Second-order conditioning and Pavlovian conditioned inhibition: operational similarities and differences.

Procedures for establishing second-order excitation (conditioned stimulus [CS] 1-unconditioned stimulus [US] trials followed by CS2-CS1 trials) are highly similar to those for Pavlovian conditioned inhibition (CS1-US trials interspersed with CS2-CS1 trials). Conditioned suppression in rats was used to identify the critical operational differences that result in second-order excitation as opposed to Pavlovian inhibition. No, few, or many CS2-CS1 trials were either interspersed with or given after CS1-US trials. CS2 proved excitatory only after few CS2-CS1 trials, either interspersed or sequential (Experiment 1). In contrast, CS2 proved inhibitory on both summation (Experiment 2) and retardation (Experiment 3) tests only after many CS2-CS1 trials, and then only when the excitatory status of CS1 was preserved. Apparently, the critical difference for establishing second-order excitation or Pavlovian inhibition is the number of CS2-CS1 pairings.