Simulated exposure therapy for spider phobia served as a clinically naturalistic model to study effects of sleep on extinction. Spider-fearing, young adult women (N = 66), instrumented for skin conductance response (SCR), heart rate acceleration (HRA) and corrugator electromyography (EMG), viewed 14 identical 1-min videos of a behaving spider before a 12-hr delay containing a normal night's Sleep (N = 20) or continuous daytime Wake (N = 23), or a 2-hr delay of continuous wake in the Morning (N = 11) or Evening (N = 12). Following the delay, all groups viewed this same video 6 times followed by six 1-min videos of a novel spider. After each video, participants rated disgust, fearfulness and unpleasantness. In all 4 groups, all measures except corrugator EMG diminished across Session 1 (extinction learning) and, excepting SCR to a sudden noise, increased from the old to novel spider in Session 2. In Wake only, summed subjective ratings and SCR to the old spider significantly increased across the delay (extinction loss) and were greater for the novel vs. the old spider when it was equally novel at the beginning of Session 1 (sensitization). In Sleep only, SCR to a sudden noise decreased across the inter-session delay (extinction augmentation) and, along with HRA, was lower to the novel spider than initially to the old spider in Session 1 (extinction generalization). None of the above differentiated Morning and Evening groups suggesting that intervening sleep, rather than time-of-testing, produced differences between Sleep and Wake. Thus, sleep following exposure therapy may promote retention and generalization of extinction learning.
Abnormal expression of fear, as occurs in anxiety disorders such as post-traumatic stress disorder (PTSD) and specific phobia, may result from abnormally strong fear conditioning (Armfield, 2006; Lissek et al., 2005; Mineka and Oehlberg, 2008; Orr et al., 2000), deficiency of inhibitory mechanisms that normally moderate fear expression (Craske et al., 2008; Hofmann, 2008; Milad et al., 2006), or both. Key among such inhibitory processes is extinction — learning that a once-feared object or event is no longer dangerous (Milad et al., 2006). Rather than erasing a fearful memory, extinction forms a new ”safety memory” that competes with the fear memory when the once-feared object or event is re-encountered (Hermans et al., 2006; Quirk and Mueller, 2008).
Formation of such therapeutic extinction memories is the neurocognitive basis for the efficacy of exposure therapy, a first-line behavioral treatment for anxiety disorders (Craske et al., 2008; McNally, 2007). In order for exposure therapy to be successful, consolidation and retention of extinction learning acquired during therapy is essential (Craske et al., 2008). In addition, such learning must generalize in order to ensure that the reduction of fearful responding to specific cues in treatment will extend to stimuli encountered outside the therapist's office (Rowe and Craske, 1998; Vansteenwegen et al., 2007).
Using an experimental fear-conditioning paradigm (Milad et al., 2007), normal sleep has been shown to promote the generalization of extinction memories (Pace-Schott et al., 2009). However, unlike such experimentally induced de-novo fears, anxiety disorders are associated with long-standing fears that have complex, multi-factorial origins and perpetuating factors ( Armfield, 2006). Specific phobias, such as spider phobia, are highly prevalent, mild anxiety disorders ( LeBeau et al., 2010; Lissek et al., 2007) in which treatment strategies, such as exposure therapy, can be studied in a non-clinical setting (e.g., Vansteenwegen et al., 2007).
Sleep enhances consolidation of emotional memory (reviewed in Walker, 2009). Here we characterize the effect of sleep on the retention and generalization of a specific emotional memory-- the extinction of spider fear produced by simulated exposure therapy. We hypothesized that sleep following simulated exposure therapy in spider-phobic subjects would lead to greater retention of fear extinction for the spider to which they were repeatedly exposed. In addition, we predicted that sleep would enhance generalization of this extinction memory to a novel spider.
