Second-order excitation mediated by a backward conditioned inhibitor.

Abstract

Conditioned suppression studies with rats explored the informational content of a backward conditioned inhibitor. Pairings of an unconditioned stimulus (US) and Stimulus 1 (US-->S1) established S1 as an inhibitor in Experiment 1. Pairing the inhibitor S1 with a novel S2 (S2-->S1) promoted excitatory second-order conditioning (SOC) to S2, which suggested S1 was well associated with the US. Degrading presumed S1-US associations in Experiment 2 by S1- (extinction) treatment eliminated S2's excitation while preserving S1's inhibition. Experiments 3 and 4 converged in showing that S2 was not an excitor when Pavlovian conditioned inhibition (CI) was the inhibitory treatment prior to the SOC phase, but instead acted as a second-order inhibitor. Results are discussed in relation to the temporal coding hypothesis, the SOP ("sometimes opponent process") and Rescorla-Wagner models of conditioning, and the associative structure of SOC. Also, the data suggest that backward inhibition is special and that not all forms of CI are equal.