Multiple Fixed-ratio Schedule Research Articles

Effects of negative incentive shifts in food reward on rats' consumption of concurrent ethanol solutions.

Frustration stress, typically operationalized as the unexpected loss of reinforcement, has been shown to engender substance use. Abrupt reductions in reinforcer magnitude likely also function as frustration stressors. These negative incentive shifts were previously shown to produce tap- and sweetened-water drinking in rats. The purpose of this study was to investigate whether these shifts in food reward would occasion oral ethanol self-administration. Nine male Long-Evans rats operated on a two-component multiple fixed-ratio schedule with signaled components producing either a large (4 pellets) or small (1pellet) reinforcer. Components were pseudorandomly arranged to present 4 transitions between past and upcoming reinforcer magnitudes: small-to-large, small-to-small, large-to-large, and large-to-small (negative incentive shift). Experiment 1 investigated the effects of negative incentive shifts on consumption of concurrent, freely available 10% sucrose, 10% sucrose plus 10% ethanol, and following sucrose fading, 10% ethanol. Experiment 2 entailed continuation of schedule contingencies with a dose manipulation of 4 ethanol concentrations (0, 5, 10, and 20%) to assess dose-dependent differences in transition-type control and consumption. A lever-press extinction condition was then conducted with 10% ethanol availability. In this novel model of frustration stress, negative incentive shifts prompted ethanol self-administration at each dose investigated, whereas the other transitions did not.

The effects of reinforcer magnitude in the preceding and upcoming ratios on between-ratio pausing in multiple, mixed, and single fixed-ratio schedules.

Hens responded under multiple fixed-ratio schedules with equal response requirements and either a 1-s or a 6-s reinforcer. Upcoming reinforcer size was indicated by key color. Components were presented in a quasirandom series so that all four component transitions occurred. Postreinforcement pauses were affected by the upcoming and preceding reinforcer size, with longer pauses after large reinforcers followed by small reinforcers than when followed by large ones, and longer pauses after small reinforcers that were followed by small reinforcers rather than large ones. Pauses increased with fixed-ratio size and the effects of reinforcer size were larger the larger the ratio. When reinforcer size was not signaled-mixed fixed-ratio schedules-pauses were shorter after small than after large reinforcers. Signalling the upcoming reinforcer attenuated the effect of the previous reinforcer size on pause duration when small was followed by small and when either small or large by large, but enhanced the effect when large was followed by small. There was no effect of reinforcer size on pause duration when single fixed-ratio schedules were arranged. The effects of reinforcer size on pauses depends on the size and range of the fixed ratios as well as the exact procedures used in the study.

Behavioral functions of stimuli signaling transitions across rich and lean schedules of reinforcement

On multiple fixed-ratio schedules, pausing is extended at the start of a component ending in a small reinforcer (a lean component) but only when this component follows a component ending in a large reinforcer (a rich component). In two experiments, we assessed whether a stimulus correlated with a lean component is aversive and how its function is affected by the preceding component. In Experiment 1, pigeons responded on mixed fixed-ratio schedules ending in large or small reinforcers. Observing responses converted the mixed schedule to a multiple one by producing a stimulus correlated with the current component. Overall, the lean stimulus did not suppress observing, suggesting that it was not sufficiently aversive. In Experiment 2, an escape procedure was used, and pigeons could convert a multiple schedule to a mixed one by pecking a key to remove the discriminative stimuli. Pigeons escaped from the lean-schedule stimulus more than they did from the rich one. For two pigeons, this effect was enhanced when a rich component preceded the lean stimulus. The results indicate that a stimulus correlated with the leaner of two reinforcement schedules can acquire aversive functions, but observing and escape procedures may differ in their abilities to detect this effect.

Tolerance to cocaine's effects following chronic administration of a dose without detected effects on response rate or pause.

