More than 3 million Americans live with disabling brain injuries. The vast majority of these individuals are lost to the medical system soon after their initial treatment, to be cared for by family or to fend for themselves, managing fatigue, attention and concentration problems with little hope of improvement.

On Saturday, a team of scientists reported a glimmer of hope. Using an implant that stimulates activity in key areas of the brain, they restored near-normal levels of brain function to a middle-aged woman who was severely injured in a car accident 18 years ago.

Experts said the woman was a test case, and that it was far from clear whether the procedure would prompt improvements for others like her. That group includes an estimated 3 million to 5 million people, many of them veterans of the wars in Iraq and Afghanistan, with disabilities related to traumatic brain injuries.

“This is a pilot study,” said Dr. Steven R. Flanagan, the chairman of the department of rehabilitation medicine at NYU Langone Health, who was not part of the research team. “And we certainly cannot generalize from it. But I think it’s a very promising start, and there is certainly more to come in this work.”