30-s Schedule Research Articles

Influence of amount and delay of reward on choice and response rate: A free-operant, multiple-schedule analogue of a discrete-trial procedure.

The current study explored a free-operant analogue of discrete-trial procedures to study the effects of amount and delay of reinforcement on choice and response rate. Rats responded on a multiple variable-interval (VI) 45-s, 45-s schedule, with interspersed choice probe trials. Comparison of relative response rates and percentage of choice revealed some discrepancies between the free-operant analogue and discrete-trial procedures. Amount of reward controlled choice behavior when the ratios of delays were similar. When reward delays were more discrepant, delay length controlled choice behavior. Whereas the percentage of choice was larger for the larger magnitude reward, the relative rate of response for the larger magnitude was less than .50. In contrast, when the percentage of choice generally fell to below 50% (with large amount and large delay differences between alternatives), relative response rate indicated a preference for the larger amount alternative. This study shows the feasibility and utility of a free-operant analogue of discrete-choice studies that could be used to develop an analysis of preference.

Effects of ketamine optical isomers, fluoxetine and naloxone on timing in differential reinforcement of low-rate response (DRL) 72-s task in rats

(S)-ketamine-induced rapid-acting antidepressant effects have revolutionized the pharmacotherapy of major depression; however, this medication also produces psychotomimetic effects such as timing distortion. While (R)-ketamine produces fewer dissociative effects, its antidepressant actions are less studied. Depression is associated with time overestimation (i.e., subjectively, time passes slowly). Our recent report suggests that while (S)-ketamine induces an opposite effect, i.e., time underestimation, the (R)-isomer does not affect timing. It has been suggested that opioid receptors are involved in the antidepressant effect of ketamine. In the present study we tested (R)- and (S)-ketamine, and fluoxetine as a positive control in the differential-reinforcement-of-low-rate (DRL) 72-s schedule of reinforcement in male rats following naloxone pretreatment. DRL classic metrics as well as peak deviation analyses served to determine antidepressant-like actions and those associated with timing. We report antidepressant-like effects of (S)-ketamine (30-60 mg/kg) that resemble fluoxetine's (2.5-10 mg/kg), as both compounds increased reinforcement rate and peak location (suggesting increased performance), reduced premature responses (suggesting time underestimation) and decreased Weber's fraction (suggesting increased timing precision). (R)-ketamine (30, but not 60 mg/kg) increased only the reinforcement rate and peak location but did not affect timing. Only fluoxetine decreased burst responses, suggesting decreased impulsivity. Naloxone pretreatment did not block ketamine enantiomers’ actions, but unexpectedly, increased fluoxetine’ performance. Thus, while all three medications produced antidepressant-like effects in DRL 72-s, fluoxetine- and (S)- but not (R)- ketamine-induced time underestimation (the subject experiences the time as passing quickly). The potentiation of DRL performance of fluoxetine by naloxone was unexpected and warrants clinical studies.

Resurgence increases with repetition

Resurgence refers to a behavioral process in which a recent response is extinguished (or reinforcement conditions worsen) and a previously extinguished response recurs. In previous research, resurgence has been reliably produced when the resurgence procedure is repeated. Changes in the degree of increase of the resurging response across iterations of the procedure have been inconsistent, however, with some studies showing increases and some showing no changes or decreases in resurgence magnitude. The present study examined the nature of resurgence across repeated iterations of the conventional resurgence procedure by exposing four pigeons to the resurgence procedure six times in succession. In the first condition of the resurgence procedure, pecks on one key (e.g., the left) were reinforced under a variable-interval (VI) 30-s schedule. In the second condition pecks on that key were extinguished, and pecks on another key (e.g., the right) were reinforced under the same schedule. In the final condition there were no programmed consequences for either response. Resurgence was observed in 21 of 24 opportunities (87.5%). Iteration-over-iteration increases in resurgence were observed in 15 of 20 opportunities (75.0%), and this increase was found to be statistically significant. These findings demonstrate that, under certain conditions, resurgence generally increases in magnitude with repeated exposure to the procedures that generate it.

