Researchers led by the University of California, Los Angeles (UCLA) have for the first time measured the activity of a brain region known to be involved in learning, memory and Alzheimer's disease during sleep.
The research team simultaneously measured the activity of single neurons from multiple parts of the brain involved in memory formation.
The technique allowed them to determine which brain region was activating other areas of the brain and how that activation was spreading, said study senior author Mayank R Mehta, a professor of neurophysics in UCLA's departments of neurology, neurobiology, physics and astronomy.
The team looked at three connected brain regions in mice - the new brain or the neocortex, the old brain or the hippocampus, and the entorhinal cortex, an intermediate brain that connects the new and the old brains.
While previous studies have suggested that the dialogue between the old and the new brain during sleep was critical for memory formation, researchers had not investigated the contribution of the entorhinal cortex to this conversation, which turned out to be a game changer, Mehta said in a statement.
"During sleep the three parts of the brain are talking to each other in a very complex way. The entorhinal neurons showed persistent activity, behaving as if they were remembering something even under anesthesia when the mice could not feel or smell or hear anything," he added.
The findings challenge theories of brain communication during sleep, in which the hippocampus is expected to talk to, or drive, the neocortex. Mehta's findings instead indicate that there is a third key actor in this complex dialogue, the entorhinal cortex, and that the neocortex is driving the entorhinal cortex, which in turn behaves as if it is remembering something.
Mehta theorises that this process occurs during sleep as a way to unclutter memories and delete information that was processed during the day but is irrelevant.
Notably, Alzheimer's disease starts in the entorhinal cortex and patients have impaired sleep, Mehta said.
The study was published in the journal Nature Neuroscience.