Massimo Scanziani, an investigator at the Howard Hughes Medical Institute's lab at the University of California, San Francisco, set out to understand how the brain creates thoughts without moving—and he found the answer by watching the eyes of sleeping animals. During REM sleep, Scanziani's team discovered something remarkable: the brain's internal compass, the neural system that normally guides animals through the physical world, continues to activate and coordinate precisely with rapid eye movements, even though the sleeping animal's body remains completely still.
This discovery matters because it opens a window into one of neuroscience's most profound mysteries: how the brain generates imagination, creativity, and the ability to mentally simulate the future. The brain is fundamentally a prediction machine. When you decide what to have for dinner tonight while lying in bed, or when a gymnast mentally rehearses an entire routine without moving a muscle, your brain is running an internal model of the world—a detailed simulation built from past experience. But how does this generative ability work? Studying the brain during sleep, when it receives no external input from the world, allows researchers to eavesdrop on this internal dialogue.
Scanziani's lab, which typically studies how the brain processes vision in awake, exploring animals, took an unexpected turn when postdoc Yuta Senzai suggested investigating eye movements during sleep. Senzai brought knowledge from previous research showing that the brain's internal compass—specifically neurons in the anterodorsal thalamic nucleus and post-subiculum—remained active during REM sleep. Using their expertise in vision science, the Scanziani team made a striking connection: the rapid eye movements occurring during REM sleep coordinated perfectly with this internal compass, just as they do when an animal is awake and navigating real space.
To understand how this works, the researchers examined the superior colliculus, the brain's motor command center that functions like a steering wheel, issuing signals to turn the head left or right. They found that during REM sleep, the superior colliculus continues issuing these motor commands—turning the head would normally follow—yet the animal does not move. Instead, the output of these commands reaches the internal compass neurons, which respond exactly as if the head had actually turned. The brain is essentially running a simulation. Motor neurons are inhibited to prevent movement, but the brain faithfully represents the consequences of those motor commands, as though the animal were moving through physical space.
"This is the dream; this is the unfolding of the internal model," Scanziani explains. The research, posted to the bioRxiv preprint server, suggests that the sleeping brain, disconnected from the external world, deploys its internal model to simulate interactions with the physical environment. The team is now investigating whether other brain systems—including the vestibular system that senses motion and the body's position in space—also coordinate with these motor commands to complete the full sensory simulation.
For Scanziani, the work answers a fundamental question about human cognition: how is it possible to unfold an entire imagined scenario in your mind without moving a muscle? The answer, it seems, lies in the sleeping brain's ability to reactivate and coordinate the same neural systems used for real movement and navigation. Understanding these mechanisms during sleep may ultimately illuminate how the waking brain generates the predictions and simulations that guide our decisions every day.