Resurgence during transitions from variable- to fixed-interval schedules

The effects of local periods of extinction on resurgence following transitions from variable-interval (VI) to fixed-interval (FI) schedules were studied using four pigeons exposed to a within-session resurgence procedure. Each session was divided into a Training (T) Alternative-Reinforcement (AR), and Resurgence Test (RT) phase. During the T phase, key pecking was reinforced under a VI 60-s schedule on one key. In the AR phase, responses reinforced in the T phase were extinguished, while responses to a different key were reinforced under a VI 90-s schedule. Next, responding to the same key that produced reinforcers in the AR phase was reinforced according to four different RT conditions: RT phase I (FI 90 s), RT phase II (FI 180 s), RT phase III (FI 45 s), or RT phase IV (extinction). The frequency of resurgence generally was an inverse function of the rate of reinforcement in the RT phase. Resurgence occurred less often when reinforcers were delivered under the FI 45-s schedule and more often under leaner schedules in the RT phase, peaking under extinction. The results show that resurgence may occur during local periods of extinction, with larger and more consistent effects occurring when the rate of reinforcement in the RT condition is leaner than it was during the preceding AR phase.

Assessment of antidepressant-like effects of dextromethorphan on differential reinforcement of low-rate 72-s performance in rats.

The effectiveness of ketamine for treatment-resistant depression along with several other clinical advantages, such as rapid onset and reduced adverse effects associated with serotonin transporter inhibition, has garnered interest in other similar acting psychedelics as novel antidepressant drugs. The antitussive dextromethorphan exhibits glutamate N-methyl-d-aspartate receptor antagonism, sigma-1 receptor agonism, and serotonin reuptake inhibition, which has exhibited antidepressant effects in limited human studies and animal models. The present study sought to further examine dextromethorphan using a differential reinforcement of low-rate 72-s schedule, which can be used to screen antidepressant drugs, in male and female rats. The tricyclic antidepressant drug imipramine and the psychostimulant d-amphetamine also were examined. Sex differences were not shown for baseline performance or for the drugs tested. Further, performance did not differ between the estrus and diestrus stages. Dextromethorphan alone and with quinidine produced an antidepressant-like effect by reducing the number of responses emitted, increasing the number of reinforcers earned, and shifting inter-response times to the right, although significant response suppression occurred at these doses. An antidepressant-like effect was shown with imipramine, but d-amphetamine increased the number of responses emitted and did not affect the number of reinforcers earned. The present findings provide additional support for antidepressant effects produced by dextromethorphan.

Assessment of the ‘timing’ function of schedule-induced behavior on fixed-interval performance.

It has been suggested that schedule-induced behaviors allow organisms to adapt better to temporal regularities of the environment. The main goal of the present study was to observe the effect of schedule-induced drinking (SID) on the performance in fixed-interval (FI) schedules. Rats were exposed to a FI 15-, 30-, or 60-s food reinforcement schedule, and only half of them had access to water in the experimental chamber. Rats with access to water developed SID, which occurred in the first part of the interval, regardless of the FI value, and was followed by an increase in lever pressing rate. There were no substantial differences in the quantitative measures of timing between groups that had or did not have access to water, except for the rats in the FI 15-s group with access to water, who showed longer postreinforcement pauses, possibly attributable to competition between SID and lever pressing. SID did not manifest the scalar property, contrary to lever pressing, but it is proposed that behaviors are displayed serially until the last behavior before the target operant response becomes a discriminative stimulus for that behavior. It is not assumed that the purpose of schedule-induced behaviors is to aid timing, but the development of behavioral patterns might determine the performance of organisms on temporal tasks. Additionally, in some cases competition between responses might exert more control on when the operant behavior occurs than timing. Timing seems to consist in the temporal organization of available behaviors that leads to a specific behavior occurring at a specified time, a single characteristic that typically had come to indicate accurate timing. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Longer operant lever-press duration requirements induce fewer but longer response bouts in rats.

Operant behavior is organized in bouts that are particularly visible under variable-interval (VI) schedules of reinforcement. Previous research showed that increasing the work required to produce a response decreases the rate at which bouts are emitted and increases the minimum interresponse time (IRT). In the current study, the minimum effective IRT was directly manipulated by changing the minimum duration of effective lever presses reinforced on a VI 40-s schedule. Contrary to assumptions of previous models, response durations were variable. Response durations were typically 0.5 s greater than the minimum duration threshold; durations that exceeded this threshold were approximately log-normally distributed. As the required duration threshold increased, rats emitted fewer but longer bouts. This effect may reflect an effort-induced reduction in motivation and a duration-induced facilitation of a response-outcome association.

