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Mechanisms of Extinction and the Return of Fear
Exposure therapy—exposure to a feared stimulus without harmful consequences—can reduce fear responses in many mental disorders. However, such relief is often partial and temporary: fear can return after the therapy has ended. Conditioning research has identified three mechanisms for the return of fear, viz. change in physical context (renewal), the mere passage of time (spontaneous recovery), and an encounter with the fear-producing unconditioned stimulus (reinstatement). To understand these mechanisms by which fear returns and thereby develop more effective therapies, we turn to mathematical learning models. In this paper we review the major classes of learning model and how well they can explain the three basic types of return of fear. An exemplar model can explain them using hand-tuned attention weights, but needs an automatic attention learning rule to make useful predictions. A latent cause model has trouble accounting for spontaneous recovery, but is promising in other respects. Variants of the Rescorla-Wagner model explain these phenomena surprisingly well, and currently seem best suited to inform exposure therapy. According to these models, exposure or extinction reduces fear by a combination of unlearning the original association and the development of context inhibition. To make the benefits of exposure robust and permanent, one must minimize inhibitory context learning in order to maximize unlearning.</p