It has been established for almost two decades that the slow, synchronized electrical waves in the brain during deep sleep play a crucial role in memory formation. However, until recently, the reasons behind this connection were unclear. Now, a research team from Charité — Universitätsmedizin Berlin has proposed an explanation in a study published in the journal Nature Communications. They suggest that these slow waves make the neocortex, where long-term memories are stored, particularly receptive to incoming information. These insights might enhance external treatment strategies aimed at supporting memory development.
It has been established for almost two decades that the slow, synchronized electrical waves in the brain during deep sleep play a crucial role in memory formation. However, until recently, the reasons behind this connection were unclear. Now, a research team from Charité — Universitätsmedizin Berlin has proposed an explanation in a study published in the journal Nature Communications. They suggest that these slow waves make the neocortex, where long-term memories are stored, particularly receptive to incoming information. These insights might enhance external treatment strategies aimed at supporting memory development.
How do lasting memories come to be? Experts theorize that during sleep, our brains replay the day’s events, facilitating the transfer of data from the short-term memory storage in the hippocampus to the long-term memory in the neocortex. “Slow waves” are vital to this operation: these are slow, synchronized electrical oscillations that occur in the cortex during deep sleep. They can be detected using an electroencephalogram (EEG). These waves emerge when the electrical activity of many neurons simultaneously fluctuates once per second.
“We have understood for years that these voltage fluctuations play a role in memory formation,” says Prof. Jörg Geiger, director of the Institute of Neurophysiology at Charité and the lead author of the new research. “When slow-wave sleep is artificially enhanced, memory improves. Yet, we didn’t know what specific processes occur within the brain during this enhancement, as it has been challenging to study information transfer in the human brain.”
Slow waves enhance synaptic connections
His team has now utilized intact human brain tissue—a rare resource—to clarify the mechanisms likely involved in memory formation during deep sleep. Their study revealed that the slow electrical waves impact the strength of synaptic connections among neurons in the neocortex, influencing their receptiveness.
In their research, the scientists examined intact neocortical tissue samples from 45 patients who had undergone neurosurgery for epilepsy or brain tumors at Charité, the Evangelisches Klinikum Bethel (EvKB) hospital, or the University Medical Center Hamburg-Eppendorf (UKE). They replicated the voltage patterns typical of slow electrical waves during deep sleep in the tissues and measured how the nerve cells responded. They achieved this by using glass micropipettes carefully positioned down to the nanometer. To monitor communication between several connected nerve cells, they employed up to ten “pipette feelers” simultaneously—a notably high number for this method known as the multipatch technique.
Timing is crucial for memory development
The researchers discovered that synaptic connections among neocortex neurons are most effectively enhanced at a specific moment during the voltage fluctuations. “The synapses operate at peak efficiency right after the voltage escalates from low to high,” explains Franz Xaver Mittermaier, a researcher at the Institute of Neurophysiology at Charité and the study’s first author. “In that fleeting time frame, the cortex enters a state of heightened readiness. If the brain recalls a memory during this optimal moment, it is transferred to long-term memory particularly effectively. This indicates that slow-wave sleep aids in memory formation by making the neocortex particularly receptive during several brief intervals.”
This newfound understanding could be harnessed to improve memory, particularly in older adults experiencing mild cognitive impairment. Research teams worldwide are exploring methods to influence slow waves during sleep using subtle electric impulses—known as transcranial electrostimulation—or acoustic cues. “Currently, these stimulation methods are being fine-tuned through trial and error, making it a tedious and long process,” Geiger notes. “Our insights regarding precise timing could facilitate advances in this area, paving the way for targeted stimulation techniques to enhance memory creation.”
Understanding slow brain waves
Slow waves, also referred to as slow oscillations, are a form of electrical activity produced in the brain during deep sleep. “Delta” waves represent a specific frequency range observed in EEG readings. These slow waves can also manifest outside of sleep, in association with certain diseases or conditions. This broader terminology can sometimes be used interchangeably with “slow waves.”
About the study
When conducting surgery for drug-resistant epilepsy or brain tumors, it’s often essential to remove small portions of the neocortex. The excised tissue can be preserved for up to two days outside the body in an artificial nutrient solution before it becomes inactive. Gaining explicit consent from patients was crucial for examining this precious tissue for the recently published study. The research group expresses deep gratitude to the patients for their willingness to participate. The study resulted from close collaboration between fundamental research and clinical departments at Charité as well as the University Clinic for Neurosurgery at Evangelisches Klinikum Bethel (EvKB) in Bielefeld and the Department of Neurosurgery at UKE.
