Using REM sleep to eavesdrop on the brain’s inner monologue

Key takeaways

• HHMI Investigator Massimo Scanziani and his team are studying what happens in different parts of the brain during REM sleep, a phase of slumber associated with vivid dreams.
• The team found that during REM sleep, the brain’s motor center still sends commands to turn the head left or right, even though the animal is immobile. These commands cause the brain’s internal compass, which allows animals to know where they are heading in the world when they’re awake, to respond as if the head had turned.
• Insights gained from the research could help scientists understand how the brain generates information without any external input — a fundamental aspect of the human brain that is essential for predicting, interpreting, and acting in the world efficiently.  

In his wildest dreams, HHMI Investigator Massimo Scanziani never imagined he’d study sleep.  

Scanziani’s lab at the University of California, San Francisco investigates how the brain processes information in the sensory circuits involved in vision, usually when animals are awake and exploring the world.  

But, when a postdoc in the lab suggested studying eye movements in sleeping animals, Scanziani’s curiosity was awakened.

The postdoc, Yuta Senzai, had come from a lab that had discovered that the brain’s internal compass, which allows animals to know where they are heading in the world when they’re awake, still turns during REM sleep. Even though the animals’ bodies aren’t moving, neurons in the anterodorsal thalmic nucleus (ADn) and post-subiculum, regions of the brain that make up part of this internal compass, are still active.

Using their expertise in studying vision, the Scanziani team found that during REM sleep,  eye movements coordinated with the internal compass, just like when the animal is awake.

“It means that essentially by looking at the direction of those rapid eye movements, you have some access right into the cognitive processes that are going on in the brain of the sleeping animal, which is quite fascinating if you think about it,” Scanziani says. “It was a very exciting way to enter into the study of the brain of sleeping animals.”

Studying REM Sleep to Understand the Brain

For Scanziani, the work opened a new area of inquiry: using sleep to investigate the brain’s ability to generate information on its own, without any input from the outside world.  It is what allows us to think about what we want for dinner tonight just by closing our eyes and concentrating.

One example of the brain’s generative ability is its capacity to create an internal model of the world — a mental representation of how the world works that is built up from experience. This mental simulation enables us to anticipate the consequences of our actions in the world without executing them. For example, a gymnast knows the results of her actions so well that she can run through her entire routine in her head without moving a muscle.

Studying the brain during sleep, when it isn’t receiving any external input, allows researchers to eavesdrop on the brain’s internal monologue and gain insight into how it generates these thoughts, which are essential for predicting, interpreting, and acting in the world efficiently.

“How is it even possible to imagine a scenario and to unfold it in front of the eyes of your mind without twitching a muscle?” Scanziani says. “Those are basic, fundamental, creative mechanisms of the human brain and the study of the brain during REM sleep allows me to understand those mechanisms.”

How the Brain’s Internal Model of the World Unfolds During REM Sleep

Scanziani and his team think that during REM sleep, the brain generates dreams based on its internal model of the world.

To test their hypothesis, the team examined activity in the superior colliculus – the motor center of the brain that sends commands to turn the head in one direction or the other, like a steering wheel.

The researchers found that during REM sleep, the superior colliculus keeps issuing motor commands to turn the head left or right. The animal doesn’t move, but the output of these commands — the ADn neurons that are part of the internal compass — respond as if the head had turned.

The findings suggest that the sleeping brain, while disengaged from the external world, is using its internal model of the world to simulate interactions with the physical world.

“The motor neurons are inhibited and yet, the brain is capable of representing the consequences of that motor command,” Scanziani says. “This is the dream; This is the unfolding of the internal model.”

The team is now testing whether other brain systems, like the vestibular system that senses motion and the body’s position in space, also work in coordination with these motor commands to represent their consequences as if the head is really turning.

The researchers are also manually turning the superior colliculus steering wheel during REM sleep and comparing these manipulations to what happens when the animal is awake to figure out how accurate these dreams are to reality.

“We are entering the dream and instead of letting it fold out as the brain would want, we’re taking that wheel and we’re turning it ourselves and seeing whether if we turn it ourselves, the rest of the brain will follow,” Scanziani says.