Mechanisms of second-order conditioning with a backward conditioned stimulus.

Five conditioned suppression experiments with rats examined the conditions under which backward pairings endow a first-order conditioned stimulus (CS1) with the ability to serve as a secondary reinforcer. Experiments 2-5B found evidence for excitatory second-order conditioning (SOC) if, during first-order pairings, the US-CS1 interval was 0 s rather than 3 s. Levels of SOC were comparable after forward and backward pairings (Experiments 1-3), and were unaffected by extinction of CS1 after SOC (Experiment 3). These results suggest that forward and backward CS1s support SOC for the same reason, and they call into question the need to invoke any special mechanism such as memory integration.