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. Author manuscript; available in PMC: 2012 Dec 17.
Published in final edited form as: Pharmacol Biochem Behav. 2009 Apr 17;93(1):67–74. doi: 10.1016/j.pbb.2009.04.008

Figure 2.

Figure 2

Progressive ratio breakpoints engendered by food reinforcers (filled bars) or obtained under extinction conditions (open bars). The first determination occurred after 24 sessions of limited-access for all groups, while the second determination occurred after either 40 more sessions of limited-access (left panels) or 40 sessions of extended-access (right panels). Nose-poke responses were reinforced with sucrose pellets (A) or liquid food (B and C), and lever-press responses were reinforced with liquid food (D and E). Each bar represents the mean breakpoint achieved (± SEM) from groups where N=6.