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Effects of daily morphine treatment on choice of the large reinforcer (upper panels) and trial omissions (lower panels). Each bar indicates the mean (+ SEM) of the first 14 days of daily administration of morphine. The shaded region indicates the mean (± 2 SD) for 3 baseline sessions. The “x2” next to a symbol indicates a condition in which that dose of morphine was administered twice daily (see “Methods” for details).
