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Effects of acutely administered morphine (15-min pretreatment) on omissions in choice trials before and during morphine treatment. The number of choice trial omissions during the session (maximum of 24) is plotted as a function of dose of morphine. Filled circles above “S” show the mean (± SEM) for 3 baseline sessions immediately prior to administration of the first dose of morphine. Open and half-filled symbols indicate effects of morphine determined during daily morphine treatment.