Effects of pre‐operant running and sucrose concentration on operant wheel running on a fixed interval schedule of reinforcement

Prior research proposed that temporal control over the pattern of operant wheel running on a fixed interval (FI) schedule of sucrose reinforcement is a function of automatic reinforcement generated by wheel running and the experimentally arranged sucrose reinforcement. Two experiments were conducted to assess this prediction. In the first experiment, rats ran for different durations (0, 30, 60, and 180 min) prior to a session of operant wheel running on a FI 120-s schedule. In the second experiment, the concentration of sucrose reinforcement on a FI 180-s schedule was varied across values of 0, 5, 15, and 25%. In Experiment 1, as the duration of pre-operant running increased, the postreinforcement pause before initiation of running lengthened while wheel revolutions in the latter part of the FI interval increased. In Experiment 2, wheel revolutions markedly increased then decreased to a plateau early in the FI interval. Neither manipulation increased temporal control of the pattern of wheel running. Instead, results indicate that operant wheel running is regulated by automatic reinforcement generated by wheel activity and an adjunctive pattern of running induced by the temporal presentation of sucrose. Furthermore, the findings question whether the sucrose contingency regulates wheel running as a reinforcing consequence.

Reinforcing effects of synthetic cathinones in rhesus monkeys: Dose-response and behavioral economic analyses

The abuse of synthetic cathinones (“bath salts”) with psychomotor stimulant and/or entactogenic properties emerged as a public health concern when they were introduced as “legal” alternatives to drugs of abuse such as cocaine or MDMA. In this study, experiments were conducted in nonhuman primates to examine how differences in transporter selectivity might impact the reinforcing effects of synthetic cathinones. Rhesus monkeys (N = 5) were trained to respond for intravenous injections under a fixed-ratio (FR) 30, timeout 60-s schedule of reinforcement. The reinforcing effects of selected cathinones (e.g., MDPV, αPVP, MCAT, and methylone) with a range of pharmacological effects at dopamine and serotonin transporters were compared to cocaine and MDMA using dose-response analysis under a simple FR schedule and behavioral economic procedures that generated demand curves for two doses of each drug. Results show that one or more doses of all drugs were readily self-administered in each subject and, excepting MDMA (21 injections/session), peak levels of self-administration were similar across drugs (between 30 and 40 injections/session). Demand elasticity for the peak and the peak + 1/2-log dose of each drug did not significantly differ, and when data for the two doses were averaged for each drug, the following rank-order of reinforcing strength emerged: cocaine > MCAT = MDPV = methylone > αPVP = MDMA. These results indicate that the reinforcing strength of synthetic cathinones are not related to their selectivity in binding dopamine or serotonin transporter sites.

Ecological Location of a Water Source and Spatial Dynamics of Behavior Under Temporally Scheduled Water Deliveries in a Modified Open-Field System: An Integrative Approach.

It has been reported in non-contingent schedules that the variety of patterns of behavior is affected by the temporal variation of water deliveries. While temporal variation is accomplished by delivering water at fixed or variable times, spatial variation is usually accomplished by varying the number of dispensers and distance among them. Such criteria do not consider the possible ecological relevance of the location of water dispensers. Nevertheless, it is plausible to suppose that the intersection of the programed contingencies (e.g., time-based schedules), the ecological differentiated space (e.g., open vs. closed zones), and the relative location of relevant objects and events (e.g., location of the water source—peripherical vs. center zone) could set up an integrated system with the behavioral patterns of the organism. In the present study, we evaluated the eco-functional relevance of two locations of the dispensers upon behavioral dynamics in Wistar rats using fixed and variable time schedules in a modified open-field system. In Experiment 1, three subjects were exposed to a fixed time 30-s water delivery schedule. In the first condition, the water dispenser was located at the center of the experimental chamber. In the second condition, the water dispenser was located at the center of a wall of the experimental chamber. Each location was present for 20 sessions. In Experiment 2, conditions were the same, but a variable time schedule was used. Routes, distance to the dispenser, recurrence patterns, time spent in zones, entropy, and divergence were analyzed. Our findings suggest a robust differential relevance of the location of the dispensers that should be considered in studies evaluating behavioral dynamics. Results are discussed from an integrative, ecological-parametric framework.

