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Spindles “shut off” prefrontal cortex from hippocampal input

We are able to acquire new memories, and to maintain them, thanks to the interaction between multiple brain areas that work together to process, filter, and store information. As we experience a new situation, a memory trace of this episode is formed in a structure deeply buried underneath the cerebral cortex: the hippocampus. The hippocampus is a very fast learner, that can form ‘snapshots’ of new environments, social situations, stories, etc. Without an hippocampus, or with a lesioned one, a person is no longer capable of forming new memories (like in the famous movie ‘Memento’) and is stuck living in a past that cannot be updated. But the hippocampus has a limited memory capacity, and it would be impossible to store a lifetime of memories in it. This permanent storage involves the cerebral cortex, the mantle of the brain, which contains many more neurons and is therefore capable of housing much more information. With time after the acquisition of a memory, the gist of the most relevant memories is gradually transferred from the hippocampus to the cortex. Sleep has been

 

proven to be very important for this: during sleep, the hippocampus ‘replays’ the memories just acquired under the form of patterns of neural activity, looking like a very complex ‘Morse code’ with a myriad of neurons playing the role of the operators. The cortex listens to this replayed information, and gradually stores it. Neural activity oscillations (those that we can see in the Electroencephalogram) are very important for this as they gate and time this exchange of data within the brain.

 

In a newly published paper on the Proceedings of the National Academy of Sciences of the USA, , in collaboration with Adrien Peyrache and Alain Destexhe (CNRS in Gif-sur-Yvette), study the mechanisms by which oscillations control information exchange. They recorded the neural activity from the hippocampus and the prefrontal cortex (a key cortical area for long-term memory) in sleeping rats. They looked at two important oscillatory patterns: the ‘sharp wave’ in the hippocampus, an explosion of activity in which almost half of all hippocampal neurons activate together in less than a tenth of a second, and the ‘sleep spindles’ of the cortex, a rhythmic activation (at around 10 Hz) of the cerebral cortex, lasting for about a second. Sharp waves are important because they concentrate most of the sleep replay from the hippocampus. They found that when the cortex is not engaged into spindles, they are clearly sensitive to the inputs conveyed by the sharp waves. During spindles however, this transmission was somehow blocked, as if the cortex got all of a sudden ‘disconnected’ from the rest of the brain. This disconnection may be important as it may give the cortex the chance to ‘reprocess’ the information just acquired, without being disturbed by new incoming data.

Furthermore, the researchers found that the inhibitory interneurons, a special kind of neurons that, when active, can ‘turn off’ their neighboring cells, are likely to be the switch permitting or disallowing information transfer, as they become very active during spindles, thus suppressing new inputs.

These new data on brain function during sleep may help understanding the mechanisms of the consolidation of memories in the animal, as well as in the human brain

 

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