When to respond? And how much? : Temporal control and response output on mixed-fixed-interval schedules with unequally probable components

Rats were trained on mixed-fixed-interval (FI) schedules, with component FIs of 30 and 60 s. The probability of reinforcement according to FI 30 s varied between conditions, across values of 0.1, 0.3, 0.5, 0.7 and 0.9. When response rate in the 60 s intervals was measured, separate response peaks, one close to 30 s, the other at 60 s, could be identified when the probability of reinforcement at 30 s was 0.3 or greater. Nonlinear regression found that the location of the earlier peak was always close to 30 s, that the coefficient of variation of the response functions at 30 and 60 s were unaffected by reinforcement probability, but that the 30 s component appeared to be timed slightly more precisely than the 60 s one. Response rate at around 30 s increased with increasing probability of reinforcement according to FI 30 s, but responding at 60 s was unaffected by reinforcement probability. The data are discussed with respect to a number of contemporary models of animal timing (scalar expectancy theory, the Behavioural Theory of Timing and the Learning to Time model), and a recent account of response output on FI-like schedules.