He emerged from the car accident alive but alone, there and not there: a young man whose eyes opened yet whose brain seemed shut down. For five years he lay mute and immobile beneath a diagnosis  “vegetative state”  that all but ruled out the possibility of thought, much less recovery.

But in recent months at a clinic in Liège, Belgium, the patient, now 29, showed traces of brain activity in response to commands from doctors. Now, according to a new report, he has begun to communicate: in response to simple questions, like “Do you have any brothers?,” he showed distinct traces of activity on a brain imaging machine that represented either “yes” or “no.”

Experts said Wednesday that the finding could alter the way some severe head injuries were diagnosed  and could raise troubling ethical questions about whether to consult severely disabled patients on their care.

The new report, posted online by The New England Journal of Medicine, does not suggest that most apparently unresponsive patients can communicate or are likely to recover. The hidden ability displayed by the young accident victim is rare, the study suggested.