Scientists recently used electrical stimulation in the brains of epilepsy patients to investigate the relationship between brain activity and memory consolidation during sleep. They found that synchronizing the firing of neurons in the medial temporal lobe and neocortex through this stimulation improved memory consolidation, particularly for recognition memory tasks.
The findings, which have been published in Nature Neuroscience, contribute to our understanding of memory processes and may have important implications for the development of interventions for memory disorders and dementia.
The motivation behind this study was to investigate how the brain consolidates memories during sleep. While it is known that sleep plays a vital role in memory strengthening, the specific processes that occur in the brain during sleep are still not well understood.
The researchers focused on the theory that there is communication between the hippocampus (the brain’s memory center) and the cerebral cortex (associated with higher brain functions) during deep sleep. This communication is believed to happen through slow brain waves and synchronized firing of neurons.
“Memory and sleep are each fascinating topics, and how sleep reshapes memories is a basic question that has always intrigued me,” said study author Maya Geva-Sagiv of the Center of Neuroscience at the University of California, Davis.
“Understanding what are the neural mechanisms underlying long-term memory consolidation during sleep is a fundamental scientific question, that is relevant for clinical domains where both sleep and memory are impaired (such as dementia) and also for the public at large.”
“The existing research was either based on studies in animal models or on non-invasive human studies that were mostly correlative. Here, we wanted to investigate human sleep and memory using an active intervention that could establish the role of synchrony between the hippocampus and cortex in this process.”
To conduct the study, the researchers had a unique opportunity to work with 18 epilepsy patients who had electrodes implanted in their brains for seizure monitoring. These patients stayed in the hospital for around 10 days, providing an ideal setting for studying brain activity during sleep. The researchers designed a closed-loop stimulation protocol, where they delivered electrical pulses to the patients’ brains during deep sleep to enhance the coordination of brain activity.
The study was carried out over two nights. On the first night, before bedtime, the participants were shown photo pairings of animals and celebrities. They were then tested on their ability to recall which celebrity was paired with which animal. The same test was conducted again in the morning after a night of undisturbed sleep. On the second night, the participants were shown a new set of animal and celebrity pairings before bedtime. This time, they received targeted electrical stimulation during sleep.
The researchers created a real-time closed-loop system that monitored the participants’ brain activity and delivered electrical pulses when they entered the deep sleep phase associated with memory consolidation. The system synchronized the firing of neurons in the hippocampus and the cerebral cortex to enhance their communication.
The results of the study showed that the participants who received synchronization-based stimulation in the prefrontal cortex white matter demonstrated improved recognition memory compared to the nights of undisturbed sleep. However, participants who received stimulation in other posterior neocortical regions or a different type of stimulation showed mixed results or even degraded performance. The stimulation did not significantly affect the accuracy of pairing associations between celebrities and animals.
“Our study provides strong evidence in support of a model that explains how experiences are transformed into lasting memories,” Geva-Sagiv told PsyPost. “The broad idea is that the hippocampus acts as a fast learner, picking up bits and pieces of our daily life. During sleep, our hippocampus replays selected snippets of our experiences, allowing the neocortex to update and reshape our understanding of the world.”
“Our study demonstrated that an active intervention that results in increased synchronization between the hippocampus and cortical areas also leads to an improvement in memory accuracy (relative to a night of undisturbed sleep). We hope sleep interventions would be further developed in the future to provide hope for people with memory impairments such as dementia. By boosting natural processes that happen during sleep, we were able to improve memory.”
To understand the underlying mechanisms, the researchers analyzed the effects of stimulation on sleep electrophysiology. They found that synchronization-based stimulation increased spindle power, a type of brain wave associated with memory consolidation, across various brain regions.
This increase in spindle power was significantly greater than that observed during sham-stimulation, where no electrical pulses were delivered. On the other hand, stimulation delivered without regard to the timing of slow-wave activity did not induce immediate changes in spindle power.
“We were surprised by the fact that many of the neural effects we found were brain-wide, and not just close to where the stimulation was delivered. We used very localized, low-amplitude stimulation in frontal neocortical areas, and found enhancing effects of sleep spindles across both hemispheres,” Geva-Sagiv explained.
Overall, this study provides evidence supporting the theory that coordinated communication between the hippocampus and the cortex during sleep is crucial for memory consolidation. By using targeted electrical stimulation to enhance this communication, the researchers were able to improve memory performance in the participants. These findings contribute to our understanding of how sleep impacts memory and may have implications for developing interventions to enhance memory consolidation during sleep.
“This provides the first major evidence down to the level of single neurons that there is indeed this mechanism of interaction between the memory hub and the entire cortex,” said study co-author Itzhak Fried in a news release. “It has both scientific value in terms of understanding how memory works in humans and using that knowledge to really boost memory.”
But, like all research, the new study includes some caveats. For example, the researchers acknowledged that epilepsy and medication could influence sleep and memory. Nevertheless, the researchers designed the study to minimize these factors.
“The participants in the current study were patients with medically refractory epilepsy,” Geva-Sagiv explained. “Their pathology and medication regime may affect sleep and memory. We tried to minimize the influence of such confounding factors on several levels – first, by comparing the behavioral effects of intervention night to those of undisturbed sleep in the same individual. Second, by comparing the electrophysiological effects (neural signals) to interleaved pause periods in each intervention session.”
“We also ran an independent behavioral study, using the same cognitive paradigm, with a group of healthy individuals to corroborate sleep’s effects on memory accuracy in this specific paradigm. The technical challenge of the setup and bedside testing limited the number of participants in our cohort – additional studies are needed to test these findings in a greater number of subjects.”
“We are grateful for the participants who volunteered to take part in this study and share their hope that a better understanding of memory processes will help others down the road,” Geva-Sagiv added.
The study, “Augmenting hippocampal–prefrontal neuronal synchrony during sleep enhances memory consolidation in humans“, was authored by Maya Geva-Sagiv, Emily A. Mankin, Dawn Eliashiv, Shdema Epstein, Natalie Cherry, Guldamla Kalender, Natalia Tchemodanov, Yuval Nir, and Itzhak Fried.