Rapid eye movement sleep key to formation of memories

Rapid eye movement (REM) sleep — the phase where dreams appear — plays a key role in memory formation, a new study in mice has found.

“We already knew that newly acquired information is stored into different types of memories, spatial or emotional, before being consolidated or integrated,” said Sylvain Williams, professor at McGill University in Canada.

“How the brain performs this process has remained unclear — until now. We were able to prove for the first time that REM sleep is indeed critical for normal spatial memory formation in mice,” said Williams.

Hundreds of previous studies have tried unsuccessfully to isolate neural activity during REM sleep using traditional experimental methods. In the new study, researchers, including those from University of Bern in Switzerland, used optogenetics, a recently developed technology that enables scientists to target precisely a population of neurons and control its activity by light.

“We chose to target neurons that regulate the activity of the hippocampus, a structure that is critical for memory formation during wakefulness and is known as the ‘GPS system’ of the brain,” Williams said. To test the long-term spatial memory of mice, scientists trained the rodents to spot a new object placed in a controlled environment where two objects of similar shape and volume stand.

Spontaneously, mice spend more time exploring a novel object than a familiar one, showing their use of learning and recall. When these mice were in REM sleep, however, the researchers used light pulses to turn off their memory-associated neurons to determine if it affects their memory consolidation. The next day, the same rodents did not succeed in the spatial memory task learned on the previous day. Compared to the control group, their memory seemed erased, or at least impaired. “Silencing the same neurons for similar durations outside REM episodes had no effect on memory. This indicates that neuronal activity specifically during REM sleep is required for normal memory consolidation,” said the study’s lead author Richard Boyce.