Acquisition of the operant discrimination task (n=10 per group) while maintained on the 2mg/kg training dose. A,B) Responding on the injection paired lever (IPL), after either vehicle or 2mg/kg MPH, increased with session number but was similar between the two groups over 29 sessions of training when examined as a percentage of total responding or as FFR. The dotted lines indicate criterion levels for the two measures. C) A comparison of FFR and IPL responding averaged over the last 3 days of this period. No significant differences were found. D) Over the last 3 days of the training period, the SENS mice as a group had more sessions (out of 30 possible) of criterion level performance (*p<0.05), but this was primarily due to 2 SENS mice that performed well. Data are mean + SEM except panel D, which are counts.