Patients in a so-called vegetative state have been shown to be conscious, and one has been enabled to communicate via an MRI scan.
The researchers found they could detect signs of awareness in four of these cases.
Patients' brain activity was then mapped while they were asked simple questions such as "Is your father's name Thomas?". They were told to imagine playing tennis if the answer was yes.
One 29-year-old man was found to be able to communicate simple 'yes' and 'no' answers, five years after suffering severe brain injuries in a traffic accident.
While the technique has been shown to be 100 percent accurate in healthy people, it had never before been tried on patients who couldn't move or speak.
"So far, these scans have proven to be the only viable method for this patient to communicate in any way since his accident," said Dr Steven Laureys, co-author from the University of Liège.
"It’s early days, but in the future we hope to develop this technique to allow some patients to express their feelings and thoughts, control their environment and increase their quality of life."