RESUMO
Progressive ratio (PR) schedules have been widely used to study motivation to work for a reinforcer. After a post-reinforcer pause, subjects engage in pressing a lever until a reinforcer is obtained. However, the discrete nature of lever presses allows alternative behaviors during inter-response time and has led to several behavioral explanations of pauses and work time. A progressive hold-down (PH) is incompatible with alternative responses and may allow a precise estimation of work time. Performance of rats trained in both PR and PH that received sucrose or intracranial self-stimulation (ICSS) as reinforcer were compared. We observed that rats mastered the PR and PH schedules. Post-reinforcer pauses (PSRP), work time and inter-reinforcer time increased as a function of the response or hold requirement. However, rats' performance suggested that the PH progression may be experienced by the rats as easier that the PR progression. Elimination of consummatory behavior with ICSS reduced post-reinforcer pause in accordance with predictions of explanatory models of fixed and variable schedules of reinforcement. In the case of PH performance, pauses showed little variation across intermediate requirements but increased rapidly on later requirements; since rats controlled their pause length and work time was close to the hold requirement, time allocation between PR and PH schedules diverged. Finally, the Mathematical Principles of Reinforcement model of Bradshaw and Killeen (Psychopharmacology 2012, 222: 549) rendered a good description of the performance in both PR and PH schedules.