Pacemaker rate in the behavioral theory of timing

The assumption by the behavioral theory of timing that pacemaker rate is proportional to reinforcer density was tested in 2 experiments. Pigeons discriminated between the 1 st and 2nd halves of a 50-s trial. Responses on a left key were reinforced at variable intervals for the first 25 s of the trial, and right-key responses were reinforced at variable intervals during the second 25 s. In Experiment 1, overall reinforcement rate was varied by manipulating the intertrial interval (ITI) duration. Pacemaker rate, estimated by fitting predictions to psychometric functions, was an inverse function of ITI. In Experiment 2, reinforcer duration was manipulated as a means of altering reinforcer density. Pacemaker rate was found to be directly related to reinforcer duration