BioEd Online

Our ability to learn language is already on the wane by our third year of life, according to a study of profoundly deaf children given cochlear implants to restore some of their hearing.

The research supports the widely held belief that there is a 'sensitive period' for language learning, during which the capacity to acquire vocabulary and grammar is heightened. "But I was surprised we found evidence that this sensitive period occurs so early in life," says Mario Svirsky, an acoustic engineer from the Indiana University School of Medicine in Indianapolis, who led the study.

Svirsky and his colleague Rachael Frush Holt looked at 96 children who had received implants during their first four years of life. The implants, which are surgically inserted in the ear, convert sound into electrical signals that the brain can interpret, allowing many deaf people to hear.

The team then tested the children's language development and speech comprehension every few months for several years after the procedure.

Svirsky found that the rate of language learning was greatest for those given implants before they turned two. This was measured with the widely used Reynell Developmental Language Scales, which test a child's vocabulary and understanding of grammar. Children given implants at three or four years of age acquired language skills more slowly, although Svirsky stresses that these children still benefited from the devices.

Implantation before the age of two is still relatively rare, but "this study is convincing evidence that implantation in the first two years of life is more beneficial", Svirsky says. He presented his results on 16 May at the Acoustical Society of America conference in Vancouver, Canada.

The forbidden experiment

Some evidence for a sensitive period of language formation comes from the tragic cases of feral or isolated children, who were cut off from any language stimulus in their formative years and never learned to speak using proper syntax. But the physical and emotional deprivation that these children suffer almost certainly affects their ability to interact with people, having a knock-on effect on their language skills. "It's hard to tease out those effects," says Svirsky.

The ultimate test would be what Svirsky calls the 'forbidden experiment': deliberately depriving children of all language input, which is obviously impossible.

Instead, many researchers in this field observe the progress of profoundly deaf children given implants. It's not a perfect way of studying language development, "but it's a new and exciting source of evidence", Svirsky says.

Not a cure

The Food and Drug Administration approves cochlear implantation in children as young as 12 months old, and even earlier in special cases. But this is rare. "Of the children who received cochlear implants at our centre in the past few years, less than a third received their implants in the first two years of life," says Svirsky. "I hope our results will have an impact on clinical practice."

Cochlear implantation is deeply controversial. The procedure has sometimes been billed as a 'cure' for deafness, says Gwen Carr, deputy chief executive of the National Deaf Children's Society in London, UK. "But a lot of deaf people don't feel the need to be 'cured'," she says.

She says that parents should not be rushed into making decisions about surgery on their child. "Early communication isn't just about speech," she says, pointing out that the child's sensitive period is just as useful for developing sign language.

"Cochlear implants are not a miracle for most children," says Elissa Newport, a language development expert at the University of Rochester, New York. She suggests that the best solution would be for children with implants to learn sign language and speech simultaneously, as a bilingual approach seems to boost general language skills.