For the purposes of their study, the team focused on the deepest stage of non-rapid eye movement (NREM) sleep known as slow-wave sleep (SWS). This state of dreamless sleep is believed to be essential to information processing and memory consolidation, specifically of declarative memories or one's memories of facts and events. During SWS, neurons fire off triple-rhythm brain waves consisting of slow oscillations, ripples, and spindles.
“Often during the night a regular pattern is manifested, where a slow oscillation from the cortex is immediately followed by a thalamic spindle and while this happens, a hippocampal ripple appears in parallel,” lead study author Charles-Francois V. Latchoumane explained to ScienceDaily.com. “We believe that the correct timing of these three rhythms acts like a communication channel [among] different parts of the brains that facilitates memory consolidation.”
These spindles are connected with memorization, and so the researchers concentrated on these forms of brain activity.
During the first part of their study, the mice were placed in a special cage and were administered a mild electrical shock after they heard a tonal noise, thus implanting a fear memory into them. Come night time, the mice were broken into three groups: the first group of mice received artificial spindles in sync with ripples and slow oscillations, the second group was given artificial spindles that were divergent from the slow oscillations and ripples, and the third served as the control group.
One day after the initial experiment, the researchers tested the memories of the mice by gauging their responses to the cage or the noise. The mice from the first group were frozen in fear 40 percent of the time even when the tonal noise was absent. By contrast, the mice from the second group and control group froze only 20 percent of the time.
Based on these results, the researchers concluded that manipulating the number of overnight spindles allowed them to either increase or reduce the fear memory in the mice. Furthermore, the study has shown the importance of timing in memory formation.
“We think that memorization during deep sleep has to do with time coordination. If the hippocampus tries to exchange information when the cortex neurons are not ready to receive it, the information could be wasted,” said Latchoumane. “Slow oscillations might be the signal used by the cortex to flag that it is ready to accept information. Then, the thalamus would alert the hippocampus via the spindles.”
Speaking to DigitalTrends.com, Dr. Hee-Sup Shin called the results “dramatic”, then added: “Inducing extra amount of spindles in the brain during sleep after fear conditioning helped the mice remember better the fear memory, twice as strongly, when assayed next day. Importantly, only the group of mice that received spindle induction in-phase with cortical slow waves revealed enhanced memory, while the two control groups did not.”
On the potential shown by the experiment, Shin remarked: “In the current experiments done in the mouse, we use optogenetic tools which involve expressing a foreign gene and implanting an optic cable in the thalamic reticular nucleus of the mouse.
“These tools may not be used for humans. In the future, however, when non-invasive tools are developed for manipulating brain rhythms in humans, a similar approach may be tried in humans to improve memory for whatever purposes.” (Related: Sound stimulation during sleep can boost memory)
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