Amplitude of the rabbit’s unconditioned nictitating membrane response in the presence of a conditioned inhibitor

Using a light or backshock as the reinforced CS (A) and a tone or backshock as the conditioned inhibitor (X), rabbits experienced conditioned inhibition training in an A+/AX? paradigm. Following training, the amplitude of the unconditioned nictitating membrane response elicited by a mild (.5-mA) paraorbital shock was measured in the presence of X and AX and expressed as a percentage of the amplitude of the UR to the shock presented alone. In Experiment 1, the effect of X and AX on UR amplitude for conditioned inhibition animals was compared with that of control animals treated to a variety of pretest procedures. In general, UR amplitude in the presence of X exceeded that observed to the US presented alone. There was no consistent difference between the experimental and control groups. In Experiments 2–5, A test trials were added as an alternative reference point. Again, UR amplitude increased rather than decreased UR amplitude. In addition, X as a conditioned inhibitor enhanced the facilitating effect of A on UR amplitude in four out of five experiments. These findings have implications for theories of the “locus of action” of conditioned inhibitors.