Scientists at Johns Hopkins University yesterday introduced a device aimed at stopping severely retarded and autistic persons from banging their heads as a result of self-injurious behavior.

The Self-Injurious Behavior Inhibiting System, which employs a mild electric shock, is designed to help about 50,000 people in the United States who suffer from the baffling affliction, which often involves repeated head-banging.

"The results are so dramatic in the first two cases we studied, we are convinced other cases will show the same results as well," said Thomas Linscheid, associate professor of pediatrics and psychology at Ohio State University.

A special lightweight headgear is worn by the self-injurious person along with an arm or leg band. When the head is struck, a beep is sounded and a mild shock is immediately transmitted to the person through the other band. This stops the injurious behavior, the scientists said.

"We have trouble speculating why it works, but then again we have trouble speculating why the individual does it in the first place," Linscheid said. "We just know the device works."

The system was developed 17 years ago by Mooza Grant, president of the American Foundation for Autistic Children, and her husband Leslie. Johns Hopkins' Applied Physics Laboratory near Columbia, using technology similar to those used in satellites, refined the device, putting thousands of electrical circuits on a tiny chip.

The system also records the number of hits, both light and heavy, allowing progress to be monitored.

Linscheid said the first person he studied using the device was an Ohio child who had been institutionalized because of the problem.

"Fifteen minutes after wearing the headgear, all hits went away, not just the light ones," he said. "That's not something we expected at all."

The system is manufactured by Human Technologies Inc. of St. Petersburg, Fla., for about $3,000 and is available to the public.

People afflicted with self-injurious behavior may suffer blindness, brain damage and death from the blows to the head. They are often given heavy doses of medication, strapped down or institutionalized.