Students have long been told they should get a good night’s sleep before exams rather than cramming up to the last minute.
New research adds weight to this, showing for the first time in humans that the brain does replay memories while sleeping.
The finding, published in the journal Cell Reports, builds on converging evidence that somehow new memories are consolidated in the brain and thus shifted to long-term memory stores to enhance learning.
“At the neural level, offline ‘replay’ is thought to underlie such memory consolidation,” says co-author Beata Jarosiewicz from Brown University, US.
This notion has been supported by research with non-human animals that showed the spiking activity from interconnected networks of neurons during waking experiences were repeated while “offline” during sleep.
Functional neuroimaging and intracranial EEG studies have indirectly suggested this is also happening in humans, but until now the recording technologies were unable to directly test for replay at the level of neural firing patterns.
Jarosiewicz and colleagues, including first author Jean Baptiste Eichenlaub from Massachusetts General Hospital’s neurology department, saw an opportunity to test this with participants enrolled in a BrainGate clinical trial.
These people have weakness or paralysis in their arms as a result of spinal cord injury or ALS, and the study is forging ways to restore communication and mobility using a brain-computer interface.
This involves recording neural activity patterns while they think about moving their hand, and decoding them in real time using machine learning algorithms that allow them to control a cursor on a computer screen, robotic arms or other assistive devices with their thoughts.
For the current study, two participants took a nap before and after playing a sequence copying game much like the popular ‘80s game Simon.
The game has four colour panels that light up in different sequences for participants to repeat, but instead of using their arms, they played the game with their minds, imagining moving the cursor to hit the correct colours in correct order as quickly as possible.
The researchers recorded their brain activity while they rested, played the game and rested again, using an implanted multi-electrode array with electrodes tiny enough to detect the firing of individual neurons – the first to enable such detailed neural recordings in the human brain.
Results showed the same neural firing patterns during and after playing the game, as if they were replaying it while sleeping afterwards. They also improved performance compared to control sequences, confirming learning-related replay in the human brain.
“This is the first piece of direct evidence that in humans we also see replay during rest following learning that might help to consolidate those memories,” says Jarosiewicz.
The next step is to confirm that this replay is enabling memories to be consolidated by, for instance, testing if there is a relationship between the strength of the replay and the strength of post-nap memory recall.
Natalie Parletta is a freelance science writer based in Adelaide and an adjunct senior research fellow with the University of South Australia.
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