Memory Improves with Sleep
Not only does “sleeping on it” help with decision-making, it also lets us memorize important information acquired during the day. Recent research throws new and astonishing light on this process.
Though apparently asleep, a healthy rat is busy memorizing its way through a specially-designed maze it had earlier been placed in.
These neurophysiological processes, specifically the mechanisms of memorization during sleep, were recently investigated1,2
by researchers from LPPA.3
According to the most widely-accepted hypothesis, during sleep a dialogue takes place between two structures in the brain—the hippocampus and the cortex—which enables long-term memorization of information in the cortex. Yet until now, this dialogue had never really been observed. To meet this challenge, the researchers implanted electrodes in the brains of rats to observe ongoing neural activity. The rodents were placed in a maze where they had to learn a specific task, such as turning right at each junction, for example. Once they had understood the task, they were allowed to go to sleep.
© G. Girardeau
Sleeping right after this exercise will help the rat remember where to find the food in the maze.
The scientists were then able to compare neural activity during wakefulness and sleep. “When the rat was carrying out its task, neuronal assemblies4
developed which were activated simultaneously in the cortex and hippocampus. Then, during sleep, we observed that the same patterns were reproduced,” explains LPPA researcher Adrien Peyrache. In other words, the rat was dreaming about the maze: the cortex and hippocampus together “replayed” the events that the animal had just experienced, letting the rat assimilate new knowledge. But there is more: “We realized that neural activity during sleep corresponded to that which occurred when the rat had understood a task during wakefulness. In other words, the brain only replayed episodes in which the rat's behavior was the most efficient.” Thus the animal first and foremost retained what would prove useful to him.
The other part of this research concerns the role of the hippocampus in this dialogue with the cortex. Cortex-hippocampus interactions occur specifically when the latter emits very rapid electrical oscillations called “ripples.” “The most common hypothesis is that ripples allow memory consolidation during sleep, probably by transferring labile memories to the cortex,” explains Gabrielle Girardeau, another researcher at LPPA. This is the hypothesis that the scientists wanted to test.
For this, they placed rats in a chamber shaped like an eight-pointed star. A reward was placed at the end of three of the points (always the same). The rats were encouraged to explore the star to find the reward, and were then allowed to go to sleep.
During their sleep, the scientists used electrodes to apply very weak electrical pulses to the rats' hippocampi. Though this did not wake the rodents or disturb other parts of their brain, it specifically blocked the ripples.
The result was that during the 15-day experiment, treated rats found it difficult to memorize the rewarding points of the star, unlike the control rats that were highly successful. “The ripples thus play a fundamental role in memory consolidation, probably by transmitting information to the cortex.”
1. A. Peyrache et al., “Replay of rule-learning related neural patterns in the prefrontal cortex during sleep,” Nat. Neurosci., 2009. 12: 919-26.
2. G. Girardeau et al., “Selective suppression of hippocampal ripples impairs spatial memory,” Nat. Neurosci., 2009. 12: 1222-3.
3. Laboratoire de physiologie de la perception et de l'action (CNRS/ Collège de France).
4. A subpopulation of coordinated cells whose synchronous discharge is associated with internal process and information encoding.