Pupil size during sleep decides if new or old memories are replayed

From a distance, it might seem like just another part of your eye, but the pupil is a gateway to understanding how and when the brain forms long-lasting memories.

Experts at Cornell University have discovered a key to understanding memory formation and retention: pupil size.

By analyzing pupil dynamics during sleep, the researchers identified a sophisticated mechanism in the brain that prevents “catastrophic forgetting,” a phenomenon where forming new memories disrupts older ones.

The study, led by assistant professors Azahara Oliva and Antonio Fernandez-Ruiz, reveals that memory consolidation occurs during specific moments of non-REM sleep. These findings offer insights into human memory enhancement and artificial intelligence training techniques.

Science of sleep and memory

Sleep plays a critical role in consolidating memories and organizing information in the brain. Non-REM sleep is particularly essential for strengthening new memories and integrating them with existing knowledge.

During this stage, neural activity reactivates learned information, transferring it from temporary storage to long-term memory networks. REM sleep, on the other hand, contributes to emotional processing and creativity.

This study highlights how the brain alternates between consolidating new memories and revisiting old ones, using brief micro-structures within non-REM sleep.

By understanding these processes, scientists gain insights into how the brain preserves knowledge and prevents interference between different types of memories.

Tracking pupils and neural activity

The researchers conducted their study on a group of mice, teaching them various tasks over the course of a month. These tasks included navigating a maze to earn rewards like water or treats.

During this period, the mice were fitted with special equipment: brain electrodes to monitor neural activity and tiny cameras positioned near their eyes to track changes in pupil size.

After the mice learned a new task, they were allowed to sleep. While they slept, the researchers recorded their brain activity and observed how their pupils changed during different sleep stages. This data on pupil dynamics provided insights into how the brain consolidates memories.

“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,” explained Professor Oliva.

Pupil dynamics and memory consolidation

The researchers discovered that during a specific substage of non-REM sleep, when a mouse’s pupil contracts, the brain focuses on replaying and strengthening newly learned memories.

This process consolidates recent experiences, making them more stable and long-lasting.

In contrast, when the pupil dilates during another substage of the same sleep phase, the brain revisits older memories, integrating them into the broader knowledge base.

This alternating pattern of pupil contraction and dilation represents a previously unknown micro-structure of sleep.

Significance of pupils for retaining memory

This mechanism is crucial because it keeps new and old memories separate, preventing them from interfering with one another.

By alternating between consolidating new information and revisiting old knowledge, the brain avoids “catastrophic forgetting,” ensuring both types of memories are preserved and well-organized.

“It’s like new learning, old knowledge, new learning, old knowledge, and that is fluctuating slowly throughout the sleep,” said Professor Oliva.

“We are proposing that the brain has this intermediate timescale that separates the new learning from the old knowledge.”

Implications for memory and AI

The study’s findings have significant implications for improving memory in humans.

By understanding how the brain consolidates memories during specific moments of non-REM sleep, researchers could develop techniques to target and enhance these critical processes.

For example, therapies or interventions might help strengthen memory consolidation by focusing on these brief but vital phases of sleep.

Additionally, the discoveries could influence the development of artificial intelligence. Neural networks, which mimic how the brain processes information, often struggle with “forgetting” older data when learning new information.

By applying the brain’s method of alternating between processing new and old memories, AI systems could become more efficient at retaining prior knowledge while integrating new data, making them more reliable and robust.

A critical function of sleep

The research was supported by multiple organizations, including the National Institutes of Health (NIH), the Sloan Foundation, and the Whitehall Foundation.

The discovery of this intricate sleep mechanism and pupil dynamics redefines our understanding of memory consolidation.

By identifying the brain’s ability to alternate between processing new and old memories, researchers have illuminated a critical function of sleep.

The study not only bridges gaps in neuroscience but also opens doors to innovations in technology and human cognitive enhancement.

The study is published in the journal Nature.

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