A clock not wound runs down.

Seven pigeons were trained to respond on one key during the first half of a 60-s trial, and on another during the second half, to test the prediction that the rate of the internal pacemaker should slow in the absence of reward. The relative probability of responding was well-described by Erlang distributions, which yielded as parameters the period of the pacemaker and the criterial number of counts for switching to the second key. During a subsequent period of extinction the period increased linearly with time in extinction, and the criterial count decreased. This slowing of the pacemaker was predicted by the Behavioral Theory of timing, but not by other theories.

Satiation causes within-session decreases in instrumental responding.

The amount of food necessary to cause within-session decreases in response rates was investigated by varying reinforcer duration (Experiment 1) and grain size (Experiment 2). Within-session response rates quickly increases to a peak, and the decrease throughout the rest of the session was greater for conditions with larger amounts of food. The decreases were greatest for the pigeons that ate the least when food was freely available. Previous studies of within-session changes in responding are reviewed and reinterpreted. It was concluded that satiation causes the decrease in responding and that this effect is not restricted to extreme manipulations of the amount of food consumed.

Models of ratio schedule performance.

Predictions of P. R. Killeen's (1994) mathematical principles of reinforcement were tested for responding on ratio reinforcement schedules. The type of response key, the number of sessions per condition, and first vs. second half of a session had negligible effects on responding. Longer reinforcer durations and larger grain types engendered more responding, affecting primarily the parameter alpha (specific activation). Key pecking was faster than treadle pressing, affecting primarily the parameter delta (response time). Longer intertrial intervals led to higher overall response rates and shorter postreinforcement pauses and higher running rates, and ruled out some competing explanations. The treadle data required a distinction between the energetic requirements and rate-limiting properties of extended responses. The theory was extended to predict pause durations and run rates on ratio schedules.

Biasing the pacemaker in the behavioral theory of timing.

In the behavioral theory of timing, pacemaker rate is determined by overall rate of reinforcement. A two-alternative free-operant psychophysical procedure was employed to investigate whether pacemaker period was also sensitive to the differential rate of reinforcement. Responding on a left key during the first 25 s and on a right key during the second 25 s of a 50-s trial was reinforced at variable intervals, and variable-interval schedule values during the two halves of the trials were varied systematically. Responding on the right key during the first 25 s and on the left key during the second 25 s was not reinforced. Estimates of pacemaker period were derived from fits of a function predicted by the behavioral theory of timing to right-key response proportions in consecutive 5-s bins of the 50-s trial. Estimates of pacemaker period were shortest when the differential reinforcer rate most strongly favored right-key responses, and were longest when the differential reinforcer rate most strongly favored left-key responses. The results were consistent with the conclusion that pacemaker rate is influenced by relative reinforcer rate.

The behavioral theory of timing: Reinforcer rate determines pacemaker rate.

In the behavioral theory of timing, pulses from a hypothetical Poisson pacemaker produce transitions between states that are correlated with adjunctive behavior. The adjunctive behavior serves as a discriminative stimulus for temporal discriminations. The present experiments tested the assumption that the average interpulse time of the pacemaker is proportional to interreinforcer interval. Responses on a left key were reinforced at variable intervals for the first 25 s since the beginning of a 50-s trial, and right-key responses were reinforced at variable intervals during the second 25 s. Psychometric functions relating proportion of right-key responses to time since trial onset, in 5-s intervals across the 50-s trial, were sigmoidal in form. Average interpulse times derived by fitting quantitative predictions from the behavioral theory of timing to obtained psychometric functions decreased when the interreinforcer interval was decreased and increased when the interreinforcer interval was increased, as predicted by the theory. In a second experiment, average interpulse times estimated from trials without reinforcement followed global changes in interreinforcer interval, as predicted by the theory. Changes in temporal discrimination as a function of interreinforcer interval were therefore not influenced by the discrimination of reinforcer occurrence. The present data support the assumption of the behavioral theory of timing that interpulse time is determined by interreinforcer interval.