A collaborative team of experts specializing in neuroscience and computer engineering has recorded what happens in the brain while a person sleeps, suggesting that a function of dreaming may serve as a way to permanently organize new memories and information recently learned.
Each task or memory that a person experiences correlates to specific neural firing patterns in their brain and those same patterns appear to be replayed when that person sleeps in a process known as “offline replay.” This replay has been observed in rodents whose hippocampal brain cells were shown to fire in the same order during rest as when they had previously conducted a task but until now, had yet to be shown in humans. Publishing their work in Cell Reports, scientists say that it is the first direct evidence that human brains also replay waking experiences when asleep.
Researchers operating under the academic research consortium BrainGate have been using neurotechnologies – namely micro-electrodes chronically implanted into the human brain – as a way to “decode” neural signals associated with limb movement and communication abilities. In cases of severe debilitation, such as spinal cord injury, test subjects have been shown to have the ability to move a computer cursor simply by thinking of moving their paralyzed hand or arm. This same technology was employed as part of a pilot clinical trial to determine how what such a brain-computer interface might reveal about dreaming.
Two participants were asked to take a nap before and after playing a computer game similar to the 1980s hit Simon. In the game, four flashing lights light up a specific pattern that players have to repeat by physically hitting the light panels in sequence. However, study participants played with their mind by imagining moving a computer cursor to different targets and “hitting” them as quickly as possible. After playing, participants were told to take a nap before playing another round and then resting again while researchers recorded the spiking activity of individual neurons from their motor cortex.
"There aren't a lot of scenarios in which a person would be implanted with a multi-electrode array, where the electrodes are tiny enough to be able to detect the firing activity of individual neurons," said co-author Jarosiewicz, noting that the study is “unprecedented” in that it employed electrode arrays detailed enough to track neural recordings in the brain. Typically, medical electrodes are too big to track the spiking activity of single neurons.
The same neuronal firing patterns were observed both when participants were playing the game and when they were resting, suggesting that participants were, in essence, playing the game in their sleep.
"This is the first piece of evidence that in humans, we also see replay during rest following learning that might help to consolidate those memories," said Jarosiewicz. "All the replay-related memory consolidation mechanisms that we've studied in animals for all these decades might actually generalize to humans as well."
Study participants were also more likely to successfully repeat the flashing pattering after their rest, further suggesting that they were replaying what they had learned in their sleep. The researchers add that this highlights the importance of sleep and its beneficial role in cognitive performance.