The debate over Terri Schiavo's fate comes at a time when researchers are deepening their understanding of the unconscious brain.
Neuroscientists now understand at least some of the physiology behind a wide range of unconscious states, from deep sleep to coma, from partially conscious conditions to a persistent vegetative state, the condition diagnosed in Ms. Schiavo.
New research, by laboratories in New York and Europe, has allowed for much clearer distinctions to be made between the uncounted number of people who at some time become comatose, the 10,000 to 15,000 Americans who subsist in vegetative states and the estimated 100,000 or more who exist in states of partial consciousness.
This emerging picture should make it easier for doctors to judge which brain-damaged patients have some hope of recovering awareness, experts say, and already it is providing clues to the specific brain processes that sustain conscious awareness.
"Understanding what these processes are will give us a better sense of how to help the whole range of people living with brain injuries," said Dr. Nicholas Schiff, an assistant professor of neurology and neuroscience at NewYork-Presbyterian/Weill Cornell hospital. "That is where this field is ultimately headed: toward a better understanding of what consciousness is."
The most familiar unconscious state is sleep, which in its deepest phases is characterized by little electrical activity in the brain and almost complete unresponsiveness. Coma, the most widely known state of impaired unconsciousness, is in fact a continuum. Doctors rate the extent to which a comatose person shows pain responses and reactions to verbal sounds on a scale from 3, for no response, to 13, for consistent responses.
As in sleep, people in comas may move or make sounds and typically have no memory of either. But they almost always emerge from this state in two to three weeks, doctors say, when the eyes open spontaneously. What follows is critical for the person's recovery.
Those who are lucky, or who have less severe injuries, gradually awaken. "The first thing I remember was telling my ex-boyfriend, who was at the foot of the bed, to shut up," said Trisha Meili, who fell into a coma after being beaten and raped in 1990, and wrote about the experience in the book, "I Am the Central Park Jogger."
In the days after this memory, Ms. Meili said, she slipped in and out of conscious awareness, "as if my body was taking care of the most important things first, and leaving my moment to moment awareness for last."
In fact, researchers say, this is precisely what happens. The primitive brain stem, which controls sleep-wake cycles as well as reflexes, asserts itself first, as the eyes open. Ideally, areas of the cerebral cortex, the seat of conscious thought, soon follow, like lights flicking on in the upper rooms of a darkened house.
But in some cases - Ms. Schiavo's was one of them - the cortical areas fail to engage, and the patient's prognosis becomes dire.
Neurologists were all but unanimous in diagnosing the condition of Ms. Schiavo, whose heart stopped temporarily in 1990, depriving her brain of oxygen. Brain cells and neural connections wither and die without oxygen, like marine life in a drained lake, leaving virtually nothing unharmed.
People with these kinds of injuries - Nancy Cruzan, whose case reached the Supreme Court in 1990 is an example - almost always remain unresponsive if they have not regained awareness in the first months after the injury.
In medical terms, they become persistently vegetative, a diagnosis first described in 1972 by Dr. Fred Plum of Cornell University and Dr. Bryan Jennett, a neurosurgeon at Glasgow University in Scotland. In a sense, the description of the diagnosis began the modern study of disorders of consciousness. "Before 1972 people talked about permanent comas, or irrecoverable comas, but we defined a different state altogether, with the eyes open, some reflex activity, but no sign of meaningful psychological responsiveness," Dr. Jennett, now a professor emeritus, said in an interview.
In an exhaustive review of the medical histories of more than 700 persistently vegetative patients, a team of doctors in 1994 reported that about 15 percent of those who suffered brain damage from oxygen deprivation, like Ms. Schiavo, recovered some awareness within three months. After that, however, very few recovered and none did so after two years.
About 52 percent of people with traumatic wounds to the head, often from car accidents, recovered some awareness in the first year after the injury, the study found; very few recovered after that. "It's the difference between taking a blow to the brain, which affects a local area - and taking this global, whole-brain hit," said Dr. Joseph Fins, chief of the medical ethics division of NewYork-Presbyterian/Weill Cornell hospital.
Yet these statistics cannot explain the stories of remarkable recovery that surfaced during the debate over Ms. Schiavo's fate. There was Terry Wallis, a mechanic in Arkansas who regained awareness in 2003, more than 18 years after he fell into unconsciousness from a car accident; Sarah Scantlin, a Kansas woman who, also a victim of a car accident, emerged from a similar state after 19 years; and several others, whose collective human spirit seemed to defy the experts, and trump science.
