Why do we sleep? Scientists have debated this question for millennia, but a new study adds new clues to solve this mystery.
The findings, published in the Journal of Neuroscience, may help explain how humans form memories and learn, and could eventually aid the development of assistive tools for people affected by neurological disease or injury. The study was conducted by Massachusetts General Hospital in collaboration with colleagues from Brown University, the Department of Veterans Affairs and several other institutions.
Scientists studying lab animals long ago discovered a phenomenon known as “replay” that occurs during sleep, says neurologist Daniel Rubin of MGH’s Center for Neurotechnology and Neurorecovery, lead author of the paper. ‘study. Rereading is theorized as a strategy the brain uses to remember new information. If a mouse is trained to navigate its way through a maze, monitoring devices can show that a specific pattern of brain cells, or neurons, will fire when it traverses the correct route. “Then later, while the animal is sleeping, you can see those neurons firing again in the same order,” Rubin says.
Scientists believe that this repetition of neural firing during sleep is the brain’s way of practicing newly learned information, allowing a memory to be consolidated, i.e. converted from short-term memory. in long-term memory.
However, proofreading has only been convincingly demonstrated in laboratory animals. “There has been an open question in the neuroscience community: to what extent is this model of how we learn things true in humans? And is this true for different types of learning? asks neurologist Sydney S. Cash, co-director of the Center for Neurotechnology and Neurorecovery at MGH and co-lead author of the study. Importantly, Cash says, understanding whether replay occurs with motor skill learning could help guide the development of new therapies and tools for people with neurological diseases and injuries.
To study whether proofreading occurs in the human motor cortex – the region of the brain that governs movement – Rubin, Cash and their colleagues enrolled a 36-year-old man with tetraplegia (also called quadriplegia), meaning he is unable to move his upper and lower limbs, in his case due to spinal cord injury. The man, identified in the study as T11, is taking part in a clinical trial of a brain-computer interface device that allows him to use a computer cursor and keyboard on a screen. The experimental device is being developed by the BrainGate Consortium, a collaborative effort involving clinicians, neuroscientists and engineers from multiple institutions with the goal of creating technologies to restore communication, mobility and independence to people with neurological diseases, injury or loss of limb. The consortium is led by Leigh R. Hochberg of MGH, Brown University, and the Department of Veterans Affairs.
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 colored lights and then must recall and reproduce that sequence. He controlled the cursor on the computer screen just by thinking about the movement of his own hand. Sensors implanted in T11’s motor cortex measured neural trigger patterns, which reflected the movement of his hand, allowing him to move the cursor around the screen and click on it at desired locations. These brain signals were recorded and transmitted wirelessly to a computer.
That night, as T11 slept at home, his motor cortex activity was recorded and transmitted wirelessly to a computer. “What we found was pretty amazing,” says Rubin. “He was basically playing the game overnight in his sleep.” On several occasions, Rubin says, T11’s neural firing patterns during sleep matched exactly the patterns that occurred while he was performing the memory matching game earlier in the day.
“This is the most direct evidence of motor cortex replay that has ever been seen during sleep in humans,” Rubin says. Most of the repeats detected in the study occurred during slow-wave sleep, a phase of deep sleep. Interestingly, replay was much less likely to be detected while T11 was in REM sleep, the phase most often associated with dreaming. Rubin and Cash see this work as a foundation for learning more about proofreading and its role in human learning and memory.
“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 injury,” Cash says, emphasizing the importance of moving this line of inquiry from animals to human subjects. “This type of research benefits enormously from the close interaction we have with our participants,” he adds, with gratitude to T11 and the other participants in the BrainGate clinical trial.
Hochberg agrees. “Our amazing BrainGate participants not only provide valuable feedback for creating a system to restore communication and mobility, but they also give us the rare opportunity to advance basic human neuroscience – to understand how the human brain works at the circuitry level of individual neurons,” he says, “and to use that information to create next-generation restorative neurotechnologies.”
Rubin is also an instructor in neurology at Harvard Medical School. Cash is Associate Professor of Neurology at HMS. Hochberg is a lecturer in neurology at HMS and a professor of engineering at Brown University.
This work was supported by the Department of Veterans Affairs, National Institute of Neurological Disease and Stroke, National Institute of Mental Health, Conquer Paralysis Now, MGH-Deane Institute, American Academy of neurology and the Howard Hughes Medical Institute at Stanford University.