The role of post-learning reactivation in memory consolidation

Memories are gradually consolidated after learning, and subsequent offline periods containing sleep are suggested to support the stabilisation, enhancement, reorganisation and integration of representations within long-term memory networks. The spontaneous reactivation of specific memory traces during sleep is proposed as a key mechanism underlying sleep-dependent consolidation, but the neurophysiological underpinnings of this ? memory replay? remain unclear. The research described in this thesis utilised a method of manipulating memory reactivation during sleep (targeted memory reactivation), in combination with behavioural experimentation, polysomnography (PSG), electroencephalography (EEG), and functional magnetic resonance imaging (fMRI), to refine current understanding of the neural processes underlying sleep-dependent memory consolidation. In Chapter 2 we developed a motor sequence learning paradigm that combined visuo-motor performance with sound stimuli, which enabled the targeted memory reactivation (TMR) of specific motor memories during sleep in subsequent chapters via the replay of the associated sounds during sleep. Chapter 3 used this task to cue the reactivation of a learned motor sequence during slow-wave sleep (SWS), which enhanced motor skill for the cued sequence relative to an uncued sequence, and also made the sequence of motor movements more available for conscious recall. Furthermore, these effects were associated with key neural features of sleep (slow oscillations and spindles). These findings indicate that reactivation not only enhances procedural memories, but plays a part in the reorganisation of representations that leads to the emergence of explicit knowledge. A great deal of research has shown that the neural systems supporting procedural memories evolve over time, particularly within cortico-striatal and cortico-cerebellar networks. Chapter 4 used fMRI to show that reactivation is instrumental to this neural plasticity by comparing brain activity at retrieval of a sequence that was cued during SWS with a sequence that was not. The cued sequence showed increased activation in bilateral caudate nucleus and left hippocampus, mediated by time spent in slow-wave sleep, while functional connectivity was also altered by TMR between caudate and hippocampus. These findings indicate that the behavioural enhancements associated with TMR of procedural learning are related to overnight plasticity in motor memory networks. Lastly, Chapter 5 expanded on the reorganisation of memories investigated in Chapter 3, asking whether reactivation mediates the generalisation of representations that can sometimes create false memories. Learned lists of semantically associated words were reactivated during NREM sleep, but revealed no evidence that TMR effected false memory formation. However TMR was found to reduce the recognition of studied items, which may indicate that certain TMR procedures can interfere with consolidation rather than enhance it.Collectively these results provide new insights to the role played by reactivation in memory consolidation. We have provided evidence for both the enhancement and reorganisation of procedural memories during sleep, and indicate that such effects are supported by alterations to underlying neural plasticity. We also show the importance of slow-wave sleep and associated neural features in this consolidation process.