Influence of frequent nightmares on REM sleep-dependent emotional memory processing

Existing literature suggests that REM sleep plays a role in the associative integration of emotional memory, and that attributes of dreams during REM sleep, particularly their bizarre and emotional nature, either reflect or even influence this associative and emotional function. The consequence of frequent nightmares on this process is unknown, although, the experience of a nightmare suggests an associative restriction imposed by intense negative emotion, consistent with research showing that negative affect tends to restrict cognitive flexibility in wake. This is consistent with existing theories of nightmare function, largely purporting that nightmares reflect temporary failures in emotion regulation.  The first objective of the thesis was to conceptually replicate prior research portraying REM sleep as enabling increased associative access to emotional memory. The use of a daytime nap allowed us to assess the effects of REM sleep, compared to both NREM sleep and waking, on participant performance on a novel task measuring Associational Breadth (AB). Results showed that only those subjects awakened from REM sleep responded with atypical emotional word associations, suggesting that REM is specific in its capacity to broadly integrate emotional memory traces (article 1). Further, REM dream reports were more bizarre than both NREM dreams and waking daydreams, and more emotionally intense than NREM dreams; these attributes are thought to reflect the hyper-associative and emotional nature of REM sleep (article 2).  The second objective was to clarify whether and how REM sleep-dependent emotional memory processing is altered in frequent nightmares sufferers. Using a similar nap protocol, our results showed that NM participants had higher baseline AB in response to emotional cue-words, contrary to predictions, but nonetheless corresponding with anecdotal reports of heightened creativity. Following REM sleep, both NM and CTL groups showed similar changes in associative access to emotional cue-words, with negative AB being restricted and positive AB being broadened; one week later, the NM group alone maintained this altered pattern of emotional semantic access (article 3). This finding suggests that, over time, nightmares may interfere with REM sleep-dependent emotional memory integration. Regarding imagery, the NM participants had heightened bizarreness, and positive, but not negative, imagery in their daydreams, but not their dreams (article 4), mirroring our AB finding that the NM group had significantly higher emotional associativity in wake, although patterns of associativity following a REM sleep nap did not differ between groups. Overall, findings support a role of REM sleep in the associative integration of emotional memory. However, our findings regarding nightmare sufferers are not entirely consistent with views that nightmares are associated with dysfunctional emotional memory processing. Although they did show a prolonged priming effect suggestive of inadequate emotion regulation, they also showed heightened semantic associativity and vivid positive imagery in wake. We therefore propose a novel Environmental Sensitivity framework for the study of nightmare sufferers, claiming that an increased sensitivity to a range of environmental contexts, not only negative contexts, underlies the unique symptoms and imaginative richness seen in frequent nightmare sufferers. Although further empirical research exploring potentially adaptive traits or sensitivity to positive contexts in nightmare sufferers is needed, the possibility that these individuals may benefit especially from supportive environments, and may have heightened creativity and semantic associativity, is particularly relevant when considering prognosis and treatment approaches.

Keyphrases: Nightmares, REM sleep, Dreams, Daydreams, Emotional memory, Associative memory, Mémoire associative