IBS researchers find that manipulating the pulses of electrical activity in the thalamus during non–REM deep sleep make mice remember or forget.

While we absorb so much information during the day consciously or unconsciously, it is during shut eye that a lot of facts are dispatched to be filed away or fall into oblivion. A good quality sleep is the best way to feel mentally refreshed and memorize new information, but how is the brain working while we sleep?

Scientists at the Center for Cognition and Sociality, within the Institute for Basic Science (IBS), enhanced or reduced mouse memorization skills by modulating specific synchronized brain waves during deep sleep. This is the first study to show that manipulating sleep spindle oscillations at the right timing affects memory.

The full description of the mouse experiments, conducted in collaboration with the University of Tüebingen, was published in the journal Neuron.

The research team concentrated on a non–REM deep sleep phase that generally happens throughout the night, in alternation with the REM phase. It is called slow–wave sleep and it seems 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.

The researchers focused on spindles because it was shown that the number of spindles is connected with memorization. It has been shown that the number of spindles increases following a day stuffed with learning and declines in the elderly, and in patients with schizophrenia. This is the first study to show that artificial thalamic spindles affect memory, if administered in sync with slow oscillations.

In the experiment, mice were put in a special cage and given a mild electric shock after hearing a tonal noise. The day after, their memory was tested, by checking their fear reaction in response to either the same noise or the same cage. Latchoumane explains that this could be simplified and compared to the experience of hearing a fire alarm in a certain location, like a cafe. The incident would be followed by either another visit to the same cafe or the sound of the fire alarm in another cafe on the following day.

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% of the time, even in absence of the noise. On the contrary, mice in the second and third groups only froze up to 20%. Instead, when the mice heard the same tone in a different location, remembered the tone and froze in fear up to 40% of the time, independently from the group they belonged to. The hippocampus is involved in spatial memories which might explain the difference.