Blocking in Pavlovian conditioning: effect of prior excitatory conditioning to a stimulus extraneous to compound conditioning.

The effect of prior excitatory conditioning to a stimulus not included in the compound conditioning phase of the typical blocking experiment was assessed in three CER studies. The first experiment, in which rat subjects received A+, C+, or a combination of A+/C+ trials prior to AB+ conditioning, showed that A+/C+ or C+ training produced as robust blocking as A+ training relative to a control group with no prior conditioning. A second experiment which was designed to assess the role of background cues in mediating the blocking effect indicated that background cues were not responsible for the A+ or C+ effects, while a third experiment showed these same effects were not mediated by stimulus generalization. The findings of these experiments are interpreted in the context of pseudoconditioning-induced rehearsal of a US representation in short-term memory.

Effects of contingency violations on the extinction of a conditioned fear inhibitor and a conditioned fear excitor.

Rats were used in a conditioned-suppression paradigm to assess the effects of contingency variations on responding to a conditioned inhibitor (CS-) and a conditioned excitor (CS+). In Experiment 1, various unconditioned stimulus (US) frequencies were equated across the presence and absence of a CS- in the context of either background cues (continuous-trial procedure) or an explicit neutral event (discrete-trial procedure). With both procedures, a CS-alone treatment enhanced inhibition, whereas treatments involving 50% or 100% reinforcement for the CS- eliminated inhibition without conditioning excitation to that CS. The latter outcome also occurred in Experiment 2, with discrete-trial training equating considerably reduced US frequencies for the presence and absence of the CS-. In further evidence that inhibition was eliminated without conditioning excitation to the CS-, Experiment 3 showed that a novel CS did not acquire excitation when 25%, 50%, or 100% reinforcement was equated across the presence and absence of that CS in the context of a discrete-trial event. Using the procedures of Experiment 1, Experiment 4 showed that a CS+ was extinguished by a CS-alone treatment but was substantially maintained by treatments involving 50% or 100% uncorrelated reinforcement. These effects for a CS+ and a CS- implicate CS-US contiguity, rather than contingency, as the factor determining the extinction of a CS.