Complex relationship between response rate and preference in pigeons: Williams (1992) revisited.

A decrease in the rate of reinforcement associated with one component of a multiple schedule is typically associated with a decrease in responding in that component as well as with an increase in responding in the unchanged component. This increase in responding, referred to as positive contrast, is thought to result from an increase in the subjective value associated with the unchanged component. Williams Animal Learning & Behavior, 19, 337-344, (1991) challenged this hypothesis in an experiment with pigeons in which Stimulus A, associated with a variable interval schedule, was always followed by Stimulus X, associated with extinction, while Stimulus B, associated with the same variable interval schedule, was always followed by Stimulus Y, also associated with a variable interval schedule. Although Williams found that most of the pigeons pecked more at Stimulus A than at Stimulus B (behavioral contrast), when the pigeons were given a choice between Stimulus A and B, they showed a preference for Stimulus B. In the present experiment (a slight modification from Williams's), we confirmed this finding. Although our pigeons pecked more at Stimulus A than at Stimulus B, they generally preferred Stimulus B, the stimulus that was not followed by extinction. This result suggests that positive contrast may not result from an increase in the subjective value of the unchanged component. Instead, it suggests that this version of positive contrast may result at least in part from the pigeons' attempt to get all of the reinforcers possible in the presence of Stimulus A before the extinction schedule begins.