FGF2 and amnesia: Specifically, we have suggested that the expression of infantile amnesia and relapse-resistant extinction in infancy may represent critical period plasticity and propose a model in which early environments that alter the age at which the developmental transitions occur (e.g., stress) might function through an HPA/FGF2-dependent activation of “critical period signals,” in turn leading to an early termination in emotional plasticity (see Figure 2 for a graphical depiction of this model).