To observe tolerance to drug effects on operant behavior, the dose that researchers have often selected for chronic administration is one that disrupts, but does not abolish, responding. Some evidence suggests that tolerance may develop after chronic administration of relatively smaller doses. The purpose of the present experiment was to assess systematically effects of chronic administration of a dose without detected effect on responding. Specifically, response rates and post-reinforcement pauses of five pigeons key pecking under a three-component multiple fixed-ratio schedule of food reinforcement were observed under chronic cocaine administration. We evaluated the effects of a range of doses (1.0 mg/kg to 17.0 mg/kg) during acute administration. The largest dose that failed to alter responding acutely then was administered chronically (1.0 mg/kg for 1 pigeon, 3.0 mg/kg for 3 pigeons, and 5.6 mg/kg for 1 pigeon). After 30 consecutive sessions of chronic administration, smaller and larger doses occasionally were substituted for the chronic dose. Pigeons then received pre-session saline administration for 30 consecutive sessions, and the post-chronic effects of the series of doses on responding were determined. All subjects developed tolerance to doses of cocaine that initially had caused large decreases in rate, with the magnitude of the effects varying across components of the multiple schedule and subjects. Specifically, tolerance generally was greatest in the components with smaller ratios. Following post-chronic saline administration, tolerance was usually diminished. Overall, the results demonstrate that under these conditions, repeated experience with disruptive effects of cocaine on food-maintained responding is not a necessary factor in the development of tolerance.

The effect of prefeeding on fixed-ratio pausing is jointly determined by past and upcoming reinforcer magnitudes

Pausing within multiple fixed-ratio schedules differing in reinforcer magnitude is jointly controlled by both past and upcoming conditions of reinforcement. Abrupt shifts from a just-received large reinforcer to a signaled upcoming small reinforcer (i.e., a negative incentive shift) produce marked disruptions in responding, as indexed by extended pausing. The purpose of this experiment was to determine if reducing the level of food deprivation via prefeeding enhanced these disruptive effects. Five Long Evans rats lever-pressed according to a fixed-ratio schedule. Half of the components ended in a relatively large reinforcer (three 45-mg food pellets) and half ended in a relatively small reinforcer (one pellet). Components alternated irregularly, yielding four transitions between reinforcers: small-small, small-large, large-small (the negative incentive shift), and large-large. During five, 1-session prefeeding probes, rats were given 12 g of food in their home cages 1h prior to the start of the session. Under steady-state conditions, negative incentive shifts engendered the longest pausing. Prefeeding produced large absolute and relative increases in pausing during negative incentive shifts, and small increases in pausing in the other transitions. The results are interpreted within a resistance to change framework.

Modeling the effects of fluoxetine on food-reinforced behavior

We propose a novel method to dissociate incentive motivation from memory and motor processes in instrumental performance. Components of a multiple fixed-ratio schedule of food reinforcement were adjusted to envelop the range of response requirements that maintained lever pressing by rats. We sought to manipulate motivation for food rewards with acute administrations of various doses (0-10 mg/kg) of fluoxetine, a 5-HT reuptake inhibitor that reduces food intake. A quantitative model of fixed-ratio performance derived from Killeen's (1994) Mathematical Principles of Reinforcement (MPR) provided an adequate account of data from individual rats. Decreases in response rate resulting from fluoxetine were reflected in changes in estimates of activation, indexed by MPR parameter a; estimates of working memory capacity and lever pressing duration were not systematically affected. These results support the use of MPR parameter a to index incentive motivation using multiple fixed-ratio schedules that are adjusted to individual performance.

Cannabinoid effects on behaviors maintained by ethanol or food: a within-subjects comparison

The cannabinoid CB1 antagonist rimonabant (SR141716A) has been proposed as a therapeutic agent for several addictive disorders, including alcoholism. Rimonabant may selectively reduce responding for an ethanol solution compared with an alternative. While this could represent a specific effect of CB1 inhibition on ethanol reinforcement, this could also result from differences in the baseline rates of behavior or experiences between comparison groups. We developed a procedure in rats that allows a within-subject comparison of ethanol and food-maintained responding and provides well matched baseline response rates. We determined the effects of acute doses of rimonabant (0.3-5.6 mg/kg, intraperitoneal) and the CB1 agonist Delta-9-tetrahydrocannabinol (1.0-5.6 mg/kg, intraperitoneal) on responding for food and ethanol under a multiple fixed-ratio schedule. To confirm that rimonabant blocked cannabinoid receptors, the ability of rimonabant to antagonize Delta-9-tetrahydrocannabinol effects in the same subjects under the same reinforcement schedule was also determined. In contrast with previous reports, rimonabant did not significantly alter responding for ethanol or food. The effects of Delta-9-tetrahydrocannabinol on responding for food were completely antagonized by rimonabant, whereas Delta-9-tetrahydrocannabinol effects on responding for ethanol were not. These results suggest that there may be neuroadaptation of the cannabinoid system following aging or chronic self-administration of ethanol.

