"Dream Snapshots": Eye movements in sleep linked to dream images

In the 1950s, scientists discovered Rapid Eye Movement (REM) sleep, the stage of sleep in which we experience colorful and vivid dreams. For decades, researchers assumed that the dreams during REM sleep and eye movement were related and associated with a specific dream image. However, despite decades of intense research by leading international scientists, this intuitive hypothesis remained unproven.

A TAU-led study based on rare neuronal data offers the first scientific evidence of the link between rapid eye movement, dream images, and accelerated brain activity. According to the study, eye movement during REM sleep correlates to sudden surges of activity in specific brain regions. This surge of brain activity occurs when we are introduced to a new image, suggesting that eye movements during REM sleep are responsible for resetting our dream "snapshots."

The research, published in Nature Communications, was led by Dr. Yuval Nir of Tel Aviv University's Sackler Faculty of Medicine in collaboration with TAU's Prof. Itzhak Fried, also of UCLA and Tel Aviv Medical Center; Thomas Andrillon of the Laboratoire de Sciences Cognitives et Psycholinguistique in Paris; and Dr. Giulio Tononi and Dr. Chiara Cirelli of the University of Wisconsin-Madison.

Depths of the brain

Nineteen volunteers- all epileptic patients treated by Prof. Fried at UCLA Medical Center- participated in the sleep study. These patients had not responded to medication for their condition and were now candidates for potential excision of the seizure-causing areas of the brain. Before the operation, they had electrodes implanted deep in their brains to monitor brain activity over the course of ten days. These electrodes were able to provide the rare data needed to prove the link between eye movements, dream imagery, and brain activity.

"Our goal was to examine what happens deep in the human brain during REM sleep, specifically when rapid eye movements occur," said Dr. Nir. "Prof. Fried's trailblazing research with epilepsy patients at UCLA offered a unique opportunity to collect the necessary data." The researchers focused on the electrical activities of individual neurons in the medial temporal lobe, a set of brain regions that serve as a bridge between visual recognition and memories. Prior research from Prof. Fried had shown that neurons in these regions become active after viewing images of famous people and places, such as the Eiffel Tower or Jennifer Aniston — even when we close our eyes and imagine them.

In addition to monitoring the patients' brain activity via intracranial electrodes, the researchers also recorded scalp EEG, muscle tone, and eye movements to identify periods of REM sleep and detect the precise moment of each rapid eye movement. "The electrical brain activity during rapid eye movements in sleep were highly similar to those occurring when people were presented with new images," said Dr. Nir. "Many neurons — including those in the hippocampus — showed a sudden burst of activity shortly after eye movements in sleep, typically observed when these cells are 'busy' processing new images."

"The research findings suggest that rapid eye movements represent the moment the brain encounters a new image in a dream, similar to the brain activity exhibited when one encounters visual images while awake," Prof. Fried said.

The researchers claim that their study is another step in decoding the role of dreams in sleep. Other recent studies showed that during sleep, brain processes occur to refresh and reorganize dreams. Dr. Nir summarized, “In our lab, we are looking into brain activity in various states of consciousness, when fully awake, when under anesthesia, in loss of consciousness, when tired, and of course during sleep."