social value signal was associated with increased activity in the ventral striatum and medial prefrontal cortex, which significantly predicted the reward parameters from the social value model. Therefore, we demonstrate that the computation of social value drives collaborative behavior in repeated interactions and provide a mechanistic account of reward circuit function instantiating this process.

Trimethoprim-Sulfamethoxazole or Clindamycin for Community-Associated MRSA (CA-MRSA) Skin Infections

In the United States, community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) has emerged as the predominant cause of skin infections. Trimethoprim-sulfamethoxazole (TMP-SMX) and clindamycin are often used as first-line treatment options, but clinical data are lacking. We conducted a retrospective cohort study of outpatients with skin and soft tissue infections managed from July 1 to December 31, 2006. Patients younger than 18 years of age were excluded, as were those who had no clinical admission or progress notes; were hospitalized within the 90 days before admission; were hospitalized with polymicrobial, surgical site, catheter-related, or diabetic foot infections; or were discharged to places other than home. Patient demographics, comorbidities, diagnoses, cultures, prescribed antibiotics, susceptibilities, surgical procedures, and health outcomes were extracted from electronic medical records. Patients were divided in 2 cohorts for further analysis: TMP-SMX and clindamycin. The primary study outcome was composite failure defined as an additional positive MRSA culture from any site 5 to 90 days after treatment initiation or an additional intervention during a subsequent outpatient or inpatient visit. Baseline characteristics and failure rates were compared using χ(2), Fisher's exact, and Wilcoxon rank sum tests. A total of 149 patients were included in this study. These patients had a median age of 36 years, 55% were men, 71% were Hispanic, 42% were uninsured, and 60% received an incision and drainage procedure. Patients who did not receive incision and drainage were twice as likely to experience the composite failure endpoint (57% vs 29%; P < .001). Failure rates were 25% for patients who received incision and drainage plus antibiotics compared with 60% for patients who received incision and drainage minus antibiotics (P = .03). When patients who did not receive incision and drainage were excluded, there were no significant differences between the TMP-SMX (n = 54) and clindamycin (n = 20) cohorts with respect to composite failures (26% vs 25%), microbiologic failures (13% vs 15%), additional inpatient interventions (6% vs 5%), or additional outpatient interventions (20% vs 20%). Our findings reinforce the belief that incision and drainage and antibiotics are critical for the management of CA-MRSA skin infections. Patients who receive TMP-SMX or clindamycin for their CA-MRSA skin infections experience similar rates of treatment failure.

Memories of reward and nonreward regulate the extinction and reacquisition of both excitatory and inhibitory associations

The memories of the unconditioned stimulus (US) and its absence (No US), symbolized as S R and S N, respectively, may be retrieved on US or No US trials giving rise to four types of associations, S R → US, S R → No US, S N → US, and S N → No US. Here, following acquisition under partial reward (PRF), rats were shifted either to different schedules of PRF (Experiment 1) or extinction (Experiments 1 and 2). Inhibitory S R → No US associations formed in acquisition survived extinction and shifts to one, but not another type of PRF schedule (Experiments 1 and 2). Excitatory S R → US associations also survived extinction (Experiment 2). These findings, as well as the acquisition findings of Experiment 2, are consistent with the sequential model but not with the only other two theories said to be able to explain PRF findings, the frustration hypothesis of Amsel and the attention hypothesis of Mackintosh [see Haselgrove, M., Aydin, A., & Pearce, J. M. (2004). A partial reinforcement extinction effect despite equal rates of reinforcement during Pavlovian conditioning. Journal of Experimental Psychology: Animal Behavior Processes, 30, 240–250]. Also, the reacquisition findings obtained here are inconsistent with two views applied to several learning phenomena, the rule-learning view and the position-item view.

Response–reinforcer relations and resistance to change

Behavioral momentum theory suggests that the relation between a response and a reinforcer (i.e., response-reinforcer relation) governs response rates and the relation between a stimulus and a reinforcer (i.e., stimulus-reinforcer relation) governs resistance to change. The present experiments compared the effects degrading response-reinforcer relations with response-independent or delayed reinforcers on resistance to change in conditions with equal stimulus-reinforcer relations. In Experiment 1, pigeons responded on equal variable-interval schedules of immediate reinforcement in three components of a multiple schedule. Additional response-independent reinforcers were available in one component and additional delayed reinforcers were available in another component. The results showed that resistance to disruption was greater in the components with added reinforcers than without them (i.e., better stimulus-reinforcer relations), but did not differ for the components with added response-independent and delayed reinforcement. In Experiment 2, a component presenting immediate reinforcement alternated with either a component that arranged equal rates of reinforcement with a proportion of those reinforcers being response independent or a component with a proportion of the reinforcers being delayed. Results showed that resistance to disruption tended to be either similar across components or slightly lower when response-reinforcer relations were degraded with either response-independent or delayed reinforcers. These findings suggest that degrading response-reinforcer relations can impact resistance to change, but that impact does not depend on the specific method and is small relative to the effects of the stimulus-reinforcer relation.

