Date of Graduation


Document Type


Degree Type



Eberly College of Arts and Sciences



Committee Chair

Michael Perone.


On a multiple-schedule of reinforcement, when a rich component is followed by a relatively lean component, responding is interrupted by extended pausing. The present experiment examined whether the discriminative stimulus correlated with this rich to lean transition acquired aversive properties. Four pigeons were exposed to a compound schedule with two fixed-ratio components programmed on the center key. One component ended with the delivery of a rich reinforcer (7-s access to grain) and the second with a lean reinforcer (1-s access). Each component was correlated with a distinctive key color. At the beginning of some components, a side key was activated. A single peck on this "stimulus termination key" replaced the discriminative stimulus on the center key with a white light regardless of whether the ongoing component was rich or lean. As in previous research, pausing was a joint function of the past and upcoming conditions of reinforcement and the FR requirement. At relatively large ratios, if the upcoming reinforcer was rich, pauses were short regardless of the past reinforcer. If the upcoming reinforcer was lean, pauses were longer and the length was directly influenced by the past reinforcer; the longest pauses were observed in the transitions from a rich to a lean reinforcer. At larger ratios, removal of the multiple-schedule stimuli occurred most often in the presence of a stimulus signaling a lean reinforcer and rarely in the presence of a stimulus signaling a rich reinforcer. The past reinforcer affected the frequency of escape for two pigeons (P822 and P830); they most often terminated the lean stimulus when it was preceded by a rich reinforcer.