Scientists largely ignored these sorts of debates for most of the 20th century. But in 1994, an interdisciplinary conference at the University of Arizona brought them together for the first time. That conference led to ongoing research studying the links between anesthesia and consciousness. Stuart Hameroff, an anesthesiologist and the director of the school’s Center for Consciousness Studies, was an organizer of the conference. Anesthesia, he told me, is a great example of why scientists have to think about consciousness. It’s not enough, he said, just to assume your patient is unconscious because she doesn’t respond to pain.

That’s because, while you need a brain to have consciousness, you might not need a brain to experience pain. In the 1990s, scientists ran tests on rats and goats, studying how the effects of anesthetics changed as different parts of the brain were intentionally damaged or removed. The amount of the drugs necessary to prevent the animals from moving in response to pain didn’t change as the cortex, the thalamus and the brain stem were destroyed. “Turns out, they were measuring a spinal-cord reflex, which is a much more primitive thing and has nothing to do with consciousness,” Hameroff said. Outward signs of consciousness may or not may not have anything to do with actual awareness.

In everyday life, it’s nearly impossible to ever know whether someone is conscious or not, Hameroff said, even if it seems glaringly obvious. Philosophers are fond of pointing out that, for all you know, you’re surrounded by people who appear to be fully conscious but who experience none of it subjectively. (They frequently trot out these beings for thought experiments and call them “philosophical zombies.”) But for those under anesthesia and the knife, such sophistry offers little solace.

Increasingly, research on what happens to the brain under anesthesia suggests that the synthesis and integration of information among many different parts of the brain is the best measure of consciousness. Some people, Mashour said, go so far as to say that this communication among regions is consciousness itself. Our subjective experience of the world might arise as a byproduct of the brain’s piecing together different sensory inputs.

It’s easy to see the connections between this idea and the “ping” study. When the brain falls asleep, drifts into a coma or comes under the influence of anesthetic drugs, the ability to synthesize information disappears, though the brain doesn’t cease to function. Figuring out a method for measuring intrabrain communication will be crucial for preventing operating-table awareness.

In the June 2013 issue of the journal Anesthesiology, Mashour proposed just that: a monitor that focuses on the brain’s ability to communicate within itself. It’s similar to the “ping” study but tracks a different signal. Activity in conscious brains has been shown to loop between sensory areas (the visual cortex in the rear of the brain, for example) and the higher-level parts of the brain associated with processing information (like the temporal lobe, just behind your ears). Mashour and others call this “recurrent processing”: Signals travel from the sensory areas to the processing areas and back again. When somebody is unconscious, the recurrent proc­essing disappears. Mashour’s study showed that this pattern — or lack thereof — is present in the brains of people anesthetized with three different classes of drugs. It’s not just a side effect of one kind of medication. His work suggests that anesthesia monitors might be more effective if, rather than measuring the presence of electrical waves produced by the brain, they monitor how electrical signals move around the brain.

Should such a device be developed, it would be good news not only for those 0.13 percent of patients but also for fans of Enlightenment philosophy. Mashour told me that the synthetic model of consciousness dates back to Immanuel Kant — his “Critique of Pure Reason” might be vindicated by neuroscience. “Kant said there has to be some process that takes individual processing and connects it together into a unified experience,” Mashour said. “Over the years, we’ve teased out the parts of the brain necessary for appreciating vision, color, motion. They’re all mediated by different brain areas. But how does the brain put all that together into single perception?” For Kant, this was clear long before EEG monitors and anesthetics. “Without this synthesis,” he wrote in 1781, “the manifold would not be united in one consciousness.”