Deep sleep: How slow brain waves boost memory formation

It has been known for nearly 20 years that slow, synchronous electrical waves in the brain during deep sleep support the formation of memories. However, the reason for this has remained unclear. 

Now, a team of researchers from Charité – Universitätsmedizin Berlin have found an explanation. According to the study, the slow waves make the neocortex, the location of long-term memory, especially receptive to information. 

The findings could help to optimize the treatment approaches that are intended to support memory formation from outside.

Slow waves and memory consolidation

How do permanent memories form? Experts believe that while we sleep, our brains replay the events of the day, moving information from the location of short-term memory, the hippocampus, to the long-term memory located in the neocortex. 

“Slow waves” are especially key to this process: slow, synchronous oscillations of electrical voltage in the cortex that occur during the deep sleep phase. 

They can be measured using an electroencephalogram (EEG). The waves originate when the electrical voltage in many neurons rises and falls simultaneously once per second.

“We’ve known for many years that these voltage fluctuations contribute to the formation of memory,” explained senior author Jörg Geiger, the director of the Institute of Neurophysiology at Charité. 

“When slow-wave sleep is artificially augmented from outside, memory improves. But what we didn’t know until now was what exactly is happening inside the brain when this occurs, because it is extremely difficult to study the flows of information inside the human brain.”

Slow waves enhance synaptic connections

Geiger and his team have now used intact human brain tissue, which is extremely rare, to clarify the processes that are very likely to underlie the formation of memory during deep sleep. 

According to their findings, the slow electrical waves influence the strength of synaptic connections between the neurons in the neocortex – and thus their receptivity.

For their study, the team of researchers investigated intact neocortical tissue samples taken from 45 patients who had undergone neurosurgery to treat epilepsy or a brain tumor at Charité, the Evangelisches Klinikum Bethel (EvKB) hospital, or the University Medical Center Hamburg-Eppendorf (UKE). 

The researchers simulated the voltage fluctuations typical of slow brain waves during deep sleep in the tissue and then measured the nerve cells’ response. To achieve this, they used glass micropipettes positioned precisely down to the nanometer. 

To “listen in” on the communications among multiple nerve cells connected through the tissue, they used up to ten “pipette feelers” at once – an extra-large number for this method, which is known as the multipatch technique.

Optimizing timing for memory formation

The experts discovered that the synaptic connections between neurons in the neocortex are maximally enhanced at a very specific point in time during the voltage fluctuations. 

“The synapses work most efficiently immediately after the voltage rises from low to high,” explained lead author Franz Xaver Mittermaier, a researcher at the Institute of Neurophysiology at Charité.

“During that brief time window, the cortex can be thought of as having been placed in a state of elevated readiness. If the brain plays back a memory at exactly this time, it is transferred to long-term memory especially effectively. So, slow-wave sleep evidently supports memory formation by making the neocortex particularly receptive for many short periods of time.”

Implications for memory improvement

This newfound understanding could have important practical applications. With the knowledge of precisely when the neocortex is most receptive, scientists and clinicians may be able to refine techniques that stimulate slow-wave activity. 

Such stimulation can be achieved using subtle electrical impulses (transcranial electrostimulation) or through acoustic cues timed to slow waves during deep sleep. 

“Right now, though, these stimulation approaches are being optimized through trial and error, which is a laborious and time-consuming process,” Geiger said.

Armed with these fresh insights, researchers can now work toward a more targeted development of external stimuli to enhance memory formation. 

For people with mild cognitive impairment or age-related memory decline, carefully timed interventions during deep sleep might strengthen or restore certain aspects of memory. This line of inquiry can also guide future studies into conditions where deep sleep or slow-wave activity is disrupted, such as in patients with neurodegenerative disorders.

In essence, this research clarifies a long-standing mystery: how deep sleep’s slow waves aid in consolidating memories. 

By illuminating the precise timing and mechanism by which the neocortex becomes primed to receive and integrate information from the hippocampus, the study stands to refine therapeutic strategies aimed at improving memory function and potentially help those facing cognitive challenges.

The study is published in the journal Nature Communications.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–