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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.
