Plasticity of defensive behavior and fear in early development.

Animals have the ability to respond to threatening situations with sets of defensive behaviors. This review demonstrates that defensive behaviors change during early life in mammals. First, unlearned responses are reorganized during early ontogeny and expressed in an age-specific way. Second, the expression of defensive responses is influenced by early experience prior to the first encounter with a threat. Third, once animals have been exposed to a threatening stimulus they subsequently modify their behavior. The neural bases of defensive behavior and the processes that alter them during development are discussed. Maturation of components and connections of the fear circuit seem to contribute to changes in unlearned fear responses. Early experience and learning modify these developmental processes and shape the expression of defensive behavior. Continuous reorganization of the neural substrate and defensive behavior during ontogeny seems to allow the animal to adjust to the conditions it encounters at a given age in a given environment. It is proposed that the developmental changes in defensive behavior can be conceptualized as phenotypic plasticity.