It’s no hidden health secret that sleep is really good for us. It helps our immune systems and supports almost every organ system in the body. We’ve also known for almost two decades that the slow, synchronous electrical waves in the brain during deep sleep supports memory formation. However, we did not know exactly how the brain does this until now. These slow waves make the neocortex–where long-term memory is stored in the brain–particularly receptive to new information. The findings are detailed in a study published December 12 in the journal Nature Communications.
How permanent memories form
Scientists believe that our brains replay the events of the day when we sleep. The brain moves information from the hippocampus–where short-term memories are stored into the neocortex. In the neocortex, they become long-term memories. Slow waves are key to this process. These waves are steady and simultaneous oscillations of electrical voltage in the cortex that happen during deep sleep. Slow waves can be measured using a test called an electroencephalogram (EEG). The waves also begin 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,” Jörg Geiger, a study co-author and director of the Institute of Neurophysiology at Charité Universitätsmedizin Berlin in Germany, said in a statement. “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.”
[ Related: How to fix your sleep schedule without pulling an all-nighter. ]
Studying intact brain tissue
In the new study, the team used extremely rare intact human brain tissue. They studied intact neocortical tissue samples that were taken from 45 patients who had undergone surgery to treat either a brain tumor or epilepsy at University Medical Center Hamburg-Eppendorf in Hamburg, Germany. They simulated the voltage fluctuations typical of slow brain waves during deep sleep in the tissue. Then, they measured the nerve cells’ response by using glass micropipettes positioned down to the nanometer. To eavesdrop on the communications among multiple nerve cells connected through the tissue, they used up to ten “pipette feelers” at once. This is a particularly large number for this nerve listening method called the multipatch technique.
The team found that the connections between neurons in the neocortex are very enhanced at one specific point in time during the voltage fluctuations.
“The synapses work most efficiently immediately after the voltage rises from low to high,” study co-author and neurophysiologist Franz Xaver Mittermaier, said in a statement. “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.”
This slow-wave sleep likely supports memory formation by making the long-term memory storing neocortex especially receptive for short bursts of time.
Finding the perfect timing
According to the team, this knowledge could be used to create better therapeutic methods for improving memory. Several research groups all over the world are working on methods of using subtle electrical impulses–called transcranial electrostimulation–or acoustic signals as a way to influence slow waves during sleep.
“Right now, though, these stimulation approaches are being optimized through trial and error, which is a laborious and time-consuming process,” said Geiger. “Our findings about the perfect timing could help with this. Now, for the first time, they allow for targeted development of methods of stimulation to boost memory formation.”
