AsianScientist (July 20, 2017) – Modifying the rhythm of brain waves of mice while they sleep can enhance or reduce their ability to form memories. These findings have been published in Neuron.
Slow-wave sleep, known commonly as deep sleep, is characterized low frequency electroencephalography readings and is thought to be involved with memory formation rather than dreaming. During slow-wave sleep, groups of neurons firing at the same time generate brain waves with triple rhythms: slow oscillations, spindles and ripples.
Slow oscillations originate from neurons in the cerebral cortex. Spindles come from a structure of the brain called thalamic reticular nucleus and spike around 7-15 per second. Finally, ripples are sharp and quick bursts of electrical energy, produced within the hippocampus, a brain component with an important role in spatial memory.
“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. We believe that the correct timing of these three rhythms acts like a communication channel between different parts of the brains that facilitates memory consolidation,” explained Dr. Charles-Francois V. Latchoumane, first co-author of the study.
The researchers focused on spindles because the number of spindles is known to be connected to memory. Specifically, the number of spindles increases following a day stuffed with learning and declines in the elderly, as well as patients with schizophrenia.
They first put mice in a special cage and gave them a mild electric shock after playing a tonal noise. The day after, the researchers tested the memory of the mice by checking their fear reaction in response to either the same noise or the same cage. At nighttime between the two days, scientists introduced artificial thalamic spindles in some of the mice using a light-based technique called optogenetics.
The mice were divided into three groups. The first group received the light input just after the slow oscillations, so their spindle could form a triple rhythm (in phase): slow oscillation-spindle-ripples. In the second group the light stimulations were applied later out of sync. The third group was used as a control and did not receive any light stimulation.
The day after, mice were placed in the same location and their movement was recorded. The mice of the first group were frozen in fear 40 percent of the time, even in absence of the noise. On the contrary, mice in the second and third groups only froze up to 20 percent. Instead, when the mice heard the same tone in a different location, remembered the tone and froze in fear up to 40 percent of the time, independently from the group they belonged to.
The opposite was also true: it was possible to make mice forget. By reducing the number of overnight spindles, the researchers could reduce the memory recall.
“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.”
The article can be found at: Latchoumane et al. (2017) Thalamic Spindles Promote Memory Formation during Sleep through Triple Phase-Locking of Cortical, Thalamic, and Hippocampal Rhythms.
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Source: Institute for Basic Science.
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