Rats work very hard for intracranial self-stimulation (ICSS) and tradeoff effort or time allocation for intensity and frequency parameters producing a sigmoidal function of the subjective reward magnitude of ICSS. Previous studies using electrical intracranial stimuli (ICS) as a discriminative cue focused on estimating detection thresholds or on the discrimination between intensities. To our knowledge, there is no direct comparison of the reinforcer tradeoff functions with the discriminative functions. Rats were trained to press and hold the lever for ICSS using the maximum reinforcing intensity below motor alterations or avoidance behavior. First, rats were trained to hold the lever for 1 s; after stability, they undergo trials where intensity or frequency was decreased on 0.1 log step. Thereafter, they undergo further training with a hold of 2 and later of 4 s to determine tradeoff with intensity or frequency. The same rats were trained on a discrimination task where the previously used ICSS signaled a lever where a 1 s hold response was followed by a reinforcing ICSS; on randomly alternating trials, a −0.6 log ICS signaled an alternate lever where a similar hold response led to a reinforcer. After mastering discrimination, generalization tests were carried out with varying intensity or frequency. Rats completed training with 2 and later 4 s hold response. After the completion of each task, the rats had different doses of a pimozide challenge while their intensity and hold-down requirement were varied. With regards to the rats’ tradeoff response time allocation as a function of intensity or frequency, sigmoid functions were displaced to the right when long responses were required. Rats that learned the discrimination task attained a discrimination index of 90–98%. Discrimination accuracy decreased slightly with the increase of hold requirement, but generalization gradients were not displaced to the right as a function of the response requirement. Pimozide induced a dose-dependent displacement of the time-allocation gradients, but it did not affect the generalization gradients. It is concluded that rats integrate response requirements as part of the reinforcement tradeoff function, but the response cost is not integrated into the discriminative function of ICSS.
Progressive ratio (PR) schedules had been widely used to study motivation to work for a reinforcer. After a post-reinforcer pause, subjects engage pressing a lever until a reinforcer is obtained. However, the discrete nature of lever presses allows alternative behaviors during inter-response time and had lead to the suggestion of several behavioral categories to explain 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, work time and inter-reinforcer time increased as a function of the response or hold requirement. However, performance of rats suggest that the PH progression may be experienced by the rats as easier that the PR progression. Elimination of consummatory behavior with ICSS reduced PSRP and in accordance with predictions of explanatory models of fixed and variable schedules of reinforcement. In the case of PH performance, intermediate requirements leveled off pauses but inceased rapidly on later requirements; since rats controlled pause length and work time was close to 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.
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