Alex’s hearing loss is a result of a rare congenital condition called Mondini dysplasia, in which the cochlea fails to form completely. It is often accompanied by a second condition, enlarged vestibular aqueduct syndrome. Because the vestibular system also controls balance, Mondini and the syndrome probably contributed to Alex’s delayed gross motor skills.

Marc Marschark, a professor at the National Technical Institute for the Deaf in Rochester, says that no matter whether deaf children were born to deaf parents who use sign language, or whether they have implants and hearing aids, “the one constant is that they are all still deaf.”

Dr. Marschark hypothesizes that deaf children use their brains differently from hearing children. They quickly learn to pay attention to the visual world, and that leads to differences in brain structure. “Deaf children are not hearing children who can’t hear,” he said. “There are subtle cognitive differences between the two groups.”

David B. Pisoni, a cognitive neuroscientist at the Indiana University School of Medicine, has been studying children with cochlear implants for 22 years. (The first F.D.A. clinical trials on pediatric cochlear implants were done at this center.) He is interested in cognitive processing — learning and memory, attention, language comprehension and production — in deaf children. For children with cochlear implants, success in these areas is highly variable, he said, stressing the “highly.”

This variability in cognitive success affects academic achievement. Although some deaf children do very well academically, an alarming number do not. The median reading level by age 18 in deaf children “has not changed in 40 years,” as Dr. Marschark put it. It remains stuck at fourth-grade level.

Studies have found that deaf children from families who identify themselves as culturally deaf, and use American Sign Language, are generally on a par with hearing children in terms of reading, because they have been exposed to language since birth. But 95 percent of deaf children are born to hearing parents. Those children have no exposure to language until they get a cochlear implant or are entered into an American Sign Language program. Early exposure to language is crucial to learning to read. Those studies of 18-year-olds may yield very different results when children who were implanted at 6 months or a year reach that age.

There are difficulties. For the 5 percent born to deaf parents, learning to read is complicated by the lack of a written form of sign language. Written English is essentially a different language.