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 &amp; 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.

Evaluating the Reinforcing Effects of Choice in Comparison to Reinforcement Rate

A concurrent-operants arrangement was used to evaluate a boy’s preference for a choice condition (in which he chose the reinforcement) over a no-choice condition (in which the therapist selected the reinforcement for him) when (a) these conditions produced equal rates of reinforcement and (b) lower rates of reinforcement were associated with the choice condition. The boy preferred the choice condition even when it resulted in a much less favorable rate of reinforcement than the no-choice condition (up to 4000% less).

THE CONTROL OF CHOICE BY ITS CONSEQUENCES

Five pigeons were trained on concurrent variable‐interval schedules in which equal rates of reinforcement were always arranged for left‐ and right‐key responses, but different overall rates were signaled by key colors. Sessions began with both keys lit yellow for the instrumental phase. If, after 20 s of this phase, the relative number of responses that had been made to the left key equaled or exceeded .75, both keys changed red for the contingent phase. The contingent phase arranged another concurrent variable‐interval schedule for a further 20 s before the instrumental phase was reinstated. However, if preference in the instrumental phase did not exceed .75, the instrumental phase continued for a further 20 s before preference was again compared with the criterion. In Part 1, the reinforcer rate arranged in the instrumental phase was held constant at 4.8 reinforcers per minute, while the reinforcer rate arranged in the contingent phase was varied across conditions from 0 to 19.2 over five steps. In Part 2, reinforcer rates in the contingent phase were kept constant at 36 per minute, while reinforcer rates in the instrumental phase were varied from 0 to 36 over seven steps. Part 3 replicated Part 2 but used reinforcer rates in both phases that were one third of those arranged in Part 2. Measures of choice obtained by summing responses across presentations of the instrumental phase became more extreme toward the left key as the reinforcer rate obtained in the contingent phase was increased (Part 1) and as the reinforcer rate obtained in the instrumental phase was decreased (Parts 2 and 3). Changes in these measures of choice were accompanied by systematic changes in the relative frequency with which the criterion was exceeded. Changes in both these measures were correlated with changes in the relative frequency with which subjects responded exclusively to the left key. These results are discussed with respect to the two choices that were concurrently available in this procedure and the response alternatives that might constitute the concurrent operants in each choice.

CHOOSING AMONG NATURAL STIMULI

Pigeons worked on concurrent variable-interval, variable-interval schedules with the alternatives signaled by slides either containing trees or not. The schedules were designed to hold both overall and relative rates of reinforcement within narrowly constrained limits, and slides were quasi-randomly ordered each day. Responding to the two alternatives was well described by the generalized matching equation with substantial undermatching. Using an adaptation of the matching law, we estimated that the subjects were correctly classifying 82% to 95% of exemplars. The matching performance transferred to new exemplars of trees and nontrees with only slight generalization decrement. The pigeons appeared to be discriminating among exemplars even when the alternatives provided equal rates of reinforcement and the average relative performances were close to 50%.

DEFINING BEHAVIORAL CONTRAST FOR MULTIPLE SCHEDULES

Two different definitions of behavioral contrast have been used for multiple schedules. One, interschedule, definition identifies contrast as changes in the rates of responding which occur when subjects move from one multiple schedule to another. The other, intraschedule, definition emphasizes changes in the rates of responding which occur relative to a baseline rate of responding. The baseline is the rate of responding emitted during a multiple schedule that supplies equal rates of reinforcement in the two components. The distinction between these two definitions is important for empirical and theoretical reasons. For example, theoretical confusion has arisen when the interschedule definition has been used to test and reject theories which implicitly define contrast by the intraschedule definition.

Behavioral contrast in multiple schedules with equal reinforcement rates

After responding was maintained on multiple fixed-interval schedules of reinforcement, a cuing procedure was added to one component. Cuing was accomplished by illuminating a lamp just prior to reinforcement availability. This procedure produced positive behavioral contrast. The rate of responding in the cued component decreased, and the rate of responding in the uncued component increased. When the cue was removed, negative behavioral contrast occurred. Rate of responding in the component from which the cue had been removed increased, and rate of responding in the other component decreased. Throughout the experiment, rates of reinforcement in both components were held constant.