What does it mean to be conscious? People have been thinking and writing about this question for millennia. Yet many things about the conscious mind remain a mystery, including how to measure and assess it. What is a unit of consciousness? Are there different levels of consciousness? What happens to consciousness during sleep, coma and general anesthesia?
As anesthesiologists, we think about these questions often. We make a promise to patients every day that they will be disconnected from the outside world and their inner thoughts during surgery, retain no memories of the experience and feel no pain. In this way, general anesthesia has enabled tremendous medical advances, from microscopic vascular repairs to solid organ transplants.
In addition to their significant impact on clinical care, anesthetics have emerged as powerful scientific tools for probing questions about consciousness. They allow us to induce profound and reversible changes in conscious states—and study brain responses during these transitions.
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But one of the challenges anesthesiologists face is measuring the transition from one state to another. That’s because many of the approaches that exist interrupt or disrupt what we are trying to study. Essentially, assessing the system affects the system. In studies of human consciousness, determining whether someone is conscious can arouse the person being studied, confounding that very assessment. To address this problem, we adapted a simple approach we call the breathe-squeeze method. It offers us a way to study changes in conscious states without interrupting those shifts.
To understand this approach, it helps to consider some insights from studies of consciousness that have used anesthetics. For decades researchers have used electroencephalography (EEG) to observe electrical activity in the brains of people receiving various anesthetics. They can then analyze the activity in those EEG readings to characterize patterns that are specific to various anesthetics—so-called anesthetic signatures.
Such research reveals that most anesthetic drugs slow the brain’s rhythms and increase the size of brain waves, effects that impair the communication between brain regions. For example, a 2024 study found that propofol, the most commonly used drug for general anesthesia, can disrupt the way brain regions typically work together to process sensory information.
Consciousness, this and other research reveals, is not simply a binary—on or off, conscious or unconscious—but instead something that can encompass a continuum of different states that involve different kinds of brain functioning. For instance, consciousness can be connected to the environment through our senses and behavior (connected consciousness), as in most of our waking hours, or disconnected from our surroundings (disconnected consciousness), as when we dream during sleep.
Unconsciousness—as when someone is in a coma—is more difficult to study than connected or disconnected consciousness, but it is generally understood as a state of oblivion, void of subjective experience or memory. When we prepare a patient for surgery, we adjust the levels of anesthetic to render them unconscious. When someone is under general anesthesia, they are experiencing a temporary and reversible coma during which they feel no pain and after which they will have no memories of their procedure.
Anesthetics have emerged as powerful scientific tools to help clinicians probe questions about consciousness.
Understanding the transitions between these states is essential to ensure adequate levels of general anesthesia and to illuminate questions in anesthesiology, consciousness, sleep and coma research. To better map the transition out of connected consciousness, we adapted a new approach for monitoring a person’s ability to generate volitional behaviors without external prompting.
Generally, researchers track the onset of sedation by issuing verbal commands and recording behavioral responses. For instance, a scientist might periodically ask someone to open their eyes or push a button while receiving an anesthetic infusion. When the person stops responding to this command, the scientist assumes they have lost connected consciousness.
This technique has proved useful in contrasting the connected conscious mind with the disconnected state. But when it comes to understanding the transition between the two, there are several drawbacks. For one, the auditory cue is not standardized: the inflection and volume of the voice, what is said, and how frequently it is repeated all vary across and even within studies. A more fundamental problem is that these commands can arouse people as they drift into a disconnected state. This limitation means that researchers often need to wait several minutes between issuing verbal commands and assessing the response, adding uncertainty to the exact timing of the transition.
In our study, we wanted a more sensitive and precise approach to measuring the onset of sedation without the risk of disrupting the transition. So we turned to a method first described in 2014 by sleep researchers at Massachusetts General Hospital and Johns Hopkins University. In that work, the investigators asked participants to squeeze a ball whenever they breathed in. The researchers tracked each person’s squeezes with a dynamometer, a tool for measuring grip strength, and an electromyography sensor, which measures muscle response. In this way, they were able to precisely track the sleep-onset process without disrupting it.
For our study, we trained 14 healthy volunteers in that same task and presented the breathe-squeeze exercise as a kind of mindfulness meditation. We instructed the participants to focus on their breathing and to squeeze a handheld dynamometer whenever they breathed in. After a few minutes of training for each person, we placed an intravenous catheter in their arm to deliver the sedative and set up vital sign monitors and a fitted 64-channel EEG cap to record brain waves throughout the experiment.
All participants reliably synchronized their squeezes with breathing during an initial baseline period without any sedation. They then received a slow infusion of dexmedetomidine, a sedative commonly used in operating rooms and intensive care units. As brain concentrations of dexmedetomidine increased, participants occasionally missed a squeeze or mistimed it. Eventually, they stopped squeezing altogether.
Following some additional tests, we turned off the dexmedetomidine infusion, allowing participants to recover from sedation. To our astonishment, after a period of 20 to 30 minutes, everyone remembered the task and began spontaneously squeezing in synchrony with their breath without any prompting. This allowed us to analyze the timing for both sedation onset and sedation offset and compare them with results from previous studies in which researchers used verbal commands to assess consciousness.
The breathe-squeeze task was clearly a more sensitive approach to measuring the transition out of connected consciousness. Participants stopped performing the task at lower dexmedetomidine concentrations than those at which people had been observed to stop responding to auditory cues in other studies, highlighting the arousing effects of external cues on the system. These findings may also indicate that connected consciousness can be further broken down into internally generated behaviors (such as reminding yourself to squeeze a ball as you inhale) and externally prompted behaviors (such as responding to verbal commands) with separate transition points—an idea that refines our understanding of the continuum of consciousness.
Previous research has characterized what the brain looks like in states of connected and disconnected consciousness, so we knew generally what to expect from the EEG recordings. But we were less sure of how our technique might align with the brain’s transition between the states of consciousness. We discovered a very clear pattern of changes in the brain around when people stopped squeezing the ball. Further, we saw no evidence that the squeezing task disrupted people’s state of consciousness. The EEG also revealed a much more specific time frame for that change than in past work, pinpointing the transition within a period that was about 10 times smaller than what has been possible with auditory cues—a five- to six-second window rather than the 30- to 120-second interval that has been common in other studies.
As an added perk, we were delighted to discover that many of the participants in our study enjoyed the breathe-squeeze task as a way to focus on calming their mind and body. For this reason, we also began implementing the method in clinical practice—that is, outside of carefully controlled studies—when inducing general anesthesia for major surgeries, which can otherwise be a stressful experience for patients.
We decided to build on this work by analyzing structural magnetic resonance imaging data from our volunteers in addition to our EEG data. These insights into the shift from connected to disconnected consciousness can help inform clinical care for patients who require anesthesia for surgery, as well as for those who suffer from a sleep disorder or coma. These studies also challenge us to grapple with the more philosophical aspects of consciousness and could thereby shed light on the fundamental question of what it means to be conscious.
Are you a scientist who specializes in neuroscience, cognitive science or psychology? And have you read a recent peer-reviewed paper that you would like to write about for Mind Matters? Please send suggestions to Scientific American’s Mind Matters editor Daisy Yuhas at dyuhas@sciam.com.
