Deaf children who are given “cochlear” implants before 30 months of age are better able to combine what they hear and what they see than those who receive their implants when they are older, investigators report in the Proceedings of the National Academy of Sciences.
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.”
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,” senior author Dr. Eric I. Knudsen of the University of Maryland in College Park and his colleagues explain.
The authors used this set-up 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 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. Among those who did not, 80 percent reported the auditory component of the stimulus.
The pattern differed 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 Six responded incorrectly to congruent syllables, and were withdrawn from the study.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.
“Thus, 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,” Knudsen and his associates write.
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.
These data “argue strongly for screening children for hearing capabilities and providing cochlear implants when necessary at the earliest possible ages,” the research team advises.