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 involving 5 homing pigeons. Ss discriminated between the 1st and 2nd halves of a 50-sec trial. Responses on a left key were reinforced at variable intervals for the 1st 25 sec of the trial, and right-key responses were reinforced at variable intervals during the 2nd 25 sec. In Exp 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 Exp 2, reinforcer duration was manipulated as a means of altering reinforcer density. Pacemaker rate was found to be directly related to reinforcer duration. Results support the assumption that pacemaker rate is determined by reinforcer density.