Cannabinoid antagonist effects on behaviors maintained by ethanol and food

The cannabinoid CB1 antagonist rimonabant (SR141716A) has been proposed as a therapeutic agent for several addictive disorders, including alcoholism. Rimonabant may selectively reduce responding for an ethanol solution compared with an alternative. While this could represent a specific effect of CB1 inhibition on ethanol reinforcement, this could also result from differences in the baseline rates of behavior or experiences between comparison groups. We developed a procedure in rats that allows within-subject comparison of ethanol- and food-maintained responding and provides well-matched baseline response rates. Objectives and Methods We determined the effects of acute doses of rimonabant (0.3–5.6 mg/kg, i.p) and the CB1 agonist delta-9-tetrahydrocannabinol (THC, 1.0–5.6 mg/kg) on responding for food and ethanol under a multiple fixed-ratio schedule. The ability of rimonabant to antagonize THC effects in the same subjects under the same reinforcement schedule was also determined. Results In contrast with previous reports, rimonabant alone did not significantly alter responding for ethanol or food. Surprisingly, the effects of THC on responding for food were completely antagonized by rimonabant, while THC effects on responding for ethanol were not. Conclusions These results suggest that there may be neuroadaptation of the cannabinoid system following aging or chronic self-administration of ethanol.

Multiple receptors may mediate the behavioral effects of benzodiazepine‐site (BZ) ligands in monkeys

These studies examined the behavioral effects of two novel BZ ligands, NsA and NsB. These compounds nonselectively bind GABAA1, GABAA2, and GABAA3 receptors with little or no efficacy at GABAA1 receptors and weak positive modulatory actions at GABAA2 and GABAA3 receptors. Effects were evaluated in squirrel monkeys (n=3–4) trained to respond either under a multiple fixed-ratio (FR) schedule of punished and nonpunished behavior or to discriminate 0.3 mg/kg midazolam from saline. NsA and NsB, 1-30 mg/kg, had mild and inconsistent effects on punished responding, and produced greater than 90% drug-lever responding in only one monkey. Neither of the novel compounds had response rate-decreasing effects over the doses tested. Further studies examined the abilities of NsA and NsB to antagonize the antipunishment and discriminative stimulus effects of three commonly used BZ agonists, midazolam (0.1–10 mg/kg), lorazepam (0.3–18 mg/kg), and zolpidem (0.1–10 mg/kg). NsA, 1–10 mg/kg, effectively antagonized the antipunishment effects of midazolam, lorazepam and zolpidem, and the discriminative stimulus effects of midazolam and lorazepam, but not zolpidem. In contrast, 1–10 mg/kg NsB antagonized the discriminative stimulus effects of midazolam but not zolpidem or lorazepam, and the antipunishment effects of lorazepam but not midazolam or zolpidem. These results suggest that ligands with varying efficacy at GABAA subtypes may differentially antagonize the behavioral effects of benzodiazepines, and may yield useful tools for understanding BZ pharmacology. (PHS: DA15723, DA11453, DA7252)

Effects of the dopamine reuptake inhibitor PTT on reinstatement and on food- and cocaine-maintained responding in rhesus monkeys.

High-affinity, slow-onset, long-acting dopamine transporter (DAT) inhibitors are being considered as potential agonist replacement therapies for cocaine addiction, and therefore the ability of these drugs to reinstate cocaine seeking and to selectively decrease cocaine-maintained responding should be assessed. The purpose of these experiments was to evaluate the effects of the active enantiomer of a high-affinity, slow-onset, long-acting DAT inhibitor, (-)2beta-propanoyl-3beta-(4-tolyl)-tropane (PTT), and cocaine on food- and cocaine-maintained responding and on extinguished responding previously maintained by cocaine in non-human primates using a within-subjects design. Rhesus monkeys (n=3) responded under a multiple fixed-ratio schedule of food (1 g) and drug reinforcement, and cocaine dose-response curves (saline, 0.003-0.3 mg/kg per injection) were determined. The effects of pretreatment with (-)PTT (0.001-0.056 mg/kg, i.v.) and cocaine (0.03-0.3 mg/kg, i.v.) were determined when the dose of cocaine that maintained peak response rates (0.03 mg/kg per injection) or saline was available. (-)PTT and cocaine reduced cocaine intake; (-)PTT affected cocaine self-administration only at doses that also decreased food-maintained responding. (-)PTT and cocaine reinstated responding that was previously reinforced by cocaine at lower doses than were necessary to decrease cocaine-maintained responding. For all studies, PTT was at least 1.0 log-unit more potent than cocaine. Compared to cocaine, PTT had a longer duration of action in all behavioral measures. These results suggest that PTT would decrease cocaine use, but only at doses that disrupted other behaviors. It appears that the potency of this class of drugs to reinstate cocaine-seeking is substantially greater than their potency at decreasing cocaine self-administration.

