Exploring the mechanisms and specificity of emotional memory consolidation over sleep

Abstract

Previous research has indicated that emotionally affective memory representations are preferentially consolidated over their neutral counterparts across time and sleep. While compelling, two broad research questions remain unanswered: 1) what are the neural mechanisms underpinning time- and sleep-dependent emotional memory processing and, 2) are focal and contextual elements of emotional and neutral memory influenced in different manners by post-learning sleep? Accordingly, the research described in this thesis makes use of behavioural experimentation, polysomnography (PSG) and functional magnetic resonance imaging (fMRI), with the principal aim of refining contemporary knowledge regarding the mechanisms and specificity of memory consolidation processes occurring over time and sleep. In chapter 2, we used event-related fMRI to compare the neural correlates of emotional image memories that had undergone 1 week (remote memory) or 30 minutes (recent memory) of consolidation. Our findings suggested that remote image memories, irrespective of emotional valence, were more dependent upon neocortical brain regions than recent image memories. In addition, strongly consolidated remote image memories appeared to be more reliant upon the precuneus, whereas weakly consolidated remote image memories were associated with increased activity in several occipital areas. In chapter 3, we used overnight PSG and event-related fMRI to expand upon the findings of chapter 2 and investigate how different stages of post-learning sleep influenced the reorganisation of emotional image memories over 24 hours (remote memory), as compared to 30 minutes (recent memory), of consolidation. We observed a slow-wave sleep (SWS)-related reduction in right hippocampal activity, and rapid eye movement sleep (REM)-related increase in hippocampal-cortical connectivity, associated with the retrieval of negative remote image memories, suggesting a complimentary role of SWS and REM in emotional memory processing. In chapter 4, we investigated how replaying learning associated sounds during diurnal SWS impacted upon the consolidation of emotionally negative and neutral picture-location memories. Our findings indicated that sound re-exposure primed memories for a targeted but dissociative consolidation process, whereby a concomitant enhancement and suppression of respective negative and neutral memories took place during the ensuing SWS period. In chapter 5, we used a napping paradigm with PSG to explore the properties of sleep that play a decisive role in the consolidation of emotionally negative and neutral context memories. The results were suggestive of a paradoxical sleep-dependent mechanism, whereby stage two sleep mediates a strengthening of negative context memories and sleep spindles support a suppression of their neutral counterparts. A potential SWS-related suppression of negative memories however, may have prevented emotion-related memory enhancements from occurring over sleep in the contextual domain. Finally, in chapter 6, we examined how environmental context-dependent memory effects were influenced by a period of post-learning sleep or wakefulness. We observed a sleep-related reduction in the extent to which environmental cues impacted upon retrieval, suggesting that sleep may mediate an active decontextualisation of declarative memories. Overall, these studies have provided new insights regarding the role of time and sleep in emotional and non-emotional memory processing.

Date of Award	1 Aug 2012
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Penelope Lewis (Supervisor)