Does DRO type matter?: Cycle versus resetting contingencies in eliminating responding

Following lever-press training on variable-interval 30-s schedules, rats were exposed to three types of schedules designed to eliminate lever pressing. The first two were variations on what is called a differential-reinforcement-of-other-behavior (DRO, “zero rate”, or [target response] omission schedule) schedule. Under both variations, reinforcers were scheduled to occur in different conditions after either fixed or variable inter-reinforcer intervals (IRIs). Under one variation each lever press reset the time interval (i.e., “resetting DRO”) and under the other a reinforcer delivery scheduled at the end of an IRI was cancelled by the first response during the IRI (i.e., “cycle DRO”). In another condition reinforcers were delivered independently of responding after fixed or variable time periods. Each of the DRO procedures reduced response rates quickly and to near zero across ten sessions. The time schedules also reduced responding, albeit at a slower rate. The results extend the analogy of omission training to freeoperant avoidance to shock-deletion avoidance schedules.

An evaluation of resurgence in mice

Owing in part to the implications of resurgence for issues surrounding human health and adaptive problem solving, a substantial amount of laboratory research has been dedicated to understanding why this form of relapse occurs and what factors affect it. This research, however, has never been extended to mice, leaving unknown the specific experimental parameters that are appropriate for studying resurgence in this species. Two experiments were conducted in which mice were exposed to a three-phase resurgence preparation in a multiple-baseline-across-subjects design. In Phase 1, pressing a target lever produced food according to a variable-interval 15-s schedule. In Phase 2, target-lever pressing was extinguished, and nose poking produced reinforcement according to either a variable-interval 15-s (Experiment 1) or fixed-ratio 1 (Experiment 2) schedule. Finally, alternative reinforcement was suspended to test for resurgence. Lever pressing relapsed for one of four mice in Experiment 1, but all four mice demonstrated relapse in Experiment 2. Thus, relatively dense schedules of alternative reinforcement may be required to reliably study resurgence of previously reinforced responding in mice. These findings are instructive for researchers who are interested in studying resurgence of mice’s behavior and further demonstrate the cross-species generality of this form of relapse.

Resurgence and downshifts in alternative reinforcement rate.

Resurgence refers to an increase in a previously suppressed target behavior with a relative worsening of conditions for a more recently reinforced alternative behavior. This experiment examined the relation between resurgence and the magnitude of a reduction in the rate of reinforcement for the alternative behavior. Groups of both male and female rats initially pressed a target lever for food on a variable-interval (VI) 30-s schedule. In a second phase, responding to the target lever was extinguished for all groups and pressing an alternative lever was reinforced on a VI 10-s schedule. Next, the rate of reinforcement for alternative behavior was reduced differentially across groups by arranging extinction, VI 80-s, VI 40-s, VI 20-s, or continued VI 10-s reinforcement. Target responding increased as an exponential function of the magnitude of the reduction in alternative reinforcement rates. With the exception that males appeared to show higher rates of target responding in baseline and higher rates of alternative responding in other phases, the overall pattern of responding across phases was not meaningfully different between sexes. The pattern of both target and alternative response rates across sessions and phases was well described quantitatively by the Resurgence as Choice in Context model.

A Comparison of Fixed-Time Food Schedules and Shaping Involving a Clicker for Halter Behavior in a Petting Zoo Goat

This study examined two procedures for establishing halter behavior (i.e., acceptance of wearing and being led via a halter) and decreasing undesired behavior in three petting zoo goats: a fixed-time 15-s (FT-15-s) schedule and shaping involving a clicker. The FT schedule was successful in training initial approximations, but over a longer period for the one goat presented in this article. The shaping procedure rapidly advanced the final approximations to the target behavior and resulted in a large reduction in undesired responses once the target response was reached. The procedures were also equally effective at reaching the target response and greatly reducing undesired responses for the other two goats not presented in this article. This study is one example of an effective way to experimentally examine shaping procedures, as well as promote the use of force-free halter training and behavior in goats.

The effects of force requirements on microstructure: Subcriterion responses and the mismeasurement of bouts.

