Tonic immobility in chickens: Is a stimulus that signals shock more aversive than the receipt of shock?

Three experiments were conducted on the effect of shock and aversive conditioning on tonic immobility. In the first study, increasing the intensity of preinduction shock was shown to produce reliable increases in the duration of immobility. Using classical conditioning procedures in the second experiment, a significant effect of UCS intensity was obtained. In the third experiment, brief confrontation with a conditioned fear stimulus was found more effective than shock for enhancing immobility duration. The data were discussed in terms of the fear hypothesis of animal hypnosis and as supporting a more general notion that the anticipation of shock may be more aversive than the receipt of shock.