Francesco Battaglia of the Swammerdam Institute for Life Sciences at the UvA and scientists at the CNRS - Collège de France show in their research that the cerebral cortex and the hippocampus communicate with one another during sleep and together replay previously acquired information.
The results suggest that not all memory traces are consolidated in the same manner, but rather that relevant information is selected and stored more effectively. With this study, the researchers have made significant progress in our knowledge of memory processes and information coding in the brain. This knowledge is essential for the development of new treatments for serious diseases that disrupt memory formation and consolidation, such as Alzheimer's.
Brain activity during sleep
When we are asleep, our brain is not idle. While our body is relatively calm and still, the neurons in our brain continue to work. The level of neural activity is even comparable to that when we are awake. This activity is important because during sleep we subconsciously process memories so that they can be stored in our mind. The process is rather like the working of a computer: when we type something the text is first stored in the memory, however it cannot remain there permanently because as soon as we switch the computer off the information disappears. Every now and then, we therefore click ‘save' to write information from the memory to the hard drive so that it is saved permanently.
In the brain, the hippocampus acts as the ‘temporary' memory, i.e. the first structure in which a new experience is stored. Over the course of time this experience is moved to the cerebral cortex for long-term storage, through a process referred to as memory consolidation. This takes place during periods of physical inactivity, for example during sleep.
The researchers measured the electrical activity of large groups of neurons in the brains of a small number of rats. They carried out the measurements during a behavioural task in which the rat was able to search for a reward and during periods where the animal was asleep. It was found that the same neuron ensembles were reactivated during sleep, indicating that information is replayed and that active communication takes place between the two areas of the brain. What was striking was that not every element of the behavioural task was reactivated in the same way.
A. Peyrache, K. Benchenane, M. Khamassi, S.I. Wiener en F.P. Battaglia. Replay of rule-learning related neural patterns in the prefrontal cortex during sleep. Nature Neuroscience advance online publication (31 May 2009).