The Potential of Brain Stimulation During Sleep to Enhance Memory Function

Post by Meredith McCarty

The importance of sleep

Sleep is essential for humans and all living species. Despite being a period of apparent vulnerability during daily life, the maintenance of sleep throughout evolution suggests that sleep is fundamental for neural and bodily function. Sleep deprivation can lead to numerous deficits including altered attention, memory, and learning (Krause et al., 2017).  

While humans spend about a third of their lifetimes asleep, the duration and nature of our sleep differ from that of our nearest primate relatives. Humans spend less time asleep than other primates, and relatively more time is spent in rapid eye movement relative to non-REM sleep (Nunn & Samson, 2018). There are many interesting theories as to the evolutionary origin of such changes in sleep quality and duration (see Nunn et al., 2016 for review), and increasing evidence for the essential role of sleep in memory consolidation. 

Memory consolidation during sleep

Memory consolidation describes the process by which information learned from the environment is transferred from temporary short-term memory into more distributed and permanent long-term memory. There is growing evidence that slow-wave sleep (SWS), a period of non-REM sleep marked by low-frequency, high-amplitude brain waves, is pivotal for memory consolidation (Klinzing et al., 2019).  

During SWS, cortical and subcortical regions, namely the hippocampus, thalamus, and neocortex, exhibit distinct patterns of neural oscillations (Ngo et al., 2020). These dynamics are described as thalamo-cortical spindles, hippocampal ripples, and cortical slow oscillations (Latchoumane et al., 2017). Hippocampal ripples, or brief periods of synchronized oscillatory activity, are thought to facilitate communication between the hippocampus and cortical and subcortical regions (Todorova & Zugaro, 2020; Brodt et al., 2023). 

The neural mechanism by which these dynamics may enable memory consolidation is through phase-locking of brain activity between different brain areas, enabling the transmission and nesting of neural signals between brain regions. Animal research has shown that when hippocampal ripples are disrupted, memory consolidation is impaired (Ego-Stengel & Wilson, 2010).

Can memory be enhanced during sleep?

Studies investigating what happens when we disrupt SWS in human and non-human animals have shown that disruption of oscillatory dynamics during SWS can lead to deficits in memory tasks.  But what about the possibility of enhancing memory through sleep?

In a recent study, Geva-Sagiv and colleagues performed closed-loop brain stimulation during sleep in human patients implanted with intracranial electrodes (Geva-Sagiv et al., 2023). The participants performed a cognitive memory task, by which memory accuracy was compared following natural sleep and sleep during which closed-loop stimulation was precisely applied during active phases of SWS. They found enhanced sleep spindles and synchronized spiking between interconnected brain regions following stimulation SWS. Additionally, stimulation during SWS correlated with improved memory accuracy in the behavioral task. These data suggest that the synchronized brain activity during SWS can be increased via external stimulation, correlating with enhanced memory consolidation.  

What’s next?

Sleep-related memory enhancement has enormous implications for clinical application. Sleep deprivation is incredibly detrimental to human health, for both the brain and the body. Insight from sleep research can enable improved treatment for the effects of sleep deprivation and insomnia, as well as the many disorders where sleep disruption occurs. Further research will help to progress our understanding of sleep-related memory enhancement, and how it can be used to make an impact in the future.

References +

Krause et al., 2017. The sleep-deprived human brain. Nature Reviews Neuroscience.

Nunn & Samson, 2018. Sleep in a comparative context: Investigating how human sleep differs from sleep in other primates. American Journal of Physical Anthropology.

Nunn et al., 2016. Shining evolutionary light on human sleep and sleep disorders. Evolution, Medicine, and Public Health.

Klinzing et al., 2019. Mechanisms of system memory consolidation during sleep. Nature Neuroscience.

Ngo et al., 2020. Sleep spindles mediate hippocampal-neocortical coupling during long-duration ripples. eLife.

Latchoumane et al., 2017. Thalamic Spindles Promote Memory Formation during Sleep through Triple Phase-Locking of Cortical, Thalamic, and Hippocampal Rhythms.

Todorova & Zugaro, 2020. Hippocampal ripples as a mode of communication with cortical and subcortical areas.

Brodt et al., 2023. Sleep-A brain-state serving systems memory consolidation. Neuron.

Geva-Sagiv et al., 2020. Augmenting hippocampal-prefrontal neuronal synchrony during sleep enhances memory consolidation in humans. Nature Neuroscience.