Fig. 2.

The influence of contingency on the response-enhancing effects of nicotine. Groups of rats (N = 8 in each group) were treated with daily injections of saline (open bars) or nicotine (0.4 mg/kg, s.c.; filled bars) and trained in an operant task reinforced by stimulus light above the active lever and extinguishing the house light. The bars represent the means ± SEM of the lever presses on the active and inactive levers when responding was reinforced using contingencies which increased from FR1:TO20 to FR5:TO20. Significantly different from rats injected with saline *P < 0.05; **P < 0.01