Ten years ago, Adrian Owen, a young British neuroscientist, was working at a brain-imaging center at Addenbrooke’s Hospital, at the University of Cambridge. He had recently returned from the Montreal Neurological Institute, where he used advanced scanning technology to map areas of the brain, including those involved in recognizing human faces, and he was eager to continue his research. The imaging center was next to the hospital’s neurological intensive-care unit, and Owen heard about a patient there named Kate Bainbridge, a twenty-six-year-old schoolteacher who had become comatose after a flulike illness, and was eventually diagnosed as being in what neurologists call a vegetative state. Owen decided to scan Bainbridge’s brain. “We were looking for interesting patients to study,” he told me. “She was the first vegetative patient I came across.”

Brain scans showed one patient was able to imagine playing tennis. BILL ARMSTRONG, “APPARITION #906” (2005)/CLAMP ART

For four months, Bainbridge had not spoken or responded to her family or her doctors, although her eyes were often open and roving. (A person in a coma appears to be asleep and is unaware of even painful stimulation; a person in a vegetative state has periods of wakefulness but shows no awareness of her environment and does not make purposeful movements.) Owen placed Bainbridge in a PET scanner, a machine that records changes in metabolism and blood flow in the brain, and, on a screen in front of her, projected photographs of faces belonging to members of her family, as well as digitally distorted images, in which the faces were unrecognizable. Whenever pictures of Bainbridge’s family flashed on the screen, an area of her brain called the fusiform gyrus, which neuroscientists had identified as playing a central role in face recognition, lit up on the scan. “We were stunned,” Owen told me. “The fusiform-gyrus activation in her brain was not simply similar to normal; it was exactly the same as normal volunteers’.”

Excited by this result, Owen resolved to try to conduct brain scans of other vegetative patients in the Cambridge area. Since 1997, he has studied several dozen people, though he decided to use speech sounds rather than photographs to stimulate their brains. (Owen was concerned that showing images of faces might not be a reliable way to test recognition, since the eyes of vegetative patients often wander. “We shifted to auditory responses because you can always put a pair of headphones on the person and know that you are transmitting sound,” he said.) Three years ago, he began using a functional MRI (fMRI) scanner, which is faster than a PET scanner, capturing changes in blood flow in the brain almost as they occur. The patients’ brains were scanned while they listened to a recording of simple sentences interspersed with meaningless “noise sounds.” The scans of some of the patients showed the same response to the sentences as scans of healthy volunteers, but Owen wasn’t sure that the patients had understood the words. “So we went the next step up the cognitive ladder, to look at comprehension,” he said.

Psycholinguists have shown that when we hear a noun at the beginning of a sentence we tend to associate the word with its most common meaning. For example, Owen said, most people hearing a sentence that begins, “The shell was . . .” think of an object typically found at the beach. But if the sentence is completed by the phrase “fired at the tank,” the listener quickly corrects himself, a process that is evident on a brain scan. “You can actually see it happening and image it on the scanner,” Owen said. “The beautiful thing about the psychological task is that we just do it automatically. When you play ambiguous sentences, areas in the inferior frontal lobe and in the posterior temporal lobe become activated, and these areas are very important for speech comprehension. They show that you understand the meaning of the word: it’s not just about perceiving speech; it’s about decoding. Your brain somehow appreciates that there are two meanings to a word like ‘shell.’ ”

Owen eventually identified two vegetative patients whose brains showed the same activity in response to ambiguous sentences as the brains of healthy volunteers. He also took brain scans of healthy physicians, who were presented with the ambiguous sentences while under general anesthesia. Owen found that, as the effects of the anesthesia increased, the physicians showed less activity in the brain regions associated with comprehension. “That, of course, is in keeping with our personal experience of consciousness, which is that as you sort of drift into sleep you understand less and less of what is around you,” he said. (An article about this experiment appears this week in PNAS, the journal of the National Academy of Sciences.)

