Neurostimulation techniques to enhance sleep and improve cognition in aging

Sleep plays a critical role in the process of memory consolidation. In particular, during non-rapid eye movement (NREM) slow wave sleep, slow-oscillations, spindles, hippocampal sharp wave ripples, and their phase coupling are involved in the process of transferring and consolidating information recently encoded and temporarily stored in the hippocampus into long-term memory stored in the neocortex. There is evidence that aging and neurodegenerative conditions, in particular Alzheimer's disease, are associated with changes to this transient grouping of NREM oscillations. Therefore, methods to enhance sleep, particularly slow wave sleep, have the potential to improve cognitive performance. Transcranial electrical and magnetic stimulation have been shown useful to enhance sleep slow-waves and sleep-dependent memory consolidation, however there is need for more information regarding proper protocols of application and applicability and efficacy in patients with neurodegenerative conditions. Acoustic stimulation during sleep has been proven particularly effective in enhancing sleep slow-waves and spindles with associated improvement in overnight memory consolidation. More importantly, preliminary data indicate that similar results can be achieved in healthy older adults and those with amnestic mild cognitive impairment. Studies are needed to optimize the modalities of acoustic stimulation during sleep, which may vary based on age group or clinical disorder. Overall, non-invasive techniques of neurostimulation may represent a valid approach to mitigate cognitive decline associated with aging and neurodegeneration. Furthermore, they offer the unique opportunity to improve our understanding of the physiology behind sleep-dependent memory consolidation.