STUDY // The secret life of the brain: What happens to our memory while we sleep

Why are we sleeping? Scientists have been debating this question for millennia, but a new study adds new clues to solving this mystery.

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The findings, published in the Journal of Neuroscience, could help explain how people form their memory and learn, and could ultimately help develop support tools for people with neurological diseases or injuries. The study was conducted by Massachusetts General Hospital in collaboration with colleagues at Brown University, the Department of Veterans Affairs and several other institutions, informs Harvard.Edu.

Scientists studying laboratory animals have long ago discovered a phenomenon known as “resumption” that occurs during sleep, explains neurologist Daniel Rubin of the MGH Center for Neurotechnology and Neurorecovery, the study’s lead author. Resumption is supposed to be a strategy that the brain uses to remember new information.

If a mouse is trained to find its way through a maze, the monitoring devices can show that a specific pattern of brain cells, or neurons, will light up as it crosses the correct path.

Scientists believe that this resumption of the onset of neural impulses during sleep is how the brain exercises newly learned information, which allows a memory to be strengthened – that is, to be converted from a short-term memory to one on long term.

The BrainGate experimental device

However, resumption has been convincingly demonstrated only in laboratory animals.

To study whether resumption 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 quadriplegia, which means he cannot move his upper and lower limbs, in his case due to a spinal cord injury. The man, identified in the study as T11, is participating in a clinical study of a brain-computer interface device that allows him to use a cursor and a computer keyboard on a screen.

The experimental device is developed by the BrainGate consortium, a collaborative effort involving clinicians, neuroscientists and engineers from several institutions in order to create technologies to restore communication, mobility and independence of people with neurological diseases, injuries or loss of a 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, then has to remember and reproduce that sequence. He controlled the cursor on the computer screen simply by thinking about the movement of his own hand. The sensors implanted in T11’s motor cortex measured the patterns of the neural impulse, which reflected the intentional movement of his hand, allowing him to move the cursor on the screen and click on it in the desired places. These brain signals were recorded and transmitted wirelessly to a computer.

That night, while T11 was sleeping at home, activity in his motor cortex was recorded and transmitted wirelessly to a computer. “What I discovered was pretty incredible,” says Rubin. “Basically, he played the game overnight, in his sleep.” On several occasions, Rubin says, T11’s patterns of neuronal burning during sleep corresponded exactly to the patterns that occurred while he performed the memory matching game earlier that day.

REM phase of sleep, the phase most commonly associated with dreaming

“This is the most direct evidence of recurrence in the motor cortex that has ever been seen in human sleep,” says Rubin. Most of the resumption detected in the study occurred during slow-wave sleep, a phase of deep sleep. Interestingly, resumption was much less likely to be detected while T11 was in the REM phase of sleep, the phase most commonly associated with dreaming. Rubin and Cash believe that this paper is a basis for learning more about resumption 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 to help people learn faster and more effectively to regain control after an injury,” says Cash. emphasizing the importance of moving this line of research from animals to human subjects. “This type of research benefits enormously from the close interaction we have with our participants,” he adds, grateful to T11 and the other participants in the BrainGate clinical trial.

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