By recording data from individual brain cells during the rapid eye movement (REM) phase of sleep, researchers from France, Israel, and the US have found that the flickering of our eyes acts as a way to “change the scene” and create a new image in our dreams.
In research published by the journal Nature Communications, the study authors recorded brain cell activity in patients with implanted electrodes used to monitor seizures, and found that these eye movements triggered the parts of out brain involved in processing visual images during our waking hours, according to BBC News and Discovery News reports.
The discovery could help explain why people are able to remember vivid dreams when they are woken during this phase of sleep, explained co-author and Tel Aviv University neuroscientist Dr. Yuval Nir. Furthermore, it could help sleep researchers solve the longstanding mystery involving the relationship between dreaming and eye movement during slumber.
“Since they discovered REM sleep, they knew that in that state of sleep people experience vivid dreams and move their eyes frantically with their eyelids closed,” Dr. Nir explained to Discovery News, “but any attempt to relate these two phenomena has been very challenging.”
Analyzing the brain activity of epilepsy patients
The researchers worked with 19 different patients over the course of a four-year study, recording activity from electrodes located throughout the brain, but primarily in the media temporal lobe of people undergoing treatment for severe epilepsy. The individuals had electrodes implanted deep within their brains to monitor electrical activity taking place during seizures.
This set-up allowed Dr. Nir and his colleagues to monitor electrical activity while these patients were sleeping, matching it with their eye movements, which were recorded using stickers placed near their eyes. They discovered that rapid eye movement is followed immediately by a burst of electrical activity in the medial temporal lobe.
The medial temporal lobe, the researchers explained, is not directly involved in vision. Rather, it plays a key role in visual image processing by signaling to the brain about concepts, Dr. Nir told BBC News. For instance, if someone closed his or her eyes and thought about a specific object, the neurons would fire, implying “a refresh of the mental imagery and the associations.”
“[It is] more of a bridge between the visual parts… and the memory systems of our brain,” he explained to Discovery News. “Neurons in these regions are active when we view a picture of the Sydney Opera House, but also when we close our eyes and imagine the Sydney Opera House, and sometimes also even when we just hear the words ‘Sydney Opera House.’”
Some mysteries solved; others remain unanswered
Dr. Nix and his colleagues found a similar pattern of activity during sleep, and especially after the eye movements that occur during REM sleep – the phase of sleep in which we dream. While it was long believed that these movements could signify the visual components of dreams, this is the first study to find evidence that this is, in fact, the case.
“We are intimately familiar with the activity of these neurons. We know they are active every time you look at an image, or when you imagine that image. And now we see them active in a similar way when you move your eyes in REM sleep, so it becomes very probable that the eye movements represent some type of reset, or ‘moving onto the next dream frame,” he told BBC News, comparing it to moving to the next slide when using an old-fashioned projector.
Dr. Nir’s team also reported that these parts of the brain are involved in abstract perception, not details, meaning that even if we dream of someone who is familiar, there are certain aspects that we may not remember. The activity in these neurons during REM sleep indicates that new images or concepts may be being formed in our mind while we dream.
However, as Imperial College London neuroscientist Professor William Wisden pointed out to the BBC, there are still unanswered questions: “Why do we have to have REM sleep? Why does our brain have all this circuitry to do that? This paper doesn’t answer that, but it does emphasize how similar being awake and in REM sleep are, for particular circuits in the brain.”
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