A rat was trained on a schedule that programmed reinforcements only when a minimum waiting time between successive responses was exceeded (DRL schedule). It was observed to fill much of the pause between lever presses with a stereotyped behavioral chain: it would take its tail in its mouth and nibble it. This behavior was shown to be functionally related to the efficiency with which the subject spaced its responses. It is thought to have served as mediating behavior, providing discriminating stimuli for appropriate lever presses.There are many descriptions of spontaneously arising response chains on reinforcement schedules that require subjects to space responses in time (Bruner and Revusky, 1961;Dews and Morse, 1958;Hodos, Ross, and Brady, 1962;Holz, Azrin, and Ulrich, 1963;Kapostins, 1963;Laties and Weiss, 1962;Malott and Cumming, 1964;Segal and Holloway, 1963;Stoddard, 1962;Wilson and Keller, 1953). This collateral behavior is usually thought to serve as mediating behavior in the sense conveyed by the Ferster and Skinner definition of the term: "Behavior occurring between two instances of the response being studied . . . which is used by the organism as a controlling stimulus in subsequent behavior" (Ferster and Skinner, 1957, p. 729). In only one case has such behavior been subjected to intensive analysis in an attempt to determine whether it is, in fact, serving a mediating role. While studying EEG correlates of the performance of monkeys on a DRL schedule, Hodos et al. (1962) found that their records were being contaminated by movement artifacts; one of their monkeys was regularly jerking its head to one side and the second animal was regularly licking its water bottle holder. Procedures that interfered with this behavior 'From the
When the lever-pressing behavior of five rats was maintained by a DRL schedule (reinforcement was scheduled only when a specified waiting time between successive responses was exceeded), collateral behavior developed that apparently served a mediating function. In two cases this behavior did not arise until the experimental environment included pieces of wood that the rats started to nibble. When collateral behavior first appeared, it was always accompanied by an increase in responses spaced far enough apart to earn reinforcement. If collateral behavior was prevented, the number of reinforced responses always decreased. Extinction of lever pressing extinguished the collateral behavior. Adding a limited-hold contingency to the schedule did not extinguish collateral behavior. It appears that the rat can better space its responses appropriately when concurrently performing some overt collateral activity. The amount of this activity apparently comes to serve as a discriminative stimulus. To assume the existence of internal events that serve as discriminative stimuli in temporal discriminations is, at least under some circumstances, unnecessary.Questions concerning the discrimination of time are often phrased in terms of the discrimination of on-going physiological events. Dimond (1964) reflected this viewpoint when, in reviewing the "structural basis of timing", he wrote:"The stream of sensory impulses gathered from the environment is distributed in time. It is supposed that the duration of stimuli and the intervals between them are compared with an internal standard.Such a standard could be represented by the steady functioning of some mechanism of the body." The notion of some sort of internal standard or "clock" appears in much of the literature in the traditional field of time perception 'Dedicated to B. F. Skinner in his sixty-fifth year.
Subjects pressed a telegraph key to illuminate a meter dial on which pointer deflections appeared at fixed intervals. Upon detecting a deflection they were required to press another key to reset the pointer to zero. This detecting and resetting operation reinforced the behavior of pressing the light-flashing key (i.e., the observing responses). The usual pattern of responding on the light-flashing key was a long pause following the reinforcement and an abrupt transition to a steady response rate toward the end of the interval. When the subjects were required to perform a concurrent subtraction task, the pattern of responding changed in varying degrees, ranging from complete loss of typical fixed-interval behavior to a slight shortening of the post-reinforcement pause. These effects were attributed to the disruption of the self-produced verbal chains (counting or reciting) that ordinarily govern human behavior on this schedule.
The fixed-interval schedule of reinforcement is one of the more widely studied schedules in the experimental analysis of behavior and is also a common baseline for behavior pharmacology. Despite many intensive studies, the controlling variables and the pattern of behavior engendered are not well understood. The present study examined the microstructure and superstructure of the behavior engendered by a fixed-interval 5-and a fixed-interval 15-minute schedule of food reinforcement in the pigeon. Analysis of performance typical of fixed-interval responding indicated that the scalloped pattern does not result from smooth acceleration in responding, but, rather, from renewed pausing early in the interval. Individual interresponse-time (IRT) analyses provided no evidence of acceleration. There was a strong indication of alternation in shorter-longer IRTs, but these shorter-longer IRTs did not occur at random, reflecting instead a sequential dependency in successive IRTs. Furthermore, early in the interval there was a high relative frequency of short IRTs. Such a pattern of early pauses and short IRTs does not suggest behavior typical of reinforced responding as exemplified by the pattern found near the end of the interval. Thus, behavior from clearly scalloped performance can be classified into three states: postreinforcement pause, interim behavior, and terminal behavior. Key words; fixed-interval schedule of reinforcement, pauses, interresponise-time analysis, microanalysis, cumulative record, factor analysis, second-order deviations, pigeonsThe fixed-interval schedule of reinforcement is perhaps the simplest schedule of intermittent reinforcement to arrange: The first appropriate response occurring a fixed time after some stimulus event (typically the previous reinforcer) is reinforced. However, the simplicity of the arrangement belies the complexity of the behavior that results.The behavior engendered by fixed-interval schedules, over a wide range of temporal parameters, has been subjected to intensive study in the experimental analysis of behavior because it models what is called temporal con-
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