Like many animals, rats enter an imaginary world when they doze off. We know this because their tiny beaded eyes flicker back and forth as their brains hum in the keys to their dreams.
Now, thanks to a study of sleeping rodent brains, we can say with more confidence why these rapid eye movements (REMs) occur.
Researchers have been studying dream-based blinking since the 1950s. Sleepers who wake up from vivid dreams tend to exhibit particularly exaggerated eye movements, making it easy for them to think their eyes are just tracking the fictional landscape of their dreams.
While this assertion may be plausible, it is a challenging hypothesis to support experimentally. Most studies to date have relied on awakened sleepers self-reporting their dreams, which researchers have linked to their eye movements — but this approach leaves too much room for skepticism.
Other researchers have noted that REM sleep can occur without dreams, especially in young infants and patients with traumatic brain injury, which may make them unimaginable. We can also dream without entering REM sleep.
Importantly, not all studies support the so-called “scanning hypothesis.”
Alternatively, the rapid movement of our eyeballs under the eyelids – described as saccades – may just be a neural response to more fundamental activity in the brain when it is no longer connected to consciousness.
To overcome the difficulty of studying the neurology of dreaming humans, scientists at the University of California, San Francisco used mice as proxies.
Specifically, they measured the activity of nerve cells in the mouse thalamus that are responsible for pointing the head in a specific direction.
In awake mice, saccades tended to coincide with head movements as the mice navigated the real world. Matching eye movements to nerve impulses in the direction of the head would be the main piece of evidence supporting the scanning hypothesis.
The team used small implantable probes to record neural activity in the mice, allowing them to freely explore their surroundings while awake. Meanwhile, a series of cameras captured their every dart and wink.
The sensors continued to record while the exhausted test subjects curled up for a nap. The baselines of neural activity and saccades established during wakefulness were then used to determine how likely eye movements during REM were to correlate with their intended orientation in the mental world.
Their results provided fairly clear, objective evidence linking rapid eye movements in mice to their brain’s control of head movements. For example, as in wakefulness, larger, more intense eye movements predicted larger changes in head orientation guided by the thalamus.
Of course, what we usually need to be careful about is linking the results of experiments in mice to the same behavior in humans.
Not having replicated similarly precise measurements on the convenient human brain — an invasive procedure not without precedent in dream research — is the most direct evidence you’ll ever get for the scanning hypothesis.
All of this points to a higher level of coordination throughout the brain during REM sleep, which continues to guide the body to move in the imagined space.
In addition to firmly supporting one side of the decades-long debate, the finding could have implications for further research.
Reading eye movements during sleep can help inform treatments to improve memory or manage trauma, according to reviews by neurologists Cathrin Canto and Chris De Zeeuw of the Netherlands Institute for Neuroscience.
It can also give us a deeper understanding of the purpose of our unconscious wanderings.
“The muscle twitches that also frequently occur during REM sleep may be related to the internal heading cues provided by rapid eye movements, and analysis of these signals may provide more information about dreams,” Canto and De Zeeuw wrote.
As for what rats see in their minds when they sleep, we can only imagine in the wildest dreams.
The study was published in science.