776 and between the last reward omission session and the first reacquisition session using paired t tests. The reward omission periods and reacquisition periods were separately analyzed using repeated measures ANOVA, and where appropriate, post hoc comparisons were made using paired t tests. Since breakpoints in the ST(PR) experiments are derived from an escalating curve, samples violate the homogeneity of variance. Therefore, we analyzed breakpoints via nonparametric testing. ST(PR) experiments where we varied the reward magnitude or tested the effects of α-flupenthixol were analyzed using a Friedman test, followed by a Wilcoxon signed ranks test to identify differences between groups. Reward omission experi- ments under the ST(PR) schedule were analyzed by planned comparisons between the baseline and the first reward omission session and between the last reward omission session and the first reacquisition session using a Wilcoxon signed ranks test. We also compared α-flupenthixol treatment with the first reward omission session with independent t test or Mann– Whitney U test for the ST(RI) and the ST(PR) experiments, Psychopharmacology (2012) 220:771–785 respectively. The criterion for statistically significant differences was set at p < 0.05. All data were analyzed using SPSS 15.0. Results ST(RI) schedule with cocaine reinforcement For the ST(RI) experiments with cocaine reinforcement, animals reached baseline taking levels with an average of 9.8 ± 0.1 rewards/session (range 8.6–10 rewards/session). Figure 1a shows that varying the reward magnitude by changing the unit dose of cocaine affected response rates on the seeking lever under an ST(RI) schedule (F3, 33=11.231, p<0.001, Fig. 1a). Post hoc analysis showed that seeking rate for 0.063 mg/infusion of cocaine differed from all other unit doses of cocaine. In addition, there was a difference between 0.125 and 0.25 mg/infusion of cocaine, but Fig. 1 Effect of altering reward size, α-flupenthixol, and reward omission on responding for cocaine under an ST(RI-120) schedule in rats. a Seeking response rates for different unit doses of cocaine (n=12). b Number of taking responses for different unit doses of cocaine (n=12). c Effect of α-flupenthixol on cocaine seeking (n=10). d Effect of α-flupenthixol on cocaine taking (n=10). e Effects of reward omission and reacqui- sition on cocaine seeking: base- line response (session 0; closed symbol) followed by 10 reward omission sessions (sessions 1–10; open symbols), followed by reac- quisition (sessions 11–15; closed symbols) (n=15). f Effects of reward omission and reacquisition on cocaine taking: baseline response (session 0; closed symbol) followed by 10 reward omission sessions (sessions 1–10; open symbols), followed by reacquisition (sessions 11–15; closed symbols) (n=15). Data are presented as mean presses/min ± SEM (seeking) or mean ± SEM taking responses. *p<0.05 for difference between groups (a–d) or sessions (e, f) (paired t test)