Experiment 1 investigated the controlling properties of variability contingencies on choice between repeated and variable responding. Pigeons were exposed to concurrent-chains schedules with two alternatives. In the REPEAT alternative, reinforcers in the terminal link depended on a single sequence of four responses. In the VARY alternative, a response sequence in the terminal link was reinforced only if it differed from the n previous sequences (lag criterion). The REPEAT contingency generated low, constant levels of sequence variation whereas the VARY contingency produced levels of sequence variation that increased with the lag criterion. Preference for the REPEAT alternative tended to increase directly with the degree of variation required for reinforcement. Experiment 2 examined the potential confounding effects in Experiment 1 of immediacy of reinforcement by yoking the interreinforcer intervals in the REPEAT alternative to those in the VARY alternative. Again, preference for REPEAT was a function of the lag criterion. Choice between varying and repeating behavior is discussed with respect to obtained behavioral variability, probability of reinforcement, delay of reinforcement, and switching within a sequence.

The effects of fixed-ratio (FR) and differential-reinforcement-of-low-rate (DRL) schedule histories on behavioral persistence were studied in three experiments with pigeons. After exposure to a multiple FR DRL schedule (baseline), either one of two prefeeding amounts were effected under the same schedule (test condition of Experiment 1), under extinction (test condition of Experiment 2), and under fixed-interval (FI) schedules (test condition of Experiment 3). FR response rates generally were less persistent than DRL response rates in Experiments 1 and 2, with the opposite occurring in Experiment 3. Regardless of the test schedules, FR response rates decreased and were less persistent under the large than under the small prefeeding amount. Rates under Experiment 1 and portions of Experiment 3 were completed by the first author, under the supervision of the second author, in partial fulfillment of the requirements of a Masters degree in Psychology at Universidade de Brasília, Brazil. The research was supported by a grant from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) to the first author, and by an undergraduate scholarship from Universidade de Brasília (Programa de Iniciação Científica [PIBIC-UnB]) to the third author. We thank Andy Lattal for his contributions to the method, and Leticia Faria for help with data collection. Portions of these data were presented at the 38 th Annual Convention of the Association for Behavior Analysis International,

The metaphor of the behavior stream provides a framework for studying the effects of responseindependent food presentations intruded into an environment in which operant responding of pigeons was maintained by variable-interval schedules. In the first two experiments, response rates were reduced when response-independent food was intruded during the variable-interval schedule according to a concomitantly present fixed-time schedule. These reductions were not always an orderly function of the percentage of response-dependent food. Negatively accelerated patterns of key pecking across the fixed-time period occurred in Experiment 1 under the concomitant fixedtime variable-interval schedules. In Experiment 2, positively and negatively accelerated and linear response patterns occurred even though the schedules were similar to those used in Experiment 1. The variable findings in the first two experiments led to three subsequent experiments that were designed to further illuminate the controlling variables of the effects of intruded response-independent events. When the fixed and variable schedules were correlated with distinct operanda by employing a concurrent fixed-interval variable-interval schedule (Experiment 3) or with distinct discriminative stimuli (Experiments 4 and 5), negatively accelerated response patterns were obtained. Even in these latter cases, however, the response patterns were a joint function of the physical separation of the two schedules and the ratio of fixed-time or fixed-interval to variable-interval schedule food presentations. The results of the five experiments are discussed in terms of the contributions of both reinforcement variables and discriminative stimuli in determining the effects of intruding response-independent food into a stream of operant behavior.

The effects of the response-reinforcer dependency on resistance to change were studied in three experiments with rats. In Experiment 1, lever pressing produced reinforcers at similar rates after variable interreinforcer intervals in each component of a two-component multiple schedule. Across conditions, in the fixed component, all reinforcers were response-dependent; in the alternative component, the percentage of response-dependent reinforcers was 100, 50 (i.e., 50% response-dependent and 50% response-independent) or 10% (i.e., 10% response-dependent and 90% response-independent). Resistance to extinction was greater in the alternative than in the fixed component when the dependency in the former was 10%, but was similar between components when this dependency was 100 or 50%. In Experiment 2, a three-component multiple schedule was used. The dependency was 100% in one component and 10% in the other two. The 10% components differed on how reinforcers were programmed. In one component, as in Experiment 1, a reinforcer had to be collected before the scheduling of other response-dependent or independent reinforcers. In the other component, response-dependent and -independent reinforcers were programmed by superimposing a variable-time schedule on an independent variable-interval schedule. Regardless of the procedure used to program the dependency, resistance to extinction was greater in the 10% components than in the 100% component. These results were replicated in Experiment 3 in which, instead of extinction, VT schedules replaced the baseline schedules in each multiple-schedule component during the test. We argue that the relative change in dependency from Baseline to Test, which is greater when baseline dependencies are high rather than low, could account for the differential resistance to change in the present experiments. The inconsistencies in results across the present and previous experiments suggest that the effects of dependency on resistance to change are not well understood. Additional systematic analyses are important to further understand the effects of the response-reinforcer relation on resistance to change and to the development of a more comprehensive theory of behavioral persistence.

The effects of reinforcement rates of alternative responding on resurgence were studied in two experiments with rats. In both experiments, left-and right-lever pressing were reinforced according to a multiple schedule in the Training and Alternative Reinforcement phase, respectively. In the Test phase, reinforcers were discontinued. In Experiment 1, reinforcement rates were similar between components in the Training phase, and different between components in the Alternative Reinforcement phase. This latter difference was manipulated parametrically across conditions. Resurgence occurred more frequently (i.e., in more sessions) and was of greater magnitude (i.e., response rates were relatively higher) in the rich (i.e., higher reinforcement rate) than in the lean (i.e., lower reinforcement rate) component. Additionally, for two of the four rats, the magnitude of resurgence in both components was positively related to reinforcement rates in the Alternative-Reinforcement phase. In Experiment 2, reinforcement rates were different between components in the Training phase, and similar between components in the Alternative Reinforcement phase. Across conditions, the This research was supported by a postdoctoral fellowship (PNPD-CAPES) to Carlos Cançado. We thank Ademar Gonçalves and Hugo Gomes Vieira for help with data collection. We also thank the students in the Experimental Analysis of