In 2003, 39-year-old Terry Wallis uttered his first word ("mom") in the 19 years since a car accident had left him with severe brain damage. He had spent much of the previous two decades in what neurologists call a minimally conscious state, somewhere in the gray area between coma and consciousness. In the years before his awakening, however, Wallis's family had noticed that he was growing more alert and responsive, occasionally nodding, grunting, or even crying, until one day he spontaneously started to speak. Though Wallis still has serious impairments in memory and movement, he continues to make remarkable gains.

No one knows what spurred Wallis's return to the waking world. But neurologist Nicholas Schiff is determined to find out. A researcher at the Weill Cornell Medical College in New York City, Schiff is one of a handful of scientists studying people like Wallis, patients who spend months or years seemingly unaware of the outside world and unable to communicate. Using new brain-imaging techniques, Schiff is hoping to better understand the complex nature of consciousness--and find ways to help treat the thousands of patients who suffer from severe consciousness disorders.