We all know this situation: trying in vain to learn something before sleeping and miraculously knowing it by heart when we wake up. What happened in our brain overnight to make the magic happen?
Scientists had already claimed several years ago that the brain consolidates memories by “replaying” at night what it learned during the day. However, how is the information stored in the long term? Michaël Zugaro and his team from the Center for Interdisciplinary Research in Biology (CNRS, Inserm, Collège de France) have finally answered this curious question. Memories are initially formed in a moonlike structure in the center of the brain called the hippocampus. But several theories on memory that have not yet been proven assume that in order to print lasting memory traces, that is to say stored over the long term, the hippocampus must exchange information with the outer layer of the brain, the cortex. And this exchange would occur only during sleep…
Thus, to get their hands on experimental proof, French neurobiologists recorded the electrical activity of the hippocampus and the medial prefrontal cortex of rats during their deep sleep (characterized by slow waves). When the hippocampus rippled, the cortex responded about 140 milliseconds later with so-called delta waves and trains of electrical impulses called « sleep spindles. » Next, the researchers trained the rats to memorize two identical objects located in two corners of a rectangular box. We left them in the box for 3 or 20 minutes. The next day, before letting them enter the box, an object was moved to another corner. When the rats had stayed 20 minutes in the box the day before, they then spent a lot of time around the moved object, which shows that they had noticed the change. Moreover, the electrical dialogue between their hippocampus and the cortex during their sleep was considerable. On the other hand, those who had stayed only 3 minutes had not memorized the objects and, conversely, presented an exchange between the two structures that was much weaker.
The researchers then stimulated the brains of the latter using electrodes to induce delta waves and sleep spindles in the prefrontal cortex 140 milliseconds after the emission of waves by the hippocampus. The result obtained is incredible since the next day, the rats remembered the location of the objects as if they had exploited the environment 20 min the day before. Moreover, this experiment made it possible to show that certain cortical neurons, which are solicited more when the rats circled around the object, change during sleep when the coupling between the hippocampus and the cortex intervenes. Thus, helping the interactivity of sleep between the hippocampus and the cortex would be a ploy to consolidate a long-term memory. But it’s not for now so in the meantime, sleep well!