Have you ever crammed for an exam the night before and woken up with new information embedded in your mind? Almost like a magic spell, your biological diagrams, calculus equations, or chemistry reactions have been ingrained in your memory as you slumbered away. Perhaps, this is why professors advocate against all-nighters before exams!
Multiple preliminary studies have conveyed that sleep enhances long-term memory consolidation. These findings suggest that memories of retrievable information like facts and events form in the brain's GPS, the hippocampus, which aids navigation and spatial memory storage and retrieval [1]. Newly acquired knowledge enters the prefrontal cortex, a subdivision of a neocortex in the brain, during the first three stages of sleep characterized by non-rapid eye movement (NREM), via subtle oscillations in frequency (~80-120 Hz) and firing of neurons in the hippocampus known as ripples. Imagine these ripples as small bursts of electrical activity that travel through the brain, helping to encode new episodic memories during sleep. These brief electrical events occur offline during sleep and are implicated in the encoding process of new episodic memories [1].
Researchers from UCLA and Tel Aviv University collaborated to study the relationship between brain wave patterns and the three stages of memory [2]. They examined individuals diagnosed with epilepsy and implanted with intracranial electrodes. These electrodes were surgically placed in precise locations of the brain, allowing doctors to monitor trigger sites for potential seizures, but patients could opt-in to use the electrodes already in their brains for further research. In a series of overnight experiments, these scientists provided brief (~50ms) electrical stimulation of the neocortex to the participants using a process called real-time closed-loop (RTCL) stimulation [2].
The results were fascinating! The RTCL stimulation had varying responses on memory consolidation within these participants depending on the timeframe of the hippocampal ripples [2]. If the neocortical stimulation was time-locked with the slow oscillation patterns of the hippocampus in NREM sleep (sync stimulation), participants responded with improved recognition memory accuracy relative to the absence of time-locked stimulation (mix-phased stimulation). This observation suggests that a significant communication pattern known as hippocampal-neocortical interaction is present [2].
Millions of neurons briefly synchronize with each other through rhythm within the existing neural circuits to communicate via electrical oscillations known as brain waves [2]. During early NREM sleep, hippocampal ripples trigger the same neural circuits active before sleep, which forms lasting connections of new episodic events and information. For instance, after learning 25 pairings between celebrities and animals, all participants receiving sync stimulation had higher performance accuracy than individuals with no RTCL stimulation on memory recall tasks the following morning. Scientists also observed that mixed-phased stimulation generally led to degraded performance accuracy on the same cognitive and visual-pairing assessments. These results allowed the researchers to conclude that synchronization between neocortical stimulation and ripples during sleep increased hippocampal-neocortical interactions and improved sleep-mediated memory storage and retrieval [2].
The findings from this study support an innovative approach to examining memory disorders like Alzheimer's disease and Vascular dementia. Researchers can utilize this communication pattern between the hippocampus and neocortex to construct deep brain stimulation (DBS) devices or neurostimulators for patients with these conditions, which can regulate brain activity similar to how a pacemaker operates for the heart. This process may also facilitate therapeutic pathways and treatments for memory disorders with ongoing advancements in neuroscience research! Next time you consider an all-nighter, consider going to sleep instead so your brain can ripple its way to a perfect exam score.
References:
[1] Lisman, J., Buzsáki, G., Eichenbaum, H., Nadel, L., Ranganath, C., & Redish, A. D. (2017). Viewpoints: how the hippocampus contributes to memory, navigation and cognition. Nature neuroscience, 20(11), 1434–1447. https://doi.org/10.1038/nn.4661
[2] Geva-Sagiv, M., Mankin, E. A., Eliashiv, D., Epstein, S., Cherry, N., Kalender, G., Tchemodanov, N., Nir, Y., & Fried, I. (2023). Augmenting hippocampal–prefrontal neuronal synchrony during sleep enhances memory consolidation in humans. Nature Neuroscience, 26(6), 1100–1110. https://doi.org/10.1038/s41593-023-01324-5