Effects of methylphenidate on sensitivity to reinforcement delay, magnitude, and probability: Implications for impulsive and risky choice.

Under rapid-acquisition, concurrent-chains choice procedures, psychomotor stimulants typically decrease the sensitivity of responding to changes in separate dimensions of reinforcement. Across two experiments, pigeons chose between outcomes that differed in terms of reinforcement delay and magnitude (the dimensions involved in delay discounting or "impulsive" choice; Experiment 1) or reinforcement probability and magnitude (the dimensions involved in probability discounting or "risky" choice; Experiment 2). Outcomes associated with each terminal link were varied independently and pseudorandomly across sessions such that in dominated sessions one terminal link was favorable in terms of both dimensions (sooner, larger in Experiment 1 and more likely, larger in Experiment 2) and in trade-off sessions each terminal link was favorable in terms of a different dimension. Response allocation in initial links tracked changes in terminal-link outcomes in a manner that suggests each dimension contributed additively and independently to choice. Methylphenidate decreased sensitivity to all dimensions of reinforcement at a dose (or doses) that did not substantially affect bias or initial-link response rates. The degree to which methylphenidate decreased sensitivity was related to baseline sensitivity for delay and magnitude but not for probability. Baseline dependency may be a more useful approach for predicting drug effects on delay/impulsive, rather than risky, choice.