Researchers say these cases can be accounted for by recent studies that indicate the existence of yet another state of subdued responsiveness, one that represents a clear break from the vegetative.
For years, doctors who specialize in rehabilitation have known that some of their severely brain-damaged patients were responsive, at least once in a while. In their good moments, these patients could track objects with their eyes. They could follow commands, like reaching for a glass, or grabbing someone's hand. They were - intermittently, unpredictably, but unequivocally - responsive.
In 2002, a panel of experts established a new diagnosis on the basis of exactly these reactions: the minimally conscious state. "It took years to get some agreement on the definition, and it's only now getting some acceptance," said Dr. Nancy Childs, at Texas NeuroRehab Center in Austin, "but we've known for years that there was this other group."
In a landmark study published in February, a team of neuroscientists in New York, New Jersey and Washington, led by Dr. Schiff, used imaging technology to compare the brain activity in two young men who were deemed to be minimally conscious with the brain activity of seven healthy men and women. The researchers recorded an audiotape for each of the nine subjects in which a relative or loved one reminisced, telling familiar stories or recalling shared experiences.
In each of the brain-damaged patients, the sound of the voice prompted a pattern of brain activity similar to that of the healthy participants. The team has since replicated the results in other minimally conscious patients.
Like an interlocking set of old Christmas lights, blinking on and then off, the neural connections in minimally conscious patients seem to be in place, the research suggests. In persistently vegetative brains, by contrast, the crucial connections are apparently shot: maybe one light blinks here, another over there, but the full network is dark.
One case, of a 26-year-old English woman named Kate who emerged from a subdued unconscious state after six months, suggests such patients may be at times acutely aware of what is happening around them. During rehabilitation, though unable to communicate, this woman had a visit from a college friend.
"I have just met an old friend from Uni and it really upset me," the woman recalled thinking, doctors reported. "I can now see how much I am missing. She has been married for five years and she has a house and a life. I just scream as I can't cry, which I would do if I could."
Recovery from severe brain damage is viewed in this new understanding as a step-wise progression: people who regain conscious awareness pass from a coma to a vegetative state to minimal consciousness - and almost always do so quickly, usually within a month or so of shaking the coma. Those who regain awareness within hours of emerging from a coma probably also pass through the same progression, but so swiftly the changes go unnoticed, some experts say.
"If you look at these cases of recovery closely, you will find that many of these patients were showing signs of consciousness much earlier" than is sometimes portrayed in news media accounts, Dr. Fins of NewYork-Presbyterian said.
Researchers know little about how to draw a person out of a minimally conscious state, which itself can last a lifetime. In one study of 124 brain-damaged patients, doctors in Philadelphia and New Jersey reported in March that amantadine, a drug for Parkinson's disease, appeared to speed recovery in some people. But the evidence was not definitive and will require confirmation, the authors wrote.
Rehabilitation, such as it is, typically includes life support, if needed, and regular visits from medical staff, typically to change the patient's position in bed and to stimulate the senses with bright lights, noises, sharp smells and tastes, including lemon and chocolate. "I always tell families that it's time and nature and God taking care of things, that what we do mostly is monitor the patients," said Dr. Childs.
Dr. Joseph Giacino, a neuropsychologist at the JFK Johnson Rehabilitation Institute in Edison, N.J., has been following a group of brain-damaged patients with both oxygen-deprivation and traumatic injuries, and finds that the group with traumatic injuries - if they become minimally conscious - are far more likely to show signs of recovery than the others. "There is a real separation between these patients and the others in terms of improvement in the first year," Dr. Giacino said.
Ms. Schiavo showed no evidence of having ever entered a minimally conscious state, either in the early 90's or later, neurologists say. An EEG of her cerebral cortex showed almost no electrical activity, said a neurologist who examined her, and a dozen experts interviewed about her case agreed that an M.R.I. scan would have added no information.
In Dr. Schiff's study comparing M.R.I. activity of minimally conscious with normal subjects, the researchers also found a striking difference. The overall rate of energy consumption was significantly higher in the normal brains than in the minimally conscious ones. This difference in idling speed may be crucial to maintaining conscious awareness, Dr. Schiff and others suggested.
Because signaling between brain cells requires one cell to overwhelm the other, Dr. Schiff said, a lower idling speed may make the signaling threshold harder to overcome - effectively damping activity throughout the brain. "The idea is that maybe if you were to activate that substrate, you may cross the threshold and generate enough activity" to produce more awareness, he said.