The fear hypothesis and tonic immobility (TI) modulation: Early studies in chickens.

The hypothesis that fear is involved in the mechanisms of tonic immobility (TI) has been supported by early studies conducted in newborn and adult chickens. The susceptibility to TI changes during development in parallel to other fear responses. TI duration increases following exposure before induction to threatening stimuli such as electric shock, loud sound, stuffed sparrow hawk, as well as in unfamiliar conditions applied before and/or during testing. TI duration and susceptibility are increased by prey/predator eye contact and inversely related with the predator distance. TI duration increases following exposure before induction to threatening stimuli such as electric shock, loud sound, stuffed sparrow hawk, as well as in unfamiliar conditions applied before and/or during testing. The fact that the experimenter presence or the experimenter eye visibility represent a potential source of fear like a natural predator in chicks and in adult hens is controversial. The likely explanations for the contradictory results are discussed in the text. The rearing conditions, for instance, seem to be critical: repeated handling in the first days after hatching reduces the fear of human beings, decreasing TI duration in adulthood with a parallel increase in proximity scores to the experimenter. In chicks, exposure to withdrawal from a positive imprinting stimulus increases and decreases TI duration, respectively.