Why do our eyes move when we dream? Study settles the mystery

Have you ever noticed that our eyes move back and forth during sleep? Well, researchers may have figured out the reason behind that.

A new study conducted by researchers from the University of California, San Francisco, suggests that rapid eye movements occur during sleep because we are looking at things in our dream world.

The results of the study provide insight into how our imagination functions as well as how we dream, according to a press release published by the institution on Thursday.

Resolving the mystery of REM sleep

REM sleep, a phase of sleep associated with random rapid movement of the eyes, has been known since the 1950s as the stage of sleep when dreams occur. The reason behind these movements, however, was a matter of much mystery and debate since then.

“We showed that these eye movements aren’t random. They’re coordinated with what’s happening in the virtual dream world of the mouse,” said Massimo Scanziani, a professor of physiology at the University of California and the study's senior author.

“This work gives us a glimpse into the ongoing cognitive processes in the sleeping brain and at the same time solves a puzzle that’s triggered the curiosity of scientists for decades.”

Examining “head direction” cells in mice brains

Some scientists hypothesized in the second half of the 20th century that rapid eye movements might be caused by scenes in the dream world, but there were few ways to verify it, and the experiments produced contradictory results. Many studies dismissed REM movements as random activities that may be performed to keep the eyelids lubricated.

Thanks to the much more advanced technology we have today, Scanziani, along with UCSF postdoctoral researcher Yuta Senzai, Ph.D., managed to examine "head direction" cells in the brains of mice, another species who experience REM sleep.

Researchers tracked the mouse's eye movements and recorded data about its heading directions from these cells. While comparing them, the team discovered that the mouse's internal compass and eye movements during REM sleep were precisely aligned, just as they are when the mouse is awake and moving around.

The study's findings support the notion that dreams are a way of integrating information gathered during the day and show that the same brain regions, of which there are many, coordinate during both we're awake and dreaming.

“It’s important to understand how the brain updates itself based on accumulated experiences,” said Scanziani. “Understanding the mechanisms that allow us to coordinate so many distinct parts of the brain during sleep will give us insight into how those experiences become part of our individual models of what the world is and how it works.”

The researchers' next steps include determining what causes the internal compass of the brain to move during REM sleep, how it moves with the eyes, and how numerous senses interact to provide a realistic dream experience.

The results of the study were published in Science journal.

Abstract:

Since the discovery of rapid eye movement (REM) sleep, the nature of the eye movements that characterize this sleep phase has remained elusive. Do they reveal gaze shifts in the virtual environment of dreams or simply reflect random brainstem activity? We harnessed the head direction (HD) system of the mouse thalamus, a neuronal population whose activity reports, in awake mice, their actual HD as they explore their environment and, in sleeping mice, their virtual HD. We discovered that the direction and amplitude of rapid eye movements during REM sleep reveal the direction and amplitude of the ongoing changes in virtual HD. Thus, rapid eye movements disclose gaze shifts in the virtual world of REM sleep, thereby providing a window into the cognitive processes of the sleeping brain.