At the age of ~16 months, the control mice quickly learned to transition from FR1 to FR20 schedule of milk reward (WT), but the MsrA−/− mice lagged significantly behind the WT mice in learning to perform under these conditions (despite the fact that they were not different when the task demand was FR1). WT, n=7; MsrA−/−, n=8. Over the 32 training sessions shown in Fig. 1, the mean number of training sessions to reach FR20 was 10.9 for the WT mice and 30.6 for the MsrA−/− mice, t13=6.14, p<0.001(t13 refers to a “between-groups” t-test with 13 degrees of freedom).