Hearing implants give best results when placed early

Investigators say that deaf children who are given cochlear implants before the age of 30 months are more able to combine what they hear and what they see than those who receive their implants when they are older.

According to Dr. Eric I. Knudsen of the University of Maryland and his colleagues, individuals with normal hearing typically report hearing the syllable "ta" when they hear an audio recording of one syllable while watching a video recording of a speaker saying a different syllable, such as "pa" or "ka".

Knudsen says this effect demonstrates that, in most people, the central nervous system combines visual information from the face with acoustic information in creating the speech percept.

The authors used this concept to test 36 children ages 5 to 14 years who were deaf since birth and who had used cochlear implants for at least a year, and 35 normal hearing children.

They presented the children with each combination of the two syllables "pa" and "ka" 10 times.

All of the normal hearing children correctly reported the syllables presented congruently, most reported "ta" when presented with the incongruent audiovisual stimulus, while among those who did not, 80 percent reported the auditory component of the stimulus.

The pattern was somewhat different for the deaf children.

Six responded incorrectly to congruent syllables, and were withdrawn from the study.

Among the remaining deaf children, 20 percent demonstrated integration of auditory and visual information.

Knudsen and his team say therefore that children can learn to combine visual information about lip movements with the highly unnatural neural activation patterns evoked by the implants in the processing of speech.

The responses were related to the age at which implants were received.

All of those who exhibited consistent integration had received implants before 30 months of age.

However, none of those receiving implants at later ages exhibited the same capability.

The team advise that the data is a strong argument for screening children for hearing capabilities and providing cochlear implants when necessary at the earliest possible age.

The study is published in the Proceedings of the National Academy of Science, online December 5, 2005.