Illustration: Maja Moden
Our most vivid dreams and vigorous brain activity come during the rapid eye movement, or REM, phase of sleep. Although scientists have long suspected that our eyes flicker in response to what our unconscious mind sees in our dreams, no one has been able to prove it. Now, an international team of researchers led by Itzhak Fried, MD, PhD ’81, professor of neurosurgery and professor of psychiatry and biobehavioral sciences at the Jane and Terry Semel Institute for Neuroscience and Human Behavior at UCLA, is the first to demonstrate that during dreams, our eyes and brains respond similarly to how they react to images when we’re awake. The findings offer a rare glimpse into the workings of individual brain cells in the sleeping mind.
Dr. Fried made the finding after implanting electrodes deep into the brains of 19 people with drug-resistant epilepsy in order to identify the origin of their seizures prior to surgery. For up to two weeks, the electrodes recorded the electrical activities of individual neurons in the medial temporal lobe, the bridge between visual perception and memories, allowing Dr. Fried to eavesdrop on the patients’ dreams. “Our earlier studies showed that these neurons fire when we view pictures of familiar people and places — or simply close our eyes to imagine or remember them,” Dr. Fried says. But when the researchers reviewed the data from the electrode recordings, they saw something unexpected in the responses of single brain cells during REM sleep.
“Brain cells in the medial temporal lobe showed a sudden surge of activity each time after patients moved their eyes in REM sleep,”
Dr. Fried says. “This electrical pattern closely resembles what happens when we view something new in waking life. We suspect rapid eye movements reflect the instant when the brain encounters a new image in a dream.” The finding implies that REM captures snapshots of dream imagery, suggesting that the same machinery that informs our conscious visual experience also operates during sleep.
Sleep, Dr. Fried says, remains one of the great mysteries of the human mind. “It’s a time when we relive and consolidate our memories,” he says. “Rapid eye movement offers a window into the human visual experience. At these moments, our sleeping brains capture snapshots of the dream world inside our heads.”
Future research will aim to decode dream content based upon brain-cell activity. “The next question is whether or not the neurons are responding to the images the sleeper sees in his dreams,” Dr. Fried says.
The researchers conducted the study on people preparing for epilepsy surgery because, in preparation for the surgery, doctors implant sensors to monitor brain activity, creating a rare collection of findings that cannot be obtained any other way. The scientists used the data to learn what happens in the brain during REM sleep.
“Single-neuron Activity and Eye Movements during Human REM Sleep and Awake Vision,” Nature Communications, August 11, 2015