Second-order conditioning. (A-C) General scheme (A), conditioning procedure (B), and time schedule (C) for appetitive and aversive second-order conditioning. (D) Stimulus schedules in the second-order conditioning group are initial paired presentation of CS1 and unconditioned stimulus (US) and subsequent paired presentations of CS2 and CS1 (paired/paired (P/P) group), and those in control groups are unpaired presentations in the first or second conditioning stage (unpaired/paired (UP/P) or paired/unpaired (P/UP) groups). (E, F) Two groups of animals were each subjected to appetitive (E) or aversive (F) second-order conditioning trials (P/P groups). Four control groups were each subjected to unpaired presentations in the first (UP/P groups) or second (P/UP groups) stage in appetitive (E) or aversive (F) second-order conditioning. Animals received four first-stage trials and then four second-stage trials for appetitive second-order conditioning, and six first-stage trials and then four second-stage trials for aversive second-order conditioning. Preference indexes for the CS2 (in the case of appetitive second-order conditioning) or control pattern (in the case of aversive second-order conditioning) before (white bars) and after (black bars) conditioning are shown with means + SEM. The results of statistical comparison are shown as asterisks (* P < 0.05; ** P < 0.01; *** P < 0.001; NS P > 0.05).