Two experiments studied responding in the rat when the first bar press after a variable period of time produced a cue light that remained on for either 10, 30, or 100 sec and terminated with the delivery of food. In Experiment I, response rate decreased and time to the first response after reinforcement increased as the delay of reinforcement increased. Similar results were obtained whether the delay consisted of retracting the lever during the delay, a fixed delay with no scheduled consequence for responding, or every response during the delay restarted the delay interval. In Experiment II, fixed-delay and fixedinterval schedules of the same duration during the delay period had no differential effect on either response rate or time to the first response after reinforcement, but differentially controlled responding during the delay periods.The typical finding in delay of reinforcement studies has been that response rate is a negatively accelerated decreasing function of the delay duration (cf., review by Renner, 1964). Exceptions to this finding suggest that the procedure for delaying delivery of the reinforcer can influence the effect of the delay duration on performance. Watson (1917), Ferster (1953), and Logan (1960, for example, found little or no effect on performance of a delay duration which, in other studies, was sufficient to markedly reduce responding. The lack of an effect has been attributed to procedural differences in delaying reinforcer delivery. Conversely, large decrements in responding have also been attributed to the 'Experiment I was based on a thesis submitted by the first author to the Department of Psychology, De Pauw University, in partial fulfillment of the requirements for the M.A.
Food mas intermittently presented to pigeons on a responseindependent basis. Concurrently, pecking at a key was intermittently reinforcecl with the presentation of stimuli which accompanied the food presentation, but not the food. Under these conditions, key-pecking was maintained indefinitely at moderate rates and control procedures demonstrated that the results could only be accounted for on the basis of conditioned reinforcement.Conditioned reinforcemenc is often employed as an explanatory concept in the interpretation of behavioral events and is usually assigned a major role in accounting for the maintenance of behavior which is temporally and/or topographically separated from unconditioned reinforcement. As a consequence, a large number of experimental studies have focused Lipon demonstrating and assessing the conditioned reinforcing function of stimuli. W i t h few exceptions, however, these studies have employed an extinction procedure as the experimental paradigm. I n general, extinction paradigms involve ( a ) pairing a neutral stimulus with an unconditioned reinforcer and ( b ) subsequently testing the ability of the originally neutral stimulus to reinforce behavior after [he unconditioned reinforcer has been removed from the experimental situation. These studies have generally demonstrated that a neutral stimulus can acquire the power to reinforce behavior by being paired with an unconditioned reinforcer [see reviews by Miller (1951), Myers (1958), and Kelleher and Gollub (1962]. They have also indicated, however, that this acquired reinforcing power diminishes rapidly in the absence of unconditioned reinforcement.The generality of the latter finding could lead one to question the emphasis placed upon the role of conditioned reinforcemenc in the maintenance of behavior. However, perhaps what should be questioned is the emphasis placed upon the role of the extinction paradigms themselves as the mechods of choice for studying conditioned reinforcement. The praccice of assessing the conditioned reinforcing function of stimuli in the absence of unconditioned reinforcement assures the elimination of the possible confounding influences of unconditioned reinforcement on behavior. The re~noval of unconditioned reinforce---
Key-pecking intermittently produced a set of brief exteroceptive stimulus changes under two-component multiple schedules of conditioned reinforcement. Throughout the study, free access to grain was concurrently provided on an intermittent basis via a variable-interval tape. Free food presentations scheduled by the tape were delivered if no peck had been emitted for 6 sec, and the brief stimulus changes produced by responding under the multiple schedules were those which accompanied food presentation. The second component of each multiple schedule was always associated with a 1-min, variable-interval schedule of conditioned reinforcement. The schedule associated with the first component was systematically varied and conditioned reinforcement was either absent (extinction) or programmed on a 1-, 3-, 6-, or 12-min variable-interval schedule. Under these conditions, rate of responding in the manipulated component decreased monotonically with a decrease in the frequency of conditioned reinforcement. In addition, contrast effects were often obtained in the constant, second component. These results are similar to those obtained with similar multiple schedules of primary reinforcement.In their review of positive conditioned reinforcement, Kelleher and Gollub (1962) suggested and gave many illustrations of an experimental approach involving the study of conditioned reinforcement under conditions in which primary reinforcement is not removed from the experimental situation. This approach involves the application of chained schedules of reinforcement and related procedures. In a chained schedule, responding in the presence of a given exteroceptive stimulus is reinforced on a given schedule with a subsequent exteroceptive stimulus. A chained schedule can involve any number of exteroceptive stimuli, each one of which is correlated with a given member or component. Since primary reinforcement terminates the schedule, and since each of the other exteroceptive stimuli in the chain can potentially serve as a conditioned reinforcer for behavior in the respective preceding member, chained schedules can be employed to assess the con-
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