D octors have known for years that some patients who become unresponsive after a severe brain injury nonetheless retain a “covert consciousness,” a degree of cognitive function that is important to recovery but is not detectable by standard bedside exams. As a result, a profound uncertainty often haunts the wrenching decisions that families must make when an unresponsive loved one needs life support, an uncertainty that also amplifies national debates over how to determine when a patient in this condition can be declared beyond help.

Now, scientists report the first large-scale demonstration of an approach that can identify this hidden brain function right after injury, using specialized computer analysis of routine EEG recordings from the skull. The new study, published Wednesday in the New England Journal of Medicine, found that 15 percent of otherwise unresponsive patients in one intensive care unit had covert brain activity in the days after injury. Moreover, these patients were nearly four times more likely to achieve partial independence over the next year with rehabilitation, compared to patients with no activity.

The EEG approach will not be widely available for some time, due in part to the technical expertise required, which most I.C.U.’s don’t yet have. And doctors said the test would not likely resolve the kind of high-profile cases that have taken on religious and political dimensions, like that of Terri Schiavo, the Florida woman whose condition touched off an ethical debate in the mid-2000s, or Karen Ann Quinlan, a New Jersey woman whose case stirred similar sentiments in the 1970s. Those debates centered less on recovery than on the definition of life and the right to die; the new analysis presumes some resting level of EEG, and that signal in both women was virtually flat.

But the EEG approach, once tested more broadly and refined, will likely change standard practice, some experts said, helping guide treatment decisions in the excruciating first days and weeks after a brain injury.