The effects of response force on microstructure were evaluated. A strain-gauge operandum permitted the manipulation of the force required to produce reinforcers (criterion responses) independently from the force defining response threshold. Thus, we could detect subcriterion forces that fell short of the force criterion. Eight rats earned food according to variable-interval (VI) 30- and 120-s schedules. The force requirements were set to 5.6 or 32.0 g; the response threshold was fixed at 5.6 g. Interresponse times were computed when subcriterion responses were both included and omitted from the analysis. Log-survivor functions of interresponse times showed that increasing force requirements elevated the mean between-bout interval of the VI 120-s schedule, but only if subcriterion behavior was excluded. Omitting subcriterion responses thus leads to overestimation of intervals separating response bouts. Increasing force requirements also increased the skewness of the between-bout distribution. A subsequent analysis found that subcriterion responses are most plentiful following reinforcer delivery, which helps to explain why their omission might inflate between-bout intervals, as this period is an important transition from reinforcer consumption to engagement in operant activity. The data suggest caution interpreting the effects of force on microstructure when subcriterion behavior is not or cannot be measured.

Activation of cannabinoid type 1 receptors decreases the synchronization of local field potential oscillations in the hippocampus and entorhinal cortex and prolongs the interresponse time during a differential-reinforcement-of-low-rate task.

Marijuana intoxication impairs neurocognitive functions. Common side effects of consuming cannabis include time distortion and memory loss. However, the underlying neurophysiological mechanisms involved in these effects remain unclear. We hypothesized that communication between the hippocampal CA1 region and medial entorhinal cortex (MEC) is essential for the transmission of temporal-associated information. We used a differential-reinforcement-of-low-rate (DRL) task, which requires subjects to press a lever at an optimal time point, to correlate the distributions of interresponse time (IRT) with local field potentials (LFPs) recorded in the CA1 and MEC under the effects of a cannabinoid type 1 (CB1) receptor agonist. We used a DRL 10-s schedule and trained the rats to withhold for 10s before pressing a lever. Our data showed that the percentage of 12.4- to 14-s IRT events rose after activation of CB1 receptors in the MEC. In addition, gamma amplitude synchronization and CA1 theta phase-MEC gamma amplitude coupling decreased during the 6- to 14-s IRT events. These results suggest that activation of CB1 receptors in the MEC disrupt the functional connectivity between the CA1 and the MEC. This inefficient communication may result in increased IRT during a DRL schedule. Overall, we postulate that marijuana intoxication impairs the communication between the CA1 and MEC and influences behavioral performances that require precise timing ability.

Resistance to change as function of different physical-effort requirements in humans

The effects of different physical-effort requirements on behavioral resistance to change were examined with undergraduate students engaging in a computer task for points exchangeable for money. In Baseline, button pressing was maintained on a multiple variable interval (VI) 45-s VI 45-s schedule of reinforcement with two physical-effort requirements: 10 N in the Low-Effort Component and 50 N in the High-Effort Component. In Test, button pressing was disrupted by a multiple extinction (EXT) EXT schedule plus anagrams as a concurrent task for five participants. For other two participants, button pressing was disrupted only by a multiple EXT EXT in one test, and both disruptors were implemented together in the second test. Baseline and Test conditions were replicated in an ABAB design. Resistance to change was greater in the presence of discriminative stimuli associated with the Low-Effort Component to six participants during the first test and to four participants during the second test. The present findings suggest differences in physical-effort can contribute to differences in resistance to change.

Extending the Specificity of DRL 72-s Behavior for Screening Antidepressant-Like Effects of Glutamatergic Clinically Validated Anxiolytic or Antidepressant Drugs in Rats.

