Training on the signaled nose poke task.

The first two columns display the proportion (±SEM) of magazine entries that occurred during elevation of the sucrose dipper across two sessions of magazine training. The last three columns display the days to criterion measure for Phases 1–3 during task training. With fixed ratio (FR) responding in Phases 1 and 2, the criterion to progress to the next phase was 25 reinforcers earned per 40-min session. With the introduction of the inter-trial interval (ITI) in Phase 3, this was reduced to 10 reinforcers per session. There were no significance differences between knockout and wild-type animals on any measure. wild-type HYB, n = 20; Oprm1−/− HYB, n = 10; Oprd1 −/− HYB, n = 8; wild-type BL6, n = 24; Oprm1−/− BL6, n = 11; Oprd1 −/− BL6, n = 10