It is challenging for deaf people to learn a sound - based language, since they are physically not able to hear those sounds. (sciencedaily.com)
The example is the children of deaf people who use «real» sign language, who learn this instead of spoken language. (amotherinisrael.com)
This performance was in stark contrast to a fourth group of deaf people who could hear as children, but lost their hearing between the ages of nine and fifteen through illness. (newscientist.com)