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Role of sleep in memory and learning uncovered

Research findings could aid development of assistive tools for people with neurological injury or disease

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New research into sleep may help explain how memories are formed and how learning is consolidated, and could aid the development of assistive tools for people affected by neurological injury or disease. 

Scientists previously studying laboratory animals discovered a phenomenon known as ‘replay’ that occurs during sleep – a strategy the brain uses to remember new information. 

Scientists believe that this replay of neuronal firing during sleep is how the brain practices newly-learned information, which allows a memory to be consolidated, and converted from a short-term memory to a long-term one. 

However, replay has only been convincingly shown in lab animals.

Now, a new study has investigated whether replay occurs in the human motor cortex — the brain region that governs movement — focusing on a 36-year-old man with tetraplegia who cannot move his upper and lower limbs due to a spinal cord injury. 

The man, identified in the study as T11, is a participant in a clinical trial of a brain-computer interface device that allows him to use a computer cursor and keyboard on a screen.

The investigational device being developed by the BrainGate consortium, a collaborative effort involving clinicians, neuroscientists and engineers at several institutions with the goal of creating technologies to restore communication, mobility, and independence for people with neurologic disease, injury, or limb loss.

In the study, T11 was asked to perform a memory task similar to the electronic game Simon, in which a player observes a pattern of flashing coloured lights, then has to recall and reproduce that sequence. 

He controlled the cursor on the computer screen simply by thinking about the movement of his own hand. Sensors implanted in T11’s motor cortex measured patterns of neuronal firing, which reflected his intended hand movement, allowing him to move the cursor around on the screen and click it at his desired locations. 

These brain signals were recorded and wirelessly transmitted to a computer.

That night, while T11 slept at home, activity in his motor cortex was recorded and wirelessly transmitted to a computer.

“What we found was pretty incredible,” said Dr Daniel Rubin, lead author and a neurologist at the MGH Center for Neurotechnology and Neurorecovery. 

“He was basically playing the game overnight in his sleep.

“This is the most direct evidence of replay from motor cortex that’s ever been seen during sleep in humans.”

Most of the replay detected in the study occurred during slow-wave sleep, a phase of deep slumber. 

Interestingly, replay was much less likely to be detected while T11 was in REM sleep, the phase most commonly associated with dreaming. 

The researchers see this work as a foundation for learning more about replay and its role in learning and memory in humans.

“Our hope is that we can leverage this information to help build better brain-computer interfaces and come up with paradigms that help people learn more quickly and efficiently in order to regain control after an injury,” said neurologist Dr Sydney S. Cash, co-director of the Center for Neurotechnology and Neurorecovery at MGH.

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