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 tremendous impact on clinical care, anesthetics have emerged as powerful scientific tools to probe questions about consciousness. They allow us to induce profound and reversible changes in conscious states—and study brain responses during these transitions.
But one of the challenges that 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 challenge, we adapted a simple approach we call the breathe-squeeze method. It offers us a way to study changes in conscious state 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 that activity with 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 their size, effects that impair the communication between brain regions. For example, a recent 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.
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