Tel Aviv University, UCLA researchers find eye movements during REM sleep reflect brain activity patterns associated with new images.
Rapid Eye Movement (REM) sleep, the period in which we experience vivid dreams, was discovered by scientists in the 1950s. Because REM sleep is associated with dreaming, on the one hand, and eye movement, on the other, it has been tempting to assume that each movement of the eye is associated with a specific dream image. But despite decades of intense research by leading international scientists, this intuitive hypothesis has remained unproven.
A new study based on rare neuronal data offers the first scientific evidence of the link between rapid eye movement, dream images, and accelerated brain activity. When we move our eyes in REM sleep, according to the study, specific brain regions show sudden surges of activity that resemble the pattern that 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 this week 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.
Deep down in the brain
“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 activity of neurons located deep inside the human brain.”
The research for the study was conducted on 19 epileptic patients at the UCLA Medical Center, who required invasive monitoring of brain activity prior to potential surgical excision of seizure-causing areas of the brain. Electrodes were implanted deep inside the patients’ brains to monitor their brain activity over the course of 10 days. These electrodes were able to provide the rare data needed to prove the link between eye movements, dream imagery, and brain activity.
“We 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,” said Dr. Nir. “Prof. Fried’s prior research had shown that neurons in these regions become active shortly after we view pictures of famous people and places, such as Jennifer Aniston or the Eiffel Tower — even when we close our eyes and imagine these concepts.”
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.
Images, awake and asleep
“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.
“How and why eye movements occur are important,” said Dr. Nir. “And these moments represent privileged windows of opportunity for the study of brain activity.”
About this neuroscience and sleep research
Source:AFTAU Image Source: The image is credited to Aweisenfels and is licensed CC By-SA 4.0 Original Research: Full open access research for “Single-neuron activity and eye movements during human REM sleep and awake vision” by Thomas Andrillon, Yuval Nir, Chiara Cirelli, Giulio Tononi and Itzhak Fried in Nature Communications. Published online August 11 2015 doi:10.1038/ncomms8884
Single-neuron activity and eye movements during human REM sleep and awake vision
Are rapid eye movements (REMs) in sleep associated with visual-like activity, as during wakefulness? Here we examine single-unit activities (n=2,057) and intracranial electroencephalography across the human medial temporal lobe (MTL) and neocortex during sleep and wakefulness, and during visual stimulation with fixation. During sleep and wakefulness, REM onsets are associated with distinct intracranial potentials, reminiscent of ponto-geniculate-occipital waves. Individual neurons, especially in the MTL, exhibit reduced firing rates before REMs as well as transient increases in firing rate immediately after, similar to activity patterns observed upon image presentation during fixation without eye movements. Moreover, the selectivity of individual units is correlated with their response latency, such that units activated after a small number of images or REMs exhibit delayed increases in firing rates. Finally, the phase of theta oscillations is similarly reset following REMs in sleep and wakefulness, and after controlled visual stimulation. Our results suggest that REMs during sleep rearrange discrete epochs of visual-like processing as during wakefulness.
“Single-neuron activity and eye movements during human REM sleep and awake vision” by Thomas Andrillon, Yuval Nir, Chiara Cirelli, Giulio Tononi and Itzhak Fried in Nature Communications. Published online August 11 2015 doi:10.1038/ncomms8884