Extending mathematical principles of reinforcement into the domain of behavioral pharmacology.

Mathematical principles of reinforcement (MPR) attempts to integrate the empirical laws of reinforcement schedules that have accumulated over the decades. MPR is based on three principles: incentives excite behavior; there are temporal constraints on responding; and coupling of responses to reinforcers strengthens behavior [Behav. Brain Sci. 17 (1994) 105]. In the present paper MPR is extended into the domain of behavioral pharmacology, specifically to model the effects of D-amphetamine on operant behavior. In Experiment 1a, a five-component multiple fixed-ratio schedule was designed to generate behavioral baselines that were subsequently used to assess drug effect. In Experiments 1b and 1c, the quality and quantity of reinforcer were manipulated. The data generated by the three experiments were consistent with MPR. In Experiment 2, MPR was used to model the effects of D-amphetamine on rats responding under the five-component multiple fixed-ratio schedule. According to the model, the rate-decreasing effects of D-amphetamine were due primarily to motor disruption and secondarily to increased impulsivity; at the highest dosages, D-amphetamine also may have decreased the incentive value of food.

Different types of GABA A receptors may mediate the anticonflict and response rate-decreasing effects of zaleplon, zolpidem, and midazolam in squirrel monkeys

The role of different types of GABA(A) receptors in mediating anticonflict and response rate-decreasing effects of benzodiazepines in primate species is not known. To examine the behavioral effects of the benzodiazepine-site, GABA(A) agonists zolpidem, zaleplon, and midazolam in the presence of two antagonists, flumazenil and beta-carboline-3-carboxylate-t-butyl ester (beta-CCt) in squirrel monkeys. Two schedules of operant responding were used: (1) a multiple fixed-ratio (FR) schedule of food presentation involving punished and nonpunished behavior, and (2) an FR schedule of stimulus shock-termination. Midazolam (0.03-1.0 mg/kg), zolpidem (0.1-3.0 mg/kg), and zaleplon (0.1-3.0 mg/kg) increased rates of punished responding and decreased rates of nonpunished responding under the multiple schedule. Pretreatment with flumazenil (0.3-1.0 mg/kg) antagonized the anticonflict and response rate-decreasing effects of all three agonists. Pretreatment with beta-CCt (3-10 mg/kg) antagonized the anticonflict and rate-decreasing effects of midazolam, as well as the rate-decreasing effects of zolpidem and zaleplon. However, beta-CCt did not antagonize the anticonflict effects of zolpidem and zaleplon; instead, these effects of zolpidem and zaleplon were apparently enhanced in the presence of beta-CCt. Under the schedule of stimulus shock-termination, both flumazenil and beta-CCt antagonized zolpidem and zaleplon; however, the effects of beta-CCt were less consistent than the effects of flumazenil. In nonhuman primates, different types of GABAA receptors may mediate the anticonflict and the response rate-decreasing effects of the nonselective GABAA agonist midazolam and the selective GABAA1 agonists zolpidem and zaleplon.

Response requirements and unit dose modify the effects of GBR 12909 on cocaine-maintained behavior.

Previous studies found that GBR 12909 can decrease cocaine-maintained responding at doses that do not affect food-maintained responding. In this study, the effects of GBR 12909 (0.3-3.0 mg/kg) were further examined by varying the response requirement and unit dose of cocaine. Rhesus monkeys earned food or cocaine under a multiple fixed-ratio (FR) schedule. The FR for food was always 30, but the FR for cocaine was varied from 10-130 and the unit dose was varied from 5.6-56.0 microg/kg per injection. Doses of GBR 12909 were tested in an ascending order, for 5 consecutive sessions each. GBR 12909 selectively decreased cocaine-maintained responding in all monkeys in at least 1 condition. These effects were enhanced with large response requirements and/or small unit doses. The results demonstrate that environmental variables can influence the selectivity of GBR 12909's effects and contribute to a growing debate concerning the evaluation of potential pharmacotherapies for drug abuse.

