Size of pupils during sleep could be key to how brain forms strong memories, mice study suggests
New Delhi, Pupil size during sleep whether contracted or dilated could be key to understanding how the brain forms long-lasting memories, according to a study in mice, findings from which could help refine interventions for improving human memory.
It is known that while one sleeps, the brain ‘consolidates’ memory by integrating new information learnt during the day with previous knowledge. Critical to this is the process of ‘memory replay’, in which brain activity related to recent experiences is replayed repeatedly.
Researchers from Cornell University, US, connected electrodes to the brains of a group of mice and tracked pupil movements during one of the stages of non-rapid eye movement sleep.
The team found that newer memories are replayed when the pupil is contracted, whereas older ones are replayed when the pupil is dilated.
“We found that memory replay of recent experiences dominated in sharp-wave ripples during contracted pupil substates of non-REM sleep, whereas replay of previous memories preferentially occurred during dilated pupil substates,” the authors wrote in the study published in the journal Nature.
Pupil size is possibly how the brain is able to separate two sub-stages of sleep and prevents “catastrophic interference”, or “catastrophic forgetting”, in which the consolidation of one memory wipes out another one, the authors said.
“Non-REM sleep is when the actual memory consolidation happens, and these moments are very, very short periods of time undetectable by humans, like 100 milliseconds,” lead researcher Azahara Oliva, an assistant professor at Cornell University, said.
“How does the brain distribute these screenings of memory that are very fast and very short throughout the overall night? And how does that separate the new knowledge coming in, in a way that it doesn’t interfere with old knowledge that we already have in our minds?” Oliva said.
The study also provides a potential solution for a similar memory integration problem while developing artificial neural networks, a type of artificial intelligence algorithms, the team said.
Over a month’s time, the mice were taught varied tasks, such as collecting water or cookie rewards in a maze. When they fell asleep at night, the electrodes captured brain activity, while cameras that hung in front of their eyes recorded pupil movements.
The recordings showed that the sleep patterns in mice are more similar to the sleep stages in humans than previously thought, the researchers said.
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