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.