Serial conditioning as a function of parametric variations of an interfood clock

Two experiments examined the effects of differences in the parameters of a clocked interfood interval on the obtained distribution of responding to the stimuli in that interval. In the first experiment, a 3×3 factorial design assessed the effects of the number of components and the durations of those components. Food was presented irrespective of behavior following 5, 10, or 20 discrete stimuli across durations of 3, 6, or 12 sec each. Responding began at the approximate midpoint of the interval. More responding occurred in earlier portions of the last half of the interval as the number and the durations of individual stimuli decreased or as the overall interfood interval decreased. The second experiment, also a 3×3 factorial design, manipulated the probability and duration of food presentation following a 60-sec trial containing 10 discrete stimuli. A 2.0-, 3.5-, or 5.0-sec food presentation followed 100%, 25%, or 10% of the trials. Responding again began at the approximate midpoint of the interval. More responding occurred in earlier portions of the last half of the interval only when both food duration and the proportion of reinforced trials increased. Both experiments therefore showed that the onset of responding occurs at the approximate midpoint of clocked interfood intervals in spite of a wide variety of CS and US manipulations.