New Insight Into Long-Term Memory Formation During Deep Sleep

Researchers from Charité–Universitätsmedizin Berlin, Germany, have posited an explanation for why slow, synchronous electrical waves in the brain during sleep support the formation of memories. The study was published in Nature Communications by Franz Xavier Mittermaier, a researcher at the Institute of Neurophysiology at Charité and the first author of the study.

Slow waves—the synchronous oscillations of electrical voltage in the cortex that occur during the deep sleep phase—happen when our brains replay the events of the day. During this stage of sleep, information from our day is moved from the hippocampus as a short-term memory to the neocortex, where the information becomes a long-term memory.

Professor Jörg Geiger noted that what was not known until now is “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.”

Professor Geiger and the research group have now used intact human brain tissue to clarify the process of memory-making during sleep. They studied intact neocortical tissue samples taken from 45 patients who underwent neurosurgery to treat epilepsy or a brain tumor, simulated the voltage fluctuations typical of slow brain waves during deep sleep in the tissue, and then measured the nerve cell’s response.

Glass micropipettes positioned precisely down to the nanometer allowed the researchers to “listen in” on the communication among multiple nerve cells connected through tissue. Up to 10 pipettes were used as feelers, which is an extra-large number for this method and is known as the multipatch technique.

The researchers discovered that the synaptic connections between neurons in the neocortex are maximally enhanced at a very specific moment during voltage fluctuation. “The synapses work most efficiently immediately after the voltage rises from low to high,” explained Mittermaier.

“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,” Explained the study researchers. “So, slow-wave sleep evidently supports memory formation by making the neocortex particularly receptive for many short periods of time.”

Source

Nature Communications