German neuroscientists have made a significant advancement in finally uncovering the long-missing link between deep sleep and memory formation.
Working out of Berlin’s Charité —Universitätsmedizin, the team performed electrification experiments on brain tissue to discover how the slow waves of deep sleep aid the neocortex in long-term memory formation. As sleep and memory are often linked, this finding may lead to new treatments for memory formation issues that frequently plague older adults or those suffering from traumatic brain injuries.
Slow Wave Deep Sleep And Memory
While many mysteries still reside in the sleeping mind, sleep experts have some ideas about what occurs while snoozing. They postulate that our brain replays our day in a process that transfers information from short-term storage in the hippocampus to long-term storage in the neocortex. During this process, the brain generates slow, synchronous oscillations of electrical voltage called “slow waves.”
Relaxed “slow waves” occur within a specific frequency observable on an electroencephalogram (EEG) test. The waves usually accompany sleep or rest. Specific impairments can cause these waves to appear prominently in wakeful states, such as in victims of traumatic brain injuries. The terms “slow wave” and “delta wave” are sometimes used imprecisely as synonyms, although some in the sleep study field are working to draw a clear distinction between the two types of events. Some recent research has even questioned fundamental assumptions about the relationship of slow waves to sleep.
“We’ve known for many years that these voltage fluctuations contribute to the formation of memory,” explained team leader Prof. Jörg Geiger, 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.”
Electrified Brains
The team’s experiments utilized rare intact human neocortical brain tissue samples from epilepsy and brain tumor surgeries. Such procedures typically require the removal of some brain matter. Nutrient solutions can keep the tissue alive for a day or two after removal. That window is just long enough to perform such experiments.
Geiger’s team electrified the tissue sample with fluctuating voltages that mimicked deep sleep’s slow brain waves. The slow waves strengthened synaptic connections in the neocortex, making the neurons more receptive to electrical signals.
Used glass micropipettes placed with nanometer precision to monitor nerve cell communications, Geiger and his colleagues utilized an unusually large number of devices in tandem, up to ten at once. At a certain point in the fluctuations, those neural connections reached a point of peak enhancement.
“The synapses work most efficiently immediately after the voltage rises from low to high,” commented 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.”
Utilzing the Discovery
With this knowledge, new doors are potentially opening toward the development of innovative kinds of treatment. The team believes that mild cognitive issues afflicting older people would be a prime target for applying this new knowledge.
The potential use of transcranial electrostimulation to modify brain waves during sleep offers hope for developing targeted methods to boost memory formation. With the new findings, future researchers can more precisely target such electrostimulation.
The paper “Membrane Potential States Gate Synaptic Consolidation in Human Neocortical Tissue” appeared on December 12, 2024 in the Nature.
Ryan Whalen covers science and technology for The Debrief. He holds a BA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.