Abstract
The sleep-to-forget, sleep-to-remember (SFSR) hypothesis states that the neurobiological environment provided by rapid-eye movement (REM)-rich sleep decouples the content of an emotional memory from its attendant emotional arousal. This decoupling allows divergent attenuation and enhancement effects (i.e., erosion of the memory’s emotional tone and simultaneous strengthening of its content). However, support for this proposal is mixed. An alternative account suggests there might be convergent attenuation and enhancement (i.e., elevated emotional arousal is positively coupled with enhanced emotional memory). We tested predictions emerging from the SFSR hypothesis using (a) individuals diagnosed with post-traumatic stress disorder (PTSD; n = 21), (b) trauma-exposed non-PTSD individuals (n = 19), and (c) healthy controls (n = 20). We included PTSD-diagnosed individuals because they typically experience altered REM sleep, impaired emotional memory, and heightened emotional arousal in response to threatening stimuli. Participants were assessed before and after both an 8-h period of polysomnographically monitored sleep and an 8-h period of waking activity. The assessment included exposure to negatively valenced, positively valenced, and neutral pictures before the 8-h delay, and a recognition task afterward. We measured emotional arousal by recording psychophysiological responses to the pictures, both pre- and post-delay. Results indicated no significant between-group differences in emotional memory accuracy or arousal. However, after a sleep-filled delay, pictures of all categories were recognized with equal accuracy, whereas after a wake-filled delay, negative pictures were recognized preferentially. Furthermore, the findings demonstrated that a sleep-filled delay was associated with attenuated emotional arousal to pictures of all categories, whereas a wake-filled delay was associated with a rise in emotional arousal across the day. Intriguingly, poorer recognition accuracy for valenced (but not neutral) pictures was predicted by an interaction of increased REM fragmentation and increased emotional arousal. In summary, we found some support for the SFSR hypothesis in the way it describes the REM- and arousal-based mechanisms that process emotional material. We also, however, found disconfirming evidence regarding the outcome of that process (i.e., sleep did not favor consolidation of emotional over neutral memory), and we demonstrated a convergence between attenuation of emotional arousal and weakening of emotional content relative to neutral content.
Highlights
Emotional memories are comprised of an emotional arousal component and a content component
Central to the SFSR hypothesis is the notion that healthy sleep allows preferential consolidation of memory for emotional events over that for neutral events
We found that, after a sleep-filled delay, previously encoded emotional and neutral stimuli were recognized with equal accuracy
Summary
Emotional memories (e.g., the memory of an encounter with a snake in a forest) are comprised of an emotional arousal component (e.g., racing heart, sweaty palms, a subjective feeling of fear) and a content component (e.g., the cognitive representation of the reptile and the spatiotemporal context in which it was encountered). The sleep-to-forget, sleepto-remember (SFSR) hypothesis proposes that a series of neurobiological processes (a) decouples the two components, and (b) attenuates the arousal component while consolidating the content component (van der Helm and Walker, 2011). The hypothesis states that emotional memory processing occurs during rapid-eye movement (REM) sleep and in a neural environment devoid of noradrenergic activity (Walker and van der Helm, 2009). Novel elements of this study are that, (a) we compare dayand night-time levels of noradrenergic metabolites, thereby examining whether night-time noradrenergic activity does, modulate sleep-dependent consolidation of emotional memory, and (b) we test predictions derived from the hypothesis in a clinical sample [a group of individuals diagnosed with posttraumatic stress disorder (PTSD)] compared to two non-clinical samples [a trauma-exposed (TE) non-PTSD group and a healthy control (HC) group]
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