Conditioned inhibition of the nictitating membrane response in decorticate rabbits

Rabbits with substantial neocortical ablation were trained in a Pavlovian conditioned inhibition task using a light as the reinforced conditioned stimulus (CS) and the same light compounded with a tone as the non-reinforced CS. The conditioned response was the nictitating membrane response, and the unconditioned stimulus was paraorbital shock. Training began 4-14 months postoperatively. With a number of procedural variations across 4 experiments, there was little evidence of impaired conditioned inhibition by decorticates compared to control animals. These results are consistent with earlier reports on Pavlovian discrimination learning and reversal involving single-element CSs from different sense modalities. Hence, they extend the basic conclusion that neocortex is not essential for Pavlovian inhibition to the conditioned inhibition paradigm. Consideration of these findings in the light of previous investigations of conditioned inhibition in rabbits, involving mesencephalic or hippocampal lesions, supports the hypothesis that the crucial neural elements for Pavlovian inhibition may be located in the brain stem.