Retention scores after single- and multiple-trial olfactory conditioning. Seven groups of animals were subjected to single-trial conditioning (open squares) to associate a peppermint odor with water reward. Another four groups were subjected to two appetitive conditioning trials and two aversive conditioning trials, in which peppermint odor was paired with water and vanilla odor was paired with sodium chloride solution for two times each with an ITI of 5 min (black squares). Relative preferences of the peppermint odor (rewarded odor) compared with vanilla odor (control odor) were tested before and after conditioning in all groups. They were measured as preference indexes (PIs) and shown as mean ± SE. To simplify the figure, the PIs for rewarded odor ware shown as pooled data from seven single-trial or four multiple-trial groups. Preferences at each time after conditioning were compared to those before conditioning in each group (Wilcoxon’s test) and preferences of multiple-trial groups were also compared to those of single-trial groups at each time after conditioning (Mann-Whitney test), and the results of the former comparison are shown at each data point and those of the latter are shown above the arrow (*P < 0.05; **P < 0.01; ***P < 0.001; NS P < 0.05). The preferences for rewarded odor remained unchanged from 30 min to 24 hours after conditioning in the multiple-trial group (P < 0.05, Mann-Whitney test). Modified from Matsumoto et al. .