AVERAGING EFFECTS IN THE STUDY OF FIXED‐RATIO RESPONSE PATTERNS

Three rats were exposed to multiple fixed‐ratio schedules in which large and small ratios alternated in an irregular order. Over a series of training phases, one ratio was held constant as the second ratio was increased to higher values. On average, postreinforcement pauses increased in duration as the ratio size was increased. Pausing was controlled by the size of the upcoming ratio; the previous ratio had smaller and less consistent effects. However, more detailed consideration of the aggregated data indicated that the pause distributions were positively skewed and that changes in average performances were more a consequence of increased skew rather than shifts of the entire distributions. Moreover, the distributions of pauses from condition to condition overlapped, and brief pauses were common even at the highest ratios. These results demonstrated that depictions of pausing based on aggregated data can be misleading without corresponding information about variations in the distribution on which the averages are based.

Pentobarbital discrimination and generalization to other drugs under multiple fixed-ratio fixed-interval schedules

Pigeons were trained to discriminate 5mg/kg pentobarbital from saline under several multiple fixed-ratio fixed-interval schedules of food presentation. The following schedules were studied: multiple fixed-ratio 40 fixed-interval 18s (mult FR40 FI18), mult FR10 FI18s, mult FR10 FI180s and mult FR90 FI10s. After responding stabilized under each multiple schedule, generalization curves were determined for pentobarbital, amobarbital, diazepam, phencyclidine and d-amphetamine. Pentobarbital generated dose-dependent increases in responding under all schedule components; however, there was more responding on the drug key after low doses of pentobarbital under FI components than under FR components, except for the FR90 component of the mult FR90 FI10 schedule. This tendency for more responding on the drug key after low doses of pentobarbital under FI components than under FR components generally was observed for low doses of all of the drugs. Examination of data from individual subjects revealed that there was a greater tendency for birds to distribute responding on both keys (mixed responding) under FI components than under FR components, where responding after each dose was confined largely to one of the two response keys. Analysis of local rates of responding within the FI component of the schedules showed that responding under the FI components developed the typical FI scallop at all FI-component durations. These data suggest that FI schedules with values between 10 and 180s generate similar dose-effect curves with higher rates of responding on the drug key after low doses of drugs than under FR schedules with low response requirements; however, under schedules with higher FR requirements, the dose-effect curves for some drugs begin to look more like those under FI schedules.

TESTS OF BEHAVIOR MOMENTUM IN SIMPLE AND MULTIPLE SCHEDULES WITH RATS AND PIGEONS

Four experiments examined the relationship between rate of reinforcement and resistance to change in rats' and pigeons' responses under simple and multiple schedules of reinforcement. In Experiment 1, 28 rats responded under either simple fixed-ratio, variable-ratio, fixed-interval, or variable-interval schedules; in Experiment 2, 3 pigeons responded under simple fixed-ratio schedules. Under each schedule, rate of reinforcement varied across four successive conditions. In Experiment 3, 14 rats responded under either a multiple fixed-ratio schedule or a multiple fixed-interval schedule, each with two components that differed in rate of reinforcement. In Experiment 4, 7 pigeons responded under either a multiple fixed-ratio or a multiple fixed-interval schedule, each with three components that also differed in rate of reinforcement. Under each condition of each experiment, resistance to change was studied by measuring schedule-controlled performance under conditions with prefeeding, response-independent food during the schedule or during timeouts that separated components of the multiple schedules, and by measuring behavior under extinction. There were no consistent differences between rats and pigeons. There was no direct relationship between rates of reinforcement and resistance to change when rates of reinforcement varied across successive conditions in the simple schedules. By comparison, in the multiple schedules there was a direct relationship between rates of reinforcement and resistance to change during most tests of resistance to change. The major exception was delivering response-independent food during the schedule; this disrupted responding, but there was no direct relationship between rates of reinforcement and resistance to change in simple- or multiple-schedule contexts. The data suggest that rate of reinforcement determines resistance to change in multiple schedules, but that this relationship does not hold under simple schedules.

