Unfortunately, a major limitation of extinction is that it is a temporary phenomenon and extinguished fear can re-emerge simply with the passage of time (spontaneous recovery) (Hermans et al., 2006; Myers and Davis, 2007; Robbins, 1990). This phenomenon demonstrates that original fear memory remains within the brain and ready to re-emerge even after extinction, suggesting that extinction is a new learning process that “overlays” the original fear memory (Bouton, 2002). The vulnerability of fear memory to recovery creates significant limitations to the durability and effectiveness of exposure-based therapies (Arch and Craske, 2009; Craske et al., 2008), and this has become a topic of intense translational science efforts to improve treatments for PTSD and other anxiety disorders (Graham and Milad, 2011; Jovanovic and Ressler, 2010; Milad and Quirk, 2012). One approach to overcoming the limitations of exposure therapy may be to enhance the strength of fear inhibitory learning through understanding of its neural and neurochemical substrates (Graham and Milad, 2011; Jovanovic and Ressler, 2010; Milad and Quirk, 2012).
