The Relation between Variable-Interval Schedule Value and Subsequent Progressive-Ratio Persistence

Abstract

The effect of variable-interval reinforcement rate on subsequent progressive-ratio responding was investigated by exposing five groups of rats to 15-session blocks of variable-interval training followed by a single progressive-ratio "test" session. Progressive-ratio test sessions began with a response requirement of 25 and incremented by 25 following each reinforcer. Sessions terminated when no responding occurred for 15 min. The reinforcement rate arranged by the variable-interval schedule was changed following exposure to each block of training and test sessions according to a Latin square design. Each subject received two blocks of training with variable-interval schedules arranging reinforcement rates of .25, .5, 1.0, 2.0, and 4.0 reinforcers per min. Response rates during the final five sessions of each variable-interval exposure was an increasing function of variable-interval reinforcement rate. Similarly, total responses emitted and response rate during progressive-ratio test sessions increased as a function of variable-interval reinforcement rate. In contrast, the amount of time required to satisfy the 15-min extinction criterion used to terminate test sessions decreased as variable-interval reinforcement rate increased. Similarly, the ratio of response rate during test sessions to the response rate during the five variable-interval sessions preceding the test was inversely related to the reinforcement rate arranged during the variable-interval sessions. Responding during test sessions persisted longer and response rate declined less following exposure to the lowest reinforcement rate variable-interval schedules than following the richer parameters. The use of occasional test sessions interpolated between blocks of training sessions is a promising method for assessing the relation among schedule parameters and performance under subsequent conditions.