Time Perception: A Test of Weber's Law in Possums

The performance of brushtail possums, Trichosurus vulpecula, was investigated in two experiments that used the peak procedure to investigate their ability to time. For Experiment 1 five possums lever pressed for food reinforcers on 3 different Fixed Interval (FI) schedules that included FI 15-s, FI 30-s and FI 60-s. The possums also experienced Peak Interval (PI) schedules that did not provide reinforcement for 20% of trials. The PI trials were 3 times longer than the FI schedule that was in effect on the other 80% of trials when responding was reinforced. Response rates increased to a maximum at about the time the responses were normally reinforced on FI trials and then decreased to a minimum at around 2 times the FI schedule duration that was in effect on a given condition. When relative response rates were plotted as a function of relative time the distributions generally superposed for the ascending, but not descending portions of the function. There was evidence of resurgence in response rates towards the end of the PI trials. The amount of resurgence appeared to be a function of the amount of experience animals had with the schedule and decreased across days, however, it was never eliminated. In Experiment 2, five possums lever pressed for food reinforcers on an FI 30-s schedule. Once again on 80% of trials responding was reinforced. For the other 20% of trials, responding was not reinforced and trials lasted either 3 (Experiment 2, Condition 1) or 10 (Experiment 2, Condition 2) times longer than the FI schedule that was in effect, and these extinction trials ended automatically when the specific time had passed. Resurgence was still evident at the end of the PI trials when they were 3 times longer than the FI, but decreased dramatically when they were 10 times longer than the FI that was in effect, resulting in the conclusion that that resurgence was dependent on the duration of the PI trials