"And there's no scheduled protocol that says you should re-evaluate a person every six months or every two years," said DeGeorgia. "Often times it's the family that recognizes that the patients are 'there,' and the family that drives to get more results."
Cindy and Steve Chiappa of Toms River, N.J., found themselves in a similar situation eight years ago, after Steve suffered a stroke on Christmas Eve.
Chiappa is still too paralyzed to speak clearly, but has now progressed to the point where he can eat a variety of soft foods and communicate through a specialized keyboard.
"Other than shining a light in my eyes and asking me to follow the beam they didn't ask me too much," Chiappa wrote in an e-mail to ABCNews.com.
For the first few weeks after he woke up in the hospital, only his family believed that he was actually aware of his surroundings.
"They would ask simple things like, 'If you know who you are, blink' but my response time was so slow, by the time I could blink my response they had turned away," wrote Chiappa.
"He was scared; the doctors did not want to believe it. It took about a week-and-a-half for me and all our kids badgering the doctors for a diagnosis," said Cindy. "Even three months later, they said, 'oh, he's just going to be a vegetable and that's it.'"
Finally, with a little help from a nurse who goaded the doctors, Chiappa was diagnosed as being "locked-in" rather than a vegetable. A massive stroke that hit the base of his brain had left him unable to move, but left his mind perfectly intact.
Neurologists say paralysis plays only one role in a complicated combination of impaired states that can trap a person in their body.
For example, a person who is in a vegetative state and a person who is "locked-in" would both be paralyzed to some degree. Both patients would likely open their eyes and look around.
But, as Dr. James L. Bernat of the American Academy of Neurology explained, these two individuals would be considered to have two different definitions of "consciousness."
"One is called wakefulness; eyes open, eyes moving -- that element is conducted primarily by the brainstem," Bernat said. "The second dimension of consciousness requires self-awareness -- they're aware of what's going on, they can feel, they can think.
"People in the vegetative state have only the wakeful dimension," he said.
A coma would be a state of full paralysis and full unconsciousness similar to the experience of going under anesthesia. Neurologists even define a third state in people with brain injuries as "minimally conscious," which is a state of semi-wakefulness and limited self-awareness.
In a minimally conscious person, "there may be parts of the brain that are able to generate certain types of thoughts similar to what a conscious person would do, but they're still quite devastated and quite injured," said Dr. Paul M. Vespa, director of Neurocritical Care at the David Geffen School of Medicine at the University of California, Los Angeles.
"There are probably a very small number of patients who are in this minimally conscious state. The exciting thing is, is that maybe there's a potential for rehabilitation," said Vespa.
Researchers, like Laureys, will have to do more work to define which brain image results from a functional MRI or PET machine are signs of a response, and which are random noises, before such progress could happen.
"We're not at the point where you can just throw somebody in the scanner and come up with the answer," DeGeorgia said. "[We] don't know how to interpret these [scans], because there have only been a few studies, and you have to know what is normal, and what is within the bell-shaped curve."
ABC News' Christophe Schpoliansky contributed to this report.