Whether a person is conscious or not may seem like a simple question, but neurologists say that determining a person's level of awareness can be a complex diagnosis based on signs that aren't always apparent to untrained eyes.
It can even be a complicated call for trained eyes.
"There is obviously no way to get inside another person's mind and know firsthand if they are awake," said James Bernat, a professor of neurology at Dartmouth Medical School in Hanover, N.H. "All we can do is introduce stimuli to the patient, do tests and reach a reasoned judgment."
The American Academy of Neurology has a set of guidelines when it comes to determining the conscious state of a patient. The disputed case of Florida patient Terri Schiavo has thrown these medical gradations into public light -- where, researchers argue, the complexity of the diagnosis is sometimes lost.
"The key point is this is a clinical judgment," said Joseph Fins, chief of the medical ethics division of New York Presbyterian Hospital, "and medical diagnosis is not an ideological activity."
Doctors have determined that Schiavo is in a persistent vegetative state. Bernat explains this is when "you have wakefulness, your eyes are open, but you are unaware. It can appear as if you are aware, but it's a state of unconsciousness."
How can a patient be awake but unaware? Bernat says extensive damage to key parts of the brain -- namely, the cerebral cortex, the thalamus and/or connections between them -- can strip a person of his or her sense of awareness, while an undamaged brain stem keeps automatic activities, such as breathing, sleep and wake cycles and eye movement, going.
"The cerebral cortex and thalamus are the most important regions of the brain in terms of personality and consciousness," said Bernat. "They're what differentiate us from lower animals in terms of our ability to have language and communicate."
In the mid-1990s, a group of medical societies, including the American Academy of Neurology, published guidelines defining a vegetative state as "persistent" after a month. In this condition, patients show:
No evidence of awareness of self or environment and an inability to interact with others.
No evidence of sustained, reproducible, purposeful or voluntary behavioral responses to visual, auditory, tactile or noxious stimuli.
No evidence of language comprehension or expression.
Intermittent wakefulness manifested by the presence of sleep-wake cycles.
Sufficiently preserved brainstem function to permit survival (i.e. ,breathing) with medical and nursing care.
Bowel and bladder incontinence.
Variably preserved cranial nerve function and spinal reflexes; that is, the person may blink or smile.
The AAN considers those who have been in this condition for a year or longer to have "almost no probability of recovery."
Ronald Cranford, a neurology professor at the University of Minnesota Medical School who has examined Schiavo, says there are a number of signs showing that her cerebral cortex has severe damage from the six-minute period in 1990 when her heart stopped and her brain was without oxygen. He said electroencephalograms, also known as EEGs or brain wave tests, of Schiavo's brain have revealed no activity. He has also reported that areas of her brain have shown shrinkage -- a sign of irreversible damage.
Despite such findings, Bernat says it's easy to still believe a patient is aware.