Abstract
Within-session temporal distributions of responding were investigated in three experiments using rats pressing a lever in a discrete-trial omission procedure. This schedule entailed 60, one-minute trials, and a sucrose solution was made available at the end of each trial in which no lever press occurred. In Experiment I, nonnaive rats acquired and maintained responding during this training. Moreover, the probability of a response during any session showed a strong and reliable tendency to increase from the beginning to the end of the session. These results were replicated in Experiment II, using naive animals. In Experiment III, alterations were made in the training procedure, including elimination of response-contingent and noncontingent stimulus changes. Results indicate that stimulus change may be sufficient to maintain low levels of responding whether or not this change is contingent on responding.