Sleep includes phases characterized by rapid eye movement (REM) that were known to be associated with dreaming. But are these eye movements related to the contents of consciousness in that sleep state?
Yuta Senzai & Massimo Scanziani discovered that the direction and amplitude of rapid eye movements during REM sleep reveal the direction and amplitude of the ongoing changes in virtual head direction. Image credit: Yuta Senzai & Massimo Scanziani, doi: 10.1126/science.abp8852.
REM sleep — named for the rapid eye movements associated with it — has been known since the 1950s to be the phase of sleep when dreams occur.
Some experts hypothesized that rapid eye movements may be following scenes in the dream world, but there was little way to test it, and the experiments that could be done provided contradictory results.
Many researchers wrote off REM movements as random actions, perhaps to keep the eyelids lubricated.
Given much more advanced technology, University of California, San Francisco researchers Massimo Scanziani and Yuta Senzai were able to look at ‘head direction’ (HD) cells in the brains of mice, who also experience REM sleep.
While awake, the activity of these cells in the mouse’s thalamus encodes the direction of the animal’s head as it explores or navigates the environment.
Changes in HD cell activity are often accompanied by fast saccade-like movements of the eyes in the same direction.
The researchers recorded HD cell activity using extracellular linear probes while also monitoring the movements of both eyes with head-mounted cameras in awake and sleeping mice.
They found that the direction and amplitude of rapid eye movements during REM sleep encoded direction and amplitude of the heading of mice in their dream environment.
The findings suggest that rapid eye movements provide an external readout of an internal cognitive process occurring during REM sleep and reveal a coordination that may underlie the realistic and vivid experience of dreams.
“We showed that these eye movements aren’t random,” Dr. Scanziani said.
“They’re coordinated with what’s happening in the virtual dream world of the mouse.”
“This work gives us a glimpse into the ongoing cognitive processes in the sleeping brain and at the same time solves a puzzle that’s triggered the curiosity of scientists for decades.”
The results were published in the journal Science.
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Yuta Senzai & Massimo Scanziani. 2022. A cognitive process occurring during sleep is revealed by rapid eye movements. Science 377 (6609): 999-1004; doi: 10.1126/science.abp8852
