Operant Variability: A Behavioral and Pharmacological Analysis

Abstract

Behavioral variability is often demonstrated in the laboratory setting by imposing a multiple schedule of reinforcement: one component requires the animal to vary sequences of responding and the other component serves as a control. An important determinant of behavioral variability is the type of reinforcement schedule present. When reinforcement is infrequent or is discontinued, as during extinction, behavior begins to vary. The extent to which intermittent reinforcement produces variability can change based on reinforcer density and schedule type. Behavioral variability also increases when it is directly reinforced. This operant variability is robust and is unaffected by interventions. The purpose of the Experiment 1 was to determine intermittency parametrically by using different rates of reinforcement (both rich and lean) on operant variability in a session. Another purpose was to assess differences, if any, between interval schedules and ratio schedules of reinforcement on variability either when it is required or permitted. Long Evans rats were trained under a multiple VARY 8:4 FR 4 schedule. In the VARY 8:4 component, all four-response sequences that differed from previous 8 were reinforced. In the FR 4 component, all four-response sequences were reinforced. There was much higher variability (based on an entropy measure) in the VARY 8:4 than in the FR 4 component in the FR 1 condition. All intermittent reinforcement schedules, both VIs and the VR contingencies always increased entropy in the FR 4 component but did not affect behavior in the VARY 8:4 component. Variability in the unit FR 4 component reflected the prevailing schedule and returned to baseline rapidly upon imposition of the baseline schedule. Intermittent reinforcement blunted the differences between the VARY 8:4 and FR 4 components regardless of the schedule parameters. The purpose of Experiment 2 was to examine intermittent reinforcement in the control condition, the utility of the timeout for non-criterion sequences in the vary condition, and the importance of similar reinforcement rates between the two components. The effects of d-amphetamine on behavioral variability under these conditions were also examined. Long Evans rats were trained to press two levers in multiple conditions. In the VARY 8:4 component, any four-response sequence distributed between these levers that differed from previous 8 sequences was eligible for reinforcement. Rats were then divided into two groups, depending on the control condition used. Two different control components were introduced. The first was a simple FR 4 procedure where every four-response sequence was reinforced. The other procedure was a Yoked FR 4 where inter-reinforcer intervals were equated between the VARY 8:4 component and the FR 4 component. To examine the role of timeouts for errors, each group was exposed to an ABA design including and removing timeouts in the VARY 8:4 component. The effects of d-amphetamine were assessed during the first two phases. Finally, to equate the rate at which sequences were reinforced in both the VARY 8:4 and FR 4 components, a Variable Ratio 3 schedule was imposed in the FR 4 component only (i.e., mult FR 1 (VARY 8:4) VR 3 (FR 4). In both the Yoke and Non-Yoke conditions, there was much greater variability in the VARY 8:4 than in the FR 4 component. The inclusion of a timeout for non-criterion sequences had no effect on entropy in both the Yoke and No-Yoke groups. The number of errors made during the VARY 8:4 component decreased only in the Yoke group. The effects of d-amphetamine on variability were not influenced by the timeout. The highest dose of d-amphetamine decreased response rates, increased entropy in the FR 4 component and decreased or had no effect on entropy in the VARY 8:4 component suggesting that effects may be baseline dependent. Entropy in the FR 4 component increased as a result of the presence of the VR 3 schedule. Intermittent reinforcement blunted the differences between the VARY 8:4 and FR 4 components.