Owen’s final experiment was the most ambitious: a test to determine whether vegetative patients who seemed able to comprehend speech could also perform a complex mental task on command. He decided to ask them to imagine playing tennis. (“We chose sports, and tried to find one that involved a lot of upper-body movements and not too much running around,” he said.) First, he took brain scans of thirty-four healthy volunteers who were instructed to picture themselves playing the game for at least thirty seconds. Their brains showed activity in a region of the cerebrum that would be stimulated in an actual match. “This was an extremely robust activation, and it wasn’t difficult to tell whether somebody was imagining tennis or not,” Owen said. He then repeated the experiment using one of the vegetative patients, a woman who had been severely injured in a car accident. The woman had to be able to hear and understand Owen’s instructions, retrieve a memory of tennis—including a conception of forehand and backhand and how the ball and the racquet meet—and focus her attention for at least thirty seconds. To Owen’s astonishment, she passed the test. “Lo and behold, she produced a beautiful activation, indistinguishable from those of the group of normal volunteers,” he said. (Another vegetative patient, a man in his twenties, also passed the test, though Owen, having learned that the man was a soccer fan, asked him to imagine playing that sport instead of tennis.)

In September, 2006, Owen, along with Martin Coleman, a neuroscientist at Addenbrooke’s, and four other researchers, published an article about the tennis experiment in Science and ignited a vigorous debate. In letters to the journal, some neurologists argued that the woman must have been misdiagnosed—a claim that Owen disputed. “She fulfilled all of the internationally agreed-upon criteria, and there wasn’t anything that she did that would lead anybody to say she wasn’t vegetative,” he told me. “Now, naturally, in hindsight she wasn’t vegetative; she was actually conscious. It’s a very interesting issue, because it means that she was in fact misdiagnosed, but not misdiagnosed in the sense that somebody made an error. Clearly, she is consciously aware of things around her. So something is missing in the diagnostic criteria.”

For decades, doctors assumed that patients who have been diagnosed as vegetative lack any capacity for conscious thought. Most are previously healthy people who suffered a traumatic brain injury, or oxygen deprivation after a heart attack or stroke, and have been regarded more or less as zombies: patients whose bodies continue to function—sometimes for decades—but whose minds are incapable of willed activity. (The term “vegetative” was proposed in 1972, by Bryan Jennett, a neurosurgeon, and Fred Plum, a neurologist, who chose it based on a definition in the O.E.D: “an organic body capable of growth and development but devoid of sensation and thought.”) In the occasional newspaper stories about someone who suddenly recovered consciousness after spending years in a vegetative state, the event was invariably described as a medically inexplicable “miracle.” The Mohonk Report, a paper prepared by a group of experts in brain injury and presented to Congress last year, cited estimates suggesting that there are approximately thirty-five thousand Americans in a vegetative state and another two hundred and eighty thousand in a minimally conscious state—a less severe condition, in which patients show erratic evidence of deliberate behavior, such as responding to a simple command or focussing on a person or an object for a sustained period. Because insurers typically won’t pay for rehabilitation, on the assumption that such patients are unlikely to improve, most are given little in the way of therapy. “These people with brain trauma are out of our view,” Joseph Fins, an internist and medical ethicist at Weill Cornell Medical College, in Manhattan, and a member of the Mohonk group, told me. “We ignore them, and we sequester them in places where we can’t see them, usually in nursing homes.”

According to several American and British studies completed in the late nineties, patients suffering from what is known as “disorders of consciousness” are misdiagnosed between fifteen and forty-three per cent of the time. Physicians, who have traditionally relied on bedside evaluations to make diagnoses, sometimes misinterpret patients’ behavior, mistaking smiling, grunting, grimacing, crying, or moaning as evidence of consciousness. A neuroscientist showed me a video on the Internet of Terri Schiavo, the Florida woman who spent fifteen years in what most doctors agree was a vegetative state—tests revealed almost no activity in her cortex—and whose death, in 2005, provoked fierce debate over the rights of severely brain-damaged patients. (Schiavo died after the Supreme Court rejected her parents’ appeal of a judge’s decision approving her husband’s request that her feeding tube be removed. An autopsy showed extensive brain damage.) In the video, a man’s voice can be heard praising Schiavo for opening her eyes in response to his instructions, and the neuroscientist told me that he was impressed until he muted the sound. “With the sound off, it is clear that her movements are random,” the neuroscientist said. “But, with the voice-over, it is easy to make a misdiagnosis.” (The prognosis for patients such as Schiavo, who suffered brain damage owing to oxygen deprivation following cardiac arrest, is much worse than for those who suffer brain damage as the result of a head injury.)

