Pupil Size May Reveal Memories Relived in Dreams, Study Suggests

A groundbreaking study has explored the relationship between pupil dilation and memory processing during sleep. Published in the journal Nature and conducted by researchers at Cornell University, the study, titled "Sleep Microstructure Organizes Memory Replay," reveals that pupil size during sleep may indicate the type of memories being relived in dreams.

Using cutting-edge eye-tracking technology alongside EEG (electroencephalogram) data, scientists monitored the sleep patterns of mice. During the day, the mice were introduced to new information, such as navigating a maze, and were then observed during their sleep at night.

The analysis uncovered two distinct substages of Non-Rapid Eye Movement (NREM) sleep. During one phase, pupil contraction suggested the replay of newly learned memories. In contrast, pupil dilation indicated the mice were likely processing or reliving older experiences. These phases alternated rapidly throughout sleep.

"It's like alternating between new learning and old knowledge in slow cycles throughout sleep," explained neuroscientist Azahara Oliva from Cornell's Department of Neurobiology and Behavior in an interview with ScienceAlert.

Preserving Old Memories While Creating New Ones

The findings offer insight into why forming new memories does not overwrite older ones—for instance, learning to play a musical instrument while still remembering how to drive a car.

"Our results suggest that the brain can multitask during sleep, facilitating continuous learning without interference," the researchers noted.

They propose that the brain uses an intermediate timescale to separate new learning from existing knowledge. This mechanism prevents what researchers describe as "catastrophic forgetting," ensuring the integration of new memories without erasing previous ones.

"This discovery offers a potential solution to the long-standing challenge of avoiding catastrophic interference in both biological and artificial neural networks while enabling memory integration," the researchers concluded.

This study opens new doors in understanding how memory processes during sleep contribute to learning and retention, a discovery with significant implications for neuroscience and artificial intelligence.