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As you sleep, your brain replays the experiences you had while you were awake


We know that animals replay waking experiences while asleep, likely to help consolidate memories. Now we have the first direct evidence that people do this too.

“This is the first time we have shown you can see the same replay in humans,” says Beata Jarosiewicz, now at California-based brain implant company NeuroPace. “The study is unprecedented.”

The brain structure known as the hippocampus appears to take a kind of snapshot of the connections involved in a memory, she says, and to reactivate them later to make that memory permanent. Previous work with fMRI scans suggest this kind of replay, seen in many animals, also takes place in people, but fMRI does not have enough resolution to detect the firing patterns of neurons.


However, Jarosiewicz previously worked for the Braingate research project, which is developing brain-computer interfaces to help people with spinal injuries and other disorders. As part of the project, two volunteers paralysed from the neck down have had arrays of microelectrodes that detect the firing of neurons implanted in the motor cortex in their brain.

The implants allow the volunteers to move a cursor on a screen just by thinking about it. In a series of sessions, the team asked them to move the cursor in certain patterns, some repeated many times.

Afterwards the volunteers napped for 30 minutes, while the electrodes continued to record their brain activity. The neuron firing patterns in the motor cortex associated with the repeated sequences occurred more often than would be expected by chance alone.

At present, the implants have to be physically connected to a computer by a technician who needs to remain present while it is being used. This is why the team looked only at what happened during a short nap.

Once the implants can be used without assistance, it will be feasible to monitor people throughout the night, Jarosiewicz says. “That’s maybe the next step.”

There’s still much we do not understand about sleep, but it is clear that it is important for the brain, she says. “Sleep is very good for your brain in general.”

Journal reference: Cell Reports, DOI: 10.1016/j.celrep.2020.107581

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