Within-Session Changes in the Preratio Pause on Fixed-Ratio Schedules of Reinforcement

Performances under fixed-ratio schedules of reinforcement are characterized by a "post-reinforcement" or "preratio" pause that precedes responding for the reinforcer. This paper summarizes views on the origins of pausing and presents a series of molecular analyses that shed new light on the conditions under which pausing occurs. Among the findings are that pausing is not limited to the delay before the first response of the ratio and that pause durations increase over time (in some cases more than doubling across a 40-ratio session). Similar findings were made also with variable-ratio schedules. The results are consistent with the view that pausing is a byproduct of a competition between the scheduled reinforcer and other sources of behavioral control. Keywords: fixed ratio, variable ratio, schedule of reinforcement, preratio pause, rats ********** Fixed-ratio (FR) schedules of reinforcement consist of the delivery of a reinforcer after every nth response. If, for example, subjects consistently are required to complete 50 responses to obtain a single food pellet, then the ratio of responses to reinforcers is 50:1 (FR 50). Performances engendered by FR schedules are characteristically biphasic: an initial delay in responding (i.e., what is termed either the postreinforcement pause or the preratio pause) precedes a period of responding at a sustained rapid pace that terminates with the delivery of the reinforcer. The preratio pause is of interest to researchers because it is a well-known feature of FR performances, yet one that is not required by the contingencies. Indeed, an optimally efficient pattern of responding would not include the pause: the sooner the subject completes the response requirement, the sooner the reinforcer will be delivered. For this reason, the preratio pause has been likened to human procrastination (Shull & Lawrence, 1998). Pausing under FR schedules might occur, in part, because the subject needs time to consume the previously delivered reinforcer or to recover from fatigue incurred in the course of responding. However, it is unlikely that either factor can explain pausing as comparisons with variable-ratio (VR) schedules show that subjects can complete equivalent numbers of responses while making much shorter pauses (Crossman, Bonem, & Phelps, 1987; Mazur, 1983). There is not a consensus as to why pausing occurs, but contemporary accounts emphasize competing sources of behavioral control (for recent discussions of the origins of pausing see Mazur, 2005; Perone, 2003; Pierce & Cheney, 2004; Wade-Galuska, Perone, & Wirth, 2005). For example, pausing has been described as a byproduct of a competition between inhibition elicited by the nonreinforcement of responses early in the ratio and excitation elicited by the upcoming reinforcer (e.g., Derenne & Baron, 2001). Alternatively, pausing might be an act of temporary "escape" from aversive features of the response requirement (e.g., Perone, 2003). Two additional explanations for pausing are highlighted by an earlier study (Derenne & Baron, 2002) in which subjects lever pressing for food on a FR schedule had continuous access to a bottle filled with a saccharine solution. Although the subjects were not liquid deprived, the presence of the saccharine solution led to polydipsia-like levels of drinking. Drinking principally occurred at the beginning of each ratio and corresponded with markedly longer pausing. Perhaps pausing occurs in part because the low probability of the scheduled reinforcer at the beginning of the reinforcer promotes adjunctive-like behaviors that compete with the response; in this case the result was excessive drinking, but other behaviors might show a similar effect (cf. Pierce & Cheney, 2004). However, the drinking during the pause also suggests parallels between pausing and the appearance of "impulsiveness" under the Ainsile-Rachlin model of self-control (Ainslie, 1974; Rachlin, 1970; Rachlin & Green, 1972). …