You’re lying on an operating table. A doctor injects a milky white liquid into your veins. Within a minute, your breathing slows, your face relaxes, and you remain limp when asked to squeeze a hand. You’ve been temporarily put to sleep.

We lose consciousness every night with the conviction that a blaring alarm or the whiff of fresh brewed coffee will drag us out of our slumber. Giving up awareness is engrained in the way our brain works. With anesthesia, doctors can artificially induce the process to spare patients from the experience of surgery.

Despite decades of research, however, we’re still in the dark about how the brain lets go of consciousness, either during sleep or after a dose of chemicals that knock you out. Finding the neural correlates of awareness—that is, what changes in the brain—would solve one of the most enigmatic mysteries of our minds. It could also lead to the objective measurement of anesthesia, giving doctors valuable real-time information about whether a patient is completely under—or if they’re beginning to float back into consciousness on the operating table.

This month, Tao Xu at Shanghai Jiao Tong University and colleagues mapped the brain’s inner workings as it descends into the void. By comparing the brain activity of 31 patients before and after anesthesia, they found a unique neural pattern marking when patients slid into unconsciousness. Connections between nine brain regions—some previously implicated in consciousness—rapidly broke down.

The results echo previous findings. But the study stands out for its practicality. Rather than using implants inserted into the brain, the team captured signals with electrodes placed on the volunteers’ scalps. With further validation, this shift in brain activity could be used as a signal for loss of awareness, helping anesthesiologists reliably keep their patients in a dream state—and bring them back.

Never-Ending Quest

Scientists generally agree that consciousness emerges from multiple brain regions working in tandem, but they heatedly debate which ones are involved.

Some researchers believe the seat of consciousness is rooted at the back of the brain. These regions receive and integrate information, giving the brain an overall picture of both inner thoughts and the outer world. Another camp fixates on the front and side areas of the brain. These circuits broadcast signals to the rest of the brain and break down as awareness slips away.

Still more scientists point to connections between the cortex, the outermost part of the brain, and a deeper egg-shaped brain structure called the thalamus, which gives rise to our sense of perception and self.

These latter conclusions come from studies of healthy volunteers looking at flashing images while researchers record their brain signals. Some stimuli are deliberately designed to not reach awareness. Conscious perception seems to rely on wave-like neural activity between multiple areas in the cortex and the thalamus. Without it, participants are oblivious to the images.

These studies tested perception and awareness in people while they were awake. Another team has compared neural activity in completely or partially comatose patients to alert participants. They found two circuits catastrophically fail in a coma. One of these is at the front of the brain, the other at the back. As results from studies converge on similar patterns, researchers are hopeful we’ll eventually reach a unified theory of consciousness.

But consciousness isn’t all or none. Previous studies capture only a single snapshot in time. To Xu and colleagues, truly understanding awareness means turning that snapshot into a movie.

Slipping Away

The authors of the new study recruited 31 people who were about to undergo surgery with the use of propofol, a popular general anesthetic. Once an anesthesiologist injects the milky liquid into a vein, it rapidly shuts down consciousness. Throughout surgery, the anesthesiologist carefully monitors a patient’s behavior (or lack thereof), heart rate, and other vital signs to adjust dosage in real-time. The goal is to keep the patient fully under without overdosing.

The team gave each person in the study a cap studded with 128 electrodes to capture the brain’s electrical chatter. This brain-recording method is called an electroencephalogram or EEG. It’s popular because the device sits on the scalp and is safe and non-invasive. But because it measures activity through the skull rather than directly from brain tissue, signals can be muffled or noisy.

To increase precision, the team developed a mathematical model to filter signals into five established brain wave types. Like radio waves, electrical activity oscillates across the brain at different frequencies, each of which correlates with a unique brain state. Alpha waves, for example, dominate when you’re relaxed but alert. Delta waves take over in deep sleep.

The team isolated signals from nine areas of the brain previously implicated in consciousness. These included most of the usual suspects: A cortex region in the middle of the brain called the parietal cortex, another cortex region in the back of the skull, and the thalamus and a handful of other deeper structures.

While the patients were alert, their brains hummed with alpha-wave activity between the parietal cortex and thalamus, suggesting the regions were synchronized. Other areas across the cortex were also highly connected, like parts of a well-oiled machine.

But a dose of propofol broke down most of these communications.

Within 20 seconds after patients received the drug, alpha waves disintegrated, and electrical signals between the parietal cortex and thalamus fragmented. Different parts of the cortex also lost connectivity. Although the patients seemed to lose consciousness suddenly, like flipping a light switch, their brain signals showed a steadier decline in synchrony—more like a dimmer that gradually shifted activity from a state of coordination to one of disarray.

The results “emphasize the critical role” alpha waves play in “reflecting the dynamic shifts associated with loss of consciousness,” wrote the team.

Further tests in 46 people undergoing mild sedation showed similar desynchronization in alpha waves. But the breakdown between the parietal cortex and thalamus was smaller. That specific connection seems especially relevant in the transition to unconsciousness, wrote the team.

The results back up other studies suggesting the thalamus is a critical node in consciousness. But they could also fuel further debate about the importance of different cortex regions and their connections. Instead of the front or back of the brain as the root of consciousness, the team thinks the middle parietal cortex is key, at least for patients taking propofol. They’re now exploring whether other anesthetics change brain wave dynamics in different and unique ways.

As the debate over consciousness rages on, the team is focused on practical gains in the clinic. They’re aiming to simplify the brain recording setup so anesthesiologists could routinely use it to measure consciousness in their patients before, during, and after anesthesia.