Even more significantly, one patient was able to answer yes and no questions using the researchers' technique—indicating the potential for communication with people previously considered unresponsive.

Researchers at two centers, in England and Belgium, used functional magnetic resonance imaging (MRI) tests on 54 patients with severe brain injury. Of these patients, 31 were diagnosed as being in a minimally conscious state, meaning they showed intermittent signs of awareness such as laughing or crying. The other 23 were diagnosed as being in a vegetative state, meaning they were considered unresponsive and unaware of their surroundings.

The study is part of a growing body of work changing how people think about the vegetative state. "There has been a kind of nihilism towards these patients. This represents a cultural shift," says Joseph J. Fins, chief of the medical-ethics division at Weill Cornell Medical College, New York. (Dr. Fins wasn't involved in the current study but is working with the researchers on a separate project looking at standardizing how brain injuries are assessed.)

Each patient in the New England Journal of Medicine study was placed in a functional MRI scanner and asked to imagine playing tennis, a task that activates the part of the brain associated with movement. Participants also were asked to imagine walking around their home, or on familiar city streets, which activates areas in the brain involved in spatial navigation. Four of the 23 vegetative patients responded to the commands and exhibited brain activity in the same areas as healthy control subjects.

Then the researchers used the technique to see if it might enable patients to answer simple yes-no questions, such as "do you have any brothers?" Patients were instructed to answer by imagining one of the two scenarios—playing tennis if the answer was yes, for example, or walking around one's home if no. One of the four vegetative patients responded correctly to the questions, said Adrian M. Owen, a neuroscientist at the Medical Research Council in the U.K. and one of the study's authors.

"There is a minority of vegetative patients who aren't what they appear to be," said Dr. Owen. "They have cognitive capabilities far beyond what they appear capable of."

The study, using a technique Dr. Owen first implemented in a 2006 study of brain activity in one vegetative patient, demonstrates the challenges of determining awareness in brain-damaged patients. Some estimates put misdiagnosis as high as 40%, Dr. Owen said. It's possible some of the vegetative patients in the study had some consciousness but that brain injury may have left them deaf or incapable of responding, he said.

In an editorial accompanying the study, Allan H. Ropper, a neurologist at Brigham and Women's Hospital, in Boston, pointed out that brain activity was found in only a small number of patients and that it doesn't necessarily mean the patient has self-awareness or the ability to reflect. But, he continued, "the line between consciousness and unconsciousness will be blurred" as scientific understanding of the vegetative state deepens.

Dr. Owen said he and his team plan to continue refining functional MRI scans as a communication tool in hopes of eventually letting vegetative patients participate in their own care.

Write to Amy Dockser Marcus at amy.marcus@wsj.com