Have you ever noticed that your eyes move back and forth during sleep? Thanks to the research conducted on the brains of sleeping mice. We can now confidently explain why these rapid eye movements (REM) occur.
The mystery behind REM sleep
REM sleep is a sleep phase associated with random rapid movement of the eyes. It is the stage of sleep when dreams occur. This phenomenon has been the subject of decades of research. However, the reason behind these dream-based eye movements has been a matter of much mystery and debate since then.
Sleepers who awoke from vivid dreams tended to display particularly exaggerated eye movements, making it easy to assume that their eyes keep track of the make-believe scenery in their dreamscape.
A challenging hypothesis to support experimentally
Most of the previous studies relied on sleepers who had just woken up from sleep to report their dreams, which researchers connected to their eye movements. However, that strategy leaves too much space for uncertainty.
Other researchers have noted that rapid eye movement (REM) sleep can happen without dreams, especially in young babies and those who have experienced brain trauma that presumably makes them unable to visualize. We are still capable of dreaming outside of REM sleep.
Not all studies support what is known as the ‘scanning hypothesis’
It is possible that the rapid ocular movements we call saccades are a neural system’s reaction to more fundamental activity that manifests when the brain is unconnected from consciousness.
Recent research conducted by scientists at the University of California, San Francisco, suggests that rapid ocular movements occur during sleep because we look at objects in our dreams.
The researchers used mice as a stand-in for humans to get around the challenges of researching the neurology of dreaming humans. Like many other animals, mice doze off to dream worlds when they fall asleep. We realize this because their perky tiny eyes flicker back and forth as their brains hum in the key of dreams.
According to the study’s senior author, Massimo Scanziani, a professor of physiology at the University of California, “we demonstrated the consistency of these eye movements. They are aligned with what is happening in the mouse’s virtual dream world.”
Examining ‘head direction’ cells in mice brains
In the second half of the 20th century, some scientists proposed the paradoxical theory that scenes from dreams might trigger rapid eye movements. These can be verified in a few ways, and the experiments produced contradictory results. Many researchers dismissed REM movements as idling activities that may be performed to keep the eyelids lubricated.
What the researchers did
Thanks to the far more sophisticated equipment available today. Scanziani and Yuta Senzai, Ph.D., a postdoctoral researcher at UCSF, investigated “head direction” cells in the brains of mice – another species that experience REM sleep.
The researchers observed the activity of the nerve cells in the mouse thalamus. The nerve cells control the head’s direction.
“This discovery solves a riddle that has piqued the interest of scientists for decades while also providing us with a glimpse into the ongoing cognitive processes in the sleeping brain.”
The mice were left awake and free to explore their surroundings while the scientists used tiny implanted sensors to record the neurological activity of the animals. In the meantime, several cameras recorded each dart and blink of their eyes. The sensors continued to record even as the worn-out test subjects (mice) curled up for naps.
Researchers monitored the mouse’s eye movements and gathered information from these cells about the mouse’s eye’s heading.
What the researchers discovered
According to a press statement released by the university on Thursday, the study’s findings offer a clear, unbiased insight into how human imagination and dreams work.
The findings support the notion that dreams are a way of integrating information gathered during the day. They demonstrated that the same brain regions, of which there are many, coordinate both awakening and dreaming.
The scientists found that the mouse’s internal compass and eye movements during REM sleep were precisely in line with each other, just as when the mouse is awake and moving around.
The relationships between the eye movements during REM and their desired orientation across their mental world were also ascertained using the baselines in brain activity and saccades established during the waking hours.
According to Scanziani, understanding how the brain updates itself based on cumulative experiences is crucial.
“We can gain insight into how those experiences become a part of our conceptions of the world and how it functions by understanding the mechanisms that allow us to coordinate so many different brain regions when we sleep.
Furthermore, the study’s results suggest that during REM sleep, a greater degree of brain coordination controls how a body moves through an imagined world.
The finding may have ramifications for future study in addition to taking a strong stance on one side of a long-running argument.
Neurologists share their viewpoints
Neurologists Cathrin Canto and Chris De Zeeuw from the Netherlands Institute for Neuroscience wrote in an editorial that analyzing sleep-related eye movements could help guide therapy for enhancing memory or treating trauma. It can also help us understand the true meaning behind our unconscious ramblings.
According to Canto and De Zeeuw, investigations of muscle cramps, which also commonly occur during REM sleep, “may be related to the internal heading signals provided by rapid eye movements – and this might yield more information regarding dreams.”
We can only speculate in our wildest fantasies about what mice could visualize while they sleep or the purpose of dreams.
The following stages for the researchers involve figuring out what makes the internal compass of the brain move during REM sleep, how it moves in tandem with the eyes, and how a variety of senses interact to provide a lifelike dream experience. The study was published in Science journal.