Beyond Hearing: Cochlear implants work best when given early

Children born deaf who receive cochlear implants as toddlers show brain activity that’s more normal than that of children getting the implants later in childhood, a new report shows. A separate study in animals reveals that the early implants foster development of the critical junction where the auditory nerve delivers messages to the brain.

Cochlear implants are small devices that are surgically embedded under the skin behind the ear. They pick up sounds and convert them into electric impulses, which then travel to the brain via the auditory nerve.

The new study of children takes advantage of the so-called McGurk effect, a perceptual illusion that pinpoints how visual and auditory information intersect in the brain. In one example, a subject presented with the sound “bah” while watching an image of a person whose mouth moves as if pronouncing the syllable “gah” will often perceive a third sound, “dah.” Blindfolded, however, the subject will usually hear the “bah” sound correctly.

Nathan A. Fox, a developmental psychologist at the University of Maryland at College Park, and his colleagues tested the McGurk effect in 35 children with normal hearing and 36 who were born deaf but had received cochlear implants.

Most of those with normal hearing experienced the illusion, indicating that their brains integrated sight and sound processing normally. Among the children with cochlear implants, this auditory-visual integration showed up only in those who had received implants before 30 months of age, the researchers report in an upcoming Proceedings of the National Academy of Sciences.

People constantly integrate sights and sounds, and they notice when the sensations don’t match, Fox says. Except in rare cases such as the McGurk effect, normal brain calculations prevent people from being fooled by hearing one thing and seeing another.

For those born deaf, early implantation “may help to make the world more comprehensible,” Fox says.

The other new study examined cats that had been born deaf. Those given cochlear implants within 5 months developed robust nerve endings at the synapses where the auditory nerve provides signals to the brain, say David K. Ryugo, a neuroscientist at the Johns Hopkins Medical Institutions in Baltimore, and his colleagues. Deaf cats that didn’t get implants failed to develop such connections, the team reports in the Dec. 2 Science.

At birth, people normally have in place basic nerve structures with the potential to grow into an efficient signal-relaying operation. “But it has to be used in a systematic, reliable way, or [the potential] goes away,” Ryugo says.

In earlier work, Anu Sharma of the University of Texas at Dallas and her colleagues found that in response to sounds, deaf children who were given implants before 3.5 years of age had brain waves indistinguishable from those of children with normal hearing. Children getting implants after age 7 had abnormal brain waves.

Combined, these three studies “show a critical period for brain development in deaf children,” Sharma says.

“The earlier they get the implants,” Fox agrees, “the better off they will be.”