INDIANAPOLIS -- Deaf children who receive cochlear implants do better learning language and speech the younger they receive the implants, according to research by scientists at Indiana University School of Medicine.
However, it's not clear whether implanting children before they turn age one is worth the potential risks associated with such early surgeries, the researchers said. The work will be presented next week at a meeting of the Acoustical Society of America.
The study, by Mario Svirsky, Ph.D., professor of otolaryngology-head and neck surgery, and Rachael Holt, Ph.D., post-doctoral fellow in otolaryngology-head and neck surgery, also supports the theory that there is a "sensitive period" for optimal language development during the early years of life. In the study, the speed at which language was learned was greater for children who received cochlear implants earlier. The implants provide congenitally deaf children with a sense of hearing, but the children must learn how to interpret the sounds the implants provide. The researchers studied 96 children who received the implants in their first, second, third and fourth years of life, evaluating their progress with language skills and speech perception every six months. Those who received the implants earlier consistently performed better on tests of language skills -- learning vocabulary, grammar, and other such language rules -- and speech perception -- their ability to understand spoken words -- than did those who received the implants later. "Not only is earlier better, but we found that language gains tended to be faster for children who received cochlear implants earlier in life," said Dr. Svirsky.
However, children implanted before they turned one year old did not appear to do any better than those implanted during their second year. Infants as young as six months old have started receiving the implants, but there are potential risks associated with such early treatment, including the use of anesthesia and the difficulties in accurately diagnosing profound deafness. The findings regarding the youngest patients may be due to sample size, Dr. Svirsky said, because only just six of the patients were in the six-to-12-month age group.
The children's language development skills were tested with the widely used Reynell Developmental Language Scales. Speech perception was tested using the Mr. Potato Head Task, a technique developed by IU School of Medicine researchers in which an instructor covers his face with an opaque screen and asks the child to do various tasks with the toy.
The Svirsky and Holt study may help doctors and families decide when to proceed with a child's cochlear implant. It may also help answer a question that would otherwise require what Dr. Svirsky calls a "forbidden experiment" -- whether there's an age after which children have significantly more difficulty learning language and speech perception skills.
The experiment -- forbidden because it would be unethical -- would involve depriving children of all contact with language for different periods of time, then testing how well they were able to learn such skills afterward.
The study by Drs. Svirsky and Holt provides evidence that there's a sensitive period for language development that starts at about age two.
"It's not an exact model of development in children born with normal hearing. We restore imperfect hearing," said Dr. Svirsky. "This is an indirect way of exploring the issue of sensitive periods."
Although the younger children gained language development faster, Drs. Svirsky and Holt did not find a similar effect for speech perception skills. Gains in speech perception were more or less uniform for children implanted at any age before four. That suggests that if there is a sensitive period for speech perception, it may start later than age four, Dr. Svirsky said.
Drs. Svirsky and Holt will be presenting their findings Monday, May 16, 2005, at a joint meeting of the Acoustical Society of America and the Canadian Acoustical Association in Vancouver, Canada. Their research was funded by grants from the National Institutes of Health.
Additional information about their research and presentation in Vancouver can be found on the Acoustical Society of America web site:
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