Theories of Sleep: The Role of Slow-Wave Sleep in Memory Consolidation

Introduction

Sleep is defined as a reversible, natural state of reduced responsiveness to external stimuli and relative inactivity, accompanied by a loss of consciousness1. This state of inactivity has been evolutionary conserved across species against many selection pressures. This suggests that sleep holds a vital function. Multiple studies have suggested that sleep enhances both declarative and procedural memory 2,3. Consequently, memory consolidation is one of the ostensible functions of sleep. The active system consolidation hypothesis and the synaptic homeostasis hypothesis (SHY) are the two main hypotheses for this putative function of sleep. The active system consolidation model proposes that the repeated reactivation of newly encoded representations in the hippocampus during slow wave sleep (SWS) drives a transfer of memory traces from short-term store in the hippocampus to neocortical regions for long term storage4. These representations are stabilized in the neocortex during subsequent periods of REM sleep by synaptic potentiation5. SHY proposes that sleep acts to homeostatically renormalize synaptic weight to counteract the potentiation that occurs during wake6,7. This occurs by an activity dependent downscaling of synaptic weight during SWS. These models are often portrayed in direct opposition, largely due to the active potentiation proposed in the active system consolidation model, in contrast to the global downscaling (depotentiation) of SHY. However it has recently been considered that these two models may not be mutually exclusive, but could act in unison8. This essay will analyse what I believe to be the best evidence in support and against each model to contend the role of sleep in memory consolidation. The rol...

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...urated during previous wakefulness. Napping studies show that SWA enhances the capacity for encoding of declarative memories47. The major function of SHY is the scaling back of energy consumption, once an unsustainable threshold of potentiation has been reached.

Further investigations will be required to elucidate the true role of sleep in memory consolidation. A large amount of evidence has been generated over the last 10 years to support the theory of SHY. There now needs to be a switch toward investigating underlying mechanisms. Altering expression of genes associated with LTP, or optogenetically altering input to specific cells in different sleep stages would provide a deeper understanding of the relationship between potentiation and SWS homeostasis. High resolution monitoring of specific synapses during wake and sleep would allow the greatest assessment of SHY.