Reinforcer distributions affect timing in the free-operant psychophysical choice procedure

In procedures used to study timing behavior, the availability of reinforcement changes according to time since an event. Manipulation of this reinforcer differential often produces violations of scalar timing, but it is unclear whether such effects arise because of a response bias or a change in temporal discrimination. The present experiment investigated the effects of the overall and relative probability of obtaining a reinforcer on performance in the free-operant psychophysical procedure. We arranged short and long trials with unequal reinforcer ratios, at high or low overall reinforcer rates. Changes in the overall reinforcer rate produced only small changes in timing behavior. Changes in relative reinforcer probability, which caused differences in the likely availability of reinforcers across time within a trial, produced a change in both bias and discrimination. We suggest reinforcers affect timing, and that discrimination in timing tasks depends on the distribution of reinforcers in time, as well as on the interval to be timed.