Differential responding as a function of auditory stimulus intensity without differential reinforcement.

Two groups of four rats each were trained to bar press on a variable-interval 2-min schedule. During training, either 3, 5, or 9 auditory stimuli of various intensities were randomly presented. A direct relationship between stimulus intensity and rate of responding was obtained, but it was more consistent in the group trained initially with three stimuli than for the group that started with nine stimuli. The results are related to the concept of stimulus intensity dynamism and the necessary conditions for the acquisition of stimulus control.