TOLERANCE TO AND RESIDUAL EFFECTS OF COCAINE IN SQUIRREL MONKEYS DEPEND ON REINFORCEMENT‐SCHEDULE PARAMETER

Lever pressing by 4 squirrel monkeys was maintained under a three-component multiple fixed-ratio schedule of food presentation; components differed with respect to ratio size. For each monkey, acute administration of cocaine (0.03 to 1.3 mg/kg, i.m.) produced dose-dependent decreases in overall response rate in each component. During repeated daily administration of 1.0 mg/kg of cocaine, tolerance developed to the rate-decreasing effects under each of the ratio contingencies, but developed to a greater extent and was evident in earlier parts of sessions for performance under the smaller ratios. Response rates of 2 monkeys increased above nondrug control levels despite the putative reinforcer not being consumed during the session. When saline or a smaller dose of cocaine was substituted for 1.0 mg/kg, response rates often were suppressed below nondrug control-level responding. This suppressive effect was observed in each monkey and was more likely to be observed and/or to be of greater magnitude in large-ratio components for 3 of the 4 monkeys. When saline was administered chronically at the end of the chronic-drug phase, response rates remained suppressed in the large-ratio component for 2 of the monkeys. There was, therefore, a schedule-dependent dissociation between behavioral tolerance and the residual effects: Tolerance was greater when small ratios were arranged, whereas the residual effects were more pronounced when larger ratios were arranged.

Situational specificity of tolerance to effects of phencyclidine on responding of rats under fixed-ratio and spaced-responding schedules

Responding of rats (n = 5) was maintained under DRL (lever) and Time-Delay (nose-key) schedules of food presentation in different experimental chambers during two separate daily sessions. Tolerance that developed to rate-decreasing effects of phencyclidine for nose-key pressing under the Time-Delay schedule did not extend to effects of phencyclidine on lever pressing under the DRL schedule. In a second experiment, both lever and nose-key pressing of rats were maintained under individual and multiple fixed-ratio schedules. One group of animals (n = 5) experienced both the individual and the multiple schedules in the same experimental chamber and another group (n = 5) experienced the individual and the multiple schedules in different experimental chambers. Tolerance that developed to behavioral effects of phencyclidine during the individual schedule did not extend to responding on even the same manipulandum under the multiple schedule in a different experimental chamber. In contrast, tolerance that developed to behavioral effects of phencyclidine during the individual schedule did extend to responding on even the different manipulandum under the multiple schedule in the same experimental chamber. Thus, tolerance that developed in the environment that was coincident with the pharmacologic actions of phencyclidine did not extend to similar operants in a different environmental condition, but did extend even to a different operant and schedule context in the same environmental condition.

Signal properties of reinforcement and reinforcement omission on a multiple fixed-ratio schedule

A series of experiments used food-deprived pigeons to examine several parameters of reinforcement omission in an attempt to control changes of keypeck response measures on a subsequent schedule. In Experiments 1 and 2, the pigeons were tested with a multiple fixed-ratio schedule on which reinforcement was occasionally omitted at the completion of the first component. The duration of the delay occurring in lieu of reinforcement was systematically varied. In Experiment 3, the stimulus that signaled the second component of the schedule was altered to appear either more or less similar to the stimulus that signaled the first component. Two principal results are reported: (1) Response latency decreased and, to a much lesser extent, terminal response rate increased as the delay occurring in lieu of reinforcement decreased; and (2) both latency decrease and response-rate increase were enhanced by a second component stimulus which was similar to the first. The results are evaluated in terms of Amsel’s frustration theory and an analysis by Staddon which suggests that reinforcement inhibits responding. The data appear to support Staddon’s argument that rate increases and latency decreases following reinforcement omission are largely a function of an attenuation of the inhibitory influence of reinforcement, an effect that is enhanced by stimulus generalization. Accordingly, it is proposed that an animal’s response to reinforcement omission is determined by a stimulus complex that minimally includes the omission event and component cues.

Duration of components and response rates on multiple fixed-ratio schedules

Pigeons were exposed to a two-component multiple fixed-ratio X fixed-ratio Y schedule of reinforcement in which X was always less than Y. Components were equal in duration and alternated at rates that varied between 2 sec and 23.5 h. Relative response rate in the FR X component: (1) increased as the duration of components increased between 2 sec and 15 min, (2) was at a maximum between 15 min and 6 h, and (3) decreased as the duration of components increased from 6 h to 23.5 h. The changes in relative response rate were attributable primarily to changes in absolute response rates during the FR Y schedule as absolute response rates during the FR X schedule were relatively invariant. These results pose complexities for several theoretical formulations.