Timing and the control of variation

Two rat experiments shed light on how variation in behavior is regulated. E xperiment I used the peak procedure. On most trials, the 1st bar press more than 40 s after signal onset ended the signal and produced food. Other tri als lasted much longer and ended without food. On those trials, the variabi lity of bar-press duration increased greatly after the 1st response more th an 40 s after signal onset. In Experiment 2, which asked whether the increa se was due to the omission of expected reward or the decrease in reward exp ectation, reward expectation had a strong effect on response duration, wher eas omission of expected reward had little effect. In both experiments, res ponse rate and response duration changed independently, suggesting that the y reflect different parts of the underlying mechanism. In Experiment 1, res ponse durations implied that timing of the signal was more accurate than th e rate-vs.-time function might suggest. Experiment 2 suggested that lowerin g reward expectation increases variation in response form.