Doctors can also miss signs of consciousness in vegetative patients, according to the British and American studies. Ten months after Owen and his colleagues completed the tennis experiment with the vegetative woman, she was brought back to the imaging center and placed in an MRI machine. “We were absolutely dismayed, because we scanned her and there was nothing,” Owen recalled. The team tested the woman again the next day. This time, in response to a command to play tennis, her brain showed normal activity in the regions that mediate arm movements. Owen now repeats scans for each patient, conducting them twice a day for three days. Patients with brain injuries have “seriously impaired attention capabilities and their levels of general arousal are likely to be shot,” he said. Recent research by Owen and other neuroscientists may eventually help make diagnoses more accurate, but it is not yet clear how the new brain-scan data will affect the medical understanding of consciousness. As Owen put it, “The thought of coma, vegetative state, and other disorders of consciousness troubles us all, because it awakens the old terror of being buried alive. Can any of these patients think, feel, or understand those around them? And, if so, what does this tell us about the nature of consciousness itself?”

Owen’s article in Science was accompanied by an editorial by Lionel Naccache, a neurologist at the Hôpital Pitié-Salpêtrière, in Paris, who called the results of the tennis experiment “spectacular.” “Despite the patient’s very poor behavioral status, the fMRI findings indicate the existence of a rich mental life, including auditory language processing and the ability to perform mental imagery tasks,” Naccache wrote. Yet he cautioned against drawing general conclusions about vegetative patients from a single case, and asked, “If this patient is actually conscious, why wouldn’t she be able to engage in intentional motor acts, given that she had not suffered functional or structural lesion of the motor pathways?” Prompted by questions like this, Naccache and several of his colleagues are conducting brain-imaging experiments with the goal of identifying objective indicators of consciousness, and thus enabling doctors to better evaluate patients who are unable to communicate their awareness of themselves or their environment.

We assimilate information unconsciously all the time; at any given moment, we process thousands of stimuli, of which we pay attention to only a few. As you read this sentence, you may not be aware of the birds singing in the back yard, but your brain has analyzed the sound and concluded that it poses no threat to you. In the past several decades, scientists have uncovered particularly dramatic examples of unconscious processing. In the early seventies, researchers at M.I.T. studied four patients who had experienced trauma to an area of the brain involved in vision and had been found to have a condition that was later called “blindsight.” These patients’ eyes functioned normally, but they did not perceive much of what was in their field of vision. When the researchers flashed a light at the patients and asked them to describe what they saw, the patients reported that they had seen nothing. Yet the researchers noticed that their eyes often located the source of the light. In a second experiment, a blindsight patient was shown pictures of faces displaying happiness, sadness, anger, and fear. The patient said that he could not see the faces, yet he was frequently able to correctly identify the emotions. The researchers concluded that, despite the patient’s injuries, pathways in his brain had been preserved which allowed him to process at least some visual data, even though he wasn’t consciously aware of doing so.

In the early nineteen-hundreds, the Austrian neurologist Hermann Zingerle described patients who, because of tumors or other abnormalities of the parietal lobe on the right side of the brain, ignored the left side of the body and objects in the left field of vision. (The right side of the brain controls awareness of the left side of the body.) For example, some of these patients would shave only the right side of their faces, since they were unaware of their left cheeks. In the nineteen-eighties, researchers determined that patients who had the syndrome—now called “neglect”—could process some objects in the left field of vision. In one experiment, a patient was shown two pictures of a house. The images were identical except that, in one, flames were emerging from a window on the left side of the façade. The patient said that she couldn’t see any difference between the images, but, when she was asked which house she would want to occupy, she almost always chose the one that was not on fire. “This is more complex than blindsight, because it means that the patient was unconsciously able to interpret and understand the symbolic meaning of the pictures,” Naccache said. “It is a powerful experiment to demonstrate that unconscious perception and unconscious cognition can reach upper levels of the brain.”