Extinction learning and its retrieval micHeLLe g. craske anD Bram vervLiet
Fear conditioning involves learning a relationship between a neutral stimulus, the conditional stimulus (CS), and an innately aversive stimulus (the unconditional stimulus, US), such that the CS elicits a conditional response (CR). Fear conditioning has been applied as an etiological model for phobias and anxiety disorders. Fear extinction involves the decay of CRs with repeated presentations of the CS without the US. Deficits in fear extinction are believed to contribute to the persistence of phobias and anxiety disorders, and the process of fear extinction parallels the clinical practice of exposure therapy for anxiety disorders. Significant advances have been made in the behavioral and neurobiological mechanisms underlying extinction, their relevance to phobias and anxiety disorders, and their translation to exposure therapy. The specific line of research covered in this article pertains to the theory that extinction involves the formation of a new inhibitory association between representations of the CS and the US, which actively suppresses the US memory. Thus, the amount of fear expressed after completion of extinction training is dependent on which set of associations is retrieved, the initial excitatory association or the newly formed inhibitory association (Bouton, 2002). This retrieval model of extinction is described in detail by Nelson (in this volume). Herein, we review the neurobiological evidence, and strategies for enhancing the formation and retrieval of inhibitory associations, and their translation to exposure therapy for anxious individuals who appear to show deficits in inhibitory learning.