Figure 2.

Mean (SE) locomotor response (number of beam breaks in an electronic array per 90 min) to 1 mg/kg d-amphetamine (i.p.) on 5 weekly sessions in groups of Sprague Dawley rats (n = 8/group) previously exposed to 15 daily conditioning sessions with sucrose reward (10% solution) delivered under 0, 25, 50, or 100% variable schedules. The conditioned stimulus was a light (120 s). Group 0 received the same number of rewards as group 100 in the absence of conditioned stimuli. ^*p < 0.05 for mean difference between group 50 and group 0 as well as group 100, based on planned comparisons.