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Researchers at the University of Amsterdam (UvA) have published the results of a study in which they investigated the mechanism by which memories travel from the hippocampus to the cortex during sleep.

Researchers at the University of Amsterdam (UvA) have published the results of a study in which they investigated the mechanism by which memories travel from the hippocampus to the cortex during sleep. They discovered that the cortex is more receptive to information transmitted by the hippocampus in the form of ‘sharp waves’ (bursts of neural activity) during periods of specific neuronal activity known as ‘sleep spindles’. At the same time, the cortex appears to cut itself off from the rest of the brain during these spindles, and thus stops listening to the hippocampus. This disengagement mechanism could be important in giving the cortex a chance to process newly received information without disruption from incoming data.

The UvA team conducted its research in cooperation with colleagues from the Centre national de la recherche scientifique (CNRS, France). Their findings, which were published in this week’s issue of the Proceedings of the National Academy of Sciences (PNAS), contribute a crucial piece in the puzzle of understanding the role sleep plays in the brain’s consolidation of memories in both the human and animal brain.

Sleeping to remember

We are able to hold onto memories over long periods of time thanks to the way in which different areas of our brains work together to process, filter and store information. Whenever we have a new experience, a memory trace is formed in the hippocampus – the part of the brain that is located just under the cerebral cortex. Whenever the hippocampus initiates the formation of a memory, the essence of that memory is gradually transferred to the neocortex.

Sleep plays a vital role in this transfer. That is because the hippocampus ‘replays’ these memories during sleep – in effect reliving the experiences. During this process, the cortex repeatedly tunes in to these replayed memories, storing the information more and more thoroughly over time. A key factor in this process is the synchronisation of oscillations in neural activity, which determines whether information about the memory actually makes it to the cortex.

Focus on two key oscillations

The researchers focused on two key oscillations. The first is the sharp wave in the hippocampus, where a burst of neural activity activates nearly half of all the neurons in the hippocampus in a fraction of a second. These sharp waves are crucial when it comes to replaying memory traces in the hippocampus. The second oscillation is the sleep spindle, comprising rhythmic neuronal activity in the neocortex (around one second per ‘spindle’).

The researchers also found that inhibitory neurons, which act to make surrounding cells less active, are probably responsible for controlling whether information from the hippocampus makes it to the cortex or not. As these cells became particularly active during the spindles, it may be that they are working to suppress incoming information from the hippocampus.