Uncertainty, as defined by the contingency between environmental events, and the adrenocortical response of the rat to electric shock.

The pituitary-adrenal system is thought to be sensitive to the degree of uncertainty in a situation. In addition, there is some question whether the pituitary-adrenal system can be conditioned in a Pavlovian sense. Three experiments are reported here. The first and third sought to define uncertainty in terms of conditioned stimulus-unconditioned stimulus (CB-US) and US-US contingencies, which vary the amount of information that can be used to predict the occurrence of discrete shocks. The second experiment examined the possibility that the adrenocortical system was subject to the laws of Pavlovian conditioning, by using a conditioned emotional response paradigm. The results showed that the magnitude of the Pituitary-adrenal response varied in a curvilinear manner along the dimension of uncertainty. Very low and very high degrees of uncertainty resulted in greater corticosterone elevations than did moderate levels. No evidence for Pavlovian conditioning of the adrencortical system was found, although behavioral measures showed fear conditioning. The data presented were supportive of the hypothesis that the pituitary-adrenal response reflects the operation of an arousal system.