Successive discrimination training with equated reinforcement frequencies: failure to obtain behavioral contrast.

In two experiments, pigeons were trained on two-component multiple schedules in which responding in one component (S(1)) was always maintained by a variable-interval schedule. In Experiment I, low response rates were reinforced in the second (S(2)) component for six master subjects. This schedule was adjusted to equate reinforcement frequencies in the two components. These subjects were compared to yoked partners, for which reinforcement in the S(2) component was made available on a variable-interval schedule whose value was determined by the master subjects. A similar procedure was used in Experiment II, where the S(2) schedule for master subjects made reinforcers contingent on the absence of responding. No evidence was found in either experiment for a behavioral contrast effect in the S(1) component attributable to response reduction in the S(2) component. A reliable contrast effect was obtained from a group of pigeons given extinction conditions in the S(2) component, which was compared to a group maintained throughout on a multiple variable-interval schedule. The results suggest that previous indications of behavioral contrast in similar situations were probably caused by uneven reinforcement distributions or reflect uncontrolled fluctuations in response rates.