Our eyes swivel restlessly in their sockets during rapid eye movement (REM) sleep—a phenomenon that has escaped explanation for decades. Research has suggested several possibilities: the eyes roll around to lubricate the inside of the eyelids; eyes jiggle to warm the brain; eyes twitch in response to stimulation from the brain stem. According to a study in the June issue of Brain, the most likely explanation is that our eyes orient their gaze to scan the imagery of our dreams—just as eyes change their gaze in response to our environment when we are awake and moving around.

In the study, neuroscientists at Pitié-Salpêtrière Hospital in Paris turned to a unique group of subjects: those with REM sleep behavior disorder. Such individuals do not enter the standard state of temporary paralysis that prevents any flailing about during dreams. Instead they physically act out their dreams: they kick, scream, grab, reach, climb and jump, enabling researchers to observe what normally remains inside a dreamer’s head. “It’s a direct window on people’s dreams,” says Isabelle Arnulf, a neurologist who specializes in sleep and a co-author of the study. “It’s kind of like having movie subtitles.”

Arnulf and her colleagues used electrodes to track the eye movements of 56 sleep disorder subjects and 17 normal sleepers, simultaneously videotaping their nocturnal behaviors. The researchers analyzed the nighttime footage of the patients frame by frame to see if their actions and gazes matched up.

And evidently they do. For 90 percent of the time, the gaze of a person with REM sleep disorder synchronized with mimed dream actions. A subject who dreamed of kissing someone to her left also looked to her left. Another participant who dreamed of climbing a ladder shifted his gaze up and down repeatedly to check his progress. Still another glanced over his shoulder as he ran from imaginary lions. If rapid eye movements were truly random twitches, then they would not match their accompanying dream actions so frequently, the researchers conclude. Apparently, when it comes to the neuroscience of dreaming, it’s best to believe your eyes.