Both an agonist and its associated prodrug for metabotropic glutamate2/3 (mGlu2/3) receptors demonstrated anxiolytic efficacy in large, randomized, multicenter, double-blind, placebo-controlled trials studying patients with generalized anxiety disorder (GAD). These mGlu2/3 receptor agonists produced robust preclinical anxiolytic-like effects in rodent models. Several different metabotropic glutamate2 receptor positive allosteric modulators have been found to produce antidepressant-like effects on several preclinical screening paradigms, including differential-reinforcement-of-low-rate 72-second (DRL 72-s) behavior [increased reinforcers, decreased response rate, and cohesive rightward shifts in inter-response time distributions]. Although mGlu2/3 receptor agonists have not been tested formally for therapeutic effects in treating patients with major depressive disorder, these compounds generally fail to exert antidepressant-like effects in preclinical screening paradigms and did not improve depressive symptoms in GAD trials. Thus, the present studies were designed to test the potential antidepressant-like effects of the mGlu2/3 receptor agonist 1S,2S,5R,6S-2-aminobicyclo[3.1.0]hexane-2,6-bicarboxylate monohydrate (LY354740) on the DRL 72-s schedule. LY354740 did not test similarly to clinically validated antidepressant drugs when administered alone or when coadministered with the selective serotonin reuptake inhibitor fluoxetine in rats. Another glutamate-based antidepressant drug, the uncompetitive N-methyl-D-aspartate channel blocker racemic ketamine, exerted antidepressant-like effects when administered at subanesthetic doses in rats. The findings further support the specificity of rat DRL 72-s behavior when screening for anxiolytic versus antidepressant drugs and extend testing of compounds with glutamatergic mechanisms of action. SIGNIFICANCE STATEMENT: The metabotropic glutamate2/3 receptor agonist and clinically validated anxiolytic drug 1S,2S,5R,6S-2-aminobicyclo[3.1.0]hexane-2,6-bicarboxylate monohydrate did not test similar to antidepressant drugs (increased reinforcers, decreased response rate, and cohesive rightward shifts in the inter-response time distribution) when tested on differential-reinforcement-of-low-rate 72-second (DRL 72-s) behavior and also did not enhance the antidepressant-like effects of the serotonin reuptake inhibitor fluoxetine. The uncompetitive N-methyl-D-aspartate receptor antagonist ketamine increased the reinforcement rate, decreased the response rate, and induced a rightward shift in the inter-response time distribution similar to antidepressant drugs; these results confirm the utility of DRL 72-s schedule of reinforcement when testing clinically validated anxiolytic versus antidepressant glutamatergic drugs.

Temporal regulation of children with autism spectrum disorder exposed to a differential-reinforcement-of low-rates schedule.

This study investigated temporal adjustment of children with autism spectrum disorder under a differential-reinforcement-of-low-rates (DRL) schedule. Sixteen participants, aged 3.2 to 7 years, were exposed to two conditions, DRL 5 s and DRL 20 s. Children participated in 7 sessions in each condition, except for 1 participant who attained the adjustment criteria in the DRL 5-s schedule. Temporal adjustment was measured with the proportion of reinforced interresponse times (IRTs) and the mean IRT. The operant response was a press on a touch screen and the reinforcers were cartoons. IQ and receptive language were measured prior to the DRL sessions. Results showed that the mean proportion of reinforced IRTs was slightly higher in the DRL 5-s schedule. The mean IRT was above the IRT requirement in both conditions. However, substantial individual variability was observed. Children with higher IQ and receptive language scores presented a greater proportion of reinforced IRTs in both conditions. Moreover, participants who adjusted their responses to the DRL 5-s schedule were more likely to adjust responding to the DRL 20-s schedule. This suggests that some children might be more sensitive to reinforcement contingencies than others. This study points at future research in the field of timing in children.

Local constraint of the contingent response controls rates of wheel-running and lever-pressing on an interval schedule of wheel-running reinforcement

Three experiments assessed whether changes in rates of wheel running and lever pressing on a schedule of wheel-running reinforcement were controlled by local or overall constraint of the contingent response. In Experiment 1 rats responded on a response-initiated variable interval 15-s schedule with wheel running as reinforcement. Overall constraint (total revolutions by session) was held constant at 600 revolutions while local constraint (revolutions per reinforcement) varied from 5 to 60 revolutions. Results showed that rates of wheel running and lever pressing decreased as local constraint decreased. In Experiment 2, rats responded on the same schedule, but local constraint (revolutions per reinforcement) was held constant at 10 revolutions. Overall constraint (total revolutions) was varied from 100 to 800 revolutions. Results showed no systematic changes in rates of wheel running and lever pressing with variation in overall constraint. In Experiment 3, rats responded on the same VI schedule for 30 revolutions as reinforcement. A 4.5-s interruption during the wheel-running reinforcement (30 revolutions) occurred after 2, 5, 10, and 15 revolutions as a manipulation of local constraint. Results showed that with overall constraint (total revolutions) and total reinforcement held constant, rates of wheel running before the interruption and overall lever-pressing rates varied in a bitonic relationship with local constraint. Comparison of the results across experiments demonstrates that local constraint of contingent wheel running controls the rate of wheel running and the rate of instrumental responding for wheel-running reinforcement.