Discounting of probabilistic food reinforcement by pigeons

Two experiments examined pigeons' discounting of probabilistic food reinforcers using a concurrent-chains procedure within sessions combined with an adjusting-amount procedure across sessions. The study is the first to evaluate probability discounting functions in nonhuman animals in which the obtained probability equaled that programmed in each session. In Experiment 1, pigeons chose between a smaller, certain and a larger, but probabilistic food reinforcer. In Experiment 2, the probabilities of receiving both reinforcement options were reduced by a common factor, creating a choice situation in which both options were probabilistic: a smaller, more probable reinforcer and a larger, but less probable reinforcer. Results revealed that regardless of whether choices involved a certain versus a probabilistic reinforcer or they involved two probabilistic reinforcers, subjective value decreased systematically as a function of the odds against receipt of the reinforcer and that the data were well described by the hyperboloid discounting function. In addition, no significant effect of reinforcer amount on degree of probability discounting was observed, consistent with results from studies of the discounting of delayed reinforcers by nonhuman animals.