Our long-term memory is consolidated when we sleep. Short-term memory traces in the hippocampus, an area deep in the brain, are then relocated to more outer parts of the brain. An international team of neuroscientists, among who Mathilde Bonnefond and Til Ole Bergmann from the Donders Institute at Radboud Universiy, now shows how a three-step brain oscillation plays an important part in that process. Nature Neurosciences published the results on September 21st.
Bonnefond and Bergmann specialize in research on oscillations: waves of brain activity. ‘Non-rapid eye movement (NREM) sleep is responsible for the memory consolidation during our sleep’, Bonnefond explains. ‘NREM is known for its very slow oscillations (SOs). Other types of oscillations are hidden inside these SOs. We discovered that three types of oscillations are nested inside each other in the hippocampus and have a joint function.’
Slow waves, spindles and ripples
Slow oscillations only happen about once per second (~0.75 Hz). In a specific time frame within these SOs, Bergmann, Bonnefond and their colleagues found clusters of oscillations of an intermediate speed: the so called spindles which happen about 15 times per second (12 – 16 Hz). And within these spindles, they found clusters of superfast oscillations called ripples, which happen about 90 times per second (80 – 100 Hz), and which reflect the local reactivation of the memory trace to be shuttled to the cortex.
Slow oscillations contain spindles, which in their turn contain ripples
Credit: Mathilde Bonnefond