You can often tell when someone is dreaming because his or her eyelids will flit about as if the person is watching a high-speed chase.
This is known as rapid-eye-movement (REM) sleep, and it describes the portion of our sleep when we experience a storm of brain activity and eye movements while the rest of our body is paralyzed.
Now, scientists have found evidence that these rapid eye jerks resemble our eye movements when we are awake, according to a small study of people with epilepsy published last Tuesday.
And it could explain why people woken from REM remember having vivid dreams.
"The electrical brain activity during rapid eye movements in sleep [was] highly similar to [that] occurring when people were presented with new images," Yuval Nir, a neuroscientist at Israel's Tel Aviv University and coauthor of the study, said in a statement provided to Business Insider. Every time we move our eyes when we're awake, our brains see a new image.
Scientists had long suspected that eye movements during REM sleep might correspond to specific images in our dreams, but there wasn't concrete evidence for this.
So Nir and his colleagues decided to test the idea. They studied 19 epileptic patients at the UCLA Medical Center, who already had electrodes implanted in their brains so doctors could monitor their brain activity and figure out the origin of their seizures. The researchers also listened to the patients' brain signals using EEG (electroencephalogram) electrodes placed on the scalp and tracked their eye and muscle movements.
The researchers noticed that brain cells in the medial temporal lobe, a region that acts as a bridge between our memory and our ability to recognize things we see, showed similar activity regardless of whether the patients were asleep or awake, including when they were fixing their gaze on images presented to them by Nir and co.
"Many neurons — including those in the hippocampus — showed a sudden burst of activity shortly after eye movements in sleep," Nir said in a statement, which is "typically observed when these cells are 'busy' processing new images."
The findings suggest that our rapid eye motions while we're asleep represent the instant we encounter a new image in a dream (i.e., a snapshot), just as when we see a new snapshot when we're awake, the researchers said.
While it's not clear that the dreaming brain activity of patients with epilepsy is entirely comparable with that of people without epilepsy, the researchers were pretty confident that the epileptic patients' brain activity was similar to the brain activity that has been observed in healthy individuals.
But it's hard to know, because you can't simply stick electrodes in a healthy person's brain. "Nobody ever recorded the brain of somebody without pathology because they don't have the ethical right to do so," study coauthor Thomas Andrillon, a sleep researcher at the Laboratoire de Sciences Cognitives et Psycholinguistique in France, told Business Insider.
As a precaution, Andrillion and his colleagues discarded any data that was recorded within eight seconds of when the patients were having seizures and from electrodes near the seizures' origin (though seizures are rare during REM sleep, as it happens).
Also, the researchers didn't actually ask the subjects whether they were dreaming. In previous studies, however, they found that most people woken from REM sleep reported having had dreams.
