Two deaf women have become the first people to undergo the risky procedure of having implants in their brainstems. The devices are designed to restore hearing by directly stimulating nerves.

The procedure is needed by people with a damaged cochlea or auditory nerve, where cochlear implants cannot help. The researchers hope that the implant, in which eight electrodes of different lengths are inserted into the brainstem, will be able to stimulate at least four channels or bundles of nerves individually to produce different frequencies — enough to understand speech.

The procedure might also help congenitally deaf children who are born without a cochlear nerve.