“…Previous studies (Ellen, Wilson, & Powell, 1964;MacDougall, VanHoesen, & Mitchell, 1969) have shown that, when interresponse times (IRTs) of septal animals barpressing on a DRL (differential reinforcement of low rates) schedule are converted to IRTs/opportunity (Anger, 1956), response probability increases as a function of the delay since the preceding response. The IRTs/OP is defined as the number of times an animal made a given delay divided by the number of occasions the animal actually delayed that long or longer, and is an unbiased estimate of response probability for equal IRT categories.…”