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Mean (SE) approach responses (nose pokes) on 15 sucrose training sessions in groups of Lewis rats (n = 8/group) exposed to sucrose reward (10% solution) delivered under 0, 25, 50, 75, 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. (A) Scores when CS was present (5 s × 20 trials). (B) Scores when CS was absent (average for 5 × 20 s while light was off).
