Early Implants Found to be Critical in Establishing Hearing Pathway

From: AScribe - 03 Jan 2003

TEMPE, Ariz., Jan. 3 (AScribe Newswire) -- Even before children talk, they are listening - and their brains are changing.

A study comparing brain activity of children in response to sound, including both normal and deaf children who have received cochlear implants at varying ages, has given solid evidence for a specific, critical period during early childhood when the neurological development of the human hearing system occurs and is dependent on auditory stimulation.

A report on the study, co-authored by Anu Sharma of the University of Texas at Dallas, and Michael Dorman and Anthony Spahr of Arizona State University, appears in the December issue of Ear and Hearing.

Looking at brain wave response to sound stimulation, the study notes that in hearing children there is a rapid development of neurological pathways in response to sound at an early age.

The study finds that deaf children with cochlear implants can show similar neurological development, but only if they have implants before the age of three and one-half. When children have implants between the ages of three and one-half and seven, less than half show a response similar to hearing children, and virtually none develop a normal response after the age of seven. Early exposure of the nervous system to auditory stimulation is apparently critical to developing the neurological pathway for hearing response.

"If you're a parent of a deaf child, this is information that you need to consider. If you're a neuroscientist, this is the first documentation of the existence of a critical period for development of the human auditory system," said Dorman.

According to Dorman, the study began five years ago when Sharma, then at ASU, suggested that the team begin collecting brain activity data on a group of children with cochlear implants that the scientists were observing. Sharma pointed out that a similar data set already existed for hearing children, and a comparison might be useful.

The brain wave dataset for normal hearing children indicated that brain responses to sound are relatively slow at birth and then become more rapid until the age of about four, when the increase begins to level off. The data collected by Sharma, Dorman and Spahr indicated that children who detect sound through cochlear implants can have brain wave responses that fall along the same development curve, but only if the implants are made before the age of three and one-half. Deaf children who had implants after the age of three and a half were likely to have brain responses that were significantly slower and outside the normal range. Almost all children who had implants after the age of seven had brain responses to sound that were significantly slower than those of hearing children.

The team drew the conclusion that the brain's response to sound develops during a specific period of early childhood and requires input, either from the natural ear or from an implanted device, in order for the neural pathway for processing hearing to form.

"This turns out to be an important part of the story for parents. If you implant under three and a half, you're catching the window of sensitivity. Responses are within the normal range within six months of implanting," said Dorman. "If you implant between three and seven, your odds of doing well are poorer, but still fair. If you implant after seven, your odds are very slim."

-30-

© Copyright AScribe - The Public Interest Newswire