Consciousness isn’t easy to define, but we know it when we experience it. It’s not so simple to decide when someone else is conscious, however, as doctors must sometimes do with patients who have suffered traumatic brain injury. Now, researchers have come up with an approach that uses the brain’s response to magnetic stimulation to judge a person’s awareness, reducing it to a numerical score they call an index of consciousness. “You’re kind of banging on the brain and listening to the echo,” says Anil Seth, a neuroscientist at the Sackler Centre for Consciousness Science at the University of Sussex in the United Kingdom who was not involved in the work.
Faced with an unresponsive patient, clinicians do their best to determine whether the person is conscious. Through sound, touch, and other stimuli, they try to provoke verbal responses, slight finger movements, or just a shifting gaze. Yet some conscious patients simply can’t move or speak; an estimated 40% of those initially judged to be completely unaware are later found to have some level of consciousness.