Mean (± SEM) response rates of each strain (SHR: unfilled squares; WKY: filled circles; WIS: filled triangles) as a function of mean rate of reinforcement, in two epochs: PND 58-62 (epoch 1; top panel) and PND 89-93 (epoch 2; bottom panel). Response rate increased with rate of reinforcement in all strains and epochs. SHR response rates were higher than those of WKY and WIS when reinforcement was delivered less than twice per minute. At higher rates of reinforcement, WIS response rates were lower than those of SHR and WKY. Curves through the data are traces of Herrnstein's hyperbola (Equation 1).